port and ocean engineering under arctic conditions - Poac.com

PORT AND OCEANENGINEERINGUNDER ARCTIC CONDITIONSEdited byW.M. SACKINGER, Ph. D., P.E.M.a. JEFFRIES, Ph. D.The Geophysical InstituteUniversity of Alaska FairbanksVOLUME I1988The Geophysical InstituteUniversity of Alaska FairbanksFairbanks, Alaska

Copyright © 1988 by the Geophysical Institute, University of Alaska Fairbanks. All rightsreserved. No part of this publication may be reproduced, stored in a retrieval system,or transmitted in any form or by any means, electronic, mechanical, photocopying, recording,or otherwise, without the prior written permission of the publisher, the GeophysicalInstitute, University of Alaska Fairbanks, Fairbanks, Alaska 99775-0800, U.S.A.ISBN 0 - 915360 - 05 - 5

PREFACEThe series of conferences on Port and Ocean Engineeringunder Arctic Conditions (POAC) is organized biennially bynational POAC committees under the long-term policy direction ofthe POAC International Committee. Previous POAC conferences havebeen held in Norway (2), Canada (2), Iceland, Finland, Greenlandand Alaska. The Ninth Conference (POAC-87) in the POAC serieswas held at the University of Alaska Fairbanks, Alaska, USA fromAugust 17-21,1987. This multi-volume book, entitled "Port and OceanEngineering Under Arctic Conditions", is a compilation of the paperswritten for and presented at POAC-87.A total of 224 people registered for POAC-87 and 122 paperswere presented during 14 sessions. The sessions were: ArcticDatabase; Ice Properties; Icebreaking Vessels; Ice Modelling;Arctic Port Design; Geotechnical; Ice-structure Interaction; IceMorphology; Ice Dynamics; Ice, Climate and Forecasting; SprayIce; Remote Sensing; and two special symposia on Noise and MarineMammals, and Steel/Concrete Composite Structural Systems.Papers submitted to POAC-87 were reviewed and edited priorto publication. All the papers in this book have been refereedby two, three, or more reviewers, and then edited, to try toensure a consistent and high standard for technical content,style and format for publication. Once accepted for publication,authors submitted a camera-ready copy of their papers. Themajority of papers in this book were verbally presented at POAC-87; a few authors were unable to attend the conference, but theirpapers have been published since they met the necessary reviewand editorial standards.iii

ACKNOWLEDGEMENTSMany individuals and organizations contributed to thesuccess of POAC-87 and to the publication of this book.The conference SPONSORS were:University of Alaska FairbanksGeophysical Institute. University of Alaska FairbanksMinerals Management Service. Technology Assessment andResearch ProgramNational Science FoundationMinerals Management Service. Environmental Studies Branch.Alaska OCS RegionAlaska Oil and Gas Association. Lease Planning and ResearchCommittee. Member Companies:Amoco Production CompanyARCO Alaska. Inc.BP Alaska Exploration Inc.Chevron USA. Inc.Conoco. Inc.Elf Aquitaine PetroleumExxon Company. USAMarathon Oil CompanyMobil Oil CorporationShell Western E & p. Inc.Standard Alaska Petroleum CompanyUnocal Corporationand the CO-SPONSORS were:v

American Society of Civil EngineersAlaska Academy of Engineering and ScienceCentre for Frontier Engineering Research (C-FER)Le Comite Arctique InternationalThe long-term policy of the conferences on Port and OceanEngineering under Arctic Conditions is directed by the POACINTERNATIONAL COMMITTEE (1987):Prof. Per TrydeTechnical University of Denmark (President)Mr. Alf EngelbrektsonVBB-SWECO Engineers, Stockholm, Sweden (Vice President)Prof. Per BruunThe Norwegian Institute of Technology, Trondheim, Norway(Secretary General)Prof. William M. SackingerUniversity of Alaska Fairbanks, Fairbanks, Alaska, USA(Past President)Dr. Pauli JumppanenOy Wartsila Ab. Helsinki, Finland (Past President)Prof. Bernard MichelLaval University, Quebec, Canada (Past President)Mr. K.R. CroasdaleEsso Resources Canada, Calgary, Alberta, CanadaProf. G .R. PetersMemorial University of Newfoundland, St. John's,Newfoundland, Canadavi

Dr. K. TakekumaNagasaki Technical Institute/Mitsubishi HeavyIndustries, Nagasaki, JapanDr. -Ing. Joachim SchwarzHamburgische Schiffbau-Versuchsanstalt, Hamburg, GermanyMr. G. ViggosonVita og Hafnamala Stjarinn, Reykjavik, IcelandDr. E. EnkvistWartsiHi Arctic Research Centre, Helsinki, FinlandDr. T. CarstensNorwegian Hydrodynamics Labs, Trondheim, NorwayProf. Xu Ji-zuTianjin University, Tianjin, ChinaDr. W.F. WeeksUniversity of Alaska Fairbanks, Fairbanks, Alaska, USAPOAC-S7 was organized by the U.S. NATIONAL· ORGANIZINGCOMMITTEE:Prof. W.M. Sackinger, Chairman; University of AlaskaFairbanks, Fairbanks, AlaskaMr. Muhammed A. AliChevron Corporation, San Francisco, CaliforniaProf. F. Lawrence BennettUniversity of Alaska Fairbanks, Fairbanks, AlaskaMr. Chris BirchState of Alaska Department of Transportation, Fairbanks,Alaskavii

Mr. Irving BoazShell Oil Company. Houston. TexasComdr. Lawson W. BrighamU.S. Coast Guard. Boston. MassachusettsMr. David ChiangScience Applications International Corp .. McLean.VirginiaProf. Jin S. ChungColorado School of Mines. Golden. ColoradoMr. Roger ColonyUniversity of Washington. Seattle. WashingtonDr. M.J. FeifarekMarathon Oil Company. Houston. TexasMr. Joseph GalateEnertech Engineering & Research Company. Houston. TexasProf. Ben. C. Gerwick. Jr.University of California-Berkeley. Berkeley. CaliforniaMr. H. Glenzer. Jr.State of Alaska Department of Transportation. Fairbanks.AlaskaMr. Roger HerreraStandard Alaska Production Company. Anchorage. AlaskaMr. Malcolm W. HowardBP Petroleum Development Ltd .. London. United KingdomMr. Jerry ImmMinerals Management Service. Anchorage. Alaskaviii

Dr. Martin O. JeffriesUniversity of Alaska Fairbanks. Fairbanks. AlaskaDr. Jerome B. JohnsonUSA CRREL. Ft. Wainwright. AlaskaMr. Austin KovacsUSA CRREL. Hanover. New HampshireDr. Thomas KozoUS Naval Academy. Annapolis. MarylandProf. Charles LaddMassachusetts Institute of Technology. Cambridge.MassachusettsDr. Malcolm MellorUSA CRREL. Hanover. New HampshireDr. Thomas OsterkampUniversity of Alaska Fairbanks. Fairbanks. AlaskaMr. Dennis PadronHan-Padron Associates. New York. New YorkDr. Robert S. PritchardIce Casting. Inc .. Seattle. WashingtonProf. Louis ReyLe Comite Arctique International, Monte Carlo. MonacoMs. Patricia SackingerFairbanks. AlaskaMr. Terry SetchfieldExxon Production Research Company. Houston. TexasProf. Lewis ShapiroUniversity of Alaska Fairbanks. Fairbanks. Alaskaix

Dr. Harold ShoemakerUS Department of Energy, Morgantown, West VirginiaDr. Charles E. SmithMinerals Management Service, Reston, VirginiaMr. Rodney SmithMinerals Management Service, Anchorage, AlaskaDr. Walter SpringMobil Research and Development Corporation, Dallas, TexasProf. William StringerUniversity of Alaska Fairbanks, Fairbanks, AlaskaMr. Larry SweetUniversity of Alaska Fairbanks, Fairbanks, AlaskaProf. Shyam SunderMassachusetts Institute of Technology, Cambridge,Massach usettsMr. Stephen D. TreacyMinerals Management Service, Anchorage, AlaskaMr. Michael UttUnocal Corporation, Brea, CaliforniaDr. Ken VaudreyVaudrey & Associates, San Luis Obispo, CaliforniaMr. Robert VisserBelmar Engineering and Management Service Co., RedondoBeach, CaliforniaDr. Vitoon VivatratEngineering Science Inc., Houston, Texasx

Dr. W.F. WeeksUniversity of Alaska Fairbanks, Fairbanks, AlaskaProf. Gunter WellerUniversity of Alaska Fairbanks, Fairbanks, AlaskaDr. J. Patrick WelshNaval Ocean Research and Development Activity, Hanover,New HampshireMr. Jonathan WiddisState of Alaska Department of Transportation, Fairbanks,AlaskaDr. Jay WiedlerBrown and Root USA, Houston, TexasAn important and vital task in the organization of POAC-87and preparation of papers for publication was the review andevaluation of abstracts and papers. In addition to all membersof the International Committee and the U.S. National OrganizingCommittee, the reviewers included:Dr. H. Burcharth, University of Aalborg, DenmarkDr. A. Chen, Exxon Production Research Company, Houston,TexasMr. Li Fu-cheng, University of Alaska Fairbanks, Fairbanks, AlaskaDr. James U. Kordenbrock, David Taylor Research Center, U.S. NavyMr. Donald Kover, David Taylor Research Center, U.S. NavyDr. C.-H. Luk, Exxon Production Research Company, Houston,TexasDr. Lasse Makkonen, Technical Research Centre of Finland,Dr. A. L. Mindich, Mirza Engineering Inc., Chicago, Illinoisxi

Mr. J. Poplin. Exxon Production Research Company. Houston.TexasDr. T. D. Ralston. Exxon Production Research Company. Houston.TexasDr. Philip A. Sackinger. Massachusetts Institute of Technology.Cambridge. MassachusettsDr. A. Wang. Exxon Production Research Company. Houston.TexasAlso helping with conference organization and thepreparation of this book were Kathryn Coffer. Nancy Smoyer.Jan Dalrymple and Kim Morris. Day-to-day conference administrationand co-ordination was by the Conferences and Institutes Office.University of Alaska-Fairbanks (Nancy Bachner and staff).Special thanks are due to Dr. S.-I. Akasofu. Director.Geophysical Institute. and to Dr. P.J. O·Rourke. Chancellor.University of Alaska-Fairbanks. for their encouragement andfinancial support; the encouragement of Dr. Harold D. Shoemakerof the U.S. Department of Energy was also appreCiated.To our sponsors and co-sponsors. the International andNational Organizing Committees. and all those individuals whohelped make POAC-87 and the publication of this book possible.our grateful thanks.William M. SackingerMartin o. JeffriesFairbanksJanuary 1988xii

FOREWORDIn 1985 I participated in the POAC Conference inNarssarssuaq, and for a multitude of reasons I was absolutelydelighted when I received Mr. Sackinger's and Mr. Tryde'sinvitation to address the POAC Conference in 1987 here inFairbanks, Alaska. The first reason is because I am keenlyinterested in the development of polar science and technology.After my appointment as Minister for Greenland in September 1982,I soon realized that the polar regions occupy an absolutelycentral position in the world picture, and the significance ofthe Arctic areas is overwhelming both in relation to theexploitation of natural resources, national security and defensepolicies and to scientific research, which includes the relationbetween polar regions and global environment. Through myinvolvement I have also established many contacts in the world ofinternational polar science.My second reason for being so delighted to be here today isthat I now have a unique opportunity to thank and take leave ofall the international researchers and scientists I have met overthe past five years. This Conference is my last officialappearance; after I return to Denmark I shall withdraw from theDanish government to take up a new position as headmaster of theEuropean School at Abingdon in the U.K., a new opportunity towhich I am looking forward.Therefore, at this final hour, I intend to give you asummary of the state of affairs after my five years' work asMinister for Greenland. I would especially like to focus onxiii

Denmark's position in the Arctic world and the possible rolesthat Denmark might play in the years ahead in relation tointernational polar science and technology.The small Arctic nations, which are primarily Norway andDenmark, are confronted with an Arctic dilemma; a dilemma whichis not facing countries without Arctic regions within theirboundaries. Countries such as Denmark and Norway must givepriority to applied research in their national budgets, whereascountries without Arctic regions, but with interests in andtraditions for Arctic research, can concentrate exclusively onthe more attractive scientific problems which may shed light ontheir particular scientific efforts, as they do not have toearmark funds for the necessary, but more routine accumulation ofscientific data. A case in point is our geologists who must, ofcourse, undertake a general geological mapping of Greenland. Asyou will understand, this is an extremely demanding task for asmall nation such as Denmark. Naturally, we could not do thiswithout the work of the efficient and competent geologistsattached to the Greenland Geological Survey. Over the years ourgeologists have been able to involve and draw on universitygeologists at home and abroad, and this has resulted in a veryfruitful cooperation for all involved. Geologists outsideDenmark are privileged-they have no general commitment withrespect to applied research and are free to focus on morespecific and attractive studies; the study of intrusions, forinstance. The paradox of this situation is becoming more obviousas the integration of state-of-the-art polar science into globalxiv

science is given greater and greater emphasis. In studyprogrammes such as the World Climate Research Programme. thenorthern component is an essential. and perhaps the ultimate.component of global studies.This is why initiatives for advanced. polar researchprogrammes are. understandably. often launched by non-Arcticcountries; a situation which is causing problems in the smallArctic countries and which has also been brought up by theCanadian Government's scientific adviser. Dr. Fred Roots.Scientific Advisor to Environment Canada. The Nordic countriesand Canada would benefit tremendously by such research projectsand would indeed very much like to participate in theseinternational science programmes. They would be able tocontribute vital information. facts and competence based on theirknowledge of local conditions. but our researchers and scientistsare often excluded from participation because of their domesticobligations. and thus they do not derive the full benefit of theprogrammes.The problems facing the small nations will have to beunderstood. and the planning of international scientificcollaboration will have to make allowances for our situation. Onthe initiative of Professor James Zumberge. who is chairman ofthe U.S. Arctic Research Commission. a brainstorming seminar washeld early this spring at the Norwegian Polar Institute in Oslo.with participants from all Arctic-rim nations. including theSoviet Union. Based on the results of this seminar. a small taskforce with representatives from Canada. Norway and Denmark wasxv

asked to draft a proposal for the creation of an InternationalArctic Science Committee under the International Council ofScientific Unions. The details of the proposal will be discussedat a meeting in Stockholm in the autumn. The task force alsosubmitted a proposal for the structure of a consulting body forthe enhancement of communications and contacts between Arctic rimnations.As I find the proposals outlined by the task force to beextremely useful. I warmly support these proposals which will, ofcourse, have to be subjected to keen scrutiny by the relevantnational and international agencies. These agencies, which arethe accepted forums, must be completely aware of the manyfacetted and multidisciplinary nature of polar research projects,of the circumpolar aspects, of the need to relate Arctic researchto global research programmes, and of the need for the establishmentof a computerized information system and the exchange of data andinformation at an international level. In addition to a polarscience organization, the existing Comite Arctique Internationalshould continue as an important forum for needed discussionsbetween scientific, administrative and business communities, asit is vital that business aspects are not ignored whendetermining the priorities of research efforts.On the other hand, our domestic obligations have resulted inconsiderable Danish competence and experience within Arctictechnology, obtained through our research activities and theprofessional support of these activities. They have also been ofimmense importance in connection with the solution of problemsxvi

within the local communities and the development of theinfrastructure in Greenland. Over a period of many years, Danishand Greenlandic undertakings have demonstrated not just theirtechnological and professional expertise, but also, and perhapsspecifically, their ability to meet the challenges of logistics,such as the establishment of a 100% self-sufficient industrialenterprise in a vast unexplored area; a tremendous challenge ofplanning, coordination and execution.Since the first of January 1987, the Greenlandic Home RuleAdministration has been responsible for the operation of theGreenland Technical Organization, but there is still a tremendousamount of Arctic experience and expertise in Danish businessundertakings, and in my opinion we must preserve the continuityand further development of this expertise for the benefit ofGreenlandic and Danish enterprises. One of the aims would be tostimulate and promote export of Arctic technology, and in January1986 I contacted Mr. Gunnar Rosendahl, who is the managingdirector of the Greenland Technical Organization, and thus, atthat time, a member of my staff, to ask him to accept thechairmanship of a committee which was to draw up a reportoutlining the future possibilities of technology exports byDanish and Greenlandic companies.The export-related expectations focus first and foremost onthe potential for utilization of the occurrence of natural gas,oil and other special minerals as well as related activities, butalso the expansion of the Arctic areas in general is attractingattention. It was natural also to ask the committee to look intoxvii

the opportunities for Danish companies to continue theirtechnological efforts in Greenland after the Home RuleAdministration had taken over the operation of the GreenlandTechnical Organization. By technological efforts I am thinkingnot only of consultancy services and related expertise, but alsoof the potential for exports of supplies, contract work andservices. In this connection, we would have to consider theinitiatives required for the companies to qualify for thehandling and undertaking of new assignments in the Arctic areas.The committee's report was finished in the autumn of 1986.During their work the committee had expanded the definition ofArctic areas to include both areas with mean temperatures ofunder 10°C in the warmest month of the year, and cold areas withmean annual temperatures of under OOC (permafrost regions) aswell as areas with very low winter temperatures (cold continentalclimate). In geographical terms it means that the committeeincluded areas such as Alaska, the Yukon, the Canadian NorthwestTerritories, the northern regions of the Soviet Union, China'salpine areas, the northern part of Scandinavia and theAntarctic.The committee found that there are two types of development:one serving to improve the local needs of residents and theeconomies of their regions, andone serving the exploitation of natural resources or theexistence of military installations.xviii

The development within the exploration and exploitation ofminerals and oil stimulates the inflow of expertise andtechnology from outside, and the interest in the utilization ofthese raw materials and the related technology probably will beincreasing well into the next century in concert with the growingdemand for energy and special minerals. This is also the reasonwhy industrialized countries such as West Germany, Japan andothers are displaying an enormous interest in Arctic technology.As I just said, we expect the future demand for extractionof oil and natural gas in Arctic regions to grow, but before wecan launch any large-scale extraction activities we will have tosurvey huge areas with special and technologically complexequipment. The committee, therefore, recommends that weaccumulate and coordinate expertise on prospecting methods inice-filled waters, on the behaviour of materials under Arcticconditions and on the effects of ice on mechanical construction;and that we then assemble and incorporate this material into aworkable concept which could be tested in Greelandic waters underextreme conditions.It would be qUite a long speech if I were to elaborate onall the conclusions and recommendations of this report, but it isworth noting that the report emphasizes the necessity to retainand expand Danish and Greenlandic businesses' knowledge ofcurrent Arctic technology if we are to hold our own in the fieldof technology exports in the years to come. This means that wemust currently follow the international activities anddevelopments in the cold regions. The committee also says thatxix

university education in Arctic technology should bestrengthened. Moreover, the committee recommends that theeducational institutions allot more time and facilities to polarscience and development.On the whole, the committee finds that polar science shouldbe mapped out, coordinated and strengthened, for instance throughthe creation of a Danish Polar Centre. The committee'sconclusion supports the initiative which I mentioned when Iaddressed the POAC Conference in Narssarssuaq in 1985, when Itold you that I had already set up a working group to examine andassess the need for, and the possibilities of establishing aDanish Polar Centre.The committee finished its report a few months after thePOAC Conference in 1985. It is therefore a personal pleasure forme to tell you that the plans for the establishment of a PolarCentre have now been discussed by the Finance Committee of theDanish Parliament which only last week decided that this questionwas so important that it should be debated in Parliament. Thisdebate is expected to take place in December or early next year.In connection with the preparatory work done by thecommittee, the idea of a Polar Centre was criticized in certainquarters with warnings that this would mean too much bureaucracy inscientific activities and that the responsibilities of existingscientific institutions might overlap. But the hearings thatwere held in connection with the committee's work revealed animmediate need for the strengthening of the secretariat of theexisting Commission for Scientific Research and a need forxx

planning of Arctic research and the setting up of an informationcentre and a service body for Danish authorities, the Home RuleAdministration and Danish trade and industry. A Danish PolarCentre would be able to handle all these functions and is in factessential if we are to continue our involvement and maintain ourposition in international polar science. The hearings alsorevealed that many institutions would be very interested incooperating within a common physical framework of an "Arcticenvironment" without having to sacrifice their currentaffiliations or integrity.For Denmark, with our special position in the Arctic area,it is natural that we are seeking to continue and promote Arcticexpertise through the establishment of a Danish Polar Centre forthe purposes of planning and coordinating Danish polar scienceand integrating Danish and international scientific activities inGreenland, as well as, ensuring our participation in internationalArctic-related scientific collaboration. The Polar Centre wouldalso be responsible for the identification and definition ofresearch projects and the initiation and support of scientificactivities, as well as the establishment of an informationcentre. Once the Polar Centre is opened, the Ministry forGreenland will have made its vital contribution to Denmark'sposition within polar science and technology well into thefuture. We will have ensured that current scientific experienceand expertise are preserved through the fulfillment of ourdomestic obligations and that Denmark can continue to play acentral role in international scientific cooperation.xxi

I am convinced that you will enjoy the POAC-87 Conferencetremendously, and I would like to thank all of you for yourefficient and inspiring collaboration during my five years asMinister for Greenland.For more than 10% of my life, I have been 100% occupied withArctic matters. I can guarantee that Arctic matters will be animportant part of my life in the future too. But moreimportant, the Arctic will play an increasing role in the worldfor all mankind.Hon. Tom Hct>yemMinister for GreenlandCopenhagen, DenmarkAugust, 1987xxii

TABLE OF CONTENTSVOLUME IPreface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . "Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..iiivForeword ............................................................ . . . . . . . . . . . . . . xiiiSEA ICE PROPERTIESUNIAXIAL AND BIAXIAL COMPRESSIVE STRENGTH OF ICE SAMPLED FROM MULTI-YEARPRESSURE RIDGES.Franz Ulrich Hausler. Edward N. Earle and Peter Gerchow. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1TIME-SERIES VARIATIONS IN ICE CRUSHING.G. W. Tlmco and I. J. Jordaan ....................................................... , 13THE APPLICABILITY OF LEFM AND THE FRACTURE TOUGHNESS (KIC) TO SEA ICE.Jukka Tuhkurl .................................................................... 21CREEP PROCESS AND RUPTURE CHARACTERISTICS OF SEA ICE IN THE BOHAI SEA.LI Zhl-jun. LI Fu-cheng and Sui JI-xue ................................................. , 33STUDY OF THE FLEXURAL STRENGTH AND ELASTIC MODULUS OF SEA ICE IN THE BOHAISEA.Sui JI-Xue. Li Fu-cheng. LI Zhl-jun. Zhang Mlng-yuan and Yu Yong-hal . . . . . . . . . . . . . . . . . . . . . .. 39STUDIES ON ADHESION STRENGTH OF, SALINE ICE.Lasse Makkonen and Eila Lehmus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 45ICE MORPHOLOGYSOME PHYSICAL PROPERTIES OF MULTIYEAR LANDF AST SEA ICE. NORTHERN ELLESMEREISLAND. CANADA.Martin O. Jeffries. William M. Sacklnger and Harold D. Shoemaker. . . . . . . . . . . . . . . . . . . . . . . . . .. 57GEOMETRY AND PHYSICAL PROPERTIES OF ICE ISLANDS.Martin O. Jeffries. William M. Sacklnger and Harold D. Shoemaker. . . . . . . . . . . . . . . . . . . . . . . . . .. 69A NEW LOOK AT SEA ICE THICKNESS.Roger Colony . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 85THE USE OF POLYSULPHIDE RUBBER MOULDS TO MEASURE ICE ROUGHNESS.R. H. Goodman. A. G. Holoboff. T. W. Daley. L. D. Murdock and M. Flngas .................... , 95SEA ICE REMOTE SENSING. CLIMATE. AND FORECASTINGTHE ALASKA SAR FACILITY.W. F. Weeks. G. Weller. J. Miller. F. D. C;arsey and J. E. Hilland ............................ , 103xxiii

AIRBORNE MEASUREMENT OF SEA ICE THICKNESS AND SUBICE BATHYMETRY.Austill Kovacs and Nicholas C. ValleauELECTROMAGNETIC MEASUREMENTS OF A SECOND-YEAR SEA ICE FLOE.Austill Kovacs and Rexford M. MoreyA RAPID METHOD FOR MAPPING SEA ICE DISTRIBUTION AND MOTIONS FROM NOAASATELLITE IMAGERY.Lewis H. Shapiro, Kristina Ahlnas and Coert Olmsted ....SATELLITE OBSERVATIONS OF THE NORTHERN BERING SEA.Kenneson G. Dean, C. Peter McRoy, Kristina Ahlnas and Thomas H. GeorgeAN EVALUATION OF AN OPERATIONAL ICE FORECASTING MODEL DURING SUMMER.Walter B. Tucker III and William D. Hibler III .A THREE-LEVEL DYNAMIC THERMODYNAMIC SEA ICE MODEL.Lu Qian-ming, Asger Kej and Erland B. Rasmussen .GLACIAL EUSTACY VS. LEVEL RISE: ITS EFFECTS ON SHORE STABILITY IN THEARCTIC.Per BruunAN ICE AND SNOW CLIMATE INFORMATION SYSTEM (CRISP).T. Agnew and T. W. MathewsSURFACE CIRCULATION PATTERNS IN YAKUTAT BAY.Gary Hufford and Ron ScheidtSHELF BREAK UPWELLING IN THE DENMARK STRAIT.John W. FoersterIII121137149159... 175. .... 187205215227ICE DYNAMICSTIME DOMAIN SIMULATION OF THE DRIFTING OF SMALL FLOATING BODIES IN WAVES.Jacek S. Pawlowski and Momen A. WishahyWAVE REFLECTION FROM AN ICE EDGE.Torkild Carstens and Arild R0sdalDYNAMICS AND MORPHOLOGY OF THE BARENTS SEA ICE FIELDS.Torgny VinjeANALYSIS OF ICE ISLAND MOVEMENT.W. M. Sackinger and H. R. TippensCONSTITUTIVE RELATIONS IN SEA ICE MODELS.Lu Qian-ming, Jesper Larsen and Per Tryde239.. 253263269. 279EXPERIMENTAL DETERMINATION OF THE FRACTURE TOUGHNESS OF UREA MODEL ICE.D. L. Bentley, D. S. Sodhi and J. P. Dempsey 289xxiv

SOME INVESTIGATIONS FOR EG/AD MODEL ICE.K. Hirayama and N. Sakamoto ..MULTIYEAR RIDGE LOAD ON A CONICAL STRUCTURE.Kazuhiko Kamesaki and Nobutoshi YoshimuraICE LOAD PENETRATION MODELLING.K. Riska and R. FrederkingMODEL TESTS FOR MULTIYEAR ICE LOADING AGAINST A FIXED CONICAL STRUCTURE.M. M. WinklerMODEL TESTS ON ARCTIC STRUCTURES IN ICE.S. S. Gowda and Risto Hakala .299307317329339ICE/STRUCTURE INTERACTIONAN INTEGRATED DESIGN APPROACH TO ARCTIC OFFSHORE PLATFORMS.Kailash C. Gulati and Jay B. Weidler "DESIGN AND OPERATIONAL CRITERIA FOR SYSTEMS SUBJECT TO ICE ENVIRONMENTALCONDITIONS.M. Nessim and T. Nasseri ...RELIABILITY ASSESSMENT OF A PRESTRESSED CONCRETE ARCTIC OFFSHORE PLATFORM.Jal N. Birdy and Irvin B. Boaz .DESIGN SEA ICE LOAD EXAMPLES USING API RECOMMENDED PRACTICE 2N.M. E. Utt. K. D. Vaudrey and B. E. Turner ..... .THE DISTRIBUTION OF ICE PRESSURE ACTING ON AN OFFSHORE CIRCULAR PILE.Sachito Tanaka. Kohoki Sasaki. Toshiyuki Ono and Hiroshi Saeki ...... .. 345353367387395A NUMERICAL SIMULATION METHOD FOR FAILURE ANALYSIS AND LOAD ESTIMATION.Tadashi Shibue. Kazuyuki Kato. Yasushi Kumakura and Yutaka Toi ...... .ELASTO-PLASTIC ANALYSIS OF ICE FORCES ON CYLINDRICAL STRUCTURES.Y. Taguchi. T. Kawasaki. S. Tozawa and S. Ishikawa ...STRUCTURAL ARRANGEMENT OF PRODUCTION PLATFORMS ACCORDING TO THEICE-INDUCED VIBRATION ANALYSIS.Meng Zhao-ying and Wang Ling-yu .. ... . ... .... 413. 427437VERIFICATION TESTS OF THE SURFACE INTEGRAL METHOD FOR CALCULATING STRUCTURALICE LOADS.Jerome B. Johnson and Devinder S. Sodhi . . ..... .449MUKLUK ICE STRESS MEASUREMENT PROGRAM.G. F. N. Cox. J. B. Johnson. H. W. Bosworth and T. J. VincentMEASUREMENTS OF MULTI-YEAR ICE LOADS ON HANS ISLAND DURING 1980 AND 1981.B. Danielewicz and D. Blanchet457465xxv

IMPACT ICE LOADS ON OFFSHORE STRUCTURES.A. Tunik ......................... . . .... 485ICEBREAKING VESSELSLOADS ON RESEARCH VESSEL POLARSTERN UNDER ARCTIC CONDITIONS.L. MUller and H. G. Payer. . . . . . . . . . . . .. ............................................. 495ICEBREAKING PERFORMANCE OF RV POLARSTERN IN BROKEN ICE-FULL SCALE TRIALSIN THE WEDDELL SEA. ANTARCTICA.Franz Ulrich Hausler. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 509EVALUATION OF THE MAXIMUM BREAKABLE ICE THICKNESS OF AN ICEBREAKING VESSELFROM RAMMING TESTS IN LEVEL ICE.Franz Ulrich Hausler ................................................................ 521FINITE-ELEMENT ANALYSIS OF THE ELASTO-PLASTIC MODELLING OF THE INDENTATIONPROBLEM IN SHIP-ICE INTERACTION.C. Jebaraj. A. S. J. Swamidas. Steven J. Jones and K. Munaswamy531ICE-RUBBLE BENEATH BARGES IN ICE-COVERED WATERS.Robert Ettema and Hung-Pin Huang ................................................... 543MANEUVERING PERFORMANCE IN ICE OF THE UNITED STATES COAST GUARD 140-FOOTICEBREAKER.Pekka Kannarl and David H. Humphreys ................................................ 557MID-WINTER 1983 SHIP TRANSIT IN THE ALASKAN ARCTIC BY THE ICEBREAKER POLARSEA.Fred Seibold and Richard Voelker ..................................................... 575TANKER LOADING AT EXPOSED ARCTIC TERMINALS.W. H. Jolles .................................... ' ................................... 589COMPUTER SOFTWARE TO ANALYZE ICE INTERACTION WITH MOORED SHIPS.Jorglto Tseng. Norm Allyn and Ken Charpentier. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 607A COMPUTER-AIDED STRATEGIC ROUTE SELECTION SYSTEM.N. R. Thomson and J. F. Sykes ..................................... . . . . . . . . . . . . . . . . .. 619SHIP/ICE PROBABILITIES IN ARCTIC SHIPPING.C. Ferregut. M. Perchanok and C. Daley ................................................ 631WINTER RELOCATION TECHNIQUES FOR ARCTIC STRUCTURES.G. A. N. Thomas ................................................................... 645xxvi

STEEL/CONCRETE COMPOSITE STRUCTURAL SYSTEMSOrganized by Centre for Frontier Engineering Research. Edmonton. AlbertaUTILIZATION OF COMPOSITE DESIGN IN THE ARCTIC AND SUB-ARCTIC.Ben C. Gerwick and Dale Berner ...................................................... 655DESIGN AND BEHAVIOR OF COMPOSITE ICE-RESISTING WALLS.P. F. Adams, T. J. E. Zimmerman and J. G. MacGregor .................................... 663THE RESISTANCE OF COMPOSITE STEEL/CONCRETE STRUCTURES TO LOCALIZEDICE LOADING.J. R. Smith and A. McLeish .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 675STRENGTH OF COMPOSITE, SANDWICH SYSTEM, ICE-RESISTING STRUCTURES.Masakatsu Matsuishl and Setsuo Iwata ................................................. 689TESTS ON COMPOSITE ICE-RESISTING WALLS.B. O'Flynn and J. G. MacGregor ....................................................... 699EXPERIMENTAL STUDIES ON COMPOSITE MEMBERS FOR ARCTIC OFFSHORE STRUCTURES.F. Ohno, T. Shioya, Y. Nagasawa, G. Matsumoto, T. Okada and T. Ota ........................ 711ARCTIC/OFFSHORE DATA BASETHE ARCTIC AND OFFSHORE RESEARCH INFORMATION SYSTEM.Harold D. Shoemaker and David L. Chiang .............................................. 721Author List ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 735xxvii

UNIAXIAL AND BIAXIAL COMPRESSIVE STRENGTH OF ICESAMPLED FROM MULTI-YEAR PRESSURE RIDGESFranz Ulrich HauslerHamburgische Schiffbau- Versuchsanstalt GmbH., Hamburg, F. R. GERMANYEdward N. EarleShell Development Company, Houston, Texas, USAPeter GerchowHamburgische Schiffbau- Versuchsanstalt GmbH., Hamburg, F. R. GERMANYAbstractA series of 60 compressive strengthtests have been performed on ice sampledfrom multiyear pressure ridges nearReindeer Island, Beaufort Sea. Threecompressive stress states have beeninvestigated: uniaxial with O 2= 0 andbiaxial with 0 2=Oland 02= O.SOI,Twotemperature - straln rate combinationshave been studied:TI = -5 DC, (I10-5 S -1 andT = -20 DC, £ = 1O- 3 s- 1F1r both T-£-~ombinations the coefficientsof an isotropic 3-parameter yieldcriterion were evaluated representingthe yield characteristics in the compressionoctant of the principal stressspace. The multiyear pressure ridge icewas considered to be isotropic on am~croscopic scale. At T = -20 DC,E I = 1O- 3 s- 1 the ice eXhlbited brittlefracture. Under these conditions themeasured compressive strengths were 5times (uniaxial) up to 5.8 times (biaxial01=0 2) higher than at TI = -5 DC,E = -10- 5 S- 1 , where a mostly ductile1mode of failure was observed. TheThis is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA , August 17-22, 1987. © The Geophysical Institute,University of Alaska, 1987.multiyear pressure ridge ice studiedexhibited a wide variety of ice types(snow ice, columnar sea ice, etc.). Ice• • - 3densltles of PI = 809 kg m up to 913kg m- 3 were found . The salinity measuredln the melted samples ranged from SI = 0up to 5.7 0/00'IntroductionThe Alaskan and Canadian Arctic hasproven to be one of the world's mostimportant sour~es of hydrocarbons.Offshore oil and gas exploration andexploitation in this region is impededby ice of seasonally varying severity.In order to achieve a long (at best yearround) drilling season and at the sametime prevent ships and structures fromdamage or loss, both ships andstruct~res must be designed to withstandice loads. Another important aspect ofsafe operation is to protect thesensitive Arctic environment fromavoidable pollution.One of the most hazardous forms ofice loads is exerted by multi-yearpressure ridges which are a commonnatural phenomenon ln this region.Besides the driving forces (current,wind) and the response characteristicsof the individual structure, the failureof multi-year ridge ice yields an1

important limi ting conditionloads . This condition maystructural design.for the icegove rn theMulti-year pres sure ridges can bedefined as thick accumulations of brokenice blocks that have survived at leastone melt seas on. The voids originallycontained in the ridge from the pile upprocess are filled with refrozen waterfrom surface melting. Thus, a multi-yearpressure ridge represents a huge pieceof massive, nearly voidless ice . Itsthickness may exceed 30 m (Kovacs, 1976,1983; Cox et al., 1984).BEAU FORTSEA~ G I~NIRE I N OEER~..,A.RGO l.Information on the mechanicalproperties of multi-year ridge ice isscarce in the literature, except thedata reported by Kovacs (1983) and theresults of the investigations by Cox etal. (1984, 1985). The latter alsocontain the only strength data availableup to now for multi-axial loadconditions: compressive strengths undervarious lateral confinements.In the present study, plane stressfailure characteristics of multi-yearridge ice have been evaluated. For thispurpose in total 60 strength tests havebeen conducted under horizontal,uniaxial compression and underhorizontal, biaxial compression with twodifferent load ratios (I: I and 2: I). Theinfluence of strain rate and temperaturewas investigated by performing the testsat two different temperature-strain ratecombinations .Ice field sampling and shippingThe field sampling was performed byCRREL personnel (CRREL = U.S. Army ColdRegions Research and EngineeringLaboratory) between 3 and 15 April 19 81in an area northwest of Reindeer Islandnear Prudhoe Bay, Alaska (see Fig. I).It was the same field campaign duringwhich the samples were collected forPhase I of the comprehensive CRREL studyon the mechanical properties ofmulti-year sea ice reported by Cox etal . (1984). Their report gives detaileddescriptions of the sampling andshipping procedures; hence only a shortsummary is necessary here .Fig.• 5 I. '"•••••••••••••••••••••GULF I . •PRUOHOE 8A~1A-,.Map of ice sampling area(after Cox et al., 1984)NIAKUK 15.The samples were collected from 10multi-year ridges of various sizes. Theridges were part of the fast ice beltand were apparently not grounded. Allridges showed rounded outline sindicating surface melt processes intheir history.Four vertical cores were drilledfrom each ridge using a 108 mm (4.25in.)fiberglass coring auger (Rand andMellor, 1985). The cores were drilled inpairs (A-B and C-D) on two sites of eachridge. Depending on the ridge's size thesites were from 14 to 46 m apart.Immediately after removal from the ice,the cores were catalogued and packed incore tubes. The tubes were then placedin insulated shipping boxes. No measureswere taken for sample refrigeration inthe field, because the ambient airtemperature of below -15°C was closeenough to the NaCl'2H 20 eutectic, -22.9°C, and because the muIti-year ice had alow salinity (usually < 4% 0 ), Nobrine drainage from the core samples wasobserved . The ice was then shipped byair freight to CRREL, Hanover, N.H.where it was stored at -30°C (Cox, et2

Fig. 2Triaxial closed-loop testingmachinea1. 1984). On 25/26 October 1982 a l o tof 10 tubes with ice samples was shippedto HSVA (Hamburg Ship Model Basin),againby air freight . During all air freightshipping the ice was refrigerated withdry ice. In Hamburg the ice core tubeswere unpacked from the insulated boxesand stored in a freezer at -35°C untiltest sample processing.Test facilitiesThe strength test program wascarried out on a triaxial closed-looptesting machine (Fig. 2). The verticalaxis of this device consists of ascrew-driven universal testing machineof 100 kN load capacity typeSchenck-Trebel RME 100. The twohorizontal axes are provided by anHSVA-designed biaxial testing devicewith servo-hydraulic actuators of morethan 100 kN load capacity each. Thebiaxial frame is mounted on the samebase-frame as the RME 100. Thus thethree a xes form a monolithic unit. Eacho f the three axes is individua llyelectronically closed-loop controlled .Coupling between the axes is performedelectronically. For load application thetesting machine is equipped with threepairs of brush-like loading platens(Hilsdorf, 1965) specially designed forice compression tests (Hausler, 1982).These loading platens consist of acluster of slender metal rods (bristles)arranged in a quadratic array. Theyexhibit a high rigidity in the loaddirection combined with a minimallateral constraint. Their transversecompliance allows the platens to followlateral deformation of the testedspecimen. Thus, even true triaxial testsare possible. The minimal lateralconstraind provides for the principalstresses to be almost perfectly parallelto the directions of the loadsthroughout the (cubic) specimen (cf .Hilsdorf, 1965). At least the edges ofthe samples always remain accessible tostrain measuring devices (Linse, 1975;Hausler, 1982 and 1986). The completeloading apparatus is installed in a coldchamber where any temperature between +2and -30°C can be set. Electroniccontrol and data acquisition devices areinstalled outside the cold room. Fors ample preparation, a second cold roomwith an independent refrigeration systemof the same capacity was used. It wasequipped with a band saw and a lathemodified for milling cubic ice samples.Data Acquisition and RecordingLoads were measured by means of HBM(Hottinge'r Baldwin) Z4 200 kN loadcells. For deformation measurements HBMW2K LVDTs were applied which have amaximum range of + 2 mm. The primaryaxis' deformation transducer wasinstalled in a parallelogram guide (see.Fig. 3), while the other LVDTs weremounted on pairs of bristles on theloading platen sides . Since the bristlesfollow the samples' deformation, theycan be used as pick up devices fordeformation measurements.The analoguous signals from thetransducers were on-line converted todigital data on a HP 21 MX-E series3

computer (Hewlett Packard) a nd stored onfl oppy discs for l a t er off-l ine dat aprocessing. The digitizing rate waslimited by the transfer rate from theAD-converter to the disc and by ~hestorage capacity of the floppy disc. Inthe present study, rates of 100 c.p.s.and 50 c.p.s. were applied in the highstrain rate tests and 4 c.p.s. in th elow s train rate tests. All other datasuch as temperatures or sampl edimensions were recorded manually .Sample Preparation and Test ProcedureSample preparation was done bycutting cubes of 69.8 + O. I mm sidelength from the ice cores.-This was doneby first cutting the cores intocylindrical pieces of 10 cm length andthen cutting these pieces in raw cubesof about 8 cm side length.This wasperformed on a band saw. Fig. 4 showsthe orientation of a n ice cube in theparental core. The raw ice cubes werethen milled down to their targetdimensions on a modified lathe. Forsample dimension control, a highprecision stage with a dial gauge of0.01 mm resolution was employed. Inorder to minimize brine drainage, samplepreparation was performed at a temperatureof about -25 °c, i.e., below theNaCl- 2H 0 eutectic (-22.9 2°c in seaice). Tne samples were then s tored forone day at their target test tempe r a turein order to achieve a homogenoustemperature distribution within thesamples when tested.The ice strengths are potentiallyinfluenced by parameters not systematicallyvaried, such as e.g., samplingsite (core number). vertical position ofa sample in its parental core orsalinity. In order to keep the resultsfree from such parameter effects, theselection of the samples for each of thesix test condition groups was performedby a random process (drawing lots).Prior to each individual tes t. thesample's dimensions were mea sured againand its weight was determined. Then theambient air temperature and the i cetemperature were recorded. The lcetemperature was determined inside aFig. 3Fig. 4Deformation transduce r withparallelogram guided LV Drattached to a pai r of opposi t ebristles of the brush-likel oading platens at the end ofa bi-axial t est (Series 1000).~ AXIS OF PARENTA~ CORECOORDINATE I SYSTEM OF SAMPLEPARENTAL CORE(106.1 MH DIAMETER)CUBIC SAMPLE169.8 MM SIDE LENGTH)Orientation of a cubic samplein its parental core4

eference ice cube of the samedimensions, which had undergone the sametemperature history as the sample to bestrength tested.In the next step, the sample wasinstalled in the loading device. Theprimary control axis' actuator wasdriven to a compressive pre-load ofabout F ~ 0.25 kN (corresponding stress0.05 MPa) and kept these under staticforce control.In the biaxial tests, the same wasdone with the secondary axis but with apre-load corresponding to the target1-2-load ratio. The secondary axis wasthen set to dynamic force control usingthe primary axis' force signal as adynamic setting means. The load ratiobetween the two axes was then keptconstant.In the subsequent steps of theprocedure, the tips of the bristles usedfor strain measurements were frozen tothe sample by a drop of water . Then theparallelogram guided deformationtransducer which was used for straincontrol was put on top of the specimenparallel to the primary axis. Its tipswere attached to two opposite bristles,such that deformation was measuredbetween the loading platens (see Fig.3). This allowed the continuation of atest even past sample fracture. Theprimary axis was then switched toclosed-loop strain control.During the test, the primary axis'strain was controlled dynamically suchthat the primary strain rate wasconstant. Test stop condition was theattainment of the target primary strain.After the test, the sample or its debriswas melted for salinity determination.Data analysisFor each sampling cycle, themeasured forces were converted intostresses using the equation0. : F./A. (i: 1,2)1 1 1( 1 )with 0. the stress and Fj. the load in i­direct£on, and Ai the in1tial cross sec-tion area normal to the i-direction.Simultaneously, ·the deformationsu· 1n i-direction were transformed intostrains E. using the simplified equation1with C. the initial distance between thepick-u~ points of the ui -deformationtransducer (basis length). The initialtangent modulus was read from the slopeof the 0i over Ei plots.Because of the irregulardistribution of ice types and crystalorientations within the ridge ice mass,the ice was assumed to behave isotropicon macro scale. Upon this assumption theice failure condition can be describedby the isotropic three-parameter yieldfunction (Smith, 1974; Reinicke,unpublished)2fCO ij) : aJ 1+ BJ; + YJ 1- 1 : 0 (3)where Oijis the stress tensor and J 1 itsfirst invariant, while J2is the secondinvariant of the stress deviator Sij1 O .. (4. I)s . . 0 .. - 3" °kk1J 1J 1JJ 1 °kk(4.2)1J' s .. s ..2 "2 1J 1J(4.3)For a given temperature-strain ratecondition, the average strength valuesfor the 3 different plane stress loadconditions CO 2ao ; 03=0; la =0,0.5,1 )were computed. These averaged failurestress states were then used todetermine the coefficients of the yieldfunctionf Co .. )1J(5)which is the plane stress case of thethree parameter yield function (eq. 3).ResultsThe averaged test conditionstemperature, strain rate, density, andsalinity are compiled in Table I. The5

Series Tempe rature Strain rate Stress Density Salinityat yiel? ra t io[e~ m- 3 STI [OC) E1 [s ) ]° 1= °2 [ 0/ ° ° )1000 5.0 ! 0.1 1. 00 ! 0.00' 10- 5 1: 1 868 ! 36 1. 8 ! 1.02000 -5.1 ! 0.1 1.00 ! 0.01'10- 5 2: 1 889 ! 15 1.3 ! 0.63000 -20.2 ! 0.1 1.00 ! 0. 09'10- 3 1:1 909 ! 4 1.3 ! 0.64000 -20.2 ! 0.1 0.98 ! 0.09'10- 3 2: 1 89 0 ! 23 1. 9 ! 0.75000 -5 .2 ! 0.1 1.01 ! 0.00'10- 5 1: 0 873 ! 31 1. 7 ! 1.36000 -19.9 ! 0. 1 0.99 ! 0.11'10- 5 1:0 886 ! 17 2.5 ! 1.610002000 -5.1 0.01'10- 5! 0.1 1. 01 ! all 880 ! 27 1.6 ! 1.0500030004000 -20.1 0.09'10- 3! 0.2 0.99 :! all 895 ! 19 1.9 ! 1.26000Table ITest conditionsSeries Primary st~ss Secondary ~ress In i ti a I tangen t Numbe r ofat yield 1 at Yleld 2 modulus te s ts[MPa) [MPa) E [GPa)1000 -2.30 :! 0.90 -2.30 ! 0.91 --- 102000 -2.33 :! 0.23 -1.12 ! 0. 11 -- - 103000 -13 .37 ! 1.87 -13.32 ! 1. 86 -- - 104000 -11. 98 ! 2.12 -5.98 ! 1. 04 --- 105000 -1.40 :! 0. 25 --- 0.87 :! 0.49 105000 -7.15 :! 2.03 --- 6.33 ! 2.40 10Table 2Average strengths and elastic properties6

average s trengths (stresses at yield orfailure) and ini tial tangent moduli arelisted in Tab le 2. The followingcharacteristics have been observed forth e six test series:Series 1000(01:02 1:1, TI = -5 °C , (I = 10- 5S -1)The s tres s-time histories exhibiteda sharp rise until about half the yieldstress. The yield stress was reachedafter a period of strain hardening atstrains () usually less than 17.. Afteryield, the stresses decreased s lightly.At strains around 47., stresses increasedagain and reached or slightly exceededthe initial yield level. During sometest runs, a considerable amount ofbrine was squeezed out of the specimenand formed icicles at the unloadedbottom surface. Cracks, if any, were ofminor size and were in plane with theloads applied (Fig. 5; (11012).up to approximately half the yieldstress followed by a period of strainh~rdening. Yield was reached at strains(~ of usually less than 1%. Typically,the stress dec reased continuous l y afteryield but apparentl y tended towards anasymptotic value to be encounte redbeyond th e end of the t es ts ( ( I > 5%).Cracks were of minor size and were inplane with the loads, similar to thepattern observed in series 1000.Series 5000(01:02 1:0, TI = -5°C, ( I- 5 - 1lOs )The specimens exibited an almostductile mode of failure. In general, nocracks were observed. Deformationperpendicular to the load was large.Yield was reached at an average primarystrain of (~ = 0.7%.Series 3000(01:02 1:1, TI = -20°C, (I = 10-3 S-I)Fig. 5Sample # 1012 after bi-axialcompression test with 01= 02 ,0 3 = 0 at _5°C and 10- 5 S -1The samples typically failed beforereaching a zero slope in the stress overtime (or strain ) curve. The failuremode was usually brittle. In some cases,th e first failure was not catastrophicbut was followed by a second stressrise , which then was terminated by finalfailure. Only one sample broke after aperiod of strain softening. The failurestress (first stress maximum) uas typicallyreached at strains of (1= 0.25%.The samples were usually crushed duringthe tests. The major surfaces of failurewere s lightly inclined to the planeexpanded by the loads. In addition, mostof the samples were filled with microcracks(Fig. 6; (130 13).Series 4000(01:02 = 2:1, TITypically, brittle failure wasobserved in the strain softening part ofstress history. The av~rage primarystrain at yield was (1 = 0 . 327.. Thecrack pattern was similar to the oneobserved in the series 3000 tests.Series 6000(01:02 1:0, TI = -20°C, ( I = 10-3 S -I)The typical stress-time his toryshowed, as in series 1000, a sharp riselessAll samples exhibited a more orbrittle failure. After the tests,7

____b) T I=-20 .1 °c , = 0.99 • 10- 3 -1£1 sf (0 . . ) 0.0476q+ 0.0262- 0.0396-1MPa (all + 0 ) 22-2 2MPa (all + 0~2)-2MPa ° 0 11 22- 1 0Fig. 6Sample #3013 after bi-axialcompression test with 01 = 02,03 = 0 at -20°C and 10- 3 S-1The strength results are presentedgraphically in Figs. 7 and 8. The upperleft halves show the individually measuredstrengths while in the lower righthalves the average values and 997.confidence interval bars are presented.Also shown for comparison are the resultsobtained at CRREL for the same iceunder uniaxial tension and compressionparallel to the parental core axis (Coxet al. 1984 and 1985).the samples were filled with cracks ortotally crushed. In four (out .of ten)cases, fracture occurred during or atthe end of the stress rise. Fracture oryield was observed at an average primarystrain of only £~ = 0.217.. The governingcrack pattern exhibited crack surfacesoriented parallel to the load, butwithout orientation in the twoperpendicular directions. In some cases,parts of the samples failed additionallyby buckling. The micro-crack density wassmaller than observed in thecorresponding tests with biaxial loadapplication (Series 3000 and 4000).The plane stress yield functions(eq. 5) for the two temperature-strainrate combinations have been determinedas followsa) T =-5.1 °c , 1.01 • 10- 5 -1I £1 = s-1f (0 .. ) 1. 78 MPa (0Il + °22)~J-2 2 2+ 1. 78 MPa (0 11 + 0 22 )-2- 1.83 MPa 0 ° 011 22-1 ="IHPAI-1-1-3-4-5Fig. 7TEHPERATURE T ~ -5.1 ·CSTRAIN RATE 1 ~ 1.01 ·10" S"DENSITY 9 ~ 880 KG Wl _~:>-

.,IMPAIo-2-4TEMPERATURE T = -20 1 ·CSTRAINRATE £=09910-' S-'DENSITY 9 = 895 KG M- 3SALINITY S = 1 9 %. --a>---,.ooo>o------+.-Approximate horizontal compressivestrengths can be calculated bycorrecting the vertical strengths withthe horizontal to vertical ratiosobtained from the 10-~ S-I test. Thiscorrection yields a1~ = -1.79 ~ 0.83 MPafor the -5 'c/IO- s-I case and all =-5.84 + 0.84 MPa for the -20 'c/IO- S-Icase respectively. These values are inreasonable agreement with the results ofthe present study.AVERAGE STRENGTH WITH99%-CONFIDENCE INTERVAL.......... THIS STUDY HOR~ COX ET AL 11984,198S) VERT-20 1L--JL--'-_-L-_L---..l_-L_L----l_-'-_-1.:::l-18 -16 -14 -12 -10 -8 -6 -4 -2., IMPA]Fig. 8DiscussionPlane stress strengths ofmulti-year ridge ice at -20 ·Cand 10- 3 S-IA comparison of the data from thepresent study with the results obtainedat CRREL for the same ice (Cox et al.1984 and 1985) suggests itself. Thediscrepancy between the uniaxialcompressive strengths seems obvious. Coxet al. (1984) report values of -2.34 +1.08 MPa for the -5 'C/I0- 5 s- 1 conditionand -9.63 + 1.39 MPa for the -20 'ci10-3s -1 condition (this study -1.40 +0.26 MPa and -7.26 + 2.03 MPa). The mainreason for this difference is the factthat the assumed isotropy of multi-yearridge ice is not fulfilled. In their"Phase II" test series, Cox et al.(1985) performed a comparison betweenhorizontal and vertical compressivestrengths at a strain rate of 10-~S-I.The vertical was 1.31 times (-5 'C) and1.65 times (-20 'C) as high as thehorizontal strength. The values of Coxet al. (1984) used above for comparisonwere obtained from vertical tests(parallel to the core's axis) while thestrengths of the present study representhorizontal results.The coefficients of the yieldfunctions evaluated, strictly speaking,represent the horizontal plane stressfailure characteristics undercompression only. An extrapolation toplane stress conditions where one orboth of the stresses exhibit tension istheoretically possible. But, due todifferent failure mechanisms undertension and under compression, strenghtspredicted by such an extrapolation candiffer considerably from the realstrengths. It therefore cannot berecommended.Concerning the initial tangentmoduli, only the value 6.33 + 2.40 GPadetermined at -20 ·C and 10- 3 s- 1 issupported by the corresponding modulus7.62 + 1.19 GPa measured by Cox et al.(1984)~ The value 0.87 + 0.49 GPameasured in the present study for the-5'C and 10-5s -1 condition appears tobe too low and is assumed to be due tolarger initial setting of the brushplatens in this case.Comparison of the failure strainsshow for the -20 'C/I0- 3 s- 1 case almostidentical values: 0.19 + 0.04% (Cox etal., 1984) and 0.21 (this study). Thelarger difference in the -5 'C/I0- 5 s- 1case of 0.38 + 0.17% (Cox et aI., 1984)compared to 0~7% (this study) may againbe explained by the larger initialsetting of the brush platens.The density and salinity valuesgiven in Table agree well with thedata reported by Cox et al. (1985):P = 891 + 26 kg m- 3 and 51 = 1.26 ~0.82 °/ •• (Phase I only) .9

ConclusionsThe present study provideshorizontal uniaxial and biaxialcompressive strengths of multi-yearpressure ridge ice. The uniaxialstrengths exhibit good agreement withthe corresponding results obtained byCox et al. (1984) for the same ice. Thisis remarkable, since Cox et al. used 254mm (10 in.) long nearly cylindricalspecimens of 101.6 nun (4 in.) diameterwith bonded synthane end caps (Mellor etaI., 1984), while in the present study,cubic samples of 68.9 nun side length andbrush-like loading platens wereemployed. The good agreement achieved,in spite of different testingtechniques, supports the credibility ofboth data sets and allows theircombination for subsequent analyses.The combined data sets provide anample picture of the mechanicalproperties of ridge ice under theconditions investigated. Nevertheless,as already recognized by Cox et al.(1985), the broad structural variationsencountered in natural pressure ridgesas well as the lack of knowledge on nearto melting conditions demand additionalresearch work.AcknowledgementsThis paper is a joint contributionof the Hamburg Ship Model Basin (HSVA)and Shell Development Company. Thesupport of the U.S. Army Cold RegionsResearch and Engineerlng Laboratory(CRREL), namely Mrs. J.A. Richter-Menge,given in sample shipping is gratefullyacknowledged. Thanks are also expressedto the members of the staff of the HSVAice engineering department whocontributed to the present study, inparticular to Mr. W. Neper who was incharge of sample preparation andtechnical assistance during testperformance.The present study is part of the"Mechanical Properties of Sea IcePhase II" program sponsored by the ShellDevelopment Company and the MineralsManagement Service of the U.S.Department of Interior with support fromAmoco Production Company, ExxonProduction Research Company and SohioOil Production Company.ReferencesCox, G.F.N. et aI., 1984. Mechanicalproperties of multi-year sea ice - PhaseI: Test results. U.S. Army Cold RegionsResearch and Engineering Laboratory,Hanover, N.H., CRREL Report 84-9, April1984.Cox, G.F.N. et aI., 1985. Mechanicalproperties of multi-year sea ice - PhaseII: Test results. U.S. Army Cold RegionsResearch and Engineering Laboratory,Hanover, N.H., CRREL Report 85-16,October 1985.Hausler, F.U., 1982. MultiaxialCompressive Strength Tests on Saline Icewith Brush-Type Loading Platens. IAHRSymposium on Ice, Proceedings, Vol.-rr;pp. 526-539, July 27-31,1981, Quebec,Canada.Hausler, F.U., 1986. Multiaxialmechanical properties of urea doped ice.IAHR Symposium on Ice, Proceedings, Vol.I, pp. 349-363, Iowa City, USA, August18-22, 1986.Hilsdorf, H., 1965. Bestimmung derzweiachsigen Festigkeit des Betons(Determination of the bi-axial strengthof concrete). Deutscher AusschuB furStahlbeton, Heft 173, Berlin 1965, 68 p.Kovacs, A., 1976. Grounded ice in thefast ice zone along the Beaufort Sea=c~0=a~s~t~70~f~A~1~a~s7k~a~. U.S. Army Cold RegionsResearch and Engineering Laboratory,Hanover, N.H., CRREL Report 76-32,September 1976.Kovacs, A., 1983. Characteristics ofmulti-year pressure ridges. POAC-83,Proceedings, Vol. 3, pp. 173-182,Helsinki, Finland.Linse, D., 1975. Losung versuchstechnischerFragen bei der Ermittlungdes Festigkeits- und Verformungsverhaltensvon Beton unter dreiachsigerBeanspruchung mit ersten Versuchen(Solution of problems on testing10

technology for the evaluation of thestrength and deformation behaviour ofconcrete under tri-axial loading withfirst tests). Technische UniversitatMUnchen.Mellor, M., Cox, G.F.N., Bosworth, H.,1984. Mechanical properties of multiyearsea ice - Testing Techniques. U.S.Army Cold Regions Research andEngineering Laboratory, Hanover, N.H.,CRREL Report 84-8, April 1984.Smith, M.B., 1974. A parabolic yieldcondition for anisotropic rocks andsoils. Ph.D. Thesis, Rice University,Houston, TX, USA.Reinicke, K.M., unpublished. Plasticityanalysis with an isotropic threeparametricyield function. Report, BEBGewerkschaften Brigitta und Elwerath,Hannover, Germany, 1977.Rand, J. and Mellor, M., 1985. Icecoringaugers for shallow depthsampling. U.S. Army Cold RegionsResearch and Engineering Laboratory,Hanover, N.H. , CRREL Report 85-21,December 1985.11

TIME-SERIES VARIATIONS IN ICE CRUSHINGG. W. TimcoNational Research Council, Ottawa, Ontario, CANADA1. 1. lordaanMemorial University of Newfoundland, St. John's, Newfoundland, CANADAAbstractAn analysis has been performed toexamine the time-series variations inice crushing during edge-loaded indentationin freshwater ice. A qualitativemodel of the crushing process is developed.It explains the load variationsin terms of pulverization, crushing andclearing events. The time-series variationsreveal that the crushing processis not continuous; instead, thereappears to be repeated crushing eventswith continual clearing.IntroductionWhat happens when ice crushes?This is one of the most important unansweredquestions in the field of icemechanics_ The crushing of ice plays aSignificant role in almost all icestructureinteraction events. Crushingoccurs as the primary failure rode onvertical-sided structures such as bridgepiers and some Arctic caisson structures,and it can be an important icefailure rode on sloping structures andThis is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987.during an ice-breaking vessel's penetrationthrough ice.During crushing events, theloads imposed on a structure can be veryhigh and dynamic. Clearly, it is desirableto understand the ice crushing processto gain insight into the reasonsfor both the high ice loads and thedynamic character istics of the interactionprocess. It is known that manyparameters can affect the crushingbehaviour includ~ng the loading rate,aspect ratio, and rigidity (or flexibility) of the structure. In this paper,one single interaction event of icecrushing is examined in detail to try toprovide some understanding of this problem.BackgroundSince the interaction process inice crushing is extremely complex, it isnecessary to simplify it as much as possible.One of the best ways to do thisis to perform tests of ice crushingunder controlled conditions in a laboratory.This approach has been used byseveral investigators in the past.Usually, a cylindrical or flat-facedpile (or indentor) is pushed through asheet of ice at a sufficiently highrate, and the load on the pile ismeasured. In these tests, several13

parameters can be varied including theloading rate (i. e. speed of the indentor),aspect ratio, surface friction,flexibility of the structure, etc.Particularly noteworthy studies in thisregard have been performed by Hirayamaet al (1974), Zabilansky et al (1975),Kry et al. (1978), Michel and Blanchet(1983) and Timco (1986) for indentationin freshwater ice, and by MMMttMnen(19791 1983), Sodhi and Morris (1984),Frederking et al. (1982) and Frederkingand Timco (1987) for indentation in"model n ice. Studies of this type areuseful, since they indicate the parametersinfluencing the interaction process.During crushing, the load on thestructure is not constant. Examinationof a load-time series of a crushingevent reveals that the loading event isirregular, cyclic and complex. Dynamicprocesses are clearly taking place. Inanalyzing time-series of this type, theapproach taken by all previous investigatorsin ice mechanics has been todetermine the mean, standard deviationand maximum load value of the ser ies,and then to use these values in theanalysis. For many purposes this issufficient. To understand the crushingprocess, however, it is necessary to tryto understand what causes the load levelfluctuations in the time-series, andwhat controls the magnitude of the loadin the crushing event. For this, it isnecessary to examine a time series indetail.The Crushing TestsTo look at the ice crushing processfor the present purposes, an appropriateexperimental approach would be to studythe edge-loaded penetration of a rigidindentor through an ice sheet. Recently,a comprehensi ve test ser ies of thistype was performed by Timco (1986) inthe ice tank in the Hydraulics Laboratoryat the National Research COuncil ofCanada (NRCC) in Ottawa. In thesetests, flat indentor bars were pushedthrough sheets of fine-grained (0.1-0.2 em) columnar 52 freshwater ice.Provided the interaction speed was highenough and the aspect ratio low enough,a crushing failure mode of the iceoccurred. The general features of thetest results were analyzed numericallyby Tomin et al. (1986).For the present analysis it isnecessary to simplify the problem bystandardizing all of the parameterswhich can affect the ice load. To thisend, a detailed test series was performedon a flat-faced rigid indentor6. 35 em wide interacting with a 0.9 cmthick ice sheet at a rate of 6 em_s- 1 •At this speed and aspect ratio, the predominantfailure mode of the ice iscrushing with radial cracks (Timco,1986). The indentor was attached to ahigh capacity (20 kN) load cell so thatthe load on it could be determined.This assembly was attached to the maincarriage of the facility which was usedto push the indentor bar through theice. A measurement of the natural frequencyand stiffness of the bar in thatarrangement indicated that they weregreater than 80 Hz and 6 MN_m- 1 respectively.With this test arrangement, thestiffness of the bar is approximatelyone-quarter of the stiffness of thevirgin, non-damaged ice sheet (Jordaanand Timco, in preparation). The outputof the load cell was sampled at a rateof 1,000 Hz and a load-time series ofthe interaction event was obtained (seeFigure 1). This high sampling rate wasnecessary to avoid aliasing effects(Chatfield, 1984) so that the informationon the frequency components of theload was obtained fully and correctly.This is extremely important.As the indentor bar moved throughthe ice, there was a continual stream ofsmall pieces of ice ejected upward intothe air and downward into the water infront of it. To get a quantitative measureof both the sizes and the distributionof sizes of these fragments, theywere caught in the air and immediatelysieved through screens with openings of0.4 em, 0.2 em, 0.12 em, and 0.071 em.The ice in each tray was weighed to givethe distribution of piece sizes. Theresul ts are presented in Table 1. Ingeneral, there was a more-or-less uniformdistribution of ice in each ofthese ranges. Note that the size ofthese particles are comparable to thewidth of the indi vidual ice columns inthis columnar-structured ice.14

Crushing Analysis54Z 3.::t:.oo 0.5 1.0 1.5 2.0TIME (SEC)Figure 1. Time-ser ies showing the loadvariations on a vertical indentor of6.35 cm width interacting with a sheetof freshwater ice of 0.9 em thickness ata rate of 6 cm-s- 1 •Sieve Sizes Total Mass Relativeof Ice Mass ofPieces Ice Pieces(cm)(gm)>0.4 170 0.2110.4 - 0.2 150 0.1860.2 - 0.12 199 0.2470.12 - 0.071 154 0.190

543Compliance-Ad/AFTime-O 006s-03m/MNo _~-'r--'__-'__-T___ Dr,~sl~a~nrce_-_0,_0_3~6~cTm __-r__ ~o 0.1 0.2 0.3 0.4 0.5TIME (SEC)Io 1.0 2.0DISTANCE (CM)\3.0Figure 2. A one-half second "timeslice"of Figure 1 showing the loadvariations in detail. Note that theinteraction speed was constant, so thisalso represents a load-distance ser iesthrough a distance of 3 cm.>-t-CJ)Zw0-.Jt-

"spring back" with, in this case, a displacementof the order of 0.05 em (basedon a typical load drop of 3 kN with abar of 6 MN_m- 1 stiffness). Thus, thetotal movement ~uld be on the order of0.1 cm. Although there are some iceparticles of this size ejected from thecrushed layer, the amount is relati velysmall. Hence, it is felt that this loaddrop is not associated with a clearingevent, instead, it is probably an icepulverization event. Note that at theload release the ice ~uld also "springback" with, however, a much smaller displacementthan the indentor bar. Onaverage, for the present test conditions,these pulverization events occurevery 0.05 sec or 0.3 em. This lengthis in good agreement with the thicknessof ~e highly damaged, whitish-colouredice directly in contact with the indentor.Note that because of the confinementof the attendant ice sheet, theload does not necessarily fall to zeroafter the pulverization occurs. Thecrushed ice particles along the indentorinter face are in a confined state, andwould form a "buffer" between the indentorbar and the intact (although damaged)ice sheet.As the indentor moves furtherthrough the ice, these particles mustclear. Since they have the possibilityof moving as separate entities, they nolonger form a solid continuum, instead,they behave more as a viscous fluid(Kheisin and Likhomanov, 1973, Kurdymovand Kheisin, 1976). In this state, theyare ejected from the region in front ofthe indentor. As these fragmentsclear, the load on the indentorchanges. An examination of the timeseries(Fig. 2) shows some high frequencyload variations occurring between thepulverization events. These variationscould represent this clearing process.For this, however, a relati vely largenumber of small ice pieces ~uld have tobe ejected simultaneously to give anoticeable change in the load. Thiswould result only if a whole "row" ofice particles cleared together. Whetherthis mechanism of large scale, simultaneousclearing occurs or not is unknown.Alternatively, these high frequency loadvariations between the pulverizationevents may represent further crushing ofthe ice pieces in the pulverized zone.It could be that this mechanism offurther crushing followed by clearingoccur virtually simultaneously.In any event, this ejection causesa reduction in the size of the pulverizedzone and a subsequent increase in thestress in the ice and load on the indentor(Jordaan and Timco, in preparation).At any time during this interactionprocess, the rate at which the loadincreases is controlled by the .. compliance(c) of both the ice sheet (includingthe locally crushed area) and theindentor bar, and it includes any viscousdissipation of the crushing process.For the present tests, a measuredcompliance for this system is of theorder of 0.3 m-MN- 1 (see Fig. 2) givinga resultant stiffness of "3 MN-m- 1 • Asthe stress in the parent ice sheet continuallybuilds, a critical level isreached whereupon pulverization occursagain and the cycle repeats itself.This total process gives rise to analternating series of ice pulverization(at a low frequency) followed by acrushing/clearing process (at a higherfrequency) which is controlled by theinteraction rate and the relative stiffnessof the ice and the structure. Thisshould be reflected in the time-ser ies.A schematic time-series representingthese idealized processes is shown inFigure 4. Note that the whole interactionevent is quite dynamic, yet reasonablycyclic. Although several authorsrefer to this process as continuouscrushing, the evidence indicates thatthe crushing is not continuous, instead,it may be more appropriate to view thisas repeated crushing with continualclearing.Since the load variations on theindentor bar are directly influenced bythe crushing/clearing processes, it maybe possible to correlate quantitativelythe sizes of the ice pieces ejected fromthe crushed layer with these time-seriesvariations. To do this it is necessaryto understand that sieving of the icepieces is essentially a filtering processwhereby the pieces are filtered (orsorted) into appropr iate ranges. Sincethe tests were performed at a constantspeed, the time and the distance areequivalent, and so each sieve may beregarded as a characteristic time (or17

2twoa:oLL0-221-15 Hz Filter0TIME -Figure 4. Schematic time-series for themodel described in the test showing thepulverization events and the crushing/clear ing events.z..:.:0

SIEVE SIZES (mm)4 2 1.2 0.71100 ,---,---,-----r--------,,--------,1-15 >0.4 0.211 0.146 0.13615-30 0.4-0.2 0.186 0.205 0.22530-50 0.2-0.12 0.247 0.312 0.28050-85 0.12-0.071 0.190 0.287 0.30585-

AcknowledgementsThe authors would like to thankE. Mansard and E. Funke for helpfuldiscussions. K. Croasdale providedhelpful comments on the text. Part ofthis work (I.J.J.) was funded by theNational Science and EngineeringResearch Council of Canada and Mobil OilCanada, Ltd.ReferencesChatfield, C. 1984. The Analysis ofTime Series. Chapman and Hall Ltd., NewYork, u. S. A.Frederking, R., Schwarz, J., Wessels,E. and Hoffmann, L. 1982. "Model Investigationsof Ice Forces on CylindricalStructures". Proc. Intermaritec 82,Hamburg, Germany, 341-349.Frederking, R. and Timco, G.W. 1987."Ice Forces on a Rigid Structure with aCompliant Base". proc. POAC 87 (thisvolume).Hirayama, K., Schwarz, J. and Wu, H.1974. "An Investigation of Ice Forceson vertical Structures". Iowa Instituteof Hydraulic Research Rept. No. 158,Univ. of Iowa, Iowa City, Iowa, U.S.&Jordaan, I. J. 1986. "Fracture Mechanicsand Damage Theory as a Basis for Calculationof Ice-Structure Loads". Proc.NRC Workshop on Extreme Ice Features,Banff, Alberta (in press).Maattanen, M. 1979. "Laboratory Testsfor Dynamic Ice-Structure Interaction".Proc. POAC 79, ~l. 2, Trondheim,Norway, 1139-1153.Maattanen, M., 1983. "Dynamic Ice­Structure Interaction During ContinuousCrushing". u.S. Army CRREL Rept. #83-5,Hanover, NH, U.S.A.Michel, B. and Blanchet, D. 1983."Indentation of an S2 Floating Ice Sheetin the Brittle Range". Annals of Glac.4, 180-187.Sodhi, D.S. and Morris, C.E. 1984. "IceForces on Rigid, vertical CylindricalStructures". U.S. Army CRREL Rept.#84-33, Hanover, NH, U.S.A.Timco, G. W. 1986. "Indentation andPenetration of Edge-Loaded FreshwaterIce Sheets in the Brittle Range".Proc. OMAE 86, ~l. IV, Tokyo, Japan,444-452.Tomin, M.J., Cheung, Mo, Jordaan, I.J.and Cormeau, & 1986. "Analysis ofFailure Modes and Damage Processes ofFreshwater Ice in Indentation Tests".Proc. CMAE 86, ~l. IV, Tokyo, Japan,453-460.Zabilansky, L.J., Nevel, D.E. andHaynes, F. D. 1975. "Ice Forces on ModelStructures".~C~a~n~. __ ~J~o~u~r~n~a~l~~o~f __ ~C~i~v~i~lEng. 2, 400-417.Kheisin, D.E. and Likhomanov, V.A.1973. "An Experimental Determination ofthe Specific Energy of MechanicalCrushing in Ice by Impact". ProblemyArktiki i Anarktiki 41, 55-61.Kry, P.R., Lucente, F.R. and Headley,R. E. 1978. "Continuous Crushing of anIce Sheet by a Circular Indentor". APOARept. #106, Arctic Petroleum OperatorsAssoc., Calgary, Alta., Canada.Kurdyumov, V. & and Kheisin, D. E. 1976."Hydrodynamic Model of the Impact of aSolid on Ice". prikladnaya Mekhanika12, No. 10, 103-109.20

THE APPLICABILITY OF LEFM AND THE FRACTURE TOUGHNESS(KIC) TO SEA ICEJukka TuhkuriWdrtsird Marine Industries Inc., Helsinki, FINLANDAbstractThe applicability of linear elasticfracture mechanics (LEFM) and thefracture toughness (KIC) to sea icewas evaluated by making in-situfracture toughness measurements withnotched three-point bending beams. Theapplied load and the crack openingdisplacement (COD) were measured. Twodifferent stress intensity factor rateswere used. At rate 7 kParm s-l thefracture toughness was 136 kParm and atrate 323 kPa.Jiii s-l the fracturetoughness was 119 kPa.Jiii. The linearityof the measured load vs. COD recordsmet the linearity requirements of thestandard ASTM E399-83 on both of thestrain rates. It is concluded that LEFMcan be applied to describe the fractureof sea ice at least at stress intensityfactor rates higher than 7 kPa.Jiii s-l.IntroductionThe application of linear elasticfracture mechanics (LEFM) to the studyof fracturing of ice has recently beenThis is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987. © The Geophysical Institute,University of Alaska, 1987.in the focus of attention. The researchon fracture mechanics of ice was startedat W

are so small that the theory of linearelasticity can be used. The applicabilityof LEFM to a material is usuallyevaluated experimentally by measuringthe crack opening displacement (COD) infracture toughness tests and plottingthe COD as a function of applied load.LEFH is applicable to a material if theload vs. COD record is of certain linearity.For metallic materials thelinearity is often judged according tothe standard ASTM E399-83.ReviewIceof Fracture Toughness Tests ofThe fracture toughness (KIC) ofsea ice has been studied by many people,e.g. Parsons et al. (1986), Shen and Lin(1986), Urabe and Inoue (1986), Urabeet al. (1980), Goldstein and Osipenko(1983a), Timco and Frederking (1982a)and Vaudrey (1977). The fracture toughnessof laboratory grown fresh water icehas been studied more systematicallythan the KIC of sea ice, e.g. Nixonand Schulson (1986), Timco and Frederking(1986; 1982b), Goldstein andOsipenko (1983b), Liu and Miller(1979), Hamza and Muggeridge (1983;1979), Goodman (1980), Goodman andTabor (1978) and Liu and Loop (1972).According to the studies listedabove it can be concluded that themeasured fracture toughness (KIC) ofsea ice varies typically between 30kPaiJD and 140 kPaJrn. Also, despite someopposite results, it is concluded thatKIC of ice increases with increasinggrain size and with decreasing loadingrate, temperature, salinity and brinevolume.The specimens for KIC measurementshave been three-point or fourpointloaded beams or compact tensionspecimens. The specimens have beenprenotched usually by a saw and, inaddition, the notch has been sharpenedby a sharp blade such as a surgeon'sscalpel. The fracture toughness (KIC)has been calculated from the maximumload and the dimensions of the specimen.In plane strain fracture toughnesstesting the specimen size is importantbecause the size of the zone of nonlineardeformations at the notch tipmust be so small compared to the otherdimensions of the specimen that thenonlinearity can be neglected. Additionally,if the breadth of the specimen istoo small the specimen is in planestress and the measured KIC is toolarge. For metallic materials, Brown andSrawley (1966, p.20) have determinedexperimentally the minimum size of aspecimen used in plane strain fracturetoughness measurements to bea, B > 2.52KIC~Y(2)where a y is the yield strength and aand B are the notch length and thespecimen breadth, respectively. For ice,a yield stress cannot be uniquelydefined because of creep (Timco andFrederking 1986) and the criterion (2)cannot be applied directly. However, theminimum size of specimen can be approximatedby replacing the yield strength inequation (2) by flexural strength ofice.On the other hand, the dimensionsof the specimen have to be large enoughcompared to the grain size of ice toensure the polycrystallinity of the ice.Recommendations on testing methods ofice by IAHR (IAHR 1984) suggest thatspecimen breadth and crack lengthshould be greater than 15 times thegrain size of ice. Nizon and Schulson(1986) stress that it has not beendetermined, either experimentally ortheoretically, what specimen size tograin size ratio is necessary forpolycrystalline behavior in ice.Goodman (1980) has measured thecrack opening displacement of freshwater ice and plotted the COD as afunction of the applied load. The loadvs. COD records seem to be linear atthe high loading rate used (KI:::: 1000kPaJ; s-l). The specimens Goodmanused were small; the breadth was 200 mmand the grain size 5-10 mm. Also Hamzaand Muggeridge (1983) have measured theCOD of fresh water ice, but they havenot published the load vs. COD records.22

Shen and Lin (1986) have measuredthe COD of sea ice but they have notpublished the load vs . COD records,ei ther. According to their results theload and COD have a linear relationshipwithin the loading rate used (KI ~1-400 kPaJill s-l).The applicability of LEFM to icehas also been studied by considering iceas a creeping material and calculatingthe size of the creep zone at the cracktip (Timco and Frederking 1986; Nixonand Schulson 1986). If the loading timeis too long or the specimen dimensionsare too small, the size of the creepzone becomes significant and the deformationcannot be considered linear.Timco and Frederking (1986) report thatat a low loading rate (KI ~ 2 kPaJ;s-l) the influence of the creepbehavior of ice and the resulting cracktip blunting start to influence significantlythe measured fracture toughnessvalues.ExperimentalThe main aim of this study was toevaluate the applicability of LEFM tosea ice . That was done by making in-situfrac ture toughness measurements withnotched three-point bending beams andmeasuring the applied load and the COD .The load vs. COD records were plottedand their linearity was judged accordingto the standard ASTM E399-83. Two loadingrates were used (KI 7 kPaJffis-l and KI = 323 kPaJill s-l).The ice field where the test serieswas made was located at the southerncoast of Finl and . The measurements weremade in the beginning of March 1986. Thethickness of the ice was ca. 450 mm, ofwhich 400 mm was columnar grained iceand 50 mm snow ice. On top of the icethere were only a few centimeters ofsnow which was removed. The salinity andtemperature of the ice were measured(Table 1).Specimens as large as possible wereused because the size of specimen mighthave an effect on the fracture toughness. The beam breadth and height wereTable 1. Measured salinity and temperatureof sea ice. Air t emperature +1.4°C.Depth range Salinity Temperaturefrom the upperside of ice 0/00 °co - 50 mm 0 . 40 - 0.3050 - 120 mm 0 . 48 - 0.35120 - 200 mm 0.48 - 0.45200 - 300 mm 0 . 27 - 0.45300 - 400 mm 0.27 - 0.45400 - 450 mm 0.27 - 0.40the same as the ice thickness,450 mm and the beam length wastimes the breadth, i.e. 3600 mm.i.e.eightThe experimental set up is shown inFigs. 1 and 2. The ice beams were cutfrom the ice sheet with a chain saw andthen supported to the surrounding icesheet so that the beams were kept in theposition where they were when floating .A notch with a length of ca. 200 mm wasmade by a chain saw in the middle of thebeams to the top side. In half the beamsthe notches were sharpened by a speciallymachined sharpening blade . All thenotches were not sharpened because theeffect of notch sharpness to fracturetoughness and to COD was also studied .The crack opening displacement wasmeasured wi th a specially designedmeasuring system comprising two supportswhich were placed on the top of the beamon different sides of the notch. A displacementtransducer was attached to onesupport and a magnet to the other . Thedisplacement transducer measures thedisplacement of a rod free to movewithin the transducer casing, connectedto one support, and fixed by the magnetto the other support. The measuringsystem worked well even when wet.The beams were loaded with ahydraulic unit fixed to the surroundingice sheet (Fig. 2). The loading force23

Figure 1. Photograph showing the loading arrangement for the fracturetoughness measurements.ADisplacementtransducerHydraulic loadingunitForcetransducertA Liz LIZBIce specimenIce sheetSection A-AFigure 2. Schematic showing the loading arrangement for the fracture toughnessmeasurements.24

ForceCODBreakingforceBuoyancyLoading timeTimeLoading timeFigure 3. Loading force vs. time and crack opening displacement (COD) vs. timerecords at the higher of the two loading rates used (Kr - 323 kPaJ; s-l).ForceCODBreakingforceCriticalCODBuoyancyLoading timeTimeLoading timeTimeFigure 4. Loading force vs. time and crack opening displacement (COD) vs. timerecords at the lower of the two loading rates used (I

was measured with a force transducer andrecorded, as was the COD, in analogueform. Later the signal was digitized andanalysed by computer.The breaking force, loading timeand buoyancy were determined from theforce vs. time records (Figs. 3 and 4).The fracture toughness was calculatedfrom the breaking force and from thedimensions of the beam using equation(3) developed by Brown and Srawley(1966).f(a/H)3PL .fa2BH2f(a/H) (3)1.96 - 2.75(a/H)+ 13.66(a/H)2 - 23.98(a/H)3+ 25.22(a/H)4where P is the breaking force and thedimensions L, B, H and a can be seenfrom Fig. 2.Vibration of the beam with thehigher loading rate (K I = 323 kPaJms-l) can be seen in Fig. 3. At thelower loading rate (KI 7 kPaJms-l) nonlinearity at the beginning ofthe loading can be seen in Fig. 4. Thisnonlinearity is believed to be caused byroughness of ice which leads to crushingunder the supports. It is assumed thatthe vibration or the nonlinearity doesnot affect the breaking force and thatthe tangents drawn to the figuresrepresent the linear growth of force andCOD from which the vibration or nonlinearityhas been filtered away.The flexural strength and strainmodulus of sea ice was also measuredwith the same experimental set-up. Thestrain modulus was calculated from themaximum deflection of the beam, so themodulus is the secant modulus. Thedeflection of the in-situ, three-pointbent beams was measured in relation toa straight aluminium profile located ontop of the beam (Fig. 5). This ensuresthat the flexibility of the supportingsystem does not affect the measuredstrain modulus. Two different loadingrates were used. The adjustments of theloading apparatus were the same as withthe fracture toughness measurements.The results are shown in Table 2.Results and DiscussionThe results of fracture toughness(KIC) and crack opening displacement(COD) measurements are shown in Table 3.Both KIC and COD decrease withincreasing loading rate.Typical load vs. COD records areshown in Figs. 6-9. The slopes of theDisplacementtransducersSupportsIce specimenFigure 5. Schematic showing the three-point loading arrangement for theflexural strength and strain modulus measurements.26

£sn£ofsnEsn2.andMeasured flexuralstrain modulus (E)strain ratestandard deviationnumber of measurementsLoading rate----------------Low Highstrengthof seas-l 2.5' 10-6 8.4· 10-5kPa 675 655kPa 51 541 6 5GPa 2.90 4.54GPa 0.54 0.121 6 5Table 3. Measured(KIC) and crack(COD) of sea ice.tsnfracture toughnessopening displacementloading timestandard deviationnumber of measurementsLoading rate---------------Low HighKI kParm s-l 7 323t s 19.4 0.4KIC kParm 136 119s kParm 12 9n 1 20 20COD mm 0.068 0.051s mm 0.012 0.011n 1 10 19tangents drawn on these records arecalculated by linear regression from thepart of the curves restricted by transverselines shown in the figures. TheOP5-lines and the forces PQ andPMAX, defined by the standard ASTME399-83, are also shown in the figures.The slope of the OP5-line is 0.95times the slope of the tangent. Accordingto the ASTM standard E399-83 afracture toughness test is valid andKIC exists only if the ratioPMAX/PQ does not exceed 1.10 and ifthe specimen size criterion, equation(2), is satisfied. The load vs. CODrecords (Figs. 6-9) does not fall tozero instantaneously after reachingPMAX due to the response characteristicsof the measuring system.At both the loading rates used(I

P1400(N)120010008006004002001// /// f/VP max = PQy---- '-P 5.02 .04 .06 .08 .1COD (mm)Figure 6. An example of a linear loading force (p) vs. crack openingdisplacement (COD) record at the lower of the two loading rates used(Kr = 7 kPaJi s-l). The slope of the tangent drawn to the record iscalculated by linear regression from the part of the curve restricted by thetwo transverse lines. The forces PMAX. P5 and PQ are according tostandard ASTM E399-83.P (N)1400 .----..----.,---,-----,----,1200 ~--+----r---+TT~~~-~1000 r---+---+-~~~---r--~600r---+-,,~+---~---r--~400 ~--.r,'---_+_--_+--__It--~200 ~~-;_---+---+_--_r--~.02 .1COD (mm)Figure 7. An example of a nonlinear loading force vs. COD record at the lowerof the two loading rates used (1

P1400(N)Kr323 kPa /iii' s-l12001000800600400200P max\= PQ~ -PsI'-.,j~ ..............'/NY.02 .04 .06 .08 .1COD(mm)Figure 8. An example of a linear loading force vs. COD record at the higher ofthe two loading rates used (Kr" 323 kPaJiii s-l). See Fig. 6 for explanationof the,symbo1s and lines used.P (N)140012001000800600400200/PmaxKr 323 kPaJiii's-lil ~ffist rLnV)fo o .02'--= PQ.04 .06 .08 .1COD (mm)Figure 9. An example of a loading force vs. COD record at the higher of the twoloading rates used (Kr - 323 kPa.Jiii s-l) where a step is noticed. See Fig. 6for explanation of the symbols and lines used.29

measured fracture toughness or COD. Liuand Miller (1979) have observed similarbehavior in fresh water ice.This behavior is interesting anddifferent conclusions can be drawn. Itis possible that the sharpened notcheswere not sharp enough and behaved asblunt notches in the same manner as thenonsharpened notches. If it is assumedthat the sharpened notches were sharpenough, it can also be possible that icebehaves plastically and the sharpenednotches become blunt when loaded. Thethird possible conclusion is that thelength of the notch has an effect on themeasured KIC. The results of Shen andLin (1986, Table 2) show a correlationbetween KIC and a/H, but in thepresent test series the correlationcoefficient r between KIC and a/H wasonly 0.65.The fourth possible conclusion andthe one favored by the author is thatbreaking of ice is analogous to breakingof concrete, rather than to breaking ofmetals. In concrete, in front of a cracktip, micro cracks are formed and not aplastic zone as in metallic materials(Hillerborg 1983). The microcrackingzone is analogous to the plastic zoneand in fracture toughness testing, alsothe microcracking zone has to be smallcompared to specimen dimensions in orderfor LEFM to be applicable. Also Sunderand Nanthikesan (1987) have suggestedthat in ice a microcracking zone isformed at the crack tip.According to Michel (1978, p.8]),micro cracks when formed in ice, extendto the full size of each grain. Anassumption is made that in fracturetoughness tests, despite the sharpnessof the notch, there will always be microcracks of length at least equal to thegrain diameter of ice in front of thenotch tip. The prob,lem now is, whetherthe breaking is dominated by theprefabricated notch or by the microcracks that form.ConclusionsThe following conclusions are made.The fracture toughness (KIC) ofthe sea ice used in the experiments was119 kPaJm at stress intensity factorrate 323 kPaJiii s-l and 136 kPaJiii atrate 7 kPaJm s-l.LEFM can be applied to describe thefracture of sea ice at least at stressintensity factor rates higher than7 kPaJm s-lThe observation that the sharpnessof the notch tip does not affect themeasured value of KIC might be causedby micro cracks formed in front of thenotch.AcknowledgementsThe author would like to thankWartsila Arctic Research Centre andWartsila Marine Industries Inc. for theopportunity to perform these experiments.Valuable discussions with Mr KariSantaoja are gratefully acknowledged.ReferencesASTM E399-83. 1983. Standard test methodfor plane-s train fracture toughness ofmetallic materials. American Societyfor Testing and Materials, 36 pp.Broek, D. 1984. Elementary engineeringfracture mechanics. The Hague: MartinusNijhoff Publishers, 469 pp.Brown, W.F. and Srawley, J.E. 1966.Plane strain crack toughness testing ofhigh strength metallic materials.Philadelphia: American Society forTesting and Materials, Special TechnicalPubl. No. 410, 129 pp.Goldstein, R.V. and Osipenko, N.M. 1983.Some aspects of fracture mechanics ofice cover. Proc. of the 7th rnt. Conf.on Port and Ocean Engineering underArctic Conditions, vol. 3. Helsinki,5-9 April 1983. Espoo: TechnicalResearch Centre of Finland, 132-142.(1983a)30

Goldstein, R.V. and Osipenko, N.M. 1983.Mehanika razrusenija i nekotorye voprosyrasrusenija Ida (Fracture mechanics andsome ice fracture problems). In: "Mehanikai fizika Ida" (R.V. Goldstein,ed.). Moskva: Nauka, 65-94. Unpublishedtranslation. (1983b)Goodman, D.J. and Tabor, D. 1978.Fracture toughness of ice: A preliminaryaccount of some new experiments. Journalof Glaciology 21 (85): 651-660.Goodman, D.J. 1980. Critical stressintensity factor (KIC) measurementsat high loading rates. InternationalUnion of Theoretical and AppliedMechanics Symp. Physics and mechanicsof ice. Copenhagen, 6-10 Aug. 1979.In: "Physics and Mechanics of Ice"(P. Tryde, ed.). Berlin: SpringerVerlag, 129-146.Hamza, H. and Muggeridge, D.B. 1979.Plane strain fracture toughness (KIC)of fresh water ice. Proc. of the 5thInt. Conf. on Port and Ocean Engineeringunder Arctic Conditions, vol. 1.Trondheim, 13-18 Aug. 1979. Trondheim:The University of Trondheim, 697-707.Hamza, H. and Muggeridge, D.B. 1983.Non-linear fracture toughness of freshwater ice. Proc. of the 7th Int. Conf.on Port and Ocean Engineering underArctic Conditions, vol. 3. Helsinki,5-9 April 1983. Espoo: TechnicalResearch Centre of Finland, 153-162.Hillerborg, A. 1983. Analysis of onesingle crack. In: "Fracture Mechanicsof Concrete" (F.H. Wittman, ed.).Amsterdam: Elsevier Science Publishers,223-249.IAHR. 1984. Recommendations on testingmethods of ice. 4th report of workinggroup on testing methods in ice. Int.Association for Hydraulic Research, ICe"Symp. 1984, 7th Int. Symp., Proc. vol.IV. Hamburg, 27-31 Aug. 1984. Hamburg:Hamburgische Schiffbau-Versuchsanstalt,1-41.Liu, H.W. and Loop, W. 1972. Fracturetoughness of fresh-water ice. Hanover,New Hampshire: U. S. Army Cold RegionsResearch and Engineering Laboratory,Technical note, 12 pp.Liu, H.W. and Miller, K.J. 1979.Fracture toughness of fresh-water ice.Journal of Glaciology 22 (86): 135-143.Michel, B. 1978. Ice mechanics. Quebec:Les Presses de 1 'Universit~ Laval,499 pp.Nixon, W.A. and Schulson, E.M. 1986.The frac ture toughness of ice over arange of grain sizes. Proc. of the 5thInt. Offshore Mechanics and ArcticEngineering Symp., vol. IV. Tokyo,13-18 April 1986. New York: The AmericanSociety of Mechanical Engineers,349-353.Parsons, B.L., Snellen, J.B. andHill, B. 1986. Physical modeling and thefracture toughness of sea ice. Proc. ofthe 5th Int. Offshore Mechanics andArctic Engineering Symp. , vol. IV.Tokyo, 13-18 April 1986. New York: TheAmerican Society of MechanicalEngineers, 358-364.Shen, W. and Lin, S.Z. 1986. Fracturetoughness of Bohai Bay sea ice. Proc.of the 5th Int. Offshore NechanicsaIi'dArctic Engineering Symp., vol. IV.Tokyo, 13-18 April 1986. New York: TheAmerican Society of MechanicalEngineers, 354-357.Sunder, S.S. and Nanthikesan, S. 1987.A tensile fracture model for ice. Proc.of the 6th Int. Offshore MechanicsaIi'dArctic Engineering Symp., vol. IV.Houston, 1-6 March 1987. New York: TheAmerican Society of MechanicalEngineers, 225-233.Timco, G.W. and Frederking, R.M.W. 1982.Flexural strength and fracture toughnessof sea ice. Cold Regions Science andTechnology 8 (1): 35 41. (1982a)Timco, G.W. and Frederking, R.M.W. 1982.Comparative strengths of fresh waterice. Cold Regions Science andTechnology 6 (1): 21-27. (1982b)Timco, G.W. and Frederking, R.M.W. 1986.The effects of anisotropy and microcrackson the fracture toughness(KIC) of fresh water ice. Proc. ofthe 5th Int. Offshore Mechanics andArctic Engineering Symp. , vol. IV.31

Tokyo, 13-18 April 1986. New York: TheAmerican Society of MechanicalEngineers, 341-348.Urabe, N., Iwasaki, T. and Yoshitake, A.1980. Fracture toughness of sea ice.Cold Regions Science and Technology3 (1): 29-37.Urabe, N. and Inoue, M. 1986. Mechanicalproperties of Antarctic sea ice. Proc.of the 5th Int. Offshore Mechanicsan;rArctic Engineering Symp. , vol. IV.Tokyo, 13-18 April 1986. New York: TheAmerican Society of MechanicalEngineers, 303-309.Vaudrey, K.D. 1977. Ice engineering -study of related properties of floatingsea-ice sheets and summary of elasticand viscoelastic analyses. Port Hueneme,California: Naval Construction BattalionCenter, Civil Engineering Lab.,Technical Report R 860, 79 pp.32

CREEP PROCESS AND RUPTURE CHARACTERISTICSOF SEA ICE IN THE BOHAI SEALi Zhi-junLi Fu-chengSui Ji-xueInstitute of Marine Environmental Protection, Datian, CHINAAbstractThis paper presents the creep process andrupture characteristics of sea ice in the BohaiSea under uniaxial compression based on thecreep tests on two groups of specimens whichwere natural ice cylinders with axes parallel,and perpendicular to sea level respectively. Theresults show that there are significant differencesbetween both the creep processes andthe rupture characteristics for different loadingdirections. These differences are related tothe effects of growth direction, grain size, brinedrainage and the anisotropy of sp:a ice. In addition,it was found that the elastic phase isvery short and lasts only about 1 minute beforeentering the visco-elastic phase.IntroductionCreep is one of the important processesoccurring in sea ice. However, few studies ofthis process have been done before in China. Tounderstand this behavior of sea ice in the BohaiSea, uniaxial compressive creep tests withconstant stress were performed.This is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987. © The Geophysical Institute,University of Alaska, 1987.Creep Test and Creep ProcessNatural ice specimens were taken fromthe sea ice in the Bohai Sea and machined intocylinders with a height of 175 mm and a diameterof 70 mm. The cylinders were then carefullyfinished and polished with sandpaper on a fiatsteel table to make certain that end surfacesof each specimen were flat and at right anglesto the axis of the cylinder. They were dividedinto two groups, denoted as A and B, whoselongitudinal axes were parallel and perpendicularto sea level, respectively. The test temperatureswere -5°C, -3°C and -1.5°C respectively,with the temperature fluctuating ±O.l°C. Thesetemperatures were close to the sea ice temperatureat the time of sampling. Different constantstresses were applied at each test temperature,50 the groups of £-t curves for different constantstresses were obtained.A special ice-snow creep test machine wasused to apply constant load in the form of leverloading with an extensometer to measure thespecimen deformation. To reduce the end effectsand to eliminate the bending moments,a pair of polished platens with spherical seatswere used and each specimen was accuratelycentered between platens to ensure that theload was applied along the axis of the cylinder.The constant stress was determined bywhere (To(T = (0.12 - - - 0.58) (Towas the instantaneous compressive33

strength. The specimens were maintained atthe test temperature for at least 14 hours priorto test to ensure a uniform temperature distribution.For the creep processes shown in Figure1 there are three cases for group A. When thec~nstant stress is great, only the primary andtertiary stages occur. Also, only the primaryand secondary stages occur for small stress. Forthe middle magnitude of constant stress, allthree stages occur. In addition, for group Athe slopes of curves are greater in the secondarystage and smaller in the tertiary stage than ingroup B. As for group B, ~hree stages are e-:ident,with a very small stram rate (near zero) mthe secondary stage. For both groups A and B,the primary creep is very short, mostly within60 seconds, with a small strain of (l-2)xIO- 3 •Creep Process EquationsA number of methods have been used toestahlish the creep process equations. One ofthem was given by Sinha (1978) who dividedthe total strain into three parts; the instantaneouselasticity, the delayed elasticity, and theviscosity, with considerations of the effects ofgrain size and temperature in the creep processequation. Gardner et al. (1984) suggesteda creep equation which includes all three creepstages and which also can go through the infle~tionpoints of the test creep curves. In practIcalengineering, however, one is often concernedwith the magnitude instead of the forms of deformation.As to the creep test results of sea ice inthe Bohai Sea, we define the creep with evidentsecondary creep as decay creep, and the creepwithout evident secondary creep as non-decaycreep. The total strain of decay creep can beexpressed aswhere01 = 0/ E is time independent02 = Al (t/tm)D,03 = A 2e D ,(I-lm )

and can be obtained by calculation from thetest results. Both equations (3.1) and (3.2)agree well with the test results and go throughthe inflection points of the creep test curves.The Rupture Characteristics of Sea Ice CreepThe significant differences of creep processesbetween group A and B as shown in Figure1 also indicate the different rupture characteristicsof these two groups. In group A, thespecimens ruptured mainly in the form of shear;the round end planes became elliptic after deformation,with the major axis perpendicularto the crystal growth direction. The ratio ofmajor axis to minor axis is about 1.0 to 1.2.This kind deformation resulted from crystalsliding and rearrangment accompanied ·bybrine drainage. In group A, the load directionwas perpendicular to the columnar crystalgrowth direction; thus, the crystals slideand the brine pockets between the columnarcrysta.ls are subject to compressive force andthe brine is rejected. This process introduceda large deformation in a direction perpendicularto the crystal growth direction (Figure 2 to4). Under small constant stress, the creep forg.rol~P A belongs to the non-decay creep, withslglllficant lateral extension (perpendicular tothe crystal axis) of the cylinder specimens. Undergreat constant stress, cracks with 30 0 -60 0angles occurred and the deformation was plastic.For group B, the load direction was parallelto the columnar crystal axis and little crystalsliding was evident; thus, the specimens fi~allysplit and ruptured in the brittle form. That iswhy the slope of the creep curves in the secondarycreep was close to zero and was large iIIthe tertiary creep (Figure 5). It was interestingthat most deformation was concentrated in oneend of the specimens. This can be attributedto the inhomogeneous ends of the specimen ando.nce the cracks occurred in one end, they contInuedto spread until the specimen split.Brine Drainage During The CreepDeformationThe salinity and melt temperature of thespecimens before and after each test was measuredto determine the brine drainage duringthe creep deformation. The results are listed inTable 2.From these data we can see that the salinityafter each test is lower than that before eachtest and it is higher for group B than for groupA. Consequently, the brine drainage must occurduring the creep and more brine was rejectedfor group A relative to group B. The melt temperaturedifference before and after each testis also consistent with this result. By linearregression, a good linear relation between thesalinity after each test and the total strain canbe obtained:group Agroup BSi :=: 3.139 - 3.3451:Si :=: 3.293 - 1.9941:r :=: 0.971r :=: 0.902Table 2. Ice salinity, melt temperature and total strain at -50 C.Specimen Salinity Melt Total Specimen Salinity TotalNumber (0/00) Temp.(OC) Strain Number (%0) Strain( Before Test)Group A 3.157 -1.30 0.0000 Group B 3.157 0.0000( After Test )A-5 2.614 -1.28 0.1331 B-5 3.147 0.0743A-4 2.596 -1.25 0.1945 B-7 3.118 0.1576A-8 2.523 -1.10 0.1862 B-6 2.835 0.2442A-{) 2.351 (-0.64) 0.2198 B-2 2.706 0.278435

0.100.05A-2(~,0= 0.56),,.II8-9(...a:. = 0.56)00,~;~t1',"A-5(JZ: cr. = 0.34) :IIII,,,;,,' B-5, ."" «(T ," 7F. = 0.52)0.0/.0/.5 t(hr.)Fig.I. Creep strain versus time (t-t) curvesat temperature of -S·C.Tb1Fig.2 Side view of ruptured specimen A-2. The radiusbefore test is 70.2l1l1I, - indicates the radiusafter ruptured.Fig.3 Over view of the ruptcredtop end of specimen A-2.36

(a)(b)Iload direct ionafter rupturedIoriginal loaddirectionFig.4 Side views of ruptured specimens A-4 and A-S.I--R-+--o.--lFig.5 Schematic diagram of ruptured specimen 6-11.37

Conclusionsnased upon the creep tests performed, itappears that the load direction has an importanteffect on the creep processes of columnarcrystal sea ice. Therefore, the creep rupturecharacteristics are very different for differentloading directions. Moreover, the creep deformationwas accompanied by brine drainage andthere is a good linear regression relation betweenthe total strain and the salinity of seaice specimens.AcknowledgementsThe authors appreciate the useful suggestionsand assistance given by Professor W.M.Sackinger and Dr. M.O. Jeffries in writing thispaper.ReferencesFish, A.M. 1984. "Thermodynamic Model ofCreep at Constant Stress and Constant StrainRa.te", Cold Regions Science and Technology,45: 143-161.Li Fu-cheng, Meng Guang-lin and Zhang Mingyuan.1986. "The Effects of Stress Rate onthe Uniaxial Compressive Strength of Sea Ice",Acta Oceanologica Sinica, 8(5).Gardner, A.R., Jones R.H. and Harris J.S.1984. "A New Creep Equation for Frozen Soilsand Ice" ,Cold Regions Science and Technology,9: 271-275.Hooke, R.LeB., Mellor, M. and ten others.1980. "Mechanical Properties of PolycrystalineIce: An Assessment of Current Knowledge andPriorities for Research", Cold Regions Scienceand Technology, 3: 263-275.Sinha, N.K. 1978. "Short-Term Rheology ofPolycrystalline Ice", Journal of Glaciology,21(85): 457-473.38

STUDY OF THE FLEXURAL STRENGTH AND ELASTIC MODULUSOF SEA ICE IN THE DOHAI SEASui Ji-XueLi Fu-chengLi Zhi-junZhang Ming-yuanYu Yong-haiInstitute of Marine Environmental Protection, Datian, CHINAAbstractThis paper presents some results of insitucantilever beam tests on flexural strengthand elastic modulus of sea ice conducted in theBohai Sea during the 1985-1986 winter. The resultsshow that both the flexural strength andmodulus change parabolically with respect tothe brine volume, and linearly with respect toice temperature. Moreover, both the flexuralstrength and modulus are sensitive to the loadrate, and increase as the load rate increases(in the ductile range). Furthermore, flexuralstrength for downward loading is greater thanthat for upward loading.IntroductionThe study of sea ice mechanical propertiesis becoming more necessary as the developmentof oil field and port facilities in cold regionstakes place. Since 1970, sea ice researchin China has concentrated on sea ice compressivestrength, with few studies of sea ice flexuralstrength, although flexural strength is also animportant mechanical index especially for thedesign of offshore structures. Results of flexuralstrength studies using field tests in theThis is a reviewed and edited version of a paper submittedto the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987. © The Geophysical Institute,University of Alaska, 1987.Bohai Sea during the 1985-86 winter are presentedbelow.Test SitesThe tests were done in Bayuquan Harporarea, in the northeast part of Liaodong Bay inthe Bohai Sea. This area is one in which annualfast sea ice is common (Fig.l). In Fig. 1 , Llrepresents the level fast ice and L2 representsthe fast ice with a. surface resembling a "lotusleaf" , formed by freezing the ice floes (we callthis kind of ice "lotus leaf" ice) The depth ofsea water is about 5 meters, at a distance of230 meters from the coast. The ice specimensare designated by group number Ml,M2, ... M7depending on different sites and test time(Table1).Test ProcedureThe field cantilever method was used forthe sea ice flexural strength tests. The longitudinalaxis of the beams is in the west-eastdirection at site Ll, and is parallel or perpendicularto the major axis of the "lotus leaf" iceat site L2. Actually, site Ll is in the harborand the current influence on ice crystal growthcan be neglected, so the selection of directionis not important. Near site L2, however, thefreezing ice floes are arranged with the majoraxis of the "lotus leaf" in the current direction(Photo. 1.).39

wall~ filled up planeoTest SiteFig. 1. The 8ayuquan Harbor and the test si tes Ll and L2.Table 1.Site Distance from Ice Ice Thickness Test GroupCoast (m) Description (em) Time Number27.0 1/14/86 M130.0 1/22/86 M2L1 210 Level fast ice 30.0 1/22/86 M333.0 1/29/86 M633.0 1/29/86 M7L2 240 "Lotus leaf" ice 30.0 1/24/86 M430.0 1/24/86 M540

Photo 1.The test apparatus.The beam dimensions are determined bythe formula suggested by IAHR (1980)L = lOhwhereL - - - length of beamh - - - ice thicknessand the beam width b is equal to h. The beamswere cut up with a chain saw with a spacing of20 cm between two adjacent beams.A special lever apparatus, continuouslyloaded by hand, was used (Photo. 2). Theload-time curve, deflection-time curves for thefree end and the middle of the beam wererecorded by a three channel plotter connectedto the load and displacement cells. The loadtime was on the order of 1 second, as suggestedby IAHR (1980) for all tests, except an additionalgroup of tests with slower load rate forcomparison. The load was applied upward atthe free ends of the beams for most tests, andalso downward for an additional group of testsfor comparison.Experimental ResultsThe formulas used to determine the flexuralstrength and flexural modulus are Photo 2. The "lotus leaf" ice.41

and6plOJ = bh2E= ~ (i)3 E.b h IiThe relationship between flexural modulus andbrine volume is similar to that between flexuralstrength and brine volume (Fig. 3). Theregression equation isOf course, these formulas give simplified resultsand we recognize that the problem is morecomplicated. Therefore, they represent indexstrength and modulus, not absolute values. Theaverage value of each group was calculated from3 to 10 beam results, with a total of 7 groups.The original data was processed by the Chauvenetvalue rejection criterion.The brine volume was calculated usingthe relation (Frankenstein and Garner 1967):Vb = SI (49.185)0.532 + lTIThe results of brine volume and flexuralstrength are shown in Fig.2. From these resultswe see the flexural strength of sea ice decreasesas the brine volume increases. If we assume thisrelation to be parabolic, the result is as follows:(J I = 4.95 - 0.28Vvbwith the correlation coefficientR = 0.95The results show that sea ice temperaturehas an obvious effect on the flexural strengthand there is a linear relationship between them(Fig.4). The flexural strength increases as thetemperature of sea ice decreases. This is consistentwith earlier results (Bainey and Tinawi1984) which show that the tensile strength ofsea ice also increases as temperature decreases.There is also a similar relationship between flexuralmodulus and temperature of sea ice.From the recorded curves of load vs. timeand deflection vs. time for two groups designatedas M2 and M3, the beams in these twogroups broke in the ductile range and the flexuralstrength and modulus decreased as the loadrate decreased.The results from groups M6 and M7showed that the flexural strength of beamsloaded upward (M6) was smaller than that ofbeams loaded downward (M7). The reason forthis can be interpreted as the existence of seawater buoyancy and tensile strength differencebetween the upper and lower parts of the seaice, due to the temperature difference; the airOf (kg/ em')321~ ____ ~ ____ -L ____ ~~ ________________ ~8 9 10 11 (y;('/ .. )Fig. 2.The relationship between flexuralstrength and brine volume.42

21•O~ ____ L-____ L-____ ~ ____ ~~ ___________8 9 10 11 12 ./Vi (%.)Fig. 3. The relationship between modulusand brine volume.Of(kg/cm')32-1 -2 -3T(·c)Fig. 4.The relationship between flexuralstrength and ice temperature.43

temperature was less than the sea water temperaturewhen the tests were performed.The results from two groups (M4, M5)of "lotus leaf" ice showed that the flexuralstrength and modulus for beams with longitudinalaxis perpendicular to the major axis of the"lotus leaf" are greater than that in the parallelcase. The reason for this is complicated , butone possible factor is the c-axis arrangementin the sea ice crystals due to the existence ofcurrents (Weeks and Ackley 1983).Weeks W.F., and Ackley S.F. 1983. "RecentAdvances in Understanding the Structure,Properties, and Behavior of Sea Icein the Coastal Zones of the Polar Oceans",POAC 83, Proc. Vol. 1: 25-41.IAHR Working Group on Ice Problems. 1980."Standardization of Testing Methods for IceProperties", J. of Hydrology Research, 18(2):153-165.ConclusionsBased upon the beam tests performed,it appears that both the flexural strength andmodulus change parabolically with respect tothe brine volume and linearly with respect tothe temperature of sea ice. Moreover, boththe flexural strength and modulus are sensitiveto the load rate, and increase as the loadrate increases (in the ductile range). Furthermore,flexural strength for downward loading isgreater than that for upward loading.AcknowledgementsWe appreciate the useful suggestions andassistance given by Professor W.M. Sackingerin writing this paper. We also wish to thankMr. Liu Yi, Mr. Gao Shu-gang, Mr. Yan Decheng,and Mr. Meng Guang-lin for their participationin the field tests.ReferencesBainey L. and Tinawi R. 1984. "The MechanicalProperties of Sea Ice- A Compilationof Available Data", Canadian Journal of CivilEngineering, 11(4): 884-923.Frankenstein, G. and Garner, R. 1967. "Equationsfor Determining the Brine Volume of SeaIce from -1.5°c to -22.9°c", J. of Glaciology.6(48).Li Fu-cheng, Meng Guang-lin and Zhang Mingyuan.1986. "The Effect of Stress Rate onthe Uniaxial Compressive Strength of Sea Ice",Acta Oceanologica Sinjca, 8(5).Feng Shi-yan. 1964. "Error Theory and ExperimentData Processing", Science Press, China.44

STUDIES ON ADHESION STRENGTH OF SALINE ICELasse MakkonenEila LehmusTechnical Research Centre of Finland, Espoo, FINLANDAbstractThe adhes i on strength of sa 1 i ne icewas studi ed by three di fferent types oflaboratory tests and by theoreticalanalysis. In one set of experiments sheartests were made on ice samples frozen ontoplates coated with various surfacematerials. The other set consisted ofpUll-out tests on stainless steel pipesfrozen into the ice sheet in a water tank.The third set of experiments was along-term test in whi ch changes of theadhesion strength with time weremonitored.It is suggested that brine expulsionfrom ice due to expansion of brine pocketsduring cooling of ice is responsible forthe low adhesion of saline ice. A model ofsalt concentration at the ice/structureinterface is developed and theimplications of the results to iceadhesion are discussed.This is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987. © The Geophysical Institute,University of Alaska, 1987.IntroductionWhen water freezes on a structuresurface the resulting ice adheresstrongly. On many materi a 1 s, such assteel, concrete and wood, the adhesionstrength of pure ice is hi gher than theshear strength of ice. Therefore,mechanical de-icing of ships and drillingplatforms, for example, is very difficult.The high adhesion strength of ice has,however, another even more significantconsequence as far as offshore structuresare concerned. This i$ the fact that, whena structure is adfrozen into an ice sheet,the bond at the ice/structure interfacecan transfer very large forces from theice to the structure.There are two situations that must beconsidered regarding the effect of iceadhesion to the design ice load ofoffshore structures. Firstly, changes inwater level induce vertical forces onadfrozen structures. This often causesdamage to small structures, such as piers(Doud 1978; Muschell and Lawrence 1980),but can affect also drilling platforms andcaissons (Vershinin, 1980).Secondly, when a structure isadfrozen and the ice field starts to move,a situation may develop in which themaximum load is transferred to thestructure just prior to the break- up of45

the adhesive bond at the ice/structureinterface. In such a case the adhesionstrength determines the design ice load ofthe structure (Hudson and Strickland 1976;Sackinger and Sackinger, 1977). Ice loadson an adfrozen conical structure have beencalculated by Croasdale (1980), Gershunov(1984) and Cammaert (1986) usinganalytical and finite-element models. Theresults show that when the ice sheetstarts to move, the initial failure occursat the ice/structure interface rather thanby ice sheet bending, if the adhesionstrength is below 200 - 400 kPa. Thesevalues are slightly lower than theadhesion strength of fresh water ice.However, the adhes i on strength of sal i nesea ice at temperatures close to thefreezing point is typically less than thiscriticial limit, and may consequently bethe factor that determines the design iceload.Due to the importance of the adhesionstrength of saline ice, a number oflaboratory experiments on the adhesionstrength of ice have been made usingsaline water (Stehle 1970; Smirnov 1971;Panyshkin et al. 1974a; Tkachev andMalyshev 1976; Sackinger and Sackinger1977; Oksanen 1982; Berenger et al. 1985;Laforte and Lavigne 1986; Lyyra et al.1986). Field tests have also been made(Panyushkin et al. 1974b; Sackinger andSackinger 1977; Saeki et al. 1981; 1986;Sackinger et al. 1986). The results arecontrovers i a 1 and di ffer from each otherby more than an order of magni tude undernominally equal conditions.This undoubtedly reflects differenttest configurations and the difficultiesin making repeatable adhesion tests ingeneral, but it also points out that themechanisms that control adhesion of salineice are basically unknown. The theoreticalapproach to the problem has been equallyunsati sfactory, predi cti ng much hi ghervalues than actually observed (Oksanen1983) .In this study results of threedifferent types of 1 aboratory tests arepresented in order to give more insightinto the effects of salt in ice adhesion.In addition, mechanisms controlling salineice adhesion are discussed and ananalytical model of salt concentration atthe ice/structure interface is developed.Shear Tests with PlatesFor these tests the ice was frozenonto 115 RIO diameter plates in styrofoamcontainers (Fig. 1). The surface of theplates was cleaned with ethanol and waterprior to freezing. The saline water wasobtained by mixing aquarium salt with tapwater.When the test samples were completelyfrozen (after - 48 h) the container waspeeled off, and the samples were kept atthe test temperature for another 24 h. Theplates were then attached to the wall ofthe cold chamber and the ice was removedfrom the plate by pulling the ice downwith a belt, as shown in Fig. 2. Thenominal deflection rate was 0.4 mm/s. Themaximum load prior to break-up wasrecorded automatically.ICE-------iCE--.. GROWTHCOATINGWATER - - STYROFOAMFig. 1. Freezing the samples for sheartests with plates.Tests for polyurethane and Inerta 160coatings were made at -10°C using variouswater salinities. Ice salinity was notmeasured in these tests. The resultsshowed that the adhes i on strength ofsaline ice drops to a value ofapproximately 3 % of its value for freshwater ice, when the water salinity is 2%0. At higher salinities the adhesionstrength did not significantly change withincreasing water salinity, except that atSw = 26 %0 no adhesion was observed.46

Tests were al so made on Inerta 160coating at various temperatures and Sw = 2u/oo . It was found that adhesion strengthdi d not change between -3 and -15°C butthen started to increase with decreasingBALL JOINT\IADJUST ABLE)ICEV+-'---PL .. TE -----t"RIBBON AROUNDTHE PIECE Of IceFORCE TO REMove THE ICE FROM THE PLATE(MEASUREO»Fig. 2. Test arrangement in shear testswhith plates.stra in gage whi ch was gl ued to the 16 mmsteel bar through which the force wastransmitted to the sample. Thedisplacement of the sample was measuredwith an inductive transducer. Theexperimental arrengement is described inmore detail in Frederking and Karri(1981) .The water was boiled in order toremove air bubbles. The top layer wasseeded with snow so that the structure ofice was the same in all tests. The roomtemperature was -10 DC duri ng the tests.The results are shown in Table 1 and inFi g. 3.The results show that the adhesionstrength decreases very rapi dly when thesalinity of ice increases. With thesalinity of 1.0 %0 the adhesion strengthis approximately 0.01 MPa, from which itdoes not seem to decrease when thesalinity of ice increases.Table 1. Results of the pUll-out tests.temperature. At -30°C the adhesionstrength was almost 50 % of the value forfresh water ice and at -50 0c it exceededthe fresh water value. In fact, at -500Cthe adhesion strength of ice formed fromsaline water was almost twice the adhesionstrength of fresh water ice.Saline ice adhesion tests on varioussurface materials at -10°C and Sw = 2%0 revealed that the ratio of saline iceadhesion to fresh water ice adhesion wasfrom 3 to 8 % on steel and coated steel,but on chloride-polymer-paint it was 24 %.The ratio was 18 % on smooth concrete and82 % on rough concrete.Pull-out Tests with Steel PipesF or these tests a tank (1000 nm indiameter and 600 mm in length) was filledwith water having different salinities.The test sample was a stainless steel pipefilled with polyurethane foam. Thediameter of the pipe was 113.5 mm and the1 ength 650 mm.Loading was done manually by ahydraulic pump and the loading directionwas upward. The force was measured with aTes SI11nHy Sll1nity Thickness Height of Measured Ad",stonof water of ice of ice adhes1011 force strengthsurface[\ ) [q [m) [ ... ) (N) (MP_)1. 0.00 0.00 80 120 7025 U.1642. 0.00 85 16U 8920 0.1563. 0.10 0.01 80 120 5405 0.1264. 0.01 80 130 5675 0.1225. 0.01 90 130 2700 0.0586. 0.20 0.03 100 140 5000 0.1007. 0.01 80 130 3380 0.0738. 0.30 0.07 35 60 675 0.0309. 0.11 85 120 650 0.01510. 0.35 0.15 80 130 755 0.01611. 0.15 75 120 550 0.01312. 0.38 0.11 110 160 1245 0,02713. 0.40 0.12 120 170 111U 0,01814. 0,13 105 155 9

~X0.20I O.15t-~ I~~010\~ 0VI 0~ 0.05o

Mechanisms Affecting Saline Ice AdhesionEstimation of adhesion strength ofsaline ice by theoretial means isextremely difficult. This is partlybecause even the mechani sms affecti ngfresh water adhesion are poorly known atpresent, and partly because the physicalprocesses related to entrapment, expulsionand movement of salt in ice are verycomp 1 i cated. For these reasons, noquantitative analysis to explain theexperimental results is attempted here.However, we wi 11 di scuss the physi ca 1processes that are involved, and try toqualitatively explain some of the observedphenomena.Saline ice consists of salt-free iceand brine pockets including salinesolution. The salt concentration andrelative volume of these brine pocketsdepend on the bulk salinity andtemperature of the ice.The reduction in the adhes i onstrength of ice caused by salt isapparently due to reduced effectivecontact area at the ice/structureinterface. It is reasonable to assume thatthe adhesion strength depends linearly onthe effective contact area. The effectivecontact area can be related to the bri nevolume assuming some geometrical form ofthe brine pockets.Oksanen (1982, 1983) used these ideasin his model for saline ice adhesion, andadopted Assur's geometry of vertical brinecylinders. Oksanen assumed in his modelthat the ice/surface interface structureis equal to that of a vertical cut of theice. The Oksanen model predicts muchhigher values of the adhesion strengththan observed. As Oksanen noted, this isprobably because of a liquid layer of saltsolution forming at the ice/structureinterface. A liquid layer was alsoobserved in the tests of the present studyat high salinities. Its existence in thecase of very low concentrations ofpotassium chloride has been conformed byYano and Kuroiwa (1978). The formation ofthe liquid layer at the interface isdiscussed below.When ice freezes it cools from theequilibrium freezing temperature to thetemperature of the environment. Duringcooling, water freezes on the interior ofthe brine cavities. This concentrates thebrine allowing phase equilibrium betweenthe ice and the brine to be maintained.The ice that forms on the cavitywalls occupies 9 % greater volume than theoriginal brine. Therefore, some brine isforced out of the brine pockets and musteventually be expelled out of the icesample. As a result of brine expulsion, alayer of high salt concentration forms onthe ice surface. When the ice sample isadhered to a structure, the concentratedsalt layer forms at the ice/sturctureinterface and reduces the adhesionstrength.Details of the brine expulsionprocess during cooling of ice have notbeen studied, but the existing dataindicate that the brine moves along thegrain boundaries (e.g. Wakatsuchi andSaito 1985). It is therefore likely thatbrine expulsion from an ice sample is notthree-dimensional, as the orientation ofgrain boundaries depends on the crystalstructure of the ice. It is also possiblethat, in contrast to long-term bri nedrainage, the preferred direction of brineexpul si on from an ice sheet is upwards,since the upward brine permeability of seaice is higher than the downwardpermeability (Ono and Kasai 1985).Whatever the factors that affect thepreferred direction of brine expulsionare, it is apparent that gravity plays nosignificant role in the process. This isbecause the forces related to expansion ofthe water in the brine pockets are muchhigher than those related to gravity.Let us consider a simple case of acube of ice adhered to a plate, in orderto estimate the amount of salt expelledform the ice to the ice/structureinterface. We will assume that the icecube consists of polycrystalline ice withrandom crystal ori entati on and that theplate on one wall of the cube does notaffect the brine expulsion process. Withthese assumptions the amount of salt, m s'expelled to the surface of an ice sampleduring cooling from temperature T1 totemperature T2 is49

where Si is the sa 1 i nity and Mi the massof the ice sample. E4 • (1) is equal toS· (T )m s (T2) = Mi (Si(Tl) - Si(Tl) ~( 2)) (2)Si T 1Cox and Weeks (1986) deri ved anequation for the salinity ratioSi(T2)/Sj(Tl)' When using Zubov's (1945)relationsnip between brine salinity andbrine density (Pb in g cm- 3 )Pb = 1 + 0.8 Sb (3)the equation by Cox and Weeks (1986)readsSi(T2) = (Sb(T2))(1- l/ppi)Si(Tl) Sb(Tl)1+O.8Sb(T2)exp(0.8(Sb(Tl)_Sb(T 2)))1+0. 8Sb(Tl) Ppiwhere Ppi is the density of pure ice ing cm-' .(4)The salinity of brine, Sb' is definedas the ratio of the mass of salt, m s ' tothe mass of brine, mb' so that-~mb(T2) - ( )Sb T2(6)When this amount of brine mb(T2)forms a liquid layer on the ice surface,its thickness h will beh = L mILPb Ai(7)where Ai is the surface area. For a cubeof ice with the density of Piand walllength of l the surface area, Ai' is 6 l2and the mass, Mi' is Pill. It thereforefollows from eqs. (2), (4), (6) and (7)that for a cube of ice wi th a dens ity of0.9 g cm- 3h(T2)(1-~)Si (T 1)6 l2(1+0.8S b(T2)) Sb(T2)(8)Inserting eq. (5) into eq. (8) andusing the value of 0.917 for Ppi gives anequation for the thickness of the liquidfilm on the ice surface at T 2 ._ 0.9 l Si(Tl)h ( T 2 ) - ~----:-'---:-7-----:----:-6(1+0.8 Sb(T2))Sb(T2)[1_(__Sb __( __ T2 __ ))-O.09051+0.8Sb(T2)Sb(Tl) 1+0. 8Sb(Tl)exp(0.872(Sb(Tl) - Sb(T2)) 1(9)The salinity Sb of brine in ice is anexplicit function of temperature. Thevalues of Sb as a function of temperatureare given by Cox and Weeks (1986) in theformSb(T) = -3.9921 - 22.700 T - 1.0015 T2 -0.019956 T3 (10)which is used in the present model.When the size, l, salinity, Si(Tl)and density, Pi' of the original icesample are known, eq. (9) gives thethickness, h, of the liquid film expelledwhen the ice is cooled from Tl to T2' Whenwe consider h as it affects ice adhesionat temperature T2, we are interested intotal brine expulsion that has taken placebefore temperature T2 has been reached.Therefore, the temperature Tl that we needto know to calculate Sb(Tl) is thetemperature from which cooling and brineexpulsion have started, i.e., thetemperature at the ice/water interfaceduring freezing, T f .The temperature T fis basicallyunknown. This is because salt rejectionfrom ice during freezing increases thesalinity at the ice/water interface. Thesalinity and temperature of the interfacelayer depend on the growth rate of ice andeffectiveness of mixing in the water(Weeks and Lofgren 1967; Makkonen 1987).These factors are generally unknown.However, the problem can be ~o~v~d bydetermining Sb(T f ) from the deflnltlon ofthe interfacial distribution coefficientk*(ll )50

While there are theoretical arguments thatsugges t that k* may depend on the growthcon d i t ion s (My e r son and K i rwa n 1977 ;Makkonen 1987), the experimental data byTsurikov (1965), Weeks and Lofgren (1967)and Cox and Weeks (1975) show that attypical grmvth rates of natural sea ice k*is approximately a constant at k* = 0.26regardl ess of the growth condi ti ons andwater salinity. It has been pointed out byMakkonen (1986) that the value of k* =0.26 is a reasonable approximation also inthe case of spray icing.Consequently Sb(T f ) can be approximatedby(12)and the fi na 1 equati on for the thi cknessof the liquid film on the ice surface attemperature T ish(Tl = LS. 0.151(1+0.8 Sb(T))Sb(T)[1_(0.26 Sb(Tl) -0.0905 1+0.8 Sb(T) (13)Si1+3.077 Siexp(3.354S i - 0.872S b (T))]where S. is the initial salinity of theice cu~e and Sb(T) is solved from eq.(10). An iterative solution for h(T) as afunction of the ultimate salinity couldalso be obtained using eqs. (5) and (13),but this woul d not be justified becauseeq. (12) is only an approximation. Forpractical purposes Si can be replaced bythe measured ice salinity in eq. (13), asthe difference between Si(T 1 ) and Si(T 2 )is always small and eq. (13) 1Sinsensitive to these small differences.Examples of the calculated brinelayer thickness, h, at varioustemperatures and ice salinities are givenin Fig. 5 for an ice cube with L = 10 cm.According to Fig. 5, the brine layerthickness starts to grow at a certaintemperature (e.g. -4.2 for Si = 20 %J.This is because there is no ice formationat temperatures higher than this limit. Eq(11) determines the brine salinity,Sb(T f)' and, together with eq. (10), givesan equilibrium freezing temperature T f ,which is an explicit function of, Si' andhigher than Si(T f ). .At temperatures lower than theequilibrium freezing temperature the brinelayer thickness grows rapidly withdecreasing temperature, as liquid waterwithin the ice matrix freezes and brine isexpelled from the ice cube. On the otherhand, when temperature drops, the brinelayer formed on the surface partlyfreezes, and this reduces its thickness,Therefore, h reaches a maximum at a certaintemperature (e.g. -6°C for Si = 10 %0in Fig. 5) and then starts to decrease. Itis interesting to note that according tothe results in Fig. 5 the maximum brinelayer thickness is almost independent ofice salinity.~:::r(/) ~ 1'" 5... 10 ... 20,..:fl 120Z:.:: 100~~ 80ffi 60~..J 40W~ 20a:ID·6 -8 -10 -12 -14 -16 -18 -20 -22TEMPERATURE ('C)Fig. 5. Theoretical brine layer thicknesson the surface of a threedimensionallycooled homogeneous10- 3 m 3 ice cube as a function oftemperature. The curves are fordifferent ice salinities.It should be noted that the values ofh calculated by eq. (13) are typically ofthe order of 10 2 I-Im, whereas theliquid-like layer thickness of fresh waterice is estimated to be of the order of10- 2 ~ only (Jellinek, 1967).What do the results of eq. (13) inFig 5. mean in terms of ice adhesion? Assuggested earlier in this paper, theadhesion strength is probably in some way51

elated to the calculated brine layerthickness, h. What actually happens to theice/structure bond when brine is expelledto the interface is unknown. It is alsouncertain whether a continuous liquid filmforms at the interface. Instead, a layerof mi xed ice and bri ne may exi stat theinterface, allowing some ice/structurecontacts to remain. Brine absorption bythe structure surface because of its poresand roughness elements may also affect thephenomenon, although one should note thatthe calculated values of h are much higherthan the height of the roughness elementson typical plastic, metal and paintedsurfaces. Finally, it is possible thatsome brine is removed from the surface byliquid flow along the ice/structureinterface.The present theory provi des aframework for further studi es on theadhesion of saline ice. The major problemthat should be studied is the relationshipbetween the liquid layer thickness and theadhesion strength. Before thisrelationship is determined we can only saythat the thicker the brine layer, thelower the ice adhesion strength. In thiscontext the results in Fig. 5 are somewhatsurprising in that they suggest that theadhesion strength at temperatures close tothe temperature of ice formation is higherthan at lower temperatures. There are nodata at present to verify this predictionof the theory.The results in Fig. 5 show that iceadhesion should decrease with increasingsalinity. This has been observedqualitatively in experimental studies(Panyushkin et al. 1974a; Tkachev andMalyshev 1976; Sackinger and Sackinger1977; Berenger et al. 1985), but nosignificant effect was found in thepresent data in the range of Sw = 2 - 120/00 (Si-0.5-3 0/00 ). This suggests thatice adhesion may be relatively insensitiveto variations in the brine layer thicknesswhen h is large. However, there may be acritical h, at which a continuous liquidfilm disappears and the adhesion strengthconsiderably increases. This criticalvalue probably depends on the roughness ofthe structure surface. When his bi ggerthan the critical value one should expectsaline ice adhesion to be almostindependent of temperature.The experimental data show that thereare situations in which there ispractically no adhesion between saline iceand a surface. The equal maximum values ofh at different salinities in Fig. 5indicate that if there is no adhesion forsome ice salinity, this should be the casefor any ice salinity at some temperature.This could be useful in mechanicalde-icing, for example. If, saY,a ship'ssuperstucture has been covered by salinespray ice and the temperature is low, onemight minimize the effort of ice removalby waiting for the ice to cool to theoptimum temperature predicted by Fig. 5.UiscussionAdhesion strength of saline ice wasobserved to be quite low in all threetypes of tests made in this study. Thevalues were typically 10 - 20 kPa, exceptat extremely low salt concentrations andlow temperatures. These values of theadhesion strength of saline ice are of thesame order of magnitude as those observedby Tkachev and Malyshev (1976), Saeki etal. (1981), Oksanen (1983) and Berenger etal. (1985). However, Stehle (1970) andSackinger and Sackinger (1977) havereported values that are more than anorder of magni tude hi gher than thosemeasured in this study.Qualitatively, the results agree withthose by Lyyra et al. (1986) in that theadhesion strength drops to only a fractionof its value for fresh water ice alreadyat ice salinities below 1 0/00. Thisphenomenon has not been observed in otherstudies with NaCl ice. Studies using othersalt solutions (Raraty and Tabor 1958;Yano and Kuroiwa 1978; Andrews andLockington 1983) have, however, showedthat the adhesion strength suddenly dropsto a very small value when the temperaturerises above the eutectic point of the saltsolution, and that this happens also withvery low salt concentrations. This isconsistent withthe fact that in thisstudy the adhesion strength of sal i ne icewas observed to be considerably higher at-30°C and -50 0c than at highertemperatures (the eutectic temperature ofNaCl is -23 °c).52

The di fferences between theexperimental results by various authorsare probably related to the formation anddisappearance of the liquid layer at theice/structure interface. Different methodsof freezing the samples, different testarrangements and varying storage times mayaffect the thickness of the liquid film.Preferred orientation of brine expulsionmay also depend on crystal orientation. Onthe other hand, bri ne drainage may removethe liquid film, although the tests ofSection 4 did not indicate that thiseffect would be substantial in the case ofvertical pipes. The data obtained byBerenger et al. (1985) suggests that brinedrainage may affect the adhesion strengthmore on horizontal than on verticalsurfaces.It was found in the tests of Section4 that the ratio of saline ice adhesion tofresh water ice adhes i on was much hi gheron the ch 1 ori de-po lymer surface and morethan an order of magni tude hi gher on arough concrete surface when compared withother surfaces. It is noteworthy that thechloride-polymer surface was much moreporous than the other coated surfaces andthat rough concrete was even more porous.Apparently, the liquid film is absorbed bya porous surface, thereby cons i derab lyincreasing the adhesion strength.Therefore, roughness of the surface playsa more important role in adhesion ofsaline water ice than of fresh water ice.In the light of the results of thisstudy it is clear that, if the liquid filmthat forms at the ice/structure interfaceis retained, then the adhesion strength ofsaline ice is very small at any salinityof ice formed under ocean conditions.Therefore, the ideas presented in thispaper of the formation of the liquid filmdue to brine expUlsion should by testedand developed further. For the samereason, the mechani sms of bri ne drai nageand movement of brine along theice/structure interface should also bestUdied.AcknowledgementsThi s study was supported by theTechnology Development Centre of Finland(TEKES) and a number of Finnishcompanies.ReferencesAndrews, E.H. & Lockington, N.A., 1983:The cohesive and adhesive strength of ice.Journal of Materials Science, 18:1455-1465.Berenger, D.~1., Edwards, R.Y. Jr. &Nadreau, J.P., 1985: Preliminaryassessment of the adhesion shear strengthof ice-steel and ice-frozen sand bonds.Arctic Petroleum Operators' Association(APOA), Research Project Report 85-1, 55p.Cammaert, A.B., Kimura, T., Koma, N.,Yashima, N, Yano, S. & Matsushima, Y.,1986: Adfreeze forces on offshoreplatforms. Fifth Offshore Mechanics andArctic Engineering Conference (OMAE), Vol.IV: 541-548.Cox, G.F.N. and Weeks, W.F., 1975. Brinedrainage and initial salt entrapment insodium chloride ice. U.S.A. Cold RegionsResearch and Engineering Laboratory, CRRELReport 354, 85 p.Cox, G.F .N. & Weeks, W.F., 1986: Changesin the sal i ni ty and poros i ty of sea-i cesamples during shipping and storage.Journal of Glaciology, 32: 371-375.Croasdale, K.R., 1980: Ice forces offixed, rigid structures. U.S.A. ColdRegi ons Reseach & Engi neeri ng Laboratory,Special Report 80-26, Working Group on IceForces on Structures, Hanover, NH, U.S.A.Doud, J.O., 1978: Ice sheet loads onmarina piles. Canadian GeotechnicalJournal, 15: 599-604.Frederking, R. & Karri, J., 1981:Laboratory tests on ice sheet adhesionstrength on piles of different materials.Technical Research Centre of Finland,Laboratory of Structural Engineering,Report 14, 52 p.Gershunov, E.M., 1984: Shear strength ofadfreeze bond and its effect on global iceload app 1 i ed to mobil e offshore dri 11 i ngunits under arctic conditions. 16thOffshore Technology Conference, OTC 4687,Houston, U.S.A.: 357-362.53

Hudson, LA. & Strickand, G.E. Jr., 1976:Low adhesional arctic offshore platform.u.s. Patent 3,972,199, 18 p.Jellinek, H.H.G., 1967: Liquid-like(transition) layer on ice. Journal ofColloid and Interface Science, 25:192-205.Laforte, J.-L. & Lavigne, L., 1986:Microstructure and mechanical propertiesof ice accretions grown from supercooledwater droplets containing NaCl insolution. Third International Workshop onAtmospheric Icing of Structures,Vancouver, Canada, May 6-8, 1986.Lyyra, M., Jantti, M. & Launiainen, J.,1986: Adhes i ve strength of spray accretedice on materials and coatings. TechnicalResearch Centre of Finland, SymposiumSeries 70, POLARTECH'86, Vol. I: 484-496.Makkonen, L., 1986: Salt entrapment inspray ice. IAHR Symposium on Ice, IowaCity, U.S.A., 14 p.Makkonen, L., 1987: Salinity and growthra te of ice formed by sea sp ray. ColdReIi ons Sci ence and Technology, 14: l"i)"j"":17.Muschell, J.E. & Lawrence, R.G., 1980: Iceuplift on piles. Michigan State UniversityReport MICHU-SG-80-506, 81 p.Myerson, A.S. and Ki rwan, D.J. 1977.Impurity trapping during dendritic crystalgrowth. 1. Computer simulation. IndustrialEngi neeri ng Chemi stry, Fundamenta' s, 16:414-420.Oksanen, P., 1982: Adhesion strength ofice. Technical Research Centre of Finland,Research Report, 123, 61 p.Oksanen, P., 1983: Friction and adhesionof ice. Technical Researcn Centre ofFinland, Publications 10, 36 p.Ono, N. & Kasai, T., 1985: Surface layersalinity of young sea ice. Annals ofGlaciology, 6: 298-299.Panyushkin, V.B., Shvaystein, Z.I. &Sergacheva, N.A., 1974a: On certainthermodynami c cri teri a for the choi se ofmaterials for coatings that reduce iceadhesion to construction materials. U.S.A.Cold Regions Research & EngineeringLaboratory, Draft translation 411: 49-57.Panyushkin, V.B., Shvaystein, Z.I.,Sergacheva, N.A. & Podokshik, V.S., 1974b:Experimental investigation of ice adhesionto construction materials. U.S.A. ColdRegions Research & Engineering Laboratory,Draft translation 411: 71-77.Raraty, L.E. & Tabor, B., 1958: Theadhes i on and strength properti es of ice.Proceedings of the Royal Society ofLondon, A245 :~4-2Or.Sackinger, W.M. & Sackinger, P.A., 1977:Shear strengh of the adfreeze bond of seaice to structures. Fourth InternationalConference on Port and Ocean Engi neeri ngunder Arctic Conditions (POAC): 607-614.Sackinger, W.M., Nordlund, O.P. &Shoemaker, H.D., 1986: Low adhesioncoatings for sea spray ice on offshoredrilling units in northern waters.Technical Research Centre of Finland,Symposium Series 70, POLARTECH'86, Vol. I:512-527.Saeki, H., Ono, L & Ozaki, A., 1981:Mechanical properties of adhesion strengthof ice to pile structures. IAHR Symposiumon Ice, Quebec, 1981, Vol II: 641-649.Saeki, H., Ono, L, Takeuchi, L, Kanie,S. & Nakazawa, N., 1986: Ice forces due tochanges in water level and adfreeze bondstrength between sea ice and variousmaterials. Fifth Offshore Mechanics andArctic engineering Conference (OMAE), Vol.IV: 534-540.Smirnov, V.I., 1971: Calculated values ofultimate strengths of freezing ice duringicing of ships. Theoretical andExperimental Studies of Ship Icing,Leningrad, USSR: 154-158 (in Russian).Stehle, N.S., 1970: Adfreezing strength ofice. IAHR Symposium on Ice, Reykjavik,Iceland, Paper 5.3., 12 p.54

Tkachev, A.G. & Malyshev, V.P., 1976:Study of ice adhesion to constructionmaterials, anti-corrosion and anti-icingcoatings. Kholodilnaya Tekhnika, 8: 15-18(in Russian).Tsurikov, V.L., 1965. Formation of ioniccomposition and salinity of sea ice.Oceanologiia, 5: 463-472 (in Russian).Vershinin, S.A., 1980: Effect of an icecover frozen to the cyl i ndri ca 1 supportsof offshore oil well platforms subjectedto water level fluctuations.Neftepromysloveoe Stroite'stvo, 4: 9-11(in Russian).Wakatsuchi, M. & Saito, T., 1985: On brinedrainage channels of young sea ice. Annalsof Glaciology, 6: 200-202.Weeks, W.F. and Lofgren, G., 1967. Theeffective solute distribution coefficeintduring the freezing of NaCl solutions.Physics of Snow and Ice, Institute of LowTemperature Science, 1: 579-597.Yano, K. & Kuroiwa, 0., 1979: Adhesivestrength of contaminated ice.International Symposium on Snow and IceControl Research, Hanover, NH, May 15-19,1978, Paper No. 185: 30-34.Zubov, N.N., 1945: Arctic Ice. Moscow (inRussian).55

SOME PHYSICAL PROPERTIES OF MULTIYEAR LANDFAST SEA ICE,NORTHERN ELLESMERE ISLAND, CANADAMartin o. JeffriesWilliam M. SackingerUniversity of Alaska, Fairbanks, Alaska, USAHarold D. ShoemakerU. S. Department of Energy, Morgantown, West Virginia, USAABSTRACTAs ice is lands calve from the iceshelves off the north coast ofEllesmere Is land, they are quicklyreplaced by deformed and undeformedmultiyear sea ice (referred to asmultiyear landfast sea ice [MLSI]).This MLSI remains fas t to the coas t andice shelves for many years to the pointwhere it might be cons idered asincipient ice shelf. Deformed andundeformed MLSI thickness ranges from2.24 to 10.0 m. Ice thicknessvariations are associated with thecharacteristic undulating surfacetopography of elongated hummocks(hills) and depressions (valleys) thatgive the ice a striped appearance fromthe air. Hummock ice is thicker thandepression ice, i.e., the surfacetopography is mirrored by an invertedrelief at the underside of the ice.Overall, ice salinity ranges from 0.01to 12.06 parts per thousand (0/00)while mean salinities of the variousice cores range from 0.08 to 2.920/00. Hummock ice is more saline thandepression ice and, likewise, iceThis is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987.temperatures in hummocks are colderthan in depressions. Annual salinitylayers that form on the unders ide ofthe MLSI are evident in many cores andsuggest ice growth from fresh, brackishand seawater according to the degree ofmixing of fresh and seawater below theice.1. IntroductionThe disintegration of ice shelvesoff the north coast of Ellesmere Island(Fig. 1) has created many ice islands,the most massive ice features known inthe Arctic Ocean (Jeffries, et al.1987a). The total area of shelf icehas decreased, but in many cases theice is lands are quickly replaced by deformedand undeformed multiyear sea ice(referred to as multiyear landfast seaice or MLSI). The term 'deformed' isused to describe ice with a rough,uneven surface that results from icedisintegration and ridging associatedwith pack ice motion. Conversely,undeformed ice has a relatively smooth,even surface with few signs of old,weathered pressure ridges. Thethickness of undeformed MLSI frequentlyexceeds the equilibrium thickness (2.5-5 m) of multiyear pack ice floes andcan be considered as incipient iceshelf. MLSI remains fast for manyyears, but there are occasional57

85-3'·?::.;-I ICE SHELFoIKILOMETRESI150IFigure 1. Location map of ice cores drilled in MLSI off the north coast ofEllesmere Island.calvings that create thick sea icefloes (cf. Walker and Wadhams 1979).The thickness, volume and mass of thesethick floes is probably exceeded onlyby ice islands and perhaps multiyearhumlllQ.Ck fields.With the exception of the formerMarkham Bay Re-entrant (Ragle et al.1964) and the Nansen Ice Plug (Serson1972) there have been no recent studiesof the properties of MLSI. These areof interest for the following reasons.First, some ice shelves originally grewfrom thick sea ice that remainedlandf as t for many years; henceinvestigations of present-day MLSIcould lend some insight into early iceshelf growth. Second, MLSI is attachedto the edges of ice islands that calvefrom Arctic ice shelves (Jeffries eta1. 1987a). Third, individual thicksea ice floes originate from MLSI offthe north coast of Ellesmere Island.In addition to ice islands, these thicksea ice floes might also be considereda possible threat to offshoreoperations in the Beaufort Sea. Thus,the design and siting of offshorestructures may need to take account ofMLSI properties. Yet, little is knownof the physical properties of MLSI.Our studies of ice shelves and iceis lands during the pas t six years haveincluded some ice core drilling andanalysis of MLSI. In this paper wepres ent some results of ice thickness,ice salinity and ice temperatureinvestigations.2. Surface Topography and Ice ThicknessThe ice shelves have acharacteristic undulating surfacetopography of long, parallel ridges(hills) and troughs (valleys) (Fig.2). The dimensions of the undulationsvary from ice shelf to ice shelf, buttypically the ridge tops are 200-300 mapart and the valleys are up to 5 mdeep. The topography is mos t strikingin summer when meltwater lakes occupythe troughs. MLSI also has anundulating topography but at a smallerscale than the ice shelves (Fig. 2).On MLSI the ridge tops are generally60-100 m apart and the troughs up to1 m deep. Since the dis covery of iceis lands and ice s hel ves it has beentraditional to refer to the undulationsas a ridge-trough topography. In the58

Figure 2. Oblique aerial photograph looking NE across the mouth of AylesFiord. Note the size difference between the undulating topography/melt-poolson MLSI (centre) and Ayles Ice Shelf (foreground).case of MLSI, the term "ridge" might beconfused with pressure ridges orsimilar features; hence, we will us ethe more standard nomenclature ofhummocks (ridges) and depress ions(troughs) .In this area, undeformed MLSItends to grow in sheltered locationswhere pack ice motion is minimized.Under these circums tances a gentle,undulating topography (Fig. 2) developsby elongation and coalescence of meltpools(Jeffries et al. 1987b). At moreexposed locations pack ice motion oftendeforms thin ice and the pressureridges subsequently weather and shrinkin size to create a rough, hummockysurface that is more typical ofmultiyear floes. In some instances,given sufficient time the melt-pools ondeformed MLSI will elongate andcoales ce and the characteris tic stripedappearance becomes apparent insummer. This is occurring on someparts of MLSI at the front of Ward HuntIce Shelf. Our ground observations andanalysis of air photographs suggestthat deformed MLSIundeformed MLSI.is less common thanWhen a sea ice press ure ridge isformed, an underwa'ter keel is createdbeneath the surface ridge and theridged ice is thicker than adjacentdepression ice (Kovacs 1972). This isthe case for deformed MLSI, but what ismore unusual is that the top and bottomtopography of undeformed MLSIapparently also has the same features,i.e. surface topography is mirrored byan inverted relief at the underside ofthe ice. (Fig. 3). This suggestion isbased upon evidence from the ice coresdrilled in Ayles Fiord and Milne Reentrant.Ice cores 85-5 and 85-6(Ayles Fiord) (Fig. 7) were drilledonly 36 m apart, with an elevationdifference of 0.62 m. Ice cores 85-8and 85-9 (Milne Re-entrant) (Fig. 5)were drilled only 38 m apart, with anelevation difference of 0.82 m. Thelength of each ice core, i. e. the icethickness, is given in Table 1.Allowing for the surface elevationdifference, the ice thickness data59

HUMMOCKDEPRESSIONNOT TO SCALEWATERFigure 3. Sc.hematic diagram of mirrored surface and undersurface topographyof deformed and undeformed MLSI.shows that the bottom of the hummockice is 2.06 m and 1.74 m below thebot tom of the depress ion ice at AylesFiord and Milne Re-entrantrespectively. Hence, our conclusionregarding the mirrored topography.Ice thicknesses are summarized inTable 1. With one exception, the iceis at leas t equal to, and often greaterthan, the equilibrium thickness (2.5-5 m) of undeformed multiyear pack icefloes (Maykut and Untersteiner 1971).However, the ice thicknesses are notunusual for this region. For example,before the massive calving from WardHunt Ice Shelf in 1961-62, the MarkhamBay Re-entrant attained a thickness of11 m in 14 years (Lyons and Ragle1962), while parts of the ice plug inNansen Sound (Fig. 1) were over 6 mthick and estimated to be 21-39 yearsold (Serson 1972).The thickest hummocks are 9.8 mand 10.0 m thick, in Milne Re-entrant(core 85-8, Table 1, Fig. 1) and at thefront of Ward Hunt Ice Shelf (core 85-4, Table 1, Fig. 1) respectively. Onthe basis of annual salinity and oxygenisotope layers in the ice and theabsence of old, weathered pressureridges at the surface it has beenconcluded that Milne Re-entrant islargely undeformed MLSI and is now 22years old (Jeffries and Krouse inpress). Conversely, MLSI at the frontof Ward Hunt Ice Shelf is consolidated,deformed ic~ and was perhaps as littleas 5 years old in 1985 (Jeffries inpress) •MLSI often remains fast for manyyears, but occasional calvings dooccur. Between May and August 1984, alarge floe, with an area of approximately35 km 2 , broke away from the MLSIat the mouth of Yelverton Bay. On thebasis of about 200 drill holes the icethickness varied be tween 6 m and 7 m(Table 1). (R. Verrall personalcommunication).3. Ice SalinityIce salinity is based uponmeasurements of electrical conductivityand values have an error of about± 0.01 0/00. Taken as a whole, icesalinity values cover a wide range,from 0.01 0/00 to 12.6 0/00 (Table1). The very low values are found inboth hummocks and depress ions, but arenot confined, as might be expected, tosurface ice alone. Very high valuesfrequently occur at the bottom of thecores and repres ent the mos t recent icegrowth. However, some very highsalinity values occur within the icemass. The mean salinity values alsocover a quite wide range, from a saltfreeminimum of 0.08 0/00 to a salinemaximum of 2.92 0/00 (Table 1).60

Table 1:Ice thickness and salinity data for MLSI.LOCATIONICETHICKNESS (M)CORE IfSALINITYRANGE (0/00)MEANSALINITY ±1 S.D.3 km east ofMarkham Ice Shelf3.77 Hummock (H)84-10.01 to 2.090.85 :I:: 0.41West Ward HuntIce Shelf7.62 Depression (D)85-30.18 to 4.391.26 ± 0.45West Ward HuntIce Shelf10.00 (H)85-40.03 to 12.062.92 ± 1.54M'Clintock Inlet7.92 (R. Verrall, Defence Research Est. Pacific [DREP1, personalcommunication).Ayles Fiord 4.22 (D) 85-5 0.01 to 0.22 0.08 :I:: 0.06Ayles Fiord 6.63 (H) 85-6 0.01 to 3.84 0.60 ± 0.58Ayles Fiord 6.26 (D) 85-7 0.01 to 3.53 1.21 :I:: 0.73MilneRe-entrant 9.80 (H) 85-8 0.03 to 4.54 1.61 :I:: 0.97MilneRe-entrant 7.24 (D) 85-9 0.03 to 3.30 1.07 ± 0.73Yelverton Bay 6-7 (R. Verrall, DREP, personal communication)Bjaare Strait3.06 (D)86-50.22 to 8.87 1.50 ± 1.52Krueger Island 2.24Nansen Ice Plug 3.75Nansen Ice Plug 5.5384-384-486-40.01 to 1.93 0.37 ± 0.330.01 to 3.41 1.44 ± 0.730.16 to 2.49 0.74 ± 0.46On the basis of ice coresalinities, Schwarzacher (1959)computed an average salinity profilefor multiyear ice, without taking intoaccount surface topography. Cox andWeeks (1974), however, computed averagesalinity profiles for hummocks anddepressions. Of the MLSI salinityprofiles, only that for core 84-1 (Fig.4) is similar to the average profile ofSchwarzacher, and the average hummockprofile of Cox and Weeks. Furthermore,while Cox and Weeks (1974) found thatdepression cores were more saline thanhummock cores, the data for MLSIindicates the oppos ite (Table I, Figs.5, 6 and 7).Annual salinity layers occasionallyform in multiyear ice floes asa result of freezing of "fresh" waterthat accumulates in a layer beneath theice in summer (cf. Weeks and Ackley1982, p. 64-65). Annual salinitylayers in MLSI (Figs. 4 and 5) indicatea mean bottom accretion rate of500 mm.a -1. Annual salinity layers andass ociated oxygen is otope variationsare frequently found in MLSI becausewater stratification or pooling of61

o~----------------------~CORE 84-1, NEARMARKHAM ICE SHElF13wwswswssww4~~~~~~~~~~~~~o 1 2SALINITY (0/00)Figure 4. Salinity profile of ahummock ice core (84-1) in MLSI3 km east of Markham Ice Shelf.Letters Sand W denote summer andwinter layers respectively.fres hwater below f as t ice is morecommon and persistent than beneath packice floes (Jeffries in press; Jeffriesand Krouse in press).Of the two thickest hummocks, onlycore 85-8 (Fig. 5A) contains annuallayers and it is also much less salinethan core 85-4 (Fig. 6A). Jeffries andKrouse (in press) conclude that thecontinued preservation of annual layersis partly due to an abs ence ofdeformation. Core 85-4 has neitherannual salinity nor isotope variations(Jeffries in press) and the very highice salinities between 4 m and 6 m canbe attributed to the trapping of brine3in the ice ridge matrix at the time ofdeformation. The preservation ofannual layers has also been attributedto minimal meltwater infiltration andpercolation from the surface (Jeffriesand Krouse in press). Thus, the verylow salinity layer at 4 m (Fig. 5a) is,like the annual salinity layers,probably the result of processesoccurring below MLSI. This will bediscussed further, with reference tothe salinity differences betweenhummocks and depressions.The maj or component of the lakesoccupying MLSI depressions (Fig. 2) issnow meltwater, hence ice at thesurface of depressions is expected tobe less saline than hummock ice.However, Figures 5 and 7 show that theentire thickness of depression ice isless saline. than hummock ice inundeformed MLSI. Elsewhere in theArctic Ocean, surface melt-pool wateroccasionally melts through sea icefloes and refreezes to create a lowsalinity ice, but these melt-holes arethought to be rare features (Cox andWeeks 1974). This, combined with thefact that meltwater percolation andassociated desalination is apparentlyminimal, suggests that hummockdepressionsalinity differences arealmos t certainly due to variations inwater salinity below the ice.Annual salinity variations inwater below MLSI are more common andfrequent than below pack ice floesbecause the latter environment providesmuch less stability and mixing betweenlow salinity runoff and seawater ismore complete. At the undulatingundersurface of MLSI (Fig. 3) it islikely that low salinity/low densitywater will accumulate preferentially ina layer beneath the depress ions. Thesalinity of this water below thedepress ions will vary according to thedegree of mixing between freshwater andseawater. The pooling of low salinitywater will give rise to horizontalwater stratification that is probablyenhanced by the "keels" penetratingwater of higher salinity. Therefore,at the base of hummocks and depressionsthere will be considerable variation inthe salinity of accreting ice.Furthermore, we have some evidence that62

0..---------------,CORE 85-8, HUMMOCK0.----------------,CORE 85-9, DEPRESS I ON2 2Wti 4 4tTl~::c:-. w:3'-"6 6W·W8 8A1 0 ................................................................................L..L.............'-'-L..L...I.......... 1 0 ............................................................................... L.L...JL..L...I....L.J....J-L .........o 1 2 3 4 5 0 1 2 3 4 5SALINITY (0/00) SALINITY (0/00)BFigure 5. Hummock .(A) and depression (B) ice salinity profiles in Milne Reentrant.Winter layers are represented by the letter W. Data points in thisgraph and in Fig. 6, 7 and 8 are plotted to allow for the elevation differencebetween hummock and depression, i.e., the depression ice depths are referencedto the top of the hummock.sugges ts that the low salinity waterlayers can be maintained year-round.Before the beginning of snowmelt, waterbelow core 85-5 (Ayles Fiord) had asalinity of only 1.20 0/00. This wouldexplain the very low salinity of thedepress ion ice compared to the hummockice (Fig. 7, Table 1).4. Ice TemperatureInbeforeearlysnowmeltspringhad(April-May) ,begun, icetemperatures were periodically measuredat the bottom of each borehole, afterpieces of ice core had been removed.The technique for ice temperaturemeasurement involved pressing athermistor against the wall of theborehole with a piece of foam until asteady value was obtained. The foamwas the same diameter as the boreholeand served to minimize air circulationas well as holding the thermis tor inplace. Data are, summarized in Table 2and plotted in Fig. 8. It should be63

O~.-----------------------.CORE 85-4, HUMMOCK0.-------------------------,CORE 85-3, DEPRESSION2 2468 8A10L:==~~~~~~~~~10~~~~~~~~~~~o 5 10 15 0 5 10 15SALINITY (0/00) SALINITY (0/00)BFigure 6. Hummock (A) anddepression (B) ice salinityprofiles in MLSI at the front ofWard Hunt Ice Shelf.Table 2.Ice temperature ranges in MLSILocationHummockTemperature (oC)DepressionWard Hunt I.S.Ayles FiordMilne Re-entrant-21.6 to -7.9 -16.4 to -6.3-17.2 to -5.3 -14.7 to -1.8-15.3 to -2.4 -14.3 to -4.264

Or-----------------------~AYLES FIORD2CORE85-5CORE 85-6TEMPERATURE-25 -20 -15 -10 -5 0o~~~~~~~~~~~A24810~------------------~-25 -20 -15 -10 -5 0O"..,. ........... ,.....~..,...... ......... r'""T"T""T""T'"....,.-.......... "'"T""16o123SALINITY (0/00)42t1tI1~485-6Figure 7. Hummock and depressionice salinity profiles in MLSI atthe mouth of Ayles Fiord.68 ................................ --'-&.. ........................................... ...L....... .......... L.J-25 -20 -15 -10 -5 o0.-------..-------------,c2Figure 8. Hummock (solid square) --t.~and depression (open square) icetemperature (0 C) profiles in MLSIat the front of Ward Hunt IceShelf (A), Ayles Fiord (B) andMilne Re-entrant (C).810 ....................... ~ ........... ...1...L. .......... --.l...L...L..L.~ ........... ....I..J-25 -20 -15 -10 -5 oTEMPERATURE65

noted that the measured temperaturerange is smaller than the actualtemperature range since ice surfacetemperature measurements were notalways made and measurements close tothe bottom of the boreholes weredifficult to obtain.data are ofthe thermallocations .Nevertheless, thesome value in describingregime of some MLSIIt is immediately apparent thatdepress ion ice is warmer than hummockice. This is to be expected in view ofthe melt-pools that are localized indepressions. During the freezing ofpools at the end of summer, some latenthea t of freezing will be los t but thiswill be lessened subsequently by theinsulating effect of snow which coversthe lake ice shortly after the initialice skin is formed.The hummock and depression ice inWard Hunt MLSI (Fig. 8A) are colderthan equivalent ice at the other twolocations. This may be related tolocal mean air temperature variations,but is more probably related to watersalinity and temperature differencesbelow the ice. Whereas the temperatureprofiles in Ward Hunt MLSI (Fig. 8A)converge and indicate a bottom icetemperature of -1.8' C (freezing pointof seawater) below the hummock anddepression, the temperature profiles atthe other locations diverge (Figs. 8Band 8C), with the ice towards thebottom of depressions being warmer thanice toward the bottom of hummocks. Asnoted in Section 3, water below core85-5 had a salinity of only 1.200/00. The freezing point of water ofthis s alini ty is -0.1' C. This value isalso predicted for the bottom of theice (4.22 m) by a temperature-depthregression line of the lowermost fourvalues in core 85-5. Thus, it appearsthat depression ice is warmed fromabove and below.5. Summary and ConclusionOur observations of MLSI are notextensive and the number of MLSI coreswe have studied to date is quite smallbut the data reveal a number of featuresthat are apparently peculiar tothis type of ice.1) Surface melt-pools evolve intolinear, parallel lakes that give theice a characteristic undulatingtopography. The topography of undeformedMLSI is smoother and lesshummocky than deformed MLSI, but ineach case it is most striking in summerand easily recognized from the air.2) The surface topography ofundeformed MLSI is apparently mirroredby an inverted relief at the undersurfaceof the ice. The top and bottomroughness of multiyear sea ice arefrequently well correlated (Ackley etal., 1976); hence, the bottomtopography of MLSI might also cons is tof linear, parallel undulations.Moreover, hummock and depression icethicknesses in undeformed MLSIfrequently exceed the equilibriumthickness of multiyear pack ice floes.3) In summer, low salinity waterpools at the ice surface and alsooccurs in a layer at the undersurfaceof the ice. MLSI depress ion ice isless saline than hummock ice and thisis attributed not only to the refreezingof surface meltwater, but alsoto ice accretion from low-salinitywater at the ice undersurface. Thestratification of low salinity waterbelow the ice provides a potential forfrazil ice growth, in addition tocongelation ice growth. Therefore, theice crystallography would be expectedto be variable, from fine-grainedfrazil ice textures to coarse-grainedcongelation ice textures.4) The presence of freshwater atthe surface and undersurface of the icealso causes depression ice to be warmerthan hummock ice. The temperaturedifferential between the top and bottomof hummock ice is greater than that fordepress ion ice. Therefore, at anyonetime there is likely to be greater heatconduction through the hummocks andpotentially greater ice accretion onthe undersurface keel. This would tendto perpetuate the undulatingundersurface topography.AcknowledgementsFieldwork was made possible66

through the logistic support of thePolar Continental Shelf Project (G. D.Hobson, Director) and the invaluableassistance of Harold Serson. Work atthe Geophysical Institute is supportedby the U.S. Dept. of Energy, MorgantcwnEnergy Technology Centre, W. Virginia.This work began when one of us (M.O.J.)was supported by ass is tants hips B.ndscholarships at the University ofCalgary together with grants andcontracts from Defence ResearchEstablishment Pacific, Arctic Instituteof N. America, Dome Petroleum, GulfCanada and Petro-Canada.ReferencesAckley, S.Kugzruk, F.F. 1976.variationsice. CRRELF., Hibler, W. D. III,K, Kovacs, A. and Weeks, W.Thickness and roughnessof Arctic multiyear seaReport 76-18.Cox, G. F. N. and Weeks, W. F. 1974.Salinity variations in sea ice.Journal of Glaciology 13: 109-120.Jeffries, M. 0., in press. Oxygenisotope evidence of freshwater poolsand ice accretion below multiyearlandfast sea ice, northern EllesmereIsland. National Research Council ofCanada, Technical Memorandum.Proceedings of Workshop on Extreme IceFeatures.Jeffries, M. O. and Krouse, H. R., inpress. Salinity and isotope analysisof some multiyear landfast sea icecores, northern Ellesmere Island,Canada. Annals of Glaciology 10.Jeffries, M. 0., Sackinger, W. M. andShoemaker, H. 1987a. Geometry andphysical properties of ice islands.POAC - 87.Jeffries, M. 0., Sackinger, W. M. andSerson, H. V., 1987b. Remote sensingof sea ice growth and melt-pool evolution,Milne Ice Shelf, Ellesmere Island,Canada. Annals of Glaciology 9.Kovacs, A., 1972. On pressured seaice. In, Sea Ice - Proceedings of anInternational Conference. NationalResearch Council, Reykjavik, Iceland,276-295.Lyons, J. B. and Ragle, R. H., 1962.Thermal history and growth of the WardHunt Ice Shelf. UIGG-IARS Commiss iondes Neiges et Glaces, Collogue d IObergurgl, 10-18 September 1962, 88-97.Maykut, G. A. and Untersteiner, N.,1971. Some results from a timedependentthermodynamic model of seaice. Journal of Geop.hYs ical Res earch76: 1550-1575.Ragle, R. H., Blair, R. G. and Persson,L. E. 1964. Ice core studies of WardHunt Ice Shelf, 1960. Journal ofGlaciology 5:39-59.Schwarzacher, W., 1959. Pack icestudies in the Arctic Ocean. Journalof GeoRDy'sical Research 64:2357-2367.Serson, H., 1972. Investigation of aplug of multiyear old sea ice in themouth of Nansen Sound. QperationTang~~y~port D Phys. R (G) Hazen42. Defence Research Board, Departmentof National Defence, Ottawa. 13 pp.Walker, E. R. and Wadhams, P., 1979.Thick sea ice floes. Arctic 32: 140-147.Weeks, W. F. and Ackley, S. F., 1982.The growth, structure, and propertiesof sea ice. CRREL Monog~pJl 82-1.DiscussionD. DICKENS: Have you considered that thesmall amplitude variations in salinityobserved with depth and interpreted as aseasonal effect may actually be naturalnoise in the profile?M. JEFFRIES: If there were no supportingoxygen isotope evidence I might agreethat this might indeed be the case. Theisotope data is not presented in thispaper, but will be published in duecourse (Jeffries and Krouse, in press,Salinity and oxygen isotope variations insome multiyear landfast sea ice cores,northern Ellesmere Island, Annals ofGlaciology 10, 1988). This data showsthat the salinity variations could havearisen only as a result of variations inwater salinity below the ice, the water67

salinity variations being related toseasonal variations in the input of snowmeltwater and its mixing with seawater.However, it should be noted that there isa need to drill more ice cores i~ multiyearpack ice floes and to analyze thesefor their salinity and oxygen isotopecontent for a comparison of the data withthat from thick, old fast ice. Then wemight learn more about the question ofnatural "noise" as opposed to naturalvariations arising from seasonal differencesin water salinity below the ice.G. VARGAS: Did you ever measure themechanical strength of the ice in thedepressions and in the hummocks?M. JEFFRIES: We have not yet measuredthe mechanical strength of MLSI. However,we have plans to undertake a studyof mechanical strength of MLSI in thenear future, and the study will includetesting of ice from hummocks anddepressions.68

GEOMETRY AND PHYSICAL PROPERTIES OF ICE ISLANDSMartin o. JeffriesWilliam M. SackingerUniversity of Alaska, Fairbanks, Alaska, USAHarold D. ShoemakerU. S. Department of Energy, Morgantown, West Virginia, USAABSTRACTIce is lands are the mos t mass i veice features known in the Arctic Oceanand are a potential hazard to offshorestructures in the Beaufort and ChukchiSeas. In this regard there are threeimportant concerns; 1) ice islanddrift, 2) geometry and 3) physical andmechanical properties. This paperexamines the geometry (surfacetopography, edge characteristics,length, width, area, volume and mass)and physical properties (electricalconductivity, density, crystallographyand temperature) of ice islands. Areview of the literature shows that thesize of ice is lands has decreas ed sincethe first discoveries 40 years ago, butlength-width ratios have remainedessentially constant with almost 80% ofvalues falling in the rage 1 to 2.99.The largest ice island known at thepresent time (commonly referred to asHobson's Ice Island) has a meanthick~ess of 42.5 m, a surface area o~26 km and a mass of about 700 x 10metric tons. Ice core analysis of thisice island reveals that it and otherice islands that calved from Ward HuntThis is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987.Ice Shelf in 1982-83 are composed ofgranular, freshwater ice with very lowelectrical conductivity. The influenceof known physical properties onposs ible mechanical properties isdiscussed.1. IntroductionIce islands are the most massiveice features known in the ArcticOcean. They are tabular icebergs thatcalve from the ice shelves off thenorth coast of Ellesmere Island. Aftercalving they become embedded in, anddrift with, the pack ice. There aretwo major drift syste1D3 in the ArcticOcean; 1) the Beaufort Gyre in whichice islands can drift for many yearsaround the ocean and 2) the Trans-polardrift by which many ice is lands driftout of the·ocean. Their size, combinedwith their clockwise average drift inthe Beaufort Gyre, makes ice islands apotential hazard to offshore drillingoperations in the southern BeaufortSea.With regard to ice islandinteraction with offshore installationsin the Beaufort Sea, there are threeimportant concerns; ice island drift,geometry and physical properties. Inthis paper we examine the geometry(surface topography, edge characteris-69

ICE ISLANDS1986+ 25 JUNE 198590·NEW·ICE ISLANDS19838 ·OLD·l.MAYICE ISLAN44 JULY 1984HOBSON'S+ ICE ISLANDoIKILOMETRESII150IFigure 1. Location map of ice shelves and ice islands off the north coast ofEllesmere Island. The drift of Hobson' s Ice Island represents the drift ofmost of the new ice islands that calved in 1982-83 since they have drifted asa cluster.tics, length, width, area, volume andmass) and physical properties (electrical conduc ti vi ty , dens i ty andcrystallography) of ice islands.Particular attention is given to thoseice islands recently discovered off thenorthwest coasts of Axel Heiberg andEllesmere Is lands, Canada.2. Ice Island Population, 1983-86Ice is land T-3 completed 3-4circuits of the Beaufort Gyre before itdrifted out of the Arctic Ocean in1983-84, via the Transpolar Drift(Sackinger in press). Based on thedrifts of T-3 (Dunbar and Wittman 1963)and the Soviet station NP-2 (Treshnikovet a1. 1977), it is believed that onecomplete curcuit of the Beaufort Gyrerequires 5-10 years. Some ice islandsdo not complete one circuit, whileothers drift for more than onecircuit. We have used the concept of afull circuit to distinguish between"old" ice islands (>1 circuit) and"new" ice islands «1 circuit).In 1982-83 a number of ice islandscalved from east Ward Hunt Ice Shelf(Jeffries and Serson 1983). The iceislands were first discovered in April1983 off the front of wes t Ward HuntIce Shelf (Fig. 1) where they wererecorded on SLAR (side-looking airborneradar) imagery on 11 May 1983.Analysis of the radar imagery shows atotal of eight "new" ice islands.In July 1984 a number of small iceis lands were obs erved embedded in packice off the north coast of EllesmereIsland. Subsequent analysis ofvertical and oblique air photographsrevealed a total of nine ice islands,mos tly located off the wes t front ofWard Hunt Ice Shelf (Fig. 1). Judgingby their dirty, dry surfaces and smallsize they are "old" ice islands. Bythis time the new ice is lands haddrifted to a position west of PhillipsInlet (Fig. 1), thus the old and new70

ice island populations were mutuallyexclusive, giving a total ice islandpopulation of seventeen.During the most recent opportunityto observe and count ice is lands, inMay 1986, the new ice island group waslocated off the northwest coast of AxelHeiberg Island (Fig. 1). On 3 May,during a flight from Hobson's IceIsland to Nansen Sound, three observerscounted 23-25 ice islands (Fig. 1).The increas e in the number of obs ervedice is lands might be due either to thedisintegration of some of the new iceislands and/or an "invasion·· ofpreviously unidentified old ice islandsinto the area.Between April 1983 and May 1986the new ice islands drifted a net distanceof almost 500 km (Fig. 1). It isprobable that this slow drift-rateapplies to all ice islands in thiscoas tal region, hence, the new and oldice island populations remain IIIltuallyexclusive. Therefore, there are from32 to 34 ice islands in the near-shorecoastal areas of northern Axel Heibergand Ellesmere Islands.3. Ice Island GeometryOf the ice is lands mentioned atthe end of the previous section, 17have been found on SLAR imagery and airphotographs. The length, width andarea of 16 of thes e ice is lands hasbeen measured. In addition, the volumeand mass of the new ice is lands hasbeen es timated from ice thickness anddensity measurements. To compare thepres ent ice is lands with ice islands asa whole during the pas t 40 years, aliterature search has been completedand the length and width of 37 otherice islands has been documented. Thisdoes not include the hundreds of small,ice island fragments observed from theair during the period 1972-76, close" toshore in the Alaskan and Canadianwaters of the southern Beaufort Sea.The ice island populations counted oneach occasion are summarized inSpedding (1977), but the ice islanddimens ions are cons ide red to beunreliable estimates for the purposesof our study. This section presentsdata for the 1983-86 ice islands anccompares it to that of earlier iceis lands; a total of 52 ice is lands.It 1IllS t be noted that the fate ofthe earlier 37 ice islands is unknown;it is highly unlikely that they stillexist in the same shape and form aswhen they were firs t dis covered. Forexample T-3 and WH-4 both grounded andbroke up into smaller fragments offPoint Barrow, Alaska, while many iceislands are known to have drifted outof the Arctic Ocean.3.1 1983-86 Ice IslandsWhen the first ice islands werediscovered 35-40 years ago they weredistinguished from the surrounding packice not only by their immense size butalso by their striking, undulatingsurface topography of long, parallelridges (hills) and troughs (valleys) •This topography originates on theparent ice shelf. Ground survey showsthat the undulations on the new iceislands that calved in 1982-83 have awavelength of about 200 m while peakto-troughdepth is about 2 m. At thetime of their discovery, the margins ofthe new ice islands were characterizedby 4-5 m high vertical, angular cliffson the shelf ice edges. On the otherhand, the edges of adjacent andattached old, multiyear sea ice(originally attached to the front ofWard Hunt Ice Shelf) were marked byrecently formed sea ice rubble andpressure ridges. By May 1986, mos t ofthe shelf ice edges of the ice is landshad also been overridden by sea ice.In the case of Hobson's Ice Island, alarge area (~5 km 2 ) of sea ice hadbecome attached to the other long,shelf ice edge. Stereo-viewing of themargins of old ice islands on airphotographs revealed little evidence ofsea ice pile-up and ridging whichsuggests that the sea ice occasionallybreaks off and does not remain as apermanent feature.The length and width of each ofthe ice islands is plotted in Fig. 2.The dimens ions of the new ice is landsrange from a minimum of 0.5 x 1.48 kmto a maximum of 5.2 x 10.31 km. Itmust be noted that the dimensions ofthree of the new ice is lands include71

3020* •10 0~6.XNEW. 1946-50NEW. 1961NEW. 1983OLD. 1970$OLD. 1984OTHERSE~J:....0~1.00.10.1 1.0LENGTH (km)10 20 30Figure 2. Scatter diagram of the length and width of ice islands. Theparallel diagonal lines are for length-width ratios of 1 to 6. Note that alogarithmic scale is used for the axes.the old, multiyear ice that wasoriginally attached to the front ofWard Hunt Ice Shelf and remainsattached to the ice islands. Weconsider this multiyear ice to be anintegral part of the ice islands. Newice island areas range from 0.6 to26 km 2 (Table 1).The new ice is lands are muchlarger than the old ice islands andthere is no overlap between their areasand dimensions (Table 1, Fig. 2). Oldice is~ands have areas of 0.003 to0.25 km while dimensions range from0.06 x 0.07 km to 0.39 x 0.79 km.To determine the volume and massof the ice is lands requires ice thick-ness and ice dens ity data. This isavailable only for the new ice islandsfor which the following assumptions aremade:1) The shelf ice of all the new iceis lands has a mean thickness of42.5 m, which is the cas e forHobson's Ice Island (R. Verrall,personal communication to M.O.J.1984).2) The shelf ~ce has a mean densityof 872 kg m (see Table 2).3) Old, multiyear sea ice attachedto some of the new ice islandshas a mean thickness of 10 m;this is common along the northcoas t of Elles me re Is land(Jeffries and Serson 1986;72

Table 1:Ice island data from SLAR imagery and air photographs (AP)Ice Island II Area (km 2 ) Volume (km 3 ) Mass (metric tons x 10 6 )SLAR-1 26.0 (0.64/0.36) 0.796 701.85SLAR-2 7.1 (0.54/0.46) 0.195 171.81SLAR-3 2.8 (0.43/0.57) 0.067 59.14SLAR-4 1.9 0.081 70.63SLAR-5 0.6 0.025 21.80SLAR-6 1.0 0.043 37.49SLAR-7 1.9 0.081 70.63SLAR-8 1.0 0.043 37.4942.3 1.331 1170.84AP-1AP-2AP-3AP-4AP-5AP-6AP-8AP-90.0050.0030.020.0750.030.250.020.03Notes 1) The proportion of shelf ice and----- old, multiyear sea ice in iceislands SLAR-l to 3 is givenin parentheses. 2) Ice IslandSLAR-l is known as Hobson's IceIsland. 3) Data is unavailablefor ice island AP-7 as it wasrecorded only on an oblique airphotograph.Jeffries et al. 1987a; 1987b).4) Old, multiyear s ea ~ce has a meandens i ty of 910 kg m , similar tothat found in west Ward Hunt IceShelf (Jeffries et al. in press).The 1982-83 calving from Ward HuntIce Shelf involved a total ice loss of42.3 km, with a volume tf 1.331 km 3and a mass of 1170.84 x 10 metric tons(Table 1). For comparison, the totalvolume and mass of los t ice isequivalent to 0.53% of the total annualiceberg production from Greenland, asestimated by Robe (1980). Although theamount of ice los t was only a smallproportion of the total Arctic calvingloss, the mass and volume of the newice islands far exceeded the mass of anaverage& medium-s ize Greenland iceberg(5x10 metric tons; Robe 1980). Acomparison of the volume and mass ofice islands and Greenland icebergs isuseful since in summer in therelatively sea ice-free waters off thecoasts of Labrador and Newfoundland,iceberg towing is common practice inthe protection of offshore drilling andproduction platforms (Bruneau et al.1977). Despite their much larger size,ice is land towing might be feas i ble inthe southern Beaufort Sea, ifsufficient ship horsepower is availablein relatively sea ice-free water.3.2 Ice island dimensions, 1946-86The dimens ions of a total of 52ice islands are plotted in Fig. 2.There is a great range of sizes from27.0 x 29.0 km (ice island T-2,discovered in 1950; Koenig et al. 1952)to 14 x 17 m, a very small fragmentgrounded in Stefansson Sound, Alaska,1983 (Kovacs 1985).Ice islands T-l to T-5 werediscovered between 1946 and 1950(Koenig et a1. 1952). These are thelargest ice islands ever found (Fig. 2)and their great size suggests that theyhad calved not long before theirdiscovery. In 1961-62, sixteen iceislands calved from Ward Hunt Ice Shelf(Hatters ley-Smith 1963) and Fig. 2shows that the dimensions of these iceislands fall in the range between thenew "1946·· and "new·· 1983 iceislands. This decrease in the size ofnew ice islands is to be expected as73

the total area of ice shelf decreasesover time, so that individual iceshelves occupy only fiords and bays anddo not extend far offshore.It is not known whether iceislands fragment as they circulate inthe Arctic Ocean, but it is certainthat they disintegrate after groundingin shallow water. For example, iceislands T-3 and WH-4 grounded a shortdi's tance north of Pt. Barrow, Alas kaand disintegrated into many smallerpieces. The possible size of thesesmall pieces can be assessed fromFig. 2. In addition to the small, oldice islands discovered in 1984, manysmall ice island fragments have beenfound aground along the Beaufort Seacoast of Alaska and Canada (Kovacs1976, 1977, 1985; Kovacs and Mellor1971). Some of t;hese ice islands areparticularly small and we consider themalso to be old ice islands (Fig. 2,"old 1970s").Figure 2 also shows the dimensionsof what we have termed "other" iceislands, found freely floating in theArctic Ocean in the 1960s and 1970s.It is difficult to classify them as oldor new, but they are all large whichsugges ts that they were relati velyyoung when first sighted.We have shown that the size ofobserved ice islands has changed overtime. It is also of interest toexamine their shape, since this has notbeen done previously in any detail andit might be useful for futurerecogni tion of ice islands embedded inpack ice. Ice islands are- sometimesirregular in shape, but are frequentlyalmost rectangular with quite straightedges. If one treats all the iceislands as rectangles, expressed asstandardized length-width ratios, forwhich isolines are plotted in Fig. 2,length-width ratio values range from1.0 to 5.51. This is quite wide, butthe distribution is positively skewed(Fig. 3). Almost 50% of the dataoccurs in class 1-1.99 alone and almos t80% in classes 1-1.99 and 2-2.99combined. Furthermore, this includesdata for old and new ice islands alikeduring the pas t 40 years. Thus, theshape of ice islands is largelymaintained as they circulate, ground,disintegrate and re-circulate in theArctic Ocean. T-3 is a good example ofthis phenomenon. In 1952, T-3 haddimens ions of 10 x 18 km (Crary 1958),with a L-W ratio of 1.8. Aftergrounding and disintegration in 1961, alarge piece remained, floated off andcontinued to circulate in the ArcticOc~an until 1984; a 1978 Seasat imageof T-3 in the southern Beaufort Sea (Fuand Holt 1982) gives dimensions of7 x 14 km, a L-W ratio of 2.Mention mus t als 0 be made of iceisland thinning by ablation as the icedrifts around the Arctic Ocean. Thereis very little data available toquantify this, but data from T-3suggest that it might have thinned byas much as 30 m, from 60 m to 30 m,during the interval 1952 to 1973 (Crary1958; Holdsworth and Traetteberg1974). The thinning of the iceincreases the probability that the iceislands will drift into shallowerwater; viz. the many small grounded iceislands noted earlier.4. Ice Island Physical PropertiesThree ice cores have been obtainedfrom the former shelf ice of thelargest of the new ice islands, Le.Hobson's Ice Island. The ice coreswere drilled close to the edge of theice island that corresponds to the lineof fracture which led to the calvingfrom Ward Hunt Ice Shelf. The ice corelocations are shown in Fig. 4 and itshould be noted that only core 85-10went right through to seawater belowthe ice island. I

1049.1%•mI NEW, 1946-508 NEW, 1962~ NEW, 1983f2J OLD, 1970s~6 30.2%8 OLD, 19841?11 OTHERS~ 4 15.1%2 2.8% 2.8%o1.0-1.99 2.0-2.99 3.0-3.99 4.0-4.99LENGTH-WIDTH RATIO CLASS5.0-5.99Figure 3. Frequency distribution of ice island Length-Width (L-W) ratios.The distribution is very similar to that observed for Antarctic tabularicebergs, most of which have L-W ratios between 1 and 2 (Nazarov, 1962 [seeWeeks and Mellor, 1978]).10 was complete (September 1985), a25 mm diameter polyethylene pipe waslowered into the borehole and filledwith a non-freezing fluid. The pipewas allowed to freeze in and not untilApril 1986 were ice temperatures loggedby lowering a thermis tor down the pipeat 1 m increments. A cons triction inthe pipe at 30.13 m preventedtemperature measurements in the deepes tice. The ice temperature profile isplotted in Figure 5.Ice temperatures range from -22.8°C at 0.13 m to -5.9°C at 30.13 m. Thegreates t rate of change of temperatureoccurs in the upper 10 m, the zone ofseasonal temperature change. Below10 m, the rate of change of temperaturewith depth is constant at 0.35°C permetre. The straight line (Fig. 5)extrapolates to a temperature of -1.68°C at 42.06 m (the length of theborehole). This temperature correspondsto those expected of the surfacewaters of the Arctic Ocean. Conversely,the straight line extrapolates to atemperature of -16.4° C at the surface;this probably. represents the meanannual air temperature at Ward Hunt IceShelf, averaged over many years.4.2 Density and CrystallographyThe mean density values for cores86-1 and 86-2 are very similar (Table2) and the combined mean density of theice is 871.8 kg m 3 • The densityprofile of core 86-1 is shown in Figure6 and, like that of core 86-2, it showsa trend to higher values towards thebottom of the ice island. In each icecore the Variation about the mean valueis quite low and is accounted for, inpart, by the trend to higher values inthe deeper ice. In core 86-1, thevariation about the mean is higher thanthat of core 86-2 due to oneparticularly low value at the surface(Fig.6); this ice was candled as a75

I~°Iw u..1 0 0zw~1 « ....J_I--z::>ICE CORE86-1(RIDGE)ICE CORE86-2(TROUGH)ICE CORE85-10(RIDGE)______ 1I~~~ 1..:: ~ __________________ ~ e: 38.67m 33.97m "_42.06mI. 400m.-...-1.__2_00m_---.j: iNOT TO SCALEFigure 4.Diagram of ice core positions at the edge of Hobson's Ice Island.1 0~ 2030o r-I ..... ----r-------,•result of melting along grainboundaries during the previous summer.Density variations also arise dueto changes in the type and frequency ofair bubbles in the ice. Large sectionsof the cores contain bubbles, mos t ofwhich are spherical, with diameters ofup to 2 mm. These are either evenlydistributed throughout the ice or foundin clus ters . Less frequent are "s trawbubbles" which have diameters of about1 mm, are long and straight, andoriented parallel to the long axis ofthe cores. Air bubbles are common inthe ice, but there are also sections ofice that are essentially bubble-free.The straw bubbles appear to bealmost exclusively associated withcoarse-grained ice (Fig. 7A), but thereis otherwis e no apparent relations hipbetween grain size and air bubbles.40y 0.3SH -16.4 (R = 1.0)-25 -15 -5TEMPERATUREFigure 5. Temperature (0 C)profile for a ridge on Hobson'sIce Island. The straight line isrepresented by the regressionequation, Temperature = 0.35Depth (m) - 16.4°C.76

Table 2:Ice core data for Hobson's Ice IslandLength (m)Sec Ran~1(11 S em )CORE 85-1042.062. 18 to 32. 87.16 ± 4.35CORE 86-1 CORE 86-238.67 33.971.52 to 252.0 1.05 to 274.011.55 ± 32.76 6.29 ± 25.2Densi§Y Range(kg m )Mean ~nsity(kg m )665 to 899 842 to 916870 ± 21.0 874 ± 12.5Temperature -22.8 to -5.9Range (·C)Note:all mean values are expressed to ±1 Standard DeviationFor example, Figures 7B and 7C showfine-grained ice and while one isalmost bubble-free (Fig. 7B) the other(Fig. 7C) contains many spherical,evenly dis tributed bubbles. Intermediatebetween fine and coarse-grainedice is medium-grained ice (Fig. 7D).This particular example is almos tbubble-free.Grain sizes vary from fine tocoarse, but the available evidence doesnot indicate any relationship betweengrain size and depth. On the contrary,grain size variations are apparentlyrandom. For example, coarse-grainedice (Fig. 7A) occurs only 0.44 m belowfine-grained ice (Fig. 7B). Similarly,the two specimens of fine-grained ice(Figs. 7B and 7C) are 7.39 m apart andthe medium grained ice (Fig. 7D) fallsbetween them.4.3 Specific Electrolytic ConductivityEach of the ice cores has a verylow mean SEC (Table 2) equivalent to asalinity of

DENSITY (kg.m 3 )800 850 900O~~~I~ SEC iii1 0-ISEC (~S.cm )Figure 6. Specific Electrolytic Conductivity (SEC) and density profiles forice core 86-1, Hobson's Ice Island. Note that the SEC scale is.logarithmic.a layer of freshwater at theundersurface of the ice shelf. Thehigh SEC values in cores 85-10 and 86-1might, therefore, be due to changes infreezing rates or a slight change inthe SEC of the freshwater below the iceshelf. The entire ice island is nowthought to be composed of granular ice(Fig. 7) as a result of bottomaccretion of frazil ice having beenbalanced by surface ablation forseveral decades.4.4 Some Thoughts on MechanicalPropertiesHobson's Ice Island apparently iscomposed entirely of freshwater ice.Mellor (1983) notes that the mechanicalproperties of freshwater ice must bespecified as functions of temperature,strain/stress rate, porosity, grainsize and structure. Since a program ofmechanical tes ting of the ice is onlyjust beginning, one must estimatevalues on the basis of data availablefor the other parameters.In general, ice strength increasesas ice temperatures decrease (Weeks andAssur 1967; Mellor 1983); hence, inHobson's Ice Island, where temperaturesincrease toward the bottom (Fig. 5),ice strength would be expected todecrease. Temperature measurements onWard Hunt Ice Shelf have shown that iceunder the troughs is consistentlywarmer than ice under the ridges,78

ABFigure 7. A: Coarse-grained ice at 7.49 m, ice core 86-1. B: Fine-grainedice with few bubbles at 7.05 m, ice core 86-1. C: Fine-grained, bubbly ice at14.44 m, ice core 86-1. D: Medium-grained ice with few bubbles at 9.0 m, icecore 86-1. A 10 mm grid gives the scale.79

largely due to the latent heat releasedduring refreezing of melt-pools (Lyonsand Ragle 1962). Therefore, ice undera ridge might be expected to bestronger than ice under a trough andfragmentation of large ice islands willtend to follow the line of thetroughs. On Hobson's Ice Island, therate of change of temperature withdepth is constant below 10 m tending toproduce · ice strengths which decreaselinearly with depth. Above 10 m,however, ice temperatures and icestrength will vary seasonally.The effect of an ice densityincrease is to increase ice strength(Mellor 1983). In Hobson's Ice Island,density increases with depth (Fig. 6)and throughout the entire thickness ofthe ice island the scatter in densityvalues will caus e some s ca t ter in icestrength values. This will becompounded. further by the grain sizevariations which show no systematicincrease or decrease with depth. Weeksand Assur (1967) note that thecompressive strength of lake icedecreases as grain size increases.Similarly, the fracture toughness ofartificial freshwater ice decreases asgrain size increases (Nixon andSchuls on 1986; Timco and Frederking1986) . Timco and Frederking als 0observed that fracture toughness seemsto decreas e as micro-crack dens ityincreases and the latter is alsorelated to increasing grain size (Cole1986). The random variation in grainsize of ice island ice makes itinhomogeneous with respect tocompressive strength.What ice strength values might beexpected for Hobson's Ice Island?Previous studies of freshwater icestrength have included glacier, riverand lake ice. Of thes e, it is likelythat granular, polycrys talline glacierice is most representative of the iceisland, rather than columnar river andlake ice. However, we emphasize thefact that the ice is land is notcomposed of glacier ice. In a study ofthe uniaxial compressive strength offresh ice core from Greenland (Shojiand Langway 1985), maximum stressvalues of 2.13 to 0.58 MPa wereobtaineci for strain rates between 10- 6-8 -1and 10 s and ice temperatures of-14.7 to -17.3 °C. Similar stressvalues were obtained in confined andunconfined uniaxial compression testsof fine-grained, polycrystalline ice at-11 °c at equally low strain rates(Jones 1982).Micro-cracks are common in iceisland ice and are probably the causeof occas ional ice core dis integrationthat we have experienced. Macro-cracksare als 0 common at the surf ace of iceislands and ice shelves and areprobably related to thermal strainsassociated with the rapid cooling ofthe upper surface of ice. Since thedepth of crack penetration is limitedby the attenuation of temperature waveswith depth (Mellor 1983), it isexpected that macro-cracks will beconfined to the near-surface ice of theice is land. However, there is evidencethat macro-cracks occur deeper than10 m. During hot-water-drilling ofboreholes, occasional and unexpecteddrops in water level occurred and onone occasion a massive loss of wateroccurred at about 20 m (A. Overton,personal communication to MOJ 1987).5. Summary and ConclusionAt the pres ent time the known iceisland population is between 32 and34. The dimens ions of 16 of thes e iceis lands are known and for 8 of thes ereliable es timates of volume and masshave been made. The data show that theice is lands are large ice bergs by anystandards and collectively repres ent asubstantial hazard to offshorepetroleum production in the deeper icecovered waters of the southern BeaufortSea.Although these ice islands, whichare presently located off the northerncoasts of Ellesmere and Axel HeibergIslands, are massive, they arerelatively small compared to the iceislands that were first sighted 25-40years ago and which have nowdisintegrated or drifted out of theArctic Ocean. The relatively smallsize of the new ice islands that calvedin 1983 is part of a pattern of a80

general decrease in ice island sizesince 1946. This trend is accountedfor by a gradual decrease in the totalarea of ice shelves and theavailability of fewer calving sites.However, a sudden disintegration of anentire ice shelf remains apossibility. For example Jeffries (inpress) notes, 1) the precariousposition of Ayles Ice Shelf and, 2) thepossible disintegration of AlfredErnest Ice Shelf, which might becontingent upon the rapid advance of aglacier.For reasons noted in the previousparagraph, the size of new ice is landshas gradually decreas ed over the pas t40 years. The size and volume of oldice islands decreases also, because ofdisintegration and thinning. New orold, though their size decreases, theytend to retain the same shape; themajority of ice islands have a lengthwidthratio of between 1 and 3. Thisis similar to the length-ratios ofAntarctic tabular icebergs and, thoughthis is speculative, it suggestssimilar fracturing rules for bothcalving and post-calving fracture.Of the many ice islands in knownlocations, the largest is Hobson's IceIsland, with an area of about 26 km 2and a mass of a little over 700 x 10 6tonnes. Three deep ice cores have beendrilled in this ice island and thephysical properties are probablyrepresentative of at least eight knownice islands. Hobson's Ice Islandapparently is composed entirely ofgranular, fres hwater ice that owes itsorigin to bottom accretion andcongealing of frazil ice when the iceis land was part of eas t Ward Hunt IceShelf.The variations of the physicalproperties strongly influence themechanical properties, but mechanicalproperties tes ts have yet to beundertaken. The ice is essentiallyhomogeneous with respect to electricalconductivity, yet there are considerablevariations in ice temperature,density and grain size. Temperature islikely to be the most importantdeterminant of ice strength, and sincetemperatures rise towards the bottom ofthe ice island, the deepest ice isweaker than that above. Thecharacteristic surface topography ofparallel ridges and troughs leads toice temperature inhomogeneities and,therefore, ice strength differencesbetween ice below the ridges and thetroughs. The linear nature of thetopography thus causes a linearconcentration of s train which influencespatterns of ice disintegration.Finally, there are clearly manyinternal weaknesses in the ice islandsand it is likely that their influenceextends far beyond the scale of laboratorymechanical properties testing.AcknowledgementsThis work was supported by theU. S. Dept. of Energy, MorgantownEnergy Technology Centre (Contract No.AC21-83MC20037). Field logisticsupport was provided by the PolarContinental Shelf Project (Canada).Harold Serson, Olli-Pekka Nordlund andJim Poplin assisted with the ice coredrilling and Craig Feyk completed manyof the SEC measurements.REFERENCESBruneau, A. A., Dempster, R. T. andPeters, G. R. 1978, Iceberg towing foroil rig avoidance. In, IcebergUtilization, Proceedings of the Firs tInternational Conference, Ames, Iowa,October 2-6, 1977, 379-388.Cole, D. M., 1986, Effect of grain sizeon the internal fracturing ofpo1ycrys talline ice. CRREL Report 86-5.Crary, A. P., 1958, Arctic ice islandand ice shelf studies, Part I. Arctic11:2-42.Dunbar, M. and Wittman, W., 1963, Somefeatures of ice movement in the ArcticOcean. Proceedings of the Arctic Basin~ymp~, October 1962, ArcticInstitute of North America, Washington,D.C., 90-103.81

Fu, L. and Holt, B. B., 1982, Seasatviews oceans and sea ice withsynthetic-aperture radar. JetPropulsion Laboratory Publication 81-20.Hatters ley-Smith, G., 1963, The WardHunt Ice Shelf: recent changes of theice front. Journal of Glaciology4:415-424.Holdsworth, G. and Traetteberg, A.,1974, The deformation of an Arctic iceis land. Proceedings of the 2ndInternational Conference on Port andOcean Engineering under ·ArcticConditions (POAC), University ofIceland, 419-440.Jeffries, M. 0., in press, Structureand growth of Arctic ice shelves andice islands. National Research Counctlof Canada. Technical Memorandum.Jeffries, M. O. and Serson, H., 1983,Recent changes at the front of WardHunt Ice Shelf, Ellesmere Is land,NWT. Arcttc 36:289-290.Jeffries, M. O. and Serson, H. V.,1986, Survey and mapping of recent iceshelf changes and landfast sea icegrowth along the north coas t ofEllesmere Island, NWT, Canada. Annalsof Glaciology 8: 96-99.Jeffries, M. 0., Sackinger, W. M. andSerson, H. V., 1987a, Remote sensing ofsea ice growth and melt-pool evolution,Milne Ice Shelf, Ellesmere Island,Canada. Annals of Glaciology 9:Jeffries, M. 0., Sackinger, W. M. andShoemaker, H., 1987b, Some physicalproperties of multiyear landfast seaice, northern Ellesmere Island, Canada,POAC-87.Jeffries, M. 0., Sackinger, W. M.,Krouse, H. R. and Serson H. V., inpress, Water circulation and iceaccretion below Ward Hunt Ice Shelfdeduced from salinity and isotopeanalys is of ice cores. Annals ofGlaciology 10.Jones, S. J., 1982, The confinedcompressive strength of polycrystallineice, Journal of Glaciology 28:171-178.Koenig, L.Dunbar, M.1952, Arctic103.S., Greenaway, K. R.,and Hatters ley-Smith , G.,lce islands. Arctic 5:67-Kovacs, A., 1976, Grounded ice in thefast ice zone along the Beaufort Seacoast of Alaska. CRREL Report 76-32.Kovacs, A., 1977, Iceberg thicknessprofiling. Proceedings of the 4thInternational Conference on Port andOcean Engineering under Arcticcondi tions (POAC), Memorial Uni vers i ty,St Johns, Newfoundland, Vol. 2, 766-774.Kovacs, A., 1985, An ice islandfragment in Stefansson Sound, Alaska.Proceedings of the 8th Internationconference on Port and OceanEngineering under Arctic Conditions(POAC), Narssarssuaq, Greenland,September 7-14, 1985, Vol. 1, 101-115.Kovacs, A. and Mellor, M., 1971, Seaice pressure ridges and ice islands.Creare Inc., Hanover, New Hampshire,Technical Report 122.Lyons, J. B. and Ragle, R. H., 1962,Thermal history and growth of the WardHunt Ice Shelf. (Union Geodesique etGeophysique International. AssociationInternational des Sciences Hydrologiques.Commission des Neiges etGlaces) Colloque d'Obergurgl, 10-18September, 1962, 88-97.Mellor, M.,of sea ice.1983, Mechanical behavior~RREL Monog!:!ph 83-1.Nixon, W. A. and Schulson, E. M., 1986,The fracture toughness of ice over arange of grain sizes. Proceedings ofthe 5th International OffshoreMechanics and Arctic Engineering (OMAE)Symposium, Tokyo, Japan, April 13-18,1986, 349-353.Robe, R. Q., 1980, Iceberg drift anddeterioration. Chapter 4 in, J2ynamicsof Snow and Ice Mass es, S. C. Colbeck(ed.), Academic Press, 211-259.Sackinger, W. M., in press, Loss, decayand recent drift of ice islands.National Research Council of Canada,Technical Memorandum.82

Sackinger, W. M. and M. O. Jeffries,1986, Arctic ice island and sea icemovements and mechanical properties.12th Quarterly Report (1 July 30September, 1986) Contract No. AC2l-83MC20037. U.S. Department of Energy,Morgantown Energy Technology Centre,West Virginia.Shoji, H. and Langway, C. C., Jr.,1985, Mechanical properties of freshice core from Dye 3, Greenland. In,Greenland Ice Core: Geop.Dysics,Geochemistry and the Environment (C. C.Langway, H. Oes chger and W. Dans gaard,eds.) AGU Geophysical Monograph 33, 39-48.Spedding, L. G., 1977, Ice islandcount, southern Beaufort Sea, 1976.Report IPRT-13M3-77 of APOA (ArcticPetroleum Operators Association)Project 99-3, 50 pp.Timco, G. W. and Frederking, R. M. w.,1986, The effects of anisotropy andmicro-cracks on the fracture toughnessof ice. Proceedings of the 5thInternational Offshore Mechanics andArctic Engineering (OMAE) Symposium,Tokyo, Japan, April 13-18, 1986, 341-348.M. JEFFRIES: I would go so far as to saythat it is possible, but add that thereare few observations and/or data availableto answer this question withoutlapsing into speculation. However. I canoffer one piece of evidence. We havephotographs of ice island ARLIS-II in1961 in open water in the southernBeaufort Sea north of Point Barrow. withan icebreaker alongside unloading equipmentand supplies for the establishmentof the ice island research station. Thephotographs also show clearly a number ofpieces of ice that have calved or are inthe process of calving from the edge ofthe ice island. Of course we don't knowhow typical this is and it must beremembered that ARLIS-II was occupied for4 years as it drifted across the ArcticOcean in the Trans-polar Drift. Thus,the calvings recorded in the 1961 photographswere not a sign of the imminentdisintegration of the ice island.Treshnikov, A. F., Nikiforov, Yeo G.and Blinov, N. I., 1977, Results ofoceanological investigations by theNorth Pole drifting stations. PolarGeogr!p.Dy, January-March 1977.Translated from Voprosy Geografii1001:49-69.Weeks, W. F. and Assur,Fracture of lake and seaCRREL Research Rep~ 269.A.,ice.1967,USAWeeks, W. F. and Mellor, M., 1978, Someelements of iceberg technology. In,Iceberg Utilization, Proceedings of theFirst International Conference, Ames,Iowa, October 2-6, 1977, 45-98.DiscussionD. DICKENS: I am interested in thepossible ways in which an ice Islandmight naturally break up in deeper water,without grounding. Could you comment onwhether or not you consider such an eventpossible or even likely.83

A NEW LOOK AT SEA ICE THICKNESSRoger ColonyUniversity of Washington, Seattle, Washington, USAAbstractMeasurements of sea ice draft were taken inthe Beaufort Sea by the SSN Gurnard during thespring of 1976. The sampling interval was 1.5 mand the track length was about 1350 km, producingalmost one million measurements of ice draft.In this paper the two-point joint probability densityfunction, f (hI' h 2 ; 3), is introduced. Thisfunction expresses the probability of encounteringice of thickness hI at location Xl and ice of thicknessh2 at location x2 where Xl and x2 areseparated by a distance of 3. Data are shown forseveral separation distances. The submarine draftdata have been previously studied-the ice thicknessdistribution describing the relative abundanceof ice of different thicknesses in a given area, thebottomside geometry in terms of second momentstatistics (spectral analysis, autocovariance, andvariance of increments), and feature analysis (keelcounting and abundance of level ice). The twopointjoint probability density function has potentialfor unifying these previous studies.1. IntroductionIce in the polar oceans ranges from a fewcentimeters to tens of meters thick. The temporaland spatial variability of sea ice thickness dependson the history of mechanical and thermodynami-This is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987. © The Geophysical Institute,University of Alaska, 1987.cal processes involved. Mechanical processescreate ridges and rubble fields, producing thick iceand giving sea ice its characteristic roughness.Associated with this mechanical deformation areleads and polynyas, regions where the ice partsand exposes the ocean directly to the atmosphere.In the winter, the exposed water freezes rapidly,creating smooth thin ice. Once the ice hasattained a thickness of 2 to 4 m, it tends to changerather slowly. It is strong enough to resistmechanical deformation, and the thermodynamicprocesses are rather slow, often altering the thicknessless than 10 cm per month. About 60% ofthe polar ocean is covered by ice between 2 and4 m thick. The principal mechanism responsiblefor forming ice of this thickness is not well understood.Thickness (or some parameterization of it) isfundamental to all descriptions and models of seaice. The topography of the underside of the ice isalso central to several physical processes: largescalemechanical processes, momentum exchangebetween the ice and the ocean, scattering of acousticenergy, capacity of the ice to entrap and transportpollutants, and thermal energy budget.On a geophysical scale, it is generally concededthat ice thickness must be described in statisticalterms. In this paper I describe the onedimensionalspatial structure of ice thickness interms of an underlying homogeneous random process.Using data taken by the SSN Gurnard, Iestimate the two-point joint probability densityfunction (pdf) for ice thicknesses separated by adistance 3. This statistic expresses the probabilityof encountering ice of thickness h I at location x I85

and ice of thickness h 2 at location x 2, where x 1and x 2 are separated by a distance o. The twopointjoint pdf is not a complete statistical descriptionof the spatial structure of sea ice; this wouldrequire a many-point joint pdf (see Section 2).However, many properties can be studied in termsof the two-point joint pdf, and its study may providedirection for future analysis. A joint pdf isalso necessary for the rational design of experimentsto measure ice thickness, for example, todistinguish sampling errors (due to finite records)from true changes in the process (due to nonstationarityor inhomogeneity).2. NomenclatureThe random variable H(x) is the ice thicknessat location x. In this paper, all statistics areassumed to be homogeneous, that is, independentof a particular location. This allows the spatialchange of thickness, S(o) = H(x + 0) - H(x), to bedefined in tenns of the separation distance o. Thetwo-point joint pdf for thickness at x + 0 andthickness at x isI (h l' h 2; o)dh Idh2== Prob {h 1 ~ H(x + 0) < h 1 + dh 1andh2~H(x)

two-point joint pdf, but for several separation distances.Only in special cases does the two-pointjoint pdf completely define the statistics of a process.3. Data BaseIn April 1976 the SSN Gurnard cruisedbeneath the pack ice of the Beaufort Sea for1350 km (Figure 1). Using an upward-lookingsonar, the Gurnard measured ice draft every 1.5 m(and often more frequently) along the entire track.IIIIII--t--------IIIIpaper. See Rothrock (1987) for an excellent discussionof the physical meaning and measurementof both ice thickness and ice draft.The ice thickness categories have beenchosen to balance three requirements: (1) torepresent geophysically important aspects of thethickness distribution, (2) to minimize the effectof sampling errors, and (3) to mask the effect ofmeasurement errors. The thickness category intervals(or bin sizes) are 0.20 m in the thicknessrange 0-2 m, 0.40 m in the range 2-6 m, and0.80 m in the range 6-20 m. A single thicknesscategory covers all ice greater than 20 m thick,giving a total of 39 thickness categories. Figure 3shows the marginal thickness distribution; the binsize can be clearly seen. In this figure (and othersimilar figures) the ordinate is density,'Yi I (hi - hi-I), with units of meters-I. Note thearea of each rectangle is proportional to 'Yi. For0.5Figure 1. Track of the SSN Gurnard, 7-10 April1976, in the Beaufort Sea.Approximately one million measurements of icedraft were taken at the separation distance of1.5 m. The sonar beamwidth was about 3°. At anominal sonar depth of 100 m, the ensonified areais about 5 m in diameter, and thus there is somelocal smoothing of the data. The ice drift datahave been studied by Wadhams and Horne (1980)and Rothrock and Thorndike (1980). Wadhamsand Horne estimate the error due to "white noise"is about 0.1 m of draft and the error at largerwavelengths about 0.3 m. A typical ice-draftprofile is shown in Figure 2. Although ice draftand ice thickness are not equivalent, or even proportional,they will be used interchangeably in this'7.s0.4z.·iii 0.3cOJ0Vl0.2VlOJC~u:cf- 0.10o a 5 10 15Thickness (m)Figure 3. The marginal ice thickness distribution,g (h). The mean thickness is 3.8 m.a 200 400Distance (m)600 800 1000Figure 2. Ice draft profile taken from the SSN Gurnard. The data are representative ofthe region, which is thought to be homogeneous.87

further analysis of the marginal thickness distributionobtained by submarine sonar, see Williams etal. (1975) and Wadhams (1981).The data set imposes some limitations. Thetrack is essentially one dimensional; there is noinformation about the two-dimensional structureof pack ice. Because of the sampling frequencyand sonar beamwidth, the small-scale structure ofthe ice cannot be resolved; this smaller lengthscale may be especially important to acoustic studiesof frequencies greater than 1 kHz. The dataare also regional and taken over a short time interval.Additional unclassified submarine data sets doexist for the Beaufort Sea and other portions of theArctic Ocean. Researchers are also beginning tomoor upward-looking sonars from the sea floor.These data may reveal something of the temporalchanges in the underlying (statistical and physical)processes.There presently exist no statistics on temporalchanges in ice thickness. Therefore theseparation of sampling errors due to temporalchanges must be based on our knowledge of thesampling errors associated with a stationary,homogeneous process.4. Observed Two-Point Joint pdfWe can estimate !u (0) and Pij(O) from thedata. We define nij(o) as the number of times icein thickness category i follows, by a distance 0,ice in thickness category j. Further, let nj (0) bethe total number of occurrences of ice in tliicknesscategory j and N (0) be the total number of observationshaving separation distance O. The sampleestimates are" nij(o)!ij(O) = N(o)n ..(o)""(0)=_"_p" nj(o)and" nj (0)'Yj = N (0)pdfFigure 4 shows the estimated two-point joint10864I'" +2xc0'iii 00 0 2 4 608-' 101iiCJ) c) Ii= 24 mCJ)OJc 8"" 0:cfo-61042o,~~~~~~.-~--~ o+-~~-~~~~-~o 2 4 6 8 10 0 2 4 6 8 10Thickness at Location x (m)Figure 4. The two-point joint pdf for ice thickness at locations x and x + 0 for (a) 0 =1.5 m, (b) 0 = 6 m, (c) 0 = 24 m, and (d) the asymptotic limit88

for B = 1.5, 6, and 24 m. Figure 4d shows theOne interpretation of Figures 4 and 5 is usefulfor sampling theory. The conditional pdf is theasymptotic two-point joint pdf. The maximaalong the diagonal line at h j = h j show the persistenceof the thickness category.is chosen. We see that the "thick" part of the mar­distribution from which the ice thickness at x + Bginal distribution appears to be sampled independentlyon a much shorter scale than the "thin" por­Figure 5 shows the estimated conditional pdffor the same spatial lags.tion of the marginal distribution.n .. (B)A(h Ih . B) __;.

5. Conditional SlopesThe conditional change of thickness,q (s I hi; 0), is no more than a change of real variablefrom the conditional thickness, p (h I hi; 0).In one sense it addresses the more pertinent questionof how the adjacent ice is changing in thickness.It also has applications to the scattering ofacoustic energy, provided geometrical optics isappropriate. The mean conditional change in icethickness as a function of thickness and separationis defined as+~Similarly, the variance of the conditional changeof thickness is

a familiar distribution or share some common distribution.The Kolmogorov-Smimov statisticallows one to test, subject to a confidence limit, ifa random sample is drawn from a hypothesizedcontinuous distribution. This procedure comparesthe two cumulative distributions. The cumulativenormalized distribution for the conditional changein thickness iss'Q (s' I hj ; 0) = f q (s ' I hj ; o)ds' ,where" s -Jl(hj ; 0)s =s (hj;o)= cr(hj;o)Figure 8 plots this cumulative distribution in comparisonwith the cumulative normalized Gaussiandistribution. The sample distribution is moresharply peaked (or, equivalently, has broader tails)than the associated Gaussian distribution. Notethe' increased tendency for slopes greater thancr(h ;0) as 0 increases. The distributions of slopesconditional on thicknesses greater than 3 m (notshown) are all quite similar to those shown in Figure8. However, the pdf for slopes conditional onthicknesses less than 2 m (also not shown) are differentfrom those conditional on the largerthicknesses.Most of the preceding results are based onthe two-point joint pdf. It may be natural to thinkthat there is a persistence of slope in ice draftprofiles. For example, the slope distribution maydepend on both H(x) and the previous slopeS(x - 0). In Figure 9 we consider three separateslope distributions each conditioned onH(x) E [4.0,4.4) and for a separation of 0 = 1.5 m.The three populations are further defined in termsof their previous slopes: (a) S(x - 0) < -cr, (b)-cr ~ S(x - 0) ~ +cr, and (c) +cr < S(x - 0). Conditionedon a 4 m thickness and at 0 = 1.5 m, (J =0.47 m. The three distributions are normalizedusing the overall mean and variance. It is seenthat a large previous change in thickness (eitherpositive or negative) signals a larger than usualnext change in thickness. There is no clear indication,however, that the sign of the slope persists.Again, these results are typical when conditionedon thickness greater than 3 m.~:g.0e~ 0.5.

6. Future ModelingOne may adopt the view that every singlethickness along a track (e.g., Figure 2) is the resultof a deterministic physical process; furthermore,this physical process, and its specific outcome, iswhat is important about ice thickness. An alternativeview is that the thickness along a track is therealization of a single random process acting atevery point, and this process, not specific outcomes,characterizes the important features of icethickness. There is some middle ground; forexample, Thorndike et al. (1975) describe the evolutionof g(h) in terms of physical processes.There are presently no models, physical or stochastic,for the spatial structure of ice thickness.In this section I offer some suggestions on statisticalmodeling.A first step in modeling the joint pdf is todetermine if the data are consistent with a firstorderMarkov process. Formally, the first-orderMarkov hypothesis satisfiesProb {H(x +o)e ci givenH(x)e Cj} =Prob {H(x +o)e Cj given [H(x)e Cjand H(x - 0) e Ck and ... ]} .This means the thickness distribution at x + 0depends only on the thickness at x , and not on thethicknesses at both x and other more distant locations.If the Markov property is valid, then we candefine the many-point joint statistics in terms ofPij (0·), where o· is a fixed small separation. Thisimplies that the large-scale spatial structure iswholly dependent on the local state. To show this,we define the two matrices P v = {pj.(vo·n andF v = {fij(V o*)} for v = 1,2,3, .... If the processis first-order Markov, then it can be shown thatF v = r P 1', where r = {Yi} is a diagonal matrix.The test of the Markov hy,rothesis is then whetherthe observations njj (v 0') I [N (v 0·) t.hj t.h)]agree with the theory rpl'- Something like a chisquaregoodness-of-fit test would determine if theobservations were likely to have come from themodeled distribution.Kozo and Tucker (1974) estimate the spectrumof ice draft in the Denmark Strait and compareit to a "red noise" process. The associatedMarkov process is a simple autoregressive model.They conclude the data are more consistent with a"red noise" model than a "white noise" model.random process. This random process is then partiallydefined in terms of the two-point joint pdffor ice thickness at two locations separated by adistance O. Estimates of the two-point joint pdfare obtained from the set of measurements.Previous statistical descriptions of ice thicknesshave analyzed (1) the distribution of the relativeabundance of ice of different thicknesses in agiven area, (2) the roughness of the bottom side ofthe ice in terms of the variance in draft (spectralanalysis or autocovariance), and (3) featureanalysis (e.g., counting keels or the abundance oflevel ice). These three approaches are essentiallyseparate. The thickness distribution, for example,provides no information about roughness, and theroughness spectrum (because the process is non­Gaussian) cannot be used for feature analysis.The many-point joint pdf is a complete descriptionof the statistical process, and these threeapproaches can be unified in terms of the joint pdf.From a superficial examination of Figure 2,one can see the motive for describing ice draft interms' of thin level ice, thick rough keels, and multiyearice with an "equilibrium" thickness of 2-4 m. But investigators must be wary lest what theyfind is unduly influenced by what they look for.Rothrock and Thorndike (1980) model ice draft interms of a class of functions characterized byvarying degrees of roughness. Using the Gurnarddata set, they conclude that roughness persists intothe smallest scales of observation (several metersin the horizontal and a few tens of centimeters inthe vertical). It may be tempting to paraphrasetheir result and say there is no level ice. Wadhams(1981) defines level ice as a change of lessthan 25 cm in draft over a separation of 10 m.Based on this definition, about 50% of the icepack is smooth and undeformed. These two conclusionsmay not be incompatible; they may justbe different ways to characterize roughness.Because the joint pdf is such a fundamental statistic,it should be regarded as a leading candidate inquantifying ice draft roughness.AcknowledgmentsThis research was supported by the Office ofNaval Technology with technical managementprovided by the Naval Ocean Research andDevelopment Activity (NORDA).7. Applications and ConclusionsIce thickness is a fundamental state variablein the description and modeling of sea ice. In thisstudy, we chose to regard a set of thickness measurementsas the realization of a single underlying92

ReferencesHibler, W. D. and LeSchack, L. A. 1972. Powerspectrum analysis of undersea and subsurfacesea-ice profiles. 1. Glaciol., 11, 345-356.Kozo, T. L. and Tucker, W. B. 1974. Sea icebottomside features in the Denmark Strait. LGeophys. Res., 79, 4505-4511.Rothrock, D.A. 1987. Ice thickness distribution- measurement and theory. In: "The Geophysicsof Sea Ice" (N. Untersteiner~ed.), Plenum Publ.Corp., Chap. 8.Rothrock, D.A. and Thorndike, A.S. 1980.Geometric properties of the underside of sea ice. LGeophys. Res., 85, 3955-3963.Thorndike, A.S., Rothrock, D.A., Maykut, G.A.and Colony, R. 1975. The thickness distributionof sea ice. 1. Geophys. Res., 80, 4501-4513.Wadhams, P. 1981. Sea-ice topography of theArctic Ocean in the region 700W to 25°E. Phil.Trans. Roy. Soc., London, Srs. A., 302, 45-85.Wadhams, P. and Home, R. J. 1980. An analysisof ice profiles obtained by submarine sonar in theBeaufort Sea. J. Glaciol., 25, 401-424.Williams, E., Swithinbank, C. W. M. and Robin,G. de Q. 1975. A submarine sonar study ofArctic pack ice. J. Glaciol., 15,349-362.93

THE USE OF POL YSULPHIDE RUBBER MOULDS TO MEASURE ICE ROUGHNESSR. H. GoodmanA. G. HoloboffEsso Resources Canada Limited, Calgary, Alberta, CANADAT. W. DaleyL. D. MurdockParks Canada, Ottawa, Ontario, CANADAM. FingasEnvironment C(Jnada, Ottawa, Ontario, CANADAAbstractThe roughness of the upper surfaceof Arctic sea ice has been measured in anumber of studies, but there are fewmeasurements of the ice roughness at theice-water interface. Some measurementshave been made using impulse radar andboring, but these are at a size scale ofseveral metres. It is well known fromthe observations of divers that the iceis rough at a scale of a few centimetreswith an amplitude of a few m1llimetres.This small scale roughness has beenproposed as having a major role in thestorage of oil under the ice and is,therefore, or concern to those involvedin oil spill counter-measures. This wasthe reason for undertaking thesemeasurements. Roughness at this scaleis probably a significant contributor tothe drag of an ice floe, and hence, tothe forces involved in the dynamics ofpack ice. This paper '01111 describe thetechniques used and the method of analysisof the resulting data. Contours ofthe small scale features of the ice willbe presented. This data w11l be interpretedin terms of oil storage volumesThis is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987. © The Geophysical Institute,University of Alaska, 1987.and ice-retarding forces involved infloe motion.1. IntroductionThere have been a number of studieson the surface roughness of Arctic seaice using a variety of techniques.Several of these were initiated duringproject AIDJEX (Arctic Ice DynamicsJoint Experiment) for the purpose ofdetermining the forces on sea ice. Thesurface roughness is a major componentin the determination of the coupling ofwind energy to the ice surface.One of the maj or drag forces on anice floe is due to the interaction ofthe ice with the water. The magnitudeof this force is controlled by the icefloe velocity and the under-ice roughness.A number of experiments have beenconducted to measure under-ice roughness.Various techniques have beenused, including the drilling of holes(Comfort, 1986), impulse radar (Kovacs,1981), acoustic sounding (Aurlande,1987) and under-ice photography usingdivers (Welch et al., 1973). The valuesobtained vary greatly and depend onlocation and time of year (i.e., thestage of the ice growth).95

2. 'Theory and ModelsIce roughness can be characterizedby the root mean square deviation of theice surface. In order to specify theroughness, it is necessary to state thelinear scale of the features (Ling andUntersteiner, 1974). Most measurementshave a spatial resolution of a fewmetres and the roughness scale ismeasured in terms of these lengths.If the ice is smooth at these sizescales, there is still a roughness ofamplitude in the order of a few millimetres,with length scales of severalcentimetres. Such features can providea significant retarding force to themotion of an ice floe. This will becalled the meso-scale roughness.The meso-scale roughness isimportant to the storage volume in theevent of an incident involving therelease of oil under the ice. Observationsof divers (Bright, 1972) confirmthat there are significant features thatoccur at this size scale, but no suitablemeasuring technique has beendeveloped.3. Selection of Experi.ental TechniquesThe method which was selected forthese measurements was to use a mouldingmaterial which could be placed under theice, allowed to set and then removed foranalysis in the laboratory. The requirementsfor the moulding material are:a) the material must set in sal twater at a temperature of lessthan -2°C;b) the material should be nontoxicto marine organisms andhumans;c) the material should be storableat temperatures of -40°C orlower;d) the material should set in lessthan 24 hours to a form with ahigh degree of mechanicalstrength;e) the setting reaction should notbe exothermic, as this woulddestroy the small scale icefeatures; and,f) the material should be easy andsafe to handle and transport.Several latex-based and epoxy-basedcompounds were tried in the laboratory,but were found to be unsatisfactory.Most of these compounds would not set upto a firm material under low temperatureconditions.During the exploration of the 16thcentury Basque whaling vessels in RedBay, Labrador, Hurdock and Da 1 ~y (l qR 1)developed an underwater mouldingtechnique for marine artifacts. Therequirements for this purpose were verysimilar to those of the ice mouldingproblem. The material used was SMOOTH­ON (*).Initial experiments conducted inthe laboratory showed that the materialmet most of the desired criteria.Further tests were conducted in a quarrynear Ottawa, Ontario to verify that thistechnique was valid in the field. Allthe above criteria were met with theexception of (c). The manufacturer hadno information on the low temperaturestorage characteristics of the material;hence, it was decided to control thetransport so that the product would notbe subjected to extremely lowtemperatures.4. Experimental Progra.The site chosen (Figure 1) for theexperimental program was near the Essoartificial island ARNAK, located in theCanadian Beaufort Sea (lat 69°45' N long134°50' W). Since this project wasdesigned to demonstrate a new technique,it was considered more important to havegood logistic support than to select aparticularly unique or critical ocean orsea ice situation. The sea ice in thearea was landfast and had experiencedlittle movement during the winter. Theexperiments were conducted just beforebreak-up, so that a vehicle could beused for access. At this time of year,the ice was at its maximum thickness andwas stable.* From Smooth-On Inc.,1000 Valley Road,Gillette, N.J.USA96

-eq.sv~~--+-----+-----t-----r"-25kmFigure 1. Location Map of the Study Site in the Canadian Beaufort SeaThere was no evidence of extensiveablation and the ice was not growingrapidly at this time of yea r. The icewas about 2 metres thick. Three sites,each about 2 km apart, were chosen forthe experiments, where the water depthvaried from 3 to 6 met res. There was asmall surface current in the area,moving to the north-east.The holes were cut in the ice usinga tractor-mounted trenching machine(Ditch-Witch) and the ice block wasremoved using a front-end loader. Thedive-holes were about 1 metre square.Four moulds were used at each hole. Thedesign of the mould assembly is shown inFigure 2. The base of the mould is a1 cm thick neoprene mat (1.5 m x 0.6 m)surrounded by a neoprene gasket. Thethickness of this gasket is determinedby the size of the features to bemoulded. The gasket was built in layersand the number of layers could be variedto increase or decrease the mouldingdepth.A preliminary survey was conductedby the divers to determine a suitablevalue of moulding thickness for theexperimental location. A value of 3 cmwas mounted on a 12 cm depth air chambermade of galvanized sheet metal. ThisBUNGair chamber wasone end.Figure 2. Mould Holder~ ",2jl~ M OlOTRAYfitted with a bung atThe rubber was mixed on-site, usinga rubber to catalyst ratio of 20:1, witha hand-held electric drill and mixingattachment. During this operation, aface mask with respirator was worn sincethe catalyst material was somewhat97

to-xic. The resulting moulding materialwas spread uniformly on the neoprenebase to the preselected 3 cm depth.The moulds were lowered into thedive holes and placed under the ice bytwo experienced divers. Air from thedivers' tanks was used to fill thebuoyancy chambers under the neoprene andthus force the moulding material againstthe ice. Additional buoyancy wassupplied by ~2 litre plastic pails whichwere placed in the chambers and filledwith air. This ensured an adequatepressure on the moulding material, sothat it would conform to the under icesurface.Four moulds were placed at eachsite as shown in Figure 1. The diverschose typical areas which were selectedduring the reconnaissance dive. After12 to 16 hours, the moulds wererecovered by the divers. The bung wasremoved and the mould lost its buoyancy.The mould was separated from theice and returned to the surface. Theneoprene was separated from the buoyancychamber and stapled for support duringshipping. The field component of theprogram proceeded without anycomplications.The initial observations in thefield revealed a fine microstructure inthe ice as shown in Figure 3. Superimposedupon this was a skeletalstructure as described by Weeks et al(1972) •ICE FEATURES5. Analysis of MouldsPositive moulds were obtained fromthe polysulphide moulds using a resinand fiberglass moulding material. Allthe analysis of the ice roughnesscharacteristics was undertaken usingthese positive moulds.5.1 Stereo photography (Photogrammetric)Stereo photography was used toprepare a contour map of each of themoulds. The methodology used was basedon that used for terrestrial mapping.The vertical resolution achieved wasbetter than 0.1 cm and the contourinterval was selected as 0.1 cm. Atypical contour map is shown inFigure 4. The rms (root-mean square)roughness, average roughness and variancewere calculated from the stereoimages using standard computer analysistechniques. Table 1 shows the averageroughness in cubic meters/square meterfor the twelve moulds.5.2 Progressive Flooding (Volumetric)While stereo photography is auseful technique for determining theaverage surface roughness, it does notaddress the interconnectivity of thechannels and thus the spreading of oilon the under-ice surface. In order tostudy this characteristic, a series ofprogressive flooding experiments wereconducted. The experimental programused the positive moulds. These werepainted white and the flooding fluid wasblack so as to present a high cont ras timage. The moulds were carefullylevelled and then flooded in 250 mlincrements (Figure 5).After each addition of fluid, theimage was captured as a digital imagefor subsequent computer imageanalysis. The digital images wereanalyzed using a Robot image analyzerconnected to an IBM-XT. A plot of theflooded area as a function of volume isshown in Figure 6.Figure 3. Photograph of Ice Mould98

PROGRESSIVE FLOODINGSITE C #2IIi ._ 250 MLSSITE BRIG 32ESCALE 1:10Figure 4. Contour Hap of Ice SurfaceFigure 5. Progressive Flooding99

TABLE 1AVERAGE ICE ROUGHNESSLocation Photo Volumem 3 /m 2 m 3 /m 2SiteHoleSiteHoleSiteHoleA234B234C2340.0090.0130.0100.0110.0120.0130.0140.0110.0100.011O.OlB0.010O.OOB0.0140.0100.0120.0110.0140.0130.0100.0110.0100.0160.011PROGRESSIVE FLOODING10.-----------------------------------~98(j) 7UJgo 62UJ 5::!3 4o> 3SITE C HOLE 3EXPERIMENTAL ....2IUNIFORM ROUGHNESSo~~==~=.--.-~~~~~~=;~o 20 40 60 80 100AREA (PERCENT)Figure 6. Area as a Function of FluidVolumeFigure 6 shows that there is asignificant deviation from a linearapproximation. This indicates that theice does not have a uniform spec-trum ofroughness. There is a lack of smallscale features as shown by the initiallow slope and then a predomi-nance offeatures of a size scale of a millimetre.This is indicated by the rapidrise in area at high flood volumes.The values of roughness obtainedfrom these two techniques, are comparedin Table 1 and shown in Figures 7 andB. The estimated errors are 20% and thetwo methods are in essential agreement.Values of oil storage volume havebeen published by a number of workers(Kovacs, 19B1; Comfort, 19 B6; Wadhams,1976). This data and the results of ourexperiments are shown in Table 2.TABLE 2COMPARISON WITH OTHER DATALocation Storage RefVojumm 1m 2St- Lawrence 0 .007 J70ARLISS II 0 .0 125 J70AIDJEX 0.001 H74Cape Peary 0.0056 W76Thermal Theory 0.007 WH74Tigvariak Island 0.03 KB1West Dock Site 0.06 KB1Reindeer Island 0.01 KB1(Flat Ice)Site A 0.024 KB1Site B 0.0239 KB1Site C 0.0574 KB1Seal Island 0.130 CB60.023Thunder Bay 0.062 CB6Harbour 0 .022Mold Experiment 0.01 G87J70--Johannsen (1970)H74--Hunkins (1974)WH74--Wolfe and Hoult (1974)W74--Wadhams (1974KB1--Kovacs (19B1 )CB6--Comfort (19B6)GB7--This work100

OIL THICKNESSPHOTOG RAM METRIC0020 _------1--------.., 0020001600120.0080004--1---------1 00160012000800047. EFFECT ON ICE FLOE HOTIONUsing the values of ice roughnessdetermined from these experiments, it ispossible using an equation of Hunkins(1975) to calculate the retarding forcedue to the meso-scale roughness. Thevalue of Zo' the ice roughness, assumedby Hunkins was 0.002. Our value of 0.012would suggest that even for landfast icethat the drag is considerably largerthan previously calculated.8. CONCLUSIONSFigure 7. Ice Roughness by StereoPhotographsOIL THICKNESSVOLUMETRICThese initial experiments haveshown that it is technically and logisticallypossible to use a polysulphiderubber molding technique to produce ahigh quality replicate of the under-iceroughness. Initial observations in thefield showed that there was a significantstructure at a length scale ofseveral centimeters, with a roughness ofa few millimetres. The ice roughnessspectrum is not uniform and consists ofa few features of size scales of a fewmillimetres. This meso-scale roughnessis a significant factor in calculatingthe retarding force on the motion of anice floe, and contributes to the abilityof ice to s tore oil spilled under theice.0020 -------1--------...., 00200016--I--... -~--J 001600120012000800080.00400049. ACKNOWLEDGEMENTSThe authors would like to thankEsso Resources Canada Limited for itssupport of this proj ect. The logisticsupport of Esso base at Tuktoyaktuk wasan essential component of the fieldactivities. The efforts of Mr. RickPeden of Esso in undertaking the fieldwork is acknowledged. Funding wasprovided by Esso Resources Canada andthe Environmental Emergencies TechnologyDivision of Environment Canada. Theinitial development of the mouldingtechnique was undertaken by ParksCanada, who provided this technology tothe program.Figure 8. Ice Roughness by Volume101

10. REFERENCESAurlande, o. 1987. Ice Roughness UsingSide Looking acoustics. Poster SessionAMOP Technical Seminar, Edmonton, June1987.Bright, C.V. 1972. Diving in theArctic. Naval Research News 28 (8):1-12.Comfort, G. 1986. Under Ice RoughnessMeasurements, Arctec Canada. ESRF(Environmental Studies Revolving F~Study 2041 Part 2.Weeks, W.F., Assur, A. and Liebowitz, H.1972. Lake and Sea Ice Fracture--AnAdvanced Treatise V7 New York AcademiCPress.Welch,DunganStudy.M., Partch, E., Lee, H. andSmith, J. 1972 AIDJEX PilotAIDJEX Bulletin #18,:31-45.Wolfe, S. and Hoult, D.P. 1974. Effectsof Oil Under Ice. Journal of Glaciology17 (69):473-488.Daley, T.W. and Murdock, L.D. 1984.Underwater Moulding of a Cross Sectionof the San Juan Hull: Red Bay, Labrador,Canada. ICOM Committee for theConservation 7th Triennial meeting,Copenhagen, September 10-14.Hunkins, K. 1974. The Oceanic Boundarylayer and Ice-water Stress During AIDJEX1972. AIDJEX Bulletin #25:109-127.Hunkins, K. 1975. Geostrophic DragCoefficients for Resistance Between Iceand Ocean. AIDJEX Bulletin #28:61-68.Johannessen, O. H. 1970. Note on SomeVertical Profiles Below Ice Floes in theGulf of St. Lawrence and Near the NorthPole. J. Geophys. Res. 75:2857-2862.Kovacs, A. 1981. Pooling of Oil UnderIce. Proceedings of POAC, Quebec City.Ling, C.H. and Untersteiner, W. 1974.On the Calculation of the RoughnessParameter of Sea Ice. J. Geophys. Res.79:4112-4114.Murdock, L.D. and Daley, T.W. 1981.Polysulphide Rubber and its Applicationfor Recording Archaeological ShipFeatures in a Marine Environment.International Journal of NauticalArchaeology and Underwater Exploration,10 (4).Wadhams, P. 1976. Sea Ice Topography inthe Beaufort Sea and its Effect on OilContainment. AIDJEX Bulletin #33:1-52.102

THE ALASKA SAR FACILITYW. F. WeeksG. WellerJ. MillerUniversity of Alaska, Fairbanks, Alaska, USAF. D. CarseyJ. E. HillandCalifornia Institute of Technology, Pasadena, California, USAABSTRACTA short description is given of thegel'leral characteristics of theice/ocean and applications demonstrationsresearch programs that are anticipatedas part of the Alaskan SAR Facility(ASF) program. Also describedare the characteristics of the threesatellite SAR (Synthetic ApertureRadar) systems that will supply data tothe ASF and the design and analysis capabilitiesof the different componentsof the ground station.INTRODUCTIONThe Alaska SAR (Synthetic ApertureRadar) Facility (ASF) is an approvedNASA program that will, starting in1990, offer an unparalleled opportunityfor scientific observations of the iceand open ocean conditions of theBeaufort, Chukchi and Bering Seas andof the north Pacific Ocean. The purposeof this paper is to describe both thenature of this developing facility andthe varied scientific opportunitiesthat it will present to the polar oceancommunity.This is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987.The management of the ASF is theresponsibility of the GeophysicalInstitute of the University of Alaskawhile the ASF-associated science programis managed jointly by theGeophysical Institute and the JetPropulsion Laboratory of the CaliforniaInstitute of Technology. The stationwill be capable of receiving, processingand analyzing SAR data from threedifferent satellite systems. The firstof these is the European Space Agency(ESA) First European Remote SensingSatellite (E-ERS-l), which is currentlyscheduled for launch during April 1990.This will be followed in 1992 by theFirst Japanese Earth ResourcesSatellite (J-ERS-l) and in 1994 by theCanadian Radarsat. In that all of theabove programs have associated followonprograms, the E-ERS-l launch willherald a period of SAR observationsthat would overlap the initiation ofthe NASA Earth Observing System (EOS)program in the late 1990s, a programthat will also include SAR sensors.Summaries of the three flight missionsthat will downlink SAR data to the ASFare listed in Table 1.The initiation of the ASF programand the selection of the GeophysicalInstitute at the University ofAlaska/Fairbanks as the receiving siteare the result of several considerations.First, it is widely realizedwithin the science community that, ofall the possible uses of SAR, its applicationto studies of the polar103

Table 1ALASKA SAR FACILITYSCIENCE REQUIREMENTSMISSIONS DESCRIPTIONSSAROrbitMissionOtherInstruments• Frequency• PoIarization• Swath• Resolutionllooks• Incidence• Orientation• On-Board Storage• Inclination• Altitude• Repeat• N

THE RESEARCH PROGRAMThe scientific justification forthe ASF was initially outlined by aScience Working Group (SWG) establishedby NASA in 1982. The findings of thisgroup were published in a report entitled"Science Program for an ImagingRadar Receiving Station in Alaska"(Weller et al. 1983). This report discussesthe application of SAR data collectedover and around Alaska to a varietyof disciplines including glaciology,geology, geobotany, hydrology andvegetation cover studies. The groupconcluded that the most scientificallyuseful application of SAR would be tostudy the ice cover of the Arctic Oceanand its surrounding seas as well as toinvestigate the open-ocean conditionswithin the region of the station mask.Following the release of that report,an additional specialist group was convenedjointly by ESA, NASA and theCanadian Centre for Remote Sensing(CCRS) to consider the application ofE-ERS-1 data received at Kiruna, Swedenand at Ottawa, Ontario as well as atFairbanks, Alaska to the study of thepolar oceans. As might be expected, therecommendations of this latter grouptook a broader view of polar ocean scienceproblems outlining " A Programmefor International Polar Ocean Research"(PIPOR 1985). As ice and polar oceanographyare of prime interest to the POACcommunity, we will focus on the recommendationsof these documents here asthey form the basis for the proposedASF ice and oceans program.The polar regions themselves are ofprimary scientific interest because oftheir role in global climate; they arethe world's heat sinks. In both the atmosphereand the ocean, heat is advectedto the polar regions to be lostto space by radiation. In addition, ifcurrent climate models are correct, itis in the polar regions where thelargest temperature increases resultingfrom anthropogenically induced increasesin the greenhouse gases such asC02 is to be expected. This increase isdue primarily to feedback processes involvingsea ice.Sea iceA most unique aspect of the polarocean is of course its sea-ice coverwhich serves to insulate the "warm" waterfrom the frigid overlying airmasses. As the heat transfer in thewinter from open water into the atmo-sphere is roughly 2 orders of magnitudehigher than representative transferthrough multiyear ice, it is clear thatto understand and predict the behaviorof the atmosphere over an ice-coveredocean, one must understand the circulationand deformation of the ice in thatthis determines where and when leadsand polynyas will form.Ice circulationSAR data is ideally suited to serveas a basis for studies of ice drift anddeformation, since both aircraft andsatellite-based SAR observations haveclearly demonstrated that in the majorityof sea-ice images there are a largenumber of characteristic ice featuresthat can be tracked sequentially. Alsonewly formed features such as leads andridges can readily be discerned and observationsare not limited by clouds ordarkness.Theories of ice stress, expressedas constitutive laws, describe the resistanceof the ice to deformation. Thestress is believed to depend upon deformationor strain rates and on iceconditions, particularly on the smallfractions of the ice cover consistingof thin ice and open water (Coon 1980;Hibler 1980). The observational demandsfor testing these laws are severe inthat they require accurate estimates ofice species that occupy only smallfractional areas of the ice cover. TheASF SAR data will provide a major improvementin our ability to observeboth these thin-ice fractions and deformation.Because ice stress and processesaffecting mass and heat balance dependon spatial differences in ice motion,itis essential to have a clear conceptualmodel of the spatial structure of theice velocity field. In the past, bothmodels and data interpretations havebeen based on the continuum hypothesis,namely, that ice velocity is spatiallydifferentiable. However, observationsfrom both LANDSAT and SAR have shownthat at times the ice moves as rigidfloes showing lateral dimensions ofroughly 100 km (Nye 1975; Hall andRothrock 1981) with the relative motionbetween floes taking place in bandsonly kilometers wide. New kinematicmodels have been proposed that idealizethis discrete behavior as motion alongcracks which separate the ice coverinto a countable number of randompieces (Thorndike 1986). Such models105

tell us how to use the observed motionsof sets of points to estimate the meandeformation over large areas and whaterrors are to be expected in such estimates.Clearly the coupling of thesemodels with the availability of SARdata will result in the improved characterizationof ice deformation.Mass balanceA large-scale problem highlyamenable to study via the use of SARdata is the mass balance of the icepack. The data required include icedisplacement and deformation, ice extentand the concentration of one ortwo ice types, coupled with surface airtemperature and pressure as obtainedfrom data buoys. Although the arcticice cover expands by millions of squarekilometers from September to April, thenet annual export of ice through FramStrait is less than a million squarekilometers. How much of this productionoccurs over the arctic continentalshelves? How much of it arises withinthe slowly-diverging central pack?In the face of insufficient data tosupport diagnostic studies, scientistshave, to date, turned to models thatcontain concentration or mass balanceas a governing equation (Hibler 1979;Parkinson and Washington 1979). At presentthese analyses suggest that thecentral Arctic and the Siberian Shelfare roughly equal producers of ice andthat ice mass anomalies are determinedby ice transport (wind) anomalies(Walsh 1986). An observational verificationof these conclusions remains tobe made. Large-scale studies of thistype will clearly need to combine SARdata collected at both Fairbanks andKiruna.Another interesting study of thisgeneral type would be an investigationof the mass balance of multiyear ice.This would appear to be simple. Itsonly source is first-year ice that survivesthe summer melt season, a termthat can be estimated via the use ofSAR; its dominant sink is export and itis rarely believed to participate inridging or rafting.Shear zone processesNearshore regions where there areextensive interactions between the iceand the land are called shear zones.These areas are characteristicallysites of strong ice-ice interactionswith attendant deformation, ridging,rafting, floe breakup and open waterproduction (Stringer 1978). Ice productionin such regions in the winter isbelieved to be 3 to 5 times that of locationsin the central pack. The shearzone is also the region where groundedice features and ice gouging occur. SARdata will clearly provide mechanisticdetails of the behavior of ice in thisoperationally-important region in thatridging patterns, floe shapes and sizesand open water and thin ice areas willbe readily discernible.Arctic polynyasThere are a number of knownpolynyas (semi-permanent regions ofopen water and thin ice) within thewinter ice pack in the region coveredby the ASF station mask. The mostprominent of these are Westwater (SSWof Banks Island), the St. LawrenceIsland Polynya (south of the island)and the thin-ice areas that characteristicallyoccur along the Chukchi coastand south of the Seward Peninsula. Todate these areas have been littlestudied even though they are known tobe locations of high ice-production,generation of arctic bottom water,transfer of atmospheric gases to thedeep ocean and generation of frazilice. Systematic observations by SARcoupled with surface information collectedby data buoys and oceanographicobservations from moored buoys wouldsignificantly expand our understandingof these regions.Summer ice processesBecause of operational difficulties,observations on sea-ice behaviorand processes during the summer arevery limited. This lack of informationis of more than academic interest as,~his is the period when shipping att~Ptsto transit through the ice. Theuse of SAR during this period should beof assistance because of its fine resolution.Processes at the ice marginA central problem in the marginalicezone is the definition of thoseoceanic and atmospheric processes thatdetermine the location of the ice edge,the ice morphology and deformation, andthe major fluxes of heat and momentumoccurring in this highly dynamic region.Fortunately several ice-edge phe-106

nomena have distinctive SAR signaturesincluding ice-edge position, leads, iceconcentration, ice types, ice floe dimensions,iceroughness, ice kinematics,oceanic fronts, eddies and upwellingareas, and surface and internal gravitywaves in both the open ocean and in theice. SAR observations should help toexpand our knowledge of these processes.The ASF station mask includesthe ice-margin areas of the Bering andChukchi Seas plus the interesting constrictionpresented by the BeringStrait.Open oceanThe theory of SAR imaging of theopen ocean is still under development.As a result, many aspects of SAR oceansensing are descriptive in nature. However,it is known from the Seas at andShuttle Imaging Radar satellite missionsas well as from aircraft SAR-missionsthat the radar return is sensitiveto the short gravity-wave field atboth C- and L-bands. The waves providingthe signal are further known toarise from, and be modified by, surfacewinds, internal waves, propagatingswell, bottom features and aspects ofcurrents, fronts and eddies. The theoryis complicated as the result of thefact that the Bragg-type elements whichare believed to be a major contributorto the scattering are known to be inmotion associated with the large-scalewave field. The SAR system interpretsany Doppler shift resulting from suchmotions as a Doppler shift due tospacecraft motion. Also, previous studieshave shown that the optimum altitudefor making ocean-wave observationsis much lower than the ERS-l altitudes.Even considering these drawbacks importantopen-ocean observations can bemade with data from the ASF. Subjectsof interest are as follows:Surface wavesERS-1 SAR data will be most applicableto conditions of long wavelengthand moderate to low sea states. Estimatesof directional wave spectra canbe obtained and used to test theorieson wave attenuation, on imaging mechanismsfor waves impinging on ice marginsand on the effects of waves on icefloes located near the ice edge. Withinthe ice pack itself, the high frequencycomponents of the wave field arerapidly attenuated with only very longwaveswell penetrating far into thepack. This attenuation process dependsnot only on ice type but also on floesize (Wadhams 1986). In addition, itshould be remembered that the ice-freeareas of the Bering and Chukchi Seasand the Gulf of Alaska are frequentlysubjected to intense storms. Any informationon the waves generated by thesestorms would be of both scientific andoperational value.Internal wavesIt is expected that in the ERS-ldata set only the tidally-generatedlarger-scale internal waves will be detectable.From the measured group andphase velocities of these waves, estimatescan be made of the wave amplitudesand energetics including dissipation(Fu and Holt, 1984; Apel andGonzalez 1983). There are also indications,based on the analysis of SIR-Bimagery, that internal-wave amplitudemay be obtained directly from thebackscat ter intensity (Gasparovic etal. 1986).Fronts and eddiesFronts and eddies have been observedin all ocean areas of the stationmask and during all times of theyear. In that the most effective sensorfor making such observations (infrared)is cloud-limited and that SAR is knownto provide useful information on suchphenomena, it is to be expected thatSAR observations of these entities willbe very useful. This is particularlytrue in ocean areas such as the Gulf ofAlaska where temperature gradients aresmall. Fronts and eddies at or near icemargins and at the edges of the Beringand Alaskan continental shelves areparticularly interesting in that theyprovide insight into both mixing processesand biological productivity(Alexander and Niebauer 1981) .Bottom topographySubsurface bathymetric features canfrequently be detected on SAR imagerydue to the modulation of gravity wavesby surface currents. In addition therefraction of ocean swell and the generationof internal waves may indicatethe presence of bottom features. Thereis every reason to believe that studiesof the seasonal and annual changes inbathymetry and sediment deposition overAlaskan deltas and over the shallowershelf areas of the Bering, Chukchi andBeaufort Seas would prove to be useful107

oth scientifically and operationally.Applications demonstrations researchIt is obvious that many of the observationsrequired by the science programalso have direct application tomarine operations that occur in Alaskanwaters, including marine transportation,offshore oil and gas exploration,commercial fishing, marine forecastingand oil spill and ship monitoring. Parametersof interest to both operatorsand designers include ice edge location,ice concentration, floe size andpressure ridge distributions, lead orientationsand locations, ice motion,ice island distributions and locations,sea roughness (oil slicks), swell characteristics,internal waves and eddyand front locations.The ASF will provide a unique opportunityto examine ways of effectivelyutilizing SAR-derived geophysicalinformation in the above operationalareas. One presumed requirementfor the effective application of SAR tosuch problems is the rapid extractionand analysis of the information containedin the SAR data plus the timelytransmission of this information to theoperator. This requirement for timelinessis not shared by most researchprograms, which tend to be more retrospectivein nature.The primary exceptions to the aboveare validation experiments in which theexperimenters will require data innear-real-time in order to test thecorrectness of different possible interpretations.A two-phase validationprogram is currently being planned tosupport the science investigations withstudies both prior to and after launch.The objectives of the early stage studiesare to acquire and utilize allavailable C-band, L-band SAR images totest the adequacy of the geophysicalalgorithms currently under development.The purpose of the post-launch activitywill be to continue testing of the geophysicaloutput of the algorithms andto clarify any interpretation problemsrelative to the imagery. In many validationtests, surface survey operationswill be carried out simultaneously aspart of the test.ASF DESIGN AND CAPABILITIESThe ASF will consist of three mainground segments: the Receiving GroundSystem (RGS), the SAR Processing System(SPS) and the Archiving and AnalysisSystem (AAS). These three systems willbe housed in a 5000 square-foot arealocated adjacent to the present VAXcomputer facilities at the GeophysicalInstitute on the west ridge area of theUAF campus. Also included in the complexwill be offices, meeting and conferencerooms and a display area. Constructionof this facility will startduring August 1987.The acquisition and recording ofsignal data are the primary functionsof the RGS which will consist of a 10-meter parabolic antenna, an attendantcontrol computer, an X-band (8000-8400MHz) receiver and several high densitydigital recorders (HDDR). There will betwo X-band telemetry channels for receivingthe real-time SAR data at amaximum rate of 105 million bits persecond. In addition, there will be anS-band channel to augment auto-trackingof the satellites. The system componentswill be integrated from existingtechnology of the LANDSAT type. In orderto provide the desired stationmask, the antenna will be located atopthe eight-story Geophysical Institute.The antenna is currently being constructedby Scientific-Atlanta and isscheduled for installation duringAugust 1988.In the SPS, signal data will beplayed back and correlated in a custompipeline processor operating at l/lOthreal time. Output from the SPS willconsist of l-look complex, full-resolution(4 looks) and low-resolutiondata. (The number of looks refers to thenumber of independent images that areaveraged together to reduce speckle intensity).All images will be geo-referenced(i. e. each pixel will be earthlocated). Post-processing consisting ofgeocoding will be performed upon selectedimages upon demand. (Geocodingis a process whereby an image is resampledto a map projection and rotated totrue north. In most ocean applicationsit is doubtful that geocoding will berequired.) The SPS will be able to outputSAR image data on computer compatibletape (CCT), high density data tape,optical discs and photographic film.All products produced by the SPS willbe permanently stored in the AAS forarchiving, duplication and disseminationto investigators.The AASconsists of three subsys-108

tems; an Archive Catalogue Subsystem, aGeophysical Processor Subsystem and aMission Planning Subsystem. The MissionPlanning Subsystem will provide the ASFstation scientist and manager withtools for predicting satellite groundswathcoverage, site viewing opportunitiesand administration of site datarequests. A daily plan tabulating dataacquisition for the next 24 hours willbe sent to the ASF operators and alsobe reviewable from the on-line catalogue.Management of SAR data and informationabout that data is the ArchiveCatalogue Subsystem's primary function.An on-line catalogue and inventory,built on the NASA Ocean Data System(NODS) and the JPL SAR catalogue experience,will provide investigators withthe capability to interactively selectand order data products. Optical discscontaining signal, l-look, and fullresolution data as well as CCTs containinglow resolution data will bestored in the off-line archive. Filmproducts will also be managed by thepermanent archive.The ASF catalogue will be accessiblevia the Space Physics Analysis Network(SPAN) and an x.25 packet-switchingsystem. These networks will provideasynchronous terminal and computer tocomputer connections for data exchange.Data product requests from ESA investigators,E-ERS-l auxiliary data (usedfor calibration) and orbit predictswill be transferred via X. 25 from theESA Earthnet Program Office (EPO) inFrascati, Italy to ASF. Catalogue updatesand ASF requests for SAR data acquisitionswill be forwarded to EPO forintegration and sequencing by the E­ERS-l Mission Management Control Center.Investigator requests for digitalimages will be filled on either CCTs ordigital optical discs (DOD) with the5.25 in. write-once-read-many (WORM)DOD envisioned to be the major mediumfor data transmission. The distributionof images via telecommunications networksmay also be feasible for browsepurposes by using a data-compressionscheme prior to transmission. A decodingalgorithm on the receiving computerwould restore the received image tonearly the original fidelity.The AAS will, as mentioned, alsocontain a Geophysical Processor Subsys-tem. At the present time algorithms arebeing developed to generate ice-motionvector fields and ice classificationand concentration information. Althoughit is anticipated that these algorithmswill be operational at the time oflaunch, it is doubtful that it will bepossible to complete an adequate validationof the products by that time.Such activities will be one of the mostimportant tasks of the ASF staff andinterested associated investigatorsduring the first two years of stationoperation. Also, as experience isgained in applying SAR to polar oceanproblems, it is anticipated that additionalalgorithms will be developed andverified and their products routinelygenerated by the GPS. The managementand dissemination of the GPS productswill also be a responsibility of theArchive Catalogue Subsystem.THE SCIENCE WORKING TEAMThe ASF will utilize a ScienceWorking Team (SWT) composed of successfulrespondents to a solicitation thatwill be published prior to launch. TheSWT will perform the scientific investigationsof the ASF Project and willbe charged with demonstrating that SARdata, used with other information, canproduce significant advances in a varietyof scientific areas. Members of theSWT will also serve the project onworking groups and with developmenttasks designed to assure the scientificvalidity and responsiveness of the ASF.Prior to the selection of the SWT, aninformal group called the PreliminaryScience Working Team (PSWT) has beenselected by NASA to assist the ASFstaff in a variety of tasks such as algorithmdevelopment and validationplanning. The PSWT and the SWT will beco-chaired by the project scientistsfrom JPL and the University of Alaska.The ASF has the potential to become animportant and effective element in theearth-imaging programs of Europe,Japan, Canada and the United States.It's potential contribution to Arcticscience is enormous. It will be the responsibilityof the Science WorkingTeams to see that this potential isrealized.Additional information on the ASFProject can be obtained by contacting:National Aeronautics and Space Admin.Office of Space Science & Applications109

Attn: Code EEC, ASF Program ScientistWashington D.C. 20546REFERENCESAlexander, V. and Niebauer, H.J. 1981.Oceanography of the eastern Bering Seaice edge zone in the spring. Limno1Oceanogr 26(6): 1111-1125.Apel, J .R. and Gonzalez, F. I. 1983.Nonlinear features of internal wavesoff Baja California as observed fromSeas at imaging radar. J Geopbys Res.88: 4459-4466.Coon, M.D. 1980. A review of AIDJEXmodeling. In 'Sea Ice Processes andModels' (R.S. Pritchard, ed.). Univ. ofWashington Press, 12-27.Fu, L.L. and Holt, B. 1984. Internalwaves in the Gulf of California:Observations from a spaceborne radar.J Geophys Res 89(C2): 2053- 2060.Gasparovic, R.F., Apel,J.R., Thompson,D.R. and Tochko, J.S. 1986. A comparisonof SIR-B synthetic radar data withocean internal wave measurements.Science 232: 1529-1531.Stringer, W.J. 1978. Morphology ofBeaufort. Chuckchi. and Bering Sea sNearshore Ice Conditions by Means ofSatellite and Aeria 1 Remote Sens ingGeophysical Institute, Univ. of Alaska,Vol. I, 218 pp., Vol.II, 576 pp.Thorndike, A.S. 1986. Sea icekinematics. In 'The Geophysics of Sea~ (N. Untersteiner, ed.). PlenumPress, 489-549.Wadhams,P. 1986. The seasonal ice zone.In 'The Geophys ics of Sea Ice' IN.Untersteiner, ed.). Plenum Press, 825-991.Walsh, J.E. 1986. Diagnostic studies oflarge-scale air-sea-ice interactions.In 'The Geophysics of Sea Ice' (N.Untersteiner, ed.). Plenum Press, 755-784.Weller, G., Carsey, F.D., Holt, B.,Rothrock, D.A. and Weeks, W.F. 1983.Science Program for an Imaging RadarStation in Alaska. Jet PropulsionLaboratory, 45 pp.Hall, R.T. and Rothrock, D.A. 1981. Seaice displacement from Seasat syntheticaperture radar. J Geoobys Res.86 (C11): 11078-11082.Hibler, W.D.III, 1979. A dynamic thermodynamicsea ice model. J PhysOceanog 9: 815-846.Hibler, W.D.III, 1980. Modeling a variablethickness ice cover. Mon Wea~ 108: 1943-1973.Nye, J.F. 1975. The usetographs to measure thedeformation of sea ice. J429-436.of ERTS phomovementandGlacial 15:Parkinson, C.L. and Washington, W.M.1979. A large-scale numerical model ofsea ice. J Geophys Res 84: 311-337.PIPOR. 1985. A Programme for InternationalPolar Ocean Research (PIPOR)European Space Agency ESA SP-1074, 41pp.110

AIRBORNE MEASUREMENT OF SEA ICETHICKNESS AND SUB ICE BATHYMETRYAustin KovacsU. S. Army Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire, USANicholas C. ValleauGeotech Ltd., Markham, Ontario, CANADAAbstractA pilot study was made in May 1985to determine the feasibility of usingan airborne electromagnetic soundingsystem for profiling sea ice thicknessand the subice water depth and conductivity.The study was made in the areaof Prudhoe Bay, Alaska. The multifrequencyairborne electromagneticsounding system consisted of controland recording electronics and an antenna.The electronics module wasinstalled in a helicopter and the 7-mlongtubular antenna was towed, beneatha helicopter, at about 35 m above theice surface. Examples of the profilingresults are presented; they indicatethat, for the electromagnetic systemused, both first-year and second-yearsea ice could be profiled, but theresolution decreased as the ice becamerough. This decrease was associatedwith the large footprint of the system,which effectively smoothed out the seaice relief. Under-ice water depth wasdetermined, as was seawater conductivity.The results of the feasibilitystudy were considered highly encourag-This is a reviewed and edited version of a paperpresented at the Ninth International Conference on Portand Ocean Engineering Under Arctic Conditions, Fairbanks,Alaska, USA, August 17-22, 1987.ing and further system developmentis therefore warranted.Introduc tionAn airborne electromagnetic (AEM)system was used for the first tlme inMay 1985 for the purpose of estimatingsea ice thickness, water conductivity,and water depths from about 1 to 20 munder the ice cOlTer. The AEM systemwas basically a standard geophysicalexploration device used by industry forairborne detection of highly conductivemineral deposits. The concept of usingthis technology for measuring coastalbathymetry was recently reviewed byMorrison and Becker (1982); its use fordetermining sea ice thickness was reviewedby Becker et al. (1983). Thefeasibility of using AEM techniques formeasuring sea ice thickness perhapsoriginated in 1968 (Anon.) but was notpursued beyond an analytical verification.The AEM system used in our fieldstudy had four pairs of coils (transmitTx and receive Rx). The coils allowedsimultaneous operation at nominal frequenciesof 530, 930, 4,158 and 16,290Hz. A fifth frequency of 32,020 Hz wasalso evaluated by replacing the16,290-Hz coils. The transmit-receivecoils were separated about 6 1/2 minside a Kevlar tube (bird) 7 1/2 m111

long and 1/2 m in diameter. The birdweighed about 200 kg, and was typicallyflown about 35 m above the ice surface(Figure 1).Figure 1. Illus tra tion of he licop terborneelectromagnetic sensing system.The transmi t coil crea tes a primarymagnetic field Hp which sets up eddycurrents in a conductive medium. Asecondary magnetic field Hs thus resultswhich is detected by the receive coil.In principle, the transmit coilproduces a primary magnetic field 11>which causes a secondary magnetic fieldHs when there is a conductive medium(e.g. seawater) below the bird. Theresulting primary and secondary magneticfields are sensed by the receivercoils. This is illustrated in Figure2. The distance to and the conductivityof the conductive medium affect theTx-Rx mutual coupling ratio Hs/Rp'Through the use of bucking coils andthe system electronics, the primaryfield at the receiver is canceled out,and highly precise measurements of thein-phase (IP) and quadrature (Q) componentsof the secondary magnetic field,in parts per million (ppm), are madeand recorded. With the aid of anArgand diagram, formulated on the basisof a given Tx and Rx coil spacing andorientation, the apparent conductivityand height of the bird above the conductivesurface can be estimated. Anexample of an Argand diagram is givenin Figure 3. If IP is 2000 ppm and Qis 850 ppm then the intercept of therelated lines on the diagram indicatesthe bird is about 26 m above the conductor,or in our case the seawater,and the response parameter is 5000. Inaddition, if the coil was operating ata frequency of 2000 Hz, then the conductivityof the seawater would be 2.5S/m (5000/2000 = 2.5). Through appropriatemultilayer analyses, the Arganddiagram estimates can be refined andextended by nonlinear regression techniquesto provide improved estimates ofthe properties of the ice, water andseabed. A more extensive discussion ofthe general theory related to this AEMsounding can be found in Kovacs et al.(1987) •Since sea ice is relatively resistive and therefore transparent a t theAEM system's low frequencies, the AEMsystem senses the conductive seawaterand thus determines the distance fromthe bird to the sea surfdce. A laserbuilt into the bird was used to measurethe distance from the bird to the icesurface. Subtracting this distancefrom that determined by the AEM systemto the seawater surface gives the apparentice thickness, or the snow andice thickness where a snow coverexis ts. Wi th the use of higher AEMsystem operating frequencies and appropriatealgorithms, we hope to determinethe apparent conductivity of the seaice as well. Then, in principle,through the use of data such as thosegiven in Figure 4, the average brinevolume of the ice can be estimated.And from data such as those in Figure5, an assessment of the mechanicalproperties of the ice sheet can bemade.112

~q=::::::-- ReceiverCoilRx\/secondary Field/.// Hs/IHigh-Conductivity MassFigure 2. I1lus tra tion of the magne tic field associa ted wi th A1!:M sens inp, us ing ahorizon tal coplanar (whale tail) coil arrangement. Other poss i ble coil arrangemen tsinclude vertical coplanar and vertical coaxial coils.ZO,10° ,0 f ~rf'I,G~16'5° ft ~£~~ I10 ~f£ I- 2 ft I101,5° I16 3 II10- 4 10-' 5 10' 5 la' 5IP, In Phose (ppm)10 6Figure 3. Example of an Argand diagram used for determining AEM bird elevation above(and the Apparent conductivity of) a high-conductivity layer. The diagram models therespnse of a horizontal coplanar coil pair, separated about 6.5 m, above a halfspace.113

E"'­(f)NI~aO.IOr----,----.----.----,---,,----,----,----r/---.--~oQ)uHoQ)(f)......o>.>-U:J"00.080.06g 0.04uQ)>-uQ)............wQ)0>o....Q)>c::r0.02o1.220.760.91Ice Thickness: 0.32 m.0.61•.0.481.83 1.51 o = I 620 ZI. 1.451ea· x ba2.13 Corr. Coeff.=0.99820 40 60 80lib a • Average Brine Volume (%0)100Figure 4.Average effective conductivity of model sea ice at 100 MHz versusaverage ice sheet brine volume with ice sheet thickness as a parameter(from Kovacs et al., 1987).N-~z'" 0'" ~~"00::!'u'" ow" >10 ~----.---r----,-----,---___,IOx 1010C,• Isochsen Annual IceC, 0 o Thule0.c,ll.Barrow StraiT900 98C 7Q;a::w"oooo 760~--~2~0--~40~---6LO---8~0--~,0glI ,Avg Brine Volurr,e (%0)boFigure 5.Relative elastic modulus ofsmall sea ice test samples versusaverage brine volume (afterCox and Weeks, 1985).114

Field ResultsField trials conducted in 1985 inthe Prudhoe Bay area of Alaska includedflights over first-year and mUlti-yearsea ice as well as a large groundedmulti-year rubble formation. Anexample AEM profile over a snow-free,relatively uniform 0.75-m-thick refrozenlead is shown in Figure 6. Theprofile for the ice is seen to vary inthickness. This variation is believedto be due in part to system noise, the~ 10-cm accuracy of the laser profilo-meter, and bird pitch and roll variations.The latter could not be fullyaccounted for in the bird pendulum potentiometerdata. However, the AEMdata did give an average thickness forthe lead ice of 0.80 m. This resultwas extremely encouraging. An improvedvertical accelerometer and pitch androll sensor package, reduction of systemnoise, and a more accurate laserprofilometer should further improvethese resul ts. The average seawaterconductivity rJw was determined to be4E- 8.£:a.CI)oCI)~1216Sea BedFiducial NoFigure 6.Example profile resulting from AEM sounding over 0.75-m-thick leadice (from fiducial no. 2890 to 3000). Complete profile is about 4kID long.115

3.0 S/m, or about 1/2 S/m higher thanthe measured value. Note that thewater depth under the ice was also profiled.We did not anticipate thisdetermination being made at this siteand thus no direct sounding verificationwas made.AEM system profiles were made overa multi-year pressure ridge and adjoininglow-lying ice. The AEM bird wasflown down a 250-m-long track establishedon the ice. This track consistedof three parallel lines spaced 111/2 m apart. Snow and ice thicknessesalong each line were determined bydrill hole measurement. These thicknessprofiles are presented in Figure7. The profiles show, as expected,that variations in ice thickness existbetween the profiles at each stationlocation. The thickest snow and iceshown occurred at the ridge location.The average snow and ice thicknesses asdetermined by drill hole measurementfor each profile were 3.56, 3.58 and3.74 m, for an overall average icethickness of 3.62 m.Two example ice thickness profilesobtained with the AEM system are givenin Figure 8. There are significantrelief differences between the two profiles,and neither clearly shows thethicker ridge ice. Three profiles wererun wi th the 32-kHz coil in the birdand two with the 16-kHz coil. Theformer profiles gave average ice thicknessesof 4.06, 3.18 and 3.52 m, for anoverall average ice thickness of 3.59m. The average ice thicknesses for the16-kHz profiles were 3.13 and 3.54 m,for an overall average of 3.3 m.Because there is substantial variationin ice thickness along each of thedrill-hole-measured profiles, andbecause the AEM bird wandered along theflight track, a good correlationbetween the AEM ice profiles and thedrill-hole-measured ice thicknessescannot be expected. However, the averageof all drill-hole-measured snow andice thicknesses along the track was 3.6m, the same as that for the combinedAEM data.The reason why the AEM data didnot show the thick ridge ice was thefootprint size and therefore the sur-face area over which the water surfacewas integrated into each AEM dis tancedetermination. Our preliminary assessmentindicates that the AEM systemfootprint diameter is about equal tothe bird's height. Therefore, the AEMdistance to the sea surface is averagedover a relatively large area of depressedrelief due to the undulatingice bottom topography. This averagingeffect smooths out variations in theice roughness, as occurred in profile ain Figure 8, but does not explain theodd ice thickness variations along profileb.A profile made over first-year seaice and a large grounded multi-yearrubble formation is shown in Figure 9.This formation had formed on a shoal inthe fall of 1983, survived the SlDnmerbreak-up and melt season, and was still:?;rounded in place at the time of ourMay 1985 survey. The shoal on whichthe formation rested was about 8 mbelow the sea surface, as indicated bybathymetry charts of the site. The AEMprofile indicates that 1 1/2 to 3 m ofwater existed under the ice rubble.Since the ice formation was firmlygrounded, there should be no watershown above the shoal. The cause ofthis ambiguity is believed to be theexistence of unfrozen seawater withinthe submerged ice keel block structure,and/or the AEM system was sensing theseawa ter off to the side of the icekeel. The thickest ice is indicated tobe about 22 m thick, which appearsreasonable based on ice surface elevationmeasurements and the apparentshoal depth. The snow-covered firstyearsea ice to the left of the featurewas found to average about 1 3/4 mthick, and the sea bottom to be about10 m deep at fiducial no. 5390. Drillhole measurements indicated a verysimilar snow and ice thickness but awater depth about 1 1/2 m greater. Theaverage AEM-system-determined seawaterconduc tivi ty was 2.6 S/m, which is ingood agreement with measured values.Flight line 6L7 was flown in anorthwesterly direction along the northside of man-made gravel islands Seal,Sandpiper and North Star, and acrossLoon Shoal. The AEM snow-ice thicknessand under-ice water depth along the116

8Cross- section A642E 4'"Q) '" 2C.:.t:.U..c. 008~~6,l-WESTI-Q) 8u ~H.......630c(f)4l 1208 A,8 and C Averaged6-42Stations~ 7.5 m0 4 8 12 16Station32Figure 7.Drill-hole-measured snow and multi-year ice thicknesses along the three250-m-long parallel lines spaced 11 1/2 m apart. Drill hole measurementswere made at 7 1/2-m intervals along each line. The three lines formed thetrack or flight corridor down which the AEM system antenna was flown.117

O""'""I~Ste 0 4 8 12 16 20 24 28 32(m)OWater Depth!! v "-- V lL _ .---~ !! !!-30m2 Water4 .9. o;,-27S/m2635 2645 2655 26656IceD. Thickness 4Ice0 4 8 12 16 20!! v V V ~_9water Depth4.Q..Woter2500 2510 2520Fiduciol NeFigure 8.Examples of two AEM profiles run along a track established on multiyearsea ice. Distance from first inverted triangle station markerto last is 250 m. Stations relate to those shown in Figure 7.20E'"16.. 12~u.ct-o 84E.ca. ..04Water08!e0 Sea 8ed~12C'.,-2.6 S/mFiducial Na5480Figure 9. Example of AEM profile run over first-year sea ice and a largegrounded multi-year rubble formation. The distance between theinverted triangle markers is about 2/3 km.118

flight line are shown in Figure 10.The snow-ice thickness is seen to varybut averages 1.89 m. The drill-holemeasuredsnow-ice thickness averaged1.74 m.The shoaling of the seabed at theislands is apparently due to the AEMsystem sensing the submerged slope ofthese islands. Our assessment of theAEM-determined under-ice water depthindicates the AEM data overestimatedthe water depth, but by less than 10%.The AEM system is undergoing specificinstrumentation development, andfurther analytical refinement is inprogress to tailor the system to airbornesea ice thickness measurement.The system currently appears to becapable of penetrating sea ice of varyingthickness and providing the relativethickness of the ice without beingaffected, as impulse radar is, by thein situ conductivity of the ice. However,because of the large footprint ofthe AEM system, relative variations inice cover relief, such as small pressureridges and leads, may not be distinguishedin the ice profile.Concluding RemarksThe airborne electromagnetic surveysystem discussed appears to have anadvantage over high frequency (50 to1000 MHz) sounding devices such as "impulse"radar (Kovacs and Morey, 1980and 1986 and Kovacs et a!., 1987) inthat the AEM system is not adverselyaffected by the conductivity of the seaice. The AEM system basically measuresthe distance to and roughness of thesea surface as created by ice bottomrelief. The limitation of AEM sensingis its large footprint, which has theeffect of smoothing local ice thicknessvariations. Thus, while the systemappears to offer a method for determiningthe relative thickness of an icefloe, it will not provide high-resolutionsite-specific ice thickness information.Another potential advantage ofthe AEM system is that it may be ableto determine the apparent conductivityof the ice sheet. Thus, from the AEMsystem-determinedconductivity and icethickness measurement it may be possibleto assess the average brinevolume of the ice sheet (Figure 5),from which an estimate of ice sheetstrength can be made.For our 1987 field program, wewill reduce the AEM antenna housing byhalf. Other improvements will includesubstantial reduction in electronicnoise and drift, which made analysis ofthe 1985 AEM data difficult, and theinclusion of a unique internal calibrationsystem.AcknowledgmentsFunding for this study was providedby the U.S. Department of Navy,Naval Ocean Research and DevelopmentActivity, under contract no.N6845286MP60003, program element 63704;and in part by Geotech Ltd., Markham,on on.. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. ..0 0 ~ 0 ~ ~ 0 ~ 0 ~ 0" 0 "" 0 " 0~ 0 N on ~N on ~ 0 N ~ 0 N~~'"~.. ~ ~'"'"~EASTSnow/Ie" ~~..FlduCla I No0~ 00N ~ 0 N ~ 0 N ~ 0 N.. .. ..'" ~'" '" '".. .. .. .." '" '"0~ 0 N on ~ 0'" N~'" on on'" :;: " 0 on :;: on 0:;: "..'"0 on 0 N N N N ~'" '" '" '" ~ ~ ~ ~ :;: on on on onFigure 10.AEM profile results obtained from flight 617 over fast ice west ofPrudhoe Bay, Alaska.119

Ontario. Canada. The authors wish toacknowledge the field assistance providedby Quincy Robe of the U.S. CoastGuard Research and Development Center.Groton. Connecticut; Brian M. Bennettof CRREL; and Chester F. Bassani andVictor R. Cole of Geotech Ltd. Thehelpful comments of Scott J. Holladay.Geotech Ltd.. and Alex Becke r. Universityof California. Berkeley. California.are also acknowledged.Morrison, H.F. and A. Becker (1982)Analysis of airborne electromagneticsys tems for mapping depth of seawater,University of California at Berkeley.Engineering Geoscience Report.Literature CitedAnon. (1968) Proposal for a feasibilitystudy of an airborne technique formeasuring sea ice thickness. GeoscienceIncorporated. Cambridge. Mass •• Proposalsubmitted to U.S. Navy OceanographicOffice, Washington, D.C.Becker, A.. H. Morrison and K. Smits(1983) Analysis of airborne electromagneticsystems for mapping thickness ofsea ice, Naval Ocean Research andDevelopment Activity, NORDA Tech. Note216.Cox. G.F.N. and W.F. Weeks (1985) Onthe profile properties of undeformedfirst year sea ice, USA CRREL, reportprepared for the David Taylor Ship R &D Center, unpublished.Kovacs. A. and R.M. Morey (1980)Investigation of sea ice anisotropyelectromagnetic properties, strength,and under-ice current orientation, USACold Regions Research and EngineeringLaboratory. CRREL Report 80-20.Kovacs, A. and R.M. Morey (1986) Electromagneticmeasurements of multi -yearsea ice using impulse radar, ColdRegions Science and Technology, 12: 67-93.Kovacs, A., R.M. Morey and G. F • N. Cox(1987) Modeling the electromagneticproperties of sea ice, Part I, ColdRegions Science and Technology. inpress.Kovacs, A., N.C. Valleau and J. ScottHolladay (1987) Airborne electromagneticsounding of sea ice thickness andsub-ice bathymetry, Cold RegionsScience and Technology, in press.120

ELECTROMAGNETIC MEASUREMENTS OF A SECOND-YEAR SEA ICE FLOEAustin KovacsU. S. Army Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire, USARexford M. MoreyConsultant, Holis, New Hampshire, USAAbstract"Impulse" radar and ice propertydata were obtained on a second-year seaice floe. These data were used to developa relationship for estimating the icethickness from just the two-way time-offlightof the impulse radar electromagneticwavelet traveling from the surfaceto the ice "bottom" and back to the surface.The relationship developed allowsestimation of the thickness of sea icefrom about 1 to 8 m, with or without asnow cover. The data revealed that theapparent dielectric constant of sea icedecreased with increasing ice thicknessuntil the thickness reached about 4 m.For sea ice thicker than 4 m, the apparentdielectric constant became relativelyconstant. With the use of a model fordetermining the electromagnetic propertiesof sea ice from its physical properties,as determined from ice cores, theelectromagnetic properties were calculatedversus depth. The model resultswere then compared with the electromagneticproperties determined from fieldmeasurements. The two results were ingood agreement.This is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987.I. IntroductionMeasurements were made on a twoyear-oldsea ice floe in the spring of1985 in order to better understand theelectromagnetic properties of this ice.The measurements included impulse radarprofiles made on the ice surface and froma helicopter and cross-borehole radarsoundings made through thick ridge ice.Over 100 holes were drilled along theradar profile lines and four ice coreswere recovered from the ridge. Thecross-borehole measurements were madefrom three of the cored holes. Ice coretemperature, salinity, brine volume, airvolume and density were determined versusdepth. A correlation between the drillhole and radar profile measurementsallowed development of a simple relationshipbetween two-way travel time and seaice thickness. The helicopter radarprofiles were not successful due to alack of the radar's electromagnetic waveletto penetrate the sea ice. A refinedmodel for determining the electromagneticproperties of sea ice was developed,using recent results for characterizingthe electromagnetic properties of coldseawater brine.121

II. Theoretical ConsiderationsSea ice consists of solid ice, brinecells, air pockets, and various solidsalts. The dielectric constant of seaice is primarily influenced by the brinevolume and geometry and orientation ofthe brine inclusions. Any analysis whichattempts to describe the electromagnetic(EM) properties of sea ice should have aquantitative description of the brine. Amixture model formulation was used byMorey et al. (1984) and Kovacs and Morey(1986) for determining the EM propertiesof first-year and multi-year sea icerespectively. However, in their analysesthese authors relied on the available EMproperty descriptions for the sea icebrine, which were extrapolated from measurementsmade on NaCl solutions at temperaturesabove O·C (Stogryn 1971).Besides the uncertainty of using NaClbrine conductivity data extrapolated toin-situ sea ice temperatures, seawaterbrine contains other salts in solution,which affects brine conductivity. Theseobvious concerns were expressed by Moreyet al. (1984) and Kovacs et al. (1987).Recently, Stogryn (1985) and Stogrynand Desargent (1985) published data onthe EM properties of seawater brine attern per a t u res fro m - 2 . 8· to - 25 . O· C .These more appropriate results will beused in this report to model the electromagneticproperties of the sea ice. Themodel results are then compared with EMfield measurements made on the sea icestudied.EM Properties of Brine. The electromagneticproperties of seawater brinedepend on the operating frequency f andtemperature T. The Debye relaxationequation for brine in sea ice can be expressedase * rb - e' - je n e +whererb rb reoe rsb- e reo1 jWf"bj-'lL(1)We0e"rbersbe reof"ba bWe orelative imaginary dielectricconstant of brinerelative static dielectricconstant of brinerelative microwave dielectricconstant of brinerelaxation time of brine (s)conductivity of brine (S/m)angular frequency (rad/s)dielectric constant of freespace (F/m)As Stogryn (1985) and Stogryn andDesargent (1985) pointed out, the dielectricconstant of brine in equilibriumwith sea ice is determined by the fourreal parameters e reob, e b' f"b and a ,r beach of which is a func~Lon of temperatureonly. Their results aree reob(82.79 + 8.19T2)/(15.68+T2) (2)e rsb(939.66 - 19.068T)/(10.737-T) (3)2~rb0.1099 + 0.13603 x 10- 2 T+ 0.20894 x 10- 3 T2 + 0.28167x 10- 5 T3 (4){: T exp[0.5193 + 0.8755 x 10-1 T]for T ~ -22.9·Ca b(5)T exp[1.0334 + 0.1100 T]for T < -22.9·Cwhere Tseconds.is in ·C and 2~f"b is in nano-EM Properties of Sea Ice. Equations2 through 5 were used in eq 1 to calculatethe real (e'b) and imaginary (e;b)parts of the complex dielectric constantof seawater brine as a function of temperatureand frequency. These values werethen used to calculate the real (e' ) andimaginary (en ) parts of the relatI~e complexdielecE~ic constant of the air/ice/brine mixture, as outlined in Kovacs andMorey (1986):e' -rbrelative complex dielectricconstant of brinerelative real dielectric constantof brine122

where* *frm - f~m - jf;m - fri +Vb -n -brine volumef * .r~depolarization factor(6)andu ' -r eal part of magnetic p ermeability(H/ m)f - real effective dielectric cons tanteVm - w/~ - phase velocity (m/ns) (12)and(13)*f •r~and- relative complex dielectricstant of brine-free air/iceVaframixture - ( V J-f-- + V . Jf; )2- a ra ~ 4air volume fractionrelative dielectric constantof air - 1V. - ice volume fraction~fi -relative dielectric constant offresh ice (z 3 . 14 for frequencies> lOs Hz).Other equations used by Kovacs andMorey (1986) and in this analysis are:whe re(S/m)DC conductivity of sea ice,where m is a constant(7)effective conductivity of seaice~ - Q + j~ - complex propaga- (9)tion constantw (\fef/ 2[(1 + a2 e ] 1/ 2 1/ 2Q -W2f2- 1] (10)e- r eal attenuation constant~ - W2f2 + 1] (11)eW C~f ef / 2[(1 + a\ ) 1/2 1/ 2real phase constantIII .Field MeasurementsIn May 1985, an Exxon Inc . researchteam laid out a grid on a second-year seaice floe near Prudhoe Bay, Alaska . Thecenter section of this grid was used tomeasure the physical and electromagneticproperties of the sea ice and to mode lthe ice EM properties using this information.Snow and ice depths, D and D.respectively, were measured ever~ z 7 . 1m along lines 26 and 27 and along a linehalfway between, designated as line 26ain Figures 1 and 2. The 1 ines were11.5 m apart. Impulse radar profileswere taken along the three lines from theground and attempted from a helicopter.Additional snow and ice thicknessmeasurements were made on top of thepressure ridge that snaked through thegridded area. In all, 111 holes weredrilled.Four holes were cored into the iceat the top of the ridge . The extractedcores were divided into 10-cm sectionsand the temperature, salinity, weight andsize of each section were determined sothat the brine volume, air volume, andbulk and brine-free ice densities as afunction of depth could be calculated.Electromagnetic cross-borehole measure ­ments were made by lowering a transmittingborehole antenna into one hole and areceiving borehole antenna into anotherto the same depth . In this way the timeof-flightand attenuation a s a functionof depth between the borehole s were recorded. Measurements were made b e tweenholes 1 and 2 (11 . 84 m apart), 1 and 3(7.87 m apart), and 2 and 3 (3 . 85 mapart) .123

WESTEAST26 x • x • x • x • x • x • x • x •260 x • x. x • x. x • x • x • x •27 x • x • x • x • x • x • x • x •'---,--J-15mx.x. x. x. x. x. x. xPressureRid g eFigure 1. Outline of and survey lines and drill hole positionson second-year sea ice floes study site.Figure 2. Aerial view of ice floe study site and locationof ridge crossing survey lines. Dark object on the far sideof ridge on line 26a is a small shelter, and two objects onnear side of ridge on line 26 are members of field party .124

aWesl~EO'"-IS tationa1012 13 14 15 16 17 18 19 20 21 22 23 24 ?~ ?C 77 ,'H .?'J 30 jl 32I i I I \ ! I I I I I I I I I ! I I I II . " , ..." '......". •- --------'Surfoce· - - - - ---- --~ - ------------------------ - -----------20~c:: 40Ei=6080......NUl020West Station Eastt 0 4 5 6 7 8 9 10 " 12 13 14 IS 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 I I I I I I I I I I I I I I I I I I I I I I I I I I I I I ItII!!!!! !!!II! !!!!!II!! !II! !III! !!!!I! !!I!!!!~ -!!!II! EL Ii!!!!!! !'I'!! !!!!I!!!!!!! E- EX- ---..._-'-V>C- 40Q)Ef-6080Figure 3. Graphic record of impulse radar profiles taken with 80-MHz antenna along survey lines 26 (toprecord) and 26a (bottom record).

IV.Results and DiscussionwhereRadar Profile Results. Radar profileswere taken along each of the threelines using an antenna with a center frequencyof about 80 MHz . In Figure 3,graphic record examples are shown of thetwo-way EM wavelet travel time profileobtained along lines 26 and 26a. Onlyone voltage polarity is printed in orderto highlight the reflection from thebottom of the ice and to help in pickingthe two-way time-of-flight of the impulseradar EM wavelet traveling from theantenna on the surface to the "bottom" ofthe ice and back. Since the antenna waspulled along the surface, the travel timeincludes propagation time through thesnow, where it existed, as well as in theice. In order to remove the effect ofthe snow, an apparent dielectric constant£ of 1.9 was assumed for the snow.U~rng the measured snow depth D at eachstation, the two-way travel tfme in thesnow Ts was calculated from,(14)c =free space electromagneticvelocity (m/ns).The calculated travel time within thesnow was then subtracted from the measuredtwo-way travel time through thesnow and ice T . at each station. Anapparent dielectr\c constant £ • for justthe sea ice was then calcullfed . Theapparent velocity V of propagation ofthe EM wavelet in theasea ice was calculatedfrom,V a_c __J£ai(15)Tables I, II and III list the measuredsnow depth and ice thickness, togetherwith the two-way travel time, determinedfrom the radar profiles (e.g . Fig . 3) foreach drill hole station. Also listed arethe calculated apparent dielectric constantsand related EM wavelet velocitiesfor the combined snow and sea ice thicknessand the sea ice layer alone.T sTable I. Snow andMHz versus stationice thicknessesalong line 26.andelectromagnetic properties at - 80TOTAL TOTAL TOTALSIoQoI ICE S&I TIME IN TIME INSf A DEPTH THICK THICK S~ S & INJ. an m m ns nsTOTALTIME IN APP DIEL APP DIEL VELOCITY VELOCITYICE CONST S&I CONST r IN S&I IN ICEns tasi tai mlns mlns9 9 1. 91 2.99 9.8 28.51 19 2.59 2.69 9.8 35 . 52 12 4 . 43 4.55 1.1 583 48 3.92 3.59 4.4 424 18 2.27 2.45 1.7 33.55 4 2.96 2 .19 9.4 396 39 2.71 3.19 3.6 497 18 3.52 3.79 1.7 478 7 3.63 3.79 9.6 48.59 7 3.58 3.65 9.6 4719 22 2.98 3.29 2.9 4311 29 3 .49 3.69 1.8 4912 9 4.11 4.29 9.8 5414 6 4.64 4.79 9.6 5815 43 4.12 4.55 4.9 54.516 4 6.96 6.19 9.4 74.517 49 4.55 4.95 3.7 5918 42 3.38 3.89 3.9 4719 9 4.21 4.39 9.8 5229 3 3.77 3.89 9.3 47.521 8 3.62 3.79 9.7 4722 7 3.63 3.79 9.6 4623 5 3.45 3 . 59 9.5 4424 9 3.41 3.59 9.8 4425 26 2.34 2.69 2.4 3526 6 2.94 3.99 9.6 3927 12 3.13 3.25 1.1 4128 7 3.18 3.25 9.6 4329 11 2.94 3.95 1.9 4939 8 3.97 3.15 9.7 4931 11 2.79 2.99 1.9 3932 7 3.23 3.39 9.6 4127.7 4.69 4.72 . 149 .13834 . 7 4.29 4.36 .146 .14456.9 3.66 3.71 .157 .15637.6 3.24 3.53 .167 .16931.8 4.21 4.43 .146 .14329.6 4.59 4.66 .149 .13936.4 3.75 4.96 . 155 .14945. 3 3.63 3.73 .157 .15547.9 3.87 3.91 .153 .15246 . 4 3.73 3.82 .155 .15341.9 4.96 4.25 .149 .14647.2 4.17 4.33 .147 .14453.2 3.72 3.77 .156 .15557.4 3.42 3.44 .162 .16259.5 3.23 3.38 .167 . 16374.1 3.36 3.36 .164 .16455.3 3.29 3.32 .168 .16543.1 3.44 3.66 .162 .15751.2 3.29 3.32 .165 .16547.2 3.57 3.53 .159 .16946.3 3.63 3.67 .157 .15745.4 3 .48 3.51 .161 .16943.5 3.56 3.58 .159 .15943.2 3.56 3.61 .159 .15832.6 4.98 4.37 .149 .14438.4 3.89 3.85 .154 .15339.9 3.65 3.65 .159 .15742 . 4 3.94 3.99 .151 .15939.9 3.84 3.96 .153 .15139.3 3.69 3.68 . 158 .15638.9 4.97 4.17 .149 .14749.4 3.47 3.51 .161 .169126

Table II. Snow and ice thicknesses and electromagnetic properties at -80 MHz versus station along line 26a.TOTAL TOTAL TOTALSNao/ ICE S&I TIME IN TIME INSTA DEPTH THICK THICK SN{M S & IKl. an m m ns nsTOTALTIME IN APP DIEL APP DIEL VELOCITY VELOCITYICE CONST S&I CONST I IN S&I IN ICEns < £ mlnsasi aimlns9 7 1.78 1.85 9.61 29 1.89 2.99 1.82 42 1.28 1. 79 4.93 27 1.58 1.85 2.54 29 2.25 2.45 1.85 8 2.97 3.95 9.76 36 2.12 2.59 3.37 49 2.59 2.99 3.78 17 2.31 2.59 1.69 25 2.75 3.99 2.319 19 3.59 3.69 9.911 38 3.62 4.99 3.5U 8 4.37 4.45 9.713 7 3.93 4.99 9.614 28 4.32 4.69 2.614.5 49 4.89 5.29 3.715 59 4.29 4.79 4.615.5 46 4.59 5.95 4.216 19 5.95 6.95 9.916.5 5 7.65 7.79 9.517 29 6.61 6.99 2.717.5 31 6.29 6.69 2.918 21 5.59 5.89 1.919 9 4.51 4.69 9.829 28 3.97 3.35 2.621 9 3.51 3.69 9.822 8 3.67 3.75 9.723 47 3.13 3.69 4.324 41 2.99 3.49 3.825 49 2.71 3.29 4.526 5 3.25 3.39 9.527 23 3.97 3.39 2.128 8 3.82 3.99 9.729 22 2.43 2.65 239 5 1.95 2.99 9.531 24 2.96 2.39 2.232 9 2.81 2.99 9.82829.52427.534.54134.53935414751.55759.55563586473.5918489725944.547.54945.54349.544.54359.535.52934.53927.4 5.15 5.17 .132 .13227.7 4.99 5.49 .136 .12929 4.48 5.52 .142 .12825 4.97 5.98 .135 .12632.7 4.46 4.64 .142 .13949.3 4.97 4.16 .149 .14731.2 4.28 4.89 .145 .137~il~4.97 4.51 .149 .1414.41 4.67 .143 .13938.7 4.29 4.49 .146 .14246.1 3.83 3.88 .153 .15248 3.73 3.94 .155 .15156.3 3.69 3.75 .156 .15549.9 3.59 3.66 .158 .15752.4 3.22 3.31 .167 .16559.3 3.39 3.43 .165 .16253.4 3.43 3.64 .162 .15759.8 3.61 3.82 .158 .15472.6 3.24 3.35 .167 .16499.5 3.14 3.15 .169 .16981.3 3.34 3.49 .164 .16377.1 3.31 3.38 .165 .16379.1 3.47 3.54 .161 .15958.2 3.79 3.75 .156 .15541.9 3.97 4.21 .151 .14646.7 3.92 3.97 .152 .15148.3 3.84 3.91 .153 .15241.2 3.59 3.93 .158 .15139.2 3.69 3.91 .158 .15236 3.69 3.93 .158 .15144 4.99 4.14 .148 .14749.9 3.82 3.96 .153 .15149.8 3.77 3.79 .154 .15433.5 4.94 4.25 .149 .14628.5 4.73 4.83 .138 .13732.3 5.96 5.46 .133 .12838.2 4.97 4.12 .149 .148The drill-hole-measured snow and icethickness along lines 26, 26a and 27 areshown as cross sections in Figure 4. Acomparison of the radar profile alonglines 26 and 26a (Fig. 3) with the relatedcross sections (Fig. 4) revealsvery similar variations in ice thickness.A plot of the measured snow and icethickness, D ., from the surface to thebottom of tli~ ice versus the two-way EMwavelet travel time through the snow andice, T ., is given in Figure 5. Theremay see~~to be a slight bias in the data,since for zero thickness one would expectthe linear curve through the data to bezero nanoseconds. A very small portion,less than I ns, of this apparent bias maybe due to system timing inaccuracies. Thedashed line drawn to the zero interceptin Figure 5 shows that the travel time inthinner sea ice becomes proportionatelylonger. This is to be expected, sincethere is proportionately more brinepresent, which would retard the EM waveletpropagation in the thinner ice. Forsea ice between about I and 8 m thick,the relationship in Figure 5 could bevery useful. Since ice-profiling impulseradars measure the two-way flight time ofan EM wavelet from the surface to the"bottom" of the ice and back to the surface,the equation given in Figure 5allows one to estimate the combined snow/ice thickness. However, in many casesonly sea ice thickness is desired.Therefore, a plot of sea ice thicknessversus total two-way travel time (throughsnow and ice) from Tables I, II and IIIis given in Figure 6. Again, a usefulrelationship is developed for estimatingthe relative thickness of sea ice betweenabout I and 8 m thick, the range of ourdata.In Figure 7, a plot of the apparentrelative dielectric constant of the seaice E • is made as a function of the meaal127

Table III. Snow and ice thicknesses80 MHz versus station along line 27.and electromagnetic properties at -TOTAL TOTAL TOTAL~ ICE S&I TIME IN TIME INSTA DEPTH THICK THICK SNOW S & IND. an m m ns nsTOTALTIME IN APP DIEL APP DIEL VELOCITY VELOCITYICE OJNST S&I CONST r IN S&I IN ICEns € asi €ai m/ns m/ns9 29 3.49 3.69 1.8 44.52 19 1.69 1. 79 9.9 253 5 1.79 1. 75 9.6 264 19 1. 75 1.85 1.1 26.57 18 1.97 2.15 1.7 298 19 3.39 3.49 9.9 43.59 7 3.23 3.39 9.6 42.519 6 3.14 3.29 9.6 4111 8 3.57 3.65 9.7 45.512 13 2.97 3.19 1.2 42.513 12 2.73 2.85 1.1 3914 13 3.87 4.99 1.2 5115.5 52 3.93 4.45 4.8 5416.5 39 6.35 6.65 2.8 8317 21 5.99 6.20 1.9 76.517.5 15 -6.95 6.20 1.4 7818 21 4.84 5.05 1.9 6319 24 3.76 4.09 2.2 5229 55 3.39 3.85 5.1 4721 6 3.79 3.85 0.6 4822 34 2.76 3.10 3.1 3923 11 3.19 3.30 1.0 4324 29 2.55 2.75 1.8 36.525 39 2.31 2.70 3.6 35.526 8 3.42 3.59 0.7 45.527 43 2.77 3.20 4.0 4928 9 3.71 3.89 0.8 45.529 9 4.16 4.25 0.8 51311 9 3.41 3.50 9.8 45.531 15 2.85 3.90 1.4 4032 38 2.72 3.10 3.5 4042.7 3.44 3.58 .162 .15924.1 4.87 5.16 .136 .13225.5 4.97 5.12 .135 .13325.4 4.62 4.66 .149 .13927.3 4.99 4.38 .148 .14342.6 3.68 3.73 .156 .15541.9 3.73 3.74 .155 .15540.5 3.69 3.76 .156 .15544.8 3.50 3.57 .169 .15941.3 4.23 4.35 .146 .14437.9 4.21 4.39 .146 .14549.8 3.66 3.73 .157 .15549.2 3.31 3.53 .165 .16080.2 3.51 3.69 .169 .15874.6 3.42 3.49 .162 .16176.6 3.56 3.61 .159 .15861.1 3.59 3.57 .169 .15949.8 3.80 3.95 .154 .15141.9 3.35 3.61 .164 .15847.5 3.59 3.53 .160 .16035.9 3.56 3.77 .159 .15542 3.82 3.87 .153 .15234.7 3.96 4.17 .151 .14731.9 3.89 4.27 .152 .14544.8 3.80 3.89 .154 .15236.9 3.52 3.81 .160 .15444.7 3.23 3.27 .167 .16659.2 3.24 3.28 .167 .16644.7 3.89 3.99 .154 .15238.6 4.09 4.13 .159 .14836.5 3.75 4.95 .155 .149sured 1ce thickness D .. Again, the higherdielectric constlnt for thin ice, asindicated by'the dashed curve, is reasonablesince the brine content is higher inthe thinner ice. As the sea ice growsthicker and ages, more brine will drainout of the ice. The result is that theapparent dielectric constant of thethicker ice seems to reach a constantvalue of about 3.47.Our attempt ~o profile ice thicknessfrom the air was unsuccessful, even whenflight altitude was down to 5 m above thesurface. We believe the ice was toolossy to allow effective profiling ateither 120 or 80 MHz, the relative centerfrequency of the radar used.Borehole Results. As previouslystated, four holes were cored into theice at the top of the ridge. The physicalmeasurements (temperature, density, etc.)made on each core section at the samedepth were averaged and are listed inTable IV. Ice temperature was used tocalculate the brine conductivity from eq5. Since the cross-borehole EM measurementswere made at about 100 MHz, thisfrequency was used in eq 1 to calculatethe real (f'h) and imaginary (f"b) partsof the rel~tive complex dielecfric constantof the brine. The brine-free icedensity and air volume were used to calculatethe brine-free dielectric constant(f i) of the ice; this, along with thebrIne volume and the complex dielectricconstant of the brine, were used in eq 6to calculate f' and f". In eq 6, thedepolarizationr~actor:mn, is a non-dimensionalvariable.Equation 7 contains a non-dimensionalterm m which is a formation variable(Morey et a1., 1984). Sen (1984) indicatedthat m and n are interrelated asfollows:m _ .2.....:....2.n3(1-n)2(16)The cross-borehole EM measurementswere analyzed as in Kovacs and Morey(1986) to provide total apparent attenuationTA , in dB, and dielectric constantf as aafunction of depth. The resultsf~r propagation between holes 1 and 3were not reasonable, due to an equipmentproblem, and were not used. Cross-bore-128

o Cross-secllon Line 2668~-L--L--L~--~~ __ ~~ __ ~~ __ ~~ __ ~-L ___ L-~o468L-~ __-L__ ~ __ L-~ __-L__ ~ __ ~~~~ __-L__ ~ __ L-~ __-L~o26,260,ond 27 Averaged2'--___ ~ __346Slollons-75m oporlSlolionFigure 4- Snow and ice thickness Dsi along survey lines 26,26a and 27 and the averaged snow and ice thickness for thethree lines.hole sounding could only be made to adepth of about 5 m due to "brine" poolingto this level in the holes. The averagevalues of the dielectric constant andattenuation as a function of depth, asdetermined from the other borehole transmissiondata between holes 1-2 and 2-3,were compared with the calculated relativedielectric constant E and calculatedrelative total attel~ation TAvalues, in dB, at the related depth. rt~total cross-borehole attenuation valuesTA and TA are for a borehole separati~nof 11:W4 m. This is the one-wayattenuation between boreholes and includesthe geometric spreading losses.The depolarization factor n in eq 6, andthus m (eq 16), was varied until the calculatedand measured values of dielectricconstant and attenuation matched. Inother words, the measured borehole resultswere used as a standard and n waschanged until both the calculated E andTA provided the best fit as a ful~tionofr~epth to the measured E and TA data.The resulting nand m vafues wele 0.02and 1.65 respectively. The measured Eand calculated E values versus ice adepth D are given \~ Figure 8. A similarplot of TA and TA versus ice depth isgiven in FIgure 9. rmEquation 5 was used to calculate thebrine conductivity u ' whereas E"b andbE'b were determined from eq 1. Tlie DCcanductivity U . and effective conductivityE of the s~a ice were calculatedDCfrom eq 7 and 8, respectively. It should129

9876E 5·wo4320-Tsi (ns)Dsi ~ -0.496 + 0.088 Tsil

Table IV.depth.Average ice properties for core holes 1-4 versusDEPTH TEMP BK DEN SAL AIR VOL BR VOL POROSITY BF DENm 'c Mg/m 3 %0 %0 %0 %0 Mg/m 31l.1l5 -22.9 1l.735 1l.1l2 21l1.2 1l.1 21l1.3 1l.7351l.1 -22.7 1l.779 9.1l2 163.1 1l.1l 163.1 1l.7791l.25 -22.1 1l.898 1l.93 122.1l 9.1 122.1 1l.8980.35 -21.6 0.81l9 0.1l4 121l.4 1l.1 120.5 0.81l90.45 -21.1 0.860 0.06 65.5 0.2 65.7 0.8590.55 -20.8 0.865 0.11l 59.7 1l.3 59.9 0.8650.65 -21l.4 1l.878 0.12 45.5 0.4 45.8 0.8780.75 -19.9 1l.888 0.16 35.1l 0.5 35.5 0.8871l.95 -19.1 1l.871 1l.36 53.6 1.2 54.8 1l.8691.Il5 -18.7 1l.856 1l.40 69.2 1.3 71l.6 0.8551.15 -18.3 0.861 0.44 63.3 1.5 64.8 0.8601.25 -17.9 0.878 0.50 46.1 1.7 47.9 0.8751.35 -17.5 0.864 0.49 60.9 1.7 62.7 0.8621.45 -17.2 0.873 0.52 51l.7 1.9 52.6 1l.8711.55 -16.8 0.884 0.42 38.7 1.5 41l.2 0.8771.65 -16.4 0.888 0.63 34.6 2.3 36.9 0.8801. 75 -16.1 0.885 0.85 37.3 3.2 40.5 0.8801.85 -15.6 0.888 0.70 34.0 2.7 36.8 0.8851.95 -15.3 0.886 0.74 36.9 2.9 39.7 0.8832.05 -15.0 0.876 1.26 47.5 4.9 52.4 0.8712.15 -14.7 0.881 1.36 42.6 5.4 48.1 0.8752.25 -14.3 0.879 1.08 45.0 4.4 49.3 0.8742.45 -13.6 0.889 1.24 33.8 5.2 38.6 0.8832.55 -13.2 0.885 0.96 38.1 4.1 41.8 0.8802.65 -13.0 0.882 0.92 40.4 4.0 44.3 0.8782.8 -12.4 0.879 1.05 44.2 4.7 48.9 0.8743.0 -11.9 0.873 0.87 50.3 4.0 54.2 0.8693.15 -11.4 0.884 1.24 38.4 6.0 44.4 0.8783.35 -10.8 0.886 1.38 37.1 6.9 44.0 0.8783.45 -10.5 0.887 1.43 36.0 7.3 43.4 0.8783.55 -10.2 0.889 1.36 33.7 7.1 40.8 0.8813.65 -9.9 0.885 1.35 37.8 7.3 45.9 0.8773.75 -9.6 0.892 1.21 29.9 6.7 36.6 0.8843.85 -9.3 9.900 1.09 21.8 6.2 27.9 0.8934.05 -8.8 0.892 1.37 31.1 8.1 39.2 0.8824.15 -8.5 9.899 1.46 22.6 8.9 31.6 0.8894.25 -8.2 0.908 1.17 12.9 7.4 20.4 0.8994.35 -8.9 0.907 0.89 13.3 5.6 18.9 1l.91l14.5 -7.6 0.992 1.Il6 18.4 7.2 25.6 1l.8954.6 -7.3 0.908 1.06 12.5 7.4 19.9 13.9004.7 -7.0 0.909 0.99 11.7 7.2 18.9 0.9014.9 -6.5 0.914 1.26 5.7 9.7 15.4 0.9045.0 -6.3 0.908 1.03 11.3 8.2 19.6 0.9005.1 -6.0 1l.912 0.91 11.3 7.5 18.8 0.9015.2 -5.7 0.91l8 13.93 12.9 7.9 20.8 0.8995.3 -5.5 1l.905 1.43 18.8 9.4 28.2 0.8925.4 -5.5 0.906 1.12 14.3 10.1 24.4 0.8955.5 -5.3 13.895 0.95 25.6 8.8 34.3 0.8875.6 -5.0 0.897 1.46 25.1 14.2 39.3 0.8825.8 -4.6 0.909 1. 79 12.3 19.0 31.3 0.8885.9 -4.4 0.913 1.15 7.0 13.1 20.0 0.9016.0 -4.2 1l.904 1.07 16.5 12.5 29.0 0.8966.1 -3.8 0.91l7 0.81 12.3 10.3 22.5 0.8966.2 -3.5 0.915 0.61 4.4 8.4 12.8 0.9066.3 -3.3 0.915 0.77 4.8 11.2 16.0 0.9036.4 -3.1 0.914 0.66 4.7 10.2 14.9 1l.9046.5 -2.8 0.908 1.05 13.0 17 .9 30.9 0.8896.65 -2.5 0.917 1.23 3.8 23.7 27.5 0.8926.75 -2.3 0.908 1.57 14.7 32.6 47.3 0.8746.85 -2.1 0.917 1.06 3.6 24.5 28.1 0.8916.95 -1.9 0.917 0.96 3.6 24.6 28.2 \l.8917.05 -1.8 \l.917 1l.56 2.4 15.2 17.6 0.9017.2 -1.6 1l.910 1.88 14.7 57.8 72.5 0.851131

8Eoi£al =8.195·2.518 D; + 0.4520;2. 0.027D~l

e noted that u will increase with frequency(Kovacs e et al. 1987). The relativeattenuation A in dBjm was determinedusing eq 13. EqJations 12 and 15 wereused to calculate E ,which is a relativedielectric con~~ant for the sea icemixture that takes into account the effectof sea ice conductivity on the EMpropagation velocity. As the conductivityof a mixture increases, it causes thevelocity of propagation to decrease, andthus the dielectric constant increases.The calculated and "measured" electromagneticproperties of the sea ice are listedversus depth in Table V. Note inTable V that E is the same as orslightly larger t~n e' . The relativereal, E', and imaginaly~ E" ,parts ofthe com~fex dielectric coK~tant werecalculated from eq 6. In Figures 10through 16, plots are presented of thecalculated electromagnetic properties,listed in Table V, versus depth.o(m)2468L-~~ __ ~~ __ ~-L __ L--L __ L-~Figure 11. Relative real dielectric constantof sea ice brine E'b' at 100 MHz,versus depth D in pressur~ ridge.(5/m)2~ __ -r ____ ~ ____ ~ __ -;~ __ -r __ ~8o400o140022o(m)4o(m)4668~--~----~----~--~-----L----~Figure 10. Conductivity of sea ice brineu ' at 100 MHz, versus depth D in pressurebridge.8~~ __ ~ __ ~~ __ -L __ L-~ __ -L __ ~~Figure 12. Relative imaginary dielectricconstant of sea ice brine E~b' at 100MHz, versus depth D in pressure ridge.133

("rmO.-__ -r ____ O,._4 ____ .-__ -. ____ ,-__ ~1.2o(m)o(m)4668~---L--~ ____ ~ __ ~ ____ ~ __ ~Figure 13. Effective conductivity of seaice a, at 100 MHz, versus depth D inpressufe ridge.Figure 15. Relative imaginary dielectricconstant of sea ice En , at 100 MHz, versusdepth D in pressuf~ ridge.2 8Ar (dB/m)O.----r----r----r----T----.--~122o(m)4o(m)4668~--~----~--~~--~----~--~Figure 14. Relative real dielectric constantof sea ice E~m' at 100 MHz, versusdepth D in pressure ridge.8~--~----~----L-__ ~ ____ ~ __ ~Figure 16. Relative attenuation A of100-MHz electromagnetic wave in sear iceversus depth D in pressure ridge.134

Table V. Calculated and measured electromagnetic properties at -100 MHz versusdepth in second year pressure ridge.Measured Calculated Measured0 A TADepth °bTA°OC1 e r rm uepth am 5/m 9 W05 42.1 .0{.10~ • ~000 .~3 3.03 21.8 3.03 0.04ii.55 5.7 1018 42.4 .0000 .O~OO .04 3.07 22.0 3.07 0.O6 0.5 3.01 24.0CJj5 5.75 1U34 42.7 .0000 .0000 .06 3.12 22.2 3.12 0.090.75 5.86 1053 43.0 .C000 .0000 .08 3.16 22.4 3.16 0.11 0.75 3.20 23.2U.95 6.03 1084 43.7 . aDOO .0002 .21 3.21 24.0 3.21 0.251."5 6.11 1099 44.0 .0001 .0002 .23 3.18 24.2 3.18 0.26 1.0 3.30 23.9l.15 6.20 1114 44.4 .UOOI .0003 .27 3.22 24.7 3.22 0.30:.25 6,28 1129 44.7 .0002 .0004 .32 3.30 25.3 3.30 0.34 1.25 3.33 27.21. 35 6.36 1143 45.1 .0002 .0004 .32 3.26 25.3 3.26 0.33,.45 6.41 1153 45.4 .0002 .0004 .37 3.32 25.9 3.32 0.371. 'i5 6.49 1167 45.8 .000, .C003 .27 3.26 24.7 3.25 0.29 1.5 3.42 28.2I.b5 6.56 1179 46.1 .0003 .0005 .48 3.38 27.1 3.38 0.441. 75 6.61 !l89 46.4 .0005 .0008 .74 3.53 30.2 3.53 0.61 1. 75 3.48 28.11.85 6.69 12~3 47.0 .0004 .0007 .59 3.48 28.5 3.48 0.521.95 6.74 1211 47.3 .0004 .0007 .65 3.50 29.2 3.50 0.552.05 6.78 1219 47.6 .0010 .0016 1.31 3.76 37.0 3.75 0.90 2.0 3.55 29.22.15 6.82 1227 48.0 .~OI2 .0018 1. 50 3.84 39.2 3.84 0.102.25 6.87 1236 48.4 .0009 .0013 1.14 3.70 35.0 3.69 0.81 2.25 3.63 30.82.45 6.95 1250 49.2 .0012 .0017 1.43 3.85 38.4 3.84 0.962.55 6.99 1256 49.8 .0~08 .0~12 1.04 3.62 33.8 3.67 0.75 2.5 3.72 35.52.65 7.0£ 1259 50.0 .0008 .0012 1.01 3.65 33.4 3.64 0.072.8 7.04 1266 50.8 .0010 .0015 1.26 3.74 36.3 3.73 0.08 2.75 3.81 38.63.0 7.06 1269 51.5 .0008 .0012 1.01 3.62 33.4 3.61 0.07 3.0 3.92 42.13.15 7.06 1270 52.3 .0015 .0021 1.75 3.95 42.2 3.94 0.113.35 7.05 1268 53.2 .0019 .0026 2.11 4.08 46.5 4.07 0.12 3.25 4.02 44.63.45 7.04 1266 53.7 .0021 .0028 2.28 4.14 48.4 4.12 0.133.55 7.02 1262 54.2 .0020 .0027 2.19 4.12 47.4 4.11 0.13 3.5 4.00 46.63.65 6.99 1258 54.7 .U021 .0028 2.27 4.14 48.3 4.12 0.133.75 6.96 1252 55.2 .0018 .0025 2.02 4.07 45.4 4.06 0.12 3.75 4.01 47.Ul.85 6.92 1245 55.7 .0016 .0022 1.81 4.03 42.9 4.02 0.124.05 6.B5 1231 56.7 .0024 .OU33 2.58 4.27 52.0 4.25 0.15 4.0 4.15 47.24.15 6.79 1221 57.3 .g028 .0038 2.91 4.43 55.0 4.40 0.174.25 6.72 1209 57.9 .0020 .0028 2.27 4.23 4B.3 4.22 0.14 4.25 4.12 47.54.35 6.67 12Q~ 58.3 .0013 .0019 1.56 3.97 40.0 3.96 0.114.50 6.57 1181 59.1 .0019 .0021 2.17 4.19 47.1 4.18 0.14 4.5 4.07 47.74.6 6.48 1165 59.8 .0020 .0028 2.23 4.24 47.9 4.22 0.154.7 6.37 1146 6~.5 .~018 .0027 2.14 4.21 46.8 4.20 0.1;; 4.75 4.13 50.34.9 6.18 1112 61.7 .0030 .U041 3.12 4.61 58.4 4.58 0.215.U 6.10 1097 62.2 .0022 .0032 2.49 4.35 51.0 4.34 0.185.1 5.96 1073 63.0 .0019 .0028 2.20 4.25 47.5 4.24 0.165.2 5.82 1046 63.8 .0020 .0030 2.33 4.20 49.1 4.29 0.185.3 5.71 1027 64.3 .0026 .0038 2.92 4.50 56.0 4.47 0.215.4 5.71 1027 64.3 .0n9 .0042 3.20 4.62 59.4 4.59 0.23,.5 5.60 1008 64.9 .0023 .0034 2.64 4.39 52.7 4.37 ~.205.6 5.~2 976 65.8 .0048 .0067 4.78 5.19 7B.l 5.12 0.325.8 5.17 930 67.0 .00;5 .0100 6.72 5.96 101.0 5.83 0.465.9 5.03 905 67.6 .0039 .0058 4.20 5.10 71.2 5.05 0.336.0 4.89 879 68.3 .0035 .~054 3.92 5.00 67.9 4.95 0.336.1 4.58 824 69.6 .0024 .0040 3.03 4.62 57.J. 4.59 0.286.2 4.33 779 70.7 .0016 .0030 2.34 4.37 49.2 4.36 0.246.3 4.15 748 71.4 .0025 .0044 3.28 4.77 60.3 4.74 0.346.4 3.97 "115 72.2 .0021 .0038 2.91 4.62 56.0 4.59 0.326. " 3.6B 663 73.4 .0048 .0080 5.51 5.68 86.7 5.58 0.586.65 3.38 608 74.6 .0070 .0116 7.42 6.53 109.0 6.37 0.826.75 3.16 569 75.4 .0111 .0175 10.30 7.14 143.0 7.42 1.146.85 2.94 529 i6.3 .g065 .0116 7.44 6.55 1l0.0 6.38 0.936.95 2.70 487 77.2 .0060 .0114 7.35 6.50 109.0 6.33 0.987.U5 2.58 465 77.7 .0026 .0061 4.41 5.18 73.7 5.12 0.6413S

V. ConclusionsA relationship was developed forestimating the relative thickness of coldwinter sea ice from just the measuredtwo-way time-of-flight of an EM wavelet,with a frequency spectrum centered atabout 80 MHz, traveling from the surfaceto the ice "bottom" and back to the surface.Knowledge of the dielectric constantis not needed. This relationshipseems to be good for ice thicknesses fromabout 1 to 8 m, the range of our data.Another useful relationship is that foundbetween the apparent dielectric constantand relative sea ice thickness. Thisrelationship shows that thin cold winterice has a higher dielectric constant thanthick sea ice and that the dielectricconstant of sea ice over 6 m thick approachesa constant value. This isreasonable since the dielectric constantof pure ice at a given frequency is aconstant, and as sea ice grows thicker orages there is less brine in it due tobrine drainage processes. The model usedby Morey et al. (1984) and Kovacs andMorey (1986) for determining the EM propertiesof sea ice has been refined. Thisrevision now uses the EM properties ofseawater brine at negative temperaturesinstead of extrapolated values for NaClbrine as previously used.Morey, R.M., A. Kovacs and G.F.N. Cox(1984) Electromagnetic properties of seaice, Cold Regions Science and Technology,9:53-75.Sen, P.N. (1984) Grain shape effects ondielectric and electrical properties ofrocks, Journal of Geophysics, 49(5):586-587.Stogryn, A. (1971) Equations for calculatingthe dielectric constant of salinewater, Ieee Trans. Microwave Theory andTechniques, 19(8):733-736.Stogryn, A. (1985) A study of some microwaveproperties of sea ice and snow,Aerojet Electro Systems Company, Azusa,Calif., AESC Report 7788.Stogryn, A. and G.L. Desargent (1985) Thedielectric properties of brine in sea iceat microwave frequencies, IEEE Trans.Antennas and Propagation, AP-33(5): 523-532.AcknowledgmentsThis study was funded by the U.S.Department of Energy under contract DE­A12l-83MC20022 and in part by the U.S.Coast Guard under MIPR no. Z5ll00-5-00004. The authors wish to acknowledgethe field assistance provided by QuincyRobe of the U.S. Coast Guard Research andDevelopment Center and Brian M. Bennettof CRREL.ReferencesKovacs, A. and R.M. Morey (1986) Electromagneticmeasurements of multi-year seaice using impulse radar, Cold RegionsScience and Technology, 12:67-93.Kovacs, A., R.M. Morey and G.F.N. Cox(1987) Modeling the electromagneticproperty trends in sea ice, Part I, ColdRegions Science and Technology, in press.136

A RAPID METHOD FOR MAPPING SEA ICE DISTRIBUTIONAND MOTIONS FROM NOAA SATELLITE IMAGERYLewis H. ShapiroKristina AhlnasCoert OlmstedUniversity of Alaska, Fairbanks, Alaska, USAAbstractTo improve the speed and accuracy ofmapping sea ice features, motion anddistribution from NOAA satellite imagery,we have developed a procedure by whichdata are mapped from an image, usingdigitizing equipment, and located andplotted on a conventional map projection.The satellite images are oriented on adigitizing board and the cursor is usedto delineate the ice edge, boundariesbetween ice types, floe positions, etc.The digitized information is thenprocessed by computer to convert from thelatitude-longitude grid of the image tothat of the map projection. The accuracyof location is within 5 km at the1 : 1 , 000, 000 scale of the imagery. Theprocedure can also be used to map Landsatimagery and to take data from maps.These can be merged with the mapping fromthe NOAA imagery and presented on thesame base map. The development of theice cover in the eastern Bering Sea inFebruary-April, 1984 is used as anexample of the type of information whichthe method can provide.This is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987. © The Geophysical Institute,University of Alaska, 1987.IntroductionIn this paper we describe a method forrapidly and accurately mapping sea iceconditions, distribution and motion fromNOAA-AVHRR (Advanced Very High ResolutionRadiometer) weather satellite images.The method permits an interpreter to mapdirectly from a photographic data productwhich is mounted on a digitizing board,simply by moving a cursor over the image.The trace of the cursor is converted tolatitude and longitude coor:-dinates by amethod descr ibed b·elow. The resul ts canbe printed out on a base map of any scaleand merged with data from other sourcessuch as Landsat imagery or geographicdata bases.The quality of the mapping from NOAAimagery,. and its applications, areillustrated through a description of thechanges in the ice cover in the easternBering Sea from February through April,1984.BackgroundOur intention was to devise a systemto improve our ability to utilize theextensive collection of locally archivedNOAA satellite imagery for sea icestudies. Mapping from photographicproducts of NOAA-AVHRR data by the use ofgrid overlays is slow and difficult to do137

with precision. Thus, we sought a systemwhich could work with the archivedimagery without the need to acquire thedata in digital tape format as is neededfor use with automatic image processingsystems. The primary difficulty is thatthe latitude-longitude grid of the NOAAimagery is not a standard map projectionand the orientation of the grid on anyparticular image depends upon the trackof the satellite and whether the pass isascending or descending along that track.Thus, a method was required for theaccurate transformation of positions fromthe distorted grid of the imagery to astandard map base.At present, the method is implementedon a main-frame computer, but it could bemodified for operation on a microcomputerfor which non-linear regression softwareis available.The synoptic views provided by NOAA­AVHRR satellite imagery can be used to(1) map the pack ice edge over largeareas, (2) track floes for relativelylong time per iods, and (3) map internalboundaries separating ice domains withinthe pack ice cover. In addition, thedata can be enhanced to show thedistribution of sea-surface temperaturesby displaying areas within a specifictemperature range in a particular imagedens i ty . The boundar ies between theseareas appear as curves on the image andcan be mapped as temperature contours.For our work we use the enhancement of(Ahlnas, 1981) in which sea-surfacetemperatures within the freezing regimeof sea ice (Le., -5° to -1°C) aredisplayed in a succession of 5contrasting gray tones (Figure la,b).The spatial resolution of the NOAA­AVHRR imagery is about 1 km, so it can beuseful to combine it with higherresolution data from Landsat to providemore detail. In addition, we haverecognized the need to merge the resultsof the mapping from NOAA imagery withinformation from maps (i.e., flight linesor ship tracks, isobars form weathermaps, sample distribution data, etc.).Thus, the procedure was designed so thatdata from these sources could bedigitized, stored and printed along withthe information obtained from the NOAAimagery.Image RegistrationAs noted, the method i nvol vesdigitizing information directly from theNOAA images and transforming it into alatitude-longitude grid which can beplotted out on a standard map projection.The problem is to relate the relativepositions of the data points on thephotographic image plane to theircorresponding geographic locations onearth's surface. During the process ofrecording the image data, several nonlineardistortions are introduced due to(1) lateral compression due to obliquescanning angle, (2) curvature of theearth's surface, (3) mot ion of thesa tell i te in its orbi t dur ing scan, and(4) rotational motion of the earth duringscan.While it is theoretically possible toinvert the distortions from a detailedknowledge of the relative geometry anddynamics of the earth's motion, thesatellite orbit and the scanning device,the technical requirements areconsiderable and expensive. A morepractical alternative is to approximatethe distortion inversion by means of ageneral parameterized geometrictransformation. The parameters can beadjusted to give a best fit ofrecognizable geographic features to theirmap locations. The resultingtransformation provides a conversion ofthe image position into latitude andlongitude on the surface of the earth.The form of the transformation ismotivated by cartographic considerationswhich suggest the oblique transversemercator prOjection (with the transversedirection being given by the path of thesatellite) as geometrically appropriate.Standard references (i.e., Snyder, 1982)provide formulas for computing imageplane coordinates (x,y) in terms oflatitude (~) and longitude (A) given the(parameter) values for central (subpoint)latitude (~o) and longitude (Ao ), scale(ko ) and angle of obliquity (ao). Therelationships can be written as(1)x = F(~o, Ao, ko, ao; ~, A),y G(~o, A, ko, ao; ~, A),138

Figure 1a. Sample of un enhanced NOAA-AVHRR imagery of eastern Bering Sea, April 17, 1984.139

Figure lb. Sample of enhanced NOAA-AVHRR imagery of eastern Bering Sea, April 17, 1984.140

or inverted to give(2)~ = H(~o, Ao, ko, 00; x, y),A = K(~o, Ao, ko, 00; x, y)In order to account for the motion of thesa tell i te and the earth dur ing the imagescan, two additional parameters (~ and 0)are introduced which apply a skew in thelatitudinal direction. Thus, havingcomputed ~ and A by (2) above, the finalgeographic position is taken to be ~', A'where(3)A' = AFor the inverse transformation to (x,y)from ~', A', before applying (1), ~ and Aare computed by(4)~ = ~' + ~(~-1 )-1 (~o-~') +0(~_1)-1 (Ao-A')A = A'Combining (1) with (4) and (2) with (3)gives the forward and reversetransforma tion between (x, y) and ~', A'as functions of the six parameters ~o,Ao, ko, 00, ~ and o. For polar satelliteimages in the vicinity of Alaska we found~ '" 0.1 and 0 0.1, where thegeographic coordinates are in degrees.Finally, we can write~' = M(~o, Ao, ko, 00, ~, 0; x,y)A' = N(~o, Ao, ko, 00, ~, 0; x,y)so that, given n known geographic tiepointsat locations ~i', Ai', whichappear at image coordinates xi, Yi(i=l, ... ,n), we may form the sum ofsquares of residuals for this data:(5)n~ {[~'-M{x ,Y l]2+[A '-N{x ,y.l]2}L, " , ,I.,=1Apply ing a Levenburg-Marquard t algor i thm(Brown and Dennis, 1912) to this nonlinearregression problem will produce aset of parameters which specify a leastsquares best fit map transformation tothe given tie-point data. Thecalculation can be done with thecommercially available computer routineZXSSQ from the International Mathematicaland Statistical Library (IMSL. Inc.),Houston, Texas.The system has been implemented onthe VAX cluster and TALOS/CALCOMPdigitizer at the Geophysical Institute ofthe University of Alaska-Fairbanks.The tie-points used in equation (5)are the latitudes and longitudes ofseveral easily recognized landmarks(capes, islands, etc.) which wedetermined from large-scale maps. In theexamples discussed below the area ofinterest was the eastern Bering Sea, butthe same procedure has been applied forsea ice studies in the Beaufort Sea andfor oceanographic work in the Gulf ofAlaska.The location routine requires onlythe positions of four widely-spacedpoints to register the NOAA imagery inthe latitude-longitude grid. Additionalpoints appear to provide only slightimprovement in registration accuracy(although we have not attempted toquantify the effect). In addition, wehave prepared transparent overlays of thecoastline as it appears on the passes ofthe imagery which we normally use. Theoverlays have been useful when cloudcover prevented us from locating enoughclearly identifiable tie-points toregister an image. In some cases,reaches of the coast or other landmarkscould be identified through openings inthe clouds. These features were thenused as references to position theoverlay on the image so that gridded tiepointscould be digitized from theoverlay to register the image. Thus, itwas often possible to map from images141

which would usually be considered toocloudy to provide usable information.The program includes an adjustablescale so that mapping can be done fromenlarged NOAA-AVHRR imagery, as well asstandard products. In addition, it isgeneral enough to be used for earlierNOAA satellite imagery, Landsat or otherpolar orbiting satell i te imagery, or todigitize information from maps ofcylindrical or conical projection.Mapping ProcedureA flow diagram of, the procedure isshown in Figure 2. The program is userfriendly, leads the operator through therequired steps, and provides theopportunity for each entry to beconfirmed before continuing to the nextstep.The image to be mapped is mounted onthe digitizing board and information on(1) its location and orientation on theboard, (2) the image scale, (3) whetherthe pass is ascending or descending and(4) the digitizing frequency (discussedbelow in the section on "Data Quality andAccuracy") is entered in the computer.The computer is then told whether to addthe data to an existing file or to createa new file. The existing file wouldobviously be used if some mapping hadalready been done from the image. Inaddition, it can be used when it isdesirable to merge mapping from differentversions of the same image (i.e., visibleand enhanced IR, etc.). If a new file isto be opened, the area to be included inthe map is specified by placing thecursor sequentially over the four cornerpoints of the area on the image.The locations of the tie-points areentered by first providing theirlatitudes and longitudes to the computer.Then, the cursor is placed sequentiallyover the tie-points (on the tracedoverlay of the coastline if necessary)which digitizes their positions.During mapping, the trace of thecursor, as it is moved over features ofthe image on the digitizing board, isrecorded as latitudes and longitudes of asequence of points. Once stored in thecomputer in this format, the trace can beplotted out on an annotated base map (inUSERENTRYPART 1POLAR SATELLITE IMAGE DIGITIZERCOMPUTER SYSTEM BLOCK DIAGRAMCREATE ORAPPEND TOA DATA FILEOPENPART 2LEASTCALIBRATE SQUARESIMAGE FITDISKFILESUSER ENTRYi~J)PARAMETERSLEGENDIMAGEPOINTSACQUIREDATAMAPTRANS-FORMDIGITALLINE GRAPHCONTROL--.. OATAIcons HARDWARE~ SOFTWAREFigure 2.System Block Diagram.142

the projection of a U.S.G.S. Series E Mapof Alaska in our case) at any desiredscale. Other commonly used projections,such as UTM or polar stereographic couldeasily be implemented.An immediate check on the accuracyof the registration can be made byprinting out the base map showing theapparent locations of the tie-points andother landmarks. If these are not closeenough to their locations on the map(acceptable limits are discussed in thesection on "Data Quality and Accuracy"below), we assume an error inregistration, and re-register the image.If the locations do not improve, theimage is rejected. This was necessary ina few cases for reasons which are notclear.The drift trajectories of particularfloes (or reference points on the packice surface, such as lead intersections)can be mapped by fixing the position ofthe floe on a sequence of images. Thisis done by placing the cursor over thefloe, identifying it by one of thesymbols available for the purpose, anddigitizing its location. The position isstored in the data file corresponding tothat image. The trajectory is plotted byprinting out the position of the floefrom the sequence of files. When printedout, the symbol and the date appear inthe proper sequence of pos i t ions. Notethat the trajectories can be combinedwith any other data in the files so that,for example, a sequence of ice edgepositions can be shown on the same figureas the floe trajectories.Linear features, such as the packice edge, isotherms, leads, internalboundaries between ice types, bands,streamers, and cloud streets originatingnear the ice edge as indicators ofsurface wind direction, are mapped bytracing the cursor along the feature.The limitations on what can be mapped arethe same as those in any other imageinterpretation exercise; resolution,cloud cover, and the ability of theinterpreter to identify and separate thefeatures to be mapped.As was the case for the point data,linear data for any combination of imagescan be printed out on a map base. Eachcurve is marked with symbols indicatingthe date and type of feature represented.Accuracy and Data QualityUnderstandably, the quality of theregression depends on the relativelocations of the tie-point landmarks aswell as the accuracy of their location onthe image. If the landmarks areclustered together or are approximatelycolinear, the resulting tranformationwill not be reliable for the data farfrom the landmarks. However, the methodgives consistent results when three ormore landmarks are spaced far apart overthe area of interest.To check on the regressionprocedure, we mapped parts of thecoastlines shown on a 1:7,000,000 scaleNOAA 7 image of Alaska and Siberia. Theresults indicated an accuracy of no worsethan 5 km with a mean error of less than3 km.Errors may be inherent in theimages, arising from distortion duringsatellite transmission or dataprocessing. In addition, slightmisplacement of the cursor while locatingthe tie-points used to orient the imagecan also result in a lack of properregistration of the image in thelatitude-longitude grid. The check onregistration is done (as noted above) byusing the cursor to locate knownlandmarks on the image and plotting themback on a latitude-longitude grid. Ingeneral, images for which the locationsdisagree by more than about 10 km (adistance of 2 mm on a base map at a scaleof 1 :5,000,000) are rejected. Exceptionsare made occasionally for landmarks nearthe margins of an image where distortionis greatest; larger errors are acceptablein those cases, provided that thelandmarks closer to the center of theimage are within an acceptable range.Given that the registration isacceptable, the accuracy of the mappingshould depend primarily on the scale ofthe image. However, the frequency ofdigitization must also be adjusted toagree with the precision with which theoperator can guide the cursor over theimage without hand motion affecting thedata. Experience has shown that an143

increment of about 0.5 mm is optimal( i . e., the cursor records a new locationwhenever it is moved through thatdistance). This establishes theprecision with which curves can belocated. On a standard NOAA-AVHRR imageat a scale of 1 :7000,000 (1 cm = 70 km)the cursor can be guided to within 3.5 kmof any point, which is comparable to theaccuracy noted above. The accuracyshould improve linearly with enlargementof the image. However, it can never bebetter than the (approximately) 1 kmresolution of the image data or the pointresolving power of the digitizer (about0.15 mm which, at a scale of 1:7,000,000,is also about 1 km). Other errors canresult from a simple inability toaccurately distinguish points or lines'which are partially obscured by cloudcover. However, we have been able tocompare the results of our mapping of theice edge from NOAA-AHVRR imagery with theedge position as shown on Landsat and·other remote sensing imagery acquired atapproximately the same time. The resultsare essentially indistinguishable at the1 : 5, 000,000 scale at which our maps arepresented, provided that the edge is notcomposed of dispersed floes which aresmaller then the resolution of the NOAAimagery. This is perhaps the bestverification of the accuracy of themethod which we can present.Beyond the technical problems, thenormal difficulties of imageinterpretation remain. Cloud cover canbe difficult to separate from ice,although this is normally recognized byan experienced interpreter. Inparticular, cloud streets formed duringperiods of off-ice winds obscure thewater so that consistent mapping of watertemperature distributions is a problem.Example of ApplicationIntroduct~onIn 'order to illustrate theapplications of the process, we willdescribe some preliminary results ofmapping ice conditions in the easternBering Sea (Figure 3) for the period fromFebruary through April, 1984. Theresults shown were developed as part of alarger study (Shapiro and Ahlnas, inprogress). They are presented only to".eo'56'51'\.SI MatthewIslandFigure 3.Sea.0-PnbllOIIsland •..~ 51 Lawrence~ISlandLocation Map of Eastern Beringdemonstrate how a history of changes inthe ice cover, floe movement and watertemperatures can be developed from acombination of mapping and examination ofweather maps.The data used were standard andenhanced NOAA-AVHRR infrared imagery,enlarged to a scale of 1:3,500,000,supplemented by standard National WeatherService charts of surface weathercondi tions. From these we mapped iceedges, island wakes, isotherms of seasurface temperature, surface winddirection from cloud streets, and floepositions on successive days as describedabove. Relatively long periods of cloudcover break the run of consecutiveimages, but it is still possible todefine sequential changes in the state ofthe ice cover.FebruaryPart of the data obtained fromimages for the month of February areshown in Figures 4 and 5. The early partof the month was generally cloudy so thatcontinuous runs of usable imagery werenot obtained, but clear weather dominatedthe second half of the month.".144

170·Figure 4. Southern limit of pack ice onFebruary 2, 12 and 28, and floetrajectories for dates shown in figure.Figure 5. Southern limit of pack ice(solid line) and _1°C isotherm (dashedline) for February 14.The positions of the ice edges inFigure 4 show the general trend ofexpansion of the ice cover through themonth. The data on floe tracks(available only for the second half ofthe month because of cloud cover)indicate that this was a relativelycontinuous process with only a few shortspurts of rapid advance separated byperiods of slow advance. No instances ofgeneral northerly movement were observed.The pattern of advance is consistentwith weather patterns for the month. Formost of the month the area was dominatedby low pressure systems in the Gulf ofAlaska, which occasionally driftednorthwestward into the lower KuskokwimRiver Valley. These systems bring cloudcover and northeasterly winds to thestudy area. High pressure and northerlywinds affected the area for short periodsof time between the 16th and 20th and24th to 27th. The effect of these windson the ice cover is shown as jogs andincreases in velocity on the drift tracksof the floes.The change in shape of the edge ofthe advancing pack-ice as shown in Figure4 is also of interest. Note that on the2nd, it was at about the latitude of CapeNewenham, but the subsequent advance wasnot uniform. Instead, a bight developedin the ice edge which is apparent on thedata from both the 12th and 28th. Thecause is suggested by the ice edge andwater temperature data shown in Figure 5.The water temperature distributionsuggests the movement of relatively warmwater from west of the study areanorthward across the mouth of BristolBay. The implication is that thepresence of this warm water prevented (orslowed) the southward advance of the iceedge. As shown below, this pattern ofwater temperature distribution remainedin place and influenced the ice edgethroughout the remainder of the periodstudied.MarchThe pattern of southerly advance ofthe ice edge described for Februaryprobably continued into early March, asindicated by the ice edge for March 3(Figure 6). The sense of motion thenreversed and the ice edge moved northunder the influence of southerly windsahead of a low pressure system whichpassed from west to east, and south ofthe Aleutian Islands during March 5-8.This motion is documented by the iceedges for March 6 and 8, and the floedisplacements over these days shown inFigure 6. Note that the bight in the iceedge which formed during the advance in145

Figure 6. Southern limit of pack ice onMarch 3, 6 and 8, and floe trajector iesfor dates shown on figure.Figure 1. Southern limit of pack ice(solid line), -1°C isotherm (dashed line)and _2°C isotherm (dotted line) for March14.February deepened as the ice withdrewnorthward.The ice edge for March 14 (Figure 1)shows that the ice edge advancedsouthward again while the area was cloudcovered. However, the weather chartsindicate that the winds during the periodfrom March 8-14 were light, suggestingthat this stage of advance may have beenaccomplished by freezing of the seasurface, rather than by advection.However, the bight was still well definedand the pattern of sea surfacetemperatures in Figure 1 is the same asthat described for February 14.From mid-March through early Aprilthe winds over the study area weregenerally strong and from the east andcloud cover was extensive. The windswere primarily on the north sides of aseries of low pressure systems whichtraversed across (or just south of) thestudy area. It was not possible to mapthe area during this time, other than foroccasional short reaches of the ice edge.However, observations through openings inthe cloud cover indicated that the packice cover was driven westward from NortonSound and away from the coast betweenCape Newenham and the Yukon River delta.AprilThe cloud cover and easterly windsnoted above continued through the firstweek of April. For the remainder of themonth the weather was dominated by lighteasterly winds from low pressure systemsin the Gulf of Alaska, with only a shortepisode of northerly winds between the12th and 15th. Some of the mapped iceedge positions and floe trajectories forthe month are shown in Figure 8.The position of the ice edge forApril 8 is far north of its location inmid-March. This reflects the westwardshift of the pack ice noted above, inwhich heavy ice from the eastern BeringSea was driven westward toward St.Matthew Island, creating a southwardbulge in that area. The subsequentsouthward advance of the ice edge in theeastern Bering Sea is apparent in thefigure. The advance was exceptionallyrapid between the 8th and 14th, andcontinued at a slower rate through theremainder of the month. The bight westof Cape Newenham, which was noted above,reformed with the same relationship towater surface temperatures as in Februaryand March (Figure 9).146

.,',Z'...5S'5S'Figure 8. Southern limit of pack icefor April 8, 9, 10, 14, 17 and 25, andfloe trajectories for dates indicated onfigure.Figure 9. Southern limit of pack ice(solid line), _1°C (dashed line) and _2°C(dotted line) isotherms for April 11.During most of the time representedin Figure 8 winds were light and the iceedge advanced faster than the floeswithin the pack ice. This suggests thatrapid freezing of cold surface water,rather than advection, was primarilyresponsible for the advance.DiscussionEven though it was not possible tomap the ice edge daily, the main featuresof the process of development of the icecover through the season seem clear fromthe data. In February, the ice advancedsteadily, probably primarily byadvection, since floes in the pack icemoved southward faster than the ice edge.In early March, the pack ice edgeretreated northward under southerly windsearly in the month and then advancedsouthward when the weather systemresponsible for the southerly winds movedout of the area. In this case freezingof the sea surface, rather thanadvection, appears to have been thepr imary mechanism for the advance, sincewinds were light during this time.During the last half of March andinto early April, strong easterly windsdrove the pack ice westward off theBering Sea coast, leaving a large area ofopen water. Subsequently, the pack iceedge advanced rapidly to the south whilewinds were generally light, suggestingagain that freezing, rather thanadvection was the primary mechanism ofadvance.An interesting feature of the iceedge maps is the bight near CapeNewenham. It was present whenever thepack ice edge was near or south of thelatitude of the cape, and was apparentlyassociated with a zone of relatively warmsea-surface temperatures which extendsalong the north side of the AleutianIslands, and across the mouth of BristolBay.Results and ConclusionsThe mapping technique described hereprovides a relatively simple method ofmapping ice information, sea-surfacetemperature distribution and otherfeatures from NOAA-AHVRR satelliteimagery. Once mapped, the data can becombined with information from othersources and printed out on a base map atany scale or common projection. Themapping is done from photographicproducts of the imagery so that digi taltapes and image processing systems arenot required, and the large volume of147

archived photographic data can easily beaccessed. The method relies upon therecognition of a few landmarks toregister the imagery in a latitudelongitudegrid, so that usable data canoften be obtained even when cloud coverobscures most of an image. In general,the mapping is accurate to within 5 km atthe 1:1,000,000 scale of the standardNOAA-AVHRR and pack ice edges mapped fromNOAA imagery at that scale agree withedges mapped from Landsat when plotted ata scale of 1:5,000,000.ReferencesAhlnas, K., 1981, Surface temperatureenhanced NOAA-satellite infrared imageryfor the Bering, Chukchi a~1 Beaufort Seasand the Gulf of Alaska, May, 1914-September, 1980, Institute of MarineScience, U. of Alaska-Fairbanks, IMS R80-2, 91 p.Brown, K. M., and Dennis, J. E., 1912,Derivative free analogues of theLevenberg-Marquard t and Gauss algor i thmsfor non-linear least squaresapproximations, Numerische Mathematik,18, p. 289-291.Snyder, J. P., 1982, Map projections usedby the U.S. Geological Survey, U. S.Geol. Survey Bull., 1532, pp. 313.148

SATELLITE OBSERVATIONS OF THE NORTHERN BERING SEAKenneson G. DeanC. Peter McRoyKristina AhlniisThomas H. GeorgeUniversity of Alaska, Fairbanks, Alaska, USAAbstractPhysical and biological oceanographyare being studied in the northernBerin,g Sea using sa telli te da ta inconjunc tion wi th ship-board measurements.The satellite data acquired bythe NOAA Advanced Very High ResolutionRadiometer (AVHRR), the Landsat Mul tispectralScanner (MSS) and the ThematicHapper (TM) sensor were used to detectsea surface temperatures and suspendedsediments. Ship-board measurements oftemperature, salinity and nutrients wereacquired through the Inner ShelfTransfer and Recycling (ISHTAR) projectand were compared to the satelliteimages.The satellite data reveals northflowing,warm water along the Alaskancoast that is highly turbid near theYukon ~iver nelta. To the west, near.the Soviet Union, cold water, that is inpart nerived from an upwelling, is alsoflowing north. The cold and warm watercoincine wi th the Bering Shelf, Anadyrand Alaskan Coastal water masses identifiedby ISHTAR investigators. TheBering Shelf ann Anadyr water masses areThis is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987. © The Geophysical Institute,University of Alaska, 1987.cold (-1 to 6°C), highly saline (31.7 to33° /00) and nutrient rich. In contrast,the Alaska Coastal water mass is warm (2to 12°C) has low salinity (less than31.7 %o) and is nutrient poor, but hashigh concentrations of suspended sediments especially in the vicini tv of theYukon de 1 ta.Analysis of archived satelliteimagery was usen to analyze the variabilityof the warm and cold surfacewater. Generally, the war", AlaskaCoastal Water forms near the coast andextends seaward as the summer seasonprogresses. Turbid water discharged bythe Yukon River progresses in the samefashion but extends northward across theentrance to Norton Sound, attaining itsmaximum extent in October. Cold waterderived from the upwelling near theSoviet Union flows northward and seaward,but the extent is highly variableand does not conform to seasonalchanges.IntroductionThe continental shelf of the Beringand Chukchi Seas adjacent to northwestAlaska and the Sovie t Union cons is ts ofa broad plateau that is typically lessthan 50 m deep. This plateau areacovers some 45% of the Bering shelf and61% of the Chukchi shelf, a total of 8.4x 105 km 2 • The inner shelf of both ofthese seas is characterized by the149

advection of three water masses, AlAskaCoastal, Bering Shelf and Anadyr (Fig.1), that are longitudinally segregatedfrom east to west, respectively, andflow primarily northward through theBering Strait (Coachman et al., 1975).Processes tha t sus tain the ecosystemin this inner shelf region arelargely unknown in detail but thepresence of a major river, the Yukon, onthe eastern side, presumably, has asignificant effect on the marine environment. A second ma 10r driving forcein this inner shelf is the Bering Shelf /Anadyr wa ters tha t origina te in thedeeper basin of the Bering Sea and flownorth parallel to the Alaska Coastalwater ad1acent to the Soviet Hnion.Because of its morphology, the innershelf of these high latitude seas iscontrolled by nutrients from the twowater bodies: the eastern source fromthe Yukon and other smaller rivers inAlaska (e.g., Kuskokwim, Kobuk andNoa tak) , and the wes tern source from theBering Sea basin. These two flows havedistinct properties and are readilyidentifiable in satellite imagery due todifferences in their sea-surface temperatures.The Yukon' derived nutrients andsedimen ts are primarily deposi ted inouter Norton Sound on the eastern edgeof the ma.10r flow axis. This suggeststhat the impact of the Yukon River isprimarily confined to the shelf south ofBering Strait ad1acent to Alaska.The western shelf contains a verydifferent ecology (Sambrotto et al.,19R4). The Bering Shelf/Anadyr watermasses, that flow from south to northacross the shelf, are basin-derived andoriginate at the shelf break in theBering Sea. This source is significantbecause the water contains a persistent,high nutrient concentration, particularlyni tra te. As a resul t of shelftopography these nutrient rich watersare below the euphotic zone until theypass around St. Lawrence Island wherethey cross the SO m isobath. From St.Lawrence Island north the water columnis well mixed and mostly shallower thanthe 1% light depth. These conditionsresult in a phytoplankton bloom thAtbegins in spring and persists throughsummer in to fall; the produc tion isapparen tly limi teil by flow veloci ty andthe Annual light regime. This system isanalogous to the oceanic upwellings ofother regions of the world oceans.Sa telli te imagery provides a synoptic,spatial view of a relatively largearea of the earth's surface. Thesensors "freeze" the !'lotion of dynamichydrologic environments and thus providea dAta source to study relationshipsbetween v'lrious fe'ltures. The diversityof orbiting satellites result in therecording of image swaths ranging inwidths from about 2700 km (AVHRR) to IRSkm (Landsat). These satellites havedetectors sensitive to visible, nearinfrared and thermal infrared radiationthat record a variety of oceanographicconditions. The sa tellites also differin the frequency of repetitive coveragefrom daily (AVHRR) to every 16 daysexcluding side lap (Landsat 4 and 5).70' ...-_-,._---!l cr 7!:..2'_--. __!116 r S _'_-,-_,.,..!: 16 r: 4 "-'--,70'6S'CHUKCHISEAAnadyr I Bering Shelf... {.§:.,/1 Ia. /62'\\L

study oceanography in northern arcticand subarctic areas including circulation,sea-surface temperatures, sedimentor thermal plumes and sea ice. AVHRRimages have been used by Ahlnas andGarrison (1984), Royer (1983) and Coachman(1975) to study All\skan coastalcurrents in the Chukchi Sea, Bering Seaand Gulf of Alaska. ~OAA imagery isroutinely used to produce sea-surfacetemperature (SST) maps (Ruttenberg,1980). Infrared enhancement techniqueshave been developed to study sea-surfacetemperature and sea ice distribution(Ahlna, 1979; Jayaweera, 1976). LandsatMSS imagery in the visible green and redbands have been used to map the generaldistribution of suspended sediments thatresult from the discharge of ma.1orstreams in near-coastal waters of Alaska(Reimnitz and Barnes, 1973; Burbank,1974; Sharma, 1979). The purpose ofthis study is to incorporate thesynoptic view of the satellite data withship-board measurements of the watercolumn to achieve an accurate assessmentof a portion of the arctic oceanographicenvironment, and to evaluate the utilityof the new Landsat Thematic Mapper datafor oceanographic studies.MethodsNOAA Advanced Very High Resolu tionRadiometer (AVHRR), Landsat Multispectral (MSS) and Landsa t Thema ticMapper (TM) data of the northern BeringSea were acquired during ice-freeperiods of 1985 and 1986. Throughoutthis same period samples and ~asurementswere collected within the watercolumn from an oceanographic researchvessel by ISHTAR (Inner Shelf Transferand Recycling) inves tiga tors (ISHTAR,1985). ISHTAR is a National ScienceFoundation (NSF) funded project studyingbiological produc ti vi ty of the a rea.The ship-board measurements were used tocalibrate the satellite data and toprovide direct field measurements oftemperature, salinity and nutrientswithin the water column. The satelliteda ta were digi tally enhanced to Oil timizethe analysis of temperature, currents,circulation and suspended-sediments.ProceSSing consisted of masking the landareas, and maximizing image contrast(contrast stretch) to enhance subtlepatterns and structures in the image.Spectral bands were enhanced individual-ly, and composited for interpretation.To quantify the extent of intensitydifferences on the sea-surface, discretelevels of image brightness were alsocolor-coded (referred to as levelslicing). Color ranges then show extentand relative differences in either watertempera ture, in the case of thermalspectral bands, or suspended sedimentconcentrations in visihle spectralhands.The variability of sea surfacetempera tures and turbid water were alsoinvestigated using archived satelliteimagery from the 5-year period 1974through 1978. The surface, tempera turesensitiveboundRries of warm and coldwater observed on thermal-infrared AVHRRimagery, and the boundaries of turbidwater observed on visible-red MSSimagery, were manually delineated. Over100 images from each da ta source wereanalyzed. Inter-annual and intra-annualvariability was summarized by overlayingthe resulting maps and generalizing thelocation of boundaries. These generalizationsminimize errors caused bymisregistration and the possible lack ofcontinuity in the interpretation of p,raytones between images.ResultsAVHRR DataThe thermal infrared channel of theAVHRR sensor provided a synoptic perspective of sea-surface tempera tures inthe Bering Sea. The da ta were registeredto a bathymetric base-map andtransformed to an Alberts Equal Area mapprojection. The distribution of warmand cold surface water was delineated onthe images (Fig. ?a, b, c & d). Thesurface tempera ture of the coas tal wa teris warm (11.5 to 2?.5°C) and is displayedas shades of yellow, oran,ge andred. The seaward boundary of thecoastal water eventually extends out to,and stabilizes in, the vicini ty of the30 m isobath. The surface temperatureof the oceanic water (I.e., that derivedfrom the Bering Sea basin) is cold (3 to11.5°C) and is displayed as shades ofblue and green. Dark blue delineatescold oceanic water that upwells near thecoast of the Soviet Union, flows northwardand remains predominantly in thewestern Bering Sea. This upwelling151

Figure 2. Thermal infrared (band 4) AVHRR im'lgery of the Bering Sea recorded in1985: 5 July (a). 22 July (b). 2 August (c). and 3 August (d). Temperatures rangefrom 3°C (dark blue) to 22.5°C. (grey).152

esul ts in low sea-surface tempera tures(3-4°C) in the western portion of thenorthern Bering Sea (dark blue in Fig.2).Landsa t TI1 T>a taThe thermal infrared TI1 data hashigher resolution (120 m) than the AVHRRrlata (1 km) and provirles more detailedinformation concerning local circulationand mixing. This can be seen by comparingthe TI1 image (Fig. 3) to the AVHRRimage (Fig. 2A) recorded on the samedate.The T.M image (Fig. 3) is locatedalong the boundary of the \~arm coastalwater (dark tones to the right) and coldoceanic water (light tones to the left).The coastal water, at least at thesurface, is seen as a ribbon of warmwater along the coast egressing NortonSound along the southern coast of theSeward Peninsula. This warm wa tercirculates around Sledge Island leavingIi pocket of cold water on the lee side,indicating that the direction of flow isto the northwest.Offshore to the southwest is colderwater (light tones) with numerousscalloped and tongue shaped structures.These s truc tures are rela terl to themixing of cold oceanic water with warmcoastal water primarily discharged bythe Yukon River. A color-corled subsectionof the imagery displays thecomplexity and temperatures of thestructures with values ranging from6.5°C (dark blue) to 10.'oC (mediumorange). The tempera ture of the warmwater discharge rlecrease as the rlistancefrom the Yukon nel ta increases. Thistempera ture change wi th dis tance is ingood agreement with trajectory and dilutionnumerical models (Gosink, manuscriptin preparation) of buoyantsurface jets described by Brochard(1985), and Jirka et al. (1981).Landsat MSS DataMSS da ta provides a more res trictive,but still regional, perspective ofthe oceanic environment compared to theAVHRR data. The visible red channel(band 2) of the MSS da ta displays thedistribution of turbid water dischargedby the Yukon River.A level slicerl image of the YukonRiver ou t-flow recorrled on July ';, 1985(Fig. 4) reveals the distribution andcirculation of turbid water. The colorsrepresen t increas ing degrees of brigh t­ness of the water caused by turbidityfrom blue through cyan, magneta, red,green and yellow. Most of the turbidwa ter is flowing north-northwes t acrossthe entrance to Norton Sound. To thewest, in the Bering Sea, the highestbrightness levels (yellow) occur offshorera ther than a t the mou th of thedistributarf.es. Shoals are present inthis offshore area and breakers formwhen waves encounter the shoals anrl thedelta-front platform. Apparently, resuspensionof sediments by wave actionresul ts in more highly turbid wa ter atthe surface than that discharged by theYukon River.niscussionThe sa telli te images provide asynoptic view of the surface of thenorthern Bering Sea. The images revealpatterns on the sea-surface causeci byvariations in sea-surface temperaturesand turbid water. These patterns areprimarily related to the general circulation and mixing of coastal and oceanicwater, anrl surface and subsurface water.Water column rlata collected byISHTAR investigators indicate thepresence of three distinct, northflowingwa ter massoes in the Bering Sea(McRoy and Springer, unpublished cia ta).The Alaska Coas tal wa ter mass is warm (2to 12°C), has low salinity (less than31.7 %0) and is nutrient-poor, withhigh concentrations of suspended sediments, especially in the vicini ty of theYukon Delta. In contrast, the BeringShelf/Anaiiyr water is colrl (-1 to 6°C),saline (31.7 to 33 %0) anrl nutrientrich. For the mos t part the exten t ofwarm and cold water delineated on satelliteimages coincides with the AlaskanCoastal and Bering Shelf/Anadyr waterswith surface temperatures being higherthan those measured in the wa ter column.The turbid water discharged by the YukonRiver is the primary source of suspendedsediments in the Alaska coastal waterthat results in the nutrient-poorenvi ronmen t in this area. The coldupwelling near the Siberian coast is amajor source of the cold, highly salineand nutrient-rich waters to the west.153

I'Figure 3. TN images, thermalinfrarerl hanrl, from S July 19R5. Theenhancerl image (top) shows the rlistrihutionand circulation of warm (lighttones) anrl cold (rl'lrk tones) of surfacewater. A level-slicerl subsection(hottom) rlisplays complex patternsrela ten to the mixing of warm anrl coldWI! ter wi th tempera tures ranging from6.SoC (dark hlue) to IO.SoC (mediumorange) .154

The extent anrl houndaries of thetemperature-sensitive water borlies anrlturhirl water as ohserverl on the imageryvary substantially throughout the icefreeperiod (Fig. '» • Generally, inearly June the Alaskan Coastal waterdevelops near shore in the immediatevicinity of the Yukon delta, to thesouth along the coast anrl in discreetareas along the shore of Norton Sound.By late June the Yukon water supplementedhy solar heating (Ahlnas andGarrison, 1984), has extended northwardalong the coast into Norton Sound incorporatingother warm bodies of wateralong shore, forming a single176' 160'6,'-:.,60'BERINGSEA--A175' 110' 165' 160'60'Figure 5. In tr'l-annual varia t ions inthe extent of warm coastal water (solidlines) and cold oceanic water rlerivedfrom the upwelling (dashed lines) basedon archived AVHRR thermal infraredimagery (1974-l97R) . The numhersindicate the typical flow sequence ofthe water bodies.Fi~ure 4. Lanrlsat MSS im'll(e, visihlererl hanrl, from ') July 19115, of turhirlwater rlischarp,erl hy the Yukon River. Thevarious colors represent hrightness ofthe water anrl hence concentrations ofsuspenrled serliments near the surface .water-body. By late July the coastalwa ter extends throughou t mos t of NortonSound and to the north along Alaska'scoast approaching the Bering Strait. InAugust and Sep tember the warm wateradvances seaward, where it stahilizes inthe vicinity of the 30 m isobath approximately100 km offshore from the Yukondelta . Ahlnas and Garrison (19114)discuss the origin of the coastal water,its variations and its dispersion155

through the Bering Strait. Their shipboardmeasurements indicate that thewarm surface waters extend to a depth of10-30 m.The cold upwelling adiacent to theSiberian coast, as observed at the seasurface on satellite imagery, does notfluctua te seasonally (Fig. 5). Generally,the upwelling is firs t observedadjacent to the southern coast of theSoviet Union on early June images. Theupwelling may occur throughout the icefreeperiod, but becomes distinct whenthe surrounding surface wa ter is warmerthan the upwelling. 'By late June andJuly the cold wa ter has flowed northwardadjacent to the coast and seaward as faras longitude 169°W. The cold wateroften extends northward through theBering Strait and into the Chukchi SeaJuly through October. The extent andshape of the surface exPression of thisupwelled water is highly variable asohserved on satellite imagery.The turbidi ty of the Alaska coas talwater is also variable depending uponriver discharge of Alaskan tributaries(Vig. 6). Discharge from the YukonRiver is the primary component in thedevelopment of the Alaska coastal waterin the northern 'Bering Sea. A turbidplume composed of silt develops in Mayin the vicini ty of the delta. In Juneand early July, when peak riverdischarge occurs, turbid water extends20-50 km offshore and across theentrance to Norton Sound. During Julythe river discharge begins to decrease.However, the extent of near-surfaceturbid water continues to expand toapproximately 100 km offshore in Augustand 150 km in October. As winterdevelops the extent of the turbid waterrecedes. The seaward limit of theturbid wa ter coincides wi th tha t of theAlaska Coastal water-mass in thevicinity of the 30 m isobath.ConclusionsThermal-infrared images recorded bythe NOAA-AVHRR satellite providedsynoptic observations of distinct waterbodies based on their temperatures inthe northern Bering Sea. These wa terbodies consist of warm coastal waternear the Alaskan coast and cold oceanic64'62'60'112' 168'St.Lawrenceleland~ AUGUSTBERINGSEA~JULY168' t64'164' 160'0160'Figure 6. Seasonal varia tions in thedistribution of turbid water dischargedby the Yukon River. The analysis isbased on archived Landsat MSS visiblered imagery (1974-1978).water, including an upwelling, ad1acentto the Soviet Union. The tempera turedependentwater bodies coincide withwater masses that have distinct temperatures,salinity and nutrients withinthe wa ter column, as iden tif ied byISHTAR investigators.The Landsa t "iSS and TM da ta wi thits rela ti vely high resolu tion comparedto tha t of the AVHRR da ta were used toexamine detailed structures that provideinforma Hon concerning local circulationand mixing on the ocean surface inselected areas or in areas too small tobe resolved on AVHRR data. The relativelysmall field of view (185 km by185 km) and low frequency of coveragelimit the utility of Landsat data forregional oceanographic studies. Landsatimages of the reflective bands indicatethat the net flow of turbid waterdischarged by the Yukon River is northwardfrom the delta across the entranceto Norton Sound with minor amountscirculated into the sound. The TIlthermal-infrared images reveal complexpatterns and str1lctures in the surfacewater along the boundary of warm coastalwater and cold oceanic water that cannot be resolved on the AVHRR data.66'64'62'156

Analysis of historical imagesindicates that the surface extent of theAlaskan coastal water, and very coldoceanic water derived from an upwelling,are variable and fluctuate throughoutthe ice-free period. Generally, thecoastal water initially develops alongthe coast in early June and progressesseaward and northward until August whenit reaches i ~s maximum extent near the30 m isobath. Variations in the extentof the coastal water are related toseasonal warming of the sea surface andto discharge of Alaskan streams. Thellpwelled wa ter is firs t observed on Juneimages; it then spreads seaward andnorthward, usually reaching its maximumextent north of the Bering Strait byJuly. The surface extent of theupwelled water as seen on the satellitedata is highly variable but does notappear to be related to seasonalchanges.AcknowledgementThis pro.1ect was funded by theNa tional Aeronau tics and Space Administrationgrant number NAS5-28769. Wewould like to thank EOSAT for permissionto publish the Landsat TM imagery.ReferencesAhlnas, K., and Garrison, G. R.. 1984.Sa telli te and oceanographic observa tionsof the warm coas tal curren t in theChukchi Sea. Arctic, 37 (3): 244-254.Ahlna s, K. 1979. IR enhance men t techniquesto delinea te surface tempera tureand sea ice distributions. In "Proc. ofthe 13th Int'l. Symposium---on RemoteSensing of Environment", Ann Arbor,Michigan, Vol. II: 1067-1076.Brochard, O. N. 1985. Surface buoyantjets in steady and reversing crossflows.Journal of Hydraulic Engineering. III(5): 793-809.Coachman,R. B.regionalsity ofpp.L. K., Aagaard. K. and Tripp,1975. Bering Strait, thephysical oceanography. Univer­I~ashington Press, Seattle, 172Jayaweera, K. 1976. Use of enhancedinfrared sa telli te imagery for sea iceand oceanographic studies. OceanEngineering, (3): 293-298.Jirka, G. H., Mams, E. E. and Stolzenbach,K. D. 19R1. Buoyant surfacejets, In "Journal of the HydraulicsDiviSiOn", Proc. of the Am. Soc. ofCivil Eng., 107 (11): 1467-1487.ISHTAR. 1985. Data report. Instituteof Marine Sciences, University ofAlaska, Fairbanks, V. 1 and 2.Reimnitz, E., and Eames, P. W. 1973.Studies of the inner shelf and coastalsedimentation environment of the BeaufortSea from ERTS-l. NASA Rep. No.NASA-CR-132240.R.oyer, T. 1983. Observa tions of theAlaskan Coas tal Current. In "Coas talOceanography", Plenum Publishing Corp.,pp. 9-30.Ruttenberg, S. 1980. Satellite seasurface temperature measurements anintroduction. In "Oceanography fromSpace" , J. F. iC" Gower, (ed. ). PlenumPress, New York, 71-72.Sambrotto, R. N., Goering J. J. andMcRoy, C. P. 19R4. Large yearlyproduction of phytoplankton in thewes tern Bering Sea. Science (14).September: 1147-1149.Sharma, G. D. 1979. The Alaskan Shelf:hydrographic, sedimentary, and geochemicalenvironment, Springer-Verlag,New York, 498 pp.Burbank, D. C. 1974. Suspended sedimenttransport and depos i tion in Alaskancoastal waters. M.S. Thesis, Universityof Alaska, Fairbanks, 222 pp.157

AN EVALUATION OF AN OPERATIONAL ICE FORECASTINGMODEL DURING SUMMERWalter B. Tucker IIIU. S. Army Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire, USAWilliam D. Hibler IIIDartmouth College, Hanover, New Hampshire, USAAbstractThe Polar Ice Prediction System(PIPS) is an ice forecasting model run ona daily basis at the U.S. Navy's FleetNumerical Oceanographic Center (FNOC).The model was originally developed byHibler (1979) and subsequently modifiedby Preller (1985) to run on FNOC's Cyber205. Atmospheric forcing fields are derivedfrom the Naval Operational GlobalAtmospheric Prediction System (NOGAPS).PIPS is run on a 127-km resolution 47 x25 grid, which covers the entire ArcticBasin and substantial parts of the Greenlandand Norwegian Seas. The system producesforecasts of ice drift, thickness,concentration and divergence at 24-hr intervalsout to 144 hr (6 days). AlthoughPIPS is run on a daily basis, the concentrationfield is initialized weekly usinga digitized version of the concentrationanalysis field prepared by the NavalPolar Oceanography Center at Suitland,Maryland. The system's ability to forecastice drift, concentration and iceedge location was assessed for the periodfrom June 15 to October IS, 1986. ThePIPS drift predictions were generally excessive,although the predicted drift di-This is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987.rections were reasonable. Mean concentrationdifferences between the PIPSforecasts and the analyses were about12%. Although ice edge location was reasonablypredicted in most cases, themodel demonstrated a trend of rapid iceretreat in the Chukchi and East SiberianSeas that was unrealistic.IntroductionThe Polar Ice Prediction System(PIPS) is an arctic sea ice forecastingsystem based upon a numerical dynamicthermodynamicsea ice model developed byHibler (1979). Basic components of themodel include a momentum balance, a viscous-plasticconstitutive law that relatesice stress to ice strength andstrain rate, an ice thickness distribution,and a strength parameterizationthat relates strength to the thicknessdistribution. A coupled thermodynamicmodel calculates ice growth rates from asurface energy balance equation. Thismodel was chosen to provide numerical iceforecasting guidance at the Fleet NumericalOceanographic Center (FNOC) becauseof its substantially improved physicsover previous ice forecasting models usedat this facility which simply empiricallyrelated ice drift to geostrophic windsand mean ocean currents (Skiles 1968;Thorndike and Colony 1982). A verydesirable feature of the Hibler (1979)159

model was that a variety of productsother than ice drift could be directlyoutput froID the IIIOdel. Other IIIOdel outputfields that are useful for Navaloperations include ice thickness, concentration,divergence and ice stress.Although other evaluations of othermodels (e.g. Pritchard et al. 1987) havebeen conducted, this investigation waslimited to PIPS because it was undergoingan operational evaluation for considerationof becoming an FNOC forecast guidanceproduct.Prt!l1er (1985) developed PIPS fromthe Hibler model. The PIPS grid has127-km resolution, and its 47 x 25 gridpoint dimensions were established suchthat they encompassed the entire ArcticBasin as well as the Greenland and NorwegianSeas. Substantial changes to theoriginal model code included an improvednumerical code, which significantly reducedthe run time. Also, modificationswere made that allowed the code to useatmospheric forcing fields produced bythe Naval Operational Global AtmosphericPrediction System (NOGAPS). All fieldsused to drive the model, except the surfacewinds, geostrophic currents andoceanic heat fluxes were obtained fromNOGAPS. Surface winds were obtained froma separate FNOC product, the marineboundary layer wind fields, while oceancurrents and heat fluxes were taken frommonthly values generated by the coupleddiagnostic ice-ocean model of Hibler andBryan (1987).PIPS is run daily at FNOC at a 24-hrtime step out to 144 hrs (6 days). Theice forecast at each 24-hr interval isbased upon forecast atmospheric forcingfor the preceding 24 hrs. In otherwords, today's ice conditions are derivedfrom yesterday's atmospheric forcingfields while the 144-hr ice forecast isbased upon the 12p-hr atmospheric forecast.Graphic forecast products are madeavailable to the Naval Polar OceanographyCenter (NPOC) via the Naval EnvironmentalDisplay System (NEDS). These IIIOdelproducts are designed to be used as guidanceto NPOC for ice forecast preparation.Typical PIPS products are shown inFigure 1.This phase of the operational evaluationcovered the period 15 June until 15October 1986. This period included theice retreat to its minimum extent and thebeginning of the subsequent advancecycle. This time period also included arecently implemented method of re-initializingthe PIPS model. The weekly iceanalysis prepared by NPOC was digitizedand transmitted to FNOC to be used forthe weekly updating of the PIPS IIIOdel.Specifically, the digitized ice concentrationsdirectly replace the modelgeneratedconcentrations after the digitizedanalysis has been received byNPOC. As might be imagined, the updatingprocedure utilized resulted in significantlyimproved model performance overprevious operational periods when themodel was updated from climatology onlyin the event of a computer system failure.Evaluation Methods and ResultsOf the IIIOdel products shown in Figure1, only ice concentration and driftcan be validated with any degree of confidence.Inadequate data exist to properlyevaluate the ice thickness fields.Likewise, divergence fields that would becomparable to the IIIOdel-predicted divergencefields cannot be calculated from thefew drifting buoys in the western ArcticBasin. It is, however, reasonable tocompare IIIOdel-predicted drifts with thebuoy drifts, although some error mayexist because these buoys are located onindividual ice floes which mayor may notbehave in unison with the surroundingice. Ice concentration and position ofthe edge were evaluated by comparing PIPSforecast concentrations to the NPOC digitizedconcentrations used to update themodel.Ice driftIce drift is the single model outputparameter that can be quantitativelyevaluated. However, it is not necessarilythe IIIOst valuable prediction field.The drifting buoys placed on the ice andmonitored by the Arctic Buoy Program atthe University of Washington (Colony andMunoz 1985) allow quantitative studies ofice drift. Unfortunately, the drift predictedby the IIIOdel can only be evaluatedat buoy locations, which were restrictedprimarily to the western Arctic for thetime period of this analysis.160

...0'1...00Z 08 SEP 86 POLRR ICE PREDICTION SYSTEMICE DRIFT OVER 6 DRYS 1-- EOURLS 80 NRUT ICR ,I:U ICE THIKCNESS fCNTR INTRVL EQUALS 0,5 MI,DriftThicknessFigure 1. PIPS 144-hr graphic forecast products.

ICE CONCENTRATION tl'l.2 TO 1.0. CNTR I~TRVL E~UALS 0.1) CNoR I NTRVL EQUALS .00001'105/5. NEGFiT I VE EDUALS ~ONVERGENCE.ConcentrationDivergenceFigure 1 (cont'd). PIPS 144-hr graphic forecast products.

Several steps were necessary to preparedata from the different sources forevaluation. Buoy data were obtained fromthe Polar Science Center (PSC) on magnetictape; these were essentially raw dataas received by PSC from Service Argos.Buoy positions were recorded approximatelyhourly and are presumably accurate towithin a few hundred meters. These datawere edited and reduced for our purposesto one position per day at 0 GMT. We didnot, however, interpolate positions tothis time; usually position times werewithin minutes of 0 Gl.T, although occasionallyposition reports may have occurred2 to 3 hours earlier or later. Positionswere given as latitude and longituderecorded to three decimal places.Eighteen buoys were used in the analysis,although not all were available for theentire period. At any given time, 12 to13 buoys were available for the evaluation,and some of these were locatedquite close together. In Figure 2, thebuoy locations, and the time periods forwhich they were used in this analysis aregiven. Unfortunately, no buoys werelocated in the eastern Arctic. The dailybuoy positions were converted to equivalentCartesian coordinates on the PIPS 47x 25 grid.Hard copies were made from microficherecords of PIPS model products andwere used for extracting predicteddrifts. A standard model output is thecumulative drift prediction at each gridpoint every 24 hrs for the 6-day forecastperiod. A four-point interpolationscheme was used to interpolate the cumulativedrifts at the 24-hr intervals tothe buoy positions. Drifts were interpolatedonly to the initial buoy positionat the beginning of the forecast interval,which is equivalent to the manner inwhich cumulative drifts are calculated inthe model. That is, the drifts aresummed at grid points, rather than by usinga dynamic scheme in which each day'sdrift would be calculated at a point correspondingto the previouR day's positionplus the 24-hr drift. The microfichecumulative drift products of both modelshad a resolution of one nautical mile.Although model forecast drifts were carefullyinterpolated for the buoy positions,an inherent error is expectedbecause the model calculates ice velocitiesfor 68 nmi-square grid elementswhile buoys are located on discrete icefloes. Floes may have differentialmotions as large as 2 km/day during summer(Colony 1977).The evaluation of drifts for alldaily model forecasts would have been amonumental task; therefore it was decidedto evaluate only two forecasts per week.Generally, the PIPS drift forecasts producedon Wednesdays and Saturdays of eachweek were subjected to the analysis.Further, the analysis was broken intothree periods of approximately 37 dayseach. Each period contained 10 forecastevaluation days. These periods wereselected such that a reasonable statisticalsample was, obtained, yet temporalvariability would not be completelyobscured. Drift was evaluated by examiningthe individual x and y drift componentsas well as the cumulative driftvectors. Statistics were calculated foreach forecast interval from 24 to 144hrs.Cumulative Vector PlotsThe most straightforward method ofcomparing the drift predictions is toexamine plots of the predicted driftvectors. In Figures 3 through 5, vectorplots of the 144-hr predicted and actualcumulative drift vectors are shown foreach of the buoys for the three timeperiods. By way of clarification, thecumulative vectors were calculated bysumming the x and y components for the 10forecast drifts within each of the threeforecast periods. This is equivalent tographically adding the vectors head totail sequentially. Then the cumulativevector is that which extends from thetail of the first to the head of thelast. Drifts were examined in thismanner to reduce data volume and to estimateoverall performance for the 10-dayforecast period. Although not shownhere, cumulative drifts were also calculatedfor 24- and 72-hr forecast intervals.In general, trends shown in the24- and 72-hr intervals are manifested inthe 144-hr cumulative drifts.The most obvious feature pointed outby these figures is that PIPS generallyforecasts excessive drifts. The largesterrors appear to be associated wi th thelargest buoy drifts, which occur in the163

3 SEfJ1986, ,-,-\) - - 7012£-)7005-[i]5097..5078..... -.~- -I- I _Ii 'Figure 2. Locations of drifting buoys used for the PIPS ice drift analysis.Circled buoys indicate they were available for the entire study. Those outlinedby triangles were available from 18 June until 29 August and squaresindicate their availability from 28 August until 4 October. Other buoysshown were not used in this study.164

i44hrs.18Jun-25Jul---Observed--'-PIPS125nml3848~3165\. 70~\'7022'"3164~,\7010"'7021 3849I144hrs.28Aug-4 Oct---Observed125nmi--- -PIPS~3848V 700670~~7002 I~ 7004 ,~ i i ~05I ........ , .....384~ __3164~ ~ 3168-7010 -~--7021~...... ~9_-Figure 4. Cumulative vector plots ofactual and predicted buoy drift for the10 forecast days between 23 July and 29August 1986 for 144-hr forecast intervals.Scale is in nautical miles.Figure 5. Cumulative vector plots ofactual and predicted buoy drift for the10 forecast days between 28 August and4 October 1986 for 144-hr forecastintervals. Scale is in nautical miles.165

June 18 through July 25 period (Fig. 3).However, these errors are not necessarilylarger than those of the two subsequentperiods because the direction of buoydrift is reasonably well predicted.Large drifts and relatively large errorsoccur for all buoys during the first period.The period from 23 July to 29 August(Fig. 4) shows considerable differencesin both actual buoy drifts and predictionsfrom the preceding period. Overall,displacements are smaller and, forthe most part, predicted drifts are correspondinglysmaller. However, duringthis period the directional errors forthe PIPS model increased significantly.This is especially manifested by thedrifts of buoys 3164, 7022, 7010 and 7021located in the central Beaufort Sea. Thepredicted drifts of two other buoys, 3161and 3165, are especially poor. Buoy 3161is adjacent to the Alaskan coast and thusmay be located in an area where windforecasts are frequently known to be inerror by as much as 180 0in direction,with large associated speed errors (Kozoand Robe 1986). Buoy 3165, in theChukchi Sea, may be affected by oceancurrent fields, which may be significantlydifferent than those in the PIPS.Drift predictions for all buoys locatedin the Chukchi and Beaufort Seas appearto have been subject to especially poorwind forecasts during this period.For the third period, 28 August to 4October (Fig. 5), predicted and actualbuoy displacements are slightly largerthan those occurring in the previous period.Directional errors between actualand predicted buoy drifts are somewhatsmaller than those of the second period.The drift magnitudes predicted by PIPSare also IIl.1ch more reasonable except forthose buoys located near the North Pole.It is noteworthy that PIPS predicts excessivedrifts for all three periods forthese buoys located near the pole.StatisticsA limited number of the statisticsthat have been calculated on these dataare included in Table I. Statistics werecompiled separately for the 37 day evaluationperiods for each of the 18 buoys.Statistics for three of these buoys arepresented in Table 1. The mean x and yerrors and correlation coefficients betweenthe predicted and actual x and ydrift components are shown. In addition,the quantity labeled Ve/Vobs is themean of the ratios of the magnitudes ofthe error vector and the observed driftvector. The error vector is defined asthat vector between the predicted andobserved final drift positions. Thesestatistics point out some interestingfeatures regarding the predicted drifts.In general, the PIPS error magnitudesverify the observations made in theexamination of the cumulative vectorplots. This is demonstrated by the meancomponent errors (fix and fly) and by themean error vector magni tude ratio (VelVobs). The PIPS Ve/Vobs ratiosgenerally range from 100 to 200%.The x and y correlation coefficients,however, are more encouraging.High correlations indicate that a stronglinear relationship exists between therespective predicted and observed driftcomponents. This then implies that, althoughmagnitudes may be in error, asimple linear correction to the model maybe all that is necessary to correct themagnitudes.The statistics clearly point outsome regional and temporal trends. Forinstance, the statistics for buoy 3161,located adjacent to the Alaskan coast,indicate very poor drift predictions bythe model. Here the Ve/Vobs ratiovaried from 1. 5 to 3.7. In addition,only one pair of x and y correlation coefficientsexceeded 0.5. Buoy 3165,located in the Chukchi Sea, exhibitedsimilar results. Poor wind forecasts orunaccounted-for current effects may beplaying a role here.Near the North Pole, as shown by thestatistics for buoy 3168, drift predictionsare improved relative to coastalAlaska, but they remain poor. Here theVe/Vobs ratio varies from 0.8 to2.2. While several pairs of x and y correlationcoefficients exceed 0.7, manyare less than 0.5. Generally poor predictionswere noted for the other buoyslocated in this region.166

Table I. Statistics for forecast buoy drifts by 24 hourly interval for thePIPS and free-drift model accumulated in 10 forecast-day intervals. Mean xand y drift errors are given by 6x and 6y. Ve/Vobs is the mean ratioof the error vector magnitude to the actual drift vector magnitude wherethe error vector magnitude is that distance between the predicted finalposition and actual final position. rx and ry are correlation coefficientsbetween predicted and observed x and y components of drift.hrBuoy 3161Buoy 3168 (cont'd)r ri\x(nm) t.y(nm) Ve/Vobs x Y hr t.x(nm) hy(nm) Ve/Vobsr xr y18 Jun - 25 Jul 28 Au~ - 4 Oct24487296120144-0.8 -1.5 2.4 .35 .56 23 -0.8 -0.4 0.90.7 0.1 1.8 .75 .51 48 -1.6 0.2 1.60.1 -0.4 1.9 .75 .21 72 -4.0 -0.2 1.5-0.9 -2.3 2.2 .69 .29 96 -5.5 0.9 1.5-2.3 -2.7 2.3 .48 .18 120 -6.7 2.7 1.5-4.7 -3.5 2.1 .30 .05 144 -8.4 2.6 1.6.83.81.84.86.84.83.59.77.60.40.29.1724487296120144hr23 Jul - 29 Aug Buoy 3164-1.3 2.9 3.7 -.06 .63 hr t.x(nm) fly(nm) Ve/Vobs-4.3 2.9 1.8 .06 .82-5.7 3.9 1.5 .24 .77 18 Jun - 19 Jul-6.7 9.8 1.7 .30 .68-9.1 11.4 2.1 .18 .72 24 -0.2 3.2 1.6-9.9 10.7 2.2 .31 .65 48 1.8 5.5 2.072 0.4 6.8 1.4Buoy 3168 96 -0.6 9.4 2.9r r 120 -3.9 11.2 1.5t.x(nm) t.y(nm) Ve/Vobs x y 144 -4.1 9.8 1.518 Jun - 25 Jul 23 Jul - 23 Augr x.62.33.43.60.44.49r y.68.66.75.89.90.8224487296120144-1.5 -0.9 1.6 .97 .36 24 -1.0 0.1 2.2-3.3 -2.5 2.2 .96 .35 48 0.1 -2.5 1.1-4.7 -3.5 1.4 .89 .55 72 3.2 -4.2 1.3-5.5 -3.6 1.5 .83 .68 96 6.3 -6.1 1.4-5.3 -3.7 1.5 .87 .77 120 6.3 -7.9 1.3-6.9 -4.4 1.3 .91 .76 144 7.5 -10.5 1.2.90.94.94.97.94.94.70.84.89.84.64.6223 Jul - 29 Aug 28 Aug - 28 Sep24487296120144-0.5 -0.7 0.9 .79 .69 24 1.1 -1.1 0.7-1.0 -1.2 0.8 .89 .78 48 2.5 -1.3 0.8-4.1 -2.9 1.2 .87 .70 72 2.5 -1.3 1.3-6.2 -4.8 1.7 .88 .51 96 -0.7 -1.3 0.6-7.9 -5.4 1.9 .87 .33 120 2.7 -0.1 0.6-11.1 -6.3 2.0 .81 .28 144 3.7 -0.4 0.7.81.91.67.72.83.86.91.91.84.87.92.84167

The drift predictions for the buoyslocated in the Beaufort Sea are somewhatbet ter than those in other areas. Thestatistics shown in Table I for buoy 3164are a representative example. During the18 June to 19 July time period the VelVobs ratios for the 24- to 144-hr forecastintervals ranged from 1.5 to 2.9 andonly two pairs of correlation coefficientsexceeded 0.6. For the two laterperiods, however, Ve/Vobs ratiosranged from 0.6 to 2.2 and averaged about1.1. The correlation coefficients forthese latter two periods are also encouraging,with all x and y coefficientsexceeding 0.6 and many pairs exceeding0.8. We are unclear as to the cause ofthe dramatically-improved results duringthese latter time periods.Concentration and ice edge locationThe PIPS ice concentration and edgelocation forecasts were evaluated by comparingthem to selected digitized iceanalyses prepared by NPOC. This is theonly quantitative means of evaluating themodel performance for these products. Aswill be demonstrated, PIPS performance isvery dependent upon the accuracy andtimeliness of the digitized analysisfields used to update the model.Ten model updates from NPOC digitizedanalysis fields were made duringthe evaluation period. As mentionedearlier, the model update is usuallyimplemented within three days of theanalysis date. For purposes of thisstudy, the day of update was also consideredto be the reference date of theanalysis. Thus, comparisons of forecastswere made for model runs valid on the updatedate. Concentration was examinedfor both 6-day (l44-hr) and 24-hr forecasts.Ice edge location error was investigatedfor 6-day forecasts only. InTable II, the analysis dates, the updatedates, and the forecast dates are giventhat were compared to these update fieldsfor the evaluation period.Once again hard copies of microficheon which the PIPS output is stored wereused to extract both model and analysis(update) concentration fields. Ratherthan examining the entire grid, we choseseven profile lines from the PIPS modelgrid. The selected profile lines areshown in Figure 6. Concentration valueswere extracted from the 6-day forecastrun, and the model update field for eachof these profiles corresponding to thedates is shown in Table II. Residualsbetween the PIPS and NPOC concentrationswere calculated for each grid cell alongthe lines. Twenty-four-hour concentrationdifferences are routinely calculatedas a model product; thus, these valueswere extracted separately for the profilelines. In this manner we were able toexamine the concentration differencesbetween the 144- and 24-hr forecasts andthe update analyses for every grid cellalong each of the lines. In addition,the distance between the model ice edgeand the analysis ice edge was calculatedfor each of the lines, for both thewestern and eastern Arctic (correspondingto the left and right sides of thegrid). As well as investigating regionaldifferences, we could examine theeffect of the length of time between updatesto a limited extent.ConcentrationIn Figure 7, one example is shown ofthe 6-day predicted concentrations coincidentwith the analyzed concentrationsfor the seven profile lines as depictedin Figure 6. This figure shows the predictedand analyzed concentrations validfor 8 August 1986. In this case themodel had been updated 15 days prior tothe model run. Several features areTable II. NPOC analysis dates, PIPS updatedates and model run dates for concentrationand edge analysis.NPOCanalysisPIPSupdate-----------------------------1 July 3 July 27 June 2 July8 July 11 July 5 July 10 July15 July 18 July 12 July 17 July5 Aug 8 Aug 2 Aug 7 Aug19 Aug 23 Aug 17 Aug 22 Aug26 Aug 29 Aug 23 Aug 28 Aug3 Sept 5 Sept 30 Aug 4 Sept19 Sept 19 Sept 13 Sept 18 Sept30 Sept 4 Oct 28 Sept 3 Oct7 Oct 10 Oct 4 Oct 9 Oct168

252423222L2.1918171615U1912lL...Q\ 111\C9B765,321Figure 6.PIPS model grid. Lines chosen for concentration and edge location analysis are highlighted.

obvious here that will also be verifiedin later figures. First, the modelgenerally forecas ts lower concentrationsthan depicted by the analysis, particularlyin the central Arctic. Althoughdramatic changes in the update fields,some of which are questionable, accountfor some of the errors, the model tendstoward lower concentrations in the centralArctic than does the analysis. Thisis obvious in Figure 7, which shows lowerconcentrations in the central Arctic.Next, the model predicts much less icethan is shown by the analysis in thewestern Arctic, specifically in theChukchi and East Siberian Seas. That themodel predicts rapid ice retreat in theseareas is made clear by Figure 7 for whichan update was made 15 days prior to themodel run. The rapid retreat of the iceedge forecast for this 21-day period (6-day model forecast plus update 15 daysprior) is evident in most of the westernArctic. Excessi ve ice is predicted inthe eastern Arctic. Evidence of aslightly extended ice edge in the EasternArctic is seen in the figure.To examine the effect of the timelinessof the NPOC updates on the qualityof the PIPS concentration forecast, meanconcentration errors were calculated foreach line and plotted as a function oftime since update. Specifically, concentrationerrors at each grid point locationwere summed for the line, and thenthat sum was divided by the number ofgrid points on the line. Figure 8 showsthese mean 6-day forecast errors for eachline as a function of the number of dayssince the model was updated prior to theforecast run. Initially, the large scatterof points makes any dependence ontime seem speculative. Closer inspec-//'/\ /\/,_/---------........I~ ......... /, ,------...." ,LINE 23LINE 20I__ oJLINE 17I~----r=======================~==~~==~~--~~----------------------~100%,.. .".---_///---------""LINE 14LINE 11LINE 8/'/I/,--LINE 5Figure 7. PIPS 6-day forecast and NPOC digitized ice concentrations for each of thelines shown in Figure 11. Solid line is NPOC digitized analysis and dashed line isPIPS prediction. The valid date for this forecast is 8 August and 15 days had elapsedsince the model was updated prior to the model run.170

tion, however, shows generally smallererrors for runs that had not been recentlyupdated. The figure also points outthat profile lines closer to the Sovietsector (lines 17, 20 and 23) generallyhave larger errors. Predictions in thisregion appear to be particularly sensitiveto timely updates.In Figure 9, a condensed version ofFigure 8 is presented. Here, a mean concentrationerror was calculated for anentire 6-day forecast (by combining alllines). This figure clearly shows atrend of increasing error with time sinceupdate. The two relatively-large errorsassociated with ° and I-day update timesare somewhat disturbing. As will be discussedlater in this report, these indicatelarge changes between the weeklyanalyses in addition to model errors.Ice Edge LocationIce edge location error was calculatedby simply measuring the distancebetween the predicted and analyzed edgelocations on the selected concentrationlines of which Figure 7 is an example.The ice edge in both cases was taken to35 I I I I I I• Line 5830 I-'""II -0 14~e 25 I-17 _0"a a 20W '1'1 23c~ 20 I--"~ca"/u "L-c.~015~ -u./' 0c 0/ .0:.......-0000::; " 10~: ~ . -.::...-00~~.",of:?5'::-0 o~ -~I I I I I I0 5 10 15 20 25 30 35Days Since UpdateFigure 8. PIPS 6-day forecast mean concentrationerror for each line as afunction of the number of days elapsedsince update prior to the model run date.Arrows designate overlying values.be the first non-zero concentrationvalue. Unfortunately, the smallest resolutionelement is one grid cell, approximately68 nautical miles. Mean dis tanceerrors were calculated for both the westernand eastern Arctic by summing individualline errors for the left and rightsides of the lines, respectively.In Figure 10, the mean ice edgelocation error is depicted as a functionof elapsed days since update prior to themodel run. A clear trend of increasingnegative error is evident for the westernArctic. A small increasing positivetrend is associated with the easternArctic. This figure also makes it veryclear that too lit tle ice is being forecastfor the western Arctic (negativeerror) while the ice edge is slightly excessive(positive error) for the easternArctic. Once again the large positiveerror for the western Arctic at zero dayssince update is puzzling. An investigationof the original NPOC analysis chartand the digitized version of that samechart revealed significant errors in thedigitized ice concentration. Specifically,the 23 August update field, which wasa representation of the 19 August NPOCanalysis, showed 95% concentrations inthe Chukchi Sea area. The analysis, however,showed the area to be ice free.20 I I I I~~•0~w l-• -• •c0~0~•c.,u••c0U101-- • -c0., r- -::;:I I I I0 5 10 15 20 25Days Since UpdateFigure 9. PIPS 6-day forecast mean concentrationerror for all lines combined,as a function of the number of dayselapsed since update prior to model rundate. All updates are plotted; thereforetwo cases exist for 0, 1 and 9 days.171

204 1 1 1 Io Weslern Are II e:;z 136 r-• Easlern-~e~wc: 68 - -0c• • •0• •0...J0'-;/. • -Q)." '"WjoioQ)00H-68 f- -0c 0Q)0:;0-136 I- --204 I _l I 10 5 10 15 20 25Days S Inee UpdaleFigure 10. PIPS 6-day forecast mean iceedge location error for all lines combinedfor western and eastern Arctic as afunction of the number days elapsed sincethe last update prior to the model rundate. Arrows indicate overlying values.This error in digitization caused thelarge error associated with the zeroday-since-updateposition on the figure.The extent to which such errors may haveoccurred in the remaining analyses wasnot investigated as part of this analysis.Standard deviations of the ice edgelocation errors were also calculated althoughthey are not shown here. They arequite large, as would be expected fromthe large variability shown in Figure 7.The standard deviations range from 23 to138 nmi for the western Arctic and from33 to 70 nmi for the eastern Arctic.The analysis up to this point hasfocused only on pointing out errors inconcentration and edge location. Thisnecessarily draws attention to areaswhere the model performs poorly. Smallor zero errors, on the other hand, indicateexcellent prediction performance,yet, because they are inherently smallvalues, they receive little attention inan error analysis. Therefore, it isworth pointing out a few examples whichdemonstrate extremely positive aspects ofthe model. Firs t, Figure 7 shows predictedconcentrations for which more than2 weeks has elapsed since a model update.The model does quite well for predictingedge location in the Greenlandand Beaufort Seas; the maximum locationerror in these regions is 68 nmi or onemodel grid cell element. A significantnumber of these errors can be attributedto the coarse resolution of the model andto the gridding procedure used in thedigitization of the NPOC analysis. Althoughnot shown here, PIPS often properlyaccounted for short-term (one-week)ice edge retreats, particularly in theeast Greenland area. Several occurrencesof this nature were observed in the data,indicating that the model can properlyforecast a moving ice edge.Conclusions and CommentsDrift evaluationThe comparison of the drifts predictedby PIPS model with those of driftingbuoys during this evaluation leads tothe following conclusions:1. This evaluation has shown thatdrifts forecast by PIPS are generally excessivefor all forecast intervals (24 to144 hr). Drift directions, however, tendto be more reasonably predicted.2. The vector error magnitudes aregenerally quite large for the PIPSmodel. Vector error magnitudes generallyrange from 100 to 200%.3. Correlation coefficients betweenthe model and buoy x and y componentswere promising. High correlation coefficientsindicate that while drift magnitudesare in error, a simple correctionmay greatly improve the magnitudes.4. PIPS generally performs poorlynear the Alaskan coast and in the ChukchiSea. It is likely that wind forecastsare poor or unaccounted-for currents mayhave a large effect here. PIPS errorsalso are pronounced near the North Pole.Several aspects of the model operationdeserve comment. PIPS is integratedon a 24-hr time step. Since the drifts172

are calculated from the instantaneous icevelocity at the time of integration, considerableerror can be accumulated by alarger time step. A smaller time stepwould tend to mitigate such error.Perhaps the most important aspect,however, is that the wind forcing forthis model is suspected to contain largeerrors. PIPS makes use of the FNOCmarine boundary layer wind fields. As aresult of this study, the accuracy of usingthe marine boundary layer winds isquestionable. Preller (pers. comm.) hasfound significant differences in airstress values generated from geostrophicwinds and marine boundary layer winds.In addition, the spherical projectionused for the marine boundary layer windfield generation apparently has a singularityat the pole; thus the larger PIPSerrors may have resulted from inaccuratelyforecast winds in that region. As afirst step in improving the PIPS forecasts,changing the forcing fields togeostrophic winds may be warranted. Furtherdrift examination by comparison tobuoy drifts during all seasons is alsosuggested although, as mentioned previously,the drift of individual buoys maybe significantly different from the averagedrift of ice in a grid cell 127 km ona side as calculated by PIPS.Ice concentration and edge locationThe following are conclusions regardingperformance of the PIPS model toforecast ice concentration and edge locationfor the two evaluation periods.1. The mean concentration differencebetween the PIPS 6-day forecast andthe digitized ice analysis for the lineswas about 12%. Much of this error wascaused by differences at the ice margins,although PIPS tends towards lower concentrationin the central Arctic than thatindicated by the NPOC analysis.2. Only slight improvements in concentrationwere noted for PIPS 24-hrforecasts, which indicate that the NOGAPSforcing fields (versus NOGAPS analysisfields) are, for the most part, sufficientfor prediction of concentration.The largest discrepancies between the 24-and 144-hr forecasts were noted in theice margin regions.3. As regards the ice edge, theforecast edge location is too far to thenorth in the western Arctic, while an excessiveice cover is predicted for theeastern Arctic. Ice concentration followsthis trend. Errors in location arelarge in the western Arctic in summer andare particularly pronounced in parts ofthe Beaufort, Chukchi and Eas t SiberianSeas.4. The study pointed out that themagnitude of the concentration and edgelocation errors is generally closely relatedto the number of days since themodel has been updated. This is especiallycritical in the western Arctic insummer and the eastern Arctic in winterwhere excessive movements of the ice edgeare predicted.5. Where large errors occurred whenvery few days had elapsed since an update,major changes between the two weeklyanalyses had occurred. A cursory investigationshowed significant discrepanciesbetween an NPOC analysis chart andthe digitized ice concentration. Theextent to which this occurs is not known,but its effect on model performance issignificant.6. The model adequately forecastsconcentrations and edge locations in manyareas. It also provides reasonableresults for these areas when many dayselapse between updates.The largest concentration and edgelocation errors occurred in the westernArctic in the Chukchi Sea region duringsummer. The Chukchi region also causedpro blems in the drif t analysis. It issuspected that currents as defined in themodel (the Hibler-Bryan current field)may be in error in this region. TheNOGAPS forcing fields also may haveproblems here. However, the largestsource of error regarding concentrationand edge location may be the oceanic heatflux in this region. It appears thatheat flux should be closely examined andchanged if the examination warrants. Asimilar examination is necessary for theeastern Arctic in terms of suppressingthe tendency of the model to advance theice edge.173

This analysis shows that the greatestimprovement in ice location and concentrationcan be obtained with timely,accurate updates. Although the modeldoes extremely well in some locationswi thout frequent updates, the errorsaccumulating in the ice margin regionscan be substantially mitigated by frequentupdates. It is suggested thatmodel performance be closely monitoredduring the winter period to properlyassess performance, particularly in theeastern Arctic.Skiles, F.L. 1968. Empirical and driftof sea ice. Arctic Drifting Stations,The Arctic Institute of North America.Thorndike, A.S. and Colony, R. 1982. Seaice motion in response to geostrophicwinds. J. Geophys. Res., 87 (C8), 5845-5852.ReferencesColony, R. 1977. Estimating the deformationof sea ice. In "Proceedings ofPOAC-77," Memorial University of Newfoundland,St. Johns, 506-515.Colony, R. and Munoz, E.A. 1985. ArcticOcean Buoy Program Report: Data Report 1January 1983 - 31 December 1983. PolarScience Center, Applied Physics Laboratory,University of Washington.Colony, R. and Thorndike, A.S. 1984. Anestimate of the mean field of sea icemotion. J. Geophys. Res., 89 (C6),10623-10629.Hibler, W.D. III 1979. A dynamic-thermodynamicsea ice model. J. of Phys.Ocean. 9, p. 815-846.Hibler, W.D. and Bryan, K. 1987. A diagnosticice ocean model. J. Phys. Ocean.,in press.Kozo, T.L. and Robe, R.Q. 1986. Modelingwinds and open-water buoy drifts alongthe eastern Beaufort Sea Coast, includingthe effects of the Brooks Range. J.Geophys. Res., ~(C11), 13011-13032.Preller, R.H. 1985. The NORDA/FNOC PolarIce Prediction System (PIPS) - Arctic: Atechnical description. NORDAReport 108,Naval Ocean Research and DevelopmentActivity, NSTC, MS, 60 p.Pritchard, R.S., Coon, M.D. and McPhee,M.G. 1987. Forecasting summer ice conditionsin the Beaufort Sea. In "Proceedingsof Sixth International OffshoreMechanics and Arctic Engineering Symposium,"Houston, TX, March 1-6, 1987,371-377.174

A THREE-LEVEL DYNAMIC THERMODYNAMIC SEA ICE MODELLu Qian-mingTianjin University, Tianjin, CHINAAsger KejErland B. RasmussenDanish Hydraulic Institute, H¢rsholm, DENMARKAbstractA three-level (thick ice, thin iceand open water) dynamic thermodynamicsea ice model is presented. It is usedto simulate the sea ice process in theEast Greenland area. The simulated resultsfrom the three-level model arecompared with that from the two-levelmodel and actual data from remote sensing.It is shown that the three-levelmodel is more sensitive to geophysicalforcing than the two-level model.Introductionsince a two-level dynamic thermodynamicsea ice model was formulated byHibler (1979), it has successfully beenused for the simulation of sea ice circulationand thickness variation in polarregions. In the two-level approach,the thickness distribution is dividedinto two categories: thick ice and thinice_ This is adequate in polar regions,but not in subpolar regions, especiallynot in the East Greenland area.This is a reviewed and edited version of a paperpreSented at the Ninth International Conference on Portand Ocean Engineering Under Arctic Conditions, Fairbanks,Alaska, USA, August 17-22, 1987. © TheGeophysical Institute, University of Alaska, 1987.In the East Greenland area, theocean surface is only partially coveredby thick or thin ice. From remote sensingit is known that the areas ofthick ice, thin ice and open water inthe East Greenland area change quicklywith the complex geophysical conditions.The thick ice comes from theArctic regions through the Fram Strait,whereas the thin ice is formed byfreezing the water or melting the thickice. Both thick and thin ices driftsouthwards along the East Greenlandcoast. In order to obtain a moreaccurate simulated result for the area,a three-level dynamic thermodynamic icemodel is developed based on thetwo-level model. Thick ice, thin iceand open water are tracked in thethree-level model. The purpose of thepresent study is to show that thethree-level model gives a betterdescription of the dynamics andthermodynamics of the ice cover in theEast Greenland area compared with thetwo-level model.In this paper the three-level dynamicthermodynamic sea ice model ispresented and it is used in the simulationof the sea ice conditions in theEast Greenland area. The results from175

the simulations are compared with theresults obtained using a two-level modeland actual data obtained from remotesensing.Description of the Three-level Sea IceModelAs in the two-level model, themodel described here also consists ofdynamic and thermodynamic parts. Thedynamic and thermodynamic simulationsare coupled through the continuityequations. The characteristics of themodel are that the thick ice and thethin ice are tracked simultaneously andthe different surface heat budget equationsfor the thick ice, the thin iceand open water are used in the thermodynamicsimulation.Relations between thick and thin iceIce thickness, compactness andveloci ty are the three main variablesin the simulation. So, the relationsbetween the thick ice and the thin iceper unit area should be described bythe main variables as follows:+ + +U U Utn tk(1)Dynamic partEquation (1) indicates that thedynamic part of the three-level modelis the same as that of the two-levelmodel (Lu and Larsen, 1986). The mainequations of the dynamic part are shownas follows:+DUm DtThe momentum balance:+ + +- mfk x U + AT a+- mg grad H + F++ ATW(5)+where f is the Coriolis parameter, k isa unit vector normal to the surface,H is the sea surface dynamic height 4g is+ the ac!=e1eration of gravity, T aand T ~re the air and water stressterms~ F is the force due to variationin internal ice stress and m is the icemass per unit. area.where PI is the ice density.The constitutive relation:(6)A tn + Atk A (2)h tn + htk h (3)where the subscripts "tn" and "tk" re­~resent the thin ice and the thick ice,U is the ice velocity, A is the icecompactness, restricted to the interval{O,l}, and h is the mean ice thicknessper unit area.h tn= h• Atn tn(4)where h k and ht: are the thicknessesof thicit ice andnthin ice, respectively.The basic idea of the three-levelmodel is given by Equations (1), (2),(3), and (4). From that, the dynamicpart, the thermodynamic part and continuityequations can be formulated.where a., is' a two-dimensional stresstensor, l.t" is a strain rate tensor,0" is Kr~Jecker delta, P is a pressuret~tm representing ice strength and nand ~ are the nonlinear shear and bultviscosities, respectively.The relation between ice strengthand ice thickness and compactness:P = P*h exp {-C(l-A)} (8)where p* and C are empirical constants.Thermodynamic partIn order to obtain the surfacetemperature field, the surface heatflux balances of the three cases areconsidered in the thermodynamic simulation,Le. open water, thin ice andthick ice.176

For an open water surface, theheat flux balance equation readsQtn=E IL w+(1-0.4I O) (l-a I)Sw+D V (T -T f )+0 V (q -q )1 was c 2 was4-EloTsfc+(KI/htn) (Tmix-Tsfc)(12)+ D 2V w(qa-qs) + F -E oT 4 (9)w w sfcwhere E is the long-wave emissivity,Lw is t~e incoming long-wave radiation,S is the short-wave radiation, V ist~e wind speed at a height 10 m ~ovethe sea surface, F is the oceanic heatflux, 0 is Stefan-~oltzmann's constant,Ta and Tsfc are the temperatures of airand surrace, respectively. D and Dare the bulk sensible and latent heattransfer coefficients, Q is the summedheat flux and I is the ~nternal energyof the mixed layer per unit horizontalarea and it readsI = d . CT.ml.X v ml.X(10)where d. is the mixed layer depth, Cl.S.theml.X vvo~umetric heat capacity of waterand T. is the temperature of themixed lay~l; 0.0, the surfacetemperatur~ is restr~cted to the interval{271.2 K, 273.05 K}. If the temperatureis higher than 273.05 0 K, whichis the melting point of sea ice, a portionof the ice should be mel ted andthe su~face temperature is set equal to273.05 K. If the temperature is below271.2 o K, which is the freezing point ofsea ice, a portion of the water is assumedto freeze and the surface temperatureis set to 271. 2 o K. The growthrates flO), f(h k) and f(h ), are calculatedby usitg the foltrowing equations.f1 (h tk) + f 2(h tk)f1 (h tn) + f 2(h tn)(13)(14)(15)where Q Iis the heat of fusion of i'ce,f1 (h tk) and f1 (h tn) are the growth rateson the top surface of the ice andf 2(h tk) and f 2(h tn) are the growth rateson the bottom surface of the ice.They are calculated fromf1 (h tk)f1 (h tn)f 2(h tk)f 2(h tn)(16)-{FW+(KI/h tk) (T f -T . )}/Qs c ml.X I-{Fw+(KI/h t) (T f -T . )}/Qn s c ml.X IAccording to the type of the icecover in every computational grid cell,Figure 1 shows a flow chart for the useof the thermodynamic model in the program.Continuity equations!or the mean ice thicknesses, htkand h , and compactnesses, Atk anaAtn' tt~ following continuity equationsare used,177

e(a) open water, thin ice cover andthick ice cover;(b) open water and thin ice cover;(c) open water and thick ice cover;(d) thin ice cover and thick ice cover;(e) only open water;(f) only thin ice cover;(g) only thick ice cover.Figure 1. Flow chart for the use of thethermodynamic model.However, it is sufficient to consideronly cases (a) and (c), becausethe other cases are included in cases(a) and (c), due to the calculations ofthe growth rates as can be seen fromthe flow chart given in Fig. 1. Cases(b), (d), (e) and (f) are covered bycase (a), and case (g) is covered bycase (c).Oh o (uh ) o (Vh )tk tk tk--+ + ox oyatShtk+ diffusion (17)For case (a)(21)OAn o (UA ) 3 (VA )tk tk--at ++ox 3ySAtk + diffusion (18)f (0) (I-A) Ih tnf (0) (I-A) Ih tn+f(htn)Atn/htnoh o (Uh ) O(Vh )tn tn tn--at ++Ox oy(23)Shtn+ diffusion (19)of(h tk)>0oA o (UA ) o (VA )tn tn tn--at ++Ox oyS +Atndiffusion (20)if f(O)

If f(O»O, the sink or sourceterms are calculated by using Equations(21) - (24) •In the calculation of the sinksand the sources, some main concepts areused. Firstly, the open water can onlyform thin ice when it freezes, but itcan melt both thick ice and thin icewhen the surface temperature of 5heopen water is higher than 273.05 K.Secondly, the compactnesses of thethick ice and the thin ice can not beincreased through ice growth; only thethickness can be increased by icegrowth. Finally, in the area whereO

Grid Spacings (30000 meter)0 1 2 3 4 5 6 7 8 99 9..::-8., CcIIpactness of 8..::-.,~7 whole ice 7i:E8 6 680 03,5 53,~4c:4~c:]3 3°gc. a.~2 2

Table 2.The simulated results with the melting conditions.Condition Three level model Two level modelareaareaA Atk (10 8 m 2 rate)(10 8 m 2 rate)-0.9 0.0 213.431 -0.561 244.67 -0.-1970.9 0.1 192.961 -0.602 278.027 -0.4280.9 0.25 182.872 -0.624 300.774 -0.3810.9 0.4 198.456 -0.593 312.335 -0.3570.9 0.5 223.637 -0.539 317.332 -0.347,0.9 0.6 248.895 -0.488 312.097 -0.3390.9 0.7 274.192 -0.436 324.037 -0.3330.9 0.8 299.528 -0.384 326.397 -0.3280.9 0.9 328.331 -0.324 328.331 -0.324!10 8 2 IIni tl al ice covered area : 486.0 x rn iarea : the lce covered area after the slmu1atlon of 336 hct:.rs,rate : (area - Inltial area) / InitIal area.IITable 3.The simulated results with the freezing conditions.Condition Three level model Two level modelareaA Atk (10 8 m 2 )ratearea(10 8 m 2 )rate0.5 0.0 540.697 1. 003 490.273 0.8160.5 0.1 545.782 1.021 490.273 0.8160.5 0.2 550.319 1. 0"38 490.273 0.8160.5 0.3 552.541 1. 046 490.273 0.8160.5 0.4 552.600 1.047 490.273 0.8160.5 0.5 557.735 1. 066 490.273 0.816Initial 270 10 8 2area : x m ,area and rate are the same as the definition in table 2.181

calculated using the thin ice compactnesscan not be melted in a fixed periodin the same melting conditions.That is of the reason why there is asection S in Fig. 3.All the results demonstrate thatthe three-level model is more sensitiveto the geophysical forcing than thetwo-level one, both under melting andfreezing conditions.Simulations in the East Greenland areaIn order to examine the validityof the new three-level model, simulationsusing actual geophysical forcingin the East Greenland area have beencarried out. The domain of the simulationis shown in Figure 4. The simulatingperiod is from February 25, 1979to March 11, 1979. The grid scale is 30km in each direction and the time-stepis 6 hours.According to previous findings (Luand Larsen, 1986), there must be a verylarge oceanic heat flux in the EastGreenland area or the thickness of icecover is very thin in some areas. It isstill an open question whether the largeoceanic heat flux is due to upwellingon the continental shelf slope ordue to the warm surface current fromthe Atlantic. Also we still do not knowthe exact magnitude of the oceanic heatflux and exact distribution of the initialice thickness field. A cons~~t ofthe oceanic heat flux, 150 WIn , isfirst adopted in the comparing simulations.Then we make a test with anoceanic heat flux distribution whichwe believe is more realistic than theuniform distribution used in the comparedsimulations. The other thermodynamicparameters used in the simulationsare the same as those used in thetest simulations (Table 1). The dynamicparameters are shown in Table 4.Table 4. The dynamic parameters used inthe simulation.3 -3C 0.0025 Pr=0.9xlO KgM e 25 0aC 0.0055 P =1.3 KgM- 3 hI 2.8 mwa3 -2C 20.0 p*=27.5xlO NM h2 0.8 m46W;----"'''"c-c---Figure 4. The domain of the simulation.Open boundaries are shown with a dashedline.The ini tial compactness fieldsobtained from remote sensing and theinitial ice thickness fields are estimatedusing Equation (27). The Coriolisparameter is calculated by using actuallatitude. The daily wind field calculatedusing the measured atmospherepressure field and daily measured airtemperature field during the period ofthe simulation are used as actual mechanicaland physical parameters, whilea stagnant ocean is used and the depthand the initial temperature of the oceanicmixed layer are assumed to be constants,60 m and 273.05 0 K, respectively.e 2 4> = 25 0The compactness, thickness andvelocity results from a full model simulationare shown in Figure 5 - Figure7. We make the following comments onthe results:(1) The simulated ice velocity fieldsfrom the three-level model do not lookmuch different from the results obtainedwith the two-level model. (Thesimulated results with the two-levelmodel mentioned in the following arefrom Lu and Larsen, 1986). The reasonis that the dynamic part in the threelevelmodel, which dominates the icevelocity, does not differ from the two­-level one. Even though the ice thicknessis the sum of the thick and thin182

ice thicknesses, it has only littleinfluence on the ice velocity field.(2) It is noted that the calculatedice thickness field strongly depends onthe initial ice thickness field. However,the present simulated thicknessfields appear more reasonable thanthose of the two-level model. Firstly,the simulated ice thickness around thenorthern open boundary between Greenlandand Spitsbergen is much thickerthan other places throughout the simulatedperiod. This is reasonable becausethe thick polar ice enters theEast Greenland area through this boundary.Secondly, after 14 days simulationwith the three-level model, thereis still some thin ice around Spi tsbergenand southern Greenland . This isnot seen in the two-level model results.Mar. 5 , 1979(3) From the compactness fields simulatedusing the three-level model, itis clear that the results in area Bandarea E are much better than that fromthe two-level model. After 14 days simulation,the simulated compactnessesaround Spitsbergen and the southernGreenland are about 0.3, but they wereabout zero when the same period was simulatedwith the two-level model.Even though the results from thethree-level model have improved as comparedwith those from the two-level model,it is difficult to improve the simulatedresults further. This is due toa poor understanding of the initial icethickness field and the oceanic heatflux field.Based on the previous simulations(Lu, 1986; Lu and Larsen, 1986), it isclear that a change of the oceanic heatflux field, or initial ice thicknessfield or both fields can change the simulatedcompactness field, and the appropriatereadjustment of the twofields may make the simulated compactnessfield more close to the compactnessfield obtained from remote sensing.Mar. 11, 1979Figure 5. The ice velocity fields simulatedusing the three-level model. Thedashed lines are the actual compactnesscontour of 0.1 obtained by remote sensing.183

'"Grid Spooings (~OOOQ rrn!t~r)~ \ 0 ,OJf,lP~~':>.p ~OJ757065oo~~j",I);,," - "' ... .ui 55 ~~OO-4-58 ~'Oc ~ .,. n~35 CI:sol25 Q.20 CIl'C15 :5105;-i-I--r.....fo""T"''''''''''-of 0Mar. 5 , 1 979Grid Spooings (~OOOQ m~t~r)\o,":l .p ~":l757065~"""""'+""''I'''"'I'"'f 0Q 'O "O "f)""fJ'I-~·;P.p ~~G rid Sped n9~ (30000 meb,,)Mar . 11, 1 979oo~55 ~~o 0458 §!§.,. 'Oc ~8.~35 .p ~":l757065oo~55 ~~o 04-58~ not calcu­~ lated area0 . 9 -1.00 . 7-0 . 90 . 5 -0 . 74-0 B, :§:35 .p I.":l75~ not calcu­70 ~ lated area65oo~ 0 .9- 1. 055 ~0 . 7-0 . 900 0~"'-+-~r-"I""T-f 0Q '0,,0 "f)""fJ 'I-~'f>'.p ~ ~G rid Sped n9~ (30000 mete r)Mar. 11, 1 9794-58 0 . 5-0 . 7c408. ~35 .. ~ 0 . 3 -0 . 5:50~25 ~20 ~15 :51050 . 1 - 0 . 3open waterlandFi gure 7 . The i ce compactness fieldssimulated using the three-level model.The das hed lines are the actual compactnesscontour of 0 . 1 obtained byremote sensing.184

Only the oceanic heat flux fieldis adjusted in this test simulation(Figure 8). The other dynamic and thermodynamicparameters and initial conditionsare the same as in the full modelsimulation described above.Gdd SpQCing9 (~OOOO mlilter)Q

ConclusionA three-level dynamic thermodynamicice model has been completed . Themain idea of the model is that the oceansurface in subpolar regions can becomposed of three parts, a thick icecovered area, a thin ice covered areaand an open water area. In the simulation,the thick ice, the thin ice andopen water are tracked simultaneously.Therefore, the sensitivity of thin iceto the physical and mechanical conditionsis incorporated in the threelevelmodel and, accordingly, thethree-level ice model is more sensitiveto geophysical forcing than the twolevelmodel.It is necessary to make more testruns and improve the model further, butfrom the results obtained so far it isconcluded that at least three levelsare needed in an ice model of subpolarregions.AcknowledgementsThe work is supported by theDanish Technical Research Councilthrough Grant No. 5.17.7.6.02.ReferencesHibler, W.D. III 1979. A dynamic thermodynamicice model. J. Physical Oceanography,9, 815-846 .Lu, Q. -M. 1986. Ice cover hindcasts forthe East Greenland area - A PreliminaryStudy . Danish Hydraulic Institute, EGC(East Greenland Current) Internal ReportNo. 23.Lu, Q.-M. and Larsen, J. 1986. Ice coverhindcasts for the East Greenlandarea A Further Study. DanishHydraulic Institute, EGC (East GreenlandCurrent) Internal Report No. 25 .Lu, Q. -M. 1987. A three-level dynamicthermodynamic sea ice model for theEast Greenland area. Danish HydraulicInstitute EGC (East Greenland Current)Internal Report No. 33.186

GLACIAL EUSTACY VS. LEVEL RISE:ITS EFFECTS ON SHORE STABILITY IN THE ARCTICPer BruunConsultant, Hilton Head Island, South Carolina, USAAbstractThis paper reviews relative sealand(RSL) movements for Arctic Areas inAmerica and Europe. It explainsmovements that occurred or are occurringas a result of combined glacial eustacyand a general sea level rise. In somecases consolidation of softer materialsin the ground has taken place or istaking place, increasing the RSLmovements.In general arctic shores arenot exposed to erosion by sea levelrise, unless they are built up on softerconsolidating materials.IntroductionChanges in sea level have beenobserved over a long period of time.Large fluctuations have occurred betweenperiods of glaciation and periods ofinterglaciation. Our concern today isnot the large (more than 100 meters)fluctuations which took place during thelast half million years, but the recent,or last 100 years, oscillations of sealevel. The only way in which we areable to accurately observe the relativeThis is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987. © The Geophysical Institute,University of Alaska, 1987.movements of land and sea is byinstrumented tide level recordings. Thefollowing is from a review by Pirazzolli(1986): "The establishment of the firsttide marks is fairly old, 1682, inAmsterdam (van Veen, 1954), 1704 inStockholm (Marner, 1979), 1807 in Brest,1811 in Swinoujcie, 1825 in Venice, etc.From the second half of the 19th centurymore than one hundred tide-gaugestations were established in Europe(especially along the Baltic coasts), inthe USA, and in a few harbours in othercontinents.Early measurements were made byreading the level of the water on agraduated staff, once or several timeseach day. However, continuous recordingdevices have replaced man: ink penscoupled to a float and a clockworkmechanism and, more recently, automaticrecords directly compatible with acomputer. Very few gauges are sited onthe open coast where a record would bemost desirable for scientific work.Generally the gauge is associated withthe operation of a major port andconsequently is often to be found in ariver or estuary. In such a locationwater density may change significantlyand sediment compaction makes the tidegauge station subside, thus alteringlong-term trends."187

Tide gauges measure local,instantaneous level of the watersurface. Over given periods of time,RSL variations can appear periodic(tides), random (meteorological andhydrological effects) or monotonic(neotectonics, eustacy). If periods,short-lived and random components areremoved by filtering processes, and ifthe series investigated is long enough,long-term trends will appear.The first comparative studies onlong-term trends at several tide gaugestations were carried out inFennoscandinavia and showed differentialuplift movements in this area. In 1941Gutenberg was the first to publish anestimation of the eustatic variations inthe world, based on data recorded at 69tide-gauge stations from 1807 to 1937.Many records analyzed by him, however,covered less than two or three decades.According to Gutenberg, the sealevel is rising at an average rate ofabout 1 mm/year and this rise ispresumed to be eustatic in origin.However, the 69 stations investigated byGutenberg were selected outside areas ofuplift movements (Fennoscandinavia,Northern Canada, Alaska), but theyincluded areas which were considered byGutenberg to be sinking (Atlantic coastsof the USA) or even rapidly sinking(Galveston). As a matter of fact,nearly one half of the 69 stations arelocated along European and Americancoasts which are known to be sinkingglacio-isostatically (Newman et al~1971; Walcott, 1972; Pirazzolli, 1977~Clark et al~ 1978).More recent investigations (Polli,1952; Cailleux, 1952; Munk and Revelle,1952; Valentin, 1954; Fairbridge andKrebs, 1978) have remained stronglyinfluenced by Gutenberg's choices andmethods and have consequently obtainedsimilar results: a "global" sea levelrise between 1 and 1.5 mm/year. Thisrise has often been ascribed to themelting of glaciers. More recentlyEtkins and Epstein (1982) and Gornitz etal (1982) have considered the thermalexpansion of the upper layers of theoceans, these phenomena beingtentatively related to an increase inthe mean surface air temperature(although this has been decreasing sincethe 1940's in the Northern Hemisphere),(Yamamoto and Hoshiai, 1980) and to agreenhouse effect which would be causedby increasing CO 2in the atmosphere.The global work by Pirazzolli(1986) "was based on records of 1,178stations provided by the PermanentService for Mean Sea Level (PSMSL) orfound in the literature" (see somereferences in Table 1). "The locationof 743 operational stations is given byUNESCO (1983). For compendiums ofinformation on available measurements ofthe sea level of the world oceans, seeLutjeharms (1980) and Lutjeharms andAlheit (1982, 1983).At the first stage, the dataavailable for each station was groupedin 5-year periods and, for each period,the 5-year average RSL was calculatedand assigned to the middle year. The 5-year values were then used to plot agraph for each station. Severalexamples of these graphs are given inPirazzolli (1986, Figs. 1-9).Sea Level/Land Level Movements InNorthern and Arctic AreasNorthern EuropeReference is made to Table 1,extracted from Pirazzolli (1986). "Longseries of tide-gauge records arefrequent in Fennoscandia, especially inthe Baltic area, where uplift movements(RSL drops) increase towards the north,reaching a maximum of about 9 mm/year inthe Gulf of Bothnia. On the southBaltic coasts, some trends show relativestability, others that a slightsubsidence is occurring.Localized large oscillations can beobserved at some stations. In Riga, forexample, a steep, 25-year long RSL risehas been followed by a RSL drop of thesame amplitude during the next 20 years.This case illustrates the misinterpretationsthat may result from usingRSL trends deduced from insufficientlylong data series. Another example isgiven for Narvik, where the existence ofsubsidence trend of 2.5 mm/year wasassumed by Gutenberg (1941), based onrecords only 10 years long; over alonger period (46 years) the trend188

ecomes a more realistic uplift ofabout 3 mm/year. A comprehensiveinvestigation of geologic Fennoscandianuplift has been published by Marner(1979).American North Atlantic, CanadaTable 2 extracted from Pirazzolli(1986) gives data from the northernNorth Atlantic.North Pacific and ArcticTable 3, extracted from Pirazzolli(1986) gives data from the CanadianPacific and Alaska.Global Results of Studies on RelativeSea Level RiseThe results are summarized byPirazzolli (1986) as follows:"The values of the secular lineartrend obtained at 229 stations are veryscattered. A histogram of the trendsfor the data, referred to a standarddatum (RLR) , scarcely resembles atypical Gaussian distribution. Only ifthe 86 series not referred to RLR (60%of which are located in Europe) areincluded in the sample does thehistogram show a peak between 1.0 and1.5 mm/year. Even in this case,however, the range of variation is wideenough to prevent any determination ofthe global eustatic factor with goodaccuracy. Only 13% of the stationsindicate a RSL rise between 1.0 and 1.5mm/year (these values correspond to the"eustatic" rise claimed by most authors)and 22.5% between 1.0 and 2.0 mm/year.On the other hand, 28.5% of the stationsshow a RSL rise greater than 2.0mm/year, 20.5% a rise between 0.1 andl.0 mm/year, and 28.5% a drop in RSL."Comparison of Relative Sea Level/LandMovements in the Arctic Including theBaltic, N.E. Atlantic and N. PacificFennoscandinavia (Table 1)All trends are negative (uplift).This undoubtedly is mainly a result ofglacial eustacy as no important tectonicmovements are found to occur.Baltic, Including Denmark and Poland(Table 1)All areas, apart from Esbjerg onthe S.W. coast of the Jutland peninsulawere glaciated during the last glacialage. Some areas in the Baltic andPoland were only glaciated for arelatively short period of time. RSLmovements are slightly positive. Thetwo northernmost stations, Vyborg inFinland (60" -42 0 ) and Hirtshals inDenmark (57"-36"), which probably werecovered by ice for a longer period thanall other areas in the Baltic and inDenmark, show a slight negative orrelative land rise. In other words, therise in sea level balances the glacialrebound. As the glacial reboundundoubtedly is decreasing while sealevel rise may be increasing the O-linemost likely is going to move northward.The American North Atlantic,(Table 2)CanadaMost stations in Canada's N.E.provinces show a relative rise of sealevel of a similar magnitude as the riseon the eastern seaboard of the UnitedStates, but the northernmost stations(Quebec and Pointe-au Pere) have landrise. The glacial rebound still is thestrongest. Most stations around theGulf of St. Lawrence show a rise of sealevel, but results vary greatly.In Greenland results are peculiarand probably not reliable. With thewithdrawal of glaciers from the coastalareas one should expect an uplift. Aslight warming period of the sea, asrecorded for some recent decades, butnow subsiding, would work in theopposite direction. But only surfacewaters warmed up. Records from thecapital of Greenland for the last 30years show a rise in sea level of about3 mm/year, or the same as in Reykjavik,Iceland.The North Pacific Including(Table 3)AlaskaThere are large variances in theresults from slightly positive in thesouthern part to negative (glacialeustacy) in the northern part. These189

Table 1 FENNO-SCANDINAVIA Long-term trend, RSI (Pirazzo1i, 1986)Long· Term Trend. of Changes in RSl.LocalityNorwayNarvikBergenStavangerOsloSwedenStromstadSmogenGOteborgVarbergKlagshamnYstadKung..l0lmsfortOIands Norra UddeMem"LandsortNedre SOdertiljeStockholmNedre Stockholm"BjornNedre GavleDraghillanRatanFuruogrundFinlandKemiOuluRaaheYkspihlajaJakobstadVaasaRannskitKaskaMintyluotoLyakkiLypyrttiTurkuLemstromDegerbyUtaJungfrusundRussswHangoHelsinkiSoder.kitHaminaLatitude68°26' N60°24' N58°58' N59°54' N58°57' N58°22' N57"43' N57"06' N55°31' N55°25' N55°06' N57"22' N58°29' N58°45' N59°12' N59°19' N59°19' N60°38' N60°40' N62°20' N64°00' N64°55' N65°H' N65°02' N64°42' Ntl3°50' N63°42' N63°06' N63°04' N62°23' N61°36' N60°51' N60°36' N60°25' N60°06' N60°02' N59°47' N59"57' N59°46' N59°49' N60°09' N60°07' N60°34' NLongitude TIlDe L-TTrendt Variability: ReC17"25' E 1929·1973 (-3.0) (1.1) P05°18' E 1883-1973 -1.0 (0.4) P05°44' E 1881-1973 +0.3 (0.5) P10°45' E 1885·1973 -3.8 (0.5) p11°11' E 1900-1965 -2.4 (0.6) p11°13' E 1911·1981 -2.4 (0.4) P11°57' E 1887·1974 -2.0 (0.8) p12°13' E 1887-1981 -0.9 (0.4) p12°54' E 1930-1981 0.0 (0.8) P13°49' E 1887-1981 +0.5 (0.4) P15°35' E 1887·1981 -0.2 (0.6) P17°06' E 1923·1981 -1.8 (1.0) P16°25' E 1864-1925 -2.6 (1.6) p17"52' E 1887·1981 -3.2 (0.6) P17°37' E 1869-1965 -3.5 (0.5) P18°05' E 1889-1981 -4.2 (0.6) P18°05' E 1825-1932 -4.2 (0.7) p17"58' E 1892-1976 -6.0 (0.5) P17"10' E 1896·1965 -6.4 (0.9) P17°28' E 1898-1974 -7.7 (0.7) p20°55' E 1892-1981 -7.9 (0.7) p21°14' E 1916·1981 -9.3 (0.9) P24°33' E 1920-1976 -7.4 (1.2) P25°26' E 1889-1976 -7.3 (1.5) p24°30' E 1923·1972 -7.5 (1.1) P23°02' E 1889-1924 (-7.6) (1.1) p22°42' E 1915-1978 -8.2 (1.1) P21°34' E 1884-1977 -7.7 (0.5) P20°48' E 1867-1936 -7.5 (0.7) P21°13' E 1927-1977 -7.1 (0.7) P21°29' E 1911-1978 -6.4 (0.9) P21°11' E 1858·1936 -5.7 (0.5) P21°14' E 1858-1936 -5.2 (0.6) P22°06' E 1922-1978 -5.0 (0.8) P20°01' E 1889-1936 (-4.8) (0.8) p20°23' E 1924-1978 -5.1) (0.8) P21°22' E 1866-1936 -3.2 (0.9) p22°22' E 1858-1934 -3.2 (1.0) P22°57' E 1866-1936 -3.2 (0.9) P22°58' E 1889-1978 -3.1 (1.3) P24°58' E 1879-1977 -2.9 (0.7) P25°25' E 1866-1936 -2.5 (0.8) P27"11' E 1929-1978 -2.0 (1.3) Pt Long-term trend (mm!yr).: Variability (± mm!yr) .• Stations not reCerred to a RLR.ReC: AB. ARUR and BAS1R, 1982 (?); Ch, CAHIERRE, 1948; Cx. CAILLEUX. 1952; D. D1S:-;EY. 1954; E. EGEDAL.1954; G. Gl"TEI'Bf.RG, 1941.H. HICKS et at. 1983; p. PrRAZZOL!. (the present study); R. RoSS!TI:R. 1954; V. VALE:'>Il.'o. 1954; WS. WIGEN and STEPHENSON. 198!190

Table 1 continuedTable 1 continued.Locality Latitude Longitude Time L-TTrendt Variability*l)enmarltGedoer 54°34' N 11°58' E 1898-1969 +0.8 (0.5)Kobenhavn 55°41' N 12°36' E 1889-1969 +0.4 (1.0)Hombaek 56°06' N 12°28' E 1898-1969 +0.1 (0.6)KOnOr 55°20' N 11°08' E 1897·1969 +0.8 (0.4)Slipshavn 55°17' N 10°50' E 1896-1969 +0.8 (0.5)Fredericia 55°34' N 09°46' E 1889-1969 +1.0 (0.6)Aarhus 56°09' N 10°13' E 1888-1969 +0.6 (0.3)Frederikshavn 57"26' N 10°34' E 1894-1969 +0.3 (0.4)Hirtshals 57"36' N 09°57' E 1892-1969 -0.4 (0.7)Esbjerg 55°28' N 08°27' E 1889-1969 +1.1 (0.4)U.S.S.R. (Baltic)Vyborg 60°42' N 28°44' E 1889·1938 -1.6 (1.3)Riga Old Iron Bridge· 56°57' N 24°07' E 1872·1935 +0.1Daugavgriva (Diinamiinde) 57°03' N 24°02' E 1872·1938 +1.1Kolkasrags· 57°48' N 22°38' E 1884-1935 +1.4 (1.9)Ventpils (Windau)· 57°24' N 21°33' E 1873-1936 +1.3Liepaja (Libau) 56°32' N 20°59' E 1865-1938 +0.7Kaliningrad 54°57' N 20°13' E 1926·1980 +1.3 (1.4)Pillau· 54°38' N 19°54' E 1898·1943 (+0.5)polandGdansk (NoW)' Port)· 54°24' N 18°50' E 1886-1938 +1.0HeI-54°36' N 18°48' E 1901·1966 +1.0 (1.2)Swinoujscie (Swinemiinde)· 53°56' N 14°17' E 1811·1943 +0.5RefPPPPPPPPPPPEEPEEPERPETable 2 AMERICAN NORTH ATLANTIC Long-term trend in RSL(Piraz?,oli, 1986 and Danish Data from Greenland)Canada (Atlantic)st. JohnN.B.st. JohnN.B.HalifaxCharlottetownPointe-au-Pere (Father Point)Quebec"Harrington HarborCanada (Arctic)(Churchill JGreealandDisko·Angmagssalik"Nanortalik"45°16' N45°16' N44°60' N46°14' N48"31' N46"50' N50°30' N58°46' N65°.6 N60°.1 N66°04' W66°04' W63°35' W63°07' W68°28' W71°10' W59°29' W94°11' W37".OW45°.2 W1894-19491929·19791920·19801912-19741897·19771894-19491940-19791940-19771848-1948+1.2+3.8+3.9+2.9-0.2-0.9(0.0)(-6.6)+10.0+2.7+3.9(0.5)(0.5)(0.6)(0.6)(0.6)(1.5)uPPPPDPPCxEE*t Long·term trend (mmlyr).Variability (± mmlyr).·Stations not referred to a RLR.Ref: AB, ARUR andBASIR, 1982 (?); Ch, CAHIERRE, 1948; Cx, CAn.LEUX, 1952; D, DISNEY, 1954; E, EGEDAL, 1954; G, GUTENBERG,1941;H. HICKS et oL, 1983; P, PlRAzzou, (the present study); R, RoSSITER, 1954; V, VALENTIN, 1954; Ws, WIGEN and $n:PIlENSON, 1981191

Table 3 NORTH PACIFIC incl. ALASKA Long-term trend, RSL (Pirazzoli, 1986)LocalityLatitudeLongitude'JUn. L-TTrendt Variability* RefAlaskaYakutatSkagwayJuneauSitkaKetchikanCanada (Paciftc)Point AtkinsonVancouverVictoria59'33' N59'27' N58'18' N57'03' N55'20' N49'20' N49'lT N48'25' N139'44' W135'19' W134'25' W135'20' W131'38' W123'15' W123'07' W123'22' W1940-1980 (-4.6) H1945-1974 (-19.7) (2.9) P1936-1980 (-12.9) H1938-1980 (-2.4) H1919-1980 -0.1 H1915-1979 +1.0 WS1911-1979 -0.1 WS1910-1979 +0.5 WSTormo (Clayoquotj°Prince Rupene49'09' N54'19' N125'55' W130'20' W1910-1979 -1.8 WSWS1912-1979 +0.7t Long-term trend (mm/yr).* Variability (± mmlyr) .• Stations not referred to a RLR.Ref: AB, ARUR andBASlR, 1982 (1); Cb, CAHIERRE, 1948; eX. CAILLEUX, 1952; D, DISNEY,1954; E, EGEDAL, 1954; G, GUTENBERG,1941;H. HICKS -toL, 1983; P, PiRAzzou. (the present study); R, RoSSITER, 1954; V, VALENTIN, 1954; Ws. WIGEN and STEPHENSON, 1981OROGOrt~... '" ~z OCEAN P\. ... f£ T£CTOfoIICS•'" ~%~...~ ~~ HVDRO • ISOSTASl'EARTH· VOlUME CHANGESMIO.OCEA",,- RIOG( GROWTHSOU '\.QOR SUISlD£NCEu BASIN0~OTHfR EAIUM MQVEMEHT$VOLUME z~SEDIMENT IN·II'ILL>", ~:!lLOCAL l!tOsr ... sy"'z ri! IMTERMAI. IDADiNG ADJUSTMENT• z % 0GLACIAL EUSTASY%au CHAHGES OCEANOGRAPHICFI""" 1. E .... tic .oriabl .. (MOONE'" 19850; cf 19830).192

areas have been glaciated until fairlyrecently. Results from the Arctic Oceannorth of Alaska and Canada unfortunatelyare missing. Churchill in the CanadianArctic (Table 2) shows a relative landrise.Discussion of the ResultsGeneral - In evaluating the resultsin tables 1, 2, and 3 one should, aspointed out by Morner (1986), one shouldremember that the general term Eustacycan no longer be defined as "worldwidesimultaneous changes in sea level", butmust be redefined as "ocean levelchanges" or any "absolute sea levelchange" regardless of cause andincluding both the vertical andhorizontal changes of the geoid surfaceas well as changes of the dynamic seasurface topography. In the field, wecan only observe the "relative sea levelchanges".Figure 1 gives a general summary ofthe "eustatic" variables according tothis definition (Morner, 1985a; cf1983a). In citation:"The ocean basins change theirvolume due to a variety of crustaldynamic processes that may be includedunder the main heading of "tectonoeustasy".Ocean basin volume changesare nothing but variations of thehypsographic distribution of continentalheights and oceanic depths. Thetectono-eustatic factors are usuallyvery slow processes. The isostaticprocesses are somewhat faster, however(Morner, 1983a) . In-filling ofsediments (sedimento-eustasy) tends toreduce the ocean basin volume, whilstthe general ocean basin subsidence tendsto increase the volume.The volume of ocean water changesmore or less in balance with the Earth'sglacial volume changes; the balanceknown as "glacial eustasy". Water insediments, lakes and clouds,evaporations and juvenile water maycontribute to minor changes in the watervolume. The glacial eustatic changesmay reach fairly high rates (some 10mm/year) but are, of course, confined toperiods of significant glacialvariations. Paleoglacial volume changescan, however, neither be directlyquantified from sea level data nor fromoxygen-isotope records (~drner, 1981b,1983b).The ocean level (or water mass)distribution is the function of thegravitational and rotational potentials.The rotational ellipsoid may bedeformed by changes in the tilt axis andthe rate of rotation. More important isthe novel recognition of rapid andirregular deformations of the geoidsurface (Morner, 1976, 1980, 1981a,1983a; Newman et al. 1980, 1981) andlarge scale "gravitational drop motions"(Morner, 1981c, 1983c). All thesechanges of the geoidal surfaceconfiguration are termed "geoidaleustasy". The deformations of the geoidrelief are often very rapid (some 10-30mm/year) and must represent rapidchanges at the core/mantle interface(Korner, 1980) and/or rapid phasetransitions in the upper mantle (Morner,1984a). The "gravitational dropmotions" represent some sort of densitywaves in the upper mantle (Morner,1983c).The deviation of the dynamic seasurface from the geodetic sea surface(i.e. the geoid) is caused by variousmeteorological, hydrological andoceanographic factors (such as airpressure, temperature, salinity, majorcurrents, etc.). The deviation from thegeoid surface reaches up to 2 m forlong-wavelength profiles (Mather et aL,1979) and up to 4-5 m for shortwavelengthprofiles such as across theGulf Stream (Morner, 1983a, Figure 5.3).Furthermore, Morner (1984b, 1985b) hasrecently shown that climatic changes ona decada1 to century basis andcorresponding sea level changes arecaused by oceanic circulation changesdue to interchange of momentum betweenthe "solid" Earth and the hydrosphere.This implies that dynamic sea levelchanges are much more important thangenerally believed.In the field, we can only observethe "relative sea level changes", i.e.the end products of all factors thathave -changed the ocean level and thecrustal level with time as illustratedin Figure 2. (cf.Morner, 1976, 1980,1981a, 19831, 1983d, 1985a)."193

Changes In LevelTectono - Eustasy -'-- TectonicsGlacial Eustasy - - IsostasySEALANDGeoldal Eustasy - - Geoid DeformationDynamic Sea Surface - r- CompactionObserved Relative Changes in LevelFigure 2. Main factors controlling the changes in level of the sea andof the land (crust). and together responsible for the observablerelative changes in level (Horner. 1983d. 1985a).-/3/l.,II10I?&5"..,..J.t,...., ANNUAL RISeN£r/tVLANL)eD- • 0 dOe--;::'~fAt'ki"J.'(O.C., 4".", ... )COHPARISON BCTWeeNSt;4 UVEL ANDIANI) HOY'EN£N7SIAI VANIDUJ dllCTIC 8~&ION..s:.194

The Arctic AreasThe results clearly indicate thatglacial eustacy has the greatestinfluence on the RSL-movements. Thelater the glaciers remained in an areathe stronger is the glacial eustacy(Alaska, Northern and EasternScandinavia, Greenland). This isclearly indicated by the comparativeFig. 3.The Influence of Sea Level Changes onShore Stability in the ArcticWhere sea level is rising relativeto land the shore retreats by erosion.Shoreline recession is 100-200 times therise (Bruun, 1962, 1980, 1983, 1986,1987) •The Arctic obviously enjoys theprivilege of a sea level droppingrelative to land. It is not likely thatthe ocean bottom, in deeper waters neverloaded by ice, behaves similarly. Itprobably experiences continued siltationwhich may balance a sea level rise.Shores in the Arctic shouldgenerally remain stable, if they arefounded on surface rock. If not, likemany arctic river deltas, consolidationmay cause a drop in elevation comparedto sea level.A drop of sea level relative toland is often accompanied by theformation of longshore sand and/orgravel barriers. Such barriers arefound on some Alaskan and CanadianArctic Shores. Like other barriershores, for example the u.S. EasternSeaboard, such barriers may rest onweaker materials, mostly bay and riversilts. They will, therefore, also tendto subside slowly by consolidation.This could possibly account for a smallpart of the RSL rise on the easternseaboard of the United States.A complex, but very interestingexample of glacial eustacy, settling byconsolidation and influence of sea levelrise is described below: The SkagenSpit, the northernmost part of Jutland,the Danish mainland.Skagen, The Northernmost Tip of Jutland,DenmarkSkagen Spit is located with itstip at 57 degrees 45 minutes North (Fig.4) . The 40 km long spit has grown outslowly since the end of the last glacialperiod, about 12,000 years ago. At thattime the ice-filled senglacial seacovered the sea bottom north of the lineextending from the marginal moraines atHirtshals and Frederikshavn (Fig. 4).Today we find the Ice-Sea shorelines atelevations exceeding 60 meters in thesemoraine hills. At this line, therelative sea/land movement is now aboutzero. South of the line, movements areupward and north of it sea levelrelative to land is coming down.The development of the Skagen Spitover the period since the retreat of theglaciers is described by P. Hauerbach asfollows:"During the last glacial period icefrom the ice-covered ScandinavianPeninsula moved from N towards Scrossing the northern part of the NorthSea, the Skager Rack between Norway andDenmark (Fig. 4) which, due to the lowersea level at that time, was so shallowthat the ice rested on the sea bottom,apart from the deeper area in the middlecalled The Norwegian Trench with depthstoday of 500-700 meters. The thick icecap exerted a huge pressure on thebottom and in the direction ofpropagation of the ice as shown in Fig.4.The glacier penetrating from thedeeper areas and the mountains towardsthe north moved up about 1,000 meters in100 kilometers or with a slope of 1 in100. The tremendous pressure generatedby the ice built up the marginalmoraines now found at the base of theSkagen Spit between Frederikshavn andHirtshals (Fig. 4).Following deglaciation sea levelrose. So did the land liberated fromthe glacial loads. The upward movementwas faster than the sea level rise. TheNorth Sea finally became ice-free andleft the western and northern part ofJutland greatly exposed to wave action.The result was erosion and build-up of195

IN+58'N"ENORWAYSWEDENNOI'lh SeaBaltic+~'N"EPOLANDDDR50 .....Fi . 4Fig, 4 ICE COVER AND ICE MOVEMENTS DURING THE LATEST GLACIAL PERIOD196

Fig. 7 gives a picture ofdevelopment of the spit in timegeometry based on the geometrytheandandelevations of the ridge system as itappears today in the dunes of theeroding west coast of the Skagen Spit.The N.E. moving littoral currentbypasses the tip of the spit carryingits loads of material for deposition inthe shallow bay waters on the east-side,generating new land by glacial reboundand material movement towards the shore.Recent decades have demonstrated anerosion trend on the northernmostsection, partly a result of the largeSkagen Harbor causing lees ide erosion.The barren sands of the spit wereinitially covered by various vegetationand even trees. The climatic decline inthe 15th century caused a degradation ofthe vegetation and a strong increase insand drift by winds from west to eastacross the spit, partly obliterating theridge geometry. The major part of theSkagen Spit is now covered by beach andother grasses and by spruce planted inthe latter part of the 19th century andmaintained since then."Perhaps the most interestingresults of Hauerbach's surveys are hisresults of a steep discontinuity in thedevelopment of elevations occurringland by sediments carried north (Fig.5). Following a stabilization of sealevel the Skagen Spit started developingby littoral drifts of a large magnitudemoving north (today it is of the orderof about 1 million cubic meters peryear). This material was deposited overdepths which were shallow close to themoraines but increasing towards thenorth. Waves built up the beach ridgesystems of coarse material which we nowfind elevated, elevations decreasingnorthward as seen from Fig. 6 giving theresults of Carbon-14 dating therebyproviding information on the growth rateof the spit. The actual elevations area result of interaction of all factorsinvolved in the relative sea/land movementsincluding general eustacy andglacial rebound. The former has beensubject to fluctuations in climate, thelatter has decreased slowly sincedeglaciation.south of the Town of Skagen. Suddenlythe gradient of elevations as a functionof time drops from about zero to 1mm/year. To find an explanation forthat one may consider a long-range geosedimentologicaldevelopment.From hydrographic maps one mayobserve that the Norwegian Trenchextending towards and coming very closeto Skagen by a north-south runningsection has depths of 100 meters fairlyclose to shore. The Skagen Spitapparently grew out in very deep waterincreasing from 100 to several hundredsof meters. The sea bottom at suchdepths is composed of mainly silt andclay washed out from eroding shores andby rivers. This material consolidatesslowly when heavy loads, e.g. by a sandspit or barrier, are placed on it.The explanation for the abruptchange in the relative movements foundby Hauerbach to occur a few kilometerssouth of Skagen may be sought in thelarge scale geological structure of thespit. Only tertiary and recent marinedeposits have been found in thesubstrata below the Skagen Spit and nolimestone, as in the rest of Denm.ark.There is some evidence that the falllineseparating the granites and oldlavas of Norway, and the granites ofSweden from the Danish depositsoverlying much deeper granites, passesthrough the Skagen Spit, perhaps closeto the Town of Skagen; deeply trenched,it unites the Norwegian Trench with thefall line which runs down the Swedishwest coast and is right in the middle ofthe sound separating the SwedishProvince Skaane from the Danish IslandSeeland (upon which Copenhagen islocated). The Skagen Spit graduallymoved out from the moraines at its rootover first senglacial marine deposits,until it finally passed out in the fallarea, where the bottom was composed ofdeep marine deposits including softsilts and clays, as it is today. Theresult was that these deposits startedconsolidating under the heavy loads ofthe growing spit.One will undoubtedly be able tofind similar conditions on "other arcticshores", including Canada and Alaska.The barriers on the Eastern Seaboard of197

INSkoge RockFrecierlicsh:rmCattegat-------- littoral driftJUTlAND - DENMARK Fig. 5Fig. 5 LITTORAL DRIFT ALONG THE COAST OF JUTLAND198

INSkoge RockCattegat.!----~---..,.~k,.Port of the Skogen SpitFig.6Fig. 6 CARBON-14 DATING OF OLD STRANDLINES ON THE SKAGEN SPIT""The Skogen Spit Fig. 7Fig. 7 THE DEVELOPMENT OF THE SKAGEN SPIT199

the United States, from Florida to NewYork, are also placed on softer baydeposits.The barrier gradually washedlandward and carne to rest on thesedeposits of greatly varying thickness.This may be one of the reasons for therelatively larger relative movements ofsea and land found on these shoresmaximizing to about 4mmm sea level risein the Cape Hatteras area in NorthCarolina according to the most recentrecords.At Skagen it will require furthergeophysical and geomechanical surveys toexplain the development of elevationsduring recent decades.Conclusion(1) Contrary to the situation inthe rest of the world, the Northern andArctic shores oJ Fenno-Scandinavia andsome shores of the Canadian and AmericanArctic are rising compared to sea level.Information, however, is still lackingfor most parts of the Canadian andAmerican arctic ice-covered shores. Itis likely that the alluvial arcticshores, particularly at river deltas,are subject to consolidation movementswhich in some cases may overpower anexisting glacial rebound.(2) At this time (1987) shoreerosion resulting from a relative sealevel rise is not likely, apart fromareas of consolidation and/or heavyexposure to wave and/or current actionoccurring in the ice free period.Erosion by ice pilings or overrides isonly sporadic.(3) Movements over the offshorebottom are not known. Most likely aslow siltation takes place, possiblybalancing a slow rise of sea level.AcknowledgementDr. P. A. Pirazzoli, CNRS-INTERGEO,Paris, Paul Hauerbach, Randers, Denmark,and the Danish Meteorological Institute,Copenhagen.ReferencesArur, M.G. and Basir, F., 1982, YearlyMean Sea Level Trends Along The IndianCoast, Proceedings of the Seminar onHydrography in Exclusive Economic Zones,Demarcation and Survey of its WealthPotential, Calcutta, Hugli River SurveyService, 54-61.Barnett, T. P., 1983, Recent Changes inSea Level and Their Possible Causes,Climatic Change, 5, 15-38.Bruun, P., 1962, Sea Level Rise as aCause of Shore Erosion, Proc. ASCE,Journ. Waterway, Port and Coastal Eng.,88, 117-130.Bruun, P., 1983, The Bruun Rule,Discussion on Boundary Conditions,Proc. Bruun Symposium, Un. of RhodeIsland, Newport, R.I., InternatGeographical Union.Bruun, P., 1983, Review of Conditionsfor Uses of the Bruun Rule of Erosion,Coastal Engineering 7, 77-89.Bruun, P. and Schwartz, M., 1985,Analytical Prediction of Beach ProfileChange in Relation to a Sea Level Rise,Stuttgart, W. Germany, Zeitschrift furGeomorphologie, Bd. 36.Bruun, P., 1986a, Sedimentary Balances- Land and Sea with special Reference tothe Icelandic South Coast fromPorlakshofn to Dyrholaey, Proc. IcelandSymposium Sediment Balances, Sept. 1985,Reykjavik, Iceland. National EnergyAuthority, Grensavegur 9.Bruun, P., 1986b, Worldwide Impact ofSea Level Rise on Shorelines, Proc.Conference on Climate Changes, UNDP andEPA/US, Washington D.C., June 1986.Bruun, P., 1988, Profile Nourishment.Its Background and Economic Advantages~'Journal of Coastal Research, 4 no. 1 •Cahierre, L., 1948, Variation du Niveaude la Mer, Revue General d'Hydraulique,48, 310-315.200

Cailleux, A., 1952, Recentes Variationdu Niveau des Mers et des Terres,Bulletin Societe Geologique de France,6e serie, t.II, 135-144.Clark, J.A., Farrell, W. E., andPeltier, W. R., 1978, Global Changes inPostglacial Sea Level; a NumericalCalculation. Quaternary Research 9,265-287.Dansgaard, W., 1985, Fast EnvironmentalChanges in the North Atlantic, Proc.POAC, 1985, Narsarsuaq, Greenland.Proceedings POAC, Trondheim, N-7034,Norway.Disney, L.P., 1954, Report of theInvestigation of Sea Level along theCoasts of America, the Hawaiian andPhillippine Islands, and Japan, Assoc.d'Oceanographie Physique, PublicationScientifique, 13, 11-15.Egedal, J., 1954, Report on theInvestigation of the Secular Variationof the Sea Level on the Coast of Europe(Except the British Isles) and of NorthAfrica. Association d'OceanographiePhysique. Publication Scientific, 13,4-10.Emery, K.O., 1980, Relative Sea Levelsfrom Tide-Gauge Records, Proc.National Academy77(12), 6968-6972.of Sciences, USA,Etkins, R. and Epstein, E.S., 1982, TheRise of Global Mean Sea Level as anIndication of Climatic Change, Science,215, 287-289.Fairbridge, R.W., 1961, Eustatic Changein Sea Level, Physics and Chemistry ofthe Earth", 4, 99-185.Fairbridge, R.W. and Krebs, O.A., 1962,Sea Level, Post-Glacial Uplift, andMobility of the Earth's Interior,Bulletin Geological Society of America,52, 721-772.Hauerbach, P., In press.Coastal Research.Journal ofHicks, S.D., Debaugh, H.A. and Hickman,L.E., 1983, Sea Level Variations forthe United States 1855-1980, Rockville,MD: NOAA, Nat. Ocean Service.Hicks, S.D. and Shofnos, W., 1965, TheDetermination of Land Emergence from SeaLevel Observations in Southeast Asia,Journal of Geophysical Research, 70(14),3315-3319.Kalinin, G.P. and Klige, R.K., 1978,"Variation in the World Sea Level,World Water Balance and Water Resourcesof the Earth", UNESCO, Studies andReports in Hydrology, 25, 581-585.Lisitzin, E., 1974, Sea Level Changes,Elsevier, New York, 286 pp.Lutjeharms, J.R.E. and Alheit, M.M.,1983, Long-term Sea Level Measurements- A Global Catalogue, CSIR TechnicalReport T/Sea 8210, 99 pp.Lutjeharms, J.R.E. and Alheit, M.M.,1983, Sea Level in the World Ocean. ACatalogue of Measurements, CSIR T/Sea,8303,t.1-7.Mather, R.S., Rizos, C. and Coleman, R.,1979, Remote Sensing of Surface OceanCirculation with Sate lite Altimetry,Science, 205, 11-17.Marner, N.A., 1971, The HoloceneEustatic Sea Level Problem, Geologie enMijnbouw, 50, 699-702.Marner, N.A., 1976, Eustasy and GeoidChanges, Journal of Geology, 84, 123-152.Marner, N.A., 1979, The FennoscandianUplift and Late Cenozoic DynamicsGeological Evidence, Geojournal, 3(3),42 pp.Marner, N.A., 1980, Eustacy and GeoidChanges, Earth Rhelogy, Isostasy andEustasy, New York, Wiley, 535-553.Morner, N.A., 1981a, Eustasy,Paleoglaciation and Paleoclimatology,Geol. Rundschau, 70, 691-701.Morner, N.A., 1981b, Space Geodesy,Paleogeodesy and Paleogeophysics, Ann.Geophysiques, 37, 69-76.~6rner, N.A., 1981c, Revolution inCretaceous Sea Level Analysis, Geology,9, 344-346.201

Mdrner, N.A., 1983a, Sea Levels Megamorphology.Oxford Univ. Press., 73-91.Morner, N.A., 1983b, Illusions andProblems in Water Budget Synthesis,Variations in Global Water Budget,Dordrecht; Reidel, 419-423.Morner, N.A., 1983c, Earth'sGravitational Drop Motions, Abstracts­Lunar and Planetary Science, 14.Morner, N.A., 1983d, DifferentialHolocene Sea Level Changes Over theGlobe, Evidence for Glacial Eustasy,Geoid Eustasy and Crustal Movements,Coastal Evolution in the HoloceneSymposium (Tokyo, 1983), 93-96. Seealso Litoralia, 1(1), 83-86, 1984, andBulletin INQUA Neotectonics Commission,7, 89-92, 1984.Morner, N.A., 1984a, GeoidalTopography; Origin and TimeConsistency, Marine GeophysicalResearch, 7, 205-208.Morner, N.A., 1984b, Planetary, Solar,Atmospheric, Hydrospheric and EndogeneProcesses as Origin of Climatic Changeson the Earth, Climatic Changes on ayearly to Millennial Basis, Dordrecht;Reidel, 483-507.Morner, N.A., 1985a, Model of GlobalSea Level Changes, Sea Level Changes,Oxford; Blackwell.Morner, N.A., Short-Term PaleoclimaticChanges. Observational Data and a NovalCausation Model, Proc. A ClimaticSymposium in Honor of Prof. R.W.Fairbridge, (New York, 1984), VanNostrand Reinold.Morner, N.A., 1986, The Concept ofEustasy: A Redefinition, Journal ofCoastal Reserach, Vol. Special No.1,1986, pp. 49-51.Munk, W. and Revelle, R., 1952, SeaLevel and the Rotation of the Earth,American Journal of Science, 250, 829-833.Newman, W.S., Fairbridge, R.W. andMarch, S., 1971, Marginal Subsidence ofGlaciated Areas, United States, Balticand North Sea, Etudes sur IeQuaternaire dans Ie Monde, INQUA, Paris,795-801.Newman, W., Marcus, L., Pardi, R.,Paccione, J., and Tomaseku, 1980,Eustasy and Deformation of the Geoid.1,000-6,000 Radiocarbon Years BP, EarthRheology; Isostasy and Eustasy, NewYork, Wiley, 555-567.Newman, W., Marcus, L.F. and Pardi,R.R., 1981, Palegeodesy; LateQuaternary Geoidal Configurations asDetermined by Ancient Sea Levels, IAHSPublication 131, 263-275.Pirazzolli, P.A, 1977, Sea LevelRelative Variations in the World Duringthe Last 2,000 Years, ZeitschriftGeomorphologie, 21(3), 284-296.Pirazzolli, P.A., 1983,Venice; A WorseningCoastal Problems in theSea, Bologna, Italy,Bologna, 23-31.Flooding inPhenomenon,MediterraneanUniversity ofPirazzolli, P.A., 1986, Secular Trendsof Relative Sea Level (RSL) ChangesIndicated by Tide-Gauge Records,Journal of Coastal Research, SpecialIssue, No.1, 1986.Polli, S., 1952, Gli Attuali movimentiverticali delle coste Continentali,Ann. Geofis, 5(4), 597-602.UNESCO, 1983, Operational Sea-LevelStations, ICC, Tech. Series 23, 40 pp.Valentin, H., 1954, Die Kusten derErde, Veb. Geographisch-KartographischeAnstalt Gotha, 118 pp.Van De Plassche, 0., 1984, Causes of aLate-Atlantic Swamp-Forest Submergencein the Central Rhine/Meuse Delta,Climatic Changes on a Yearly toMillenial Basis, 205-214.Van Veen, J. ,Subsidence-Gaugesthe Netherlands,16(6), 214-219.1954, Tige-Gauges,and Flood-Stones inGeologie en Mijnbouw,Vignal, J., 1935, Le changement duNiveau moyen des Mer, Ann. Ponts etChaussees, Mem. Doc., Paris.Walcott, R.L., 1972, Past Sea Levels,Eustasy and Deformation of the Earth,Quaternary Research, 2, 1-14.202

Yamamoto, R. and Hoshiai, M., 1980,Fluctuations of the Northern HemisphereMean Surface Air Temperature DuringRecent 100 Years, Estimated by OptimumInterpolation, Journal of theMeteorological Society, Japan, 58(3),187-193.203

AN ICE AND SNOW CLIMATE INFORMATION SYSTEM (CRISP)T. AgnewAtmospheric Environment Service, Downsview, Ontario, CANADAT. W. MathewsScarborough, Ontario, CANADAAbstractCRISP Climate Research in Ice andSnow Software Package) was originallydeveloped to meet arctic climateresearch needs. However it has beenused more as an ice information systemto service requests from the engineeringand marine transportation community.The mapping, high quality graphicsoutput, and flexible statistics makethe system valuable for producingregional atlases as well. Severalapplications of the system are describedin the paper. The main effect of thesystem has been to increaseaccessibility to digital ice informationfor a wide range of users.This is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987. © The Geophysical Institute,University of Alaska, 1987.IntroductionRecent production of a globaldatabase of sea ice and a hemisphericsnow cover data base, has stimulated thedevelopment of an ice and snow climateinformation system referred to as CRISP.Although these databases were originallyproduced to study cryospheric parameterssensitive to climate variability, thedata and CRISP software system havebecome more useful as a data summarytool to service requests for sea iceinformation to meet engineering, marinetransportation and governmentregulatory needs. The high qualitygraphics and mapping capabilities of thesystem allow direct output from thesystem to go into atlas publicationswithout redrafting. The most recentpublication-is the Marine ClimatologicalAtlas for the Canadian Beaufort Sea(1987). Other applications includedesign of transportation routing throughice, and probabilistic ice climatologyin ice engineering design (Jordaan,1987).CRISP is just one of severalclimate information systems (Saulesleja,1985) which allow easy access to theCanadian Climate Centre archives. Allthe systems have interactive sessionswhich are menu driven or simple205

esponses to questions such as locationname and area. The major benefit of theCRISP system is ease of access to largeamounts of ice and snow data and highquality graph and chart summarieswithout any knowledge of computeroperations or computer programmingrequired.Data BasesThe CRISP system accesses a varietyof databases. The first is the weeklydigitized NOAA-Navy Joint Ice Centre icecharts from 1972 to 1984 for the Arcticand from 1973 to 1984 for the Antarcticregion. The gridded data are spaced atno greater than 15 nautical mileintervals on an evenly divisiblelatitude-longitude grid from 45° to 90 0 Nand 50° to 85°S. Sea ice concentrationsand ice type are coded using theproposed World MeteorologicalOrganization standard SIGRID (sea icegrid) system (Thompson, 1981). Thesecond data base is a set of regionaldata bases covering the Canadian Arctic,the East Coast, Hudson Bay, and theGreat Lakes. The period covered foreach region varies but together theregional data spans the period 1959 to1984. All ice data sets are convertedinto an internal CRISP format for moreefficient access and display of thedata.The system also accesses theNOAA/NESDIS Northern Hemispheredigitized snow cover database describedby Dewey and Heim (1981,1982) whichcovers the period 1966 to 1985. Becauseof the physical relationship between icecover and thickness to freezing andmelting degree days, a fourth dataset ofaccumulation of degree days using onlyCanadian station temperature data isavailable.CRISP FeaturesCRISP initiates an interactivesession by prompting the user to specify1) the area of analysis,2) the specific input dataset to be analyzed,3) the time span of theanalysis and,4) the nature of the analysisAny area of the world can beselected provided that the databasecovers the area.CRISP will display statistics onice concentration (maximum, mLnLmum,median, etc.) by date, frequency ofeach ice concentration by date, iceconcentration by each year by date,break-up and freeze-up dates which theuser can define, and map any of theabove statistics over an area.Graphical display of the data is akey factor in the utility of the CRISPsystem since large volumes of data canbe condensed into a simplerepresentation. Figure 1 illustrates thedisplay of mean ice concentrations bydate for 5 locations along a radial outfrom the Tuktoyaktuk Peninsula towardsthe permanent ice pack. The periodextends from May to November as shown onthe x-axis and the data is averaged from1972 to 1984. The insert in the upperright indicates the position of eachlocation in relation to the TuktoyaktukPeninsula. Each shaded graph in Figure 1indicates the fraction of ice coveredfor that area by month. Area 5 isclosest to the permanent pack whilearea 1 is closest to the coast. Asexpected, area 1 has the longest openwater season extending from mid-July tomid-September. As one approaches thepack, each area has increasing ice coverduring the July to September period. Onewould also expect a higher fraction offirst year and older ice. This isillustrated in Figure 2. As one movesfrom area 1 to area 5 there is anincreasing fraction of the total icecover composed of first year and olderice. A summary of ice conditions foreach year for a particular area can alsobe produced as illustrated in Figure 3for area 1, the Amauligak site that GulfCanada will be developing. From thedisplay, one can see that 1974 was a badice year for this area of the BeaufortSea. Figure 4 summarizes the percentfrequency of various categories of icecover for the Amauligak site. The rapidonset of 9 tenths or greater ice coverduring the ice formation season(September) is in contrast to the moregradual melting of the pack during themonths of June and July.206

AREAICE CONCENTRATIOf\JS1972 TO 1984o~0 40 3~0>N1-7.3N0 2.. ,-O~,rY.~· -69N270.5N 134W371N 1.35W471.4N 136W572N 137WM J J A SMO~~THoNFigure 1. Ice Concentration by date and area.207

AREAI CE CONCENTRATIONSOLD ICE OR OLDER1972 TO 19840 50OJ4sOlC\l1-73N0 21," p,i, " -69N~70N 133W270.5N 134W371N 135W471.4N 136W572N 137WMAMONTHFigure 2. Ice Concentration of first year and older ice bydate and area.208

ICE CONCENTRATION1972 TO 1984;:n '"'I;:n'I- 71NYEAR1972~-69NAMAULIGAK197219731973197+197+19751975197619761977197719781978197919791980198019811981198219821983198+AMONTH5NFigure 3. Ice Concentration for consecutive years for aparticular area.209

I CE CONCENTRATION1972 TO 1984!:CDnI"~nI-TNac-~-69NAMAULIGAKa01aOJa(DrJ-'"Qo .. ,>- '" .. ,5 >n "'I~""i"=" ,::­z --It''' ::-o :--anaNaaMAMONTHsFigure 4,by date.Percent Frequency of categories of ice concentrations210

The system also allows mapping andcontouring of data over a user specifiedarea using 4 possible projections;Mercator (including Transverse),Lambert, Stereographic, and Equidistant.Figure 5 is a map of the median iceconcentrations in the Cabot Straitbetween Nova Scotia and Newfoundlandfor the first week in March. The shadingpatterns used for open water, 2 and 3tenths cover, 4 to 6 tenths cover, 7 and8 tenths cover, and greater than 8tenths come from shading displayconventions described in the Manual ofIce Reporting (MANICE, 1980). Figure 6is a map of the percent occurrence ofice in the Canadian Beaufort Sea for thelast two weeks of August. In this Figurea series of user defined shadings areused to separate frequency contours.Figure 7 is a contour map of thefreeze-up dates of ice cover for HudsonBay. Freeze-up is defined as the timewhen ice cover becomes greater than5jlOth and remains that way for 14 days.MEDIAN ICE CONCENTRATION (1972-84)MAR 1 -MAR 7Similaravailable fordegree days.types of displays aresnow cover and freezingThe degree day option hasFigure 5. Median Ice Concentration. ThisFigure illustrates the shading capabilitiesof CRISP.PERCENTAGE OCCURRENCE OF ANY ICEAUG 16 - AUG 31Figure 6. Percent occurrence of any ice.211

MEDIAN FREEZE-UP DATES (>5/10 ICE COVER)size data, ice movement and moreinformation on ice type. The system hasproven to be flexible, easy to use, andhas been used by Canadian governmentdepartments, oil companies consultantsand members of the research community. 'AcknowledgmentsThe authors thankGovernment's Panel onand Development fordevelopment of the CRISPthe CanadianEnergy Researchfunding thesystem.ReferencesAgnew, T., Spicer, L., and Maxwell, J.B. (1987): Marine Climatological Atlas­Canadian Beaufort Sea, to be publishedas a CCC Report, Canadian ClimateCentre, Downsview, Canada, pp. 285Figure 7. Freeze-up Dates for Hudson Bay.been generalized so that one candetermine either the date that a certainnumber of accumulated degree day hasbeen reached or one's own threshold atwhich to start accumulating degree days.Individuals or companies may access theCRISP system directly on the EnvironmentCanada computer in Downsview,Ontario, Canada. This requires anaccount code for billing purposes and aCRISP user's manual which describes indetail all options and types of outputavailable. A CRISP run for a specificrequirement may be requested bycontacting the Canadian Climate Centrein Downsview, Ontario, Canada.ConclusionsThis interactive software systemallows sea ice and snow cover data to beeasily summarized in forms appropriateto make engineering design, planning,and policy development decisions as wellas for research. Additions to thesystem will be the introduction of floeDewey, K. F. and Heim, R. Jr. (1981):Satellite observations of variations inNorthern Hemisphere seasonal snow cover.NOAA Technical Report NESS 87, U.S.Department of Commerce, Washington,D.C., pp. 83Dewey, K. F. and Heim, R. Jr. (1982):A Digital Archive of NorthernHemisphere Snow Cover November 1966through December 1980. Bulletin ofthe American Meteorological Society,v. 105, p.1594-1597.Environment Canada (1980): Manual ofIce Reporting (MANICE), AtmosphericEnvironment Service, Canadian ClimateCentre, 80p.Environment Canada (1986): CRISP User'sManual, Version 1, Downsview, Ontario,pp. 241.Jordaan, I.J., Nessim, M.A., Gheneim,G.A. and Murray, A.M., (1987): ARational Approach to the Development ofProbabilistic Design Criteria forArctic Shipping, Proceedings 6th OMAESymposium, Vol. IV, pp. 401-406.212

Saulesleja, A., Agnew, T., Swail, V.and Mathews, T. (1985): ClimateInformation Systems for Applicationsand Research, InternationalConference on InteractiveInformation and Processing Systems forMeteorology, Oceanography andHydrology,1985.Los Angeles, January 7-11,Thompson, T. (1981):for Gridded Sea(SIGRID), prepared forProgram, WMO, pp. 27.Proposed FormatIce Informationthe World Climate213

SURFACE CIRCULATION PATIERNS IN YAKUTAT BAYGary HuffordRon ScheidtNational Weather Service, Anchorage, Alaska, USAAbstractForty-six LANDSAT images wereanalyzed to obtain surface water flowdirection and circulation patterns inYakutat Bay from suspended sedimentpatterns and floating glacial icedistribution. The imagery gives acoverage from April through November.Because of low sun angle, and lack offreshwater discharge during Decemberthrough March, winter circulation isnot discussed. Surface circulationinferred from the imagery is comparedto currents inferred from temperatureand salinity distributions obtained inOctober 1980 and April 1981. Theeffects of ~~inds and tides on surfacecirculation patterns observed in thisstudy are considered.IntroductionYakutat Bay is the only natural,protected deep-water harbor bet~veenPrince William Sound (400km north) andCross Sound (280km south) along theGulf of Alaska coast. As plansprogress for increased use of YakutatThis is a reviewed and edited version of a paperpresented at the Ninth International Conference on Portand Ocean Engineering Under Arctic Conditions, Fairbanks,Alaska, USA, August 17-22, 1987. © TheGeophysicailnstitute, University of Alaska, 1987.Bay, more information will be neededregarding oceanographic conditionsthere. As yet, there have been nosystematic studies of the oceanographyof the bay. The purpose of thisinvestigation is to describe thesurface circulation patterns in YakutatBay and consider the effects of windsand tides on these patterns.Regional SettingYakutat Bay is a large embaymentsurrounded by mountains to the northand northeast which rise rapidly over1000m (Fig. 1). On the western side,the Malaspina Glacier extends to within6km of the bay. The southeastern sideof the bay consists of a coastal plain.The bay is 46km long with an averagedepth of 90m although water depthsexceed 280m in closed depressions(I~right, 1972). The bay narrows from theentrance (3lkm wide) to the head of thebay (4km). Russel Fjord, located at thehead of the bay, is in the process ofbecoming isolated from Yakutat Bay bythe advance of the Hubbard Glacier(Reeburgh et al., 1976). Thus for thisreport, Russel Fjord will not beexamined.At the entrance to Yakutat Bay, a10-20m arcuate submarine ridge forms asill between Ocean Cape and Pt. Manby215

HUBBARDGLACIERFigure 1. Geographical map of Yakutat Bay showing depth in meters.(Fig. 1). This submarine ridge isbreached by narrow channels in twoplaces: one, near its western end thathas a maximum depth of 76m: andanother near the eastern end with amaximum depth of 32m (Hright, 1972 ).This eastern channel is used as themain route over the ridge into YakutatBay by the small draft vessels locatedat the City of Yakutat. lvater depthincreases rapidly to more than 70m oneither side of the submarine ridge.The major source of fresh waterinto the bay is numerous small streamscarrying runoff from the many glaciersin the nearby mountains. The majorityof these streams are located on thewest and north sides of the bay andthey discharge glacial sediment insufficient quantity to act as anatural tracer in the surface ~~atersof the bay from April throughNovember. In the winter, the waterdischarge is low and the suspendedsediment distribution becomes muchless obvious in the surface waters.There are no streamflow recordswithin Yakutat Bay so freshwaterinputs can only be estimated. Neglectingorographic effects of the mountains andassuming the snowfields and glaciersto be in a steady state, precipitationmeasured at the Yakutat airport weatherstation can be used to determine runoffestimates. The total annual precipitationat Yakutat is 300cm yr- l with amaximum in October (SOcm) and a minimumin June (14cm). The drainage areasurrounding Yakutat Bay is estimated tobe 3.1 x 10 3 km 2 . Applying the precipitation figure to the drainage area gives amean annual fresh~~ater input of 322216

m 3 s -1 •Tides in Yakutat Bay aresemidiurnal. The difference betweensuccessive low and high water isgenerally 1.0 - 1.4m with a meandiurnal range of 3.lm at the City ofYakutat (NOS, 1985). No systematicmeasurements of tidal currents areavailable. The u.S. Coast Pilot (NOS,1985) indicates that strong outgoingcurrents have been observed just offshoreof Pt. Manby. In addition, thecurrent east of Knight Island flowssouth on the flood tide and north on theebb tide.The importance of tides in relationto freshwater can be estimated using the3.lm range for Yakutat. The average9tidal prism is 3.19 x 10 m or about 3%of the bay volume assuming average depthof 9Om. On an annual basis, therefore,tides contribute about 200 times morewater to the bay than does runoff.Meteorological conditions in thenortheast Gulf of Alaska region aredominated by seasonal variability.During the winter, severe cyclonicstorms originate to the west along theAleutian Islands and migrate northeast\"ardwhere they tend to slow andintensify over the northeast gulf. Thecoastal waters near Yakutat are subjectedto a series of wind events \"hich,averaged over the winter season, yielda predominantly southeasterly wind.During the summer, the eastward migratinglm"s are much weaker than in thewinter; the resulting winds are lightand variable although a net easterlycomponent still exists in the summer(Brower et al., 1977).Locally the mountains north andnortheast of Yakutat Bay have extensiveglaciers. These glaciers not only havean orographic influence but also providefor katabatic flow (Reynolds et al.,1981). The magnitude of these winds isdependent on the cold-air reservoir,the local topographic focusing, and theexternal pressure field. The Yakutatairport provides the most continuouslong-term wind record near the bay. Thepredominate direction durinp, the winteris southeast while in the summer, it isfrom east-southeast. Across Yakutat Baynear Pt Manby, a one year study showedthe predominant direction of the windsto be from the north-northeast when thewind at the airport was from the southeast(Reynolds et al., 1981). this suggeststhat the mountains on the westside of the bay are steering the windsseaward. In addition, sea breezes developingby mid day and continuing intothe evening, are also known to occurover the bay during periods of strongsolar heating (Endicott, personal communication).ApproachLandsat imagery has proven to beparticularly useful for synoptic viewsof circulation processes where waterturbidity exists (Klemas et al., 1974,Burbank, 1977; Gatto, 1982). Forty-sixscenes, all HSS4 and 5 imagery with lessthan 50% cloud cover and taken between1972 to 1983, were obtained from thefiles at the Geophysical Institute,University of Alaska and the EROSoffice of the U.S. Geological Surveyin Anchorage.Flow direction and circulationpatterns of the surface waters wereinferred from the configuration ofpatterns of turbid water and, in somecases, floating glacial ice movement.The imagery gave a coverage over thebay from April through November. Lowsun angle, frequent cloud cover andlack of freshwater discharge duringDecember through March reduced thenumber of useful images. Thus wintercirculation in Yakutat Bay will not bediscussed in this paper.'·lind data from the NationalClimatic Center and tide predictionsfrom the National Ocean Survey werecompiled for the dates of the Landsatimagery so that descriptions could bemade of the influence of wind and tideon the surface circulation. Resultsfrom the imagery analyses are comparedto currents inferred from temperatureand salintiy distributions obtained inOctober 1980 and April 1981 in the bay.ResultsFlood: There were twenty-one imagesobtained during flood (defined here as217

1~3 hours before high tide) from Aprilto November. Of these images thirteenoccurred during predominantly southeastwinds, five during weak and variablewinds, and three during southwest winds.A composite circulation pattern duringflood is shown in Figure 2a. Thegeneral circulation is largely rotary,flowing northward along the easternshore and .southward along the ,,,esternshore. A northwesterly along-shorecoastal current is seen flowing intoYakutat Bay, with the most concentratedflow through the eastern channel inthe submarine ridge. The actual aerialextent of this along-shore coastalcurrent into Yakutat Bay varies with theseason. In April, the along-shore flowof the coastal current, as defined bythe turbid water in the imagery, is arelatively narrow band extending about4km offshore froM the coast. By June,this band has increased to about 7km andreaches a maximum in October of aboutllkm. In November, the width of theband rapidly decreases to about 4km andreflects the beginning of freeze-up inthe area. This apparent increase in flowof the coastal along-shore current intosummer is also seen in Yakutat Bay. InApril, this current, as defined by theturbid water, is constrained to the eastfifth of the entrance. During July toOctober, the coastal current occupiesthe eastern half of the entrance. InNovember, the coastal inflow decreasesto less than one fith of the entrancewidth.The suggested southward currenteast of Knight Island during flood 'vasobserved in the imagery for all months.The tide front moves up the eastern bayand is diverted to the west side ofKnight Island by the shoals south ofthe island. This flmv then abuts themain shoreline and part of the flowmoves east and south around the islandresulting in the apparent countercurrentobserved by the local populace.Strong southward flow near Pt.}1anby was evident at all stages of theflood; its intensity was indicated bythe shoreward packing of the turbid wateras it moved parallel to the isobaths.An anticyclonic gyre was also observedat the head of Yakutat Bay near theface of Hubbard Glacier in all theimagery obtained during flood tide.Floating pieces of glacial ice formingcurved stringers were observed in eightimages, also indicating the presence ofthe gyre. The diameter of the gyreaveraged 3. 2km.Ebb: During ebb (defined as theimagery obtained 1-3 hours before lowtide), the rotary flow pattern wasagain evident, in seventeen images. Asexpected, the southerly flow along thewestern shore was reinforced (Fig. 2b).northerly flow on the east side,although present, appeared greatlyreduced by the lack of obvioussuspended sediment patterns in thatarea. The along-shore coastal currentshows most of its flow continuingnorthwest across the mouth of the bay.The anticyclonic gyre observed atflood near the Hubbard Glacier is alsopresent during ebb (Fig. 2b) as is theweak northward current northwest ofKnight Island. In all the images forebb tide, a countercurrent was observednear Pt. Manby. This extends southeastalong the coast west of Yakutat Bay andagainst the shoreline. It then penetratesabout 18km into the bay 'vhere itbecomes entrained in the outflow of thebay further offshore.Slack: Eight satellite images wereobtained during slack tide (defined asimages obtained between 1-3 hours afterhigh or low tide). The rotary patternis still evident but much weaker (Fig2c). There is more cross-bay flow fromeast to west in the wide portion of thebay. Freshwater turbidity plumes areclearly visible at the mouths of thestreams, fanning out in 180 degree arcsinstead of streaming either up or downthe bay during flood or ebb as a verynarrow band. The countercurrent observednear Pt. Manby at ebb tide is alsopresent at slack tide with followingdifferences. At ebb, the countercurrentat its furthest penetration in the baybecomes entrained in the southerly outflowfurther offshore. During slack thecountercurrent at its northern endturns west into a small embayment andforms a small lee-side cyclonic eddybehind an unnamed point of land about 7km from the mouth of the bay.218

Figure 2a. A composite surface circulation during flood tide in Yakutat Bay.(b)-/---Figure 2b. A composite surface circulation during ebb tide in Yakutat Bay.Wind Effect: The effects of windon the surface waters were examinedusing the winds measured at YakutatBay airport, Ocean Cape, Pt. Manby andby calcualting winds from the synopticsurface pressure analysis produced bythe Anchorage, Alaska NWS forecastOffice.During northeast winds at Pt.Manby the general rotary circulationpatterns described above were observedwith some additional notes. The outflowon the west side of the bay becamemore compressed in horizontal extent inconcert with strong northeast along-shorewinds. In addition, during ebb when the219

northeasterly winds are greater than 15ms-l , this outfloH continues southwestacross the continental shelf from thebay to up to 35 km offshore.When there are south'vest winds atOcean Cape, an anticyclonic gyre occursa short distance northwest of the Capeon the ebb tide (Fig 2d). The eddy canreach a dia~Iter of 15km when the windsexceed 10ms At Imver speeds the eddyis smaller. A 9km eddy was observed "henthe winds were less than 6ms- l . The eddyis not present on the flood; instead, ameander occurs.Winds from the southwest cause theinflow on the east side of the bay tobecome compressed against the islandsand they also push turbid water throughthe numerous channels betvleen islandsall the way to the shore. The observedcountercurrent near Pt. Hanby on the ebband slack extends further up the bay byan additional 12km or so.Precipitation Effect: The surfacecirculation pattern shown in Figure 2ewas obtained from six October images, atime of high discharge from coastalrivers and streams (Royer, 1979). Royerhas measured a threefold increase intransport and speed in the along··shorecoastal current in the Gulf of Alaska.This seasonal amplification in thecurrent is due to the precipitationmaximum that occurs in the autumn.In the October satellite imagery, theincrease in the along-shore co as tal flmvappears to reach a maximum: horizontalextent of the turbid flm] along thecoast is over llkm \Vide. However, theamount of coastal \Vater flowing intoYakutat Bay does not appear to differfrom the observed flow in July throughSeptember; it occupies the eastern halfof the bay. Instead, most of the coastalwater continues across the mouth ofYakutat Bay and continues toward thenorthwest along the coast, with only asmall inshore component into the bay,even at flood tide.TemperatureuSalinity Distribution:Thirteen CTD stations were occupied inYakutat Bay on 23 October 1980 and thenrepeated on 2 April 1981. The firstcruise occurred ,,,hen maximum annual accumulationof freshwater was assumed tobe present in the coastal marine system.-',;' ~/1"­/Figure 2c. A composite surface circulation during slack tide in Yakutat Bay.220

Figure 2d. Effect of northeast winds on surface circulation in Yakutat Bay.Figure 2e. Surface circulation in Yakutat Bay during maximum river discharge in autumn.The second cruise took place in earlyspring when accumulated freshwater inthe system is considered minimal.Specific details of these cruises canbe reviewed in Huench et al. (1982).The temperature and salinityfields are presented as verticaldistributions along a transect acrossthe mouth of Yakutat Bay (figs. 3 and4). The most prominent feature is the221

cross-bay temperature and salinitygradient with colder, less salinewater on the west side of the bay. Itappears that the horizontal andvertical extent of freshwater appearsabout the same for both April and inOctober, recogn1z1ng the seasonal shiftin temperature and salinity. However thevertical profiles in April occurredduring a slack, while the October datawas collected during a low tide.Observations from satellite imageryduring slack period showed meanderingof the turbidity plumes and increasedcross-bay flow. Thus, the apparentsimilarity in freshwater distribution onthe west side of the bay is probablydue to totally different processes.The April salinity distributiondoes show a narr8'~ band of relativelyhigher salinity (31 100) water on theeast side of the bay that agrees withsatellite imagery taken during themonth of April. This coastal waterflowing into the bay increases considerablyin its horizontal surfaceextent in the October profile; inagreement with the results from theimagery data and for compilation ofrelative currents from the temperatureand salinity data. The results andconstructions indicate that somecoastal water enters the east side ofYakutat Bay during low tide. Therelative velocity of this coastalinflow is quite slow in the surfacewaters but then increases almosttwo-fold between 12m and 20m waterdepth.DiscussionThe surface general circulation,as inferred from suspended sedimentpatterns, and to some degree by thetemperature-salinity distributions, isthe same from spring through autumn.Perturbations to that general patternwere observed and attributed primarily,to the tide and to some extentthe wind. It appears that most coastalwater is added with each flood tideover the sill into Yakutat Bay, withmost of the less-dense surface waterremoved on the subsequent ebb. However,there appears to be a continuous inflowand outflow at all other times,PTMANDYOCEANCAPE~~60f< =S '" 80)1\20406080100120°c~::~:~:: ...... ,;:gif;~~.:.\:~;"."~: . ..•.... , ".' "~':';;i~j",~:j1:;100120Figure 3a. Vertical distribution of temperature (OC) across the mouth of Yakutat23 October, 1980.Bay,222

2040PTMANDYOCEANCAPE_2728 /~---n\,':;'.:,:."g::::-----:.,.2040~60=:f

PTMANBYOCEANCAPE2020.040e 60:z:f

ReferencesBrower, W.A., Jr., H.F. Diam, A.S.Prechtel, H.W. Searby and J.L. Wise,1977. Climatic atlas of the outercontinental shelf waters and coastalregions of Alaska; Vol. 1 - Gulf ofAlaska. AEIDC publication B-77,Anchorage, Alaska.567.Wright, F.F., 1972. Marine geology ofYakutat Bay, Alaska. U.S. GeologicalSurvey Professional Paper, 800-B; B9-B15.Burbank, D.C., 1977. Circulationstudies in Kachemak Bay and lower CookInlet. Environmental Studies KachemakBay and Lower Cook Inlet. Vol. IIIAlaska Department of Fish and Game,Anchorage, Alaska.Endicott, M., 1985. Personal communication,National Weather Service OfficeYakutat, Alaska.Gato, L.W., 1982. Ice distributionand winter surface circulation patterns,Kachemak Bay, Alaska. RemoteSensing of Environment. 12:421-435.Klemas, V., D. Bartlett, and W.Philpot, 1974. Coastal and esturarinestudies \vith ERTS-l and Skylab. RemoteSensing of Environment. 3:153-174.Muench, R.D., P.R. Temple, J.T. Gunn,and L.E. Hachmeister, 1982. Coastaloceanography of the northeast Gulf ofAlaska. Report to OCSEAP, RU 600January 1982. 137 pp. Unpublishedmanuscript.National Ocean Survey, 1985. UnitedStates Coast Pilot; Pacific and ArcticCoasts of Alaska: Cape Spencer toBeaufort Sea, Vo1. 9, Rockville,Maryland.National Ocean Survey, 1984. TideTables 1985. West coast of Northand South America. NOAA, Rockville,Maryland.Reeburgh, W.S., R.D. Huench, and R.T.Cooney, 1976. Oceanographic conditionsduring 1973 in Russel Fjord, Alaska.Estuarine and Coastal Harine Science. 4:129-145.Royer, T.C., 1979. On the effect ofprecipitation and runoff on the coastalcirculation in the Gulf of Alaska.Journal of Physical Oceanography. 9:555-225

SHELF BREAK UPWELLING IN THE DENMARK STRAITJohn W. FoersterU. S. Naval Academy, Annapolis, Maryland, USAAbstractInvestigations of historical oceangraphicrecords and recent infraredsatellite scans in the area betweenlongitude 24° west to 32° west andlatitude 62° north to 66° north led tothe discovery of a zone of apparentunstable water. This zone was betweenthe 200-m and 1,000-m bathymetriccontours west of Iceland in the DenmarkStrai t. The study area continues to bevery active in biological production andhas provided the majori ty of the catchfor the Icelandic whale fishery. Asurvey expedition investigated this areain June 1981. Calculations from thesurvey data revealed that water wastransported to the northwest at 2.3 x10 3 m 3 /sec with an average Ekman layerdepth of 74 m. An anomaly in thevertical Sigma-t distribution indicatedwater movement towards the surface.This physical information, results ofwater chemistry for ortho-phosphate,di s tribution of the biota and theinfrared satellite scans led to theconclusion that upwelling conditionsexist during the polar sunmer in thisarea of the Denmark Strait.This is a reviewed and edited version oj a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987.This upwelling is believed to be afunction of the particular air-seainteraction that exists at this time ofyear. The result of this interactionhas been the development of a 40,000 km2area of high biological productivity.IntroductionThe Denmark Strait separatesIceland and Greenland in the NorthAtlantic Ocean. On the eastern side ofthe Strait along the west coast ofIceland and out over the island shelf isa feeding ground for whales (Rorvik eta1.. 1976; Watkins et aI., 1981;Foerster and Thompson 1985). Why thisfeeding ground exists where it does, andthe evidence leading to describing theboundaries of the area are emphasized inthis study.Foerster and Thompson (1985) havedescribed the biology and itsdistribution in the proposed productivepatch. Patterns resulting from theiranalyses indicated that a possibleupwelling system was operating duringthe arctic sunmer in this area. Theorganisms showed patterns of response toenvironmental conditions that includedwind, water temperature, and nutrients(Platt, 1972; Steele, 1976; Foerster andThompson, 1985). A review of historical227

studies performed in the Denmark Straitis given by Foerster and Thompson(1985) .bathythermographrecorded using atemperature probe.(XBT)T-llprofile wasfine structureAfter reviewing historical recordsof physical oceanography, meteorologyand archived satellite infrared data ofthe region, a testable hYpothesisdevelops. Winds from the north andnortheast directions, responding to thenortherly summer location of theIcelandic Low Pressure Center (Rodewald,1967), results in mass transport ofwater to the northwest. This windmovement induces upwelling, creating thefeeding ground over the Iceland shelf.The mixing created by this air-seainteraction is an example of shelf-breakupwell ing.MethodsSatell He infrared scans using anAdvanced Very High Resolution Radiometer(AVHRR) on the NOAA-6 and NOAA-7 weathersatellites were examined over a threeyear period from 1979 through 1981. Thedata collected from these sources wereanalysed using a Hewlett-Packard 9020Digital Image Processing Station.Infrared images (AVHRR) were enhancedboth in black and white as well as inpseudocolor. The acceptable images werecompared to seasonal wind direction,wind speed and ground truth datacollected during cruise 270681 of theU. S. Naval Oceanographic Ship "Kane".All ground truth data werecollected during June 1981. A modifiedsaw-tooth cruise pattern was employed(Figure 1). The purpose was to cover asmuch surface area off the west coast ofIceland as possible in the allotted shiptime.Measurements of salinity andtemperature were obtained from 21vertical stations and one YO-YO station.A Neil Brown conductivity, temperature,depth (CTD) system coupled to a PDP-9computer collected the temperature,depth and salinity data. These datawere analysed to determine densitystructure as Sigma-t and temperaturedistribution. Samplings from allvertical stations were made 4 times perday (0300, 0900, 1500, 2100). Beforeeach vertical station an expendableFigure 1. Study area wi th cruise track(dashed line). Lines A and B are areasfrom which vertical analyses will bepresented.Coupled to the CTD system was arosette sampler. Water sampling wasdone at the surface and at depths of 10,25, 50 and 100 me ters. These sampleswere analysed for chlorophyll-a andortho-phosphate (Strickland and Parsons,1972).Meteorological data collections forcloud cover, wind speed and winddirection were made hourly. The windspeed was used to calculate Ekmantransport and the depth of influence(Ekman, 1905; 1923; Dietrich et al.,1980).ResultsMeteorology.Storm fronts moving into theDenmark Strait had winds backing to thenorth and in most instances intensifying.On a single occasion (5 June 1981)the winds backed to the north northwest,but any effect on the sea surface wascance lled in the next 6-hours as thewind veered to the north northeast andintensified. During this study the winddirection was predominantly from thenorth to northeast with an average speedof 9.0 m/sec (high=l1.8 m/sec, 10w=3.6m/sec).228

In Figure 2, a summary of windpatterns for the area under study isshown. It was evident during the cruiseand from the review of the archived winddata that the patterns are predominantlynortheasterly. Peaks of wind arerelated to the seasons and thesubsequent movement of the Icelandic LowPressure Center into and out of the areawest southwest of Iceland (Rodewald,1967). From the da ta records it wa sno ted tha t the wi nds became d i spe rsedmore evenly between the north to eastdirection from June to September. Afte rSeptember, as the Icelandic Low movessouthward .into the mid-North Atlantic,the winds had a more northeasterlycomponent. The wind patterns anddirectional persistence, therefore, werekey factors in this study.~ ~ ~ ¢:JANUARY FEBRUARY MAR CH APRILCRUIS[PERIODJUNE 1981~tV~MAY~~ "'Ii !''''' .., ...••.w , ..... 1 ..". '100 OI~ ( CT ' OIoo.. ". ,"'lOllJUNE~~JULY .. ," [ sc ..... [ AUGUS Td ~ ~ ~SEPTEM8ER OCTOBER NOVEMBER DECEMBERFigure 2. Wind patterns for the areaunder study off the west coast ofIceland. The center wind rose is forthe cruise period.PhYSical OceanographyIn addition to the position of th-evertical sampling stations sholyn inFigure I, two transect lines are given,along which analyses of the physicalcharacteristics of the upper 100 metersare plotted. Initially thedistributions of the water density asSigma-t and the temperature were made inthe horizontal (Figure 3). Satelliteimages from NOAA-4 (historical) andNOAA-6 (during the study) had a faintlydefinedregion of colder water west ofthe Irminger Current off the IcelandicCoa st.In Figure 4 three satellite AVHRRimages are enhanced with the digitalimaging processing equipment. Thelighter area of water identified by thewhite arrow WaS the zone of interest.From the cruise data it wasdiscovered that the surface isopycnals(Figure 3a) and the surface isotherms(Figure 3b) were deformed, with a centerat 64°N latitude, 26°101 longitude.Satellite AVHRR from 1981 at the sametime (Figure 5b-e) confirmed the shipdata. A zone of cooler water with ashoreward projection has appeared. Atthis time the winds we re predominantlyfrom the northeast (Figure 2).All stations were analyzed in thevertical for distribution of density(Figure 3c). Sigma-t was calculated foreach vert ical station at meterintervals. The resulting data wereplotted in vertical and horizontalprofiles which resulted in theaxonome t ric (3-dimensiona 1) mode 1 inFigure 3c. The densi t y laye rs we re ben tsurfaceward along the 27.50 Sigma-tla ye r. Thi s appa ren t surf acewa rdbending of the isopycnals was near theIceland shelf between 63°- 64° Nlatitude, 25°- 27° 101 longitude.To provide more information tosubstantiate the possibility of anupward movement of water in the areadepicted in the model, furtherhorizontal and vertical analyses weremade along transects A and B (Figur e 1).The vertical distributions oftemperature and density along these two229

,"~ ;'!ft 'SIGMA-T(surface)TEMPERATURE(surface)Figure 3 . Horizontal distribution ofdensity (A) and temperature (B). Athree dimensional plot of the densitydistribution is presented with theupwelling zone identified (C) (modifiedfrom Foe rster and Thompson (1985).Figure 4. AVHRR satellite photographsof western Iceland and the DenmarkStrait. A=May 3, 1979 (79123), B=August22, 1980 (80234), C=August 27, 1980(80239). egc=East Greenland Current,ic=Irminger Current, white arrow=zone ofupwelling.230

AFigure 5. AVHRR satellite photographs of the Denmark Strait andwestern Iceland. A=April 30, 1981 (81120) B=June 4, 1981 (81155)C=June 5, 1981 (81156) D=June 6, 1981 (81157) E=June 9, 1981 (81160).egc=East Greenland Current, ic=Irminger Current, white arrow=zone ofupwelling.231

transects are presented in Figure 6.Along transect A (Figure 6a and 6c) thepattern was one of upward movement ofthe isotherms and the isopycnals. Thisindica ted tha t in the reg ion a round 64 0N latitude, at the edge of the Icelandshelf, a zone of upwelling waterexisted .Along transect B (Figure 6b and 6d)the pattern was the reverse. Thereappeared to be more of a downwelling orsinking structure to the isotherms andthe isopycnals. This indicated that inthe region along the Irminger Current tothe east of the Iceland shelf there wasa zone of convergence.Developing the physIcal datafur the r, a sta bil i ty analysi s wa s made.The stabil -tty (Will iams, 1966) of theproposed center of the upwelling zone(Station 8105) is -1.95 X 10-6 at 30m.This depth corresponded to the zone inFigure 3c where the Sigma-t is deformedupward toward the surface. Stations8106, 8107 and 8108 to the north, atdepths of 20 meters or shallower, alsohad stability values that were negative.Negative values could be interpreted asthe water column having the potential tobe displaced upwards. All otherstations had positive stability values .Finally, the physical data wereexamined qualitatively using the Ekmanwater mass transport analysis. The windblew predominantly from the north tonortheast during the study. The averagepersistent wind speed was 9.0 m/sec. Itwas an offshore wind that had apotential for producing a northwesterly0TE MPERATURE C·C I...~~'~a~~25~~ ~ 7.5 Q;:~~~ ;::E"~ 1 0 __ ~'0:x:~i:'>-~~a.w ~75\0 ...6.0 6.10.6A~"6' 6. 63 62NORTH LATITUDETEMPERATURE C'CI...~~\8~"~ ~w~~"~7.5 __...~"• "E , ~ "- i!~ ,~:x:~~>-n.w "...\" " '" ,6.'B~100••50••63 .2NORTH LATITUDE~"~SIGMA-T~ '''':~m21.5gL, j l 2'~2 7.6010O-j--r--~-r--,----,-l-.L,--'--r--r--'6:5 64c •• .2NORTH LATITUDE/.E 50D65SIGMA- T••64 6'NORTH LATITUDE.2Figure 6. Temperature and density distribution on a cross sectionin the study area. A and C are data from transect A in theupwelling zone. Band D are data from transect B in the easterndownwelling zone.232

mass transport of water.The Ekman layer depth wascalculated using the empirical equationDE = 7.6W/(SINTheta)~W = wind speed in meters/ secondSIN Theta = latitudeA typical value for DE was 72.1meters when the wind speed was 9.0 m/secand the latitude was 640 N. Thecalculated DE was then placed into theEkman water mass transport equation(Dietri.ch et al., 1980 p. 319). Theaverage value obtained for the watertransported in the study area was 2.3 x10 3 m 3 / sec.ORTHO- PHOSPHATE(surface)Chemical OceanographyOrtho-phosphate and chlorophyll-awere used as chemical tracers of theactivity in the water mass . Figure 7depicts the mapping of the orthophosphate(soluble phosphorus) (7a) andthe chlorophyll-a (7b) along thehorizontal. The trend in both was acenter of higher concentration near theproposed zone of upwelling and asoutheast-to-northwest distributionalong the proposed axis of the Ekmanwater mass transport. Chlorophyll-a wasindicative of the phytoplankton biomassproduction centered in the upwellingzone and its distribution outward .Vertical analyses of the orthophosphatein c ross-section alongtransec t s A and B were made (Figures 8 aand b) . The vertical profile depictedin Figure 8a has the soluble phosphorusmoving upwards to the surfaceparticularly in the region between 64 oNto 65°N along the Iceland shelf. To theeast of the shelf near the Irmingercurrent the pattern of solublephosphorus movement is reversed (Figure8b) . In this instance the micronutrientpa t te rn appea red to indica te a zone ofdownwe 11 i ng .DiscussionIn his physical oceanography of theIrminger Sea and Denmark Strait,Dietrich (1957) plotted a horizontalsurface temperature map (Fig.3, p.284).CHLOROPHYLL a(surface)Figure 7. Surface distribution of thetracer nutrient phosphorus (A) and thephytoplankton biomass expressed asconcentration of chlorophyll-a (B)(modified from Foerster and Thompson1985).In the area analyzed in this study (64°Nby 26°W-Station 8105) Dietrich hadmapped a zone of water cooler than thesurrounding areas. There was nodiscussion of the anomaly. The AVHRRsatellite photographs (Figures 4 and 5),although not all from the same year, canbe used to reveal a pattern and depictboth Dietrich's and this study'sindication of anomalous condi tions.Dietrich's study has been the onlycomprehensive source of publishedoceanographic data for the study area.233

A., ..NORTH LATITUOEobtained between 1930 and 2130 GMT. Thefinal photograph (Figure Sf) was 16 July1981 and little structure is discerned.To confirm the contention that thestructure is \~ind-driven and a functionof the air-sea interface conditions, theJune information can be compared to thewind conditions at the time the AVHRRscanned the area:Fig. 5b-north wind at 11.3 m/secFig. 5c-north wind at 11.3 m/secFig. 5d-northeast wind at 9.5 m/secFig. 5e-northeast wind at 10.3 m/secB..NORTH LATITUDEFigure 8. Phosphorus (ortho-phosphate)distribution in cross section in thezone of upwelling (A) and in the zone ofdownwelling (B).In Figure 4a (May 3, 1979) nostructure appears to have developed.The anomaly is developed in Figures 4band 4c (area shown by the white arrow).The dates of these two AVHRR scenes areAugust 22 and August 27, 1980. In thetwo August 1980 scenes the structureappeared to intensify. Thesephotographs were indicative of ananomaly that appeared to develop intothe arctic/sub arctic SUlOOler. Itsextent appeared to be influenced by themeteorological conditions.In the Figures 5a-f are a sequenceof photographs developed from AVHRR dataon the NOAA-6 satellite. In Figure Sa(30 April 1981) a suggestion that thestructure is beginning to form wasnoted. On 4 June 1981 the zone ofcolder water is developed (Figure 5b).In Figures 5c through 5e, data collectedon 5, 6, and 9 June, respectively, aregiven. Again, as in the 1980 satellitedata, the structure appeared to increaseand decrease in intensity as themeteorological conditions in the areachanged. All sate 11 i te da ta we reThis information suggested that the seasurface temperature (SST) under studywas flexible in shape and responded tothe wind direction in several ways. Thedirection of the wind appeared to assistthe structure in terms of intensity anddistribution. The more northerly is thewind direction, the more westerly is thespread of the apparent colder water. Ifthe wind was frOID the northeast and,thus, more offshore than longshore, thespread of the colder water had a morenortherly and more confined aspect. TheAVHRR data helped estimate the movementsand spread of the anoma ly but, becauseof extensive cloud cover, composites ofinformation have to be used.The anomalous SST structure was atthe edge of the Iceland shelf. Therefore,shelf break upwelling is proposed.Upwelling at the edge of a shelf,particularly at the continental shelf,has been reported for the Canary Currentby Tomczak (1981), for the Bering SeaShelf (Goering and McRoy, 1981), overthe Continental shelf of Nova Scotia(Petrie, 1983), and in the sunnner offthe coast of Florida (Atkinson et al.,1984). It was deduced by Hart andCurrie (1960) and Bang (1971) for theBenguela system and modeled by Hill andJohnson (1974) and Hseuh and au (1975).Hidaka (1954) in his paper oncoastal upwelling discussed theinfluence of wind direction on thestrength of an upwelling zone. Heconcluded that offshore winds such asexisted in this study (northeastcomponent) would stimulate upwelling buthave a weaker intensity than longshorewinds (northerly component in this234

study). One of Hidaka I s conclusionsabout offshore wind was that the bestangle for stimulating the most offshoreupwelling was 21.5° to the coast. Theangle of the winds that are the mostpersistent during the year are between0-45 degrees true, with the northnortheastcomponent of 31.5° to theshore being the most persistent.Through use of satellite and shipdata collections, as compared torealtime and historical meteorologicalconditions it was determined that anupwelling developed just to the west ofthe Iceland shelf break (200 m). At theouter boundary of the upwelling,convergence or downwelling would occur.Equilibrium in the water column must bemaintained. The temperature, Sigma-t andortho-phosphate data depicted an eastside convergence zone at or near theedge of the Irminger Current (Figures 6and 8). The Satellite AVHRR recorded thewestern boundary between 28°W and 29°Wlongitude. These conve rgenceobservations were similar to the modelproposed by Hill and Johnson (1974, Fig.3). The water mass transport appeared tospread the effects of the upwellingalong a southeast to northwest axis.On the basis of the data analyses,a model of the proposed circulation offthe west coast of Iceland can beproposed (Figure 9). The overalleffective depth of the upwelling wasgenerally less than 100 meters. Themajor impact of the wind appeared to bein the region centered about 64° N, 26°Wand the rising water is circulated tothe northwest. Convergence occurred tothe east and west of the upwelling zonewith nutrients apparently being suppliedfrom the east and south by the IrmingerCurrent and the north Atlantic Drift(Figure 8). The total zone offrictional or horizontal effect (Dh) inHidaka (1954) is confined by the majorcurrent system of the East GreenlandCurrent to the north. Frominvestigation of the horizontal nutrientdistribution (as indicated by the orthophosphate)and the phytoplankton biomass(as indicated by chlorphyll-a) (Figure7) the zone of effect apparently reachedup to the East Greenland Current.100 0rTI~--I:J:200I;;Figure 9. Hypothetial model of theupwelling zone. The black areasrepresent Iceland and the shelf .The data suggests a relativelyshallow upwelling system. It is winddrivenand to some extent nutrient resuppliedby surrounding currents. Thefeature is persistent and is a result ofair-sea interaction.Inspection of the data in Figures5d and 5e, 6a and 8a revealed thateddies may be infusing nutrients andwarmer water into the upwellingstructure. The eddies would be releasedby the Irminger Current and the NorthAtlantic Drift.no t de te rmined.An overall effect wasSo as not to be confined to asingle idea on upwelling other resultsof oceanic dynamics were considered.These included:1. convergence created by geostrophicflow;2. convergence developed by a thermohaline circulation;3. a frontal system developed betweenthe Irminger and East GreenlandCurrents;4. quasi-permanent wind drivencyclonic circulation.~235

Geostrophic flow was believed notto exist in the study area because nosignificant dynamic topography existed.Sigma-t densities "'ere decreasingnarrowly towards the Iceland coast.Dietrich (1957, Fig . 30, p. 297) had nodynamic topography depicted in hissurvey results.Thermohaline circulation would bewater sinking across a density gradient(Defant, 1961; Dietrich et al., 1980 ).No moderate or strong gradients asdescribed by Houghton and Marra (1983)existed in the study area. However,there appeared to be a slight gradient(Figure 3c) and thus, a thermohalineci rculation stimulated by the proposedupwelling is suggested. On the east ofthe study area the north-flowing warmIrminger current is noted. To the westof the proposed upwelling zone no southwardflo\~ing colder current has beendetermined. Neither Dietrich (1957) northe Pilot Charts for this area (Anon .1982) have indicated a southerly currentcomponent.The AVHRR satellite informationshown in Figures 4b-c and 5d- e have theEast Greenland Current located at theupper edge of the photograph (heavywhite structure area-egc) . It is wellremoved from the proposed upwelling zoneand the west Iceland component of theIrminger Current. A front createdbetween the two currents is notindicated. The data from Dietrich'sstudy (1957. Fig. 3, p.284) also had nofronts for this area.Finally, quasi-permanent winddrivencirculation was considered. Inorder for this to be a reality it wouldrequire steady mono-directional, reasonably-uniformwind speeds as found in thesub-tropical Trade Wind system. In theregion under study the wind is multidirectional(Figure 2) with a persistentnortheasterly component. Therefore,development of a cyclonic wind stressable to bring about development ofhorizontal currents that would create afeature as reported in this study wasunlikely.The air-sea interaction is believedto interlock in two major ways. First,a she 1 f break upwell i ng is es tab Ii shed.Thi s upwe lling, although reasonablyshallow, slowly injects nutrients intothe surface waters. Second, the wind,acting in the role of water masstransporter, moves this "ater slowly tothe northwest, distributing the effectof the nutrient enrichme nt over anestimated area of about 40,000 squarekilometers (120 Km east to west, 360 Kmsou th to nor th) • Thi s es tima t ion isfrom navigating satellite data, andplotting whale catch statistics andnutrient distribution.To further support the upwellinghypothesis Foerster and Thompson (1985)discussed the distribution of the biota(plankton to whales) of the study area.The biota of the area, in terms of biomass and standing stock (numbers),appeared high. Rorvik et al. (1976)reported the area to be used annually a sa feeding ground by an estimated 6100fin whales . Watkins et al. (1981)reported swarms of krill at the surfacein and near the proposed upwelling zone .The major biological characteristics ofan upwelling zone are reported as (1)increased productivity, (2) low speciesdiversity, and (3) shortness of the foodchain (Boje and Tomczak, 1978) .Foerster and Thompson (1985) showed boththe zooplankton and phytoplankton tohave low diversity but large biomass.The presence of the large baleen whalesfeeding in the area was evidence of asho rtened food cha in and a high foodorganism concentration.Further study and a mo re detailedsurvey over other mo nths o f the yearwould yield a bette r understanding ofthe operational mechanisms. It alsowould aid in answering questions suchas:1. Is the anomaly a year-roundphenomenon? The persistent northnortheastwind component would lead tothe speculation that it might exist as asomewha t permanent feature.2 . Does the ice and/or loweredwater temperature during the arctic/subarctic winter slow down or ca ncel theeffect of the wind? This is a possibilityin that satellite reconnaissance ofthe area in late winter had no seasurface temperature fronts. However,the water temperatures at 100 meters and236

a t the surface would be simi 1a r, andthus any circulation created by the windwould not be easily discerned by thescanning AVHRR of the satellite.If a knowledge of this upwelling isgained, and its pE'riods ofestablishment, intensification anddissolution are understood, thenpredictions on fisheries harvest, datesfor vessel deploymE'nt andpresence/absence of competitor speciessuch as the whales can be predicted.The polar-orbiting weather satelliteand/or some basic temperaturemeasurements in the area could go far increating an inexpensive nowcastingpredictability for this increasinglycostly industry.SummaryUsing archived satellite gE'neratedinformation integrated withoceanographic and meteorological datafrom the Denmark Strait, anomalies inthe temperature patterns off the westcoast of Iceland were identified. InJune of 1981 an oceanographic survey wasconducted in and around the area shownto have de formed tempera ture pa t terns.From this survey, AVHRR polar orbitingsatellite scans, 30 years of historicalrecords on the oceanography and weatherconditions of the area, as well asinformation on the biological activity,the existence of a zone of upwelling offthe west coast of Iceland betweenlatitudes 63°- 64 oN and longitudes25°-26°W is suggested. This upwellingis estimated to influence an area of40,000 square kilometers. Thepumping/replenishment of nutrients intothe area of the shelf break hascontributed to large fisheries harvestsin Iceland. This small mesoscaleoceanic feature has added considerablyto the gross national product ofIceland.AcknowledgementsGratitude is expressed for thesupport of this program by the DefenseMapping Agency, The Office of NavalResearch, The National EnvironmentalSatellite Service, The Marine Mammalsand Endangered Species Program of theNational Oceanographic and AtmosphericAdministration, The Naval OceanographicOffice, the Naval Academy ResearchCouncil and the Research Fund of theExplorers Club. Special thanks are forassistance with the satellite imageprocessing from Dr. Alan Strong and Lt.Eric Coolbaugh of the Naval AcademyOceanography Department, for assistancewith data collection and computerizationboth at sea and ashore from ProfessorPatricia Thompson of the Johns HopkinsUniversity Applied Physics Laboratory,and for assistance with the meteorologyof the study area from Commander GeorgeSchwenke of the Office of theOceanographer of the Navy. The fieldresearch was performed as part of cruise270681 of the U.S. Naval OceanographicShip "KANE", Mr. Kenneth Countryman,Chief Scientist. Finally gratitude isexprE'ssed to The Bulletin of MarineScience for permission to modify and usethe Figures presented as 3C, 7a and 7b.ReferencesAnon. 1982.Atlantic.Washington D.C.Atkinson, L. P., P. G. O'Malley, J. A.Yoder and G.A. Paf5enhofer 1984. Effectof summertime shelf break upwelling onnutrient flux in southeastern UnitedStates continental shelf waters. J.Mar. Res. 42:969-993.Bang, N. 1971. The southern Benguelacurrent region in 1966. Part II.Bathythermography and air-seainteraction. Deep Sea Res. 18:209-244.Boje, R. and Tomczak, M. 1978. Ecosystemanalysis and the definition of boundariesin upwelling regions. In:"Upwell ing Ecosystems" (R. Boje and M.Tomczak, eds.). Springer-Verlag, NewYork. pp. 3-11.Defant, A.Vol. 1.729pp.1961. Physical Oceanography.Pergammon Press. New York.Dietrich, G. 1957. Schichtung undZirku1ation der Irminger in Juni 1955.Ber. Dtsch. Wiss. Komm. Meeresforsch.14:255-312.237

Dietrich, G., K. Kalle, W. Krauss and G.Siedler. 1980. General Oceanography.2nd. ed. John Wiley and Sons, New York.629pp.Ekman, v. W. 1905. On the influence ofthe earth's rotation on ocean currents.Archiv. Matematik, Astron. Och. Fysik.2:1-52.Ekman, v.Zirkulationstromungen.Och. Fysik.W., 1923. Uber Horizontalbei Winderzeugten Meeres­Archiv Matematik, Astron.17:1-74.Foerster, J. and Thompson, P. 1985.Plankton and whaling ground dynamics inthe Denmark Strait. Bull. Mar. Sci.37:504-517.Goering, J. J. and McRoy, C. P. 1981. Asynopsis of Probes. Trans. Am. Geophys.Un. 62 :44.Hart, T. J. and Currie, R. I. 1960. TheBenguela Current. Discovery Rep.31:123-298.Hidaka, K. 1954. A contribution to thetheory of upwelling and coastalcurrents. Trans. Am. Geophys. Union.35:431-444.Hill, R. B. and Johnson, J. A. 1974. Atheory of upwelling over the shelfbreak. J. Phys. Oceanogr. 4:19-26.Houghton, R. W. and Marra, J. 1983.Physical/biological structure andexchange across the thermohalineshelf/slope front in the New York Bight.{. Geophys. Res. 88:4467-4481.Rodewald, M. 1967. The NortheastAtlantic pattern of atmospheric circulationduring the first six months of1960. In "The Iceland-Faroe Ridgein ternat iona 1 (ICES) overflowexpedition, May June 1960." (J. Tait,ed.) 11157. Int. Per. Con. Expl. Mer,Copenhagen. pp. 18-23.R6rvik, C., J. Jonsson, o. Mathisen andA. Jonsgard. 1976. Fin whales,Balaenoptera physalus (L.), off the westcoast of Iceland. Distribution,segregation by length and eXploitation.Rit. Fiskideildar 5:1-30.Steele, J. H. 1976. Patchiness. In "TheEcology of the Seas." (D.H. Cushing andWalsh, J. J. eds.) W. B. Saunders Co.,Philadelphia. pp. 98-115.Strickland, J. D. H. and Parsons, T. R.1972. A Practical Handbook of SeawaterAnalysis. #167, 2nd. ed. Bull. Fish.Res. Board of Canada. Ottawa:-3"l0 pp:-Tomczak, M. 1981. Longshore advectionduring an upwelling event in the CanaryCurrent area as detected by airborneradiometer. Oceanol. Acta 4:161-169.Watkins, W. A., K. E. Moore, J.Sigurjonsson, D. Wartzok and G. N.diSciara. 1981. Finback whale,Balaenoptera physalus, tracked by radiofrom Icelandic to Greenland waters.Int. Per. Con. Expl. Mer. Cm 1981/N:16:1-19.Williams, J. 1966. Oceanography. Little,Brown and Co., Boston. 242 pp.Hsueh, Y. and Ou, H. W. 1975. On thepossibilities of coastal mid-shelf andshelf-break upwelling. J. Phys. Oceanogr5:670-682.Petrie, B. D. 1983. Current response atthe shelf break to transient windforcing. J. Geophys. Res. 88:9567-9578.Platt, T. 1972. Localabundance and turbulence.19:183-187.phytoplanktonDeep-Sea Res.238

TIME DOMAIN SIMULATION OF THE DRIFTINGOF SMALL FLOATING BODIES IN WAVESJacek S. PawlowskiNational Research Council, St. John's, Newfoundland, CANADAMomen A. WishahyNORDCO Limited, St. John's, Newfoundland, CANADAAbstractThe drifting in waves ofsmall icebergs, bergy bits andgrowlers presents significanthazards to navigation and offshoreoperations in ice-infestedwaters. In this paper, acomputational algorithm, based onthe equivalent motion method, ispresented, for the prediction ofmotions in waves of isolated icemasses in time domain. Predictionsare shown for spherical ice massesin unidirectional regular waves.The computed values are found tobe in good qualitative andquantitative agreement withexperimental data obtained fromsmall scale model tes ts, wi th theexception of the heave resonancecondition where the motions arelargely overpredicted.IntroductionNavigation and offshoreoperations in ice-infested watersThis is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987.are exposed to hazards of theinteraction of ships and offshorestructures with continuous or packice fields, and of collisions withisolated ice masses ranging insize from icebergs to bergy bitsand growlers. The danger ~fcollisions with floating icemasses has drawn attention mainlyin connection with the drift andwave-induced motions of icebergs.Models of iceberg drift areapplicable to predicting icebergtrajectories (in the horizontal)over periods of tens of hours, andtake into account forces resultingfrom the action of currents, windand added-mass effects, as well asthe Coriolis force and, related toit, effects of the pressuregradient due to the sea surf aceslope (Sodhi and EI-Tahan 1980;Smith and Banke 1981; Gaskill andRochester 1984). A short term(over trajectories of hundreds ofmeters) model of iceberg drift,which includes wave-inducedmotions and the influence of thepresence of a large fixedstructure has been developed byIsaacson (1986 a,b,c). In thismodel the drift motion expressedby linear displacements (surge andsway) in the horizontal, isinduced by the current and wavedrift forces. The effects of239

wind, the Coriolis force and seasurface slope, are neglected.However, the disturbance of theuniform current velocity field bythe presence of the fixedstructure, and added-mass effectsare included, both by means oflinear diffraction theory. Thedrift trajectory is solved by atime-stepping procedure independentlyof wave-induced oscillatorymotions of the ice mass. Thewave-induced oscillatory motionsin the six degrees of freedom arederived at a series of pointsalong the drift trajectory usinglinear, frequency domain,diffraction theory and assumingzero average speed. Again,diffraction effects resulting fromthe presence of the fixedstructure are taken into account.The application of the lineardiffraction theory to theevaluation of wave-inducedoscillatory motions of ice mallsesnecessitates the assumption ofsmall motion amplitudes (withrespect to the characteristicdimensions of the ice mass) andsmall wave steepness. Such anassumption cannot be adopted for awide range of practicallyimportant situations, whengrowlers, bergy bi ts or smallicebergs are driven by heavyseas. The size of the ice massesof interes~ varies approximatelybetween 10 to 10 5 tonnes, wi thcharacteristic dimensions between10 and 60 m. The relevant rangeof wavelengths extends approximatelyfrom 80 to 300 m. Itshould be noticed that ice massesof such dimensions are difficultto observe and monitor, especiallyunder adverse weather conditions,and therefore chances of theirdetection and interception oravoidance by a change of locationare low. Results of model testscovering a similar range ofconditions in regular periodicwaves, with steepnesses between0.077 and 0.034, were presented inLever et al. (1984). It wasdetermined that the ice masses mayachieve high values of kineticenergy due to wave-inducedmotions, thus creating significanthazards to offshore operations.It was also found that for waveslonger than approximately 13 timesthe characteristic dimensions ofthe ice mass, the wave inducedmotions are close to the orbitalpaths of fluid particles in wavesof finite amplitude. In addition,complete submergence and largesurfacing displacements of themasses were observed at conditionsof heave resonance.It follows from the abovediscussion that the development ofa theoretical and numerical model,suitable for the prediction ofmotions of small ice masses underthe influence of waves andcurrent, should be beneficial fordesign applications. The modelshould predict motions of isolatedice masses floating in theproximity of an offshore structureor ship, thus making possiblethe determination of kinematicparameters of possible collisionsand of the range of occurence ofthe collisions around the waterline.It is also inferred that insuch a model large motions withrespect to the dimensions of theice mass must be reckoned with,and that the drift needs to beconsidered as the resultant of acontinuous asymmetric oscillatorymotion, so that under certainconditions a similarity to a fluidparticle motion could be achieved.Simultaneously, the diffractioneffects must be adequatelyevaluated in order to avoidsignificant limitations ofapplicability with respect to theratio of the characteristicdimension of the ice mass to thewave length. In broad terms themodel must fill in the gap betweenthe conditions appropriate for theapplication of Morison's formulaand those suitable for the use oflinear diffraction theory.In the absence of a full,time domain solution of thenonlinear diffraction problem,which, even if it were available,240

would appear to be very costly toimplement, the model presentedhere relies on the equivalentmotion concept developed by thefirst author in connection withthe evaluation of ship motions inwaves, in frequency and timedomains (Pawlowski 1982, 1987).The model makes possible anapproximation of the disturbanceof an incident potential flow, dueto the presence of an impermeablebody, by a finite number ofpredetermined "distortion mode"potentials and their speedamplitudes (the equivalent motioncomponents) in such a way that abest fit in the impermeabilitycondition on the body surface isobtained. In the remaining partof the paper a summary of thecurrent version of the model isprovided, together with severalcomparisons of predicted motionswith the results of speciallyconductedexperiments (Wishahy andPawlowski 1987).The Governing EquationsAn isolated ice mass isrepresented by a rigid body ofuniform specific density 0.9 andof a form determined by itsexternal, impermeable surface ClB,which has piecewise continuousnormal. The motion of the body isobserved in a right-handedCartesian system of coordinateswhich is stationary with respectto the earth. The unit vectorsof the coordinate system aredenoted by ~i,i=1,2,3 and thecorresponding coordinates byx,y,z. The coordinate axes x andyare placed on the undisturbedfree surface. The z axis isdirected vertically upwards. Inthe reference configuration thecentre of gravity of the body, CG,coincides with the origin of thereference system. The radiivectors of the points P of thebody in the referenceconfiguration are denoted by X,and the normal vectors at the bodysurface are signified by N.Instantaneous configurations ofthe body are defined by XCG' theradius vector of CG, and by thesecond-order tensor of rotation R,such that:'X' R-X (la)-"N -R • N (l b)with X" and N/ denoting,respectively, instantaneous radiiof the points of the body withrespect to CG, and instantaneousnormals at the body surface. Aninstantaneous configuration of thebody and velocity field of itsparticles are therefore given bythe formulae:-/xCG + X(2a)x xCG + W" j( (2b)with x(X,t) representing theinstantaneous radius vector ofthe point p(X), the dotssignifying derivatives withrespect to time t, and w denotingthe angular velocity of the body:w ..... = I (2c)-=where I represents the unitsecond-order tensor, T signifiestransposition and A denotes vectormultiplication.The fluid flow around thebody is assumed to be irrotationaland described by a single-valuedvelocity potential~. Viscouseffects are taken into accountsemi-empirically by means ofexperimentally determined dragcoefficients. No lift effects areincluded. The potential ~ isconsidered to result from thesuperposition of an incident flowpotential ~I and potential ~Bdue to the disturbance of theoncoming flow by the submergedpart of the body:~ = ~I + ~BIn the present~I is chosen tosecond-order waveamplitude, such that:(3)applicationrepresent aof finite241

HI. cosh 211(z+d)/A~I sin 8 +2T sinh 211 d/ Asatisfy the linear freecondition, the potentialbe set equal to zero.surface~o can311 H2 cosh 411 (z+d)/ A+ sin 2816T sinh 4 211 d/ A (4a)with8 x/ A - tiT (4b)andgT2 211dA tanh (4c)211 Awhere H, A and T denote,respectively, the wave height,wave length and wave period,whereas d signifies water depth.The time variable is denoted by tand g signifies the acceleration ofgravity. The correspondingexpressions for the waveelevation "I and wave-inducedpressure PI can be found inWeigel (1964).Following the equivalentmotion method (Pawlowski 1982,1987), the potential ~B isexpressed in terms of anapproximating finite series:m~B = ~u + ~o+L Pi¢ii =1 ( 5)where ~u satisfies theimpermeability condition on aBcorresponding to the steady motio~of the body, ¢i represents modalvelocity potentials correspondingto unit amplitude distortion modesof aB, Pi denote speedamplitudes of the modalpotentials, and ~o fulfils thehomogeneous Neumann condition onaBo The potentials on the r.h.s.of (5) are also required tosatisfy appropriate free surfaceand regularity or initialconditions. In the presentapplication, the potential ~urepresents mainly the potentialflow due to the steady drift ofthe body and is assumed to havesufficiently small influence uponthe motion of the body to beneglected. In addition, since themodal potentials are assumed toAccording to the equivalentmotion concept, the impermeabilitycondition on the wetted surfaceaBw of the body is imposed uponthe velocity potential ~ in theintegral form:L = J «a/an) 41 - v n )2 dS = 0aBw(6a)with account being taken of theseries representation (5). Informula (6a) (a/ an) 41 is definedas the normal derivative of 41:(a/Cln) 41 =-"N • (a/Cli) 41 (6b)with (a/ax) signifying thegradient operator, and vndenoting the normal speed of thesurface. Therefore, in order tocompensate for the neglectedcontribution of the potential~u' vn is taken in the form:-I !.. _vn = N • x (X --"N • U(6 c)with U denoting the averagevelocity of drift. Inapplications U can be obtainediteratively or can be approximatedby the drift of a fluid particlein finite-amplitude waves. Theminimization conditionsi=l, ••• m ( 7 )yield the normal equations whichdetermine speed amplitudes Pi atevery instant of time, providingthe quantities a¢i/an, whichdefine the distortion modes onaB w ' are given and are linearlyindependent (Pawlowski 1982,1987). Here, the distortion modesare chosen to correspond to rigidbody displacements parallel to theaxes of the stationary system ofreference, and to rotations aboutthe point CF of the body, which,in the free floating condition,coincides with the centre offlotation.242

Therefore:-/a ~i I an = ~i· N i=1,2,3-"[!:.i "ex - i C F ) 1 • Ni=4,5,6(8a)(8b)The resultant generalizedhydrodynamic forces are consideredto be obtainable by the summationof pressure forces resulting fromthe potential flow described bythe potential ~, and forces due tovis c 0 use f f e c t s • As i sin d i cat e dabove, the viscous forces areevaluated semi-empirically usingexperimentally-determined dragcoefficients. Lift effects areneglected. In the presentapplication, the pressure forcesare expressed as a sum ofFroude-Krylov forces, correspondingto the potential ~I' anddiffraction forces, correspondingto the potential ~B. TheFroude-Krylov forces are obtainedby a direct integration ofthe pressure Plover theinstantaneous wetted surface ofthe body, using a discretizationinto quadrilateral isoparametricelements of second order. Forthis purpose the instantaneousfree surface elevation is taken asdefined by wave elevation nI'Apart from neglecting theeffects of potential flow whichresults from the steady componentof body velocity, and isrepresented by the potential ~u'a major simplification isintroduced here in the evaluationof diffraction forces. Thepressure contribution, which isnonlinear with respect to themodal potentials, is discarded andthe modal potentials are replacedby the corresponding radiationpotentials for the bodyoscillating at its free-floatingconfiguration, with the frequencyof encounter determined relativeto the first-order wavecomponent. Consequently, thediffraction forces are expressedin terms of speed amplitudes Pifound from formulae (6), (7) and(8), and radiation added-mass anddamping coefficients pertinent tothe free-floating configuration ofthe body at the frequency ofencounter. In addition to thedetermination of the speedamplitudes at the instantaneousconfiguration of the body, andaccording to the instantaneousbody motion and wave flow,possible large displacements ofthe body are taken into account byconsidering the added mass anddamping coefficients asproportional to the instantaneousvolume of the body submerged belowthe undisturbed water level, forAID ~ 10, and as proportional tothe instantaneous submerged volumebelow the wave elevation forAID> 10, with D denoting thecharacteristic dimension of thebody.The diffraction forces arethen determined by the followingformulae:6 "-I [d I d t ( p . Ak .) +. 1 J JJ=(9a)where for k=1,2,3, Fk denotesforce components in directionsek respectiv~ly, and fork=4,5,6, Fk signifies momentcomponents with respect to pointP(XCF) correspondingly indi rections ~k-3' The instantaneousadded-mass and /dampingcoefficients Kkj and Bkj aredefined by:/Akj = A kj V" IVandwi th Akj andrespectively,k,j=1,2, •• ,6k,j=1,2, •• ,6(9b)(9c)B kj representing,the added-mass and243

damping coefficients in the freefloatingcondition, V denoting thevolume of displacement in the"free-floating condition, and Vsignifying the instantaneoussubmerged volume. Therefore, theterm describing the rate of changeof momentum in (9a) takes theform:(9 d)For the linear displacementmodes of motion, the viscous dragforces, Fvk' are assumed to bedetermined by means of generalformulae:(10 )for k=1,2,3. In the aboveformulae uk and ~k denote,respectively, speeds and accelerationsof the linear displacementsin the directions of versors ~k'appropriately defined in terms ofspeed amplitudes p'; Surepresents the area Jof theprojection of the wetted surfacedB w on a plane perpendicular tothe velocity vector u ofcomponents uk' and C signifiesthe unsteady-motion dragcoefficient corresponding to thedirection of u. In addition, Mdenotes the mass displaced by thefully-submerged body, Sw representsthe instantaneous wettedsurface, S is the total surface ofthe body and p signifies waterdensity.The application of formulae(10) follows from the resultsobtained in experiments withdeeply-submerged spheres oscillatedin viscous fluids(Karanfilian and Kotas 1978), forwhich a good fit of a regressionalformula determining thecoefficient C was established.This application is also inagreement with the assumptionadopted here that the forces dueto viscous effects are consideredas additive corrections to thepressure forces induced by thepotential flow. In addition tothe empirical origin of theformulae it is also obvious thattheir applicability is problematicin the case of large surfacingdisplacements of the body.Results of Computations andExperimentThe evaluation of thecomputational method describedabove requires a sufficientlyextensivecomparison of computedand measured motions of ice massesdrifting in waves. Below, such acomparison is presented for an icemass of the form of a smoothsphere, for which parameters ofmotion were obtained from a smallscaleexperiment. The choice ofthe s hap e 0 f the mo dell e d ice ma s sis dictated by the availability ofthe experimentally-determined dragcoefficients (Karanfilian andKotas 1978), and by resultingsimplifications of the computationalalgorithm and experimentalprocedures.Taking into account thesymmetry of spherical form,rotations of the body about CGleave the form of body surface dBunchanged. Therefore, R=I can beinserted in formulae (1) and (2),and the rotational modes of motiondo not contribute to the r.h.s. offormula (6). Consequently, thedescription of body motion can bereduced to linear displacementsand velocities of CG in the planeof wave propagation. Thishowever, does not eliminate therotational distortion modes informulae (5), (6), (8) and (9).For the computations presentedhere, added-mass and dampingcoefficients All, B ll , A33, B33,AlS and BlS are evaluated by meansof a standard panel methodalgorithm (Murray 1987).The forces FVl andare determined from formulaewith:244

andu 1 P 1 - P5 rXCF I(11 a)U3 = P3 (11 b)Following Karanfilian and Kotas(1978) the unsteady dragcoefficient C is determined by:C = (An + 1) 1. 2 Cd(12a)with acceleration number Andefined as:An I tr I D/(u)2 (12b)where D denotes the diameter ofthe sphere. In formula (12a) Cdsignifies the steady-motion dragcoefficient which is a functiononly of Reynolds number Re:Re = ,ui D/V(12c)where v is the kinematic viscosityof the fluid.The dependence of Cd uponReynold's number Re is provided bythe seventh-degree polynomial inlog Re identified in K~ranfiliarsand Kotas (1978) for 10- ~ Re .(.10and by formula (12d) from Chow(1979) :0.500.083.66for 30.18for 10 5 < Re5 • 6• 10 < Re ,2 • ~ 0 ,for Re>2 • 10 •(12d)The hydrodynamic forces arecombined with the force of gravityand a version of the predictorcorrectormethod (Bass 1985) isused for the numerical integrationof the equation of motion in time.In order to obtain necessaryexperimental data, a small-scaleexperiment, consisting of twoseries of tests, was carried outin the wave tank of the MemorialUniversity of Newfoundland. Inthe first series a smooth solidsphere of paraffin wax, ofdiameter D=30 cm and of specificdensity 0.9, was allowed to driftfreely in waves ranging from 61 cmto 545 cm in length and from 1/32to 1/9 in steepness, at the waterdepth of 180 cm. The motion ofthe sphere with respect to ascaled grid were recorded on videotape using an arrangementanalogous to the one described inLever (1984). In the secondseries of tests a smooth woodensphere of the same diameter, andballasted to the same weight wastowed, free to heave, in therepeated wave conditions and atforward speeds equal to thecorresponding speeds of free driftwhich were determined in the firsttest series.A complete description of theexperimental arrangement and fullresults of the tests are providedin Wishahy and Pawlowski (1987).Here, a summary of the results ofthe free drift series of tests isshown in Table 1, in comparisonwith the corresponding quantitiescomputed by means of the methoddescribed above. The mean driftspeeds U av were measured using astopwatch, whereas the othermotion parameters, i.e. maximumspeeds of surge U max ' maximumspeeds of heave V max ' and heaveheights 2zA were obtained froman analysis of the video records.In several cases, either due tothe smallness of the observedmotions or owing to theirirregularity, reliable estimatesof the motion parameters could notbe found, which is indicated inthe table by symbol NA. In someother instances, only roughestimations were possible, whichis shown in the table by means ofthe sign of approximate equality". Occurrences of deep submergencesof the sphere and of overtakingof the sphere by waves wereobserved in the range of heaveresonance, i.e. for A/D between 5and 13, in particular in steepwaves i.e. for H/A=I/10.To facilitate the comparison,the computed and observed valuesare presented in Figures 1 to 4 ina normalized form. In addition,245

WAVE EXPERIMENT COMPUTEDT ). HIA U U V 2Z U U V 2Z av max max A av max max Asec em - em/sec em/sec em/sec em em/sec em/sec em/sec em.625 61 1/24 4.7 " 4.7 ,,1 ,,0 4.6 8.6 1 0.2.625 61 1/11 13.7 "13.7 :1 ,,0 13.6 22 2 0.4.8 100 1/28 "2.0 NA ,,0 ,,0 2.4 9 1 0.3.8 100 1/19 6.0 NA NA "1.0 5.8 16 2.5 0.7.8 100 1/10 11 .4 27 NA 2.7 11. 3 30 7.7 21.0 156 1/30 2.5 NA NA ,,1 2.1 12 13 3.81.0 156 1/20 6.0 25 NA 1.3 4.5 19 17 4.61.0 156 1/10 8.6 41 22 7.0 8.7 36 28 81.2 225 1/30 3.6 15 NA 1.7 2.6 15.9 28.5 10.21.2 225 1/19 6.5 30 10 5.6 4.6 27 44 161.2 225 1/10 6.8 32 ,,18 NA 9 47 63 21.71.4 305 1/30 5.4 22 23 5.1 2.3 19.7 27.5 12.51.4 305 1/20 5.7 NA NA NA 4.2 31 44 19.51.4 305 1/9 7.9 NA NA ,,7.1 14 63 69 351.6 393 1/32 6.5 28 "26 10.5 2.9 24 26 131.6 393 1/20 6.8 47 39 14.8 6.4 39.2 40 20.51.9 545 1/30 4.9 33 32 19.2 3 31 31 18.51.9 545 1/20 9.6 50 50 30.4 8.3 48 46 28Table 1.Measured and Computed Parameters of Motion246

~\7 V l36 MEASURED COMPUTED00HI).0 1/10 01/5 ~ 20 I;!1/30• •4 \7 LEVER (1984)00"- 0>000::> 2, •• • •1/2S\7j~Iii! ~I;! 0N ~1/21 \7 '/ 162f"0Iii!~.~Iii!S0 2 4 6 10 12 14 16 18AjoFigure 1.Measured and computed drift speeds~14MEASUREDCOMPUTEDHI )..120 01/ 10 0~ 1/20 I;!010 0 1/300• •0 0 \7 LEVER (1984)0\7 't 168\71/21"- ~> 60 I;! I;!Iii!N2Iii! IiiiiI \7 11214•I;!2 ~• •••1"z5ZIii!I• ••02 4 6 10 12 14 16 18)../0Figure 2.Measured and computed maximum surge speeds247

141210D~DMEASUREDHI A-COMPUTED01/ 10 D1/~ 20 ~•1/30•\l LEVER (1984)~....." 0E>864DD0~•11\71/21•0~1/211~6\l~..1•/25 \l••BNQ:r:"-0NC\J1.21.0o 8060.4o 20iD~,-...r-.•0 -~2 4•~D0D! •6 8• ~D•o1012•~MEASURED0~•14COMPUTEDHI)...1/10 01/20 ~1/30•16 181/13~iii1.i0 46 81012 14 16 181.. IDFigure 3.Measured andcomputedmaximum heave speeds14Figure 4.AIDMeasured and computed heave amplification factors248

MEASUREDT : 1.9 sec.HI),: 1/30T = t:O sec.HI)" = I 110T = 0.8sec.H 1)...= I 110COMPUTEDFigure 5. Measured and computedmotion trajectoriesin Figure 5 the match betweentypical trajectories of CG ascomputed and observed is shown.The comparison is hampered by theuncertainty related to theexperimental results. In Lever(1984), the relative errors of themotion parameters were estimatedat ±0.15. The present results atA/D=5.2, which are directlycomparable with the correspondingvalues reported in Lever (1984)and also shown in Figures 1 and 2,seem in general to confirm thisestimation, although a discrepancyof approximately 0.3 relativedifference is observed for theUmax value at the wave steepness1/20.It is seen from Figures to4 and Table 1 that a largemajority of the computed valuesare in good agreement withthe corresponding experimentalresults. In particular thecomputed values of average driftspeed U av ' shown in Figure 1,follow closely their measuredcounterparts over the whole rangeof wavelengths and wav~steepnesses, with the exception ofsteepness 1/10 at A/D = 10. Goodagreement is also found for thevalues of maximum horizontalspeeds Umax (Figure 2) over thewhole range of relativewavelengths for wave steepnessesof 1/30 and 1/20. For the wavesteepness of 1/10, although two ofthe me as u red value s are clos e tothe corresponding computedresults, the other two indicatelarge discrepancies.For the vertical motions thecomparison of maximum verticalspeeds Vmax show good agreement,with the exception of theresonant value of A/D = 7.5 wherelarge differences are observedat steepnesses 1/20 and 1/10.A similar pattern of discrepancyis observed between the measuredand computed values of heaveheights, (Figure 4) where ingeneral large over-predictions bythe computational method are foundin the resonant range of heave249

for AID between 5 and 13.The comparisons of computedand observed motion trajectoriesshown in Figure 5 indicate theeffectiveness of the computationalmethod in predicting the motionoutside of the range of heaveresonance.Although more detailedanalysis of the computationalmethod will be possible relativeto the forces and motions measuredin the second series of tests, itappears that the systematicdiscrepancies between the computedand observed motion parameters,reflect the limits of theapplicability of the algorithm inits current form.ConclusionsThe results presented aboveindicate that the equivalentmotionmethod can be used forpredicting motions of smallspherical ice masses in waves,assuming that the ice mass isrepresented by the sphere of thesame specific densi ty and weight.The method gives reliableestimates of average drift speedspractically over the whole rangeof relative wavelength ratios2 ~ AID" 18. The maximum surgespeeds are also well-predictedwithin the whole range of AID forwaves of steepnesses 1/30 and1/20. For waves of steepness 1/10the method closely predicts thelargest observed speeds of surge,but appears to overpredict thespeed values in the other twocases.Good agreement between thepredicted and measured values isalso observed for heave maximumspeeds and motion amplitudesoutside the respective ranges ofrelative wavelengths 7 ~ AID:!: 9 and5 , AID ~ 13. The success of themethod in determining motiontrajectories outside of theseranges is illustrated in Figure5.The motion within thespecified ranges is significantlyaffected by flow phenonema due tooccurrences of the deepsubmergence of the sphere and theovertaking of the sphere bywaves. These phenomena are notmodelled adequately by the presentmethod, which results in the largeoverprediction of the heaveparameters. It should however benoticed that in the same range ofrelative wavelengths, heaveamplification factors predicted bya linear diffraction theory mayreach values above 4, (Kobayashiand Frankenstein 1986).Considering impact loadingsdue to possible collisions withice masses as mainly dependent onthe relative horizontal velocityof the ice mass with respect tothe structure, in practical termsthe present inadequate modellingof heave amplitude leads to overpredictionsof the exposed area ofthe structure around the waterline.The results presented herehave been obtained with theapplication of the simplestversion of the method which can beupgraded in several ways, e.g. byincreasing the number ofdistortion modes in thedetermination of equivalent motioncomponents. However, in order toimprove predictions in the rangeof ext reme heave responses, acloser examination of thepertinent physical phenomena andan appropriate formulation oftheir mathematical descriptionappears to be necessary.ReferencesBass, D.W., 1985, "The Solution ofthe Equations of the Motion for aRigid Body", Institute for MarineDynamics, National ResearchCouncil Canada, Report MTB-167.Chow, Chuen-Yen,Introduction toFluid Mechanics",Sons, New York.1979, "AnComputationalJohn Wiley &250

Gaskill, H.S. and Rochester, J.,1984, "A New Technique for IcebergDrift Prediction", Cold RegionsScience and Technology, Vol. 8,223-234.Isaacson, M. de St. Q., 1986a,"Ice Mass Motions Near an OffshoreStructure", Proceedings of FifthOMAE Symposium, Tokyo, Japan,Vol. 1, 441-447.Isaacson, M. de St. Q. and DelloStritto, F.J. , 1986b, "Motion ofan Ice Mass Near a Large OffshoreStructure " , Proceedings OTC'86,Vol. 1 , 21-27.Isaacson, M., 1986c, "Ice MassMotions Near a Large Of fshoreStructure", Proceedings of OceanStructural Dynamics Symposium '86,Oregon State University, 262-276.Karanfilian, S.K.1978, "Drag onUnsteady MotionRest", J. FluidPart 1, 85-96.andain aMech. ,Kotas, T.J.,Sphere inLiquid atVol. 87,Pawlowski, J.S., 1987, "TimeDomain Sim,ulation of Ship Motionsin Waves by the Equivalent MotionMethod", Institute for MarineDynamics, National ResearchCouncil Canada, Report IMD-HYD-Ol.Smith, S.D. and Banke"A Numerical ModelDrift", ProceedingsVol II, 1001-1011.E.G., 1981,of Icebergof POAC 81,Sodhi, D.S. and El-Tahan, M. ,1980, "Prediction of an IcebergDrift Trajectory During Storm " ,Annals of Glaciology, Vol. 1 , 77-82.Wiegel, R.L., 1964, "OceanographicalEngineering", Prentice­Hall, Inc./Englewood Cliffs, N.J.Wishahy, M., Pawlowski, J. andMuggeridge, D., 1987, "TheNonlinear Heave Motion and WaveForces on a Partially SubmergedSphere", Proceedings of OCEANS~, Vol. 1, 42-47.Kobayashi, N. and Frankenstein,S., 1986, "Interaction of Waveswith Ice Floes", Proceedings ofIAHR Ice Symposium 1986, Vol. 1,101-112.Lever, J.H., Reimer, E. andDiemand, D., 1984, "A Model Studyof the Wave-Induced Motion ofSmall Icebergs and Bergy Bits",Proceedings of the Third OMAESymposium, Vol. III, 282-290.Murray, J.J., 1987, "A ComputerProgram to Calculate HydrodynamicLoading and Response of FloatingBodies Using Green's FunctionPanel Method", Institute forMarine Dynamics, National ResearchCouncil Canada, Report RR-HYD-02.Pawlowski, J. S. , 1982, "TheEstimation of Diffraction ForceComponents from the EquivalentMotion Concept", InternationalShipbuilding Progress, Vol. 29,62-73.251

WAVE REFLECTION FROM AN ICE EDGETorkiJd CarstensNorwegian Hydrotechnical Laboratory, Trondheim, NOR WAYArild R0sdalFellesdata AIS, Oslo, NOR WA YAbstractBecause waves change speed when theycross the ice edge, some of the waveenergy is reflected. The flexibillty ofthe ice cover determines the reflection,which is studied by means of several hundrednumerical experiments with six differentice models. These models lnclude icecovers that are rigid, elastic, viscoelastic,and without any stiffness. Also, someice floes or bands are included in theanalysis, which is confined to linear wavetheory.The experlmental results show thatthe reflection coefficient (ratio ofreflected to incident wave amplitude) ishighly dependent on wave period for anykind of ice. High reflection coefflcientsare only found for short period waves,while waves with period 10 seconds or morehave reflection coefficlents less than10\. The more flexible the ice, the lessreflection.This is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987. © The Geophysical Institute,University of Alaska, 1987.IntroductlonThe wave cllmate in seas wlth anannual ice cover differs from that ofotherwlse comparable seas WIthout ice onseveral counts. The most obvious dlfferenceis the variable fetch. The shore-toshorefetch is reduced by the ice cover toa shore-to-ice-edge fetch which, throughoutthe year, may vary between 0 and 100 \of the ice-free fetch. Another significantdifference is in the prevailing wind. Thelarge cyclones associated with the polarfront are less frequent than in, for example,the North Sea. However, the heatingfrom below of cold air blowing off the icetriggers smaller, but Intense cyclonesknown as polar lows (Carstens 1985). Aless obvious process is the reflection atwaves from an ice edge. Physically, thisreflection is due to the change in wavespeed or, in another jargon, "the impedancemismatch" across the lce edge.A llbrary search revealed that whilea good number of papers have been writtenon wave propagation under an ice cover,the reflection mechanIsm has not beendealt with in more than two rather theorethicalpapers (Keller & Weitz 1950;Keller & Goldstein 1953). In view of themany conceivable practical implications ofreflected waves we felt a need for anassessment of the range of reflections tobe expected. The natural startlng POlntwas a mathematical study.253

Althouqh the lIterature contains fewresults dealIng expliclty wIth reflectIonfrom an Ice edge, there dre many studIesof closely related problems For lnstance,stIff horIzontal plates at or near thewater surface are known to generate strongreflections (Stoker 1953). Reflectionsfrom floatIng bodles, Important to waveforces on these bodIes and theIr absorptIonof wave power, also yield resultsthat apply to ice floes and Ice bergs.The ModelsWe have chosen the 6 different icemodels shown in Table 1 and run severalhundred numerical experIments. Each modeldefines the type of ice, through a constitutivelaw or otherwIse. Each experimentspecifIes the geometry and the relevantmechanical properties of the ice. Theoutput of an experIment is the reflectIoncoeffiClent.Four of the models deal with icecovers extendIng from an ice edge at x = 0to x = -. The remaIning two models SImulateIce floes. The models of the firstgroup are all flexible:1. Elastic plate2. Visco-elastic Burgers plate3. Visco-elastIc Maxwell plate4. Non-elastlc, non-viscous floatIngmatterThe models of the second group are stiff:5. Fixed plate at the surface (Immobile).6. FloatIng plate (with displacements androta tions) .The reflection coefficient R ISdefined as the ratio between the amplitudeof the reflected (a) and incident wave(a ), respectivelyriRa ra.I( 1 )We use linear wave theory assuming avelocity potential. satisfying Laplace/sequationowith the impermeablecondition~- - 0oz - z = -dbottom(2)boundary(3 )and the kInematic free surface condItionwhere ~z = 0 (4 )is the surface displacement.Ice covers1. Elastic plateTable 1_ Ice models.Mechanical analoguemE~ E zm E 1 ryRheology2. Burgers plate 43. Maxwell platem24. floating matter m•-_.----_. - ---------Ice floes and ice bandsmotionNo of Degrees ofparameters freedom5. Fixed plate o6. Floatlng plate 3o254

By further assumIng the dynamIc freesurface condItIon for constant pressuregO + ~: = 0 z = 0 (5)we get the linear free surface conditIonz = 0 (6 )Equations (1) - (6) are valid tor allmodels used here. They form two groups:Group 1 Continuous ice coversGroup 2 Broken Ice coversElast.ic PlateA floating elastic plate is supportedby an elastic foundation. The fourth orderordinary differential equat.ion governingthis case worked out by Hetenyi (1946) is~aZwD~+ \1.h - = p (x,t) (7 )ax~ I atZDh 3 E12 (1 - i)is the flexural rigidity for the platewIth elastic modulus E, Poisson's rat.io vand thickness hw = plate deflectionp fluid pressure exerted on theice, obtained trom Bernoulli'sequation.The ice is assumed to float on thesurface, taking the shape of the wave forz = O. This geometry is shown in Fig. 1.ai~domai n 1zdomain 2Fig. 1. The two-dimensional ice cover.x:>We consider a two-dimensional spacewith two semi-infinite domains:Domain 1 contains the incident waveplus the reflected wave and has a velocitypotential~ = (A - lkx+ B e- Ikx ) e-kZe-Iwt(8)1 Iefor an incident waven = a (kx + lut) (9 )with amplitude d, wave number k and circularfrequency w.Domain 2 has a velocity potentialit x -ik x -k Z -Iwt+2 = [( A e 2 n + B e 2 n ) e n e ( 10)nnnwhere k are the wave numbers for thewaves tr~Rsmltted under the ice and A, Bdescribe amplitudes.andorCondition ( 6) yieldsa,Clw 2at liZiHi 2w = (-w)liZw = -!-. k 40UI 2 n z(11 )( 12)( 13)Expressions 13 combine to givethe dispersion relat~on for domain 2Dk 5 + (p g - \1. hi ) k - \1 w 2 = 0 ( 14)2n 0 I 2n 0which has five roots. The acceptable rootsdefine the velocity potential.ik X(AI e 21 e-k Z21-Ik x -k z+ B e 23 e 23 ) e - iwt3( 15)The first term represents the transmittedwave, while the other two termsdescribe waves that die out with x eventhough there is no energy dissipation.andandThe expressions (8) and (10) for.for 40 contain 4 unknowns: R, A, AlB3 ~e need 4 boundary conditiohs ana255

usea) + 4> x 0 , z '" 01 2b)0+ 1 0+2x ,ax axz '"a 2 wc) 0 x 00/0 0('6 )FollOWIng SqUIre and Allan (1980) wetest a four-element (Burgers) model ofFig. 2. The differential equation for thedeflectIon caused by a pressure excitatIonP(x,t.) isd)oJ w0 X 0ox J( '8)a) and b) presume continuIty across theplane x = 0, which is not possible WIthoutadditIonal terms in (8) and (,0). However,these terms are needed only close to theice edge and do not affect our solution.c) and d) are unproblematIc and presumezero bendIng moment and shear, respectIvely,at the end of the plate.After lengthy computations one obtaInsRand Rr (R0sdal 1985). The result we area~ter is the magnitudewhereE1 E, E2 E,E 2a, = -- + --_. + a 2"1 "2 "2 "'"2b, ElEl E2b =--2 "2IntroduCIng again p from theBernoulli equation one obtains the dispersionrelationJRI = fR 2 + R 2H I('7 )of the reflection coefficient.Visco-elastic plateThe two basic models for visco-elastICmaterials are the Maxwell and Kelvin­Voight 'units' shown in Fig. 2. With theseunits one can build multi-element models.However, the number of elements one canprofitably introduce is limited to 3 or 4.LINEAR VISCO-ELASTJC(Maa •• II'( a-Conl'an'The velocity potentIal for domaIn 2 isIk x -k z ik x -k z21 11 22 224>2 = (Ale e +A 2e e(19 )-k x -k z .+ B 23 23) -Iwt (20)3e e eThe wave number k21 is now complex,allowIng all waves to be aamped away fromthe ice edge. RR and HI are obtained fromthe system of equations in R0sdal (1985).LINEAR VISCO-ELASTIC(K.I.,n-Voi9t1__ ~r[!.~ __Again, a solution for a single waveis given by the first term in (20).Broken ice coversAs Ice edges composed of broken iceare more common than a continuous plateedge, we have investigated the availablemodels of broken ice.Fig. 2. Two basic visco-elastic models.256

Table 2. Test condlt~ons for numer~cal experlmentsI eTestth~ck- lengthNo Mode I nessElast~c plate h m 1 m E N/m2 v12345678910111213141516171819202122---------- ._---Const+dec ay~ng wave ampl 0.5+2~Alexp (~k xl1~A2 exp (~k~l xl+B exp (lk 12 2xl5lJ1,Canst wav e ampli+2~Alexp (ik 1, xl2 Burgers p late3 Maxwell p late,0.51250.51251-0.51251---6· 10 9 0.3" ",","6· 10 4 . "6· 10 96 '10",6.10 9 0.36" 0.3"","RheologyE, N/m 2 n, Ns/m 2 E2N/m21.2'10 9 1.46'10'2 2.18' 10 9, ,"" · ",""2 '10 8 ). 10'2 2.18' 10 91.2' 10 9 1.46'10'2" " " ·2 '10 8 ) '10'2,WaterI deptd m---- Ideep""",,".",Ns/m 2 n 25.57 '10" "2.6' 10"",,,,""23242526272829JO4 Float~ng matter0.51250.5125",""shallow,,,3132333435365 FlXed pIa te (icefloel1040100104010010""20",1738394041424344456 Floating plate ( lcetloel0.51251.,,40","10100104010010,"20".,257

Non-elastic. non-VISCOUS floating matterIce coversKeller and Weltz (1950) and laterKeller and GoldsteIn (1951) introduced anice cover with no stiffness and no viscosity,consisting of uncoupled mass pointsor floating matter with a total ice massper unit surfaceThe Import.ant factor IS the stIffnessD .: h 3 /6 alt-the general plasto-visco-elastic casefororis valid,tionsm = ,-\h(21 )The tranSItion from open to icebelinear andcovered water is assumed tofor both domains the same velocitypotentialhD =3 E212 (l-v )for the elastIC case. The reflection IncreasesWIth IncreaSIng D (Figs. 3, 4 and5 )~ = +(xz)exp[l(ky+wt)]subject to the boundary condi­and'"20,z = 04>2 a~, z = d, x < 0+2 tlf , z d, x < 0• o(e~lxlB < k, 1 x 1-0 (22)k 211/L sinala = Ii w2 / (Ii g-mlt,2)o 013 ul 2 /gFor normal Incidence B. = 0, k = 0the reflection coefficientlbecomesR = (f3-a)/(f3+a) (23)for deep water (f3d » 1, nd » 1), andR = (/f3-/u)/(/f3+/o) (24)for shallow water (f3d « 1, ad « 1).ResultsThe model results are presented inFigs. ) - 11. The test conditIons areshown in Table 2. The ice edge acts as alow pass wave filter. High-frequency wavesare mostly reflected, while long wavescross the ice edge without significantchanges. As a rule of thumb the reflectioncoefficient is less than 10% when the waveperiod exceeds 10 seconds.0::1.0~0.8:zw-- ICE THICKNESS. mLJu.. 0.6u..w0LJo 4:z0xw 0.2-'u..w0::O. 0 0 2 8 10WAVE PERIODFig. 3. ElastIC plate E = 6'10 3 N/m2c:~o 8:zwLJu.. O. 6u..w0LJo 4:z0xw-'u..w0::0.2O. 00 -- ELASTIC MODULUS0 2 4 6 8 10WAVE PERIOD ssFIg. 4. ElastIC plate h = 1 m258

0:I-zUJ!::!u..1.0 -- ICE THICKNESS0.'0u.. 0.6UJ0LJ:z 0.40xUJ-'u..UJ0:O. 25.0 m0.0 0 2 4 6 '0 10WAVE PERIOD s0:I-zUJwu..1.0O. '0u.. 0.6UJLJW:z O. 40xUJO. 2-'u..UJ0:0.0-- ICE THICKNESS5.00 10 15 20WAVE PERIOD sFig. 5. Elastic plate. constant waveampl. E = 6'10 9 N/m2The model results confIrm our expectatlonthat an elastIc ice is a better reflectorthan a vIsco-elastIc Ice. TYPIcalreflectIons from the vIsco-elastic Ice are10 20% less than for the elastic lce(Flgs. 6 and 7)0:I-0.'0zUJLJu..u..UJ0wz0xUJ-'u..UJ0:1.0 --ICE THICKNESSO. 60.40.2O. 0!5+----r--~----.--=._---,0 2 4 6 '0 10WAVE PERIOD sFIg. 7. Maxwell Pl~te'2£2=2.18'10 N/m , "2 =5. 57Ns/mThe four-element Burgers model lS acomblnation of a two-element Maxwell modeland a two-element KelvIn-Voight model. Itturns out that the reflectIon tram theBurgers model is governed by Its Maxwellelement so that the two vlsco-elastlcmodels 2 and 3 gIve identlcal results.The disIntegrated lce cover of model4 (Flgs. 8 and 9) displays a resonanceeffect that makes It a surprlslngly goodreflector for5.0m1.00:- ICE THICKNESS10mI-z 0.'0UJ!::! 2.0mu..u.. 0.6UJ0w:z 0.40xUJ-'u...0.2UJ0:20.0 0WAVE PERIOD sFlg. 8. FloatIng matter. Deep water.259

c::f-zUJ~L.L.1.0O. S0.6-- ICE THICKNESSL.L.UJ0LJ:z: 0.40xUJ-'L.L.0.2UJc::o 0,0 2 4 6 B 10WAVE PERIOD sFlg. 9. Floating matter. Shallow water.Ice floesFor the lce floe models 5 and 6 thedomlnatlng lnfluence comes from the lengthI of the floe ln the dlrectlon of wavepropagation. The flxed plate (model 5) lSa strong reflector. An example lS shown lnFlg. 10. When compared wlth the resultstor lce covers lt becomes clear that theflxed plate lS an unreallstic model of leefloes.c::1.0f- O.BzUJ~ 0.6L.L.UJoW 0.4zo;:s o. 2-'L.L.UJcr:-- PLATE WIDTH,m0.0 +---~--~~--~--~~o 5 10 15 20 25 30WAVE PERIOD sFig. 10. Fixed stlff plate.Water depth d = 10 m1004010Even the stitf tloatlng plate appearsto be too good a reflector, at least whenlt lS long (Fig. 11). Physlcally, theassumptlon of a long plate rotating wlthoutdeflectlons does not make sense. Onthe other hand the results for short floesseem acceptable.cr:f-zUJLJ1. 0O. BL.L.0.6L.L.UJ0LJz 0.40xUJ-'L.L.UJcr:0.2--ICE THICKNESS0.00 2 4 6 B 10WAVE PERIOD (s)Flg. 11. Floating stiff plate.Water depth d = 10 mComparlson w1th observatlonsOnly one set of observations of reflectedwaves lS reported ln the openliterature (WADHAMS et a!. 1986). Theseobservations (Fig. 12) were taken in frontof an lce cover with the size and thicknessdistrlbution shown 1n Table 3.Table 3. Tce floe distribut10nsd - floe diameterhi thicknessL1 - edge length fractionp~ - areal fractiondi,m12.51525355580100150250~i,m li Pi1.5 0.002 0.00011.5 0.004 0.00052 0.006 0.00100.030 0.00652 0.103 0.03762.5 0.087 0.04142.5 0.196 0.12560.293 0.26390.277 0.5035260

ENERGYREFLECTIONCOEFFICIENT%15,-,---------------------,ReferencesHetenYl, M. 1946: Beams on Elastic Foundation,Univ. of Michi

DYNAMICS AND MORPHOLOGY OF THE BARENTS SEA ICE FIELDSTorgny VinjeNorwegian Polar Research Institute, Oslo, NOR WAYAbstract.A short review of the morphology ofthe ice fields of the Barents Sea isgiven. Drift statistics from automaticbuoys have been calculated, and sonarprofiles of the bottom topography are reproducedtogether with autocorrelationfunctions. The ice ridges of first-yearice are observed to act as sediment trapsover the shallow shelves. This mayexplain the characteristic distributionof sediment-laden surfaces of ice floesin this area.General Conditions1. DistributionThe maximum southern extension ofthe sea ice in the Barents Sea islimited by the oceanic polar front whichgenerally follows the 200-300 m isobaths.The lighter polar water resides in theshallow northern shelves while warmer.more saline and denser water from theNorwegian Sea fills the deeper. southernpart of the Barents Sea. Ice melting bywarmer currents thus occurs in theThis is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987. © The Geophysical Institute,University of Alaska, 1987.margins only during the winter maximumextension. This means that it is themeteorological conditions which determinethe seasonal extent of the ice border inthis region. The most importantdisintegrating effect 1S the radiativeheating of the surface polar water which1n turn washes and melts the ice floes.The degree of heating and washing isdirectly dependent upon cloud cover andwind conditions.i.e. the intensity of theso-called Barents Sea Low. The cyclonicactivity in the area may varyconsiderably from year to year and thereis a clear correspondence between thedeviation in the atmospheric circulationfrom its mean and the anomalies in thesea ice distribution (Vinje. 19801. Thelatitudinal position of the ice edge inthe central part may thus vary from yearto year within a range of 200-300 kmduring the cold season and within thebroad range of 600-700 km during June andJUly (Vinje. 19851. Anomalies in the iceextension also correspond fairly wellwith the temperature anomalies found inthe warmer water masses passing from thewest into the Barents Sea (Sztersdal andLoeng.19841.2. Ice formsThe ice fields may freeze to acontinuous layer in sheltered areas onlyduring periods with long spells of cold.263

calm weather.Such periods are relativelyrare in the Barents Sea. Northerly w1ndswill carry the ice southwards and acontinuous production of new ice thenoccurs in the exposed areas on the leesides of the island. Compared w1ththe neighbouring, less disturbed part ofthe ice fields,these ice producing reg10nsare characterized by a higher surfacetemperature, i.e. a darker grey tone onthe IR satellite pictures (Fig.l). Archingalso occurs in the various straits duringsuch conditions, resulting in theproduction of a great number of parallelep1ped-shapedice floes. These crackfeatures have been modelled successfullyby Erlingsson (1987).3. Source of iceMost of the ice in the Barents Seais produced locally and, on average, thismarginal sea also seems to be an icesource for the Arctic Ocean (Zacharov,1976). This general picture is alsosupported by other calculations for theperiod 196B-1977 (Vinje, 1985). However,there may be a small average inflow fromthe north during the warmer seasonThe average annual net export of ice fromfrom the Barents Sea to the Arctic Oceanis found to be 35 km 3 .The average w1nd field suggests aconsiderable 1mport of ice also from theKara Sea Wh1Ch is an order of magnitudelarger than the above export to theArctic Ocean (Table 1). We note alsothat an export of ice occurs 1n theopposite direction during the summermonths, from the Barents to the Kara Sea.In addition to these exchanges withneighbouring seas, occasional summeringoverof ice occurs in the area.Altogether, the ice fields of the BarentsSea may be quite complex, cons1sting offirst-year and multi-year ice of variousorigins and ages.4. ThicknessLong-term observations of thethickness of fast ice at themeteorological station at Hopen shows alinear relationship between the icethickness (T) given in cm and theaccumulated degree-days ([dd) (Vinje,1985) .T = 15 + 0.045 [ddWhen applying this equation we cantabulate the expected average thicknessof level ice at the various latitudes inthe Barents Sea (Table 2).Figure 1. A NOAA-6 IR image illustrating typical ice conditions in the Barents Sea on 8December 1986. The surface temperature is highest 1n the darkest areas. Received atTroms. Telemetry Station.264

Table 1. Monthly average wind induced ice import (.) 1n km 3 from the Kara to the BarentsSea during the perlod 196B-1911. The assumed ice thlckness is 2 m. The ice drift speedis taken to be 1 I of the geostrophic wlnd speed which has been read from weather maps.Jan feb Mar A r Ma Jun59 51 80 141 37 -8Jul-35Au Se Oct Nov Dec YEAR-18 11 56 194 -11 527Table 2 illustrates the effect ofthe large latitudinal thermal gradientwhich generally is observed in thismarginal sea. It is noteworthy that atthe end of the extremely cold 1968-1969season, the fast ice at Hopen, 16.5° N,reached a thickness of 173 cm at the endof April. This gives an indication ofthe maximum deviation from averageconditions as given in Table 2.Table 2. The average level ice thicknessas calculated from the accumulateddegree-days at various latitudes in theBarents Sea.Latitude,deg.: 15 76 17 78 19 80Thickness.cm.: 95 105 120 130 145 1705. Ridges.Ridging acticivty is particularlyhigh during conditions with SSE winds,since the islands in the SvalbardArchipelago then act as a barrier,damming the movement of vast ice fields.At Hopen, the most frequent maximumheight of ridges is 2 m with acorresponding ridge draft of about 12 m.~~ekm~Vs~i~~e~u:::i:~~g:fd~~S~~~liS ~~~~~anchored ridges caused by shoreward windshave been observed to be as high as 10-15m above the sea surface at some of theislands (Vinje 19851.Special Observations1. Ridges as traps for sediment andnutrient saltsThe occurance of ice with a brownishappearance characterizes the ice fieldsin the Barents Sea and the eastern sideof the fram Strait. Some of this icemay have formed during the fall when thenear-shore, upper layers receive amaXlmum amount of suspended materialcarried by the rivers. freezing duringsuch conditions should result in theinclusion of terrigenous material over avariable column of ice, dependent uponthe amount and the endurance ofsuspension and freezing. This type of iceis often observed in the areas mentioned.Another, markedly different feature,is the scattered ocurrence of small icefloes with extremely muddy surfaces. Itwas not until recently that the origin ofthis ice type could be explained. Whenbreaking through unconsolidated ridgeson Svalbardbanken in March 1987, itbecame evident that terrigenous materialhad accumulated between the rafted iceblocks. This was clearly seen when a pileof four ice blocks revolved. Watersamples showed that the whole watercolumn was filled with suspended materialwhich, according to the bacteriologists,was of terrestrial origin. The diversreported that accumulation of darkmaterial also took place on the currentwardside of the keels and that theaccumUlation was most intense in theareas with crystals on the surface. Onthe lee side, dark material accumulatedonly close to the bottom of the ice keel.It is assumed that an increased intensityof the atmospheric clrculation, causingdisintegration and redistribution ofridges, will give an increased percentagecoverage of muddy ice in the area.2. Ice floe topographyIn connection with possiblespills in the Barents Sea it isoilof265

lnterest to estlmate the absorblngpotential of the lce fields. As the oilalso may gather In domes under the lce Itis necessary to measure the bottomtopography. A Mesotech 971 sonar scannerwas used for this purpose. The sonar wasattached to a JOO m long cable so thatthe processlng units could be operatedfrom the laboratories on board. After thesonar had been lowered through a hole inthe ice, the scanning scheme wascontrolled by a computer. Measurementswere performed on a series of multi-yearand first-year ice floes (Fig.2). Thecorresponding surface topography wasmeasured by theodolltes and by stereophotographicmethods of a closer study ofthe relatlonship between top and bottomfeatures.On an average, isostatic equilibriumshould exist for an lce field. Asillustrated by Figure 2, this is farfrom the case in the two-dimensionalplan. We observe also a marked differencein the smoothness of top and bottomtopography for the two ice typesconsidered.A series of effects may play animportant role in this connection.Thermal and dynamlc effects operate forlnstance on different time scales, oldlce is less plastic than young ice,thermal processes affect prlmarily thesurface and the sides of an lce floe,consolidation of ridges and melt waterdralnage affect multl-year ice only, snowcover varlatlon on the surface causes avariatlon of the heat transfer throughthe ice, etc. All these effects have tobe taken into consideration when studyingthe small-scale variation of the top- andbottom-side of the ice.The autocorrelation (Fig.J) showsthat there is very little correspondencebetween lce thicknesses more than 25 mapart (Johnsen, 1987). This number varieswith the instrument resolution and theroughness of the ice floe. Rothrock andThorndike (1980) used data that wasaveraged over 20 square meters while ourinstrument averages over 0.1 square200150SEALEVEL100-5..0 •• ":::50-40 -30 -20 -1010 20 30 40 50HORIZONTAL 1m)10 15 20 25 30HORIZONTAL DISTANCE 1M)35SEALEVEL200-5-40 -30 -20 -1010 20 30 40 50HORIZONTAL 1m)~ 150~zoi 1008~ 50Figure 2. Typical samples of bottom andsurface profiles for first-year ice(above)and for multi-year ice. The sonarhead operated at a depth of 15 m belowthe surface.10Figure J. Theobserved formulti-year ice.15 20 25 30autocorrelatlon (dm 2 )first-year (above)35asand266

meters only when looking vertically. Theyfound that the autocorrelation becamevery small at distances greater than 150meters.We observe a markedly larger scatter1n the autocorrelatlon for the multi-yearlce as compared with the first-year lce(F 19. 3). Th1S may reflect the fact tha tmechanical and thermal processes haveacted over a longer period on the former.This is also expressed by the markedlyhigher autocorrelatlon observed for thefirst-year ice when compared with themulti-year ice within distances of 15-20m.The autocorrelation functions havebeen used to perform an objectiveinterpolation when modelling theunderside topography from the scanningsonar profiles (Fig.4).3. Orift featuresThe average wind pattern in theBarents Sea indicates a southward driftof the ice over the northern shallowareas. Approaching the southwestern areaof the shelves the cold water will turnnorth-northwestward and will ultimatelynourish the cold coastal current west ofSpitsbergen. the main island of theSvalbard Archlpelago. The tidal effect onthe current is very pronounced and eddyformation on the oceanlC polar front willalso affect the drift to a varying degree(Fig. 5).Three automatlc ice drift buoys(ICEXAIR) were parachuted on to the ice atdifferent latitudes in the Barents Sea inDecember 1986. The northernmost stationdeployed at 80° N-400 E drifted into thePolar Ocean and passed through the FramStrait during Hay 1987. The ice In them03t southern deployment area. at 77 Nand40 E. drifted southward and melted incontact wlth the warmer water. The buoydeP10ye& in beiween these two locations.at 78.5 Nand 40 E. drifted southwestwardsand was still on the ice at the end of Hay1987 at 76.1 0 N and 22 0 E.The average drift and standarddeviations for this buoy do not seem tochange markedly when approaching themargins of the ice fields (Table 31. Thiswas unexpected and may indicate that thewhole ice field has moved in a more orless uniform way during the drift. Towhat extent this is a characteristicfeature for the ice fields in the BarentsSea will be studied closer during thecontinuation of the buoy program.Figure 4. The underside of first-year ice with some ridging as estimated with the aid ofa scanning sonar. The ice floe is turned upside down. The dimensions of the box are 40 mby 40 m by 10 m.267

effect (Fig.5) is supposed to be ofparticular lmportance in th1S connection.ReferencesErlingsson. B. 1987. Sea ice rheology. Aphysical study of internal forces in seaice. M.S. Thesis (in Norwegian).Inst1tute ofOslo.Geophysics. University ofFigure 5. Dr1ft of Argos buoy with sail.The buoy was deployed in the beginn1ng ofJune and recovered at the end of August1984 after hav1ng drifted from easttowards west along the oceanic polarfront. We note the numerous tidal orinertial nodes. Eddy format10ns on thepolar front make the buoy stay longer insome areas than in others. (Modifiedafter Loeng. 1986).Table 3. Average ice drift speeds (m s-1 )and standard deviation 1n d1fferentlatitudinal intervals. The d1stance fromthe ice margin is given in kilometres.The total ice drift is denoted by U andthe components are Ux and Uy. x positiveeastwards and y positive northwards. Therespective standard deviation 1S denotedo.Interval: 79-78 78-77 77 -76Distance: 250 150 50U 0.177 0.194 0.195a 0.108 0.119 0.114Ux -0.037 0.007 -0.061a 0.137 0.186 0.161Uya-0.0170.150-0.0250.130-0.0140.147The average drift s~fed. varyingbetween 0.177 and 0.195 m s is nearlytwice the average drift speed of 0.105 ms-1 observed by Vinje and Finnekasa(1986) in the area with a maximum icevelocity in the Fram Strait. The standarddeviation is also about twice as highcompared with that observed in the latterarea. This reflects the greatervariab11ity of the driving forces 1n amarginal sea. Because of the shallownessof the Barents Sea. the tidal or inertialJohnsen. A.s. 1987. Imaging the undersideof sea ice using a scanning sonar. NorskPolarinstitutt Internal Report (inNorwegian) .Loeng. H. 1987. Investigations in theBarents Sea necessary for the analysis ofthe consequenses of o11-activities.Institute of Marlne Research InternalReport (1n Norweg1an).Rothrock. D.A. ~ Thorndike. A.S. 1980.Geometric properties of the underside ofsea ice. J.Geophys.Res.Vol 85. No C7.Sztersdal. G. ~ Loeng. H. 1984.Ecolog1cal adapt ion of reproduction 1nArct1c cod. in ·Proc. of the Soviet­Norwegian Symp.: Reproduction andRecruitment of Arctic Cod' (Gode. O.R. ~Tilseth. S .• eds.). Institute of MarineResearch, Bergen.V1nJe, T. 1980. On the extreme sea iceconditions observed in the Greenland andBarents Seas in 1979. NorskPolarinstitutt Arbok 1979.Vinje, T. 1985. Drift, composition,morphology and distribution of the seaice fields 1n the Barents Sea. NorskPolarinstitutt Skrifter Nr 179 C.Vinje,T. ~ F1nnekasa,0. 1986. The icetransport through the Fram Strait. NorskPolarinstltutt Skrifter Nr 186.268

ANALYSIS OF ICE ISLAND MOVEMENTW. M. SackingerUniversity of Alaska, Fairbanks, Alaska, USAH. R. TippensCalifornia Institute of Technology, Pasadena, California, USAAbst.ractTrajectory and environmental data fromHobson's Ice Island are examined for the periodMay 6 th to May 16 th , 1986. During thistime the ice island moved a distance of over 20kilometers to the southeast. Wind shear, waterdrag, and Coriolis forces are computed andcompared with the forces estimated from theobserved motion. The total forces acting on anice island are found to be a balance of relativelylarge forces opposing one another. Mechanismsfor pack ice and ocean current forces are proposed.Introd\lctionIce islands are the largest ice featuresin the Arctic Ocean. They represent the sin­"Ie largest hazard to offshore structures in the;outhern Beaufort Sea. An understanding ofthe forces that cause ice island movement is desiredto minimize the chance of a collision betweenan ice island and an offshore structure.During the summer of 1986, two Argosbuoys which had been deployed on one end ofHobson's Ice Island reported position, windThis is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987. © The Geophysical Institute,University of Alaska. 1987.speed and direction, barometric pressure, andair temperature several times per day. Thisdata is used to compute the wind shear, waterdrag, and Coriolis forces. These forces arethen compared with the forces that apparentlyacted on Hobson's Ice Island as calculated fromthe observed motion. The difference betweenthe modeled forces and the forces calculatedfrom the observed motion represent forces thatact on an ice island that are not well understood,including ice pack and ocean currentforces. Certain properties of these forces canbe inferred from their relationship with otherobservable aspects of ice island motion.Description of Hobson's Ice IslandHobson's Ice Island is the largest piece ofice in a group of ice islands and fragments locatednorthwest of Axel Heiberg Island. Thisgroup is believed to have calved from the eastside of the Ward Hunt Ice Shelf in the fallof 1982 (Jeffries and Serson 1983). Hobson'sIce Island is roughly rectangular in shape, hasa surface area of 26.0 km 2 , a mean thicknessof 42.5 m, and weighs approximately 7.0185 xlOll Kg (Jeffries et. aI., 1987).Movement Event of May, 1986A large movement event occurred betweenMay 6 th and May 16 t h, 1986, when the ice269

... ~~ .. ~... ~.~~ ~ard Hunt Ice Shel~jzo~r------r------~--~~----~~~~~------~zozoo~r---------~---------t~----~~~L-----~oa)zMEIGHEN I LANDooa)10~O~o~W-----i9-8Lo-W--------9~6io~w~~--~9~41o-w--------9-12°WFigure 1. Track of Hobson's Ice Island from May 6 th to May 16 th is shown against coastal outline ofAxel Heiberg and Meighen Islands. Small map shows relative positions of the study area and WardHunt Ice Shelf.270

island moved 20 km to the southeast. The trackof Hobson's Ice Island during this episode isshown in Figure 1. The movement data were dividedinto parallel- and perpendicular-to-shorecomponents for analysis. These components areshown in Figures 2 and 3.Forces Acting on the Ice IslandThe forces acting on an ice island to beconsidered here are wind shear on the top surface,water drag from the differential movementbetween the ice island and the water, Corioliso~I ~----,-----.-----,-----,-----.6.0 8.0 10.0 12.0 14.0 16.DateFigure 2. Parallel-to-shore components of theMay movement event.oI"-N~ ____ .-____ ,-____ ,-____ ,-__ -,6.0 8.0 10.0 12.0 14.0 16.0DateFigure 3. Perpendicular-to-shore componentsof the May movement event.Winds shifted from the north-northwestto the southeast on May 6 th • These winds persisteduntil May 9 th , when the wind shiftedback to the north or northwest for the remainderof the movement event. Surface windswere measured at a height of 2 meters. Thesedata are shown in Figures 4 and 5 brokeninto parallel- and perpendicular-to-shore components.(Positive on the graph is toward thenortheast and offshore respectively). Hobson'sIce Island started moving parallel-to-shore andslightly offshore late in the day of May 9 th • OnMay 9 th and May 10 th the ice island turned andstarted moving rapidly away from shore and inthe opposite direction along shore. It is interestingto note that most of the offshore movementoccurred from May uth to 13 t ", during aperiod of onshore wind. On the 13 th and 14ththe ice island moved back toward shore and bythe 16 th its movement was arrested, apparentlylocked in place by surrOl,mding pack ice compressedagainst the shore.force, and the force from the interaction betweenthe ice island and the surrounding icepack.Wind shear force is given bywhere Po. = 1.3 gm/m 3 is taken as the densityof air, ·co. = 0.0012 is the skin drag coefficientfor air over smooth ice (Pease et. al. 1983),A, = 26.0 Km 2 is the surface area of the top ofthe ice island, Vo. is the velocity of air, and v,.is the velocity of the ice island. The mildlyundulating surface of the ice island is usuallylocally smooth, with blown snow drifted againstits edges; form drag for the air is considered tobe negligible compared with surface drag.Water drag is made up of both surfacedrag and form drag. The surface water dragrelation is similar to the wind shear equation271

0N0N00UOJ~OE0UOJ~OE0o~4------.-----.-----r-----r----,6.0 B.O 10.0 12.0 14.0 16.0DateFigure 4. Parallel-to-shore wind data from AR­GOS buoy 2996.where Pw = 1.032 gm/cm s is taken as the densityof sea water and c w • = 0.00132 is the surfacedrag coefficient for water and ice (Langleben1982). The second component of the water dragis caused by form drag and is given bywhere h = 0.36 is the pressure drag coefficientand AI is the frontal area of the wetted portionof the ice island (Shirasawa et. aI., 1984).These forces are turned 24° to the right toaccount for the Ekman spiral layer (McPhee1982), as an initial approximation.Consider a two-dimensional Euclidian coordinatesystem with the origin at the NorthPole and the x-axis aligned with the 0° longitudeline. Points from the Earth's surface couldbe projected onto the plane as follows:x = 2R. sin 90 - "')(-2- cos(tl)y = 2R.sin (90; "') sin(tl)where R. = 6,356 km is the radius of the Earthand '" and tl are degrees latitude and longituderespectively. In this coordinate system theCoriolis force can be writteno~4------'-----.-----.-----r----,14.0 16.06.0 B.O 10.0 12.0DateFigure 5. Perpendicular-to-shore wind datafrom ARGOS buoy 2996.where I'ni, is the mass of the ice island, w. isthe angular velocity of the Earth, and II" andII~ are the speeds of the ice island in the fi andfi directions respectively. These forces are allresolved into components and plotted in Figure6 as a function of time during the movementevent.Residual ForcesResidual forces are the forces acting onthe ice island that were not considered in theprevious section. It is expected that two ofthese forces arise from; (1) pack ice forces thatact on the boundary between the ice pack andthe ice island; and, (2) ocean current forces, relatedto mesoscale sea surface tilt. No directmeasurement of currents beneath the ice islandwere made, nor were pack ice boundary forcesmeasured independently, so that these must beinferred from the trajectory analysis.Residual forces can be estimated in astraightforward way. The Argos positioningsystem locates the ice island to within one kilometer,several times per day. The positiondata often comes in groups where the individualmeasurements are only minutes apart. We useda weighted averaging scheme (Chambers et. ai.1983) to reduce error variance in the positiondata. Regularly spaced estimates of position,velocity, and acceleration were obtained by fittinga set of cubic spline interpolating polyno-272

0rigzC!Perpendicular0 0Wind Shear Force0 0on0rigonzC!~o ~oParallel0 0onI0 0gg0 0ri

mials to the x and y components of the positiondata and taking the first and second derivativesof these polynomials.The magnitude of the residual force appearsto be linearly related to the speed of theice island. We calculated a correlation coefficientof 0.98 for a linear least-squares fit of thisrelationship. The plots in Figures 7 and 8 showthis residual force compared to the negative ofthe Corio lis force. There is a strong correlation,suggesting that the residual force generallyopposes the Coriolis force. Other investigatorshave found a significant internal packice stress directed opposite to the Coriolis force(Hunkins 1975; Yan 1986). Thus, an elementaryconcept for the residual force could be aforce equal in magnitude and opposite in directionto the Coriolis force.added mass of consolidated pack ice floes tothe right of the direction of motion, and thesea surface tilt.Mountain Barrier EffectThe total force acting on a moving ice islandappears to be the sum of several largerforces acting against each other. Thus, forcesthat are smaller than some of the largest forcesshould not be discounted as insignificant. Forexample, the ocean surface tilt force may contributesignificantly to ice island movementnear shore. The surface elevation of AxelHeiberg Island rises very abruptly from sea levelto over 1,500 meters. This could cause a mountainbarrier effect (Parish 1983). When thegeostrophic wind blows offshore a low pressureResidual ForcesC.9IjoLis_ ForceoIf)ooResidual ForcesC.9IjoLis_ Force0vi0viz~~oz~~o0viI0viI0ci6If)I6.0 8.0 10.0 12.0Date14.0 16.00ci6viI6.0 8.0 10.0 12.0Date14.0 16.0Figure 7. Perpendicular-to-shore residual forcecompared with the negative of the Coriolisforce.Figure 8. Parallel-to-shore residual force comparedwith the negative of the Coriolis force.The part of the residual force that is notmodeled as the opposite of the Coriolis forceis obtained by simply adding the residual andCoriolis forces together. This may convenientlyand arbitrarily be called the non-Coriolis residualforce. In Figures 9 and 10 this non-Coriolisresidual force is plotted with the total force actingon the ice island. This is a measure of thecombined effects of the variability of the packice force contact area with the ice island, thearea forms near the shoreline, causing the sealevel to rise in that area. Conversely, when thegeostrophic wind blows onshore, a high pressureregion forms against the mountain barrierand the water level drops. The resulting oceancurrents could explain why the ice island movedoffshore immediately after the wind shifted onshore(after a period of offshore winds). Analysisof data from the entire array of pressure sensorson other islands in the area of Hobson's IceIsland is currently underway, to quantify this274

olflNon-Coriolis Residual ForcesJotQL[orceNon-Coriolis Residual ForcesJotQL[orce~lflz~~o0viI0viI0ci6viI6.0 B.O 10.0 12.0Date14.0 16.00ci6lflI6.0 B.O 10.0 12.0Date14.0 16.0Figure 9. Perpendicular-to-shore non-Coriolisresidual force compared with total force.Figure 10. Parallel-to-shore non-Corio lis residualforce compared with total force.possibility.Pack Ice ForcesPrior to the movement event, the ice islandwas held in place by surrounding pack ice.At this time the pack ice force was simply theopposite of the wind force. This can be seen inFigures 9 and 10, from May 6 th to late in the dayof May 7 th • The beginning of a movement eventis correlated with offshore wind. However, thereis a time lag between the offshore wind and thebeginning of the movement event. In this particularevent the wind had shifted back onshorebefore the ice island made its main offshore motion.Thus, there was an offshore force that wasnot caused directly by the surface wind. Theforce may have been caused by the momentumof a large area of coupled pack ice, or it mayhave been caused by the mountain barrier effect.This process is still under investigation.When an ice island moves in the same directionfor a long period of time (approximately2 days) a triangular area of broken pack iceforms against the side of the ice island. Themechanism for transferring force from the icepack to the ice island is through shear alongthe boundary of this area. The length of theice wedge could reach 2.5 to 3.5 ice island diameters,based on observations of grounded icemasses (Barrett and Stringer 1978). The icewedges will also transmit their own wind shearand water drag forces to the ice island.ConclusionsThe forces that initiate an ice islandmovement event originate with air pressure gradients,but are not simply correlated with windshear forces. Rather, the net force is a resultantof larger forces that act nearly oppositeand partially cancel one another. Relativelysmall forces are important to accurately modelthe trajectory of an ice island for some transientmovement sequences. The pack ice force on theboundary of the ice island, together with thesea surface tilt effects, can be combined into aresidual force term which is of the same orderof magnitude as the Coriolis force, but whichis oppositely directed. Evidence suggests thatthe mountain barrier effect may produce transientsea surface tilt conditions which move theice island occasionally against the direction ofthe prevailing surface winds. Once the possiblecontributions of the mountain barrier effect arebetter understood the pack ice forces may bemodeled by the ice wedge shear mechanism.AcknowledgementsThe support of this research by the U.S.275

Department of Energy, Morgantown EnergyTechnology Center (Contract No. DE-AC211-83MC20037) and also by the Polar ContinentalShelf Project (Canada), is appreciated by theauthors.ReferencesBarrett, S.A., and Stringer,W.J.,1978, "GrowthMechanisms of Katie's Floeberg", Arctic andAlpine Research 10(4): 775-783.Chambers, J.M., Cleveland, W.S., Kleiner, B.,and Tukey, P.A., 1983, Graphical Methods forData Analysis, Duxbury Press, Boston, pp.91-124.llunKlllS [("'\ and Ocean Engineering UnderArctic Condi1.i()II~,Filirhanks. Alaska,1987,W.M.Sackingerand 1\1.0 . .Jeffries, eds., (in press).Langleben, M.P., 1982, "Water Drag Coefficientof First Year Sea Ice" ,Journal of GeophysicalResearch 87(C1): 573-578.Me Phee, M.G., 1982, "Sea Ice Drag Laws andSimple Boundary Layer Concepts, IncludingApplication to Rapid Melting" , USA Cold RegionsResearch and Engineering Laboratory,CRREL Report 82(4).Parish, T.R., 1983, "The Influence of theAntarctic Peninsula on the Wind Field OverThe Western Weddell Sea" ,Journal of GeophysicalResearch 88(C4): 2684-2692.Pease, C.H., Salo, S.A., and Overland, I.E.,1983, "Drag Measurements for First-Year SeaIce Over a Shallow Sea" ,Journal of GeophysicalResearch 88(C5): 2853-2862.Shirasawa, K., Riggs, N.P., and Muggeridge,D.B., 1984, "The Drift of a Number of IdealizedModel Icebergs", Cold Regions Science andTechnology 10: 19-30.Yan, M., 1986, The Relationship Between IceIsland Movement and Weather Condition~,M.S. Thesis, University of Alaska-Fairbanks.DiscussionD. DICKENS: Is the ice island beingsimply advected with the surrounding packice or is there evidence of relativemotion between the island and thesurrounding ice.W. SACKINGER: Open water has often beenobserved on one side of the ice islandduring movement events, indicating thatit moves with a velocity different fromthe surround ing pack ice. The calculationspresented in our paper show thatour drag, water drag, and especiallyCoriolis forces are different than thosefor pack ice .D. SODHI: In the literature on themodeling of iceberg drift, computation ofCoriolis force acting on a driftingiceberg is based on the relative velocityof iceberg with respect to water. Thisis done to incorporate the forces causedby this lateral pressure gradient as aresul t a f sea surface slope. This hasnot been done in your paper, resulting ina large imbalance of forces.W. SACKINGER:The approach which we have adopted, usedin modeling sea ice motion, is to computeCorio lis force based upon the ice islandvelocity relative to the sea floor. Thisis the form of our actual data. Since nocurrent meters were deployed beneath theice island, the relative velocity betweenthe ice island and the water beneath wasnot known. As pointed out in the paper,the computed residual force contains boththe pack ice force on the ice islandboundary, and also the portion of thewater drag force which arises from seasurface tilt. The sea surface tiltarises from the passage of atmosphericpressure systems, and the resultingcurrent flow, a dynamic process which wehave not attempted to model as yet. Itmight also be noted that the concept ofcalculating ice island velocity withrespect to the water beneath it isfraught with complications because of thetransient nature of the ice island276

driving forces and pack ice forces. Thewater velocity profile beneath the iceisland is not constant in time, andcertainly varies with depth beneath theisland and at different locations withrespec t to the submerged leading edge.Without a longtime series of detailedcurrent measurements. or a set ofcomputed transient solutions to theNavier-Stokes equations for turbulentflow beneath the ice island, the separationof sea surface tilt forces from packice boundary forces is difficult.277

CONSTITUTIVE RELATIONS IN SEA ICE MODELSLu Qian-mingTianjin University, Tianjin, CHINAJ esper LarsenConsultant, Danish Hydraulic Institute, H¢rsholm, DENMARKPer TrydeTechnical University of Denmark, Lyngby, DENMARKAbstractThe paper gives a common form forthe constitutive relations that havebeen used in existing ice models. A newconstitutive relation based on floe collisionsis considered in some detail.Again the stress strain rate relationcan be given in a form that conformswi th previous theories. Merits of thedifferent relations are discussed.IntroductionThe time development of a sea icecover is a complex dynamic-thermodynamicproce ss. Dynamically the l.ce cover isforced by wind and currents and thermodynamicallythe ice volume must be inbalance with the net short and long waveradiation and heat exchanges across theice-ocean and ice-atmosphere interfaces.In the dynamic analysis of polarice drift Hibler (1984) found that theresidual force in a dynamic balance ofthe external forces has a magnitudewhich is about 55% of the largest force,i.e_ the wind. This residual force isThis is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22,1987. © The Geophysical Institute,University of Alaska, 1987.due to the deformation of the ice cover.In order to compute the internal forcefrom the deformation field a constitutiverelation is needed.Constitutive relations characterizethe individual material and its reactionto applied loads. For sea ice itshould reflect a variety of factors,among them the type of ice, the grainstructure, the density, the salinity,the porosity and the temperature. Researchon ice constitutive relations canbe separated into two groups accordingto the scale of the problem, small scaleand large scale. On a small scale seaice is studied in the same fashion asmetals, whereas on a large scale sea iceis studied as a geophysical phenomenon.Hence, research on the large scale isalso called a phenomenological study.Though these two groups of research aredifferent in concepts, methods and results, the two groups are often interconnected,mutually beneficial and e­qually important to the general advancementof ice mechanics. However, thelarge scale research will be emphasizedin this paper, because the constitutiverelation that is applicable in a sea icemodel is used for the calculation of theice drift and thickness change on thegeophysical scale.In this paper, areview of the279

esearch of the constitutive relationsused in sea ice modelling is presented.A new constitutive relation based on icefloe collisions is also reported. Finally,a common form of the constitutiverelations for large scale sea ice modellingis shown and some details of thecoefficients in the relations are discussed.Constitutive Relations Usedin Existing Ice ModelsThe internal force with componentsF., i = 1,2 is given in terms of thestress tensor a ij, i,j = 1,2, byElastic modelA simplified constitutive relationfor sea ice is obtained using the linearelasticity theory. An ideal linear elasticprocess is isothermal and the bodyreturns to its initial state as soon asthe external forces are removed. Theconstitutive equation for a linear elasticisotropic material is defined byHooke's lawa ..~J(4)and·the inverse equation takes the formF.~__ a a ..ax. ~JJ(1)Here and throughout the paper Einstein'ssummation convention is used,i.e. the expressions are summed overrepeated indices.The state of deformation of the iceis either described by the strain tensoraU. au.1£ .. (-~+ -1)~J 2 aX ax. j ~or the strain rate tensorau. au.1 (-~£ij +2 aX _J)ax. j ~(2)(3)where we have used the small strain(rate) approximation and U. and u., i =1,2, are the displacement Jomponents andvelocity components, respectively. Constitutiverelations linking a.. toeither £ .. or ~ .. will be considel~.~J ~JSeveral different constitutive relationshave been used previously in thesea ice model e. g . an elastic model(Mellor 1983), a viscous model (Glen1970), an elastic-plastic model (Coon etal. 1974; Pritchard 1975), and a viscous-plasticmodel (Hibler 1977). All ofthese models are based on continuum mechanicsand the ice is considered to bea two-dimensional continuum.£ ..~J(5)E is Young's modulus and v is Poisson'sratio.In general, the factors which influencethe magnitude of the elasticconstants of sea ice can be divided intotwo groups, the physical group and themechanical group. The physical groupconsists of salinity, density, porosityand temperature and the mechanical grouppertains to the loading characteristics,i.e. stress, strain and the type ofstress state. It is apparent that thedependence of ice behaviour on the physicalparameters reflects the naturalcorrelation between the micro and macroproperties of the material. However, asfar as the mechanical factors are concerned,their pronounced influence onthe magnitude of the elastic constantsindicates that at least some of the assumptionsof the linear elastic theoryare not valid for sea ice. For example,the deformation of sea ice on a largescale usually can not be recovered. Thisillustrates that not only the linearelastic assumption but also a non-linearelastic assumption is not always validfor large-scale sea ice.Viscous modelA viscous model is also a simplifiedconstitutive relation for a sea icemodel. It was first used in a sea icemodel (Campbell 1965) in which the icewas supposed to behave as a two-dimensional,highly-viscous fluid. In asimple Newtonian viscous fluid, the con-280

stitutive relation reads(6)where 0 .. is a deviatoric stress tensor,~.. is 1f strain rate tensor and n is aN~~tonian viscosity.Glen (1970) proposed a new form ofthe relation, called a physical viscousmodel. In the new model, it is assumedthat the viscous relation is not onlybetween shear stresses and shear strainrates, but also between "hydrostatic"(two-dimensional) compressions and areachanges. The strain-rate tensor can besplit into two parts in the same manneras the stress tensor. Thus, the constitutiverelation from the physical viscousmodel reads0ij = 2n~~j + ~~kkOij(7)where n and ~viscosities.are the shear and bulkIf it is necessary to abandon thelinear viscous model, it can be shownthat the generalized relation betweentwo second-rank tensors of stress andstrain-rate in two dimensions is of theform(8)where Band B are general functions ofthe tW$ invariants of the strain-ratetensor, the first and second invariants.This relation is called the double parameterviscous model.Elastic-plastic modelFrom experiments and observations,it is quite evident that ice does notrecover completely to its initial undeformedconfiguration, when the load isremoved. The ridging process especiallyis irreversible. These phenomena arevery similar to the behaviour of plasticmaterials.In the classical plasticity theory,it is assumed that plastic deformationoccurs when the stress is equal to acertain stress limit, which in the uniaxialcase is defined as the yieldstress, a . When the stress is below theyYlcld stress, a , no plastic deformationyoccurs.Several idealized stress-straincurves that are simplified material modelsin plastic theory are illustratedin Fig. 1 (from Malvern 1969).Figure 1. Idealized stress-strain curves.a) rigid, perfectly plastic model,b) elastic, perfectly plastic model, c)rigid plastic with linear hardening model,d) elastic-plastic with linearhardening model.The curves in Figs. la) and lc),neglect elastic strains altogether andthey describe the rigid, perfectly plasticand the rigid linear hardening behaviour.The curves in Figs. Ib) and Id)consider elastic strains and they describethe elastic, perfectly plastic andelastic-plastic with linear hardeningbehaviour. However, even without deformation,but with the thermodynamic process,the sea ice may harden and a increasewith freezing, and a may ~akenand decrease with melting. yTo obtain the constitutive relationof an elastic plastic model of seaice requires that a yield criterionwhich determines whether plastic behaviourcan occur, a flow rule for plasticbehaviour, and a deformation law for e­lastic behaviour of sea ice, are established.A yield criterion can be describedby a function of the stresses,F(I ,I ) which is named the yield conditioh.!t gives a yield curve in the twodimensionalstress space and is analogousto the yield surface in a typicalthree-dimensional stress space. An elastic-plasticmaterial is defined as follows,ffF(I ,I ) l 2< 0 elasticity(9)if F(I ,I ) 0 plasticityl 2where II and 12 are the first and thesecond stress lnvariants, which read in281

two dlmenslons{I ~O +0112I ~ - 0 02 1 2(10)where oland o? are the f lrst and thesecond principaI stresses.A new yield criterion for a sea icemodel was developed by the AIDJEX group(Coon et a1. 1974; Coon 1980), i.e.(11)where the parameter P denotes the compressivestrength considered as a functionof the ice thickness distribution.The hardening and weakening of sea iceare described by the parameter P.Figure 2.shapes.0',LENS SHAPECIRCULARTEAR DROPYield curves of severalAny yield curve that satisfies thefollowing constraints In the principalstressspace is physically acceptablefor sea ice.(a) the yield curve lS symmetric aboutthe II aXlS,(b) the Yleld curve passes through thepOlnts (0,0) and (P,P),(c) randomly oriented cracks shouldpreclude tension in any direction,(d) the yield curve should be largelyconfined to the quadrant in whichthe principal stresses oland O 2are negative.In Fig. 2, several yield curves aregiven that satisfy the constraints mentionedabove. The tear drop curve hasbeen suggested in the AIDJEX sea icemodel (Coon 1980), and the cone with oneray coinciding with the P axis was suggestedby Coon and Pritchard (1974).A flow rule in common use is theplastic potential function; it reads, P10, ,lJ A a!~, IF~OlJ(12)where ~~ is the plastic strain-rate, Ais a no~lnegative scalar function to befound as part of the solution. Thisequation states that the plastic strainrate is normal to the yield curve.In elastic-plastic models, theelastic behaviour is also assumed to belinear and isotropic.The yield function for the Clrcularyield curve is°(13)From (12) the constitutive relationfor the plastic part is obtained asP 10, ,lJ X 10, lJ ,2 P lJ0, ,where• 2 '2 ~2 ~2 )}1/2f'> {2 (Ell + 1022+ +12 21(14)(15)As indicated by Coon et al.(1974), Coon and Pritchard (1974) andPritchard (1975), the elastic strain isusually small compared to the plasticstrain-rate and the material approximatesrigid plasticity. But the elasticresponse must be included to solve thesystem of equations. The effect of thetype of sea ice was not considered inthe discussion above; in reality, theyield curve varies with different typesof sea ice (cf. Timco and Frederking1984) .282

Viscous-plastic modelA viscous-plastic constitutive relationwas proposed by Hibler (1977) andsince then the viscous-plastic constitutiverelation has been used successfullyin several sea ~ce models (Hibler1979; Tucker 1982; Preller 1985; Larsenand Lu 1986) .The foundation of the viscous-plasticmodel is a two-dimensional rigidplastic law described by an ellipticalyield curve. The elliptical yield functionis specified bywhere e is the eccentricity of the ellipse.Using the flow rule to equation(16), the explicit stress-strain raterelationship is obtained asP -2' 1 -2' 1a . . =-{e E .. +-2 (l-e )EkkO .. }--2PO.. (17)~J ~ ~J ~J ~Jwhere(18)The viscous-plastic constitutiverelation is obtained from (17) by carryingout a statistical average assuming~.. fluctuates according to a GaussiandI~tribution about a mean value .Dropping the mean value sign the r~Iationreads (Hibler, 1977)where n and ~ are called nonlinear shearand bulk viscosities, i.e.~ (~ij ,P)n(~ij'p)P /2~2~/ewhere the strength of sea ice,function of the thickness and(20)P, is acompact-ness of the ice. A semi-empirical equationof state relating the strength withice thickness and compactness has beenput forward by Hibler (1979).*-p = Ph· exp {-C(l-A)} (21)where P * and C are empirical constants,h is the mean thickness of ice and A isthe compactness.Constitutive Relations Basedon Floe CollisionsThe constitutive relations presentedin the previous section relied onphenomenological modelling of the iceinteraction term, with only little attentionto the micro-scale mechanicsthat gives rise to the internal forces.As such it can only be expected to describea uniform ice cover with a highdegree of compaction. In the marginalice zone the ice cover appears on asmaller scale as individual floes and itis believed that an understanding of theinteraction of the individual floes isnecessary for developing macro-scalecontinuum models.A new constitutive relation basedon the idea that all the momentum transferand energy dissipation take placethrough floe collisions has been 'developedon the basis of granular flow theory(Shen et al. 1984). A detailed derivationof a constitutive relation ofthis type for the use in ice models hasbeen given by Lu and Larsen (1986).Fundamental conceptsThe new constitutive relationsbased on the floe collisions are obtainedin terms of the granular flowtheory which has been greatly developedin recent years (Shen and Ackermann1982). The basic equation was establishedby Bagnold (1954), in whichstress and average momentum transfer byparticle collision are related. It readsa P .f.~M (m)mn m nwhere a(22)is the stress on the surfacenormal fS X acting in the X direction,P is the ~umber of particl~s per unitm283

area normal to the X direction, f 1St~1myOllision frequenc~ per particle andMl is the average momentum transferpe¥ collision in the X direction.nTherefore, to obtain the stressfield of a sea ice cover using the granularflow theory, three necessary factors,i.e. the number of floes in theconsidered area, the collision frequencyper floe and the momentum transfer percollision, need to be known. Because theshape and size of the floes vary, somesimplifying assumptions are necessary atthe present stage of the research. Theassumptions are:a)b)c)All the floes have the same shape,viz. the shape of a disc.All of the floes have the same physicalproperties, i.e. the samedensity, salinity, porosity andtemperature.In a computational grid the floeshave the same thickness.Below we give constitutive relationsbased on collisions of discs ofthe same size and multiple-size disccollisions.Constitutive relation based on the collisionof uniform sizeIn Lu and Larsen (1986) it is shownthat the constitutive relation can bewritten asaijThe fluctuation velocity V' isdetermined from an energy balance whichreduces towhereo (24)(25)" "where II and I are the first and thesecond 1nvarian£s of strain rate.As an intermediate step, the constitutiverelation based on the collisionsof disc of two different sizes wasderived in Lu and Larsen (1986) to whichthe reader is referred. Here, only theresult for multiple-size disc collisionsis given.Constitutive relation based on the collisionof mUltiple size discs.Again the constitutive relationcan be put in the form(23)a . . =1JPh (1+8) A 0 V'S"A'm Om(26)in which p is the density of the icefloe, 8 is the restitution coefficient,i.e. the degree of elastic recovery fromthe collision, A is the ice compactness,i.e. area of ice cover relation to totalarea, O:>A:>l, A is the compactness atclosest packing~ 0 is the disc diameterand v'is a random disc fluctuation velocity.For the deriv?t~on ~f 2(2,#) it is~ssumed"that V'»O (E + E2 ) , wherelEl and E2 are the major and minor princ1palcomponents of the strain rate.where h is the mean thickness of ice perunit area, S is the average gap sizebetween neighbouring discs, v'is arandom fluctuation velocity of ~fsc withan average diameter 0 . F , F , A' andlo 2are defined as foll~ws,m284

F =1F =2DJrnaxD rnHlDJrnaxD .m~nP(D) 'P(D')dDdD']max h+D2/D,2D. (D,2+D2)D'D'rn~nD DF = Jmax Jrnax33 D. D.m~n rn~nDiscussion(P(D)P(D')D' dDdD'(D,2+D2)DAll the constitutive relations consideredin this paper can be put in thecorrunon formDA'= J maxD .m~nP(D)'P(D')dDdD'(27)wherefor the elastic case(31)DD = (maxm J D minD'P(D)dDwhere P(D) is the probability density ofdisk sizes.Again the mean fluctuation velocityis determined from an energy balanceDmV'K V' 2+ K V' + K3 = 0lDm 2Dmwhere(28)(29)E ..~JE .. all other cases~J(32)and the coefficients, P, Q and Raregiven in Table 1.Form of the yield curveThe yield condition must be consideredfor any constitutive relationwith plastic properties. In section 2.3,several yield curves have been given(see Fig. 2). Other physically acceptableyield curves can be proposed. However,no criterion has been formulatedto determine the yield curve which mostrealistically describes behaviour ofpack ice. But there are some obviousdifferences between the curves, eventhough they are all acceptable.in whichFllF22D2Fl(12 + I 2)311 1D= Jmax Jmax P(D)P(D' )D. D.m~n m~nDD= Jmax Jmax P(D)P(D' )D. D.m~n m~ndDdD'(D,2+ D 2)D,3 D(D,2+D2)D'DdDdD'(30)(I)The different yield curves allowfor different tensile strengths.The circular yield curve gives thelargest tensile strength among thecurves given in Fig. 2, while notensile strength is allowed by thetear drop and the square-shapedyield curves.(II) The circular curve gives thelargest range of stress becausethe area of a circle is thelargest under the same constrainedconditions. The ranges given bythe ellipse and the lens-shape arevariable with the different ellipticeccentricity and differentsine function, while the rangesgiven by the square and the circle285

Table 1. TI~ Coefflcients of Mechanical Properties in the Constltutive Relatlons.Mechanics Model P Q RElasticE\IE(1+\1) (1-2\1) 1+\10Viscous 1;' 2n' DViscous- t;-n 2nplastic12 PUniformsize disccollisionp (1+ t3 ) _A'-..3..,./_2_v_,-,- 2D4 A1/2_Al/2 3n'op (1+/3)2nA 3 / 2 V'Al/2_Al/2ol"iv'Multiplesize disccollisionph(l+B)A D V' F *) ,3n'S'A' m Dm 12ph(1+B)A D2V,2F *)n2'S'A' m Dm 2*) See eq. 27.are not variable under the givenconstrained conditions.mated by the angle a between the N directionand the V A/ Bdirection (Fig. 3).(III) The circular and elliptic curvesgive the simplest forms of themathematical expression, i.e. theycan be defined by a single expression.With an elliptic curve, therange of the stress state underthe yield condition is easilychanged by giving a different ellipticeccentricity.Questions about the new constitutiverelationsBecause the constitutive relationsbased on the collision of ice floes havebeen constructed only recently, thereare many unsolved problems and the relationsneed to be examined and improved.A major assumption in derivinS2 tJ;12 I~~stitutiverelation is V' »D (e:h+e: 2) .The assumption leads to a simp e estimateof collision frequencies and enablesthe angle Ci1between the N directionand the V' dlrection to be approxi-Figure 3. The relation between fluctuation~elocity, V', and relative velocity,V / ' in the collision between twoA Bdiscs.But it is evident that the assumptionseriously affects the applicabilityof the results. In order to avoid thisproblem Monte Carlo simulation has been286

used by Shen et a1. (1987). However, theconsequence of the assumpt~on should beconsidered further, espec~ally in concreteapplications, before the analyticalapproach can be discarded.Concluding RemarksSeveral constitutive relations havebeen discussed. They can be divided intwo categories, i.e. a phenomenologicaltheory and a floe collision theory. Theconstitutive relations from the phenomenologicaltheory have been used inseveral large scale sea ice models,whiJ e the others are new relations forthe Marg~nal Ice Zone (MIZ).From the experience obtained fromactual simulations and from the discussion,it is concluded that the viscousplasticconstitutive relation is adequatefor the simulation of the centralpart of a sea ice cover. It can describethe behaviour of large scale sea ice.The new constitutive relationsbased on floe collisions are believed tobe a good description of the main mechanismof dynamics of sea ice in the MIZ.But, as mentioned above, there are severalquestionable assumptions in thederivation of these relations. Some ofthe assumptions may lead to inaccurateresults. Therefore, the relations shouldbe exam~ned and improved for use in thesimulation of the MIZ.AcknowledgementThis work is supported by the DanishTechnical Research Council throughgrant No. 5.17.7.6.02.ReferencesBagnold, R.A. (1954) Experiments on gravity-freedispersion of large solidspheres in a Newtonian fluid undershear. Proceedings of the Royal Societyof London, Ser. A, Vol. 225, pp. 49-63.Campbell, W.J. (1965) The wind-drivencirculation of ice and water in a polarocean. J. Geophys. Res., 70, pp. 3279-3301.Coon, M.D., Maykut, G.A., Pr~tchard,R.S., Rothrock, O.A. and Thorndike, A.S.(1974) Modeling the pack ice as an elastic-plasticmater~al. AIDJEX Bullet~nNo. 24, pp. 1-106.Coon, M.O. and Pr~tchard, R.S. (1974)Application of an elastic-plastic modelof Arctic pack ice. In, The Coast andShelf of the Beaufort Sea, edited byJ.C. Sater and J.C. Reed, pp. 173-194,Arctic Institute of North America,Washington, D.C.Coon, M.D. (1980) A review ot AIDJEXmodeling. In Pritchard, R. S. (ed): Seaice processes and models. University ofWashington Press, Seattle and London,pp. 12-27.Glen, J.W. (1970) Thoughts on a viscousmodel for sea ice. AIDJEX Bulletin, No.2, pp. 18-27.Hibler, W.O. III (1977) A viscous seaice law as a stochastic average of plasticity.J. Geophys. Res., 82, pp. 3932-3938.Hibler, W.O. III (1979) A dynamic thermodynamicsea ice model. J. of PhysicalOceanography, 9, pp. 815-846.Hibler, W.O. III (1984) Ice dynamics.USA-CRREL, Monograph 84-3.Larsen, J. and Lu, Q.-M. (1986) Sea icemodelling in the East Greenland Area.ITC-86 proceedings, Cambridge, Mass.USA, pp. 147-156.Lu, Q.-M. and Larsen, J. (1986) A constitutiverelation for the maginal icezone (MIZ) Theory and Equations.Danish Hydraulic Institute, EGC (EastGreenland Current) Internal Report No.28.Malvern, L.E. (1969) Introduction to themechanics of a continuous medium. EnglewoodCliffs, New Jersey: Prentice Hall.Mellor, M. (1983) Mechanical behaviourof sea ice. USA-CRREL, Monograph 83-1.Preller, R.H. (1985) The NORDA/FNOC polarice prediction system (PIPS) - Arctic:A technical description. USA-NORDAReport 108.287

Pritchard, R.S. (1975) An elastic-plasticconstitutive law for sea ice. J. ofApplied Mechanics, 43E, pp. 379-384.Shen, H.H. and Ackermann, N.L. (1982)constitutive relationships for fluidsolidmixtures. J. of Mech. oiv. ASCE,Vol. 108, No. EMS, pp. 748-763.Shen, H.H., Hibler, w.o. III and Lepparanta,M. (1984) On the rheology of abroken ice field due to floe collision.MIZEX Bull. IV, USA-CRREL, pp. 29-34.Shen, H.H., Hibler, w.o. III and Lepparanta,M. (1987). The role of floe collisionsin sea ice rheology. J. Geophys.Res., 92, pp. 7085-7096.Timco, G.W. and Frederking, R.M.W.(1984) An investigation of the failureenvelope of granular/discontinuous-columnarsea ice. Cold Regions Science andTechnology, Vol. 9, pp. 17-27.Tucker, W.B. III (1982) Application of anumerical sea ice model to the EastGreenland Area. USA-CRREL Report 82-16.288

EXPERIMENTAL DETERMINATION OF THE FRACTURE TOUGHNESSOF UREA MODEL ICED. L. BentleyClarkson University, Potsdam, New York, USAD. S. SodhiU. S. Army Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire, USA1. P. DempseyClarkson University, Potsdam, New York, USAAbstractThe use of different types of model ice inexamining ice/structure interactions requires abetter understanding of the fracture behavior ofthese materials in order to accurately interpretthe results of model tests. There have been onlya limited number of fracture tests performed onmodel ice. A preliminary experimental study ofthe fracture toughness of the urea-doped modelice used in the test basin at CRREL has beencompleted. An "in-situ" wedge-loaded TDCB(tapered double-cantilever-beam) specimen geometrywas chosen. An expression for the fracturetoughness as a function of applied load,specimen geometry, and ice thickness was developedusing a finite element program.Several preliminary test series were conductedto examine the effects of specimen taperand size, ice thickness, loading rate, and sideloadedflexural strength on the fracture toughnessof the model ice. The results of these testsand the trends associated with them are presented.The experimental measurement of severaldifferent parameters proved to be useful incalculating the fracture toughness values. Theloading rate appeared to have the most effect onthe measured fracture toughness and no clear de-This is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987.pendence on either thickness or side-loaded flexuralstrength was found. The taper and scalingdown to a 2/3-size specimen seemed to have negligibleeffect, but any significant pre-load that thespecimen experienced measurably increased thefracture toughness.IntroductionTo reliably predict ice forces on structuresor the interaction between ice and ice-breakingships, for example, a better understanding of therole of ice fracture in these situations is needed.Physical modeling of such ice-structure interactionsis conducted in ice modeling basins in severalcountries, yet the fracture behavior of differenttypes of model ice is not well understood.In model tests the emphasis is on maintainingthe important forces in the interaction processin the same ratio as in the prototype, butreducing the problem to a much smaller scale.Typically, this has involved scaling the linear dimensions,strengths, stiffness, and thickness ofthe ice while the density and frictional characteristicsare kept the same as in the prototype.Atkins (1975) recognized the importanceof scaling ice fracture effects properly in modeltests and proposed the use of the "Ice Number,"the ratio of the inertia and ice fracture forces.Jones (1986) also suggested the use of anotherdimensionless number, M n , which should be satisfiedwhen modeling ice problems in which fractureand gravitational effects are both important.The ice fracture force used in both cases is a289

function of the fracture toughness of the ice, aquantitative measure of its resistance to crackpropagation.At the present time, there are no ASTMstandards for fract .Ire toughness testing in ice.The development and standardization of experimentaltechniques suitable for ice (Schwarz etal. 1981; Earle et al. 1984) have relied heavilyon the knowledge and experience gained in fracturemechanics measurements for metallic materials.However, the grain size in ice is severalorders of magnitude larger than that in metalsand the brittle-to-ductile transition in ice occursat a temperature very close to its melting point.Thus, it is important to realize that standardsdeveloped for metallic materials do not necessarilyapply to ice.Other investigations of the fracture toughnessof urea model ice (Parsons and Snellen 1985;Timco 1985) have used either compact tensionor three-point bend specimens. Here, a wedgeloadedfloating tapered double-cantilever-beamspecimen (TDCB) was used for static fracturetoughness tests of the urea-doped model ice inthe test basin at CRREL. The specimen geometrywas chosen to provide both crack path andcrack growth stability. The TDCB test specimenwas large and the ice was thick (50-90mm) in anattempt to obtain plane strain fracture toughnessvalues.Since it was not a standard specimen, aK-calibration to determine the Mode I stressintensity factor as a function of the appliedload, ice thickness, specimen width, and nondimensionalizedcrack length for the particularTDCB geometry used was needed. A modificationof the Kanninen (1973) elastic foundationprovided a check for the final K-calibration expressionderived using the J-integral capability ofABAQUS (1985), a finite element method program.Short series of tests on several ice sheetshave been conducted to ensure the repeatabilityof the results for a given ice sheet and to obtaina preliminary understanding of some of the testparameters. In these tests, the effects of the specimensize and shape, the loading rate, and theice thickness on the measured values of fracturetoughness were examined. In addition, the relationshipbetween side-loaded flexural strengthand fracture toughness was considered. A dramaticeffect of pre-load was also noticed.Double Cantilever Beam SpecimensThe choice of the wedge-loaded doublecantilever-beam(DCB) specimen (see Figure 1)for fracture toughness testing of the urea modelice was governed by several different criteria.Considerations in designing the experimentalprogram included a sufficient specimen thicknessand size to assure not only a state of plane strainat the crack tip, but also to promote crack pathand crack growth stability and the practicality ofworking on a floating ice sheet.In fracture toughness testing, Kle, the planestrain fracture toughness of a material, is independentof the specimen thickness and theamount of crack growth. This is because thetriaxial constraint is such that a state of planestrain is attained and the size of the zone ofnonlinear deformation is small compared to thethickness of the specimen. When the specimenthickness is not large enough, the fracture toughness,K 1e , will depend upon the thickness. Here,the fracture toughness of the model ice will bedenoted K 1e since no thorough thickness studyhas been completed.An examination of the requirement for crackgrowth stability for the DCB specimen revealsthat under prescribed displacement loading itshould be feasible to investigate both the initiationand steady-state propagation of a crackand to collect multiple measurements of the resistanceto fracture. The use of a wedge-loadedDCB specimen ensures crack growth stability,provided kM is large enough (a rigid loading device),but not necessarily crack path stability(Benbow and Roesler 1957). For the static K 1etests performed on urea model ice here, wedgeswith a large included tip angle (90°) were usedto provide a sufficient crack-parallel compressiveforce which encouraged the crack to propagatealong the plane of symmetry of the test specimen.Further considerations about the nature ofthe stress field around the crack tip and othersuggested size requirements (see Dempsey et al.1986) led to the choice of the TDCB as the initialtest specimen. Additional tests of similarsizedspecimens with no taper (named the RDCBhere) and the corresponding finite element analyseswere also performed to examine the actualeffect of the taper.Double Cantilever Beam ModelsSeveral different analytic approaches wereconsidered to derive expressions for the Mode I290

stress intensity factor for the TDCn since it isnot a standard specimen. The work discussedhere involved the extension of an elastic analysisofDCn specimens (Kanninen 1973). In addition,a finite element ]( -calibration of the TDCB specimenusing the J-integral capability of ABAQUSwas completed.Kanninen (1973) extended the simple beammodel of the DCn by considering the uncrackedportion of the specimen as a beam resting onan elastic foundation (see Figure 2). Subsequentrefinements of the cantilever beam approach(Fichter 1983; Foote and Buchwald 1985)and corroborating experimental and numericalresults (Srawley and Gross 1967) have confirmedthe accuracy and viability of Kanninen's approach.In adapting Kanninen's analysis to fit thespecimen configuration used in the experiments,there were two important changes. First, unlikeKanninen's case, there was no stress-free endwhich the crack approached; rather, at some distance(c) (see Figures 1,2) ahead of the crack tip,the tangential displacements along a boundaryline perpendicular to the crack path must vanish.For our specimen, a value for c was establishedusing ABAQUS, the finite element program usedto calibrate the specimen. The amount of theHoating ice sheet included in the analysis was increaseduntil the extension of the boundary intothe sheet (increasing c) made virtually no differencein the results of the analysis. The valueof L = a + c thus obtained was approximately3m. The other adaptation concerned the crackparallelcompressive force Q. It was not appliedat the centerline of the beam arms, so it gaverise to a negative moment, M Q , of magnitudeMQ = -Q(w a /2 - e), where Wa is the specimenhalf-width at the tip of a crack of length a, asseen in Figure 1.After incorporating these modifications intothe model used by Kanninen (1973), for a DCBspecimen of constant width wand thickness h,~)~I(--------------------- c ------------------~-4d~IE-------------------L------------:>V'{Figure 1. TDCn specimen and loading configuration. (L' =333mm, L" =784mm,d =35mm,e =38mm, Wo =308mm, wL=648mm, a =45.6°, f3 =45°)291

subjected to a prescribed force P and a prescribedmoment ]0.1, the stress intensity factorfor the RDCn isJ{=P(12)1/2[(!!:..._MQ)A+~],whereandh w w Pw >.wA = (C 2 - C 2 )/(C 2 + c 2 ),B = (CS - cs)/(C 2 + c 2 ),(la)(lb)(Ie)>. = (6)1/4/W. (ld)Here, C = cosh(>.c), S = sinh(>'c),c = cos(>.e),and s = sin(>.e).If e > 2w, as in our case, A = B ---+ 1,and then, for the dimensions given in Figure 1,if P = Q,J{ = h P(12)1/2[ -:;;- 0.20 + 4.75]Y . (2)The final calibration of the TDCB specimenwas completed using the J-integral capability ofABAQUS, a finite element program. The programhas a procedure to evaluate the J -integralbased on the virtual crack extension method ofParks (1977). For the purposes of the program,it is simplest to interpret the J-integral as afunction of position along the crack front, whichgives the decrease in total potential energy ofthe loaded specimen caused by an increase in thecrack opening at that same position on the crackfront. The relationship between J and J{ is givenbyJ{2J--- E e ,,'(3)where Eel I is the effective Young's modulus, beingso-named because of the composite natureand associated transverse isotropy of the ureamodel ice (Gow, 1984).Eight-node, biquadratic, plane strain finiteelements were used with a focused mesh ofquarter-point crack tip elements to allow for theproper strain singularity (1/ vr) as the crack tipwas approached. The value of J was computedfor each of three different contours surroundingthe crack tip, for seven different initial cracklengths, with applied loads of just P, just Q, andboth P and Q. The numerical results of theseruns were then used to develop a least-squaresfitpolynomial to express J{ as a function of thenon-dimensionalized crack length (a = a / L) forthe TDCn specimen with w = 0.21L,J{F = h F(12)1/2 -:;;- fp() a ,fp = 1.11 - 3.65a + 48.30a 2 - 100.06a 3 ,fQ = 0.55 - 3.03a + 17.79a 2 -30.01a 3 •(4)(5)(6)(a)..L~p6--QlQeT-1PIIIIIIiW1--Idl~E---a ---~)~IE----- c "I~~~(b)Figure 2. (a) DCn specimen and (b) Beam on elastic foundation model of DCB specimen.292

By superposition, the expression for K inthe combined loading case can be determinedfromK=Kp-KQ,(7a)whereQ=",P,(7b)'" being the coefficient of friction, a function ofthe included wedge angle and the materials involved.Then, if it is assumed that the contactbetween the wedge and the roller bearings is virtuallyfrictionless, for the 90 0 -wedge, P = Q andfp=Q = 0.56 - 0.62a + 30.51a 2 - 70.05a 3 • (8)K1.41.21.0P 12 1/2.8 - (-)h w.6. 4.2 0 .10 .20 .30Figure 3. Non-dimensionalized K vs. (a)from ABAQUS and the revised Kanninen analysis.ABAQUS runs were also performed for theRDCB specimen of the same size as the TDCBspecimen. The corresponding polynomials, assumingP = Q for the RDCB are:fp = 1.03 - 2.82a + 46.23a 2 - 99.24a 3 , (9)fQ = 0.50 - 2.49a + 15.98a 2 - 28.19a 3 , (10)f P=Q = 0.53 - 0.33a + 30.2M 2 • - 71.05a 3 . (11)A comparison of the more accurate K­expressions from ABAQUS (8 and 11) and thesimpler K-expression from the Kanninen analysis(2) shows them to be reasonably close in therange of 0.05 < (a) < 0.25, where our experimentswere conducted (see Figure 3), and providesa check for the finite element work.DCB ExperimentsAs described in Dempsey et al. (1986), thespecimens were cut from model ice sheets grownin the test basin of the Ice Engineering ResearchFacility of the U.S. Army Cold Regions Researchand Engineering Laboratory, Hanover, NH. Theexperimental procedure has since been modifiedslightly. Templates continue to be used to assureproper alignment of the new loading postsand the wedge. The initial crack was cut witha handsaw and held open by a thin ('" Imm)plastic sheet. Two holes were bored out to accommodatethe lucite loading posts which havecamroll bearings,. attached to the top and bottom.A small collar and spacers allowed propervertical alignment of each of the posts. A crackmouth-openingdisplacement (CMOD) gage wasinserted on the spacers between the rollers to givethe deflection of the beam arms along the lineof loading. On specimens with an initial cracklength greater than 0.2m a crack-opening displacement(COD) gage was placed along the cutcrack in a 38mm diameter hole at 0.2m from thefront edge of the specimen. Both of these gageswere clip ga~es modeled after one used by Schraet al. (1973). They consist of two beam armsseparated by spacer blocks with strain gages locatedat one end of the beam arms and on eitherside of each arm .The velocity and displacement of the loadingplaten were also monitored during the tests. Theuse of an acoustic emission (AE) transducer gaveadditional data on when cracking of the specimenoccurred. Typically, it was placed in a '" 12mmdeep, 22mm diameter hole which was bored at50mm in front of and 50mm off to the side of thecrack tip.The initial tests were concerned with crackpath stability and examining the effect of cracktipradius once the TDCB specimen configurationwas decided upon. In later tests, at higherloading rates, the effects of size and shape wereconsidered by testing 2/3-sized and untapered,full-sized specimens. The ice thickness and flexuralstrength also varied from one ice sheet toanother.Experimental ResultsThe test results to date are given in TableI. Each test is identified by a test numberwhich tells which ice sheet and what testin the sequence it was. The test type denoteswhether it was the "traditional-sized" TDCB(T), the scaled-down TDCB (S), or an untaperedRDCB (R). The initial crack length has been293

Table I. Experimental Test Data for DeB SpecimensType ##T 2.01 .075 78T 2.02 .046 78T 2.03 .046 7763 545o 455o 51320 11 53 2015 3 2117 3 22T 11.09 .121 65 183 225T 11.10 .153 67 54 140T 11.11 .187 63 206 23012 18 82 248 22 2516 21 25T 2.04 .071 86T 2.05 .121 83T 2.06 .096 87T 3.02 .055 88T 3.03 .089 89T 3.04 .122 88T 3.05 .154 94T 3.06 .055 94T 3.07 .055 94T 3.08 .057 94T 3.09 .059 93T 4.01 .055 76T 4.06 .188 82T 4.08 .089 80T 4.09 .122 80o 555o 38352 459o 36829 379o 29067 253o 345o 360o 36022 414o 36720 247o 298o 27118 .5 46 2516 2516 2511 40 2413 2411 2411 2410 .5 51 2510 1 2510 1 2512 .5 2514 3 54 1714 3 2011 3 2412 6 24T 12.03 .121 66 123S 12.04 .115 67 94 131T 12.05 .153 67 106 137S 12.06 .148 71 3 91T 12.07 .187 72 0 78S 12.08 .181 70 0 61T 13.U2 .088 81S 13.03 .115 79T 13.04 .153 82S 13.05 .148 80T 14.01 .055 82T 14.02 .088 71T 14.03 .088 83T 14.04 .121 75T 14.06 .088 73o 10:1o 74o 97o 73o 340o 2494 2076 1964 1886 44 61 238 22 238 26 236 28 235 62 235 40 234 32 53 244 31 245 55 244 53 2411 34 36 410 41 187 51 37 249 50 238 31 23T 6.01 .055 81 144 331T 6.02 .089 77 89 323T 6.03 .122 79 58 271T 6.04 .154 86 23 231T 6.05 .188 87 181 242II 4 68 2413 7 2312 6 23II 5 2713 12 25T 15.0U .088 55T 15.01 .088 55T 15.02 .088 55T 15.04 .088 56T 15.05 .088 5717 1267 1204 1284 11410 1247 29 57 86 34 137 39 206 54 226 56 23T 7.03 .089 70 109 412 18 4 54 24T 7.04 .188 66 0 278 19 5 24T 7.05 .055 73 16 283 10 5 71 24T 11.01 .088 75 0 115R 11.02 .088 67 111 171T 11.03 .121 67 153 183R 11.04 .121 68 12 72T 11.05 .153 72 0 65R 11.06 .153 69 129 160T 11.07 .055 68 0 209T 11.08 .088 69 144 209UNITS5 41 55 238 28 239 28 244 27 234 34 249 14 248 44 82 219 16 24T 16.01 .088 50T 16.06 .088 57T 16.07 .088 51T 17.00 .088 53It 17.01 .088 53T 17.03 .121 53It 17.0,1 .121 53T 17.05 .153 54It 17.06 .153 56T 18.01 .252 52T 18.02 .252 48T 18.0·1 .088 53T 18.07 .088 541321214 1235 10221 1042 9429 1012 10512 594750o 52o 608 36 48 86 58 237 25 236 35 53 86 19 76 29 107 31 118 31 94 19 95 27 51 116 61 123 14 123 50 12h (mm)Po and PJ (N)u/ (kPa)T (OF)294

non-dimensionalized with respect to L. Po is thepre-load applied when first aligning the wedgeand ensuring proper contact with the rollers. Itwas typically applied for about 30 seconds beforethe test was started. In the later tests, the experimentaltechnique was refined and the wedgewas firmly attached to the loading platen whichresulted in very small amounts of pre-load.PI refers to the load at which fracture wasbelieved to have occurred. For most of the teststhis represents an average between two loads associatedwith behavior of the CMOD gage. Onewas the load associated with the time at whichthe CMOD (CMOD= 28, see Figure 2) readingbecame nonlinear (in the CMOD vs. timecurve). The second load, which was usuallyslightly higher, was obtained from the P vs.CMOD curve - the initial slope of the linear portionof the curve was found and then a 95% secantline was drawn to allow for some nonlinearbehavior at the cracktip. The true load atwhich fracture occurred probably lies within therange of the two values unless there was significantnonlinear deformation occurring. The twovalues typically differed by less than 10%, andso, were averaged. The corresponding loads correlatedquite well to the onset of detectable AEactivity in most of the tests.In these tests the initiation of fractureoccurred well before the maximum load wasreached. Typical values of P max / PI are 2 - 5.After an initial straight crack propagated, thenature of the test is such that it became more ofa flexural test and the load continued to rise untilit was great enough to break off the two beamarms. The results of the earlier tests (ice sheets#2-4), reported in Dempsey et al. (1986), havebeen corrected by returning to the test recordsto establish PI and by using the value of c foundin the finite element calibration.](1c and k were computed using (8) or (11),depending on the type of test and accounting forthe change in scale (2/3) for the S tests. Theside-loaded flexural strength tests were done witheither three-point-bend or cantilever beam specimens.Tests were usually also done concurrentlyto find the downward and upward-loaded flexuralstrengths, but for our tests there was no cleardependence of the side-loaded flexural strengthon the downward-loaded strength. Due to thenature of the fracture experiments, it is the sideloadedflexural strength which should be of interestand, therefore, this strength is included inTable 1.It is difficult to draw any conclusion aboutthe effect of the taper in tests on ice sheets #11and #17. There was more of an effect of pre-loadin tests on sheet #11, with the pre-load causinghigher fracture toughness values. For #17 therewas no consistent tendency for the taper to eitherincrease or decrease the fracture toughness.The taper was used to provide crack path stability,but there was little difference in the crackpatterns observed.The measured fracture toughness values forthe 2/3-scale tests, denoted (S), on ice sheets#12 and #13 are not significantly different fromthose for the full-scale tests (T). The larger specimenwas chosen to ensure that the initial cracklength, a, could be more than 15 times the averagecrystal diameter, even for the shortest cracks.This limi ted series of tests suggests that usingsmaller specimens may still give reliable results.However, tests on much smaller specimens shouldstill be performed to check the validity of this assumption.The experimental results are displayed ingraph form in Figure 4 with the fracture toughnessplotted vs. the rate of change of stress intensityfactor, the ice thickness, and the side-loadedflexural strength. The range of values is quitesimilar to those found by Timco (1985) and Parsonsand Snellen (1985). From Figure 4(a), thegeneral trend is for decreasing fracture toughnesswith increasing k (or, increasing loading rate, inlooser terms). The only extreme outlying pointswere those tests for which there was a significantamount of pre-load compared to the fractureload. This pre-load allowed the growth ofthe process zone around the crack tip and, hence,gave larger values for the fracture tou~hness (e.g.the value for test #11.11 is identified).Figure 4(b) highlights the assertion than anincrease in the loading rate decreases the measuredfracture toughness. Comparing ice sheets#4 and 13 from Table I, for approximately thesame thickness of ice (but relatively thick,

thin), a layer of randomly-oriented crystal structure(isotropic, ~ 5-7mm thick), and a columnarlayer (transversely isotropic, ~ (h-7)mm thick).·We found no clear dependence of fracturetoughness on the side-loaded flexural strength ofthe urea model ice, as seen in Figure 4(c). Thereare the confounding effects of the loading rateand the ice thickness as well as some variabilityin temperature at the time of testing. Insome cases, because the emphasis was in obtainingreliable results which could then be used inconjunction with other property tests performedto characterize the ice for a model test, an icesheet was tested on two successive days. So, theice was not always tempered in exactly the samemanner, as seen by the air temperatures at thetime of testing listed in Table I.It is hoped that with further refinement, theTDCB tests will provide a basis for comparisonof future testing configurations in the developmentof an index test for the fracture toughnessof model ice sheets. The index test will be onewhich can be performed more quickly, most probablyon a much smaller specimen with a simplerloading apparatus, with a loading rate and orientationsimilar to those used in the associatedmodel test.AcknowledgementsThe first and third authors (D.L.B. andJ.P.D.) would like to acknowledge the supportof the U. S. Army Cold Regions Research andEngineering Laboratory under Grant DACA 89-84-K-0008.ReferencesABAQUS - Version 4.5a, 1985, Hibbitt, Karlsson& Sorenson, Inc., Providence, RI.Atkins, A. G. 1975. "Icebreaking Modeling,"Journal of Ship Research, Vol. 19, pp. 40-43.Benbow, J. J., and Roesler, F. C. 1957. "Experimentson Controlled Fractures," Proceedings ofthe Physical Society, Vol. B70, pp. 201-211.Dempsey, J.P., Bentley, D.L., and Sodhi, D.S.1986. "Fracture Toughness of Model Ice," Proceedingsof the 8th International IAHR Symposiumon Ice, Iowa City, Vol. I, pp. 365-376.Figure 4. Fracture toughness vs. (a) rate ofchane;e of stress intensity factor, (b) ice thickness,and {c) side-loaded flexural strength.K 1C(kPa m112)(b)(c)K 1C(kPa m112)201510205x (11.11)00 20 40 60K (kPa m 112 S·1)15 \0 < K < 1~5o 40LJ.....14

Earle, E.N., Frederking, R.M.W., Gavrilo, V.P.,Goodman, D.J., Hausler, F.-U., Mellor, M.,Petrov, I.G., and Vaudrey, K., 1984,"IAHR- Recommendations on Testing Methods inIce," Proceedings of the 7th International IAHRSvmpium on Ice, Hamburg, West Germany, Vol.IV, pp. 1-41.Timco, G. W. 1985. "Flexural Strength andFracture Toughness of Urea Model Ice," Proceedingsof the 4th International OMAE Symposium,Dallas, Texas, Vol.lI, pp. 199-208.Fichter, W.B. 1983. "The Stress Intensity Factorfor the Double Cantilever Beam," InternationalJournal of Fracture, Vol. 22, 133-143.Foote, R. M. L., and Buchwald, V. T. 1985. "AnExact Solution for the Stress Intensity Factor fora Double Cantilever Beam," International Journalof Fracture, Vol. 29, pp. 125-134.Gow, A. J. 1984. "Crystalline Structure of UreaIce Sheets Used in Modeling Experiments inthe CRREL Test Basin," CRREL Report 82-24,Hanover, New Hampshire.Jones, N. 1986. "A Note on Ice Scaling," Journalof Ship Research, Vol. 30, pp. 134-135.Kanninen, M. F. 1973. "An Augmented DoubleCantilever Beam Model for Studying CrackPropagation and Arrest," International Journalof Fracture, Vol. 9, pp. 83-92.Parks, D. M. 1977. "The Virtual Crack ExtensionMethod for Nonlinear Material Behavior,"Computational Methods in Applied Mechanicsand Engineering, Vol. 12, pp. 353-364.Parsons, B. L., and Snellen, J. B. 1985. "FractureToughness of Fresh \-Vater Prototype Ice andCarbamide Model Ice," Proceedings of the 8thInternational POAC Conference, Narssarssuaq,Greenland, Vol. 1, pp. 128-137.Schra, L., Boerema, P. J., and van Leeuwen, H.P. 1973. "Experimental Determination of theDependence of Compliance on Crack Tip Configurationof a Tapered Double Cantilever BeamSpecimen," National Aerospace Laboratory Report,NLR TR 73025 U, Netherlands.Schwarz, J., Frederking, R., Gavrillo, V., Petrov,I.G., Hirayama, K.-I., Mellor, M., Tryde, P., andVaudrey, K. D. 1981. "Standardized TestingMethods for Measuring Mechanical Properties ofIce," Cold Regions Science and Technology, Vol.4, pp. 245-253.Srawley, J.E., and Gross, B. 1967. "Stress IntensityFactors for Crackline Loaded Edge-crackSpecimens," Materials Research and Standards,Vol. 7, pp. 155-162.297

SOME INVESTIGATIONS FOR EG/AD MODEL ICEK. HirayamaN. SakamotoIwate University, Morioka, JAPAN1 In troduc tionThe repetition of ice/structureinteractions under controlled conditionsin a refrigerated ice test basin is aneffective and economic technique inexamining failure modes, size of iceforces and effects of ice properties andgeometries of the structure. Forsuccessful model experiments, a testfacility has to furnish good controland reproducibility of ice propertiessuch as ice thickness and strength;uniform distributions of theseproperties in the tank are of primaryimportance. It is also necessary forthe model ice to be structurally andmechanically similar to naturalsea/river/lake ice. In addition, it isnecessary that the ice or the materialbe able to endure continuous use, anddoes not cause any corrosion to theequipment and instrumentation or anyharm to the operator. Large ice tanksalso require cheap and easy handling ofmaterial during the preparation of theice.Starting with the use of an aqueousThis is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22,1987. © The Geophysical Institute,University of Alaska, 1987.solution of sodium chloride salt, manyefforts have been made to obtain anideal model ice for experiments. Abrief summary of model ice is given byTimco (1986). A recent, big advance inmodel ice technology is 'urea-doped ice'proposed by Timco (1979); this shows anexcellent similarity with the mechanicalproperties reported by Hirayama (1983),and is free of corrosion problems.The only defect of urea-doped iceis the two-layer system of the icesheet. A relatively thick and strongtop layer results in a rather long icetempering time and in a loss of materialhomogeneity from top to bottom,whichcauses a large difference of iceresistance depending on the loadingdirection. In this respect, Timco'simproved material, 'EG/AD/S ice' is muchmore promising as ice for modelexperiments, since it can successfullyhave a very thin and weak top layer.In his paper(Timco 1985), severalmechanical properties of this new typeof model ice are investigated and rolesof additives such as ethyleneglyco1(EG), aliphatic detergent(AD) andsugar(S) are described. EG is a lowmolecular weight dopant which is trappedin the ice as impurity pockets andreduces the strength of the ice; AD is asurfactant, which reduces the surface299

tension of the solution allowing moreof the low molecular weight dopant to betrapped in the ice and; S is a longchain organic molecule of high molecularweight to inhibit the lateral growth ofthe individual ice platelets in the icesheet. Timco presented ice propertiesfor a concentration of EG: AD : S =0.46 : 0.032 : 0.049 when Rexonic P-1 ofHart Chemical Company of Guelph,Ontario, Canada was used as AD, andsuggested that this ratio of theconcentrations may not be the optimum/In this paper, effects of EG and ADconcentrations on several model iceproperties are examined experimentallyand roles of each additive areinvestigated. Since an ordinaryseeding is sufficient to create smallenough crystals in the ice sheet, and toconstrain the lateral growth of iceplatelets, an use of sugar is notconsidered in the present study.2 Experimental Facility and IcePreparation ProcedureThe low temperature laboratory ofIwate University has a small wooden icetank, 1.55 m long, 1.0 m wide and 0.5 mdeep in a refrigerated room in which thetemperature can be lowered to a minimumof -15°C.The aqueous solution in the tank ismixed by an air bubbler system until itstemperature is close to the freezingtemperature. The freezing temperatureof the pure EG solution was measuredwith a O.Ol°C resolution thermistor andthe results are shown in Table 1. Inthe table, column (2) indicatestheoretical decreases of the freezingpoint from O°Cand columns (3)-(5) showobserved temperatures for variouscooling conditions. Since the solutionwas continuously stirred, super-coolingof the solution was observed before thefirst crystal originated; fora case without seeding, see column (3).Wet seeding is employed when a calmsurface exists in the tank, to giveuniform and small crystals in the icesheet and, therefore, uniform propertiesof the ice sheet. Seeding should beperformed at a temperature slightlyabove the freezing temperature with anair temperature of about -3°C. A fan inthe ice room is started when the icesheet is strong enough to withstand theair current over the ice sheet.After the ice reaches a targetthickness,the thickness distribution andtop layer thickness are measured by acaliper. Then, cantilever beams are cutto test in-situ flexural strength,following the recommendation by the taskcommittee on testing methods in ice, ofIAHR Ice Committee. Force is normallyapplied downwards at the tip of the beamand the breaking load is measured by apush-pull scale. Simple elastic theoryis used for the calculation of thestrength. For the measurement of theelastic modulus, beams of differentlength are prepared and a methodpresented by Tatinclaux andHi,rayama(l9S0) is applied.Growing temperature in the room was-15°C throughout the experiments.Tempering of the ice sheet was performedby setting the dial for room temperatureto 4°C, which is obtained within 10 to20 minutes starting from the temperatureof the growing period.A suitable aliphatic detergent wasfound after consulting severalcompanies, and PBC-44 of NikkohChemicals Co. Ltd., Osaka, Japan waschosen as AD. Concentrations of ADand EG are in a range of 0 - 0.1 % andO.lS - 2.0 % respectively.3 Results of ExperimentsVariation of ice thickness in thetank was less than 2 mm for 3 cm thickice sheet, and the rate of ice growthfor 0.6 % EG concentration was about0.023 cm/ °c hr ( approximately 2. Smm/hr ), which is similar to the ratefor 0.6 % urea concentration in thepresent cooling system.Typical crystal structure of EG/ADice for EG 0.6 % and AD = 0.1 % isshown in Figures 1, 2 and 3. From thesepictures it is seen that the size ofcrystals at the top surface is less than1 mm, and 3 to 5 mm at the bottom of 3cm thick ice sheet. It is alsonoteworthy that the thickness of the top300

Figure 1Cross-section View of EG/AD ice.Figure 2Top s urface of EG/AD ice with seeding.Figure 3 Bottom surface of 3 cm thick EG/AD ice .301

________________•Table 1Freezing Point of Aqueous Solution of EG.without seedingwith seedingEG Temperature of the Te mperature of Te mperatu re of theCo ncentration T first crystal solid/liquid f i r st crystal(%)(OC) o r igination (OC) equilibrium(OC) origination (OC)0. 10.30.61 . 52 .0- 0.03 - 0.07 - 0.02 - 0.04-0.09 - 0. 18 - 0.09 -0.08-0.18 - 0.35 -0.19 - 0.21-0.46 - 0.48 -0.45 - 0.46-0.6 1 -0 . 67 -0.6 1 - 0 . 61Table 1Freezing Point Aqueous Solution of EG.5. 0 TOTAL ICE THICKNESSEG (%)0 o. 188u~trltrlw:z:x:u......;=w84. 0 9 60O. 30~0 0~tfj 0 O. 6003. 0~8 .&TOP LAY ER THICKNESS• O. I 8... O . 3 02. 0•O. 601.0o ~,• J• •!~________________o 0. 05 0. 10DETERGENT CONCENTRA nON (%)________ ~Figure 4Relation between Top Layer Thicknessand AD concent ration.302

layer is very thin, only 1 to 2 mm, andhas a columnar structure.Generally, the thickness of the toplayer varies from zero to 4 mm, as shownin Figure 4, and it was sensit~ve to theseeding condition. The rat~o of thetop layer thickness to the totalthickness was in the range 4 to 10 % for0.18 2.0 % EG ice sheet. It is alsoobserved that the thickness remainsconstant with increasing total icethickness and ~s independent of ADconcentration. Considering that theratio for urea ice is 25 to 30 %, it isclear that EG/AD ice has a very thin toplayer.Relations between flexural strengthand AD concentration for ice of 0.3% and0.6% EG concentration are depicted inFigure 5. Data were obtained for icethicknesses of 2.9 3.7 cm, and themagnitudes of the strength are about 170and 100 kPa respectively for both EGconcentrations, which roughly correspondto 0.45 and 0.95% urea ice respectively.Also it is clear that the effect of ADconcentration is negligible.The relationship between flexuralstrength and EG concentration is shownin Figure 6. This shows that thestrength decreases significantly withincreasing EG concentration. Decreaseof strength was not observed for EGconcentrations larger than 1.5 %.F~gure 7 shows the effect ofloading direct~on on the strength, where0' indicates the flexural strength forupwards loading i.e. the bottom side intension and a for ordinary downwardloading. Ratios of 0' to a are in therange 0.5 0.9 ( thick line in thefigure). The same experiments for ureadoped ice (Hirayama 1980 ) shows thatthe ratios are in the range 0.33 - 0.66.The difference in the strength dependson the loading direction and is due tothe effect of a temperature gradient inthe ice sheet as well as the existenceof the rather hard top layer. Thus,non-homogeneity of the model ice in thedirection of ice thickness is greatlyreduced for EG/AD model ice.The tempering of ice strength withtime is shown in Figures 8 and 9. Soon~c.. '"~250aa200ax a aI->< 100w...J"-aga0a8a-----.---:-----• •a0-g850OEGO.3%• E G 0.6%oo0.1DETERGENT CONCENTRATION (%)0.2Figure 5Relation between Flexural Strengthand AD co~centration.303

~-;uc..~::.....~z:..... '"en ""..J->

250200oaoc.O!)EG 0.3AD 0.0-0.2;::liwa:....en..J>

after the warm-up starts, the strengthbegins to decrease and reaches 50 kPa in60 minutes for 0.6 % EG ice while ittakes about 100 minutes for 0.18 % EGice. Since the flexural strength ofthe ice sheet during the tempering isgreatly enhanced by the strength of thetop layer, it is expected that thetempering time could be shorter for icesheets with a thin top layer.Elastic modulus for EG/AD ice is inthe course of analysis and presentlyseveral results have been obtained. Thepresent conclusion for the elasticmodulus indicates that E/a ratio forEG/AD ice is 700 to 1000 for a = 120kPa, whose magnitude is similar to ureadoped ice.As a conclusion, model ice, whichis structurally very similar to naturalsea ice and has the flexural strength(100 kPa), with reasonably high elasticmodulus and strength ratio, will beeasily obtained by freezing an aqueoussolution with 0.6 % EG concentration.4 ConclusionsReferencesHirayama, K. ,(1983) Properties ofUrea-doped ice in the CRREL Test Basin,US Army CRREL Report 83-3, Hanover,N.H.,USA.Tatinclaux, J.C. and Hirayama, K.,(1982)Determination of the FlexuralStrength and Elastic Modulus of Icefrom In-situ Cantilever-beam Tests,Cold Regions Science and Technology,6:37-47Timco, G.,(1979) The Mechanical andMorphological Properties of Doped Ice,Proc. POAC 79, Trondheim, Norway,Vol.I, pp. 719-739.Timco, G.,(1980) The MechanicalProperties of Saline-doped andCarbamide (urea)-doped Model Ice,Cold Regions Science and Technology,4:269-274.Timco, G.,(1985) EG/AD/S: A New Typeof Model Ice for Refrigerated TowingTanks, Cold Regions Science andTechnology, 12:175-195.Though the present experimentalinvestigations are not completelyfinished, the results of the experimentsand subsequent comparison with the ureaice conclude that EG/AD ice has muchbetter properties than other model ice,including urea doped ice. The thin andalmost negligible top layer, withcolumnar structure, of EG/AD ice is agreat improvement. This avoids thenon-homogeneity of the ice sheet in thedirection of increasing thickness andreduces the tempering time to obtain areasonable strength for model testing.However, the influence of aliphaticdetergent, as well as of sugar, was notclear in the present experimentalconditions and the ice properties, suchas the top layer thickness and flexuralstrength, were simply a function of theconcentration of ethylene glycol.306

MULTIYEAR RIDGE LOAD ON A CONICAL STRUCTUREKazuhiko KamesakiNobutoshi YoshimuraNippon Kokat. K.K., Tsu, JAPANAbstractFor designing a conical structureoperat ing in the Beaufort Sea, it iswidely accepted that a multiyear ridgeembedded in a multiyear floe gives thedesign global load. The authors conduc teda series of model tests simulating acone/ridge interaction in order toinvestigate the applicability of existingcalculation methods employing twodifferent models designed for operationin the Beaufort Sea . The comparisonbetween the results of model tests andthose of the existing calculati on methodsare presented and an empirical formula topredict the ridge clearing load for along ridge is proposed. The tests programalso included ridges moving onto thestructure with an ang le of attack and theeffects of the skew hit are alsodiscussed .I.IntroductionA multiyear ridge embedded in amultiyear floe is well-acknowledged as acritical ice feature that governs thedesign global ice load for a bottomfoundedconical structure in the deeperThis is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987. © The Geophysical Institute,University of Alaska, 1987.water of the Beaufort Sea. To develop astructure which can withstand an iceload due to such a ridge, the designerhas to carefully choose the calculationmethod capab le of predicting the maximumridge l oad during the whole ridge/coneinteraction process . To date, Lewis andCroasdale(1978) have conducted modp ltests; Abdelnour(1981) presented botnmodel tests and a calculation formulabased on the elastic method, whichregarded a ridge as a beam, with aneffective width of the ice sheet on anelastic foundation . Wang(1984) developedthe plastic limit analysis, applying thevelocity field observed in model tests,and obtained good agreement with modeltests . He pointed out that the elasticmethod tends to underestimate ridgeloads. Winkler and Nordgren(1986)presented a short ridge scenar iOcombining a ride-up mechanism and anelastic solution for a rid ge with acritical l ength . The authors conducted aseries of model tests to evaluate theapplicability of these theoreticalmethods, using two different types ofconical structures designed for theBeaufort Sea operation. The test programwas designed to investigate firstly theeffects of the ridge length and the rati~of a kee 1 depth(H) to ice sheetthickness(T), and secondly the effects ofa skew hit .307

2. Model Tests2.1 Model goemetryThe model geometries are depicted inFig.l. Model A was mainly designed tooperate in variable water depth rangingfrom 18 to 60 meters, and model B wasdesigned for a fixed water depth of 36meters. The model scale is 1/100 and thefriction coefficient was adjusted to be0.1 to 0.2.2.2 Model arrangementEach model was set on an underwatercarriage through a multi-component forceblock as shown in Fig.2. The carriagewas towed through the ice cover at aspeed corresponding to 0.5 knot in fullscale,model.instead of driving the lce to the2.3 Ridge constructionIce sheet& were formed by applying acontinual w~ter-spraying technique.Firstly, a trapezoidal portion of a ridgewas made by a saw cut from a thick lcesheet, and placed at a specifiedlocation with a desired angle of attack.Secondly, an upper portion as thick asthe lce sheet was formed. The typicalridge cross section is shown in Plate 1.Ridge geometries were chosen according toaverages given by Wright(l979) et al.Several sections were cut out to measureactual sizes after experiments and theaverage values are indicated in Table 1 .UNIT = em-+ : TESTED WATER DEPTH .... ~f-.....,.~ 200~OCLA ~OCL BFig.l Model geometriesRICONNECTED WITHMA N ARAIAGE7:::

Table 1lee propertiesEXP .I D.A1 / A2IA3A4 / A5 / A6A7/A8 / A9A10/All / A12A1381/B3/8586/87/89B10/811/B13814/B16/B17B18/B19/B21822 thru B24B25 thru B2B28 thru B3RIDGE GEOMETRIEST 8t 8b H(c III ) (c III) (c m) (c III )6. 23 48.0 33.0 12.06.26 76.0 48.0 17.56. 11 75.0 44.0 16.36 . 12 45.8 27.5 12.35.91 49.5 21.0 12. 03.46 55.0 25.0 10.55.89 76.0 34.0 17 . 57.03 77. 3 41.0 16 . 35.95 45.3 25.3 1l. 83.38 88.8 38.6 17. 36.0 6 78.5 36.2 17.96. 17 80.2 37.0 17.66. 17 77.6 38.8 17. 0T= ICE SHEET THICKNESS8t=RIDGE TOP WIDTHBb=RIDGE BOTTOM WIDTHH=RIDGE KEEL DEPTHL=RIDGE LENGTHIICE PROPERTIESL SF.s SF.r SC.s Es Er(c m) (Kpa) (Kpa) (Kpa) (Mpa) (Mpa50/200/400, 32.3 21. 6 NA 7. 9 10.7100 /200/ 40~ 27.2 26.3 28.3 7. 0 7 . 1IDO /' DD/'D~ '0.0 22.8 35.4 8 .3 10. 150/200/40 12.4 30.2 18.9 4.0 13.8400 2l.5 32.2 27.5 5.9 15 . 5100/200/40 12 . 4 30.2 18.9 4.0 13.8100/200/40 26.6 23.3 40 . 9 10.0 15 . 1100/200/400 25.5 29.8 40.4 5.0 16 . 950/200/400 27.6 30.5 4l.5 8.9 10.4100/200/400 13 . 1 13. 1 22.3 3.6 5.6400 35.3 30.0 38.2 19.5 28.9400 22.8 25. 1 38.1 5 . 1 10.0200 24.4 26.9 37 . 1 6.9 9.0SF.s=FLEXURAL STRENGTH OF ICE SHEETSF.r=FLEXURAL STRENGTH OF RIDGESC.s=COMPRESSIVE STRENGTH OF ICE SHEETEs=EFFECTIVE MODULUS OF ICE SHEETEr=EFFECTIVE MODULUS OF RIDGE2.4 Ice property measur ementThe results are listed in Table 1.The compress i ve strength of the lcesheets was est imated from indentationtests with a 15 cm-wide flat indentor,assuming that indentation factor *contact factor is 0.36. In general, itlS imposs ible t o conduct sufficient l ceproperty t ests utilizing unbrokenportions of ridges. Therefore, weprepared a two-meter-wide r ec tangular iceplate that was formed using the sameprocedure as the tested ridge. Cantilevertests we re used to measure flexura lstrength and effective modulus, andvalue s were regarded as representative ofthe ice properties of t es ted ridges .Although ridge dimensions weres uccessfully scaled, the flexuralstrength of model ice was approximatelythree times stronger than that of thetarget value because of scaling problems.3. Experimental ResultsThe experiment s were conducted atNKK ICE MODEL BASIN . The details of thefacility a r e referred to in Sudo( 1983) e tal. We constructed three ridges withdifferent ridge lengths a nd an identicalkeel dep th in an ice s hee t measuring 6meters wide and 20 meters long .3 .1 Broadside interactionIn Fig.3, the variation of failuremodes is given in terms of HIT ratio andthe ridge length for the case of abroadside interact ion using model B. Thefailure mode called "very short ridge" or" short ridge t ype II" def ined byWang( 1984) was predominant for the ridgeswhose l engths were less than 1.0 meter.The mode denoted as "long ridge type I"was frequently observed in t he t ests with4.0-meter long ridges, and the failur emode is shown in Plate 2 . If the l ceshee t was sufficiently thin, the ridgetended to rotate i n-plane after a centercrack was deve l oped , and the high HITr a tio resulted in a s impl e failure mode .On the other hand, if the surrounding icesheet was s ufficient l y thick, the failuremo de tended to become comp l ex and alocal bending failure or acircumferential crack was likely toeme r ge in the middle of the ridge becauseof the high confinement by thesurrounding ice sheet. The crackingsequence was not clear; however, a centercrack always emerged first and propagatedfrom the leading edge to the trailingedge except in case of the short ridges .In Fig .4, the typical load trace curvesrecorded in the model B t es ts are shown.The model ran through the ice sheetbefore hitting a ridge. The r efo r e, the309

~UII-6t:i 5wI(f)f-LEGENDL_ '~:::OO _ __ J"I I 008 18RA K IN1 MODELMOVING DIRECTIONOOIJ o (::-1 ::] 821819"I~II-a.. 3wo-.JWS!:! 2w

Table 2Test results for a broadside hitEXPERIMENTS HASTI CITY MET HO D PLASTI CITY MET HO DEXP.( Fy I Fhb FYi Fhb FyiID ( hf F~~ Kd (Kgf) (Kd (KI1f) (KdA1 39.2 26.6 101. 7 83.2 32.2 26.4A2 53.4 30.3 32.3 26.4 39.6 32.4A3 38.3 30.7 34.6 28.3 39.6 32.4A4 52.9 HA 168.8 138.0 86.4 70.7A5 89.0 NA 94. 3 77.1 106.8 87.4A6 112.2 83.2 90.1 73.7 106.8 87.4A7 70.5 38.3 149.2 102.5 93.3 63.7A8 131. 0 51.2 82.6 56.7 98.2 67.0A9 122.4 55.5 77.3 53.1 98.2 67.0A10 34.4 13.0 165.6 113.8 35.2 24.0All 55.7 17.3 52.9 36.4 56.9 38.8A12 47.2 19.6 56.9 39.1 56.9 38.8A13 28.6 28.3 42.1 34.4 43.5 35.6B1 11.4 11.1 37. 0 35.2 24.4 23.2B3 16.4 16.2 23.4 22.3 28.2 26.8B5 28.6 26.9 24.0 25.2 31.5 30.0B6 36.9 32.5 120.5 114.7 66.0 62.8B7 61.3 54.7 64.3 61. 2 75.7 72.0B9 86.4 75.5 53.8 51.2 75.7 72.0B10 55.4 53.1 138.4 131. 7 73.8 70.2B11 54.1 53.8 73.7 70.2 87. 0 82.9B13 80.0 83.7 61.6 58.6 87. 0 82.9B14 21.0 19.0 110.0 104. 7 25.0 23.8B16 30.4 32.8 36.2 34.5 40.0 38.0B17 33.9 35.7 39.2 37.3 40.0 38.0B18 16.5 15.9 55.6 53.0 47.0 44 .8B19 34.7 31.8 31.9 30.4 54.6 52.0B21 58.7 54.2 31.8 30.3 54.6 52.0Fhb=HORIZOHTAL RIDGE LOADFyb=VERTICAL RIDGE LOADMODEL WATERLINE DIAMETER=121.0 em for A1 thru A6 and A13MODEL WATERLINE DIAMETER=66.6 em for A7 thru A12HODEL WATERLINE DIAHETER=45.6 em for B1 thru B31Table 3Test results for a skew hitEXP . 8ID !rDEG.B2 0B2 30B2' 59B2! 0B21 45B2 90B21 0B2! 30B31 60B3 90RIOGE LOADS ELASTICITY HEYHO PLASTICITY HETHOFyb a Fhb Fvb FhbKtf Ktf (K~ f) (DEG. (Kg!) (Kgf) (Kgf) (Kd FV~)71.0 71.0 74.5 0 66.1 63.0 97.4 92.776.1 74.4 77.8 1964.3 55.S 67.4 3277.8 77.S 77.3 0 63.7 60.6 84.1 80.060.4 56.7 60.8 2587.8 87.8 74 .2 060.4 60.4 53.3 -2 149.2 102.5 84.3 80.268.4 67.7 59.7 2256.1 49.4 55.4 2764.3 64.1 48.1 2/hg /h~)Fbb,Fvb= Referred to Table 2Fhrb=IN-PLANE RESULTANT RIDGE LOAD8 =ANGLE OF ATTACKa =DIRECTION OF Fhrb311

ecorded ridge load consisted of the icesheet rideup component and the ridgeclearing component. The depression in theload trace curve corresponding to theinstant shortly before the model hit theridge was regarded as the rideupcomponent. In Table 2, the vertical andhorizontal ridge loads are indicated withthe lce sheet rideup componentsubtracted. In Fig.S, the relationbetween the ridge length and thevertical load recorded for model B testsis shown. In every ice test, the ridgeload tended to increase with increase ofthe ridge length.3.2 Skew hitRidges were placed with an angle ofattack to the moving direction and themodel struck the center of the leadingedge as shown in Fig.6. We define thiscondition as a skew hit to discriminateit from a broadside hit. The failuremodes varied depending on the angle ofattack as depicted in Fig.6. At an angleof 30 deg., the failure mode was similarto that in a broadside hit. At an angleof 60 deg., the portion between a leadingedge and a center crack was not broken,although the portion between a centercrack and a trailing edge was completelycracked along the moving direction. Forthe head-on impact, an angle of 90 deg.,the ridge broke like a semi-infinite beamloaded at the end. The broken ridgeslabs, approximately as long as the ridgewidth, frequently piled up in front ofthe models. For a skew hit, the lateralload Fy was expected to be significant;....0>:.c10050810-813. 86_89.D>u.. 814-817Fig.S200 400RIDGE LENGTH (em)81-85Vertical ridge load vs. ridge lengthI IiiI'"I / .....)I II I/I~(, / t MOVING DIRECTION'oJe= 60· e = 90·Fig.6 Variation of the failure modefor a skew hi ttherefore, the in-plane resultant loadand its direction defined in Fig.lS werecalculated from the measured data andare listed in Table 3. In Fig.4, thehorizontal load trace curve is presentedfor the ridge hitting at an angle of 60deg., together with that for a broadsidehit. As for the duration necessary toclear a ridge, a skew-hit ridge neededlonger time than a broadside-hit ridge.This implies that the clearing energyrequired for a skew hit is larger thanthat for a broadside hit4. Analysis4.1 Comparison with the existing methodsTheoretical loads corresponding tothe experiments were calculated byapplying the elastic method ofAbdelnour(198l) and the plasticity methodof Wang(1984), respectively. The measuredand calculated values are listed in Table2 and compared in Figs.7 and 8. Theexperimental results with a ridge lengthless than 100 cm were excluded fromFig.7. The elastic method underestimatedboth vertical and horizontal loads formore than half of the cases investigated.On the other hand, the plasticity methodoverestimated the vertical loads exceptin three caseS and gave a betteragreement than the elastic method. As forhorizontal loads, the plasticity methodgave approximately the same trends;however, it gave underestimation in halfof the cases for model A. TKis312

150r-------------.-------------r-----------~150r------------,.------------,.------------n~~:>u..JU"LlU. '"1005000000010000I'!0u..J 0

discrepancy was caused by the rubbleaccumulated in front of the model testedat the shallow water depth, or bycrushing of the rideup ice sheet againstthe deck tested at the deep water depth.These are considered to be similar to theeffect of a slope angle increase. Exceptfor these phenomena that are not treatedin the theory, the plasticity method isthus preferable to the elastic method fordesign purposes.4.2 The effects of ridge lengthBoth the elastic and plasticitymethods predict the maximum load at aspecific length. Fig.9 shows the relationbetween non-dimensional vertical ridgeload and ridge length which arenormalized by (Md/a) and keel depthrespectively. The term (Md/a) is laterdiscussed in eqs.(l) and (2). The figuredemonstrates that the ridge loadincreases with increasing ridge length.It reaches a constant value if L/H, theratio of ridge length to a keel depth,exceeds approximately twenty, . where theridge length may be regarded as infinite.1or-------r-------.-------~------~4.3 Empirical formulaAn empirical formula was derivedusing the results where the ridge lengthwas regarded as infinite, as wasdiscussed in section 4.2. Fig.IO showsthe relation between the vertical ridgeload and the downward limit moment of thetruncated trapezoidal section given byeq. (l). The figure indicates that thevertical ridge load is mainlyproportional to the downward limitmoment, regardless of H/T ratio. Thisimplies that the effects of cracksemerging on the trailing edge and the icesheet can be neglected. Therefore bysetting crack distance "d" and "b",(seeFig.ll) equal to zero in the equation forthe failure mode of a long ridge type Ipresented by Wang(I984), one can obtainthe crack distance "a",(see Fig.ll)expressed as eq.(2).oo'"..J100,------------,------------.------------,/yfJ 0 5 ~~,~::oa.A' 0 --- MODEL B() [) ----3 0 ( HIT~ 00oI )!' I- .----H/T~ ...ll.V ."00 ---20 (WT~3!1°O~----------~O=OO~----------~,OOO~--------~,~OOOFvb(Mod)5DOWNWARD LIMIT MOMENT Md (Kgf-em)Fig.IO Fvb vs. Md°O~----~------~20~----~------~40L/HFig.9 Fvb/(Md/a) vs. L/HORIGINAL L.R. TYPElPROPOSED MODEFig.II The definition of crackdistance "a","b" and lid"314

whereafr=I =rH =Y =nUsing thevertical loadexpressed in theIrH-Y )nflexural strength of ridgemoment of inertia of the(1)truncated trapezoidal sectionridge keel depthdistance of a neutral axismeasured from the ridge bottomridge top widthdensity of icecoefficientscrack distance "a"was assumed toform of eq. (3).thebe(3)Non-linear regression analysis wasperformed to determine the coefficients.The final result is given in eq. (4) andFig.12.4.4 The assessment for a skew hitThe vertical ridge load Fvb and thehorizontal resultant load Fhrb werenormalized by the results for a broadsidehit and are plotted in Fig.13. The datawas scattered, and a definite qualitativetendency was not found. In Fig .14, therelation between the vertical loadnormalized by (Md/a) and an angle ofattack is given to investigatequantitative effects. The normalizedvertical load coefficient A4 in eq.(3)was less than the value 6.61 obtained fora broadside hit. Fig.lS shows therelation in terms of Fhrb/Fhr, thedirection of the resultant load and anangle of attack. Both the resultant loadand its direction increased with increaseof the angle of attack and are estimatedto have peaks somewhere between 60 and 90deg. Even at an angle of 60 deg.,Fvb(9)Fvb!()')or 1.0Fhrb(9)Fhrb«(f)1.5,------------------,LEGEND0---- Fyb!S)/Fvb(O")8---- Fhrb(S)/Fhrb(O")F =vb(4)0.5'---0;;-------:;30~---::'60~---9::':0:--...J9 (DEG)Fig .13Fbv( e)Fvb( 0°) orFhrb(e)Fhrb( 00) vs. e1oor---------,---------.--------,en'" 50.cLotFvb: 6.6IMd/aBtl/000 8'5 08"a: / J.618Md B9 ,0 A6I.J78Md/Bt' .1.1,.9H/n''0' 0817/ A9AlOor EJA13 A12, A1l. 822 are excluded/ ~ 0 frOfT1 regression purpose/ AI2°0L---------~5~------~1~0~-------1~5Fig.12 Fvb vs. Md/al0r---r----r----r---~--.5~ .. "LEGEND0---- L' 400cm• ----- L· 200cmO'--~---~----L---~ _ __'o 30 60 90Skew Angle 9 (DEG.lFig.14Fvb( e )/(Md/a) vs. e315

Fhrb/Fhb is approximately 1.2 and thedirection of the resultant load 1S 30deg. The effects caused by a skew hit canbe relatively small and the load 1n themoving direction was st i 11 predominant.1.2.0~ 1.0"-u.-e~"- 06n~Fhb Fhrb /~'~o 30 60 90e (DEG)60,b 40 ci20\\\,g 0w0lSrepresented the ridge clearing load,defined as the maX1mum load recordedduring the cone/ridge interaction. Theelastic method significantly underestimatedthe ridge load and was notsuitable for design purposes. Theempirical formula (4) proposed by theauthors predicts the ridge loadconservatively and can be useful for thesimple calculations.In this test program, the ridgeswere supposed to encounter a cone at thecenter of the leading edge. In reality,eccentric hits on a ridge by a cone aremore frequent than centric hits and thiseffect should be studied further topredict a ridge load more precisely.ReferenceAbdelnour,R. 1981. "Model tests of multiyearpressure ridges moving onto conicalstructures",IAHR Ice Symposium, vo1.2,pp728-75l,QuebecFig.15FhrbFhbvs • a and a vs • eHnatiuk,J. and Felzien,E.E. 1986."Performance of Beaudril's new BeaufortSea drilling system" ,Fifth OMAESymposium, vol.4, pp183-191,Tokyo5. ConclusionsAs far as the ridge load wasconcerned, the load increased withincrease of the ridge length and issaturated for L/H ratio> 20. Also, theridge load increased in proportion to thelimit moment of the ridge cross-section,and was found to be independent of thesurrounding ice thickness. However, themode of failure varied, depending on thesurrounding ice thickness and the ridgelength.For a skew hit, the ridge load wasalmost at the same level as that for abroadside hit, and no significantdifference was found with respect to theload value. Such a skew hit is, however,expected to give more severe effects to astructure foundation when considering thedamages caused by high repeated loads, aspointed by Hnatiuk and Felzien(1986);itmay cause a massive rubble composed ofthe broken ridge slabs. The plasticitymethod of Wang(1984) gave good agreementwith the experimental results andLewis ,J.W.and"Mode ling thepressure ridgesstructures", IAHRpp165-196,LuleaSudo,M.Ice TankOffshoreHelsinkiCroasdale,K.R. 1978.interaction betweenand conical shapedIce Symposium, part 1,et al. 1983. "Outl ine of a Newfor Ice-going Vessels and PolarStructures",POAC 83,pp644-653,Wang ,Y.S. 1984. "Analysis and model testsof pressure ridges failing againstconical structures",IAHR Ice Symposiumvol.2,pp67-76,HamburgWinkler,M.M.and Nordgren,R.P. 1986. "Iceridge ride-up forces on conicalstructures",IAHR Ice Symposium, vol.l,pp171-183, Iowa CityWright,B.,Hnatiuk,J. and Kovacs,A. 1979."Multiyear pressure ridges in theCanadian Beaufort Sea", POAC 79, ppl07-126, Trondheim316

ICE LOAD PENETRATION MODELLINGK. RiskaTechnical Research Centre of Finland, Espoo, FINLANDR. FrederkingNational Research Council, Ottawa, Ontario, CANADAAbstractA laboratory test program wasconducted to measure the deformation andstrength properties of multi-year ice attemperatures of -2, -10 and -20°C. Thestrain rate dependence of strength overthe range 10~ s-l to 10- 2 s-l wasdetermined for uniaxial, biaxial artdtriaxial loading conditions. These datawere fit by means of a regressionanalysis to a failure surface. TheTsai-Wu criterion was found to be mostsatisfactory for describing thissurface.A penetration model based on contactcoefficients as suggested by Varsta wasformulated to physically describe thepenetration process. The coefficientswere determined empirically from testdata at 2 scales. The penetration modelrelates the actual average pressure (Pav)acting on the contact area (A) to anominal ice pressure (Pnom) calculatedfrom ice properties and the penetrationgeometry. The stress distribution withinthe ice for the penetration geometry isdetermined with a finite element method.This is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987. © The Geophysical Institute,University of Alaska, 1987.A reference stress rrumber which relatesthe stress state to the failure stresss tate is used to determine the nominalstress on the contact surface for theonset of failure of the ice. Anempirical relation of the form Pav =C (A/Ao)n Pnom was found to represent theaverage pressure on the contact surfaceduring structure penetration into ice.The value of the exponent n was -0.41 forboth large and small scale penetration.IntroductionIce forces generated on a structurearise from the relative movement of theice feature with respect to thestructure. If the movements are small orslow they may be accommodated by elasticor creep deformation within the icefeature. In many cases, however, thestructure penetrates into the ice featurewith an accompanying total disintegrationof the ice. This is the loading processwhich will be addressed in this paper.There are two ice load penetrationcases of interest. The first one, thatof a ship penetrating into an infiniteice feature, results in a contactgeometry such that the contact areaincreases throughout the penetrationprocess, with the highest load occurringat the end of the penetration.317

Generally, the penetrating structure(ship's bow) is inclined to the directionof movement. This scenario has led to anice force formulation where the icepressure is assumed to decrease withincreasing contact area; the so calledpressure-area relation (Sanderson, 1984).The other case is of an ice sheetindenting a fixed vertical-facedstructure (pile) where the contactgeometry remains constant throughout theprocess. The maximum load occurs whenthe structure is completely enveloped bythe ice. In this scenario, the ice forceformulation is assumed to show themaximum load to decrease with increasingratio of ice thickness to structurewidth; the so called indentationcoefficient-aspect ratio effect (Afanasevet aI, 1971). This latter formulationwas developed by Korzhavin (1971) andrelated the maximum ice pressure to aform factor, a contact factor, anindentation coefficient and the uniaxialcompressive strength of the ice. Maximumice force was obtained by multiplying theice pressure by the contact area.The case of a structure penetratinginto an ice feature has been studied moreas an interaction process since themaximum pressure, indentation and loadare a function of the size and velocityof the vessel as well as the propertiesof the ice. Varsta (1983) carried outlaboratory measurements of ice pressuregenerated during increasing contact areapenetration with a crushing mode offailure. This led to a formulation ofaverage ice pressure based on a contactcoefficient which varies during thepenetration process.Both of the above cases are based onessentially empirical factors and arethus difficult to generalize. There is areal need for an ice force formulationbased on an understanding of the physicalprocesses involved. This formulationwould take into account the geometry ofthe structure and the appropriatedeformation and strength properties ofthe ice.The objective of this paper is toindicate how to incorporate ice strengthinto analytical models for ice pressureduring penetration. Multi-year icestrength data will be presented and thenincorporated into a failure criterionfrom which a nominal contact pressure canbe calculated for a particularpenetration geometry. This nominal icepressure, together with an empiricalcontact coefficient, can be used topredict average contact pressure duringthe penetration process.Development of Failure Criterion forIsotropic IceThe overall approach here was todetermine strength properties ofisotropic ice for different stresscombinations and loading orientations.The testing was concentrated in thecompression-compress ion-compressionoctant as the intended applications werein this zone. Because more data areavailable than the minimum required fordescribing the failure criterion, aregression approach was used to obtainthe best fit.Strength testsThe ice used for these tests wasrecovered from the eastern CanadianArctic. Block samples were recoveredfrom two multi-year floes frozen into thelandfast ice adjacent to the south shoreof Lancaster Sound. One was tabular inform, about 30 by 50 m in size andgrounded in 10 m of water. The other wasan irregular block about 5 m across andgrounded at the shore line. Thetemperature of the ice at the time ofsampling was about -30°C. Aftertransporting to the south it was storedin a cold room at -10° to -20°C.Three different series of tests werecarried out. These included uniaxial andbiaxial (confined) compression tests atthe National Research Council of Canada(NRCC) in Ottawa and triaxial compressiontests performed (under contract) byGeotech in Calgary. Grain structure ofthe ice was examined by placing thinsections of the ice between crossedpolarized sheets. The tabular floe had avariable grain structure which could bedescribed as type R, agglomerate (Micheland Ramseier, 1971). The other floe hada more columnar structure, but was stillvariable. It fit the description of"breccia" by Richter and Cox (1984). The318

average densi ty of the tabular ice floewas 875 kg/m 3 and the block 900 kg/m 3 •Salinity of the ice tested at NRCC rangedfrom 1.2 to 3.0 ./•• while that ~ested atGeotech was 0.1 to 0.3 ./•••This lowervalue may have been due to brine drainageduring transportation.behaviour preceding premature failure canbe described as linearly elastic. Figure4 summarizes the results of the triaxialtesting which was carried out underproportionate loading.8r--------,---------,---------,Prismoidal specimens 250 mm long by100 mm wide and 100 mm thick for theuniaxial and 70 mm thick for the biaxialtests were used. For the triaxial testscylinders 250 mm long by 95 mm indiameter were used. All testing wascarried out on servo hydraulic machineswith closed loop control on strain rate.For the uniaxial and biaxial tests anextensometer was attached directly to thespecimen to provide the control signal.In the case of the triaxial tests,relative movement between the loadplatens was used for control, in additionto the confining pressure which wasmaintained at a constant ratio to theaxially applied stress. More details onthe test procedures can be found in Riskaand Frederking (1987).';;c..~CJlCJlUJa:I- CJlE.2-10- 3 1/51/5642°O~--------~S----------~10~--------~1SSTRAIN. 10- 4 -€Testing was carried out attemperatures of _2·, -10· and -20·C, andat strain rates ranging from 10-6 s-l to10- 2 s-l. For strain rates less than10-4 s-l the oode of failure was ductilewith an upper yield strength. For higherrates the stress-strain curve was linearup to an abrupt failure. This behaviouris illustrated in Figure 1 for tests at atemperature of -lO·C. It can be seenthat at a strain rate of 10- 2 s-l thecurve is linear up to failure, but thatthe slope of the curve, 8.5 GPa is stillless than the Young's Modulus value ofabout 9.5 GPa. The results of theuniaxial and biaxial tests are summarizedin Figures 2 and 3. These results showan increase in strength with increasingstrain rate for the ductile mode offailure similar to that found for freshwater ice by Gold and Krausz (1971). Forstrain rates of about 10- 3 s-l and higherthe failure behaviour is brittle andthere is no consistent dependence ofstrength on strain rate. This region hasbeen termed one of premature failure(Sinha,1981). The actual failurestrength is most likely influenced bysmall imperfections in the ends of thespecimens. While there is doubt aboutthe actual strength values the iceFigure 1. Stress vs strain for constantstrain rate uniaxial compressive strengthof multi-year ice at -10·C.(ij"-c..10ICE TEMPERATURE~T = - 2°C 0I 8.... o T= -10·C -~--~P=-; ~

VIVIW~I­VI2010520.52ICE TEMPERATURE l--Black symbols corre'pol'ld10 conl,n,ng sireno T=-2°Co T=-10°C T=-20°C0•----:--------t-iIo i!j •f-~t.1---- ---tIIi10- 5 10- 4STRAIN!RATE------- -~l&!--~-~I•..--•yield or failure criterion. Bothcriteria define a stress region withinwhich the material does not fail. Thisregion is represented by the formulaf(a) - "'.D.=. 0w'" =>V)V)wa::l>t!>Z15.0

(2) is valid only if the material isisotropic; i.e. the stress componentsare independent of the orientation of thecoordinate system. As discussed earlier,multi-year ice is considered to beisotropic at the scale of thestructure/ice penetration problem.There are two requirements for anisotropic failure criterion of multiyearice; i) the ability to account fordifferent tensile and compressivestrengths and, ii) adequately describethe increased strength in multiaxialcompressive stress states. A verygeneral criterion is that of Tsai and Wu(1971) which has a quadratic formf(a) = L Fij a ij + L G ijkli,j i,j ,k,l(3)where Fij and G ijkl are second and fourthorder strength tensors. In an isotropiccase presented with principal stressesRiska and Frederking (1987) showed thiscriterion reduces to(4)where J 2is the second invariant of thedeviatoric stress. This is essentially athree parameter criterion since G 1212= 12(G 1111- G l122). Recently another threeparameter criterion based partly onphysical reasoning has been proposed byNadreau and Michel (1986). It has theform(5)where a, b, c, and d are derived from aknowledge of T and e, tensile andcompressive strengths under thehydrostatic stress state and 6 is afitting parameter related to the stresslevel required to induce pressure meltingat a given temperature.Either of the failure criteriaobtained are macroscopic ones. Theypredict only the maximum stress state thematerial can sustain and consequently themaximum load ice can exert on astructure. They do not predict thebehaviour of the material once the stressreaches the failure state. This is anarea where fracture mechanics may be afruitful approach.Failure surface determination for thetest casesThe determination of strengthconstants requires selection of thesevalues so that the failure surface bestfits a set of llX!asured strength values.A number of sets were obtained fordifferent ice temperatures and strainrates. For illustrative purposes, datagroup IV for -10°C and 2 x 10- 3 s-l ispresented in Table 1. Note that all thedata in Table 1 are for compressivestresses.Where more data than the minimumdata required to determine thecoefficients in equations (4) or (5) areavailable a regression is performed toobtain the best fit. This is done byminimizing the following function overthe independent parametersL = L (). - 1) 2i i(6)where ). is the strength numbercorresponding to the i'th strengthmeasurement. The strength number for theTsai-Wu criterion iswhere/Ai Z + 4 Bi - AiI Z1.(-+ + J 2)~(7)and 11 and JZ are the stress invariantsi ifor the i' th llX!asurement. It is also321

Table 1. Strength test results used indetermination of failure surface[MPa] 02 [MPa] 03 [MPa]° 1Data set I· , -2°C, 0.0002 s-l2.4 0 03.2 0 05.2 1.1 03.9 2.4 08.0 5.9 5.94.9 2.4 2.420.4 20.4 20.46.7 5.0 5.05.7 2.8 2.820.7 20.7 20.7Data set II; -2°C, 0.002 s-l4.0 0 04.2 0 08.8 2.0 07.9 3.9 05.8 1.2 06.0 2.8 2.84.2 1.1 1.15.2 1.3 1.3Data set III; -lOoC, 0.0002 s-l5.0 0 03.7 0 04.0 0 02.8 0 08.7 4.3 04.8 2.4 011.8 8.8 8.87.7 3.8 3.820.4 20.4 20.48.7 6.5 6.56.5 3.2 3.24.6 1. 1 1.13.7 1.1 1.120.3 20.3 20.3Data set IV; -lOoC, 0.002 s-l7.5 0 07.0 0 05.8 0 08.2 0 013.0 1.5 011.3 2.7 09.6 3.4 010.8 5.4 5.410.9 2.5 2.5possible to derive the strength numberfor the Nadreau criterion. Theregression gave a flat minimum whenapplied to either criterion. Because thedifferent failure surfaces differed onlyin the tensile quadrant it was decidedthat the calculation could be performedmore easily by fixing the tensilestrength ST. This allowed theelimination of one independent parameterfrom the regression. For the Tsai-Wucriterionand for the Nadreau criteriontan 6(C + T) 2 ST 2-......"...----,:----=---".-- (9)9 TC 2 (1 - as 3 - bS 2 - cST T TThe val ue of ST' if in the range 0.5 to1.5 MPa, did not have much influence onthe root mean square error at the minimumof equation (7). A value of ST = 0.9 MPawas chosen.The regression of equation (7) ,taking into account equations (8) and(9), gives the results presented in Table2. The final failure surfaces can bevisualized by drawing the intersection ofthe surface with the 01 - cr2 plane (03 =0), Figure 6. The differences betweenthe Tsai-Wu and Nadreau criteria aresmall and thus, in subsequent analysis,the Tsai-Wu criterion will be used solelybecause of its ease of use and simpleextension into anisotropic cases.The strength number A used in theregression is also helpful in practicalapplications of the failure criteria. Itcan be thought of as a non-dimensionalnumber which defines the ratio of thelength of a line from the origin to apoint in stress space to the length of aline from the origin to the failuresurface and passing through the stresspoint. In this instance the factors Aand B are defined in terms of the stresspoint ° ij rather than the stressinvariants as above. When the strengthnumber is 1 the stress state is on thefailure surf ace and the material fails.The actual method is to assume a unit322

Table 2. Results of failure surface regressionIIIIII Tsai-Wu IcriterionlSet IFU [MPa-1 ] 0.736G UU[MPa-2 ] 0.417G U22[MPa-2 ] -0.188Sc [MPa] 2.7(A -l)RMS 0.35P HYDR[MPa] 18.0Set II Set III Set IV0.885 0.835 0.9880.251 0.307 0.137-0.062 -0.129 -0.0244.4 3.6 8.10.20 0.31 0.147.3 17.1 U.6C [MPa] 31.4T [MPa] 0.73620.6 30.6 29.50.616 0.655 0.563NadreauI criterion II II II II Itie 0.026Sc [MPa] 2.7(A -l)RMS 0.290.089 0.043 0.1384.4 3.8 8.20.21 0.30 0.155.0 -12.0SET IVTSAI-WUNADREAU...........................-12.0+---jboundary load or displacement and then tocalculate the values of A within the bodyof the material. The unit boundaryconditions can then be linearly scaled,either up or down, to give a maximum A of1. This method provides an efficientmeans of incorporating the failurecriterion into calculations of the stressstate in a body. In actual applicationsof A there are some limitations. Forexample, if the extension of the linethrough the stress point contacts thefailure surface at a point where thesurface is far from normal to the line,then small uncertainties in defining thestresses or failure constants could leadto a large variation in the strengthnumber.Modelling Ice Pressure DuringPenetrationFigure 6. The failure surface on a planeof principal stress. The Tsai-Wu andNadreau failure surfaces are compared.When a structure penetrates into icethe average ice pressure has beenobserved to decrease with increasing323

penetration (Varsta, 1983). This averageice pressure is defined as the total loaddivided by the total contact area derivedfrom the geometry of the structure andthe depth of the penetration. It is 4Postulated that this average pressure is ~• f3'" .~ 3"'"related to a contact function and a ~ >..nominal ice pressurePav = g(A,v,,) Pnom (10)~. . ~KEMI 198245'. MEASURED- REGRESSIONThe contact function, g, has beenobserved to be a function of contactarea. It may be additionally related toindentation speed and aspect ratio, butnot ice properties. On the other hand,the nominal pressure is related to theice properties and the contact geometry.These ideas are here incorporated into ananalytical model of ice penetration.Figure 7.(Pav) asz.2 3 4 567~[cMJ50 100 150 200 300 A [em":!Peak values of ice pressurea function of penetration depthFormulationThe nominal ice pressure on thecontact surface between a structure andan ice feature is derived from the iceproperties and the geometry of thecontact. It is the minimum uniformpressure on the ice required to initiatefailure; i.e. A 1. This nominalpressure is scale independent and assumesthat the ice is intact. There isjustification for this assumption fromthe test results of Varsta (1983) whichshowed that for the initial stages ofpenetration of a 45° plane into an iceblock, the pressure remained constantwith increasing contact area in the earlypart of the penetration (Figure 7).The next concept is that of acontact function. This was inves tigatedby Varsta (1983) who proposed thatpressure was transmitted through thecontact layer by solid-to-solid contactand also a viscous layer. Varsta'sresults can be interpreted to imply thatthe contact function varied through thepenetration process. It can be describedby an empirical r~lation of the formg(z) = C zn (11)where z is depth of penetration and C andn are coefficients. Depending on theindentation geometry the contact areavaries directly with z or z2. Thisformulation ignores velocity which mayalso effect the contact function.Calculation of nominal pressureThe actual calculation of thenominal ice pressure can be bestdemonstrated by an example. Consider thepenetration of a ship's bow into an icefeature. The configuration of thecontact surface is that of an inclinedwedge as shown in Figure 8. The stressesin the ice feature are calculated withthe finite element method usingthree-dimensional solid elements andinfinite elements (Klinge, 1985) for thefar boundaries. A uniform nominalpressure of 1 MPa is assumed for thecalculation and the ice behaviour priorto failure is taken to be linearlyelastic. A particular set of iceproperties is selected and the yieldcriterion parameters determined, in thiscase -10°C and 2 x 10-3 s-l (set IV ofTable 2). Within each element of thefinite element model the strength numberA is calculated from equation (7) and Band A as defined for the stress state inthat element. From these calculationscontour plots of A can be obtained.Figure 9 shows the contours of A on thecontact surface for a uniform nominal icepressure of 1 MPa. A values at depth inthe ice feature were lower. The maximumvalue of A on the contact surface was324

Bow penetrating a mul t IyearIce ridgeA=C·X0.088. For the purposes of calculatingthe nominal pressure, however, theaverage value of ).. is taken to be morerepresentative of the condition offailure on the contact surface. It canalso be seen that the ).. contours arerelatively uniform so that this is not acritical assumption. In this instancethe average value of the strength number).. is 0.071, which when scaled up to thefailure condition value of 1 gives auniform nominal ice pressure Pnom = 1/)"or 14.2 MPa.calculations for thedescribed in Tablessummarized in Table 3.that higher straintemperatures lead toPnom·Results of Pnomfour data sets1 and 2 areIt can be seenrates and lowerhigher values ofTable 3. P nomvalues calculated for foursets of dataInclined planecrushing an Ice edgeFigure 8. Test configuration of a bowpenetrating an idealized multi-year icefeature.Set I II III IVP nom 8.9 8.4 11.3 14.2 MPaASECTIONIT --10.0 degCd"/dt. - 2.0"10" II.x- 0.51~- 0.31Contact functionThe contact function, g(A, z,v, •• ), which could be a function of anumber of factors including contact area,A, penetration, z, velocity, v, etc. asdiscussed previously, will be assumedhere to be a function of contact areaonlyg(A) (12)o0.0 0.10.2 0.3 0... O.S 0.6 0.7 0.8OC0.9 1.0 1.1 1.;cFigure 9. The strength number ).. on thecontact surface for Pn m = 1 MPa. Theunits for the geometry of the contact aresuch that the horizontal indentation isunity (distance OC on Fig. 8).where Pav and Pnom are as defined above.Pnom is independent of contact area.Pav' on the other hand, has been found tobe a function of contact area, A, duringthe penetration process. Certain dataare available from which empiricalrelations between pressure and contactarea can be determined. The resultingcontact function calculated from fullscale data obtained with the M.V. Arctic325

in multi-year ice (Anon, 1985) andlaboratory crushing tes ts (Vars ta, 1983)are presented in Figure 10. Regressionequations of the contact function giveg(A) = 0.1 (AlA )-0.42 (13)ofor the crushing tests andg(A) = 0.3 (AlA )-0.41 (14)ofor full scale tes ts, where Ao = 1 m 2 •The value of the exponent is practicallythe same for two tests of very differentgeometries. This implies a more generalvalidity of the value of the exponentwhich characterizes the area dependency.The values of the constant, 0.1 and 0.3,are different which may reflect, forexample, the different indentationspeeds.0.80.60.4E00.1:...... >0.'"0.2TEST CASE• SHIP BOW INDENTo INCLINED PLANE0.10.002 0.01 0.1 10CONTACT AREA [m']Figure 10.plotted vscases.ConclusionsThe contact function g(A)the contact area for two testA method has been developed forpredicting the average ice pressure onthe contact surface during thepenetration of a structure into an icefeature. This average pressure is afunction of two factors, a nominal icepressure and a contact function.The nominal ice pressure can becalculated explicitly by applying ananalytical model which requires aknowledge of the geometry of thepenetration and the failure surface ofthe ice. The failure surface can bedetermined from strength measurementsunder uniaxial and multiaxial compressiveloading. Extensive tests were carriedout on multi-year ice and, for strainrates corresponding to ramming tests byships, behaviour was initially elasticfollowed by brittle failure. Icestrength was found to be sensitive tomultiaxial stress states; i.e. anincrease of strength occurred inmultiaxial tests compared to uniaxialtests. The ice features from which theice was sampled could be described asbeing globally isotropic, althoughindividual specimens did showanisotropy.The Tsai-Wu failure criterion, whichis of a general quadratic form, proved areasonable fit of a failure surf ace tothe strength data. Starting with elasticbehaviour of the ice, a finite elementcalculation of the stress states in thecontact zone was incorporated with thefailure criterion through a strengthnumber A to predict a nominal icepressure on the contact surface. This isthe pressure at which intact ice wouldfail. For example the nominal icepressure at -lO·C and 2 x 10- 3 s-l was14 MPa •To relate this nominal ice pressureto the average pressure on the contactsurface during the penetration process anempirical expression (the contactfunction) of the form C (A/Ao)n was foundto provide a good representation. Forboth small and full scale tests the valueof the exponent was about -0.41. Thecoefficient was 0.1 for small scale testsand 0.3 for full scale tests. There isstill not an adequate understanding ofthe contact function.A proper physical understanding ofice behaviour in the contact zone isrequired before a rigorous analyticalmodel can be developed for the contactfunction. Some of the approaches which326

might be taken include treatment of thecontact zone as a discontinuoussolid/viscous material, application ofdamage mechanics to the disintegration ofthe intact ice, and application ofnon-simultaneous failure concepts.AcknowledgementThe partial funding of this work,provided in Canada by the Panel on EnergyResearch and Development throughTransport Canada and in Finland by theMinistry of Trade and Industry, isgratefully acknowledged. The continuinginteres t and support of the proj ect bythe Canadian Coast Guard is appreciated.Also the assistance of numerouscolleagues at our respectiveorganizations is gratefullyacknowledged.ReferencesAf anasev, V. P. , Dolgopolov, Y. V • andShyeishtein, Z.1. 1971. Ice pressure onseparate supporting structures in the sea(in Russian). Arkticheskii i AntarkticheskiiNauchno-Issledovatel'skii.Trudy, Institut, Vol. 300, pp. 61-80.u.s. Army Cold Regions Research andEngineering Laboratory, Hanover, N.H.,Draft Translation 346 (1972).Anon. 1985. M. V. Arctic dedicated fieldtests: test results and analysis, finalreport. Prepared for Transport Canada,Coast Guard Northern, by German and Milneand the Technical Research Centre ofFinland, Transport Canada ReportNo. TP6270E.Gold, L.W. and Krausz, A.S. 1971.Investigation of the mechanicalproperties of St. Lawrence River ice.Canadian Geotechnical Journal. 8(2):163-169.Michel, B. and Ramseier, R.O. 1971.Classification of river and lake ice.Canadian Geotechnical,Journal. 8: 36-45.Nadreau, J.P. and Michel, B. 1986. Yieldand failure envelope for ice undermultiaxial compressive stresses. ColdRegions Science and Technology. 13~75 82.Richter, J.A. and Cox, G.F.N. 1984. Apreliminary examination of the effect ofstructure on the compressive strength ofice samples from multi-year pressureridge. Proceedings of ThirdInternational Offshore Mechanics andArctic Engineering Symp., New Orleans,Vol. 3, pp. 140-144.Riska, K. and Frederking, R. 1987.Modelling ice load during penetrationinto ice. JRPA No.1, Ice LoadPenetration Model, Report No.2. JointReport of the Technical Research Centreof Finland and the National ResearchCouncil of Canada. Transport CanadaReport No. TP8237E.Sanderson, T.J.O. 1984. Theoretical andmeasured ice forces on wide structures.Proceedings of 7th Symposium of IceProblem, International Association forHydraulic Research, Hamburg, F.R.Germany, Vol. IV, pp. 151-207.Sinha, N.K. 1981. Rate sensitivity ofcompressive strength of columnar- grainedice. Experimental Mechanics. 21(6):209-218.Tsai, S. and Wu, E. 1971. A generaltheory of strength for anisotropicmaterials. Journal of CompositeMaterials. 5(1):~5~8~-~8~0~.~~~~~~~~Varsta, P. 1983. On the mechanics of iceload on ships in level ice in the BalticSea. Technical Research Centre ofFinland, Publications 11, Espoo, 91 p.Klinge, P. 1985. Infinite elements (inFinnish). M.Sc. Thesis, HelsinkiUniversity of Technology, Espoo, 110 p.Korzhavin, K. 1971. Action of ice onengineering structures. Cold RegionsResearch and Engineering Laboratory(CRREL), Draft Translation, Hanover, NewHampshire, 319 p.327

MODEL TESTS FOR MULTIYEAR ICE LOADINGAGAINST A FIXED CONICAL STRUCTUREM. M. WinklerShell Development Company, Houston, Texas, USAAbstractIntroductionAn experimental model testing program to determinemultiyear ice ridge and floe loads with a conicalstructure is described. Tests were performed to modelthe interaction of a uni.form thickness ice floe against afixed cone and to model the interaction between an iceridge and floe during the ride-up of a short ridgeagainst the cone. The ice floe was modeled as a verythick uniform ice sheet and both the ridge and structurewere modeled as rigid bodies. Tests were performed usingtwo different model materials: ure

programs were considered using two different modelmaterials.Tests were performed by HamburgischeSchiffbau-Versuchsanstalt (HSVA) in Hamburg, W.Germany and by Arctec Engineering Inc. in Columbia,Maryland USA. The HSVA tests were performed usingurea doped ice and the Arctec tests were performedusing their waJt-based material. Each of these materialswas perceived to offer different compromises to that ofan ideal model ice material.ter of 80 feet. ThiS structure is not a true cone, butformed of flat elements (facets). The structure shownin Figure 1 has been modeled using 24 equally spacedfacets. These facets have been included in the modelstructure in both tests.The model structure used ineach of these tests was instrumented to measure globalforces in the x, y, and z directions.Both the urea-doped ice and wax ice have beenused for modeling first-year ice conditions. In selectingthese materials and in carrying out the actual tests, noattempt was made to distinguish differences betweenfirst-year and multiyear ice types other than thicknessand selection of target mechanical properties.Thesetests were undertaken with the purpose of trying toestablish the overall feasibility of transferring technologydeveloped for modeling first-year ice conditions tomodeling multiyear ice conditions.Both the uniform-thickness ice sheet and ridgetests were successfully completed with the urea ice;however, the results from these tests are inconclusivedue to material property scaling deficiencies and unrealisticdeformational behavior.The wax ice tests werecompleted only through the testing of a uniform-thicknessice sheet. These results showed totally unrealisticmaterial behavior. Conclusions given in this paperregarding the adequacy of the urea and wax tests areunique to a specific perception of multiyear ice behaviorunder the given loading conditions. Results from thesetest programs are presented with the intent of trying tofoster discussion and encourage ideas that can be usedto improve upon our experience and benefit future testingprograms.The experimental test setup is described, followedby a discussion of the test results from the wax and ureaice tests. A compariso!1 is given between the urea iceresults and results computed using alternative analyticalmethods.Experimental Test ProgramThe test structure geometry is shown prototypescale in Figure 1. This structure has a constant slope of40° with a base diameter of 360 feet and a neck diame-EL.120·---40'120'~----------------~I360" .Figure 1. Test structure geometry.The structure shown in Figure 1 is based on thegeometry of an exploration drilling structure designedfor the Beaufort Sea, Alaska and has an operationalwater depth range of 60 to 120 feet. Structures designedfor these water depths have to be capable of resisting iceforces generated by both first-year and multiyear icefeatures. Level first-year ice thicknesses in theBeaufort Sea may reach greater than 6 feet in thicknessand multiyear floe thicknesses may reach 20 to 30 feetand may include ridges of greater than 60 feet in thickness.Both tests were performed at a scale of 1 :50 for auniform 25-foot uniform thickness ice sheet. Becauseof problems with the wax ice, ridge tests were performedusing only the urea ice. These tests employedthe same ice sheet as the uniform thickness ice sheettests but also contained an embedded short ice ridge of65 foot thickness. The test geometry for the ridge testsis shown in prototype scale in Figure 2. The objectiveof the ridge tests was to evaluate the capability of the330

FLOW ~---...."THICKNESS \SIDE200':IIIIIPLANfr!() .~-y\/./--- /Table 1. Target Multiyear Ice Material Properties -Prototype ScaleProperty Symbol ValueCompressive Strength ac 900 pSITensile Strength at 100 psiFlexural Strength af 80 psiModulus of Elasticity E 10 6 psiModulus/Flexural Strength E/af 10,000Ice-Structure Friction Coeff. ~Is 0.15\ELEVATIONFigure 2. Ridge test geometry.surrounding ice sheet to push the ridge onto thestructure.These short ridges have sufficient flexuralcapacity to resist the bending moment developed asthey are lifted out of the water and are not expected tofracture. Since previous finite element study has shownthat the ridge will first break free from the sheet prior toriding a substantial distance onto the cone (Winkler andNordgren 1986) it was decided for simplicity to modelthe ridge as initially free floating within the surroundingice sheet or floe. The ridge was modeled as a rigid element.Tests for the uniform ice sheet were planned tostudy the effects of velocity and waterline elevation onstructure load. Tests for the sheet containing the embeddedice ridge were planned at a waterline elevationto prevent interaction between the ridge and basinfloor.The target material properties were constant forboth tests and are tabulated in Table 1. These propertieswere selected to be representative of the propertiesobtained from the Mechanical Properties of Sea Ice testprogram (Cox et al 1984; Cox et al 1985).Recognizing that it is not possible to scale all ofthese properties simultaneously, the flexural strengthwas selected as most important since the predominantfailure mode for both the floe and ridge tests wasexpected to be bending failure. The expected failure inthe floe tests is by upward bending failure of the icesheet as it lifts onto the cone surface. The expectedfailure of the ice sheet in the ridge tests is by downwardbending failure of the ice sheet resulting from the frictionalforces developed at the ridge-floe interface.Laboratory tensile test results available for multiyear iceshow an average tensile strength value of about 100 psi(Cox et al 1984; Cox et al 1985). A target flexuralstrength value of 80 psi was selected for these tests toaccount for discontinuities which may occur in sea icefound in the field.No distinction was made betweendifferences in flexural strength in downward versus upwardbending of the ice sheet.Modeling a 25- foot-thick ice sheet at a scale of1:50 results in a model ice sheet which is 6 inches thick.Based on the tests which have been reported in the literaturethis thickness is at least two times greater thanthe thickness at which all previous known tests havebeen performed. Aside from the differences betweenmultiyear and first-year ice types, these tests representthe first time, to our knowledge, that anyone hasattempted to test ice of this thickness in the laboratory.Many of the problems which will be discussed next forthe individual test results appear to be thickness related.331

Wax Ice Test ProgramTests were performed at Arctec Engineering Inc.'sColumbia, Maryland wax ice test basin during Januaryand February of 1985. The material used in this basinhas been described by Schultz and Free (Schultz andFree 1984). The salient feature of the wax basedmodel material for these tests was that the flexuralstrength could be scaled to the target value. Additionally,the composition of the wax ice is essentiallyisotropic which is felt to be in line with the bulk propertiesof multiyear ice.Two level ice sheets were tested having averagematerial properties summarized in Table 2. Values forflexural strength given in this table are for downwardbending. The fact that the compressive strength is lessthan the flexural strength in both ice sheets is probablymore a reflection on test technique than on the actualmaterial properties.Table 2. Wax Ice Material Properties - Model ScaleProperty Sheet No. 1 Sheet No. 2a( 2.91 psi 2.12 psiEla( 1070 837ac/ a( 0.89 0.75J.lis 0.17 0.182For both ice sheets tested, the behavior of the waxice was judged to be unrealistic because of the lowcrushing strength to flexural strength of the material.Tests were conducted at prototype velocities of 0.12and 1.2 ftlsec and waterline elevations of 60 and120 feet. The initial failure of the wax ice resulted inbroken piece sizes which appeared realistic; however,any semblance to realism decreased with increasingpenetration. With subsequent penetration, the brokenpiece size deteriorated until the rubble pile that formedin front of the structure consisted of essentially crumbledmaterial. The cohesiveness of the crumbledmaterial may have hindered clearing of the rubble pastthe structure resulting in the formation of a vertical icefoot at the waterline of the cone. The presence of thisattached ice foot moved the failure zone away from thestructure in the direction of the approaching ice sheet.Because the ice could not ride up the cone, the sheetfailed in compression against the vertical ice foot. Theresulting failure of the ice sheet was characterized bythe formation of wedges which were extruded in frontof the ice foot, similar in appearance to passive earthpressure wedges.Urea Ice Test ProgramThe testing program at HSV A was performed duringJanuary 1985 using a 2% urea doped solution. Adescription of this basin can be found in Wessels (Wessels1984). The salient feature of the urea doped iceused by HSVA was that realistic ratios of Ela( could beobtained; however, the available flexural strength wouldprobably be higher than the target value. Prior to performingthe tests it was estimated that the minimumavailable flexural strength would be about twice that ofthe target value.The ice sheet thickness considered in these testsposes some practical problems, including the amount oftime required to grow and temper the ice sheet and thequantity of material which is required to perform materialproperty tests. The time interval between the twoice sheets during the test performance period was overone week. This included cleaning broken ice from theprevious test out of the basin, growing the new ice sheetand finally, warming the basin to reduce the icestrength, or temper the ice. The useable test area isalso reduced in thicker ice sheets because of the needto test larger size material property test specimens.Floe test resultsFour floe tests were performed at two differentwaterline elevations and two different velocities. Tests101 and 102 were performed at the 60 foot water depthelevation at velocities of 0.12 and 1.2 ft/sec. Tests 103and 104 were performed at the 120 foot water depthelevation at velocities of 0.12 and 1.2 ft/sec. Each ofthese tests were run for sufficient duration to achievesteady state conditions. Force histories for test 102 areshown in Figure 3. Results from all four of these testshave been analyzed by separating the floe breaking332

Table 3. HSVA Material Properties - Model ScaleProperty Floe Test Ridge Testa( 1.89 - 1.02 psi 3.05 - 2.47 psiEta( 6000 - 900 3500 - 3000act a( 15.7 8.2fils 0.06 0.15Figure 3. Force histories for floe test 102 (model scalein Newtons).flexural strength since compressive strength tests wereonly performed at the conclusion of the model tests.Tests for coefficient of friction between the structureand ice were performed using a separate test setup priorto the tests.component from the floe ride-up component to facilitatecomparisons with an analytical model.The floe breaking component results from theforce of the floating deformed ice sheet acting on thecone prior to fracture. The floe ride-up componentresults from the force required to equilibrate the previouslybroken ice blocks resting on the cone surface.These components have been determined from theforce histories by assuming that the peak forces correspondto the summation of the two individual forcecomponents and the magnitude of the troughcorresponds only to the ride-up component (see Figure3). The floe breaking component can then beestimated as the difference between these two values.The floe breaking component can also be estimated bythe initial force peak.The model material properties for these tests havebeen summarized in Table 3. Values for flexural tests.strength given in this table are for downward bending.The range of values for the flexural strength andmodulus of elasticity represent the range of values frommechanical property tests performed prior to the modeltest performance and property tests performed at theconclusion of the model test performance. The timeinterval between these tests was over 8 hours. A singlevalue is given for the ratio of compressive strength toThe horizontal (x) and vertical (z) force componentsfor both the peak and trough forces have beenvisually determined from the force histories for each ofthe tests. These forces have been compared with forcespredicted using plastic limit analysis given by Ralston(Ralston 1980). A comparison of the measured to predictedfloe breaking forces for the tests is summarizedin Figures 4 and 5 for the horizontal and vertical forcecomponents respectively. The range in predicted forcecoincides with the range of flexural stresses given inTable 3. Comparison of the measured to predicted floebreaking components shows a large disparity withalmost all of the measured values substantially greaterthan those predicted from the limit analysis. The valuespredicted by the limit analysis should represent an upperbound on load. Velocity effects cannot bedistinguished from these results. For tests 103 and 104the ice sheet contacted the vertical structure neck andthe failure was categorized by crushing and crystalspalling. This resulted in the horizontal forces beingmuch greater than the vertical breaking forces for theseA possible explanation for the large measuredforces for tests 101 and 102 can be obtained by examinationof the flexibility of the ice sheet. The ratio ofmodulus of elasticity to flexural stress, Eta(, provides ameasure of the deformability of the ice sheet relative toits strength. For elastic material property behavior, theice sheet strength is inversely proportional to this ratio.333

60,-------------------------------------~TEST 101 WD = 60 FT V" 0 12 FT/SECo TEST 102 WD· 60 FT, V" 1 2 FT/SEC50 CJ TEST 103 WD - 120 FT, V" 0 12 FT/SECo TEST 104 WD = 120 FT, V '" 1 2 FT/SEC60 ,---------------------------------------~TEST 101 wo = 60 FT V'" 0 12 FT/SECo TeST 102 WD.o 60 FT, V = 12FT/SEC50 CJ TEST 103 WD"" 120 FT, V· 0 12 FT/SECo TeST 104 WD" 120 FT, V = 1 2 FT/SEC4040302010g~ b~ (PI g (51(51 (51 (PI (PI%(51302010XXX(51 (PI (51 (PII(PII I(51 (5120 406020 4060MEASURED FORCE (KIPS)MEASURED FORCE (KIPS)Figure 4. Comparison of measured to predicted horizontalfloe breaking forces (P == initial peak force, S ==steady state force).Figure 5. Comparison of measured to predicted verticalfloe breaking forces (P == initial peak force, S == steadystate force).Since cone force is a direct function of the weight of iceon the cone, a more flexible ice sheet will result in morestructure load as a greater mass of ice is lifted out of thewater prior to failure.The ice sheet tested had a relatively low ratio ofEla" but more importantly, the increased deformabilitymay have resulted from the deformation being domInatedby shear rather than bending.Work done byLainey and Tinawi (Lainey and Tinawi 1983) andTinawi and Gagnon (Tinawi and Gagnon 1984) hasshown that transverse anisotropy can have a significanteffect on the deformational behavior of first-year seaice, and for this ice type, deformation is betterdescribed using shear theory rather than bendingtheory. At a temperature of -5°C, Tinawi (Tinawi andGagnon) reports that the measured central deflection ina circular ice plate under short term loading is about 4times the predicted deflection based on elastic platetheory.The urea ice sheet was highly anisotropichaving long columnar ice crystals on the order of acouple of millimeters in diameter. During the performanceof the sheet tests, the deformation of the ice sheetwas estimated at times to be greater than 4 inchesmodel scale or greater than 16 feet in prototype scale.A comparison of the measured to predicted floeride-up forces for the tests is summarized in Figures 6and 7 for the horizontal and vertical forces, respectively.For test 102, there is about a 45% differencebetween the measured and predicted horizontal ride-upforces and about a 50% difference between the measuredand predicted vertical ride-up forces.Thisdisparity can be partly explained by the difference inride-up ice coverage between the test and the assumptionscontained in the predictive model. Ralston's limitanalysis model assumes the leading half of the cone surfaceto be covered with ice between the waterline andstructure neck.Ice coverage during the test alsoextended onto the vertical neck portion of the structureas well as covering the leading half of the cone betweenthe waterline and neck.The additional vertical load component of this icehas been estimated to be on the order of 13,000 kipsprototype scale. The corresponding horizontal forceacting against the 40° slope required to equilibrate thisforce is 12,400 kips. Including this additional neck334

(i.!wua:~.... 5lu~50.---------------------------------------~40302010o TEST 102 WD" 60 FT, V = 12FT/SECo TEST 103 WD'" 120 FT. V '" a 12 FT/SEC/::). rEST 104 we· 120 FT. V '" 1 2 FT/SEC6.INIINIoINIo70 .---------------------------------------~605040302010o TEST 102 we = 60 FT, V" 1 2 FT/SECo TEST 103 WO"'120FT,V-O.12FT/SEClJ TeST 104 we,. 120 FT, V '" 1 2 FT/SEC6.INIo(NIooINIo2040204060MEASURED FORCE (KIPS)MEASURED FORCE (KIPSIFigure 6. Comparison of measured to predicted horizontalfloe ride-up forces (N == additional neck load).Figure 7. Comparison of measured to predicted verticalfloe ride-up forces (N == additional neck load).load reduces the difference between the measured topredicted ride-up force for test 102 to 2% and 16% forthe horizontal and vertical components, respectively.Reasonable agreement should be expected between themeasured and predicted floe ride-up forces, regardlessof material properties, since ride-up forces are simply afunction of the weight of ice on the cone.Ridge test resultsA second urea ice sheet containing a short iceridge was tested with test geometry shown in Figure 2.The ridge was constructed of fresh water ice and thetests were performed by inserting the ridge into a precutslot in the ice sheet. This loading condition hasbeen previously found by Winkler and Nordgren(Winkler and Nordgren 1986) to represent an extremeloading condition due to both the magnitude of thestructure load as well as the elevation to which the ridgecan potentially load the structure.Four ridge tests were conducted. The first test,test 211, was performed at a waterline elevation of75 feet and ice velocity of 0.12 ft/sec. In this test, theridge lifted onto the cone surface and rotated approx-imately 20° prior to what appeared to be a punchingfailure of the supporting ice sheet directly behind theridge. The ridge then became jammed between thecone and •'seafloor" and subsequent penetration of theice sheet resulted in override onto the top surface of theridge. Tests 202 and 213 were also performed at awaterline elevation of 75 feet but for these tests the falsebottom was lowered to prevent jamming of the ridge.Test 202 was performed at a velocity of 0.12 ft/sec andtest 213 was performed at a velocity of 1. 2 ft/sec.Force histories for test 202 are shown in Figure 8. Apunching failure of the ice sheet was also observed ineach of these tests, similar to test 211; however, becauseof the increased water depth, jamming of theridge did not occur. Instead, the ridge slid underneaththe advanCing ice sheet. The final test, test 204, wasperformed at a waterline elevation of 120 feet and at avelocity of 0.12 ft/sec.A comparison of the measured (M) to predicted(P) forces from the ride-up model (Winkler andNordgren 1986) is summarized in Table 4. Predictedridge ride-up forces are given at a ridge rotation angleof 20° which corresponds to the observed rotation angleat peak force in Tests 211, 203 and 213. For these335

24"'0j..a.0mi. J1847 •Table 4. Comparison of Measured to Predicted RidgeRide-Up Forces - Prototype Scale in 1000 Kips·:::1~"Tt., ==~·:::::10.0 ......-400.0 FTTrNI -212.0 _I.'1110.-201.,:- •.:1.HI)-I.':t, ';;.olll.!lL...--=;-----;:r.-;-----::z::-:-----::>.SHEll 40 OED 80/120 FT CONICAl STftUCTUItENil I 202 Yon 26' I.e" 1,26Figure 8. Force histories for ridge test 202 (model scalein Newtons).tests, no apparent relative displacement was observedbetween the floe and the surrounding ice sheet. Thecoefficient of friction between the ridge and floe used inthe ride-up analysis was selected at 1111 =0.5. This valueeffectively prevents relative vertical displacementbetween the ridge and the surrounding ice sheet.From the comparison shown in Table 4, fairly goodagreement is seen between the measured and predictedresults for tests 202 and 213. For test 211, the ice sheetremained attached to the sides of the ridge during rideupresulting in increased load. The size of the ridgecutout was increased for the subsequent tests to preventattachment. For test 204, the large horizontal force resultedfrom the ridge jamming between the structureneck and ice sheet.In all cases the surrounding ice sheet failed at amuch greater load than predicted. From analysis of theice sheet as a plate on an elastic foundation (Nevel1965), it was predicted that fracture would be initiatedwhen the vertical force acting on the ice sheet behindthe ridge reached 20 kips/ft. This analysis assumed thesheet to be loaded along at its free edge by a 200 footlong line load and any additional stiffness resulting fromHorizontalVerticalTest M P M P211 140 96 101 85202 116 96 81 85213 106 96 93 85204 490 96 45 85placement of the ridge within the cutout was neglected.The vertical force predicted from the ride-up analysis atfailure of the model ice sheet was 137 kips/ft, or almost7 times greater.Again a possible explanation for this large disparityis that the deformational behavior of the ice sheet isgoverned by shear slip along the ice crystal boundariesrather than elastic bending. Because of this increaseddeformability, the model ice sheet can carry a greaterload prior to fracture. During the performance of thesetests, flooding on the top surface of the ice sheet wasobserved prior to fracture indicating that the sheet haddeflected over one half inch model scale. Unfortu·nately, measurements of the ice sheet deflection atfailure are not available.ConclusionThe results from the wax ice tests have been considereda failure because of the unrealistic behavior ofthe wax ice material at the test thickness. Previousexperience with thinner wax ice sheets did not extrapolateto a 6 inch wax ice sheet. Performance of the waxice was further aggravated by the low crushing toflexural strength ratio of the material.The results from the urea ice tests are of limitedvalue, since the behavior of the urea ice, at the testthickness, was not considered to be representative ofmultiyear ice behavior. These results are judged to beinconclusive due to the flexibility and ductility of the icesheets tested.The thickness of the ice sheets grownusing the urea ice resulted in a pronounced crystalstructure that appears to be responsible for the observedflexibility of the ice sheets.336

Aside from these technical problems, testing usingthick ice sheets is both costly and poses other practicalproblems.For both test facilities these problemsincluded ice material waste for performance ofmechanical property tests, material handling and disposalproblems and for the case of the urea ice, lengthyice growth and warming times.Prior to performing additional tests of this type, aseries of simpler tests should be performed to validatethe behavior of the model ice material against analyticalmodels and basic judgement. Results have been presentedin this paper to show some of the problemsencountered in testing thick ice sheets and to share theresults using two of the available model ice materials.Ralston, T. D. (1980). Plastic Limit Analysis of SheetIce Loads on Conical Structures, in "Physics andMechanics of Ice" (P. Tryde, ed.) Proc. IUTAMSymposium, Copenhagen, Denmark, 1979.Schultz, L. A. and Free, A. P. (1984), RecentExperience in Conducting Ice Model Tests Using aSynthetic Ice Modeling Material, Proceedings Int.Assoc. Hydraul. Res. Ice Symposium, Hamburg, WestGermany, August 27-31.Tinawi, R. and Gagnon, L. (1984). Behavior of Sea IcePlates Under Long Term Loading. P~oceedingsInt.Assoc. Hydraul. Res. Ice Symposium, Hamburg, WestGermany, August 27-31.AcknowledgementThe testing program described in this paper wasfunded by Shell Western Exploration and ProductionCo. and permission to publish these results is acknowledged.ReferencesCox, G. F. N., Richter-Menge, J. A., Weeks, W. F.,Mellor, M., and Bosworth, H. W. (1984), TheMechanical Properties of Multi-year Sea Ice. Phase I:Test Results, Report 84-9, Cold Regions Research andEngineering Laboratory, Hanover, NH.Wessels. E. (1984). Model Test Investigation of IceForces on Fixed and Floating Conical Structures.Proceedings Int. Assoc. Hydraul. Res. Ice Symposium,Hamburg. West Germany. August 27-31.Winkler. M. M. and Nordgren. R. P. (1986). Ice RidgeRide-up Forces on Conical Structures, Proceedings Int.Assoc. Hydraul. Res. Symposium on Ice. Iowa City.Iowa. August.Cox, G. F. N., Richter-Menge, J. A., Weeks, W. F.,Bosworth, H. W. Perron, N. Mellor, M., and Durrell,G. (1985), The Mechanical Properties of Multi-yearSea Ice, Phase II: Test Results, Report 85-16, ColdRegions Research and Engineering Laboratory,Hanover, NH.Lainey, L. and Tinawi, R. (1983), The Importance ofTransverse Anisotropy for the Bearing Capacity of IceCovers, Proc. 7th Int. Conf. Port Ocean Eng. UnderArctic Cond., Helsinki, Finland, v. 1, April 5-9.Nevel, D. E. (1965), A Semi-Infinite Plate on anElastic Foundation. U. S. Army Cold Regions Researchand Engmeering Laboratory, Research Report 136.Hanover, NH.337

MODEL TESTS ON ARCTIC STRUCTURES IN ICES. S. GowdaRisto HakalaTechnical Research Centre of Finland, Espoo, FINLANDAbstractThe paper describes a series ofexperiments carried out on differentmodels in various ice conditions. The iceload tests were performed on four types ofmodels, viz. a rectangular indentor, acylinder, a cone and a caisson structure.Both level ice and first year ridges wereused for testing. During experiments, iceforce records were obtained for all thetests and the failure behaviour of icea round the models was monitored andvideo-recorded. This paper only deals witha vertical cylindrical structure in levelice conditions.1. IntroductionThe successful performance ofoffshore structures in the Arctic andSub-Arctic environments mainly depends onthe ability of the structures to resistlateral ice loads encountered during theirservice . For better understanding of theice- structure interaction and accurateestimation of ice forces on Arcticstructures, small-scale experiments arevery useful. The results obtained fromThis is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987. © The Geophysical Institute,University of Alaska, 1987.laboratory experiments under simulatedArctic environmental conditions provideuseful information in testing theoriesthat are used in estimating ice forces andprocesses associated with full-scale iceresistant offshore structures.The experiments were conducted at theWartsila Arctic Research Centre ofHelsinki- The model ice used duringexperiments was the standard fine-grainedmodel ice developed at the Wartsila ArcticResearch Centre. The details of the testbasin and model ice are given in Hakala etal. (1986).The ice load tests were conducted onfour types of structures, viz., a caisson,a cone, a circular cylinder and arectangular indentor. The caisson modelconsisted of a 2 .4 m wide caisson and a6.0 m wide berm. Two caisson modelsrepresenting different scale factors of1:50 and 1:40 were used duringexperiments. The scale model of theconical structure was 1 :15, with top andbottom diameters equal to 440 mm and 1150mm respectively. The cylinder had adiameter of 600 mm while the rectangularindentor had a width of 600 mm. This paperincludes only the tests carried out usingthe circular cylindrical model with adiameter of 600 mm in level ice and ridgefields.339

2. Test Set-up and Experimental ProcedureThe test arrangement consisted of aninstrumented cylinder attached to amotorized carriage. The cylindrical modelwas pushed against the ice sheets. Theattachment of the cylindrical model to themoving carriage is shown in Fig. 1. Only a600 mm diameter model was used during theexperiments and pushed through the levelice, ridge fields and partly- consolidatedridges with pushing speeds of 1, 5, 30 and160 mm/s. Only two level ice thicknessesof 42 mm and 46 mm were used. Before thes tart of the experiments, the flexuralstrength, characteristic length, effectivemodulus of elasticity and the ave ragethickness of ice sheets and ridge fieldswe r e me asured for each experiment . Thedata on model ice properties are given inTable 1.The total force on the 600 mmdiameter cylinder during indentation testsin level ice and ridges was recorded witha special measuring system. The measuringsystem consisted of two rigid platesinterconnected by three bearings. Thesupporting system was fixed to the towingcarriage so that the lower plate was freeto move longitudinally while all otherdegrees of freedom we re elimi nated. Thetowing force was then measured using oneforce transducer. In some cases, asix-component measuring sys t em was used.This consisted of two relatively stiffrings interconnected by six forcetransducers, three of the transducers areplaced in a vertical position and theremaining three in a horizontal position(Figure 2) .During the measurements, all forcecomponents were transmitted via thetransducers from the l ower ring to theupper one. This system made it possible toresolve the forces in whatever coordinatesystem was necessary from the readings ofsix channels .Fig. 1. The cylindrical model in themoving carriage .Table 1. Model Ice Propertiesverllcallransducer)horlzontaltransducerParame t ersicepropertiesCrushing s trength of ice 28 °ckPaFlexural strength of ice of 12,5 kPaYoung I s modulus ofelasticity E40.0 MPaInner friction of ridgeblocks 200 ... 350Ridge thickness500 mmRidge porosity 0 .39Ice/Structure frictioncoefficient 0 .4010800Figure 2 . Six-component Measuring System340

3. Analysis, Results and DiscussionA considerable amount of work hasbeen carried out on the determination ofice forces on cylindrical structures byrna ny inves tiga tors (Morris and Sodh i,1984; Frederking et al. 1982; Nakajima etal. 1981; Kato and Sodhi, 1983; Hirayamaet al. 1975). These include bothlaboratory and field studies. The earlierwork on the es tima tion of ice forces onvertical cylindrical piles by Korzhavin(1971) consisted of an empirical equationof the form:F = K a c D twhereF interaction forceK empirical factora c unconfined compressive strengthD diameter or width of structuret ice thickness.( 1)The empirical factor K takes severalfactors into account, such as biaxialindentation stress (I), shape of theindentor (m), contact coefficient (k),etc. To include many of these factors,equation (1) has been modified by manyresearchers (Ralston, 1979; Michel andToussaint, 1977; Afanasev et al. 1973;Hoikkanen et al. 1984) into the form:F = I m k D t (2)The value of the shape coefficient mfor a cylindrical structure was takenequal to 0.9 (Afanasev et al. 1973), andthe value of the indentation factor wastaken equal to 1.0 (Kovacs and Sodhi,1981). The changes in aspect ratio andvelocity lead to changes in the maximumnormalized force on the structure (Morrisand Sodhi, 1984; Ralston, 1979). Afanasevet al. (1973), have proposed the followingequation for obtaining the value of theindentation factor which is given by:k = Jl + 5(t/D) (3)Normally it has been found that themaximum normalized forces decrease withincreasing aspect ratios. The maximumnormalized force is given by the followingexpression:(4 )For crushing failures, a c is theunconfined uniaxial compressive strength.The selection of compressive strength iscomplicated and depe'nds on many factors,such as ice temperature, salinity, grainsize, crystallographic structure, strainrate etc. The effective strain rate isdefined by an empirical equation as givenby Michel and Toussaint:f. =J4 D.where e:Veffective strain rateindentation velocity.(5)Abou t 10 impact tes ts we re carriedout by towing the model in level ice andridges of different configurationsincluding partly-consolida ted ridges.The ice forces on the cylindricalmodel were calculated using equation (2)and compared with the experimental data.In Figure 3, a comparison is given betweentheoretical and experimental values. Ascan be seen from the graph, there is somevariation between the two results. Inaddition, the small amount of data isinsufficient to arrive at solidconclusions. However, the study indicatedthe need for further tests with moreparameters included.!1.8"/1.6V1.4v.;; 1.2]g"-1.00.8 • /1/0.6VV/V/V1/0.6 0.8 1.0 1.2 1.4 1.6 1.8Fig. 3. Comparison of theoretical andexperimental results in level ice.The force-time record using aone-component system in 46 mm thick levelice at a speed of 160 mmls is shown inFigure 4./341

Fx [NJ1000800600I. I ~~ ~Max. force - 996 N\I~t I! IL~ IMit \(W~~ rtl~ ['1 11 • 1 W W~.~ilr~W MA typical recordinghistory from this typesystem is shown in Figureat a speed of 160 mm/s.Fx [NJ500x-ax i s forceof force-timeof measurement6 for level ice400200-2008180-sea-1_-1500oFy [NJzea28 38 48 58I V V vv 'VII 'V r-J\1 .... [.J -zea ':~ [IV 11 ~ WFig. 4. Force-time record using singletransducer in level ice.In Figure 5, the force-time record isShown with one transducer at a very lowspeed of 1 mm/s in level ice of 46 mmthickness.Very few tests were conduQted inunconsolidated and partly-consolidatedridges using the cylindrical model. Henceforce calculations are not made for thesecases. However, force-time records arepresented in this paper for comparison.1200Fx [NJMax. force = 977 N-4ea\A nlyIVY" VtAlI'V" iVY ~ ..J~. ry-axis force1011. LUJ 11 ..n 1\!V VI V '"-sea o 10 IZFz [N)4eazeaIIz-ax is forceId dA-zea If'I 'Mv !1MIIV'VWI. IJ.-J' I~ u-.ea-sea oIAIWIZr\rJ1 "" 1I"'" .,I"v, \II.r10 IZtime (s]Fig. 6. Force-time history in level iceusing the three-coordinatemeasuring system.1414141000800600In addition to level ice, a few testswere conducted in ridge fields, includingpartly consolida ted ridges. The typicalforce-time records of ice forces in ridgefields and partly-consolidated ridge areshown in Figures 7 and 8 respectively. Thetest parameters are given in Table 1.4002000-2008 48 88 128 168 2BBtime (s]Fig. 5. Force-time record using singletransducer in level ice.342

Fx [N]600500400300200100-1008~28IMax. force = 530 N\J\.../ \"'"rV \ '\ .,J)i"J1/68 88 188tiM [el4. Summary and ConclusionsThe indentation tests were performedwith a vertical cylindrical indentor usingHartsiUi fine grained model ice wi thimproved mechanical properties. Theavailable empirical equations are veryuseful in predicting ice forces onvertical cylindrical models. However,large variations between calculated andexperimental results were noticed in somecases. This indicates the need forimprovement in theoretical expressions aswell as improved model ice properties.Further tests are required to arrive atsignificant conclusions. More tests areplanned in the newly constructed ice tankat the Laboratory of StructuralEngineering of Technical Research Centreof Finland (VTT).Fig. 7. Force-time record in head on ridge(not consolidated).5. AcknowledgementFx (N]SI!0-500-100ax-axis force\ ~~I'~~ ~Mif-!V"vJThe financial help for the projectfrom Technology Development Centre ofFinland (TEKES) is gratefullyacknowledged.6. References-IS0a oFy (N]10aa-11!00-2_-JI!0eoFz (N]400zoo-zoor-400-S000Fig.JJ.10 Z0 J0y-ax i s force\ r~~ III'"I1UvrI'10 Z0 J0z-ax I s forceA1.'ll. ~ j, llJdflil I~n AJ 1111 I~w\ ~A I \1. rVYlfN rr y \ .~ TJ \~~ 11'1111 ~Il ~10 Z8 J0part1y-8. Force-time record in aconsolida ted ridge.tIme (s]40'040Afanasev, V.P., Dolgopolov, Y.V. andShraishtein, Z.I (1973).: "Ice Pressure onIndividual Marine Structures", Ice Physicsand Ice Engineering, Israel Program forScientific Translation, p. 50 - 68.Croasdale, K.R (1978).: "Ice Forces onFixed Rigid Structures", Report preparedfor the Horking Goup on Ice Interaction onHydraulic Structures Committee on Iceproblems, IAHR.Frederking, R., Schwarz, 1., Wessels, E.and Hoffman, L (1982).: "ModelInvestigations of Ice Forces onCylindrical Structures", In Proceedings,International Conference on MarineResearch, Ship Technology and OceanEngineering (INTERMARITEC'82) , Institutefor ship and Marine Technology, TechnicalUniversity of Berlin, Hamburg, Germany, p.341 - 349.Hakala, R., Joensuu, A., Eranti, E. andS.S. Gowda (1986).: "Analysis of IceForces on Caisson Type Arctic Platform",343

Paper OTC 5130, Offshore TechnologyConference. Houston, Texas, U.S.A.Hirayama, K., Schwarz, I., Wu, H.C(1975).: "Ice Force on Vertical Piles:Indentation and Penetration", InProceedings, 3rd IAHR Symposium on IceProblems, Hanover, USA.Hoikkanen, J., Krankkala, T., Maattanen,M. and Pulkkinen, E (1984).: "CalculationMethods for Loads Against OffshoreStructures" , Finnish-Soviet ScientificCommittee Report No.4, University ofOulu, Ou1u, Finland.Kato, K. and Sodhi, D.S (1983).: "IceAction on Pairs of Cylindrical and ConicalStructures", CRREL Report 83 25,U.S.Army Cold Regions Research and EngineeringLaboratory, Honover, USA.Korzhavin, K.M (1971).: " Action of Ice onEngineering Structures", Draft Translation260, U.S. Army Cold Regions Resarch andEngineering Laboratory, Hanover, USA.Kovacs, A. and Sochi, D.S (1981).: "SeaIce Piling at Fairway Rock, Bering Strait,Alaska-Observations and TheoreticalAnalyses", International Conference onPort and Ocean Engineering Under ArcticConditions, Laval University, Ruebec,Canada.Michel, B., and Toussaint, N (1977).:"Mechanisms and Theory of Indendation ofIce Plates", Journal of Glaciology, Vol.19 (91), p. 285 - 300.Morris, C.E. and Sodhi, D.S (1984).:"Crushing Ice Forces on CylindricalStructures", In Proceedings, 7thInternational Association of HydraulicResearch Symposium on Ice Problems,Hamburg, Germany.Nakajima, H., Koma, N. and Inoue, M(1981).: "The Ice Force Acting on aCylindrical Pile", InternationalConference on Port and Ocean EngineeringUnder Arctic Conditions, Lavel University,Quebec, Canada, p. 517 - 525.Ralston, T.D (1979).: "Sea Ice Loads",Technical Seminar on Alaskan Beaufort SeaGravel Island Design, Anchorage, USA.344

AN INTEGRATED DESIGN APPROACH TO ARCTIC OFFSHORE PLATFORMSKailash C. GulatiSYNERTECH, Houston, Texas, USAJay B. WeidlerBrown & Root, Inc., Houston, Texas, USAAbsractThis paper examines the state-of-theartof arctic offshore structure designfrom a designer and builder's viewpoint.Arctic platform configuration and detaileddesign are determined by many, and oftenconflicting, design considerations. Theseinclude the operating requirements, theenvironment-structure interaction and theconstraints due to available technologyand resources. The paper examines the influencesof the three categories of designrequirements on platform design. It alsoidentifies design configurations suitablefor various water depths in the Arctic.IntroductionArctic offshore platform configurationand detailed design is influenced bynumerous parameters. Predominant amongthese are functional requirements and constructionconsiderations. This paper examinesthe influences of various designrequirements for oil and gas explorationand production platforms. Only fabricatedstructures suitable for application in 20mThis is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987. © The Geophysical Institute,University of Alaska, 1987.to 100m water depth in the Beaufort Seaare considered.Require­General Design Consideration andmentsBroadly speaking, the design requirementsand considerations for arctic platformsmay be divided into threecategories; the operating requirements,the environment-structure interaction andthe constraints due to the available technologyand resources. Operating requirementsare comprised of such parameters asthe topside facilities definition, weightand spatial requirements, oil and gas dispositionmode, personnel safety and evacuationconsideration, and the platformresupply and servicing philosophy. Interactionwith ice during the constructionphase and subsequently on site togetherwith oceanographic, meteorological andgeotechnical considerations may be includedin the environment-structure interaction.Finally, the available fabricationfacilities, the technical feasibilityof marine operations necessary during thefabrication, transportation and installationoperations and the availability ofnecessary skills and materials constiutethe constraints introduced by eechnology.In Figure 1 is illustrated conceptuallythe design development process for345

dn arctic platform. While the presence ofsea ice in various forms is a major influenceon platform design, other considerationsidentified above cause significantmodifications to the resultingconcept and details. Generally speaking,an "ideal" shape of a platform is associatedwith each particular design consideration.For example, under pseudostaticice movement conditions, e.g., inthe fast ice zone, from the viewpoint ofglobal loads, the ideal shape of thestructure may consist of a circular cone,provided that adfreeze can be eliminatedfrom consideration and adequate ice clearingcan be achieved. The circular cone,however, may not fare too well with regardto fabrication and installation considerations.Practical shapes of platforms involvecompromises to ideal shapes to accommodatethe conflicting requirements ofvarious design considerations. Theseshapes do not perform in an ideal fashionwith regard to anyone aspect of thedesign requirements, but they are acceptablewith regard to every considerationand they perform best overall with regardto platform costs. This may be illustratedby considering the shape of astructure, vis-a-vis its accessibility andease of operation. These considerations,especially during the emergency evacuationoperations, require that the vicinity ofthe platform be free of undesirable iceaccumulations. Accordingly, adodecahedron in the horizontal plane maybe an ideal shape to facilitate the clearingof ice and reduce the extent of rubbleat the leading edge of the structure.However, if the increased rubble extent infront of a beveled square (in the horizontalplane) structure is acceptable fromthe operating point of view, it may berated superior overall.Arctic design is not a straight forwardextension of prior offshore experiences.The remoteness and rela tiveinaccessibility of the arctic sites duringmost of the year result in long periodsbetween resupply and long-term life supportrequirements. The cold temperaturesand the large temperature range placestringent requirements on topsidefacilities design and general materialperformance. The sensitivity of thearctic environment to extraneous eventsputs severe restrictions on permissibleOPERATING REQUIREMENTS• TOPSIDES FACILITIESr-- • HYDROCARBON DISPOBITION• SAFETY / EVACUATION• SUPPLY/ SERVIC)NG• INSPECTION / MAINTENANCE / REPAIRENVIRONMENT-STRUCTUREINTERACTIONAVAILABLETECHNOLOGY& RESOURCES• ICE fORCES • STATE-Of-ART DESIGNCANDIDATE DESIGNMETHODS• ICE PILE-UP/CLEARINGCONFIGURA nONS• WAVE RUN-UP• MATERIALS• fAB. FACILITIES• SCOUR EFFECTSITECHNIQUES• CONST. EQUIPMENTITECHNIQUESWORKABLEPLATFORM DESIGNFIGURE 1DESIGN DEVELOPMENT PROCESS FOR AN ARCTIC OFFSHORE PLATFORM346

operations. These and other factorsrequire a re-evaluation of the platformdesign and operation philosophies.In the following, the three categoriesof design requirements, as identifiedabove, are examined. Following this, asummary of design requirements, design objectivesand an illustration of platformconcepts for a range of water depths ispresented. The authors' conclusions anddiscussion of the current platform designpractice complete the paper.Operating RequirementsThe predominant operating requirementfor arctic platforms is represented by thetopside facilities 2rocess definition,weight and spatial requirements. The topsidedeck directly influences the platformstability and buoyancy requirements, andby defining one extremity of the platformstructure, determines its size. In principle,the process facilities in thearctic are no different than those in moretemperate regions. Complexi ties in thetopside design are, however, introduceddue to factors such as cold temperatures.No multi-platform complexes are expectedin the arctic. All drilling, production,enhanced production, storage, utilitiesand life support systems must, therefore,be located on a single structure. Someconsiderations that influence facilitieslayout and design are:o Arctic facilities must be completelyenclosed, with insulations to resist-S7°e.o All vents,doors mustbuild-up.drains, pipes,be protectedvalves andfrom iceo Arct ic process units are essentiallythe same as in other regions.However, the large temperature rangeresults in design changes in the heatingand cooling systems and operatingequipment such as pumps and compressors.In general, the packaged unitscommercially available for crude oildehydration, gas treatment, gasdehydration, gas compression and gasrefrigeration can be adapted to arcticconditions. Gas dehydration andrefrigeration requirements in theArctic may be more stringent. Dewpoint suppression for gas to -S7 0 C maybe required. Other important considerationsthat contribute to arcticprocess design include avoidance offreeze-up during shutdown. For example,the injection water used forwater flood may require preheating,continuity of flow at all times andthe ability to displace water with anon-freezing fluid may be necessary.o The design philosophy concerningutility systems perhaps varies themost in comparison to applications intemperate regions. These differencesare primarily caused by the wide temperaturerange and the requirements ofreliability and redundancy. Thedesign of the support systems withseveral levels of back-up illustratesthe complexity of utility systemsdesign. The building heat, for example,may be provided by either wasteheat from turbine exhaust or a processheat recovery system. The finalstandby for building heat will bedirect-fired heaters. The dependability and reliability of direc t­fired heaters will be backed up by amulti-fuel system, designed to utilizeall stored fuels, including refrigerantsif required.o Large intervals between resupplies areexpected to result in large warehousingand storage of consumables.Design studies have shown that thespace required for storage of consumableson an arctic drilling andproduction platform is approximatelyhalf of the total deck space. Somefactors to consider in warehousing arethat selected chemicals should haveadequate shelf life and freeze-thawrecovery.o Waste disposal methods for the Arcticmust not adversely affect the existingecosystems. Combustible solid wastesmay be incinerated. Liquid hydrocarbonwastes could be burned as fuelsupplements. Special provisions may,however, be made for storage and disposalof hazardous wastes.o Safety systems on arctic platformsgain special importance due to thefact that facilities are totally347

enclosed. Fire and safety hazards maybe caused by accumulation of hydrocarbonvapors, limited access for manualfirefighting, increased congestion ofequipment and limited personnel escaperoutes. Safety considerations may,therefore, result in special requirementsfor equipment segregation, fireand gas detection and fighting systems,fire and explosion proofing,pressurization and ventilation etc.Factors such as those discussed aboveresul t in complexi ty, and size and cos tescalation of arctic facilities. To meetthe constraints of economics, relativelylarge production rates are anticipated toconsti tute the floor of economic feasibilityin the Arctic. Accordingly, theauthors have examined requirements for50,000 and 100,000 barrels per day (b/d)oil production rates. For a 50,000 b/ddrilling and production facility, containinga two rig drilling operation, the sumsof individual equipment package (footprint) areas and weights are approximately7500 square meters and 24,000 metric tons(including deck structure). The correspondingfigures for a 100,000 b/dfacility are anticipated to be 8500 squaremeters and 30,000 metric tons. The totaldeck area for a particular platform, includingspace required for storage of consumablesis anticipated to be 3 to 4 timesthe package areas listed above. Thus, thetotal area of a 100,000 b/d capacity topsidedeck is anticipated to be in therange of 30,000 to 35,000 square meters,and with first year supplies on-board, thedeck weight is anticipated to be approximately50,000 metric tons.It should be emphasized that an importantconsideration in the layout of arcticfacilities will be vertical integration ofprocess equipment using, for example, 4 to5 deck levels. This will reduce the arealsize of the deck, thus possibly reducingthe overall size of.the structure.Environment-Structure InteractionA unique aspect of the arctic environmentis the presence of sea ice in variousforms. The influence of ice on the platformshape and size and, hence, its constructionfeasibility and cost, ismanifested in two ways. The first con-cerns the modification of the sea iceregime in the platform vicinity, i.e., theagglomeration, pile-up and rubble formation,or clearing of ice. From a structural/founda tion strength point of view,the effects of ice accumulation may bedesirable or detrimental, depending onwhether advantage is taken of a groundedrubble pile to expand the effective foundationsize of a platform or not. From anoperational point of view, i.e., consideringgeneral accessibility, safety evacuationand supply/servicing requirements,ice accumulation is not desirable. Sinceice accumulation is caused by the presenceof a platform, platform configuration is aprimary factor in determining the type andextent of the accumulation. Thus, beforeconfiguring an arctic structure, an engineeringevaluation should be made todetermine an acceptable ice accumulationfrom an operating viewpoint.The other significant influence of seaice on the platform design is through theforces of interaction induced on thestructure. Again, the presence of theplatform as an obstruction to the naturalice movement causes ice forces and theplatform configuration determines theirmagnitude and distribution. The followingconsiderations rela ted to ice loads influencethe shape and overall dimensionsof the platform and the layout and designof its internal framing.o Ice loading is influenced by threefactors associated with the structuraldesign: (a) each platform shape, bycausing specific modifications to icemovement patterns, coupled with theselection of specific ice failuremechanisms, influences the ice loadingmagnitude and distribution; (b) themanner of transmission of ice forcesto the foundation influences the sizeand design of the platform by determiningthe stress flow patterns in allmembers of the structure; and (c) theplatform structure must dissipatehighly-concentrated local loads withsomewhat distributed foundation reaction.The ability to do this effectively,especially for shallow to midwater depth range, places significantrestrictions on admissible platformshapes for sloping structures. Whilea sloping ice wall possibly reducesglobal ice loads and enhances founda-348

tion performance by reducing load eccentricityat the seabed level, thenormally specified local loads (Bruenet al., 1981) for these platformslead to large diaphragm sizes andframing difficu1 ties. The optimumshape of a platform must, therefore,be established by considering thebeneficial and undesirable effects ofsloping walls.A state-of-the-art discussion on iceforces is presented in API (1987),Carstens (1980), Sanderson (1986).Generally, for a particular structure anda postulated ice-structure interactionscheme, global ice forces can be estimatedby established techniques. The currentunderstanding of the distribution of theseforces, i.e., local loads, while based onsome laboratory experience and fieldstudies, is somewhat arbitrary and possiblynot applicable to all different iceinteraction schemes. For safe design, adesigner must, however, assume a distributionthat is both compatible with the postulatedice interaction and failuremechanism and the applicable principles ofmechanics and material behavior. For example,in the authors' experience,specification of forces on sloping structuresbased on the current understandingof the subject can lead to situationswhere the entire global load can be appliedas local load on an area of the orderof 2 percent of the total area of theice interaction zone. In si tuations suchas these, the authors believe, a morerealistic specification of the local forcewill be to assume a reasonable distribution,for example an appropriately truncatedlog-normal distribution, of the estimatedglobal load on the applicable zoneof ice contact and failure.Two other aspects of environmentstructureinteraction that have major influenceson platform design are the waverun-up and geotechnical considerations.Wave run-up is a function of the size andshape of the structure and roughness ofthe structural surface, vis-a-vis wavecharacteristics. Published guidance forestimating wave run-up on break-waters isavailable in the Shore Protection Manual(1977). The authors' limited experiencewith model tests indicates that thisguidance provides reasonable estimates ofwave run-up for fabricated structures. Byignoring the three-dimensional aspects ofstructures, the manual-based proceduresperhaps over-estimate the run-up.However, the lower friction of the structuresurface compared to that consideredin the manual, and a general inability toproperly model friction forces at themodel scale, possibly cause an underestimationof the run-up when determinedwith the procedures outlined in themanual. These two effects possibly canceleach other.Soil strength and foundation characteristicsinfluence the size and spacingof the foundation skirt elements. To efficientlytransmit ice forces to the foundations,skirt design then influences internalframing of the structure and thusthe structural configuration.Constraints Dueand Resourcesto Available TechnologyThe state-of-the-art of constructiontechnology and the availability of requisitematerials and skills place significantlimitations on platform designconfigurations for the Arctic. Some constructionrequirements are obvious andapply uniformly to all structures deployeda t any loca tion. For example, thestrength of the structural componentsshould be adequate to resist applied loadsat every stage of construction. Theseverity and inaccessibility of the arcticenVironment, however~ lead to some specialconsiderations for offshore arctic platformdesign.o All platforms must proceed to thearctic location completed and precommissioned,with all equipment requiredover the platform life installed andpossibly first year supplies on board.This requirement has important consequencesfor the platform design andconstruction. It influences the modeof topside fabrication and integrationwi th the substructure and subsequentcompletion and precommissioning. Italso determines the transportationdraft and floatation stability and establisheslimits for platform motioncharacteristics, as they influence thestructural integrity of the installedequipment, piping and other systems.Draft limitations are particularly349

severe around Pt. Barrow, Alaska.Table 1 indicates the offshore distancefrom the coast to some specificisobaths at Pt. Barrow. General iceconditions in this region indicatethat icebreaker support will be necessaryfor all major tows. The authorsbelieve that preferred routes past Pt.Barrow for most major platform towswill be along the 30m isobath. Thus,a draft limitation of 20 to 30m willimpose restrictions on permissibleplatform configuration for deepwaters.Table 1Water Depths in Vicinity ofPt. BarrowWater DeEthDistance from Pt. Barrow10 m 15 km20 m 23 km30 m 34 km40 m 47 km50 m 53 kmo The fabrication of arctic platforms isexpected to require graving-dock-typefabrication facilities. The size ofthese facilities may be limited by thepractical considerations of providinggates and keeping the facilitiesdrained of seeping sea water. Thishas important consequences for pIa t­forms with large base sizes. Basesections of such platforms must befabricated in sections, followed by anassembly of the sections in a floatingmode into a monolithic structure.Subsequent fabrication of these structuresmust be carried out in floatingmode at inshore facili ties. Figure 2illustrates this concept.o The feasibility of marine operationsthroughout the construction phase,e.g., assembly of base section unitsinto a monolithic structure or themating of an integrated deck with thesubstructure may influence structuralconfiguration and platform design.o Long duration tows, for example fromthe Orient, will require adequate1. FABRICATION OF BASE SECTIONSIN GRAVING DOCKS2. ASSEMBLY OF BASE SECTIONS INTOMONOLITHIC BASE3. CONTINUATION OF PLATFORM FABRICATIONIN FLOATING ;40DE4. COMPLETION OF STRUCTURE FABRICATIONFIGURE 2FABRICATION SEQUENCE OF A LARGE BASE ARCTIC PLATFORM350

strength and motion and stabilitycharacteristics to weather anticipatedstorms in a floating or submerged modeof the platform.o The platform structure must be inherentlystable for all operations inthe Arctic. The ability to installthe structure with water ballast alonemay also be an essential feature ofdesign.o The construction operations in theArctic must be designed to be simpleand, to the extent possible, failsafe.Thus, for example, mechanical installationaids, if required, should beinstalled prior to the platform entryto the Arctic.o Finally, the ease and feasibility offabrication of thick concrete wallsand highly-stiffened steel sections,or the abili ty to weld large,pre f a b ric a t ed s t e el sec t ions inenclosed spaces in the floating modeinto a monolithic unit may determinecertain platform configurations aspreferred over others.Summary of Design Objectives, DesignRequirements and Some Arctic PlatformConceptsThe objective of arctic platformdesign is to engineer and build structuresthat are functionally adequate for allrequirements, including those originatingfrom operating considerations, interactionwith the environment and construction withthe state-of-the-art technology, with aminimum of initial capital and risk costsexpenditures.Typical design requirements for arcticoffshore platforms may be summarized as:o Minimum production rates of 50,000 bIdto upwards of 200,000 bId.o Simultaneous drilling (2 rig) andproduction operations.o Self-sufficiency and redundancy ofprocess facilities and increasedreliability of life support systems.o Large storage and warehousing require-ments with possibly yearly re-supply.o Accommodation for 200 to 350 people.o Stringent requirements to withstandcold temperatures, fire and explosionproofing and personnel and environmentalsafety.o Topside area of 30,000 to 35,000square meters and a tow-out weight of50,000 metric tons.o Satisfactory interaction with environmentwi th regard to ice accumula tionand strength and stability requirementsagainst environmental forces.o Structural concepts to permit design,analysis, fabrication, transportation,installation and continued inspection,repair and maintenance activities withavailable technology, or reasonableextensions of it with normal developmentalprocess.No one design concept will provide theoptimum solution for different sites anddiverse design requirements of the arcticsi tes. Rather, for each particular application,trade-off studies will be madeto accommodate the often conflictingrequirements of the various design considerations,thus arriving at optimumsolutions that, with all risks considered,indicate maximum pay back.Figure 3 presents some general observationson arctic platform configurationsbased on several studies performed by theauthors. For shallow waters, where mostof the ice is first-year and the ice featuresize is limited by the water depth,both vertical and sloping structures areexpected to find application. However,all things considered, vertical-sidedstructures are expected to provide a morecost-effective solution to platformdesign. For the mid-range of waterdepths, for example 50m, sloping wallstructures appear to be more attractivewith regard to overall performance andcosts. For deeper waters, sloping wallsin the ice interaction zone result inlarge base sizes and installation difficulties.For these applicationsvertical-sided structures provide optimumsolutions when considered against different design requirements.351

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DESIGN AND OPERATIONAL CRITERIA FOR SYSTEMSSUBJECT TO ICE ENVIRONMENTAL CONDITIONSM. NessimT. NasseriDet Norske Veritas (Canada) Ltd., Calgary, Alberta, CANADAAbstractProbabilistic methods offer arational approach to the choice ofdesign and operational criteria whichsatisfy safety and economic requirements.The overall approach begins bycharacter izing the environment with anappropriate set of parameters anddeveloping a corresponding set ofprobabilistic descriptors. These arethen used in system response models tocompute basic probability distributionsof the demand on the system due to thepresence of various ice conditions. Toobtain a probabilistic assessment ofthe maximum demand (e.g. load) during acertain period of time extremalanalysis is utilized. By selecting theappro-pr iate parameters for the extremalanalysis, one can develop usefultools for decision-making regardingdesign and operational criteria whichresult in a system capable of meetingthe environmental demands. For example,lifetime extremal analyses lead todesign tools, whereas seasonal extremalanalyses can be used to selectoperational criteria. The methodologyis illustrated by an example dealingThis is a reviewed and edited version of a paper submittedto the Ninth International Conference on Port andOcean Engineering Under Arctic Conditions, Fairbanks,Alaska, USA, August 17-22, 1987. © The GeophysicalInstitute, University of Alaska, 1987.with ice loads on a fixed offshorestructure. The example utilizes a comprehensivecomputer package developedfor probabilistic assessment of iceloads on arctic structures. Probabilitydistributions of the annual,seasonal and monthly extremal loads aregiven and their relevance to theselection of design and operationalcriteria is illustrated.IntroductionArctic energy and transportationprojects depend on safe and economicengineering and operation of complexman-machine systems. In order to meetthe technoeconomic challenges of thesecostly and technology-cum-informationintensiveprojects, an integrated engineeringprocess needs to be developedand implemented. The objective of sucha process is to ensure that a projectachieves functionality, safety andeconomy, i.e. "quality·, during itsentire lifetime. Engineer ing cr iter iashould therefore be formulated so thatthey meet the requirements of integratedengineering (Nasseri 1985).It is well known that allengineering problems can be defined, ifnot always implemented, as an optimizationprocess. In this process, theoptimum solution is sought by selectingthe problem's variables so that given353

desirable goals are achieved, subjectto specified constraints. For example,minimization of the total project costis clearly a desirable goal, and allthe assumed and actual constants of aproject, together with functional andsafety requirements, constrain theoptimization. It can be demonstratedthat all criteria by which systems areengineered, regardless of what label isattached to the design method, may beregarded as the constraints which willdetermine the optimum solution; theterm criteria is used here in a generalsense to include the hierarchy ofexplicit requirements specified tosatisfy the constraining conditions.The formulation and implementationof engineering criteria will thereforecr i tically influence the optimali ty ofan engineering system. An integratedapproach seeks to identify all theoptions and evaluate them to establishthe most efficient allocat'ion ofresources (human and mater ial) toachieve the objectives. The integraapplyto thetion is envisaged tofollowing (Nasseri 1984):- Subsystems (e.g. platform, soil, topside,monitoring equipment).- Project phases (e.g. conceptual,engineering, operation).- Project activities (e.g. risk management,quality assurance, engineering)•In Figure 1, the weakly-integratedand strongly-integrated projectactivities are shown. Strong integrationof quality assurance/quality control(QA/QC)and risk control measuresenable the full consideration of theimpact of these measures in establishingengineering criteria. For example,acceptable damage for a system isclosely limi ted to QA/QC and risktolerance and it influences designloads. Further-more, the quality ofanalysis and the confidence in theengineering that derives from acomprehensive QA would affect thecriteria considerably.The integrated engineering approachcan be applied to eitherdeterministically or probabilisticallyspecified variables of the system. Theinevi table uncertainties that attend(a) Weekly Integrated Engineering(b) Strongly Integrated EngineeringPig. 1 Integration of ProjectActivitiesoffshore engineering leave us no choicebut to adopt a probabilistic approach.The systems deployed for the recoveryand transportation of hydrocarbonresources in the Arctic offshoreregions are to be engineered subject tothe following conditions:- unique environmental constraints andhazards.- relatively insufficient operatingexperience and field data.It is noted that lack of specificengineering standards that reflect goodengineering practice derived from354

extensive and successful application isa consequence of insufficient experience.The foregoing characteristics ofArctic offshore projects and the desirdabilityof integrated engineeringclearly suggest that a more fundamentalapproach is needed for the developmentand implement-ation of engineeringcriteria. This will require directapplication of optimization methods,risk and reliability analysis, anddecision theory. The eventual aim isto incorporate the results of theseefforts into technical specificationsof future projects. It is through thespecifications that technological advancescan most effectively benefit thesafety and economics of the projects.The scope of this paper is confinedto the presentation of a methodby which design and operational criteriaare developed to allow for theimpact of ice hazards on structuresused for the development of Arcticoffshore resources. However, themethod has been developed to fit thegeneral method of developing optimumcriteria for integrated engineering.Therefore, safety criteria are firstoutlined to provide the context for theselection of design ice loads.Design for SafetyArctic field developments clearlyrequire complex and expensive systems.The failure of these systems may havecatas-trophic consequences. The Arcticenvironment, characterized by longperiods of darkness, extreme cold,isolation, lack of nearby support,navigation and communication problems,can significantly affect the performanceand reliability of men, equipmentand their interaction.In order to produce systems thatprovide an acceptable protection topeople, environment and the materialresources, safety measures are designedfor the systems and may be categor izedbroadly asHere, the term accident is used todenote all incidents that cause"undesirable" consequences; so, forexample, the range of incidents includelocal damage as well as global collapse.It is relevant to recall thatsafety measures are to protect alsocapital resources and, therefore, accidentssuch as structural damage areincluded though they may have noadverse effect on people or theenvironment. The consequence controlmeasures are to ensure that undesirableconsequences would be unlikely tobecome "unacceptable"; what is unacceptablehas to be quantified to be auseful safety criterion.Safety measures are built intosystems through engineering, qualityassurance and risk control capabilities.These affect risk by reducingthe probability of accidents and theseverity of their consequences. InFigure 2, the categorization of hazardsor sources of failure and of thecorresponding safety measures is shown.The basis of categorization in Figure 2is as follows:Failures due to gross errors that arecaused by an inadequate QA/QC system,or the improper implementation of thesystem, thereby allowing gross errorsto lead to failure.- Failures due to accidents that arecaused by risk control measures Le.protection, monitoring, redundancy,contingency plan.- Failures that are caused by deficientengineering standards and specifications.It is not of course always easy todecide how to assign a failure to oneof the categories. Notwithstandingdebatable cases and some interdependence,it is assumed that these threeevent categories are mutually exclusiveand the total probabili ty of failure ofa given system Pf is computed as:- accident prevention measures- consequence control measures.where:Pfg = Pf/g •Pf/a Pa;= Pf/e • Pe;355

HAZARDSFAILUREPROBABILITYSAFETYMEASURES""GROSSERRORSPfgQAJOC ~PRISKACCIDENTS""fa CONTROl~EXCEEDANCE.. OF DESIGNCONDITIONSPIeENGINEERING ~Fig. 2Determinants of Total Failure ProbabilityP g , P a and P e are probabilities ofoccurrence of gross errors, accidents,and deficient standardsrespectively;P~/g probabili ty of failure,glven gross errors;Pf/a probability of failure,given accidents;Pf/e probabili ty of failure,given deficient standards andspecifications.The safety criteria may beexpressed asP f- P f~ 0where Pf is the acceptable totalprobability. Failure statistics onoffshore systems clearly show thatfailures due to gross errors andaccidents are considerably more frequentand serious than those due toinadequate engineer ing. Allocation ofresources to safety-related researchand development should reflect thisfact.The specifica tion of Pf is essentialfor a working safety criterion.It is clearly dependent on the severityof the consequences of failure. Instructural design, acceptable annualprobabilities of 10- 2 and 10- 4 havebeen suggested for moderate damage (orunservice-ability) and collapse respectively.Several technical, socialand psychological factors are thoughtto influence the selection of thisvalue. Despite the sensitivity of thesubject, as human health and life isinvolved, there is no reason why atleast a range of values cannot beestablished that reflect experience andpractice in other fields of humanactivity in which there are risks andbene fits. For example, the Norweg ianPetroleum Directorate (1987) stipulatesa target value of 10- 4 for total annualprobability of failures due tospecified accidents; it should be notedthat accidents are the only category offailures considered in this stipulation.The safety criterion represented bythe inequality may be satisfied byspecifying target values either forPf9' Pfa and Pfe' or for Pf. Thelatter is suitable for integratedeng ineering and allows the flexibilityfor more efficient allocation ofresources to implement the safety356

criterion. These target failureprobabili ties can be achieved ei ther byadjusting the structural resistanceprobability distributions throughdesign or by influencing the hazarddistributions through operationalcriteria. In Figure 3, the safetycriterion is shown in an integratedhierarchy of criteria which would beused to select materials, designcomponents and subsystems and finallyconfigure the ensemble so that thesafety criteria are met most costeffectively.Selection of Design and OperationalCriteria Against IceThe selection of design andoperational criteria for an offshorestructure against ice hazards hingesupon the development of probabilisticdescriptions of load maxima for variousexposure times. Such loads are determinedby the following factors:- the severity of loading, which isdetermined by such parameters as icethickness, veloci ty and mechanicalproperties, and;- the extent of exposure of thestructure to ice loads as determinedby the frequency of occurrence ofdifferent ice interaction events andthe length of time during which acertain ice feature is present.The essence of the methodpresented here is to characterize theice environment by defining distinctiveloading scenar ios and assessing thelevel of exposure of a structure toeach scenar io as well as the sever i tyof the resulting interaction. Thesefac-tors are then combined into anoverall statistical approach to producethe required probabilistic designtools.Fig. 3 Safety Criteria Hierarchy forIntegratred EngineeringThe remainder of this paper dealswith the development of design criteriato ensure acceptable protection againstice hazards. With reference to Figure2, the application of these cri teria isprincipally in the engineering and riskcontrol categories.The method described combines probabilisticmodelling of ice masses andice-structure interaction to developthe information needed to makedecisions for design and operation ofArctic offshore systems. The resultsof the analysis are essential for thedevelopment of rational engineeringcriteria and specifications.Characterization of the environmentBased on the variation in iceconditions throughout the year in thearctic offshore, the following distinctiveconditions can be identified:- Open water conditions: occurring inthe summer when floating multiyearice is present but the first-year icecover has melted.- Closed water conditions: characterizedby a full first-year ice coverentrapping some multiyear ice in thewinter season.- Break-up conditions: representingtransition between (b) and (c) above,in the spring or early summer.Boundary dates between differentseasons, and consequently the length oftime during which each of the aboveconditions prevail, are uncertain and357

are thereforecally.treatedprobabilisti-Considering the types of icefeatures present in arctic regions (theBeaufort Sea is considered here as arepresentative area), the main loadingscenarios considered are as follows:- Areal feature interactions, includingice islands and multiyear floes withembedded ridges.First-year level ice movement.- First-year ridge interactions.These scenarios are illustrated inFig. 4. Their presence during differentseasons and ice covers is given inTable 1.Statistical approachesTwo basic typesprocesses can be usedloads resulting fromdiscussed in Sec. 3.1.of statisticalto model icethe scenariosFirst, thereare discrete processes, for whichloading occurs at small localized timeintervals, with no loading in between.These include multiyear floe, iceisland and first-year ridge scenarios(Fig. Sa). Second, there are continuousprocesses, where the load is constantlyapplied to the structure butfluctuates with time, such as the caseof constant movement of first-year icein the active ice zone during theclosed water season (Fig. Sb). It isnoted that the continuous process inFig. Sb has an increasing mean whichrepresents the growth of ice thicknessthrough the winter (Le. non-stationaryprocess) • For an incomplete ice coverin the break-up season, the continuousprocess in Fig. Sb would be interruptedby periods of open water. According tothe load scenarios possible during anyperiod of time (see Table 1), the iceload is modelled by a combination ofthe above-mentioned statistical processes.For design purposes it is requiredto estimate the maximum load that isTable 1Ice Loading Scenarios During Different Ice ConditionsLOAD SCENARIOICECONDITIONS(SEASON)AREAL FEATURE(MY RIDGE - ORICE ISLAND)LEVEL ICEFY RIDGELandfast Active Landfast ActiveZone ZOne ZOne ZOneLandfast ActiveZOne ZOneOpen WaterClosed Water (I)xxXXXXBreak-upXXXXXX(1) The small magnitude of ice movement in the landfast zone duringthe winter season makes multiyear floe and first-year ridgeinteractions unlikely.358

STlUCTUIEDIRECTIONOf /OIOTiON1_ OPEN WATER COlLISIONr-----------,i ICE COVel ii/"~V:ES~ ~-i- / i--I~-II 0 ~:ii STluCTUIE ~-!i/ !IL------------i2_ FUll ICE COVElr--'----------lI_MULTI-YEAR flOE INFIRST-YEAR ICE._ 'ARTIAL ICE COVERFig. 4Scheaatic Representation of Main Ice Load Scenariosexpected to occur during a specifiedperiod of time. For a discrete processthis amounts to determining the probabilitydistribution of Y wherewhere Xi is the load during theinteraction event i and N is the numberof events. The latter is random and ismodelled by a Poisson process:e-xt ().t)nn!where t is the time period consideredand X is the rate of interaction. Therate X can be calculated on the basisof the ice concentration and speed ofmovement, as well as the dimensions ofboth the iceTo calcula tetion of theFXi (Xi) , onefeature and the structure.the probabili ty distribuloaddur ing one collisionneeds an ice-structureinteraction model in the form:X = X(Pl' P2' •••••where Pl' P2 represent environmentalparameters affecting the load X(e.g. ice velocity, size and mechanicalproperties) and design parametersdescr ibing the structural geometry andcharacteristics. Knowing the probabilitydistributions of Pl' P2 ••• andthe form of the function X (Pl' P2, ••• ),FXi (Xi) can be calculated. Fordetailed interaction models, a mathematicalsolution cannot be found and a359

LOADLOADI, ••.....TIMETIME(.) DISCRETE (b) CONT .. UOUSFig. 5 Discrete and Continuous Statistical ProcessesMonte Carlo simulation procedure isunavoidable.Assuming that the cumulativedensity functions (CDF), FXi(Xi) areidentical and that Xi's are independent,it can be shown that Fy(Y), for aPoisson-distributed N is given by thesimple formula (Maes 1985):Fy(Y) = exp {- At (1 - Fx(Y) )}A continuous process can be handledin the same manner as a discreteprocess, if it is character ized byoccasional high peaks. This was foundto be thecase for level landfast icemovement where the large movements arerelated to storms (Spedding 1975). Forother continuous process scenarios, themaximum load was modelled by a doubleexponential distribution (Gumble 1958).This is suitable for modelling theextreme value of a stationary processwith a large number of fluctuationsduring the time period considered,regardless of the distribution type ofeach individual peak. This is givenby:fy(Y) = exp {- exp (~)}where a and b are distribution parameters.This model was used for levelice during break-up, where the icethickness does not change significantlyand the process is stationary. For theclosed water season, where the processis not sta tionary, the season isdiscretized into ,smaller time periodssuch that the ice thickness can bereasonably treated as constant withineach period.If more than one loading scenariois considered, one is then interestedin the largest of the load maxima dueto each scenario. The (CDF) of themaximum load resulting from anycombination of load scenarios is givenby:Fy(Y)j1, 2, •.. mwhere, m is the number of scenar ios.This assumes that the maximum loadsresulting from different scenarios areindependent random quantities.Ice-Structure interactionThe function X = X (PI' P2' •••• )introduced in Sec. 3.2 represents anice-structure interaction model whichshould be selected on the basis of theice feature type, the speed of theinteraction and the structural geometry.A detailed description of allthe models used is given in Nessim etal. (1987). In this section a briefoutline of these methods is given, withspecial emphasis on cases whereadditional development was undertaken:Areal ice feature collisions: Aglobal energy dissipation model wasused in a time-stepping iterative360

procedure. This model takes intoaccount the limits imposed on theforce exerted on the structure byglobal considerations such as loss ofcontact and clearing of the featuredue to an initial eccentricity of thecollision. This was applied to bothver tical and conical structures. Forvertical structures, the local contactforce was calculated on thebasis of a constant average contactpressure. However, the contactpressure can easily be made afunction of the contact area. Forconical structures, the local forcewas estimated on the basis of aseries of crushing and ride-up eventscontrolled by the out-of-planebending of the ice feature. Themodel was also applied to wintercollisions by consider ing the effectof the forces exerted by the icesheet on the feature. This accountsfor the possibility of ice featureclearing even in closed watercollisions. Multiyear ridges aretreated within this overall model asthickened parts of the floe. Localforces are calculated on the basis ofcrushing against vertical structuresand crushing or out-of-plane bendingfor conical structures. This approachfor multiyear models was basedon a probabilistic assessment ofridge loads which indicated that inplaneridge-bending against a verticalstructure is an unlikelyfailure mode since the increasedcontact loads tend to stop or clearthe floe before a bending failureoccurs. Sample force his tor ies ofthe results of this model forinteractions between a multiyear floewith a ridge and a vertical structure,and a multiyear floe on a downwardbreaking cone, are shown in Fig.6 (a) and (b). In the statisticalanalysis the peak force is used indeveloping the design load distribution.- For level ice movement scenarios acreep solution is used for low strainrates, while an average crushingpressure is used for higher strainrates. The reference stress methodwas used to produce a general creepsolution using the results of twofinite element solutions (Nessim etal. 1966).- For first year ridges, a Coulomb-Mohrmaterial model is used (Prodanovic1961) . The ridge fails either bydeveloping a velocity field to clearthe rubble around the structure or byshearing along two vertical planesparallel to the direction of icemotion. The interaction mode isgoverned by the lower of the twoforces.Results and applicationThe overall methodology isillustrated in Fig. 7 which alsorepresents an abbreviated flow chart ofthe computer package developed on thebasis of the discussion in Sec. 3.1 to3. 3. The package allows the user todefine all input parameters such asseason boundary dates, ice velocities,geometric properties and sizes, as wellas mechanical properties. Each parametercan be defined probabilisticallyusing standard mathematical distributionsor by a data histogram. Thismakes the model applicable to differentsites and allows improvement of theinput parameters as more data iscollected.In order to make it applicable tooperational as well as design decisions,the model provides the option ofdealing with any individual scenario orcombination of scenarios. Possibleapplications are illustrated by anexample of a vertical-sided circularstructure 60 m in diameter in the shearzone of the Canadian Beaufort Sea. Theresults can be used as follows:- An overall extremal load distributioncan be used in selecting a designload according to the criteriadiscussed in Sec. 2. For thisexample, the extreme load correspondingto 100-year return period isapproximately 320 MN.- In Figure 8, the cumulative densityfunctions and return per iod relationshipsof the monthly extremal loadsand the overall maximum load resultingfrom first year ice loading. Themonthly extremals reflect higherloads as the winter progresses due tothe increase of ice thickness withtime. Such a result can be used inmaking decisions regarding the month361

RidQe I Structure Interaction300i•~250200Peak Force' 275.33- PrOQlom End150100--- Ridge EncounterSummer SeasonStructure Shope - Circular- 100m diom.t-$-.Ridge FaceSpeed - 0.35 m/s°0~----~20~-----4~0~----6~0~----8~0~----~100~Tirne(wc}a) Vertical Sided Structure (Floe with Ridge)20lBend'Falur~IIl BendinQ FailuretFloe Diameter' 2 kmEccentriCity • 400minitial Floe Velocity a 0.5m/sInteraction Time • 550 secMaximum Horizontal Force a15.5MNMo>.imum Total Force· 22.9MN10Total ForceHorizontal Force0~--------~100~------~200~------~300~~------~4~00~------~500~----(b) Downward Breaking ConeFig.6 Sample Results of Multiyear Floe Interaction Model362

ICE lOAD~SCENARIO Ii) "tPR06ASllISTIC MODElS OFENVIRONMENT aASED ON ICE DATALoad ~t Procea Ice Feature Size, EnvironmentolCor.idering Un· Nor~OOY. Motion Driving Forces~rtointy in OIorocteristics &Sc.norio Ourotion MechoNcaI· "';_,:..I~~lli I~I TIME~•• ICE THlCI ... f" ~ •• a Mel I'C»CHtr.+- ICE - STRUCTURE ~ STRUCTURALMECHANICAl.aEHAVIOUROf ICEINTERACTIONMODELSHAPE ANDpiARACTERISTlCS~ ..PRQ6A8ILlSTlC ~SCRIPTIONOf CE LCW> FOR SCENARIO Ii)Pl~~ KE LOAD •tDESIGN lOAD REPEAT FOR i &',2 ....CtOCE CRITERIONfOR SCENARIO Ii)tFINAl ~SlGN CRITERIAfOR ALl. SCENARIOSFig. 7 OVerall Ice Load Calculation Approach363

....>-t:: .JK•: ..oa:L••...~~.. ..~~~ .au.1.,--- MONTHlY EXTREME"aEASON EXTREMESFORCE...(a) Cumulative Density Function-aMiDa:c...~Q0iiiIIILZa:~...IIIa:'M.0ao10,0---- MONTHLY EXTREME8•.------ aEASON EXTREMES, .. -r---------r--------~--------~--------~--------_,LOAD(b) Return Period RelationshipsFig_ 8 Illustration of the Probabilistic Description of the Winter FirstYear Ice Loads364

to month operations during thewinter. For example, by reading theloads corresponding to a certainreturn period in different monthsfrom the figure, decisions can bemade regarding how long into thewinter a certain operation can becontinued.v)seasonal extremals orextremals can be usedoperational decisions.scenarioto makeThe probabilistic load descriptionscan be used with consequenceanalysis to develop and implementstructural design criteria.-In Figure 9, the extremal loadresulting from multiyear interactionsin all seasons is shown. This resultcan be used to make decisions specificto the protection of the structureagainst multiyear floes, e.g. icemanagement or operation interruptionfor larger floes. The load distributiondue to multiyear floes can thenbe modified accordingly beforecombining it wi th loads due to otherscenarios.Summary and Conclusionsi) An integrated engineering approachis advocated as the most suitabletool for the development of designand operation criteria whichachieve optimal structural systerns.Such fundamen tal approachesare needed particularly for arcticoffshore systems due to the highlyuncertain nature of environmentalhazards, the limited operationalexper ience and the lack of appropriateengineering standards.ii) Probabilistic design approachesare valuable tools for the implementationof integrated optimizationtechniques. The probabilisticdescription of environmentalloads is an essential inputto these models.iii) The development of probabiisticdefinitions of global ice loads onfixed offshore structures has beendescr ibed. Statistical, extremaland ice-structure interactionmodels used in the analysis arediscussed.iv)Probabilistic loads can be appliedto both design and operationdecision making problems. Overallextremal loads can be used tochoose design loads, whileReferences"Acts, Regulations and Provisions forthe Petroleum Activity", NorwegianPetroleum Directorate, 1987.Gumble, E.J., 1958, "Statistics ofExtremes", Columbia University Press,New York.Nasseri, T. 1985, "Quality Assurance ofOffshore Systems", Canadian OffshoreResources Conference, Halifax, NovaScotia, Paper tlO.Nasseri, T., 1984, "Reliability andDesign of Structures for ArcticOffshore Environments", NEFTGAS 84Conference, Moscow.Nessim, M.A., Cheung, M.S., andJordaan, I.J., 1987, "Ice Action onFixed Offshore Structures - A State-ofthe-ArtReview", Canadian Society ofCivil Engineers Journdl, VoL 14, pp.381-407.Nessim, M.A., Jordaan, I.J. Lantos,S.L., and Cormeau, A., 1986,"Probability-Based Design Criteria forIce Loads on Fixed Structures in theBeaufort Sea", Det norske Veritas(Canada) Ltd. Report 86-CGY-43.Prodanovic, A., 1981, "Upper Bounds ofRidge Pressure on Structures",proceeding·s of Conference on Port andOcean Engineer ing under ArcticConditions, Quebec, Canada, Vol. 3, pp.1288-1302.Maes, M., 1985, "Extremal Analysis ofEnvironmental Loads on EngineeringStructures", Ph.D., Thesis, TheUniversity of Calgary, Alberta, Canada.Spedding, L.G., 1975, "Landfast IceMovement Mackenzie Delta 1974/1975".Arctic Petroleum Operators Association,Report No. 83-1.365

·1..>-!: .7::!IIICIII .10a:Q..1101>~ ..C..J:>:I .1:>U.1.t•FORCE(a) Cumulative Density Function-...Iia: tOOCIII~ ..00it 10IIIIL 10Za::>l- toIIIII:I1I,..LOAD(b) Return Period RelationshipFig. 9Illustration of the Probabilistic Description of Multiyear Floe Loads366

RELIABILITY ASSESSMENT OF A PRESTRESSEDCONCRETE ARCTIC OFFSHORE PLATFORMJaJ N. Birdy*Consulting Engineer, Houston, Texas, USAIrvin B. BoazShell Oil Company, Houston, Texas, USAAbstractThis paper evaluates the reliabilityof individual components in an offshoreconcrete platform designed for theBeaufort Sea. The original design wasbased on deterministic ice loads andconventional partial safety factors. Thework described shows the sensitivity ofthe original design to statisticalvariations in ice loads and pressures.Typical outer shell and base slab panelsof the conically shaped structure wereinvestigated. Monte Carlo simulationprograms were developed for evaluating theflexural and out-of-plane shearreliability. The variations in materialstrength, section geometry and workmanshipwere allowed for. Load effects whichcould not adequately be represented instatistical terms, e.g. hard-spot soilpressures produced during set down, etc.,were investigated parametrically. Thesafety index approach was used to assessreliability and the beta factors producedin this evaluat ion were compared withthose sugges ted for other st ruc tures. Ingeneral, the reliability of the panels inshear was found to be higher than in flex-This is a reviewed and edited version of a paper submittedto the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987. © The Geophysical Institute,University of Alaska, 1987.ure. Modifications necessary to thedesign for achieving a more uniformreliability have been identified.IntroductionThe reliability assessment wasundertaken for the Arctic Cone ExplorationS t ruc ture (ACES) wh ich was des igned inprestressed lightweight concrete (AOGA197, 1984). Since a combination ofconvent ional methods and new approacheswas used in the design, it was decided tocarry out a preliminary reliability-baseddesign check to estimate the level ofsafety inherent in critical components ofthe structure. The change in reliabilityto different loading conditions was alsostudied.This paper summarizes the work ontypical panels of the outer shell and baseslab of the ACES. The effects of globaland local ice loading on the safety inflexure and out-of-plane shear is studied.A method of approach and associatedcomputer programs have been developedwhich may be used to study othercomponents of the ACES or similarstructures.*Now Project Manager, Moffatt andNichol, Engineers, Long Beach,California.367

Method of ApproachGeneralAn artist's impression of thestructure is shown in Figure 1. Theglobal and local forces and momentsproduced in the structure due to iceloading were taken from the originalfinite element analyses (AOGA 197, 1984).available on a number of individuals t r u c t u res, the s tat i s tic a 1 pa r arne t e r scould be readily computed. Since this isnot so, particularly in the case of arcticconcrete structures, we have usedapplicable values from existinginformat ion developed for conventionalbuildings, e.g., concrete and rebarstrength, modulus of elasticity, etc. Thestatistics of geometrical variables suchas section depth, position of rebar, etc.,have been derived from expected tolerancesthat might be achieved in actual practice.The original design was based on nonvaryingdeterministic nominal ice loadsand pressures using appropriate partialload and material factors. Thereliability check performed here usedstatistics of ice loads and pressuredeveloped by Shell Oil Company.A Monte Carlo simulation was thencarried out by generating random numbersfor each statistical parameter. This setof random numbers was used to compute theload effects and section resistances. Thetest/computation bias was also included.The random number generation was carriedout a sufficient number of times (1200 to120,000) to obtain statisticaldistributions of loads and resistances.Figure 1.Artist's impression of ACESThe evaluation of reliabilityrequires a knowledge of the type ofdistribution, the mean value and standarddeviation of each parameter used tocompute the load effect and memberresistance. For instance, the shear loadon a panel depends on the variability ofthe ice pressure and panel dimensions,while the shear strength depends on thevariability of such parameters as concreteand rebar strengths, section depth,position of rebar and membrane compressiongenerated by the global ice loading. If asufficient number of measurements wereFlexure and out-of-plane shearresistances were computed separately, asis common practice. ACI 318 (1983) andACI 357 (1984) methods were used inaddition to experimental work. Theflexural resistances in thecircumferential and radial directions wereeva luated independent ly, as is common inbuilding slabs. Out-of-plane shearresistance was evaluated allowing for thecombined effects of circumferential andradial membrane compressions.The reliability of the section wasthen evaluated using the approachdescribed below. The sensitivity of thereI iability to changes in the statisticsof key parameters was studied and measuresfor improving the reliability assessed.Definition of ReliabilityThe safety index (beta) approach wasused to characterize reliability.Figure 2 shows the distributions ofcharacteristic load effects (Q) and368

PROBABILITYDENSITYNOTE:Q AND R ARE CHARACTERISTIC VALUESillustrated in Figure 4. The cumulativedistributions of load effect andresistance were first obtained using aMonte Carlo simulation. The meancharacteristic values of the load effectand resistance were assumed as the 99percentile and 5 percentile valuesrespectively. The respective coefficientsof variation were taken as VQ and VR' Thebeta factor was then computed usingequation 4.PROBABILITYDENS ITYV· BayCHARACTERISTIC VALUES OF Q AND RFigure 2. Strengths and maximum lifetimeloads for a family of similar members.PROBABILITY OFFAILURE'peR - Q < OJcharacteristic strengths (R) for a familyof similar members.Figure 3.Y • R - QDefinition of failure(SAFETY MARGINJIf the means and standard deviationsof Rand Q (a, 6 R , Q, and 6Q) are known, anew function, Y, can be defined asfollows:v = COEFFICIENT OF VARIATION_ STANDARD DEVIATION- MEAN VALUEwith meanY = R - Q safety margin 0)and standard deviation6y = (6 2 R + 6 2 Q) 1/2Y = R - Q (2)This distribution of the safetymargin is plotted in Figure 3. The shadedportion represents cases where R - Q isless than 0; it represents cases wherefa i 1 u reo c cur s • Th e pro b a b i 1 i t y 0 ffailure is the probability that R - Q isless than O. If the type of distributionis known, the probability of failure canbe readily computed. Beta, which isreferred to as the "safety index" and alsoas a measure of the re1 iabi1 ity of astructural member, is defined as follows:~ = Y/6y (4)The use of this approach isCHARACTERISTIC,{RESISTANCEI 1 IN 100 YR LOAD \1.05 .99CUMULATIVE FREQUENCYFigure 4. Reliability based on betafactormethod.369

Galambos et al. (1982) has evaluatedbeta factors for conventional buildingcomponents under various loadingconditions. Beta typically ranges betweena low of 1.0 for short term earthquakeloads and a high of 3.5 for long term deadand live loads. Based on this research,MacGregor (1983) has proposed beta valuesfor revisions to ACI 318. These valuesare summarized in Table 1 and correspondclosely to values of beta computed forindividual components of existingbu i ld ings. A higher beta is proposed formembers failing in a sudden, brittle modethan for members exhibiting ductilebehavior. Al so, a lower beta is proposedfor earthquake loading, since significantdamage to components may be tolerated andalso because a number of components (i.e.,columns, beams, etc.) are simultaneouslyaffected under this loading condition.conical surface. The global moments arecaused by the structure deforming on thefoundation soil, while the local effectsare caused by the concentrated icepressure locally deforming the shellbetween bulkheads. The "load effect" istaken as the combined bending momentcaused by the global and local ice forcesin each of the two directions.Figures 5 and 6 show the envelope ofcircumferential and radial membrane forcesproduced by ice loads in differentpositions. These values come from theglobal analysis using the whole structureon its foundation and a coarse grid model.The reference ice load used in the designwas 110,000 kips (vertical). The membraneDUCTILEFAILUREBRITTLEFAILUREDead + Live 3.0 3.5Dead + Snow 3.0 3.5Dead + Live + Wind 2.5 3.0Dead + Wind 2.0 2.550 0I • "-1LEGENDN. COIIPRESS 10"JMAX. TENS ION.. MAX. COMPRESSI0~Dead + Live + Earthquake 2.0 2.5Table 1.proposed1983).Safetyfor newindexes (beta values)ACI 318 (MacGregor,Similar work is currently underwayfor offshore steel platforms (Moses, 1983)but final recommendations have not yetbeen made. For this study a beta factorof 2.0 was used as a target.Outer Shell FlexureLoad Effect and ResistanceThe outer shell panels are subject toa combination of global and local momentsas the multi-year ice ridge rides up the........ -saoE-

forces due to other random values ofglobal ice loads generated by the MonteCarlo simulation were pro-rated withrespect to the values obtained for thereference ice load.Figure 7 shows the local moments inthe shell panel near the toe. Themembrane forces and local moments shown inFigures 5, 6 and 7 constitute the "loadeffects" on the outer shell panel.The "resistance" is taken as themoment of resistance of the shell crosssectioncomputed independently in each ofthe two directions. This is influenced bythe geometry of the section, the materialstrengths and the membrane forces induceddue to the deformation under the globalice forces.The flexural resistance was computedusing conventional methods for sectionssubjected to axial load and bending asrecommended in ACI 318 (1983) and ACI 357(1984). An iterative process was used tocompute the moment of resistance for agiven membrane force. The procedure isembodied in program REMPRECS which wasvalidated with respect to ACI designcharts (ACI SP-17A, 1985).1000500BB1000'--1L[C[~D500 • HIN. CONPRESS10:I rMAX. TENSION.. MAX. COMPR[SSION"-.000-1500V1\.--- ~r-...,1\ \/ \ \"""\ V ~""1\......,~......,\-2000215 250 225 200 115 150 125 100 75 SO 25RADIAL LOCATION (FT)Figure 6. Radial membrane forcesgenerated in outer shell by thecombination of ice, thermal, andprestressing loads.,.....E-

idge load of 110,000 kips and a local icepressure of 1000 psi over the whole panel.As explained earlier, this moment dependson global ice load, local pressure andsoil reaction.For simplicity and to beconservative, the computed moment 'o1asassumed to vary with the ice pressurealone. The effects of changes in thecomputed moments were studiedparametrically.Global Ice LoadAn annual and a 25-year exposureglobal ice load was developed by Shell andis shown in Figure 8. The cumulatived ist ribut ions are log-normal. The I in100 year load is 125,000 kips for annualexposure and 251,000 kips for 25-yearexposure. The deterministic load of110,000 kips used in the original designis also noted on the figure for reference.Local Ice PressureThe pressure area relationship isshown in Figure 9. The range of tributaryareas appropriate to the ACES panel sizeis marked on the figure. Figure 10 showsthe distribution of ice pressures used int his stu d y as the bas e cas e • Th einfluence of other pressure distributionswas studied parametrically.:!etiltilI>:!e

Statistics of Variables Influencing theResistance MomentConcrete Compressive StrengthThe "nominal design strength" ofconcrete in compression, fh, is based oncompressive strength tests on 4" diameter,8" height cylinders at 28 days. Thisdesign strength represents a value belowwhich not more than about 10 percent ofthe results should fall when the specimensare tested in accordance with ACI 318(1983). No distinction is made betweennormal weight and lightweight concrete inregard to the statistics of compressivestrength.Several researchers have studied thevariation of concrete compressivestrength, a summary of which can be foundin Mirza et al. (June 1979). The strengthdepends upon several factors which includevariations in the properties andproport ion of the cons t i tuent materials,variat ions in the mixing, transporting,placing and curing methods, variations inthe testing procedures, degree of control,rate of loading and variations due to theconcrete being in the structure ratherthan in control specimens. The abovere ference recommends that the in-placeconcrete strength variation can be assumedto follow a normal distribution with themean, fc' and coefficient of variation,VR' approximated by the following:fc = (0.675 flc + 1,100) [0.89 (1 + 0.0810g10 R)] psi (5)The expression in the first parenthesisshould not exceed 1.15 flc.VR = (Vcyl + 0.0084) (6)where Vcyl is the coefficient of variationin lab tests. When Vcyl is not known, thefollowing values may be used (MacGregor etal., May-June 1983):0.12 for precast concrete0.15 for ready-mixed in-situ concrete0.18 for site-mixed in-situ concrete0.85 flc (7)0.15 (8)assuming a rate of loading, R, of 100psi/sec, Le. full ice load being appliedin one minute.The 28-day nominal design strengthspecified in the ACES design was 7000 psi.This yields a mean in-situ strength of5950 psi and a standard deviation of 893psi. Figure 11 shows the actualdistribution of the in-situ concretestrength used in the analyses.The beneficial multi-axial stressstate in the concrete has been ignored.While a truly 3-dimensional stress state109 .'68 •....i

increases the ultimate strength, the biaxialstress state in this applicationyields an increase of about 22 percentwhich is not considered significant.Instead, the effect of different concretestrengths was studied parametrically.Modulus of Elasticity of Concrete inCompressionThe initial tangent modulus ofelasticity of the concrete, Eci, is neededto compute the flexural strength of thesection. Mirza et a1. (June 1979)recommends the following relationships forthe mean, Eci, and coefficient ofvariation, VR, for normal concrete:0.560,400 fc (1.16 - 0.08 10glO t)• • • • • • • • (9)v cy1 2/4 + 0.0085 (0)In the absence of Vcyl from lab tests, avalue of 0.08 has been recommended for VR'effects of the above factors on rebarstrength. The static yield strength basedon nominal bar area follows a betadistribution but a log-normal distributionwas a Iso found to be a good fi t to thetest data in all but the upper end. Inthis study, the log-normal distributionwas used. The values for the mean andcoefficient of variation for Grade 60 barswere taken as 67.5 ksi and 0.098respectively, as suggested by MacGregor eta1. (May-June 1983). The actual staticyield strength distribution used as inputin the Monte Carlo simulation is shown inFigure 12.The modulus of elasticity of thesteel has a small dispersion and is moreor less insensitive to the rate of loadingor the bar size. Mirza et al. (May 1979)suggests a normal distribution with a meanof 29,200 ksi and a coefficient ofvariation of 3.3 percent which was used inthis study.100000Assuming a load application duration,t, of six minutes, Eci reduces to57,000 fc 0.5 which corresponds to the ACI318 recommendation. ACI also recommendsthe following relationship for lightweightconcrete:1.5 - 0 • 5Eci = 33 w f c •• (ll )where ~ = concrete density, pcf. For thisstudy Eci reduces to 43,380 f 0.5 for 120pcf lightweight concrete used in the ACES.10000.01~~/'- 67,500 PSI,V = 0.098~ ~ 60,000 PSI• 05.\ l .51 I .10la"! IOn':I ,0 IS s ".,!,10 SO "99 .99CUMULATIVE PROBABILITYRebar Strength and ModulusA summary of the work on variabilityof rebar properties is reported by Mirzaet al. (May 1979). The principal sourcesof variation in the yield strength are:aoaaathe variation in the strength of thematerial itself,the varia t ion in the area of crosssection,the effect of rate of loading,the effect of bar diameter, andthe effect of the strain at whichyield is definedThe reader is referred to Mirza'swork for more complete details of theFigure 12. Cumulative distributionrebarstrength.ULTIMATE TENSILESTRENGTHMODULUS OFELASTICITYNOMINAL MEAN(KS!) (KSI)28,400270 281COEFF. OFVARIATION0.0250.02Table 2. Statistical parameters of posttensioningtendons.374

Prestressing Tendon Strength and ModulusThe statistics of tendon strength andmodulus is described by Mirza et al.(July-August 1980). The distributions arenormal in both cases. Table 2 gives thevalues used in this study.Geometry VariationsA section through the ACES outershell is shown is Figure 13. The nominalshell depth is 66 inches. Four 1-3/8"diameter. bar bundles are used at 12"centers near the top and bottom surfacesin the circumferential and radialdirections. Prestressing tendons areseven - 0.6" diameter strand at 12"centers spaced symmetrically about thecenterline. The section in the radialdirection contains twice the number oftendons. This allows for curtailing oftendons as the rad ius reduces toward thecenter and also for accommodatingtemperature stresses.The statistics of concrete thickness,placing of rebar, etc., in conventionalbuilding slabs is described by Mirza etal. (April 1979). The recommendations forbuilding slabs do not apply to the ACESbecause of the substantial differences insection sizes between the two types ofstructures. For this study, the means andvariations of the various parameters havebeen based on applicable sections of thework by Mirza et al. (April 1983),combined with judgement on achievabletolerances.The mean, coefficient of variationand the range of values for the sectiondepth, rebar (or tendon) area, rebarpo sit ion and the prestress ing force areshown in Figures 14 through 17. The rebar~ -i---\--f=I- -II-'---+~~ ~-r4=F=9F~==F=~~~F=~d:.'1~L.ec.-7 T!>NDONS(Oil. EQUI'Jtl.LEt-if67.2,....., 66.8zH •enenw~ 66.4uH~66.065.665.2-.01/I/// --/ ~/ -lMEAN = 66.32 INV = 0.015I/I• I "J.!, "n u 10 U ~,ITI.t~50 99 .99CUMULATIVE PROBABILITY.$!.1' .S .)Figure 13.outer shell.Typical section through ACESFigure 14. Cumulative distributionshellthickness.375

10,...ZH0'CI)-----I'"MEAN---- ~- 6.24 SQ INV = 0.044 II I J I I I.05.1.2 .S I 1 5 10 H)o 40 "'0J. !. 1.101.10!IS1.It I.! S.L.,...01 50 99 .99CUMULATIVE PROBABILITYFigure 15.area.-27.,... -28.z ....~~tl-29 •~f-

original ACES analyses are shown inFigures 5 and 6. The principal effect isf rom the appl ied globa 1 load of 110, 000kips. The values of the membrane force forany other global load generated during theMonte Carlo simulation was pro-rated withrespect to the membrane force obtainedusing the above value. Similar commentsapply for the prestressing force andtemperature. The base prestressing forceused was 228 k/ft. per layer. The effectof temperature close to the toe wasconsidered negligible.Test vs. Calculation BiasMacGregor et al. (May-June 1983)describes the methods used to evaluate theaccuracy of the resistance computationmethods for different categories ofconcrete melllbers. The moment ofresistance of the ACES shell is computedUS ing the program REMPRECS. Th is programgives values which coincide well with thePCA charts for combined axial load andflexure used commonly in design. FromTable 2 of MacGregor's work, the mean biasvalue of 1.05 and a coefficient ofvariation of 0.10, corresponding toprestressed concrete flexural members, wasused in this study.Monte Carlo Simulation with Program ROCSIFThe program ROCSIF (Reliability ofConcrete Structures in Flexure) wasdeveloped specially for this study. Itincorporates the main Moment of Resistanceprogram REMPRECS, subroutines for normaland log-normal distributions and therandom numbe r gene rat or rou tine RANDUavailable on the VAX 11/785 computer. Theflow chart used for developing the programis shown in Figure 18.Outer Shell Flexure -ResultsThe shell panel nearest to the toe onthe centerline was studied. This isbecause the global component of theapplied shell moments is likely to be themaximum at this location. Also, since themoment of resistance in the radialdirection is greater than thecircumferential direction and the appliedmoments are nearly equal, only thecircumferential direction wasinvestigated. The effect of increasedpanel moments and membrane forces wasCO~PUTE,­IIIIIII IDENS ITY FUNCTIONS OF INPUT/OUTPUT PARAMETERS(50 DATA POI/ITS)Figure 18. Flow chart for Monte Carlosimulation program ROCSIF (flexure)studied parametrically to give anindication of the behavior of other panelson the shell centerline.Panels on either side of thecenterline of the shell experience reducedtotal moments, but will have some in-planeshears and torsions. These actions shouldfirst be converted to equivalent momentsand membrane forces in the two directionsusing Clark's and Wood's methods (AOGA197, 1984), prior to using ROCSIF.The ACES was designed on adeterministic basis with discrete valuesof global load (1l0,000 kips) and icepressure Cl ,000 psi) using a load factorof 1. 3. With these fixed values, thecorresponding beta factor is 2.87indicating that the ACES has adequatereliability for the design conditionsspecified, i.e., a beta of377

greater than 2.0. The Base Case usingvariable loads and resistances results ina beta of 0.955. The Base Case is definedas one using the I-year exposure globalload (Figure 8) and the local ice pressurevariability shown in Figure 10.a) Sensitivity to Local Ice Pressures andExposure ConditionsFigure 19 shows the variation of betawith local ice pressure variability forthe 25-year exposure conditions. Threedifferent median values of the icepressure and a range of variability werecons ide red • The Base Case beta and theACES deterministic design value are alsonoted to provide re ference point s. Theice pressure statistics significantlyinfluence the results.8·7.6. \5.4. \" \~ 3 .LI

2 · "~r BASE CASE1·~-1·/~ ..........,~· '"--.I--- ~1.6 2.0 2.4 2.8 3.2COMPUTED PANEL MOMENT (K FT/FT) * 10 33 .2I ~ ~~/... '- --- ~//. ,.....-;;YI ~ :/'+~y----I---I-'b~ .-- f-;I----- ~~~/r- BASE lob,/-'r- CASE ....... ,/0- f--~1 .'17'o6. 7. 8. 9. 10. 11. 12.REBAR AREA (SQ IN)Figure 21. Outer shell flexuresensitivityto computed panel moment(I-year)Figure 22.sensitivity toarea (25 years).Outershellshell flexurethickness& rebard) Sensitivity to Shell Thickness andRebar AreaReliability can be improvedsignificantly by increasing shellthickness, rebar area or both, as seen inFigure 22. Increasing the shell thicknessfrom 66 in. to 72 in. and rebar area from6.24 sq. in. to 9.0 sq. in., causes thebe ta factor to improve from 0.72 to 2. afor the 25-year exposure condition.Outer Shell ShearLoad Effects and ResistanceThe "load effect" in this case is thetotal shearing force occurring on thecritical perimeter around the appliedpatch load. The critical perimeter issituated a certain distance from the faceof the supporting bulkhead walls towardsthe center of the panel. This distance iseither 0.5 or 1.0 times the effectivedepth of the outer shell depending on themethod used for computing the resistance.The "resistance" is the sum of theshearing resistances of the concrete andthe shear steel at the critical preimeter.Load Effects in Panel under External LoadsThe membrane forces and panel momentsgenerated by the global and local iceloads, prestressing, thermal effects,etc., were described earlier and apply forshear also. The local pressure causingshear on the critical perimeter depends onthe area within the critical perimeter andis taken from Figure 9.Computation of Shear ResistanceThe shear resistance of a panel iscomputed using methods outlined by Birdyet al. (1985). Since neither the slab northe beam method recommended by ACI 318(1983) was directly applicable to theACES, a series of ten tests were performedon slabs and shells with aspect ratios andshear steel density similar to the ACES.The panels were tested to failure and theultimate loads predicted by severalavailable methods. Two methods based onACI 318 appear to predict the failureloads well and have been considered inthis study. The salient features of thetwo methods are summarized in Figure 23.379

TOTAL SHEAR RESISTANCEVTOT = VCONC + VREBAR ;::~NuV CONC= 3.5 * 0.8 * {f[ bwd 1 + SOOA gVREBAR = I'wbw d * fy -

65·4·3 .,2·1 ·oBASE CASE SHEAR REINFORCEMENT1% CONCRETE AREAjII!JifBASECAfE~ V1//1 ~.£ ~I'\. 1/~~~

d) Sensitivity to Prestressin~ ForcesThe improvement in beta values withincreasing prestressing force is modest.This is due to a higher proportion of thetotal shear resistance being provided bythe shear rebars which is not influencedby membrane forces.e) Sensitivity to Concrete StrengthAs with flexure, the effect ofconcrete strength is modest. This is alsodue to the high proportion of the totalshear resistance being provided by theshear rebar. Another explanation is thatthe shear resistance of the concretevaries with the square root of thecylinder strength. Increasing thecylinder strength from 3000 psi to 11,000psi, a factor of 3.67, only improves theconcrete shear resistance by a factor ofless than 2.0.f) Sensitivity to Shear Rebar QuantityThe most dramat ic improvement inshear resistance comes from the shearreinforcement quantity, as shown in Figure24. The Base Case shear reinforcement(current ACES design) is 1% of the planarea of concrete. This yields a betafactor of 4.633, which is considerablymore conservative than the flexural betaof 0.955. Based on this, a case can bemade for reducing the shear reinforcementto about half the current design quantitywhich would still provide a beta of over3.0.Base Slab ReliabilityGeneralWork on the re liabi 1 i ty of the baseslab was carried out as an extension tothe main study, which was originally aimedat the outer shell only. There wereseveral differences between the twocomponents which led to this studyextension:a) The base slabtens ions and notglobal ice loading.is subject to globalcompressions, underb) The base slab is subject to local soilpressures which vary with the global iceload and not local ice pressures.The e f fec t of hard-spot soi 1 loadsmust be considered.d) The base slab is a thinner member andis differently reinforced. Cross sectionsthrough the base slab are shown in Figure26.Both flexural and shear reliabilitiesof the base slab were evaluated usingprocedures described earlier. ProgramsROCSIF and ROCSIS were modified to allowstatistics of the soil pressures to beinput to the Monte Carlo simulations.The outermost cellon the centerlineof the structure was investigated forflexure in the circumferential direction.The radial direction is expected to yieldsimilar results. For shear reliability,both circumferential and radial effectswere considered.tVSl E6-7 TENDONSTOP a BOT.5 HEADEDSTIRRUPS ORBENT BARSSECTION SHOWING CIRCUMFERENTIALREINFORCEMENTAll.4--11 TO P- . 8 BOT.~ t===::=tti~~tn~~~~~~~~ADD·l.VSlE6-7TENDONNOTE: SPACING "A" OF RADIAL. R[BARS ANDTENDONS vARIES WITH LOCATION DuETO FLARING. RA~G( OF ","-12"TO 18','..5 HEADED STIRRUPOR BENT BARSECTION SHOWING RADIAL REINFORCEMFN:Figure 26.details.Base slabreinforcement382

Base Slab -LoadingGlobal EffectsLoad Effects under ExternalThe toe region is subject toc i rcumferent ia 1 tens ions wh ich range fromzero to about 750 kips per foot. Theradial direction experiences peakcompressions of approximately 1000 kipsper foot when the ice load acts near thetoe.Soil Pressures under Ice LoadingPeak soil pressures of about 8 ksfare produced in the toe region when theice load acts at the toe. The soilpressure under the weight of the structurealone is approximately 1.7 ksf.Base Slab Bending MomentsThe combined global and local momentsproduced by ice loads and soil pressureswere obtained from the original finiteelement analysis. The peak moment nearthe toe region was approximately250 k-ft-ft.The base slab can also experiencelocalized hard spot soil pressures duringinstallation as local undulations areevened out. The maximum value used in thedesign was 25 ksf, although higher valueshave been known to occur in dense sand s(Eide et a1., 1983-84). Thecircumferential panel moment underthis local pressure is 368 k-ft/ft.Moments for other pressures in thesimulation were pro-rated with respect tothis value.Base Slab -Input StatisticsThe input distributions of global iceloads, materials and section propertiesare essentially the same as used for theouter shell. The only difference relatesto soil loads. The soil pressures andmoments are caused by (1) the structureself-weight, (2) ice loads, and (3) thehard spot loads. Items (1) and (3) areassumed to be statistically non-variablein the simulations. Item (2) follows theglobal ice load distribution. The hardspot effects are assumed to besuperimposed on those due to self-weightand global ice load. The latterassumption is very conservative, sincehard spot soil pressures are associatedwith a failure condi.tion in the soil. Inreality, once the bumps have beenflattened and the soil has failed, thesoil pressure may not increasesignificantly under application of the iceload.Base CaseThe base case assumes the sectiongeomet ry i llus t rated in Figure 26, forcesand moments described previously, a oneyear exposure to global ice load and zerohard-spot soil pressure. The soilpressures and moments due to structureself-weight are constant, while those dueto the global ice load are variable.Base Slab Flexure -ResultsMonte Carlo simulations wereperformed yielding a Base Case beta of2.37. The results are insensitive tosmall variations in the computed membraneforce. Figure 27 shows sensitivity tochange 1n computed moments from soil8·"-6··2 ·o·100.'\"\" '"I~BASE CASE"~-r-------,140. 180. 220. 260. 300.COMPUTED MOMENT (K FT/FT)Figure 27. Base slab flexure-sensitivityto computed moments (I-year).383

pressures caused by the global ice load.The behavior is similar to that for theouter shell where the reliability reducessharply with increasing moment.Figure 28 shows sensitivity to hardspotpressures combined with the dead load4·l-4R ~xpbsukE..........(e-...- -(2-• \.. BASE CASE ~-2··-4·-I- "- '-.----r------,~~ £"25-YR EXPOSURE'"" ~o. 4. 8. 12. 16. 20. 24.HARD SPOT PRESSURE (KSF)Figure 28. Base slab flexure-sensitivityto hard spot pressure.and global ice load effects. Both theone-year and 25-year cases for exposure toglobal ice load are shown. The 25-yearexposure produces drastic reductions inbeta since very high soil reactions andmoments are produced. It should be notedthat the effect of the hard-spot load isstudied parametrically only. The valuesrange from zero to 25 ksf, each valuebeing non-variable in the simulation.Also, it is assumed that the effects ofthe moments caused by hard spots, deadload and ice load are simultaneous, asnoted earlier.Figure 29 illustrates the improvementin beta values achieved by increasing thesection depth and reinforcement; the basecase va lue is marked for re ference. Anincrease in section depth from 27 in. to33 in. and reinforcement increase by 75percent improves beta from -3.6 to +2.0,for a 25 ksf hard-spot load and 25-yearexposure condition. This increase wouldIINCRE~SE~ T~ICkNE'sS 'AND REINFORCEMENT4'1--[,c T = 33 IN'; 'RE:[NF' & PREimu:ss'-.1 1. 75 X BASE CASEI--- .-0...~n 1-----.'-->--2 .l-'I\-r-t-;B"'A""'SE,.--',;C"'AS~E;-f--+--+--+--+-+-+'~ ----I-I-- (l-YR EXPOSURE ~' __< HARD SPOT PRESSURE = 0 KSF)~O.r-+-~~-+-+I~I __ ~I+-L~J~L~i+-+-~~ f BASEl SLAB AS IDESIGNED---'------,r---2 .+-+--t--t-t-+-==t'=--hr+--j--~+--l--1"",-O. 4. 8. 12. 16. 20. 24.HARD SPOT PRESSURE (KSF)Figure 29. Base slab flexure-sensitivityto slab thickness & reinforcement (25-years)apply to the outer perimeter cells only.Conditions would improve toward thecenter, since both global tensions andsoil pressures due to ice loading reducesignificantly.Base Slab Shear -ResultsThe Monte Carlo simulation for shearyields a beta value of 8.69 which isconsiderably higher than the correspondingbeta value in flexure of 2.37. Thedifference is again due to theconservative nature of the shearresistance calculation, as explainedearlier.The sensitivity of the shearreliability to hard-spot soil loading andexposure conditions is shown in Figure 30.Although beta values reduce significantlyfor the 25-year exposure condition, aminimum value of over 2.0 is achieved evenfor a 25 ksf hard-spot soil load. Thisindicates that shear is unlikely to be aproblem for the base slab.384

10-- '----cL8·-e....---6·- ---'--,I(1 YR EXPOSURE---...., r---.-.,.- 25 YEAR EXPOSURE4·--r------....,-.2·o ·1 1O. 4. 8. 12. 16. 20. 24.HARD SPOT PRESSURE (KSF)Figure 30. Base slab shear-sensitivity tohard spot loads.ConclusionsA methodology and associated computerprograms have been developed forevaluating the reliability of concretearctic structure panels in flexure andout-of-plane shear. The method can beapplied to other structures composed ofplate-type components. However, where inplaneshears and torsions are present,these actions must first be converted toequivalent moments and membrane forces inthe two directions.Outer ShellThe global and local loads influenceboth the load effects and the resistance.Based on the design values provided forthe original deterministic design (nostatistical variation), the ACES outershell as designed has beta factors of 2.87in flexure and 5.8 in shear.For the 25-year exposure global loadstatistics, the corresponding beta factorsbecome 0.72 for flexure and 4.9 for shear.This is to be expected since the 1 in 100year value of the global load in this caseapproaches 251,000 kips compared with the110,000 kips used in the original design.The flexural reliability of the outershell can be improved most effectively bychanging the thickness and rebar quantity.For example, a change in thickness from 66in. to 72 in. and rebar quantity from 6.24sq. in. to 8.7 sq. in. increases the betafactor from 0.72 to 2.0.The reliability of the outer shell inshear is more than adequate. This is dueto the conservative nature of currentshear des ign methods. There is scope forreducing the shear rebar quantitysubstantially and still retain the betafactor at an acceptable level.Base SlabThe base slab is influenced by soilpressures and global tensions caused bythe ice load. Its behavior issignificantly different from that of theouter shell.Hard spot soil pressures (up to25 ksf), produced by flattening ofseafloor bumps, were combined with soilpressures generated by global ice loading.This assumption is conservative, sinceflattening of bumps is associated with afailure condition in the soil. Furtherwork is recommended to produce morerealistic results.The Base Case beta values for theI-year exposure condition are 2.37 1nflexure and 8.69 in shear. For the25-year exposure condition, these reduceto -1.0 and 4.732.The flexural reliability can berestored by increasing the thickness from27 in. to 33 in. and a 75 percent increasein the reinforcement. This change is onlyexpected to be necessary near the toeregion where the global tensions and soilpressures are highest.Shear is not expected to be a problemfor the base slab, even for the 25-yearexposure condition and the maximum hardspot soil load.385

Acknowledgments:The authors express theirappreciation to Shell Oil Company,Houston, Texas for providing theopportunity and funds to carry out thisproject. Thanks are also extended tomembers of Shell for providing review,guidance and computer support and to Dr.Finbarr J. Bruen, Consulting Engineer,Houston, Texas for reviewing this paper.ReferencesAmerican Concrete Institute, "BuildingCode Requirements for ReinforcedConcrete", ACI 318-83, 1983.American Concrete Institute, "DesignHandbook in Accordance with the StrengthDesign Method of ACI 318-83, Volume 2-Columns", ACI Publication SP-17A (85),1985.American Concrete Institute, "Guide forthe Design and Construction of FixedOffshore Concrete Structures", ACI 357R-84, 1984.AOGA 197, Arctic Cone ExplorationStructure Phase II - Final Report, byBrian Watt Associates, Inc., Houston,Texas, for Exxon Company USA, Shell OilCompany and Standard Oil Company ofCalifornia, April 1984. Volumes 1 to 9,Appendices A to G.AOGA 230, "Developmental Design andTesting of High Strength LightweightConcretes for Marine Arctic Structures,Program Phase III," Final Report, by ABAMEngineers, Inc., August 1986.Galambos, T.V., Ellingwood, B., MacGregor,J .G. and Cornell, C.A., "Probability BasedLoad Criteria: Assessment of CurrentDesign Practice", Journal, ASCE StructuralDivision, ST5, May 1982.MacGregor,Factors forProceedings279-287.J.G., "Load and ResistanceConcrete Design", ACI Journal,80-27, July - August 1983, pp.MacGregor, J.G., Mirza, S.A. andEllingwood, B., "Statistical Analysis ofResistance of Reinforced and PrestressedConcrete Members", Title No. 80-16, ACIJournal, May-June 1983.Mirza, S.A., Hatzinikolas, M. andMacGregor, J.G., "Statistical Descriptionsof Strength of Concrete", Journal, ASCEStructural Division, ST6, June 1979.Mirza, S.A., Kikuchi, D.K. and MacGregor,J. G., "Flexural Strength Reduc t ion Fac torfor Bonded Prestressed Concrete Beams",ACI Journal No. 77-26, July-August 1980.Mirza, S.A., MacGregor, J .G., "Variabilityof Mechanical Properties of ReinforcingBars", Journal, ASCE Structural Division,ST5, May 1979.Mirza, S.A., MacGregor, J .G., "Variationsin Dimensions of Reinforced ConcreteMembers", Journal, ASCE StructuralDivision, ST4, April 1979.Moses, F., "Utilizing a Reliability-BasedAPI RP 2A Format", API PRAC Project 82-22,for American Petroleum Institute, Dallas,Texas, November 1983.Birdy, J.N., Bhula, D.N., Smith, J.R. andWicks, S.J., "Punching Resistance of Slabsand She 11 s Us ed for Arct ic ConcretePlatforms", Paper No. OTC 4855, OffshoreTechnology Conference, Houston, Texas, May1985.Eide, O. and Anderson, K.H., "Foundat ionEngineering for Gravity Structures in theNorthern North Sea", NorwegianGeotechnical Institute, Oslo, Norway,1983-84.386

DESIGN SEA ICE LOAD EXAMPLES USINGAPI RECOMMENDED PRACTICE 2NM.E.VttUnocal Corporation, Brea, California, USAK. D. VaudreyVaudrey and Associates, San Luis Obispo, California, USAB. E. TurnerUnocal Corporation, Brea, California, USAAbstractThe first edition of the AmericanPetroleum Institute's Bulletin 2N,"Planning, Designing, and ConstructingFixed Offshore Structures in Ice Environments"was published in January, 1982.The API Subcommittee on Offshore Structuresin Ice Environments has been atwork on a second edition for the past twoyears and a new document is expected tobe published as a Recommended Practice inlate 1987 or early 1988. The RecommendedPractice will provide more specificguidance on the selection of design seaice parameters than did the Bulletin. Inmany cases, the Recommended Practice willcontain all the information required tocalculate conservative design sea iceloads for an offshore arctic platform.This paper presents four examples ofcalculating design ice forces on structuresfor offshore Alaska locations usingthe prov~s~ons of the new RecommendedPractice. The four structures are: (1) afour legged tower platform for CookInlet; (2) a monotower structure for theBering Sea; (3) a gravel island for thenearshore Beaufort Sea; and (4) a conicalstructure for the Chukchi Sea. The paperThis is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987. © The Geophysical Institute,University of Alaska, 1987.presents total horizontal design loads(so-called "global" loads) for sea iceacting on each of these structures, basedonly on the information contained in theRecommended Practice.IntroductionThe American Petroleum Institute(API) issued the first edition of Bulletin2N, "Planning, Designing, and ConstructingFixed Offshore Structures in IceEnvironments" in January 1982. TheRecommended Practice was drafted by theAPI Subcommittee on Fixed OffshorePlatforms in Ice Environments over thepast two years and was issued in draftfor the API's annual meeting in June1987.The Recommended Practice presentsmore detailed methods of load calculationthan the first edition of the Bulletin.The purpose of this paper is to presentsome design examples using the RecommendedPractice and to compare theresults to previously published designice loads. Loads are calculated forstructures located in four Alaskanoffshore basins; Cook Inlet, Bering Sea(Navar in Basin), northern Chukchi Sea andnearshore Beaufort Sea.387

EnvironmentIce feature geometry and kinematicsare presented in Table 1, taken from theGuideline Ice Features table in theRecommended Practice. The ice thicknessused for Cook Inlet for rafted ice was 5feet. The average ice temperature was-9.8°C, derived from air temperaturesand wind speeds given in the AlaskaMarine Ice Atlas (1983). In all casesthe design ice temperature was theaverage of the temperature at the topand bottom of the ice sheet. We used athickness of 12 feet for the first-yearridge condition and a floe width of 18feet for the ice sheet buckling case.The rafted ice thickness for theNavarin Basin case was 20 feet, with anaverage ice temperature of _4°C. Weused a thickness of 100 feet for thefirst-year ridge. The ridge was consideredto have a width of 500 feet. Weused a floe width of 100 feet for theice sheet buckling case.The Beaufort Sea case had a raftedice thickness of 20 feet and an assumedwater depth of 50 feet. The average icetemperature derived from the AlaskaMarine Ice Atlas (1983) was _9°C. Thecontact width of the ice sheet for thebuckling case was taken to be 300 feet.For the mUltiyear ridge case, the keeldepth and sail height were 60 feet and20 feet, respectively. We assumed athickness of 65 feet for a first-yearridge.The rafted ice thickness for theChukchi Sea was 20 feet, with an averageice temperature of -18°C. To calculatethe load due to a first-year rubblefeature with an average thickness of 70feet, we assumed that this was equivalentto a sheet ice thickness of 35feet. The ride-up load on the conicalstructure was then calculated by themethod of Ralston (1977). For themultiyear ridge case, the keel depth andsail height were assumed to be 64 feetand 16 feet, respectively.StructuresFour structures were used as examplesto calculate design ice forces foroffshore Alaska locations. The dimensionsof the structures are presented inTable 2. A four-legged tower platform inthe Cook Inlet was used as an example ofa narrow, multi-leg structure type. Thedesign load was calculated for a singleleg and the results applied to loading ofthe entire structure.A concrete monotower, similar tosome North Sea platforms, was consideredappropriate for the Navarin Basin of theBering Sea, where water depths are over300 feet. The shallow water of thenearshore Beaufort Sea has been exploredusing man-made earthfill islands. Thewater depth at the island location wasassumed to be 50 feet. A narrow conestructure was used to withstand thedynamic, thick ice in 150 feet of waterin the Chukchi Sea.TABLE 2Structural GeometryCook Inlet - Four leg tower structure- Leg diameter of 18 feet- Center-line width betweenlegs of 80 feetNavarin Basin - Monotower- Waterline diameter of 100feetBeaufort Sea - Earthfill island- Top diameter of 300 feet- Waterline diameter of 390feet3 to 1 side slope- 15 foot freeboardChukchi Sea - Conical gravity platform- Throat diameter of 150feet- Waterline diameter of 240feet- 35° side slope388

TABLE 1GUIDELINES FOR EXTREME ICE FEATURES FROM API RP2NBeaufort Sea Beaufort Sea Chukchi Sea Chukchi Sea Norton Navarin St. George CookFeature 60 Ft. (lB.3m) North South Sound Basin Basin InletSheet IceThickness. It. (m) 6·7 (1.8·2.1) 6·7 (1.8·2.1) 6.7 (1.8·2.1) 4·5 (I.H5) 3-4 (0.9-1.2) 3-4 (0.9-1.2) 1·2 (0.3-0_61 2-3 (0.6-0.91Rafted IceThickness. ft. (m) 15-20 (4.6-6.1) 20-25 (6.1-7.6) 15-20 (4.6-6.1) 15-20 (4.6-6.1) 12-20 (3.7-6.1) 10-20 (3.0-6.1) 8-15 (2.4-4.6) 4-5 (1.2-1.5)Multi-year FloeThickness. ft. (m) 25-30 (7.6-9.1) 25-30 (7.6-9.1) 25-30 (7.6-9.1) 15-20 (4.6-6.1) N.A. N.A. N.A. N.A.~QOI,QFloating First-yearRidge Thickness. ft. (m) WD+15 (WD+4.6) 100-140 (31-43) 100-140 (31-43) 80-120 (24-37) WD+15 (WD+4.6) 80-100 (24-31) 70-90 (21-27) 10-12 (3.0-3.7)or WD+15 (WD+4.6) or WD+15 (WD+A.6) or WD+15 (WD+4.6)Multi-year RidgeThickness. ft. (m) WD+ 10 (WD.+3.0) 70-80 (21-24) 70-80 (21-24) 60-70 (18-21) N.A. N.A. N.A. N.A.Multi-year FloeDiameter. miles (km) 0.6 (0.9) 1-3 (1.6-4.8) 1-3 (1.6-4.8) 1-3 (1.6-4.8) N.A. N.A. N.A. N.A.Ice Movement RatesMidwinterknot (km/h) 0.01-0.02 (0.02-0.04) 0.2-0.6 (0.4-1.1) 0.8-1.8 (1.5-1.9) 0.8-1.0 (1.5-1.9) 1-2 (2-4) 2-3 (4-6) 2-3 (4-6) 6-7 (11-13)Freezeup/8reakup.knot (km/h) 1-2 (2-4) 1-2 (2-4) 1-2 (2-4) 1-2 (2-4) 1-2 (2-4) 2-3 (4-6) N.A. 6-7 (11-13)Summer.knot (km/h) 2-3 (4-6) 2-3 (4-6) 2-3 (4-6) N.A. N.A. N.A. N.A. N.A.Notes:1. WD = Water Depth2. Ridge thickness Is the tolal thickness (Including Ihe sail helghl and keel deplh)3. N.A. = Nol applicable

Design CasesFollowing the Recommended Practice,for application of ice loads to structuretype, we calculated global loads for seaice acting on each of the four structures.We calculated loads due to ice sheetcrushing, buckling, and bending, and dueto first-year ridges or rubble, multiyearridges and impact events. Where referenceis made to specific sections of theRecommended Practice, we have includedthe parent reference from the RecommendedPractice for the convenience of thereader.Ice Sheet or Floe Buckling, Crushing, orBendingLoads due to floe buckling werecalculated according to Section 4.4.2c(Sodhi, 1979) of the Recommended Practice.However, either crushing or bending wasalways found to be the first (lowestforce) mode of failure. Therefore, loadsdue to buckling are not presented.The loads due to ice floe crushingwere determined using the KorzhavinEquation in Section 4.4.2a (Korzhavin,1971). Unconfined compressive strengthwas found by calculating the brine volumeusing an average ice temperature determinedfrom air temperatures given in theAlaska Marine Ice Atlas (1983). TheBeaufort Sea strain rate was determinedbased on a velocity of 0.02 knots (Table1) and using a relationship suggested byExxon (1979) for a gravel island inshallow water. Horizontal unconfinedcompressive strength was found as afunction of strain rate by interpolatingbetween the appropriate brine volumecurves in Figure 3.5.3-2 of the RecommendedPractice (Vaudrey, 1977).A load due to floe bending wascalculated only for the Chukchi Sea coneusing a method developed by Ralston(1979) and presented in Section 4.4.2b ofthe Recommended Practice. The horizontalload includes the force required to causethe ice to fail in bending and the forcedue to ice friction on the surface of thestructure during ride-up.First-Year Ridges or RubbleLoads from first-year ridges (rubble)may occur from crushing or shearing. Wecalculated loads from both failure modesand found shearing to be the governingmode of failure for all cases. We calculatedthe shear load by taking the area ofa wedge shaped ice ridge, multiplied bythe apparent cohesion as presented in theRecommended Practice in Section 4.4.2d(Weiss, 1981). The width of the wedge wastaken to be five times the depth. Valuesfor cohesion in psi ranged between 0.35and 1.0 times the thickness of the iceblocks in the rubble. We used the midpointof this range, 0.675 times the iceblock thickness. The result was multipliedby two for failure of the ice ridgein double shear.First-year ridge ride-up was notconsidered to be a problem for the BeaufortSea gravel island. However, it wasconsidered relevant for the Chukchi Seacone. An equivalent vertical ride-up loadwas calculated for the Chukchi case byusing Ralston's equation for the weight ofice on a structure surface, as presentedin Section 4.4.2b of the RecommendedPractice (Ralston, 1979). We assumed that70 feet thick rubble was equivalent to 35feet of solid ice. The horizontal componentof rubble force during ride-up wascalculated using the equation presented inWang (1984) and restated in Section4.4.2e.6 of the Recommended Practice.Multiyear RidgesLoads due to multiyear ridge encounterswere calculated for the Beaufort andChukchi Seas using Section 4.4. 2e (Wang,1984) of the API Recommended Practice. Aridge profile was assumed based on Kovacs(1973) .Impact EventsLoads on a structure due to icefeature momentum were calculated byobtaining the appropriate pressure fromthe multiyear or first-year ice pressureareacurves in the Recommended Practice,using the contact area between the icefeature and the structure. The method ofimpact event load calculation described inSection 4.4.3 of the Recommended Practice390

is based on the use of a pressure-arearelationship, such as Figure 4.4.6 of theRecommended Practice or Figure 3 ofSanderson (1984). A similar method isdescribed in Vivatrat and Slomski (1984).We calculated impact loads for first-yearice in the Navarin Basin and Cook Inletand for multiyear ice in the Beaufort andChukchi Seas.ResultsHorizontal design loads are presentedin Table 3 for each of the fourcases. The controlling design load for afour-legged Cook Inlet platform was13,100 kips, assuming three legs areloaded simultaneously by ice crushing.The governing load for the Navarin Basinmonotower is 117,000 kips due to failuredue to ice sheet crushing. The icefeature was found to fail in crushing at417,000 kips for a gravel island in thenearshore Beaufort Sea. A floe impactevent of 149,000 kips was found to governfor the conical structure in the northernChukchi Sea.Comparison with OtherPublished Ice LoadsJahns (1985) reported calculated iceloads for offshore Alaska structures.Figure 1 is a reproduction of a figurefrom Jahns' paper. The largest ice loadin Jahns' paper is 200,000 kips for aBeaufort Sea cone, representing a rangeof 100,000 to 200,000 kips. This structureis comparable to the Chukchi Seacone in Table 3 which has a load of149,000 kips. On the other hand, themaximum ice force on Jahns' Navarin Basinmonotower is 10,000 kips, compared to117,000 kips in Table 3 for the NavarinBasin monotower. One significant differencein the Navarin Basin structures isthat Jahns assumes a cone on the leg ofthe monotower, while we have assumed thatthe leg has vertical sides. However, thecalculated load is about 12 times largerthan that reported by Jahns. This showsthe advantage of a sloped structuralsurface at the waterline. Similarly,compare the Beaufort Sea ice crushingload to ice bending on a Chukchi Sea conein Table 3.Typical gravel island loads of 250to 300 kips/ft of waterline diameterhave been used for exploratory islandsin the Beaufort Sea. For a 390 footdiameter island, our calculated load of417,000 kips is 3 to 4 times greaterthan typical design loads. The primarydifference between exploratory gravelisland loads and permanent island loadsis the use of the 20 foot rafted thicknessinstead of the 7 foot sheet thickness,and instead of the (less probable)30 foot multiyear thickness, in determiningthe crushing load.Ice IslandsOne class of ice features is notconsidered in this paper. Large tabularicebergs, usually called "ice islands",are generated from the glaciers ofEllsmere Island, and can exis t in theArctic Ocean for decades. The APIRecommended Practice refers to these inSection 2.3.1:"Large ice features may representan exceptional class of loading. It maybe decided to accept the risk of damageor loss of the structure and facilitieswi thout loss of life or harm to theenvironment."That is, the Recommended Practicerecognizes that it may not be realisticto design a structure to resist the loadfrom an ice feature as large as an iceisland.ConclusionsAlthough API Recommended Practice2N provides more guidelines than did theprevious ~ulletin, selecting ice featuresand calculating governing design iceloads still requires a working knowledgeof ice properties and behavior toprovide reasonable assumptions. Adherenceto design guidelines presented inthe Recommended Practice may, in somecases, lead to very conservative results.The Recommended Practice will, ofcourse, be improved as the oil industrylearns more about ice and ice loads.391

TABLE 3: DESIGN ICE LOADS (KIPS)COOK INLET NAVARIN BEAUFORT CHUKCHI1 LEG 4 LEG BASIN SEA SEAICE SHEET OR FLOECRUSHING 4,370 13,100 117,000 417,000 NABENDING NA NA NA NA 9,230BUCKLING > CRUSHING > CRUSHING > CRUSHING > BENDINGFIRST-YEAR RIDGES OR 211 633 15,040 3,900 48,900RUBBLEMULTIYEAR RIDGES NA NA NA 72,400 118,000IMPACT EVENTS 3,370 10,100 57,600 203,000 149,000FIGURE 1ENVIRONMENTAL LOAD COMPARISONFOR REPRESENTATIVE GRAVITY STRUCTURESNORTH SEA BERING SEA NORTON SOUND BEAUFORTCON DEEP MONOTOWER CONCRETE ISLAND CONEWATERDEPTH 1FT)o- WIND 1-5- EARTHQUAKE 1-5- WAVES-ICE300-500 300-600 45-90 &0-200TYPICAL HORIZONTAL LOADS (10 6 LBS)1301001707011001001503010100100-200(AFTER JAHNS,1985)392

The Recommended Practice will, nodoubt, be of continued use, especially tothose who do not have access to the largebody of proprietary sea ice data held bysome oil companies. As the RecommendedPractice itself states, however, "Thisdocument is intended to supplement ratherthan replace individual engineering judgment."ReferencesArctic Environmental Information and DataCenter 1983, "Alaska Marine Ice Atlas",University of Alaska.American Petroleum Institute 1982,"Planning, Designing, and ConstructingFixed Offshore Structures in Ice Environments",Bulletin 2N.American Petroleum Institute 1988,"Planning, Designing, and ConstructingFixed Offshore Structures in Ice Environments",Recommended Practice 2N.Exxon 1979, "Technical Seminar on AlaskanBeaufort Sea Gravel Island Design" ,presented by Exxon Company USA, Houston,Texas.Jahns, H.O. 1985, "Offshore Outlook -Technological Trends in the AmericanArctic", Arctic News - Record, Vol 4.2.Korzhavin, K.N. 1971, "Action of Ice onEngineering Structures", CRREL Translation260, Hanover, New Hampshire.Kovacs, A. 1973, "Structure of a Multi­Year Pressure Ridge", Arctic, Journal ofthe Arctic Institute of North America,Vol. 26, No.1, pp. 22-31.Sanderson, T.J.O. 1984, "Theoretical andNeasured Ice Forces on Wide Structures",Proceedings, Internat"iona1 Associationfor Hydraulic Research Ice Symposium,Hamburg, West Germany, August, Vol. IV,pp. 151-208.Sodhi, S.D. 1979, "Buckling Analysis ofWedge-Shaped Floating Ice Sheets", Proceedingsof the 5th Internationa1Conference on Port and Ocean Engineeringunder Arctic Conditions, Trondheim,Norway, Vol. I, pp. 797-810.Vaudrey, K.D. 1977, "Ice EngineeringStudy of Related Properties of FloatingIce Sheets and Summary of Elastic andViscoelastic Analyses", U. S. NavalCivil Engineering Laboratory, TR860,Port Hueneme, California.Vivatrat, V. and S. Slomski 1984,"Probabilistic Selection of Ice Loadsand Pressures", ASCE, Journal of Waterway,Port, Coastal and Ocean Engineering,Vol. 110, No.4, November, pp.375-391.Wang, Y. S . 1984 , "Analysis and ModelTests of Pressure Ridges Failing AgainstConical Structures", Proceedings, In ter-"national Association for HydraulicResearch Ice Symposium, Hamburg, WestGermany, August, Vol. I, pp. 67-76.Weiss, R.T., A. Prodanovic, and K.N.Wood 1981, "Determination of Ice RubbleShear Property" , Proceedings, InternationalAssociation for HydraulicResearch Ice Symposium, Quebec, Canada,July, Vol. 2, pp. 860-870.Ralston, T.D. 1977, "Ice Force DesignConsiderations for Conical OffshoreStructures" , Proceedings of the 4thInternational Conference on Port andOcean Engineering Under Arctic Conditions,St. Johns, Newfoundland, September,Vol. 2, pp. 741-752.Ralston, T.D. 1979, "Plastic LimitAnalysis of Sheet Ice Loads on ConicalStructures", International Union ofTheoretical and Applied Mechanics Symposiumon Physics and Mechanics of Ice,Copenhagen, Denmark.393

THE DISTRIBUTION OF ICE PRESSURE ACTINGON AN OFFSHORE CIRCULAR PILESachito TanakaKawasaki Steel Corp., Chiba, JAPANKohoki SasakiToshiyuki OnoHokkaido University, Sapporo, JAPANHiroshi SaekiHokkaido University, Sapporo, JAPANAbstractFor the design of a pilesupportedstructure in arcticareas, It IS very Important toestimate how large the local icepressure is which acts on thecontact surface of the piles. InadditIon, structural analysIs forbucklIng of the contact surface ISnecessary when the thickness ofthe pile IS determined.Systematic experiments wereconducted to clarify the distrIbutionof ice pressure acting on acircular pile. This paperdescribes the principal results ofthese experiments which aresummarized as follows:(l) themaximum total ice force In all ofthe Indentation tests wasdetermined to be about 2.6 timesthe force in penetration tests; (2)the time elapsed from load startto the peak of tolal ice force foreach indentation test Increased asthe strain rate decreased; (3) themaximum radial component of localice pressure in the transitionThis is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987. © The Geophysical Institute,University of Alaska, 1987.range occurred at the center ofthe contact area and was about 5times the uniaxial compressivestrength of sea ice; (4) thecircumferential component dependson the angle from the movement ofthe pile to the measured point.The maximum component occurs inboth the transition and theductile ranges when the angle is45 degrees; and, (5) it wasconfirmed that the distribution oflocal ice pressure depends onstrain rate as defined by Micheland Toussaint(1977).IntroductionPile structures such aswharves,' platforms and sea berthshave been bUIlt in ports andoffshore in cold regions as thebases for supporting offshoreexploration and productionfacilities in the Arctic.Recently. offshore breakwaters andjetties have been installed sothat drifting ice does not inflictextensive damage on shellfish andaquaculture facilities.To tal ice forces, such as thehorizontal static force due to themovement of ice, and the verticalforce along the pile axis due to395

adhesion of the ice sheet andchanges in water level, havebeen previously discussed(Saeki etal. 1980 ;Saeki et al. 1983).These external forces have beencons idered in des i gn so that theentire structure can withstandthese forces. It is known,however, that very high local icepressure acts on the contactsurface between the ice sheet andthe pile structure, particularlyin the case where the aspect ratioof pile diameter to ice thicknessis small. If the radial componentof the local ice pressure exceedsthe yield stress of the pile forlocal buckling, the contactsurface of the pile to the icesheet buckles out of plane and thehigher component of the local icepressure is concentrated at thepOint of buckling. In addition,fatigue on the contact surface isalso considered because arepeated, local ice pressure actson the structure due to the intermittentfailure of the ice sheet.It is also important to estimatethe durability of coatings forboth frictional force and abrasionand estimate the magnitude of thecircumferential and verticalcomponents of local ice pressureacting on the contact surface.This paper deals with thedistribution of ice pressureacting on a circular pile.Experimental ProcedureLocationAn experimental investigationwas carried out at Saroma lagoonon Hokkaido island located innorthern Japan. In this lagoon,the sea ice normally attains athickness of approximately 40 cmevery winter.Newly-refrozen ice sheet andcontact conditionsSnow on the ice sheet acts asan insulator and restricts thevertical growth of the sea ice andoften turns to snow ice. For thisreason, tests were carried outusing a newly-refrozen ice sheet.Just before each test,a hole wascut in the ice sheet, and thefield test equipment was loweredinto the hole(Fig.1). Based on aconsideration of contact conditionsbetween the pile and the icesheet, experiments were conductedfor two types of contact conditions(plane and circular) asFig.1Test equipment396

sho~n in Fig.2. Experiments inthe former condition (plane) havebeen carried out by manyresearchers and are calledpenetration tests. The experimentsfor the circular condition haverarely been conducted. In boththe plane and circular cases,chain sa~s or a core drillingmachlne ~ere used for cutting and,as a result,the initial contactsurfaces ~ere not smooth. Theaverage density, average salinityand average grain diameter in thene~ly-formed sea ice ~ere 860Kg/m 3 , 0.8 % and about 11.1 mmrespectively. The averageuniaxial compressive strength ~as1.19 MPa at a strain rate ofapproximately 10-3 sec-1 usingcylindrical samples ~hich had a100 mm diameter and 200 mmheight.Test equipmentThe equipment used for thefield testing is sho~n in Fig.3.The equipment ~as portable andcould be lo~ered into a pre-cuthole in the ice sheet by a liftingframe ~hich could be skidded fromone test location to another. Acircular pile ~as pushed into theice sheet to simulate the actionof ice against the pile. It ~asattached horizontally andvertically to the contact surface.ICESHEETICESHEETPenetrationIndentationThe pile ~as pushed into theice by means of a hydrauliccylinder. The stroke of thehydrauliC cylinder ~as 300 mm andit had a load capacity of 0.5 MN.The cylinder had a fixed pressuretransducer of 49 MPa capacity toallo~ the total ice force to betransformed to oil hydraulicpressure. It ~as actuated by ahydrauliC supply using an electricoil pump.Circular pileFig.2 Initial conditions of contactThe circular pile had thestructural rigidity to avoid beingDisplacementtransducerCircularpilePressure transducerfor total Ice forceSensing surfaces forlocal ice pressureFig.3 Side vie~ of test equipment397

deformed under high local Ice outer cylinder, ... hich ... as 545 rnmpressure. The pi I e cons 1 s ted 0 f In diameter, 40P mm In height, 12an outer cylinder, five pressure mm thIck and made of stainlesstransducers and 15 strain columns steel, had dIrect contact "'Ith thefur local Ice pressures,an Inner Ice sheet. The outer cylinder ... ascylInder and several shear supported by several shear ... alls... a II s, as sho ... n In Fig.4. The bet ... een the outer and Innero10,~i101c:>Directionofmovement(unit:mm)Fig.4Cross section of a test circular pilel.strain column 2.outer cylinder3.shear ... all 4.pressure transducer5.inner cylinderem_-$-_:::...,..-J4 40em--$---t-I2.8emFig.5Arrangement of sensing surfacefor local ice pressure(Development for a circular pile)398

cylinders. High local Icepressure on the outer cylinder,therefore, acted on the Innercylinder through the shear wall.which was 216 mm in dlameter,400mm In height and 16 mm thick.Pressure transducers and straincolumns for the local ice pressureLocal ice pressure wasmeasured by five pressuretransducers and 15 strain columns.The arrangement of the measurementpOints IS shown In Flg.5. Onlyradial components of local icepressure act on the line of thecontact surface where the anglebetween the movement of the pileand the pOints of measurement ISo degrees. Pressure transducerswhich had a 8.2 mm pressuresensingsurface diameter, 19.6MPa capacity and 0.58 TPa/mstiffness were used for themeasurement of local ice pressureon this line only. At the otherpOints of measurement, where theangle was 15.30,45,75 and 90degrees. 15 strain columns wereused to measure the threedimensionalcomponents of localIce pressure,namely the radial.the circumferential and thevertical components.The strain column made ofstainless steel as shown in Fig.6had a 20 mm p~essure sensingsurface diameter and four waterproofstrain gages on the surfacesof the rectangular section(section B-B in Fig.6). Thestrain column, which could beconsidered as a cantilever, couldmeasure the accurate radialcompressive component of more than0.8 MPa acting vertically on thesensing surface. Similarly, theaccurate Circumferential andvertical shear components. from0.01 to 3.11 MPa actinghorizontally on the surface withinthe allowable accuracy of thestrain gage and the measuringapparatus. and the deflection ofthe strain column caused by theshearing components. could bemeasured.Before testing, a pressuretransdUcer and a strain columnembedded into a stainless steelplate were examined to see whetherstress concentration occurred atthe transducer site. As a resultof calibration. it was confirmedthat the pressures measured withthe pressure transducer and thestrain column were nearly equal tothe average pressure acting on545 Dia.COuter16245cirt\8I~A-A Section B-BSection C-C SectionCunit: mm)Fig.6Schematic diagram of strain columnfor three-dimensional componentof local ice pressure399

each test piece such as ice,and hard rubber.wooduniaxialtestingcompressive strengthand crystal examination.Displacement transducerThe motions of the pile weremeasured with a displacementtransducer, which was set up onthe movement side of the sheetimmediately in front of the pile.MeasurementLocal ice pressures,total iceforce and the motion of the pilewere scanned at the same time in avery short interval and alsorecorded on the same flexible diskas the digital data. Continuousrecordings were obtained duringeach tes t .The temperature of the icewas measured after each tes t. Thegeometry of the ice failuresurface was also observed in theportion cut from the ice sheet infront of the pile after the test.Finally, ice was collected forResults and DiscussionThe tests were carried outbased on a consideration of thefollowing: (1) the contactcondition between the circularpile and the ice sheet,namelyindentation and penetration testsas shown in Fig.2;and, (2)thestrain rate of the interactionbetween the pile and the icesheet. In addition, the testswere conducted with the newlyrefrozenice sheet which was 100to 150 mm in thickness so thatthe aspect ratio of the pilediameter to the thickness of theice sheet was greater than four.The results of the twelvetests are tabulated in Table 1.The Run from Al to A9 correspondsto the indentation test andsimilarly the Run B1 to B3 thepenetration tes t. Considering theTable 1Result of testRunh V i X 10- 3 F CTe te Ti(em) (em/sec (sec-I) (kN) (MPa) (sec) ("C)AI 12.0 0.565 2.59 126 1. 93 4.73 -2.4f----A2 12.0 0.019 0.087 84.1 1.29 228 -2.2A3 12.0 0.021 0.096 66.0 1.01 47.8 -2.5A4 12.0 1. 38 6.31 135 2.07 1. 17 -2.7------ -----~t--A5 14.0 1.45 6.65 106 1.39 0.735 -4.5---A6 17.0 0.451 2.07 34.9 0.377 1.11 -4.31-- -------- ---A7 10.0 0.318 1.46 89.3 1.64 4.86 -2.01-----AO 9.0 0.678 3.11 121 2.27 31.3 -1.9A9 9.3 0.023 0.106 43.2 0.852 107 -2.081 13.5 0.263 1.21 63.0 3.5 16.3 -3.21-- ---82 13.5 0.209 0.959 43.2 11.5 4.24 -5.2f----1-----83 14.5 0.399 1.83 63.0 0.97 42.0 -3.3Note: CTe=1.19(MPa), D=54.5(cm)400

effect of strain rate,the velocityof the circular pile wascontrolled so that the strain ratewas either in the range oftransition (10-3~f:.

In Figure 7. the relationshipbetween the strain rate and thenon-dimensional total Ice force,dividing each experimental totalice force by the estimated totalice force, IS shown. According tothe results uf penetration tests,the maximum experimental total iceforce satisfies the followingequation:(Flexp./(Flesti. (=1 (21Therefore,Eq.(1) is satisfactoryfor the equation to estimate themaximum total ice force.if thepile comes into contact with theice sheet. as was the conditionfor the penetration test. It isclear that the maximum total iceforce In the indentation tests is2.6 times greater than the maximumforce In the penetration tests.The total ice force in Indentationtests was twice as large as theforce measured In the penetratIontests for both the Indentation andpenetratIon tests which wereconducted by KawasakI eta1.(1986).In Figure 8, the relationshipbetween the relative displacementand the peak of the average Icepressure In both the Indentationand penetration tests is given.In this figure, it IS shown thatthe average Ice pressure for theindentation tests IS quitedifferent from the pressure inpenetration tests at the samerelative displacement. Higherlocal ice pressure acts on thecontact surface In penetrationtests compared w~th Indentationtests because the contact surfacein the penetration test isrelatively small. The localfailure, however. occurs where thehigher local pressure acts. The2010~8eTs6(MPa )4..... ,"-'" " " (()2~)""- ~~""1/ b0.80.6 /"""'"0.4IIUTest D/h -0.20.1",0 Indentation 3.8A Penetration 5.9---f-68 2 4 6 8 2 4 6 810- 2 10-1V·h/DFig.8 Relative displacement vs.peak of average ice pressure402

total Ice force In the penetratIontest, therefore, IS relativelysmall at the same relativedisplacement compared with theIndentation test. The confinementof the ice sheet conducted by thepile acting in indentation issmaller because of a largercontact surface. The total iceforce measured in the indentationtest IS larger because the pilemust deform to the circumferentialcontact surface at the same time.Considering the initial conditionof the contact between ice sheetand pile,the equation for estimatingthe maximum total ice forcederived from this experiment is asfollows:where, f is the coefficient of thecontact condJtions (1.0 forpenetration and 2.6 forindentation).(2)Elapsedteststime inindentationIn Figure 9, the relationshipbetween strain rate and elapsedtime is indicated. The elapsedtime decreases as strain rateincreases. The following equation,in a similar range to the aspectratio, where tB=KE: n and K and nare constants, is obtained in theindentation tests referred to inthe figure.( 4)for the indentation test. Thestrain of ice on the contactsurface can be obtained fromEq. (4) ,for the indentation test. The icechanges in its fai lure mode, fromelastic brittle fracture toductile deformation, as the strainrate decreases. It is clear, asindicated in Eq. (5), that thestrain of the ice increases andthe ice deforms more as the strainrate decreases.Localtestice pressure In indentation

Top view~/~ICESHEETSide viewD~] yerrru;bzxmode (6=15 and 45 degrees on theline of Z=2.8 cm) and in theshearing mode (6=15 degrees on theline of Z=O cm) in the transitionrange for Run A7.(3)Maximum radial componentIn Figures 13 and 14, therad i al componen tat tB for thetransition and ductile range ofthe indentation tests is shown.The components are scattered foreach test;however, it seems thatthe higher component isconcentrated at 6 =0 and 45degrees in the tests in thetransition range and at 6=30degrees in the test in the ductilerange. Comparing the distributionof the components in the directionof ice thickness,the component onthe line where Z=O cm is greaterthan the others in the transitionrange. In the ductile range,thecomponent on the line where Z=2.8cm is somewhat greater than theothers.Fig.lO Three-dimensional componentof local ice pressureand global coordinatesystem(2)Failure modeIn Figures 11 and 12, thechanges of the radial componentsat 6 =15 and 45 degrees on theline of Z=O, 2.8, 4.0 cm forboth Runs A2 and A7 are related toelapsed time. In the range wheret/tB is less than 0.8 for bothRuns, the rad i a I eomponen t seemsto be unstable because the contactsurface is not smooth. Afterthat,it is considered that plasticfailure of the contact surfaceoccurs, judging from the stabilityof the radial component in theductile range of Run A2. It isclear that the radial componentschange rapidly and that thefailure occurs in the crushingEach line in both figuresconnects the maximum of theexperimental radial components onthe same line,namely Z=O, 2.8 and4.0 cm. Judging from the linesconnec t i ng these maxi ma, thelargest component of the maximaacts on the line of Z=2.8 cm at6=0 degrees in the transitionrange and it is about 4.8 timeslarger than the uniaxial compressivestrength. Similarly,in theductile range it acts on the lineof Z=2.8 cm at 6=30 degrees andis approximately 2.5 times aslarge as the uniaxial compressivestrength. In experiments with acircular pile (Saeki et al. 1986),the largest radial componentoccurred on the center line of theice thickness at 6=0 degrees forthe condition when D/h=4.4,£=1.66*10- 3 sec- 1 and it was about5.5 times larger than the uniaxialcompressive strength. Hausler(1981) indicates that the biaxialcompressive strength increases to4.5 times the uniaxial compressivestrength under a biaxial loadingcondition perpendicular to the404

Run A2, £=S 7x 10-!5 lsec- l ), CTB = 1.29(MPa)6CTr / CTB48(deg ~)0 2.S 4.02IO.S0.60.40.20.1.. _, 15 0--0 l':r-i::. 0-0~, ... .-~ ~, 45• ' .... ,tr--

~ ~err/ere108642:~'\~,..lP~D/p-~-h--/ / D;~\ ~ i V-~D\ i~I\0.8\\~ (D0.6LZ0.4 (em) Max.q-/~ f-0.2I~L:\[0 0---02.8 6------t:, f-0 pO~4.0 D---o(0.1o 15 30 45 75 908(deg)Fig.13Distribution of radial componentof local Ice pressure(Indentation test, Transition range)err /erc 108642V'/l'I0.8~ \ /0.6 ~ ..riL 'I~ "t~0.4 lk0.20.1f-- f8 , E-"8 7x 10-e",11 xI0- 4 (see- l )',' erc-1.19(MPo) -ruI=-- ~) p, "-''I/~D\ J1 / tJZ(em Max.err/eTcR\ ~ /0 0---0\ II2.8 bo-----IJ.(P 4.0 0----0Io 15 30 45 75 908(deg)Fig.14Distribution of radial componentof local ice pressure(Indentation test, Ductile range)406

direction of growth. The Ice sheetImmedIately in front of the pileIS confined In a bIaxial stresscondition due to the Indentationof the pile. JudgIng from theser' e sui t sin the t ran sit ion ran g e, I tIS considered that the largestcomponent In the maxIma acting onthe contact surface is the same asthe compressive strength in thebiaxIal confIned condition,if theaspect ratio is in the range from3.8 to 5.9. In addition, it canbe assumed that a higher componentthan the largest experimentalcomponent occurs on the contactsurface due to attaining thetriaxIal confined condition as theaspect ratio decreases. The pilewall,therefore, should be checkedfor buckling based on theassumption that the largest radialcomponent acts on the contactsurface if the aspect ratio IS inthe range from 3.8 to 5.9.(4)Maximum of the clrcumferenti a I and vertical components inthe trans I t I on rangeIn FIgures 15 and 16, theresults obtained for thecircumferential and verticalcomponents actIng on the contactsurface in the transition rangeare Indicated. Based on theseresults,It was determined that thecircumferentIal components wereconcentrated at 8=30 to 45degrees and the ver t I ca 1 componen tat 8=30 degrees.According to the elastoplasticanalysis fur theIndentation of the circular pileusing finite element methods, themaximum circumferential componentoccurred in the range from 8=15to 35 degrees and the ratio ofthe maxImum to the uniaxialcompressive strength was 0.190"8 2(MPalI --0.80.60.40.20.10.080.060.04/--)'\Zem MaX.0"80 ~/ '\ 2.8 l>---6PLl.:0 .)//.fc .(",'9'/ / ~/ / I; "!I ,/I0.02Ii' 00.01 [ 00o~15 30[ Ll.L45\ 4.0 0---0\~75 908(deglFig.15Distribution of circumferentialcomponent of local Ice pressure

( MPa) 0.80.60.40.2I t=tB, E =1.5 ....... 6 7xI0- 3 (see-')I~ //e (\ /0.10.08/J. J&.. \ ./ (D,,"T/ \ "0.06\0.04/1 0 \ Z[ \ ( DO (em Max. CTZ\j' (\ 0 ~0.022.8 fr.-----t:,.[V~ 4.0 Q---O0.01o /5 30 45 75 908 (deg)0Fig.ISDistribution of vertical componentof local ice pressure(Indentation test, Transition range)(Taguchi et al. 1987). Thisanalysis using plate elementsassumed that the coefficient offriction was zero between the iceand the pile, the velOCity of thepile was 0.2 cmlsec, the aspectratio was 10, the uniaxialcompressive strength was 2.59 MPaYoung's modulus of ice was 1132MPa work hardening of ice was11.32 MPa and Poisson's ratiowas 0.3. In this experiment,theaverage Young's modulus was 630MPa and the ratio of the maximumof the circumferential componentto the uniaxial compressivestrength was 0.84. It isconsidered that the maximumcircumferential component isconcentrated at 8=30 to 45degrees due to the frictionalforce, because the pile hadactually a unique coefficient offriction. Summarizing the resultsof this experiment, the maximum ofthe circumferential and verticalcomponents was 1 and 0.4 MPa,respectively. The coating mustbe able to withstand these maximumforces.(5)Distribution of the radialand circumferential componentIn Figures 17 and 18, thedistribution of the radial andcircumferential components,respectively, are given. Eachlocal component in thedistribution is the average of thelocal components obtained at thesame pOint of measurement for thesame f ai I ure range. I n thetransition range,the radialcomponent is concentrated at 8=0and 30 tu 45 degrees. It isobserved that the higher radialcomponent acts on the line of Z=Ocm, namely the center of the icethickness. In the ductile range,408

the radial component isdistributed more uniformly at eachpOint of measurement, comparedwith the distribution In thetransition range. Thecircumferential component isconcentrated at 8=45 degrees, andsimilarly on the line of z=o cmin both the transition and theductile ranges.For the indentation test,thecontact surface of the ice sheetdeforms uniformly. The circularpile indents the ice sheet in thedirection of the Y-axis,Er =Ecus eE8 = E sin e (6 )where E IS the strain of thecontact surface in the directionof the Y-axis due to theI n den tat ion 0 f the pi] e, Er i s theradial component of the strain,and Ee is the circumferentialcomponent of the strain. Accordingto E'1.(6), the maximum radial andcircumferential strain occurs at8=0 and 90 degrees, respectively.In the transition range where theelastic crushing fracture occurs,therefore, the radial component ISthe largest at 8=0 degrees andthe component decreases as_:c-;-__ -,o x 0[Transition ~ rR:::------,---r---F------'j=-£. 1.5-6.7x 10'3(sec'l)I Transition range Ii ·1.5~6. 7 x 10'3(sec' l)ou, (MPa)0.6u,(MPa)0.20.4I Ducti Ie rangel£*S.7xI0' s1.1 x 10. 4 Y(sec' l )0.6I Ductile range I£ -S.7xI0's~1.1 x 10'4(sec' l )y02u,(MPa)0.20.430.6-'-----Fig.I7 Distribution of averageradial component of localice pressure at tB(D/h=3.8-5.9, 0c=l.I9 MPa)Fig.I8 Distribution of averagecircumferential componentof local ice pressure at tB(D/h=3.8-5.9)409

6 reaches to gO degrees un thecenter line of the ice thickness.This high radial component,however. can no t ac t on the lineof Z=2.8 and 4.0 cm because ashear fai I ure occurs on the upperand lower edge of the Ice sheet Inspite of the small strain. ThecircumferentialcomponentIncreases in propor t i on to 6 asshown in Eq.(6). The component onthe line of Z=O cm isparticularly greater than theother component on both lines ofZ=2.8 and 4.0 cm due to the mostconfinedcondition on the centerline of the ice thickness.However, the component at 6=75degrees is smaller than the othercomponent because of the lessconfinedcondition of the pile.The contact surface is in theYield stress condition in theductile range where the ice sheetdeforms plastically. The factthat the radial component dependson 6 as expressed in Eq.(6) wasnot observed. The circumferentialcomponent in the ductile range,however, increases as 6 increases.The same tendency, as the resultof the circumferential componentin the transition range, isindicated, because that componentis much smaller than the radialcomponen t.(6)Distribution of thecomponentverticalIn Figure 19, the distributionof the vertical component inboth the transition and ductilerange IS shown. For both ranges,the component is concentrated at6=30 degrees. A comparison of bothdistributions shows that thecomponent in the ductile range isgreater than the component in thetransition range. It is consideredthat the larger component acts onthe pile because the failure modedoes not show an elastic brittlefracture but a micro crack occurson the contact surface in theductile range.ConclusionsTotal Ice force and elapsed time(1) The maximum of the total Iceforce resulting from theIndentation tests is about 2.6times larger than that from thepenetration tests.(2) The time required to reachthe peak total ice force increasesas the strain rate decreases. Theice shee t, therefore, deforms moreas the strain rate decreases.ITransltion range]E =1.5-6.7xI0-3(sec-I)0.2C1"z(MPa) 0.4I Ducti Ie ra nge IE '8. 7x 10-~1.1 X 10- 4(SeC-I)(MPa)Fig.19° x °C1"Z(MPa)0.2 0.4 0.60.6° """'___ X--,O,...--_t--_t'----'-+_''y°0.20.40.6Distribution of averagevertical component oflocal ice pressure at tB(D/h=3.8-5.9)410

Local IceIndentatIonpressuretestfromthe(1) The maxImum radIal componentin the transItion range is nearlyequal to the biaxIal compressivestrength of ice and isapproximately 4.8 tImes largerthan the uniaxial cumpresslvestrength of ice, If the aspectratio is in the range from 3.8 to5.9. The maxImum componentoccurs at 8=0 degrees.(2) The maximum circumferentialand vertical components are 1 and0.4 MPa respectIvely under thesame aspect ratio as mentionedabove. The former componentoccurs at 8=15 to 45 degrees.(3) The radIal component in thetransition range decreases as 8increases from 0 to 90 degrees.The maximum circumfprentlaJcomponent occurs at 8 =45 degreesand the component decreases as8 reaches 75 or 0 degrees.(4) In the ductile range, theuniform radial component isdistributed un the contact surfacecompared with the same componentin the transition range. Thecircumferential component in thisrange shows a similar tendency tothe same component in thetransition range.Michel, B. and ToussaInt. N.(1977) "MechanICS and theory ofindentation of loce pJates" • J. ofGlaciolojU". Vul.l9, t\Jo.8J.pp.285-300.Saekl, H. and OzakI, A. (1980)"Ice force on piles", PhysIcs andMechan I cs 0 f I c~. ed., Per TI'yde,SprInger-Verlag, Berlin, pp.342-350.Saeki. H., Ono, T., Nakazawa, N.,Sakai, M. and Yamada, M. (1983)"Ice forces due to changes inwater level and adhesion strengthbetween fresh water ice andvarious pIle materials", Proc. ofthe 20th IAHR Congress, Moscow,USSR, Vo1.2, pp.1-8.Saeki. H., Takeuchi, T., Tanaka,S. and Ono, T. (1986) "Icepressure distribution acting onvertical structures", Proc. of thE'2nd Cold Region Tech. Conf.,Sapporo, Japan. pp.160-165(inJapanese) .Taguchl, Y., Kawasaki. T. :andTozawa, S. (1987) ; the 2nd Int.Sym. Okhotsk Sea and Sea IceAbstract, Monbetsu, Japan, pp.31-32(in Japanese).ReferencesHausler, F., V.

A NUMERICAL SIMULATION METHOD FOR FAILURE ANALYSISAND LOAD ESTIMATIONTadashi ShibueKazuyuki KatoIshikawajima-Harima Heavy Industries Co., Ltd., Yokohama, JAPANYasushi KumakuraIshikawajima-Harima Heavy Industries Co., Ltd., Tokyo, JAPANYutaka ToiUniver~ity of Tokyo, Tokyo, JAPANAbstractA numerical method is developed toestimate the failure behaviour of ice,and experiments are carried out toexamine the method. The method uses athick-walled shell element based on theconcept of the Rigid Bodies-SpringModels. This method uses some functionsin order to deal with characteristics ofice, such as material nonlinearities,geometrical nonlinearities, differencesof material properties through thethickness, buoyancy and self-weighteffects, and cracking behaviour. Theverification tests of this method arecarried out in the IHI ice tank on bothlevel ice sheets and composite icefeatures including a model ice ridge,and make evident a good agreementbetween experimental results andsimulation results.IntroductionEfforts have been made to estimateice loads acting on ice breakers andoffshore arctic structures by fieldmeasurements, model tests in iceThis is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987. © The Geophysical Institute,University of Alaska, 1987.tanks (Kato, 1986) and numericalanalyses (Watanabe and Kawai, 1980;YOShimura and Kamesaki, 1981; Shibata,Kumakura and Ma tsushima, 1983). Preciseresults are obtained with fieldmeasurement methods but they involvehigh costs. In ice tanks, owing to theuniformity and reproducibility of modelice, well-designed experimental studiescan also be performed. However, thereare limitations on model ice properties.Numerical analysis may be used toestimate behaviour and they may includeice properties which are not availablein model tests. The main problem in theestimation of the failure behaviour ofice by numerical analysis comes from thecomplexity of its material properties.That is the reason why information onthe properties of the analysed ice sheetand use of a suitable calculation modelfor representing its characteristics arenecessary.This paper presents a numericalsimulation program developed for theestimation of ice failure behaviour andits verification. This program uses athick-walled shell element and take as abasis the concept of the Rigid Bodies­Spring Models (abbreviated as RBSM)(Toiand Kawai, 1982) • It also providesseveral functions which are required forthe prediction of failure behaviour ofice. To examine the proposed simulationmethod, simulation calculations are413

performed on the results from modeltests carried out in the IHI ice tank.Thefollowingfeatures.program deals withcharacteristicstheofice(1 )Ice features are afloat and may havenon-uniform thickness.(2)The material properties may change inthe direction of the thickness due totemperature distributions and otherfactors.(3)Cracks which may initiate under thelarge external forces are considered.The influence of the rigidity ofice features depends on the size ofgenerated cracks. When the cracked facescome in contact with each other, normalforce is transmi t ted. The fac t tha t thefailure strength depends on thehydrostatic components of stress istaken into account (Kato, Kishimoto andMita, 1986).Figure 1. Thick walled shell elementSimulation programFunctionsThe mathematical model employed inthis program is a triangular thick-walled shell element(Toi and Kawai, 1982)(Fig.1) based on the concept of theRBSM( Kawai, 1977; Kawai, 1979) • Eachrigid triangular element has 6 degreesof freedom at the center of gravity, andis connected to adjacent elements by twotypes of springs distributed on theelement boundary. One spring resists theaxial deformation and the other thetangential deformation. This simulationprogram provides the followingfunctions;-Multi-layer structure: Differences ofmaterial properties of ice in thedirection of the thickness can be takeninto account.-Variable thickness: Ice features havingnon-uniform thickness (ice ridges andicebergs, for example) can be taken intoaccount.-Failure conditions: The Drucker-Prageryield criterion is employed to representthe failure conditions of ice whichmainly depend on the hydrostaticcomponent of the stresses.-Buoyancy and self-weight: The influenceof buoyancy and self-weight, changingwith vertical displacement, isconsidered in the equilibrium equationas non-linear springs. In addition, theunbalanced force, corresponding to thechange of buoyancy and self-weight, isalso evaluated.-Cracks(splits): Initiation of cracks,contact of split faces and separation ofcontact faces, are treated on certainassumptions • An unbalanced force due tocrack initiation is evaluated as well asforces due to buoyancy change, etc.Surface cracks, not penetrating in thedirection of thickness, are evaluated bya number of failed springs distributedon the element boundaries.The element stiffness matrix iscalculated by numerical integration overthe springs distributed on the elementboundary. Therefore, the advantages ofthe present program include the featuresthat cracks along the element boundariescan be easily treated by the failure ofsprings and that, even where severalelements are lamina ted, only ari thmeticsummation is necessary. However, adisadvantage associated with thisprogram is that no cracks can occurinside the elements.Flow chart of the calculationIn the simulation calculations, nonlinearphenomena are simultaneouslytreated. Therefore, calculationprocedures of this program areconsiderably complex. In figure 2, theflow chart is given. Iterations for loadincrements are set in the outer loop,while setting iterations up toconvergence in the inner loop. The414

ITERATION FORLOAD INCREMENTITERATION FORCONVERGENCEI DATA INPUTI SET UP VARIABLESJI SET UP LOAD VECTORIIICALCULATE ST IFFNESS MATRIX ICONSIDERINGI BOUYANCY & CRACKI SOLVE STIFFNESS EQUATIONCALCULATE DISPLACEMENT,I STRESS and STRAINI EVALUATE FAILUREI CALCULATERES I DUAL FORCENo CONVERGED?I OUTPUT RES ULTSNoTERMINATE?I ENDFigure 2. Flow chart of the presentprogramprocedure for construction of thestiffness matrix is included inside theinner loop. When the stiffness matrix isbuilt up, the influence of cracks,buoyancy and self-weight is evaluated bythe stresses and strains obtained in theIIIIIIIIprevious i tera tion. New stresses andstrains can be calculated as a solutionof the equilibrium equation. Cracks areevaluated with these values whileestimating unbalanced force vectors,associated with the change ofconfiguration of cracks. Convergence ofthe solution is assumed to be achievedwhen no new crack is produced and theunbalanced force vectors are lower thanthe permissible limit.Treatment of ice failureInitiation of cracks and subsequentbehaviour are not yet theoreticallyclear. The authors assume that a crackis initiated when a tensile force actson an element boundary and the plasticstrain calculated from the Drucker­Prager yield criterion reaches acritical value.Based on this assumption,initiation of cracks and subsequentbehaviour are evaluated as follows (andas shown in Fig. 3) • When allcalculations with the same loading stagehave converged, the occurrence of crackinitiation is defined by referring tothe equivalent plastic strain and thesign of the relative displacement. Whenthe equivalent plastic strain exceedsthe preset value of the failure plasticstrain, and the relative displacement isPARTFAILURE STIFFNESS MATRIX.. 0STRESS VALUES + 0FOR RECONTACT RECONTACT TRANSMIT NORMAL COMPONENTOF ELEMENT BOUNDARY FORCEFOR SEPARATIONSEPARATION: SAME AS FAILUREWHERE oR :NORMAL COMPONENT OF RELATIVEDISPLACEMENT OF ELEMENT BOUNDARY'P :EQUIVALENT PLASTIC STRAIN'PE :EQUIVALENT PLASTIC STRAINWHEN FAILURE OCCURSFigure 3. The procedure of simulation415

positive (when the spring is elongated),the crack will initiate. In addition,behaviour subsequent to the initiationof a crack is assumed as follows:-After initiation of cracks: Thefailure coefficient (preset value) ismultiplied by the stiffness matrix. Witha failure coefficient of zero therigidity and the stresses of the springbecome zero.-Recontact: An element boundary, oncecracked, would come in contact when thenormal component of relativedisplacement of the spring becomes zeroor less. After recontact, the forcesnormal to the cracked faces would betransmitted again.-Separation after recontact: Arecontacted element boundary would beseparated again when the normalcomponent of relative displacement ofthe spring exceeds zero. The stiffnessmatrix associated with this situation isthe same as the one associated withinitiation of cracks.Simulations for failure of ice featuresGeneral procedure of simulationThe objective of the simulationdescribed below is to estimate the morecomplicated failure behaviour and loadfor an ice feature whose materialproperties, including failureconditions, are already known. In orderto satisfy the objective, an experimentis performed first. Then, a simulationcalculation corresponding to theexperiment is carried out using thedeveloped program. Finally, theexperimental result is compared with thecalculated result to examine the programas a simulation method.The experiment includes materialand model tests. In the former, materialproperties, including the failurecondi tions of the ice sheet, are to beobtained. The latter are supposed todetermine whether complicated crackingbehaviour could be estimated by thesimulation calculation. Cantileverbending tests and/or three-point bendingtests are applied as material testsbecause of their simplicity. The modeltests are performed to simulate theoperating conditions. The materialproperties of each model ice may beEXPERIMENTSIMULA'fIONMATERIAL CONSTANTS I[MATERIAL-TesT -; fMATE-RIAL- TEST -- 1I I I I:,1 SCANTLINGS OF 1:1 MESH U,'!LTEST PIECE Ii!:11:! CORRECT I: FAILURE CONDITIONS;: RESULTS I NO I ReSULTS I"---T- ---___ J CO"PA~~ L: -- ~ -'-'SAME MATERIAL I SAME ME-moo I:-MODE'L-TEST--~ FIX l"MoDEL-T-EsT------lI I I IilsCANTLINGS OFI: liliESHU': MODEL & ICE:I I. MATERIA-L-cONS-TANTS :I I I' FAILunE CONDITIONS II ' I I:1 RESULTS ~ ~ RESULTS II"- - - :!J UQMPA!!D ~ -- - - ,figure 4. The procedure of simulationsomewhat different even when the sameambient temperature is used to grow eachmodel ice. In order to avoid such adeviation, the same model ice is usedboth for the material tests and themodel tests. The same program is usedfor the simulation of the material testsand the model tests, of course.The simulation calculation is firstapplied to the material test. Theconditions for calculation aredetermined according to the testconditions and results. The materialproperties are determined by trial anderror to obtain better correspondencebetween the calculated results and thetest results. Then, the simulationcalculation for the model test iscarried out with the material propertiesthus obtained. Provided that the resultsobtained by the simulation calculationagree with the results of the modeltest, the validity of the simulationcalculation is examined. Therefore, itis possible that the failure behaviourof a different ice feature can beestimated by the program, if thematerial properties are given. Theprocedures above are shown in Fig.4.Experiment with level iceThe experiments have been performedin the IHI ice tank, where model ice416

8·/ CANTILEVER TESTMODEL TESTCALCULATIONFigure 5. General sketch of experimenton level ice sheet(a) MESH AND CRACKSfeatures were grown using a urea-watersolution of 1% by weight under specifiedambient temperatures. Measurements havebeen carried out wi th computercontrolledAID converters, whilerecording failure phenomena wi th a 35mmcamera and a VTR system.A thin transparent PVC sheet with a10cm square lattice was placed on themodel ice and photographed to record thescale of progressing failure.Cantilever beam bending tests havebeen performed as material tests, whilemodel tests were carried out using apyramid frustum. In figure 5, a generalsketch of the experiment on the levelice is shown.The cantilever beam bending testswere carried out in such a manner that afloating ice sheet was cut out tocantilever shape. Then, the cantileverbeam was broken by applying a downwardbending force at the free end. Adisplacement was applied at a constantspeed (5.4mm/sec) as a bending force.The forces were measured at the loadingpoint and displacements were measured atthe same time at 3 other locations, onebeing at the center of beam and the(b) SECTION DETAILFigure 6. Calculation Model and results(level ice)other two at the end.The model tests with the pyramidfrustum have been performed with afloating level ice sheet. Three sides ofthat ice sheet had been frozen to theice tank wall leaving the other sidefree. The sloping face of the pyramidfrustum was pushed horizontally at aconstant speed (5.0mm/sec) at the centerof the free edge until the ice sheetfailed. The pyramid frustum in use was awooden model with a 60 slope angle fromthe horizontal plane and a contactlength of 285mm. The loads were recordedusing a triaxial load cell. Failureoccurred in such a way that cracksspread first radially and thencircumferentially, connecting the radialcracks, thereby forming a typicalfailure mechanism. In figure 6, theshape of spreading cracks is indicated.417

Simulation for the experiment with leveliceThe test piece of the cantileverbeam bending test was supported by theremal.nl.ng ice sheet. Therefore, thecalculation model included thesupporting ice sheet (210mm x 240mm) byusing left/right symmetrical conditions.The model consisted of a layer with thesprings distributed in 3 rows along theelement boundary and 5 rows in thedirection of thickness.After some trial and errorprocedures, the best fit was obtainedwith a modulus of elasticity of 52.9MPa,a cohesion of O. 0324MPa and a failureplastic strain of 100 . Good agreementbetween calculated and measured valueswas obtained at the loading point.However, rather poor agreement wasobtained at the other measuring points.This could be explained by the fact thatthe absolute value of the measureddisplacements at the root of thecantilever beam were as small as 0.1-0.3mm, bringing about insufficientmeasurement accuracy.The simulation calculations havebeen carried out for the model test withthe material properties obtained by thesimulation calculation of the materialtest. The model ice sheet was supportedby the wall of the ice tank. Therefore,half of the ice sheet was used for thecalcula tion model, fixed at a posi tionof 1700mm from the free end,corresponding to half of the width ofthe ice tank, while using the left/rightsymmetrical conditions shown in Fig.6.The model consisted of a layer inwhich springs were arranged in the sameway as for the material test. A load ofvertical component 1 and horizontalcomponent of tan (68 + 6 ) was appliedto loading point (P). An angle of 6 , acompensation for the horizontal loadincrement due to friction between theice sheet and the model surface with acoefficient of friction of about 0.1,was made.The shapes of fracture obtained bythe calculation are also shown in Fig.6along with the observed shape offracture during the test. When 3 of 5rows in an element boundary failed, itwas assumed that the element boundaryhad failed completely. By comparing theshape of fracture obtained by the testwith the calculated one through 12iterations of the 4th loading step, itis understood that the shapes of thefracture is similar. In figure 7, themeasured and the calculatedrelationships between horizontal andvertical loads and horizontaldisplacement are compared at the loadingpoint. Although no convergence was400CALCULATION300~o~ 200...J- EXPERIMENT--6,-CALCULATION--0--CANTILEVERTEST10 20 30 40HORIZONTAL DISPLACEMENT (mm)Figure 7. Experimental and calculatedload-displacement curves (level ice)Figure 8. General sketch of experimenton composite ice feature including anice ridge418

obtained in the 4th loading step, it wasdecided that the maximum load wasachieved from consideration of the steepincrease in the displacement. Themaximum horizontal load (350N) was about83% of the experimental value (420N) inspite of a relatively large loadingstep.Experiment with composite ice featureincluding an ice ridgeThe cantilever beam bending testwas applied to the level part of theice, while the ice ridge was tested bythe three-point bending test. A generalview of the test is shown in Fig.8.First, 3 ice ridges were grown then theice ridges were floated to a suitablelocation in the ice tank while growinglevel ice in the surroundings.In the cantilever beam bending testfor obtaining material constants for thelevel ice, a loading speed wasmaintained at a constant value(2.85mm/sec), as a displacement loading.The three-point bending tests ofice ridges were performed wi th 2 of 3ice ridges grown at the same time. Thesetests were carried out with a spacing of800mm between supports in a floatingcondition. A downward displacement wasapplied to the center of the supports ata constant speed (14.5mm/sec). It wasexpected that the ice ridge might showdifferent behaviour depending on theloading direction. Therefore, it wasdecided to measure, in advance, thestrength characteristics of the iceridge in various bending directions.For this purpose, 2 types of test weremade: bottom-in-tension(BIT) and topin-tension(TIT)bending; in the former,CUPSIDEJo 50rIFigure 9. Typical cross section of modelice ridgethe test was performed in the directionthe ridge had been produced and, in thelatter, the ridge was turned upsidedown.The shape of the broken surface ofthe ice ridge was photographed with thelattice pattern of a 5cm square. Anexample of the broken surface for theice ridge is sketched in Fig.9.The BIT bending and the TIT bendingshowed different load-displacementbehaviour with an ice ridge that shouldhave substantially the same iceproperties. It appears possible that,because of the temperature during icegrowth (ambient temperature -15 C), thetop part was stronger because of lowerambient temperature while the bottompart, subjected to substantially highertemperature (-1 to -2 C) was weaker.However, the phenomenon that bringsabout different load-displacementbehaviour cannot be explained only by afact that the strengths are different inthe top and bottom of the ridge. Thephenomenon implies that the top andbot tom should be considered to bedifferent materials, and when eachmaterial is in compression 01" tension,different responses (namely differentmoduli for compression and tension) wereobtained.For the model test, the slope faceof the model was pushed horizontally ata constant speed (5.0mm/sec) untilfailure occurred at the center of thefree side of the ice ridge with thelevel ice sheet behind it. The modelconsisted of an aluminum plate with aslope angle of 30 from horizontal, acontact surface width of 100mm and athickness of 10mm. A triaxial load cellwas used to measure the load history.Simulation for the experiment withcomposite ice features including an iceridgeThe cantilever beam bending testwas modeled in the same manner as thesimulation with level ice. The trial anderror procedure was also employed toobtain better agreement with the testresults. The best agreement was broughtabout with a modulus of elasticity of1600MPa, a cohesion of 0.346MPa and afailure plastic strain of 100 Goodagreement was obtained betweencalculated and measured values at the419

loading point.For the three-point bending test,the entire body of the ice ridge wasmodeled for a calculation with simplysupportedboundary condi tions. Thesimulation calculation for the threepointbending test was carried out forboth one-layer and two-layers modelsbecause obviously different results wereobtained from the BIT bending and theTIT bending. The one-layer model had thesame spring arrangement as the model forthe cantilever beam test. For the twolayermodel, the number of springs inthe ice thickness direction wasincreased to 9 rows.1700.ICE RIDGEOBSERVEDCRACKS(BOLD LINE)First, a simulation calculation forthe one-layer model was processed todetermine material properties, on theassumption that materials becomedifferent due to the loading directionsof the BIT and TIT bending tests. As aresult, a satisfactory approximation wasobtained. However, the properties of theice ridge did not vary with differentloading directions. Therefore, anotherapproximation was introduced in whichthe ice ridge was separated into 2layers with different material constantsand failure conditions. Morepractically, an ice ridge was separatedinto 2 layers of equal thickness underthe assumption that each layer consistedof different materials with individualmaterial constants and failureconditions. Under such an assumption, asimulation calculation was performed. Atthat time, each of the upper and lowerlayers had different moduli ofelasticity to tension and compressionwith a view to simulate precisely thefailure behaviour as already described.However, the program was not providedwi th such functions; therefore, themoduli of elasticity for the TIT and BITbending, obtained with the one-layermodel, applied directly to the upper andlower layers. In consequence, theresults of the calculation provided goodagreement with the test results for themaximum load but rather poor agreementin terms of displacement.With the one-layer models,calculated values agreed well withmeasured values. However, with the twolayermodel, the calculated deformationbehaviour was far from the experimentalCALCULATEDCRACKS(a) MESH AND CRACKSLEVEL-;:t-MODEL ICE RIDGE~- ~14"(b) SECTION DETAIL\MODELSTRUCTUREFigure 10. Calculation model and results(composite ice feature)results, because the modulus ofelasticity was taken at the averagevalue between those for the BIT and TITbending. The trends of the calculateddeformation were such that they becameinsufficient in BIT bending andexcessive in TIT bending, with the sameloads.The calculation model for the modeltest is shown in Fig.10. The thicknessof the ridge part was taken as theaverage ice thickness of the section.The calculation model was separated intotwo layers, while providing the springfor three rows along the element420

250__ -b----- __________ -- - - -- __ .erA-200~o

Table 1. Assumed material constantsLEVELICE ICE RIDGE ----1Level IceFrozen layerSat!Elastic modulus (MPa) 1950 2100 600KeelPOisson's ra tiC 0.3 0.3 0.3Cohesion (MPa) 0.303 0.326 0.093Angle of Internal fnctlon 25· 25· 25·Hardening Parameter(MP.) 19.5 21.0 6.0Limit plastic strain (~) 10 10 10Figure 14. Section detail of ridge part30Z 20~o«o-110o 10 15VERTICAL DISPLACE MENT (m)Figure 15. Calculated load-displacementcurve,.. -- - ------------- - -----------,1 STEP2 11 F,=24.2MN: Fv=14.0 MN :~~_'_1J __________ ____________ 1r--- - - ---- - - -------- - ----.,I , I 'I STEP3-t !F,=27.7 MN:.. Fv=16.0MN 1KJll ____________________ :, r--------------------------~ ,, I, I, I: STEP3-2 :, IlA- 1:~_L L _____________________ ~r-- -- ---------------- ---~, ,: :: STEP3-4 :, ,hl1 _____________________ Jwere about 91% and 98% of them.Figure 16. Calculated failure sequenceAn exampleIt is worth showing how the programpredicts results in the case of aprototype situation. The calculationmodel is shown in Fig.13, and thesection details of the ridge is shown inFig.14. The assumed material constantsare summarized in Table 1. The structureassumed in this example is a conicalstructure with a 60 slope angle and adiameter of 20m at the water line. Thecoefficient of friction between ice andstructure surface is assumed to be zero.The calculated load-displacementrelations are shown in Fig.15. It hasbeen decided that the critical loadshould be achieved at the loading stepwhere no convergence is obtained; aninfinite displacement would take placeby any small load increment.The progression of failure is shownin Fig.16. The failure patterncorresponding to STEP 3-4 is consideredto be a failure mechanism. This failuremechanism is also seen in the left sideof Fig.13. It is seen that the failure422

mechanism provides the failure of theridge part and level ice part behind theridge.ConclusionsA computer program was developedfor simulating the failure behaviour oflevel ice sheets and composite icefeatures including ice ridges. Thisprogram includes some functions whichare able to deal with multi-layerstructures, variable thicknesses,failure conditions for ice, buoyancy andself-weight, and cracks.With this program, simulationcalculations were carried out for modeltests performed in the IHI ice tank inorder to examine its abilities as asimulation tool. The results obtainedfrom the simulations showed reasonableagreement with the test results.It is shown that the presentprogram can apply to failure problemsassociated with level ice sheets andcomposite ice features including iceridges when the material properties andthe failure conditions of ice are given.It is concluded that this program canestimate the failure loads and failurepatterns which are necessary for thedesign of icebreakers and offshorearctic structures.The program provides good agreementbetween the calculation and testresults, when the variation of elasticmodulus due to loading direction istaken into consideration.AcknowledgementThe authors would like to deeplythank Professor T.Kawai of ScienceUniversity of Tokyo for his kind adviceand encouragement. Dr. K.Fujii and Mr.Y.Matushima of Ishikawajima-Harima HeavyIndustries Co.,Ltd. are alsoacknowledged for their contributions.ReferencesKato K.: Experimental Studies of IceForces on Conical Structures,Proceedings, IAHR InternationalSymposium on Ice, Iowa City (1986), pp.185-196.Kato K., Kishimoto H., Mita S.:Triaxial Compression Tests on SalineIce, Proc., Cold Region TechnologyConference '86, Sapporo (1986),pp. 148-153.Kawai T.: New Element Models in DiscreteStructural Analysis, J. of the Societyof Naval Architects of Japan, Vol 141(1977),pp. 174-180.Kawai T.:EngineeringDiscreteColloquium,Collapse Load Analysis ofStructures by Using NewElement Models, IABSECopenhagen (1979)Shibata K., Kumakura Y. and MatsushimaY.: The Method of Predicting Ice­Induced Forces against Marine Structure,Proceedings, Int. Conf. Port and OceanEngineering under Arctic Conditions(POAC'83), Helsinki (1983), pp. 812-821.Toi Y. and Kawai T.: Discrete Limi tAnalysis of Shell Structures (Part 4) -Finite Deformation Analysis of Thick­Walled Shells-, "SEISAN-KENKYU" MonthlyJournal of Institute of IndustrialScience Uni versity of Tokyo, Vol. 34No.8,(1982),pp. 19-22.Watanabe M. and Kawai T.: Simulation ofthe Bending Collapse of Ice Plate Usinga New Discrete Model, J. of the Societyof Naval Architects of Japan, Vol 147,(1980), pp. 306-315 •.Yoshimura N. and Kamesaki K. : TheEstimation of Crack Pattern on Ice bythe New Discrete Model, Proceedings,IAHR International Symposium on Ice,Quebec City (1981), pp. 663-672.DiscussionL. FRANSSON: Even though you did notobserve cracks in your model study, youbased your numerical model on discreteelement where cracks were propagating.Is it possible that the numerical modelis closer to reality than the physicalmodel for full scale global loads onstructures?423

T. SHIBUE: I think it is possible thatthe well-designed numerical model simulatesloads and failure mechanisms betterthan a model test. But, a well-designednumerical model must be equipped withenough functions to simulate and evaluateall of the phenomena occurring in thefull-scale model. We may not evenappreciate some of the phenomena whichoccur in the full-scale model.c. JEBARAJ: I do appreciate the author'snew simulation method which accounts forpropagation of the cracks.My questions are:1. What is the stiffness value assignedfor the spring element which is connectingthe neighboring elements? What isthe difference between the presentnumerical model and the conventionalfinite element method?2. If the stipulated failure criterionis for tensile strength only, how do youaccount for the comprehensive failurewhen the ice breaker hits a multiyearthick ice?3. ,Justify the boundary conditions ofthe ice sheet (three edges fixed) whilecomparing it with the real ice sheetfloating in water which is almost infinitein length. If you are going to fixit, then the radiation boundary effectsare to be included.T. SHIBUE:1. The s tiffnes matrix of a spring isdefined to correlate stress increment andrelative displacement increment of thespring. These relations are expressed asfollows.{flo}where {flo}[K J {flo}/flOX} flayflozstress increment[KJE3(l+V) (lO ]030[ o (l+v)stiffness rna tr1.xlA' IB = distances from center of gravityA,B to element boundaryE Young's modulusV Poisson's ratioX in-plane shear directionY normal pressure directionZ out-plane shear directionThe differences between the present modeland the conventional finite elementmethod are as follows.o All the deformation is representedby the elongation and distortion ofthe springs distributed on theelement boundary in the presentmodel.o Element body is defined as rigid inthe present model. According to thedefinition, the continuity ofdeformation between elements is notattained with this model.0 All the degrees of freedom areconcentrated to the center ofgravity of a rigid element in thepresent model. Total degree offreedom comes much less than that ofthe conventional finite elementmethod.o The cracks may be easily representedby cutting some springs distributedon the element boundary in thepresent model.2. We have not considered such afailure mode in this program. I thinkthat the improvement of the failurecriteria is necesary in the future, andin such a case, the limitation that thefailure occurs only under the tensilestress shall be removed.{M}/Mx} MyMzrelative displacementincrement3. I agree that the boundary conditionsof the ice sheet are very important whenwe try to simulate actual sea ice behaviour.In this case, we set the widthand depth of the ice sheet large enoughto approximate actual behaviour. Because424

the effects of applied loads come to zeroor to uniform at the point far from theloaded edge. The problem is what is thesufficient width and depth for the model.The sufficient length changes accordingto the rigidity of the ice sheet in thedirec tion of length. Here. we used theconcept of the characteristic length toestimate these values. The length of themodel was set more than three times thecharacteristic length here.425

ELASTO·PLASTIC ANALYSIS OF ICE FORCESON CYLINDRICAL STRUCTURESY. TaguchiT. KawasakiS. TozawaS. IshikawaMitsubishi Heavy Industries, Ltd., Nagasaki, JAPANAbstractThe authors attempted to apply theFEM (Finite Element Method) forevaluating the ice force at theindentation of a column model against anice sheet which had been sampled fromthe landfast ice off the east coast ofHokkaido in Japan. The analysis wasperformed on the basis of the twodifferent assumptions. One was that thematerial characteristics of the icefollow the von Mises yield criterion,while the other was that the ice may beregarded as a parabolic Mohr-Coulombmaterial, using plane elements. Planestress elements were used for largestructure-breadth/ice-thickness ratio(D/h of 10), while plane strain elementswere used for smaller D/h of 1.Additionally, an analysis using solidelements was performed to compare withthe result obtained from the methodusing plane elements. The results aresummarized as follows (1) it wasconfirmed that the FEM can be appliedsatisfactorily for estimation of iceforces when a column model is indentedinto the ice sheet, even if planeelements are used and the ice is treatedThis is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17·22, 1987. © The Geophysical Institute,University of Alaska, 1987.as a von Mises material; and (2) forsmaller D/h of 1, the two assumptions asto whether the ice follows von Misesyield criterion or Mohr-Coulomb' slawdid not give a large difference for theload-displacement relation because acompressive stress field was dominantunder the indentation condition.1. IntroductionEstimation of ice forces isimportant to establish design guidelinesfor marine structures to be operated inice-infested waters. In order tocollect and accumulate information onice force evaluation technology, theauthors conducted a series offundamental studies, including fieldtests for ice forces and sea icecharacteristics and indoor ice tanktests using model ice. At the sametime, the authors have been engaged inefforts to further information ontheoretical estimation technology usingthe FEM.Ice forces acting on structures areoften estimated by means of empiricalformulas obtained from field tests orice tank tests. However, these formulasare based on the results of small-scaletests using models of simple shapes,apparently containing such problems as427

deficient estimation accuracy forcomplex-shaped structures. On the otherhand, the FEM is regarded as aneffective estimation method for iceforces because it can take account ofarbitrary structure configuration,elasto-plastic or visco-elastic characteristicsof the ice and plate thicknesseffects of the ice sheet.This paper describes the results ofelasto-plastic analysis using the FEMfor evaluating the ice force at theindentation of a column model againstan ice sheet which had been sampled fromthe landfast ice off the east coast ofHokkaido in Japan (Yamashita et al.1985;Kawasaki et al.1986; Kawasaki etal.1987). The results have been foundto be satisfactory for estimation of iceforces when the column model is indentedinto the ice sheet.characteristics of the ice wereconsidered to follow von Mises yieldcriteria and it was presumed that thetangent modulus after yielding was 1/100of Young's modulus and Poisson's ratiowas 0.3.Fig.3 shows t,he result ofcalculated stress and load-displacementcurve.yco~,... '"'\,~Fx --------- x0Indentor(Circular)ColumnIce Sheet(h=40)co,... '"2. Elasto-Plastic Analysis Usin~ FEM2.1 The case of large D/hThe outline of the indentation testand the analysis model of the ice sheetare shown in Fig.l and Fig.2respectively.yFig. 11,500Indentation Test(mm)Calculations were conducted for theconditions of a column model (D=400mm),with velocity V=2mm/sec and icethickness h=40mm (Case 1). That is, thestructure-breadth/ice-thickness ratio,D/h, was equal to 10. The calculationmodel was made to be symmetric about theX-axis and one half of the model wasanalyzed. Plane stress elements wereused in that analysis. As for theapplication of load, it was decided tosimulate the state in which a rigid bodyis being indented into an ice sheet.For this purpose a constant displacementox in the direction of the X-axiswas applied to all nodes in the contactline of the column and ice sheet. Here,the frictional force between the columnand the ice sheet was neglected. Yieldstress and Young's modulus of the icewere 2.59 MPa and 1132 MPa respectively.These values were obtained from theuniaxial crushing test by usingspecimens from which were sampled fromthe same ice block that the ice sheetwas taken. Here, the material908070Fig. 2Analysis Model~"./'e /'/' .a- h 3.0-l~"'-:;?;~~-~~aIYSiS .,60 tr-' 5 6:f:: ; 40..:E~z: 50 'f" 2.0 t!;;;"" ¥ [[ .-Fx maxexp=45.1 KN... I :M40 ' " 3oJ:f1530 IfI--e-- : ax---,,--- : aeq MISES-0-: Fx1.0 2.0 3.0ax (mm)1.0Fig. 3 load-Displacement Relation428

From this figure, it is seen that thelocal yielding begins from the point Fx=33 kN and the Fx-8x relation begins todiverge from the straight line at thepoint past Increment 3. The maximumforce is considered to be around Fx.max=55 kN, but when the Increment reaches 5or 6, plastic strain becomes as much as3% and it is considered that the actualdeformation of the ice sheet will not becontinued to such an extent. It isestimated that crushing may occur at anintermediate point between Increment 3and 4. In such a case, the result iswell matched with the test value ofFx.max.exp=45.1 kN.distribution pattern of cry changeslargely as the plastic region of thematerial proceeds. The stress crychanges to compressive stress fromtensile stress just in front of thecolumn as the material becomes plastic.-406The enlargement of the plasticregion is shown in Fig. 4. In Figs. 5through 8, the stress distribution isshown.The distributions of crx and cry along theX-axis are shown in Fig.5 and Fig.6respectively. Although there is atendency for the distribution of crx toreach a maximum, with the materialbecoming plastic just in front of thecolumn, there is no great difference inthe distribution pattern. In thesefigures, curves for Inc.=5,6 are shownin dotted lines for reference as theseare considered to be unrealistic. Onthe other hand, it can be seen that theIncrement ox(mm) Fx(KN)34561.051.732.743.0042.950.254.154.6at- 0).:;.2:---:::0'-:-.4--:::0.'="6-""'0.'="8 -71.0( .(m'Fig. 5 Stress Distribution( 0" x along the X -axis)1.51.0-2.0Fig. 4 Spreading of Plastic RegionFig. 6 Stress Distribution( O"y along the X-axis)429

In Fig. 7, the distribution of surfacepressure along the contact line of thecolumn and ice sheet is shown. Thereason why the value becomes large forInc.4 is due to work-hardening with theincrease of plastic strain. The Fig.8shows how circumferential stress, whichcauses radial cracking, is distributedalong the contact surface and it can beseen that the maximum value occurs inthe range of 9=15°-35°. In the fieldtest, it was observed that cracksoccurred in the direction of 9=0° atfirst, then cracks occured around 9=30°in many cases.The results of calculations conducted inthe same way for conditions of D=200mm,V=2mm/sec, and h=41.5mm (Case 2) areshown in Fig.9.4.03.0~2.0:Ez'"300=2001'11.0 2.0 3.0ax (mm)Fig. 9 load· Displacement RelationWhen taking the load value ofFx=23-25 kN for Inc.=4-5, at which pointthe tendency of the plotted line beginsto diverge from the straight line, it isconsidered that the value is almostmatched with the test value ofFx.max.exp = 28.1 kN if the scatteringof ice strength is taken into account,although the value is a little lower.2.2 The case of small D/hFig. 8Fig. 70- '":E0.5Surface Pressure (MPa)Distribution of Surface PressureIncrement No.2(58 (MPa)Distribution of Circumferential Stres!(Elastic Stress)In the previous section, ane1asto-plastic analysis by the FEM forthe case of a large D/h ratio of 10 wasdescribed, assuming that the icecharacteristics follow von Misesyielding condition. In this section, anattempt is made to apply parabolicMohr-Coulomb's law, which is used in thefield of soil and rock-like materials,to the elasto-plastic analysis of an icesheet, with the aim of assessing theeffects due to differences in theyielding condition. For this purpose,column model conditions of D=200mm,veloci ty V=2mm/ sec and ice thicknessh=200mm (case of D/h=1.0 : Case 3) weretaken for the study. The analysis modelwas prepared in a similar way to that inthe previous section for mesh division.For finite elements, plane strainelements were used. Although there isno experimental data for tensilestrength of ice, it was established byreferring to the results of Braziliantest. Here, tensile strength, compressivestrength and Young's modulus of the430

ice were 0.27 MPa, 2.27 MPa and 1132 MParespectively.The yield surface is shown inFig.10 and the calculated result for theload-displacement relation is shown inFig.11.In this figure, the result obtained byvon Mises law is also shown forcomparison. Local yielding begins justbefore Inc.=3 by von Mises law, and justbefore Inc.=2 by Mohr-Coulomb's law. Inboth cases, the tendency of the curvebegins to diverge from the straight lineat the neighboring point of Inc. =3.However, no significant difference is--Mises_.- Parabolic Mohr·Coulomb~.,\1.01\2.0 -1.0 o{(J = «(Jl+(Jy) 12 /r2 = «(Jl-(Jy)2/4+riyFig. 10YI250511. 0'- ~--xO=200~ h=200 -~i200 i50~ _(mm)150r (MPa)./-1.0Plane Strain Yield Surface1.0 (J (MPa)INC 7.,,?-,/seen between thes~ two curves. Thereason for this is believed to be thatthe different choice of a yield functiondoes not appear to be as large in thecase where the compressive stress fieldis dominant as in the indentation test.The maximum load is rising with theincrease of displacement and very largeplastic strains; as much as 2% occuredat the point of Inc. =7, for example.But it is considered that the ice sheetwill not continue deformation until thispoint. Crushing of the ice sheet willprobably occur at the neighboring pointof Inc.=3-4 where the tendency of thecurve begins to diverge from a straightline in the same way as Case 1 or 2.1.0. "-0.5 ?piJ , ,. "-1.0 ~9ij , .'.,'.'':9-1.5 ~!-20. :: ~.-2.5 :! I 1.015" 0.5I 9/0.4 0.5X (m)0.6 0.7Parabolic Mohr-Coulomb0.7100-0-- MJses---,6--- Parabolic MoII'·Co~ombFig. 1110 20 30ii, (mm)load-Displacement Relation-2.5Fig. 12 Stress Distribution (6y along the X-axis)431

In Fig.12, the distribution of cryalong the X-axis is shown. In thisfigure it is seen that the distributionpattern changes remarkably with theplasticizing of material just in frontof the column, the same as in thecalculated result of Case 1 shown inFig.G. However, the point to be noticedhere is the difference of von Mises lawand Mohr-Coulomb' s law in thedistribution of cry. In this case wherecrT=O.27 MPa is assumed as the tensilestrength, there is a possibility thatcracks may occur on the X-axis in theelastic stress condition, according tovon Mises law, but it can be seen thatthe distribution of cry reaches a maximumat the established value of the tensilestrength by Mohr-Coulomb' slaw. InFig.13, which shows the distribution ofsurface pressure along the contact lineof column and ice sheet, no greatdifference between the results by vonMises law and by Mohr-Coulomb's law isseen.3. Results and Discussion(1) Strain rateIce strength depends upon strainrate and the strain rate of acylindrical structures at indentation isnormally expressed approximately by thefollowing formula in many cases, but itsproof is not necessarily clear.EvV2D or 4DHere, the strain rate against maximumstrain Ex.max in front of a column iscalculated from the result of Case 1.When strain rate is expressed asVE =-­cx"D5.0Misesand a is determined from the calculatedresult of Fig.14,cx = 0.96 ;; 1.0is obtained.Surface Pressure~EW10-1/'10-210- 3 15.0Parabolic Mohr-Coulomb10-'0.1 0.5 1.0 5 10yax (mm)Fig. 13a 2.0 3.0 4.0 5.0Surface Pressure (MPa)Distribution of Surface PressureFig. 14 1 ex. max 1- Displacement Relation(CASE 1)432

That means that the strain rate againstmaximum strain in front of a columnbecomes larger than the value obtainedfrom the former formula. It isnecessary to verify the formula by thecalculation using a more preciserelationship between cr and e in thefuture.(2) Effect of struct ure-breadth/ icethicknessratio = D/hIn Fig.lS, a non-dimensionalexpression of Load-displacement relationfor Case 2 and 3 is shown. Here F/Dhcrcis called the indentation coefficientand is normally considered as a functionof D/h. According to the plastic limitanalysis undertaken by Ralston (1978),the value of the indentation coefficientdecreases with the increases of D/h andit approaches a constant value aroundD/h=2-3. When taking both limits ofD/h=O and 00, these correspond to thestates of plane strain and plane stressrespectively. The change of indentationcoefficient by D/h is considered toappear as the difference of increasingload after yielding in the calculationsunder assumption of each plane strainand plane stress which are shown inFig.1S.(3) Plate thickness effect of the icesheetTo this point, all calculationshave been conducted using planeelements, but in the case where thevalue of D/h is small, it is supposedthat the plate thickness effect of theice sheet can not be neglected. Thus,we examined the extent of the platethickness effect by conductingthree-dimensional analysis (elasticanalysis) using solid elements. Case 3was adopted as the object for analysis.In Fig.16, the FEM mesh divisionfor the calculation model of the icesheet is shown.As for the load, a forced displacementof Ox=O.1Smm, which corresponds toInc.=1 when plane elements were used forthe calculations, was applied to allnodes in the plane which touches thecolumn and the ice sheet_ The deformedstate is shown in Fig.17 and it can beseen that the ice sheet is stretchedupward and downward in front of the201.5_---0--------..&-Fig. 16FEM Mesh Divisions--0- Plane Stram Il( Mlses)----.0.---- Plane Stress2~MISIS)1) : 0=200mm, h=200mm2) . D=200mm, h=41 Smm(a) X-V PlaneFig. 150.5 10 lSX10·21$.10Comparison of the load-Displacement Curve(b) X-Z PlaneFig. 17Deformation of Ice Sheet433

column due to Poisson's effect. InFig.1S, the stress distribution alongthe X-axis is shown, as well as theresult of a two-dimensional analysis(crx, cry) which used plane-strainelements. Although the results of thestress distribution pattern using two-orthree-dimensional analysis are not verydifferent, it can be seen that the valueof crx by three-dimensional analysis isslightly smaller than that bytwo-dimensional analysis at the pointadjoining the contact surface, butconversely cry is somewhat larger. Asfor crz, it becomes a compressive stressfield in front of the column. It isbelieved that this arises because theice sheet is restrained by distantelements when it is expanding upward anddownward due to Poisson's effect.If the ratio of D/h is about 1, asdescribed here, the plate thicknesseffect is not so remarkable as far aselastic stress is concerned. It issupposed that the effect may appear withthe plasticizing of material as shown in(a) X-Axis, (Jx1.0o l~~tr::;=::::::=:::~~~1.1.!5 X (m)(b) X-Axis, (J y~c.. '"::;:\5'--0- Middle Surface of Ice Layer} Solid Analysis----0---- Top Surface of Ice Layer_.-

(iii) Calculation method which takesthe visco-elastic effect intoaccount.ReferenceKawasaki, T., et al., "Indentation Testsof Laboratory and Field Ice Sheets",POLAR TECH '86, Helsinki, Finland, Oct.,Vol.2, 1986, pp.712-724.Kawasaki, T., et al., "Study of IceForces for Offshore Structures",Mitsubishi Technical Bulletin No.174,Jan. 1987.Ralston, T.D., "An Analysis of Ice SheetIndentation", 5th InternationalAssociation for Hydraulic Research(IAHR), Lulia, Sweden, Aug., Part 1,1978, pp.13-31.Yamashi ta, M., et al., "Model Test andAnalytical Simulation on FractureMechanism of Ice", 8th Internat ionalConference on Port and Ocean Engineeringunder Arctic Conditions(POAC'85) ,Narssarssuaq, Greenland, Sept., Vol. 1,19B5,pp.195-204435

STRUCTURAL ARRANGEMENT OF PRODUCTION PLATFORMSACCORDING TO THE ICE-INDUCED VIBRATION ANALYSISMeng Zhao-yingWang Ling-yuTianjin University, Tianjin, CHINAAbstractCompliant structures (jacket, monoormulti-legged framed structures) areconsidered to be appropriate for thoseareas with first-year ice. The att ractionsto designers of these structuresare the high transparency to waves andcurrent as well as their flexibility forsoil conditions. On the other hand,problems may emerge due to the highsensitivity of these structures toice-induced vibrations.This paper presents the principlesof structural arrangement according tothe results of ice-induced vibrationanalysis, which have been employed in theconcept design of a production platformin the Bohai Gulf. The experienceindicates that there are a number offactors which can have an influence onthe vibration. These factors are: thediameter and stiffness of the structure(both the global stiffness and itsdistribution along the height) , thevelocity and thickness of ice, and so on.It is also indicated that through adeliberate adjustment among thoseThis is a reviewed and edited version of a paper submittedto the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987. © The Geophysical Institute,University of Alaska, 1987.factors, the ice-induced vibration couldbe suppressed.IntroductionIn accompaniment with the explorationof offshore petroleum resources,there are more and more offshore structuresbeing built in cold regions.Considering the factors of economy andthe soft bottom conditions, flexiblestructures are often selected (includingjacket and other framed structures). Asice sheets move and crush against thestructures, significant vibrations mayoccur, and the structures may be damaged.There already have been such examples inthe Bohai Gulf, where two jacket platformshave been totally destroyed byice-induced vibration.When a production platform isdesigned for such an environmental condition,the designers have to be concernedwith the action of ice loads. It is notonly because of the large ice forceitself, which can be three to five timesas large as the wave loads, but alsobecause of the obvious dynamic effect ofthe ice even in shallow water. Throughanalysing the ice-induced self-excitedvibration process for three differentstructures, the principles of structuralarrangement and some typical reasonable437

structural patterns are offered.Analysis of Self-excited OscillationProcessIce-induced self-excited oscillationis due to specific mechanical propertiesof ice, especially the nature of icecrushing strength which varies withstrain rate. In a certain range ofstrain rate, ice compressive strengthdecreases with increasing strain rate.This implies a negative damping effect inthe vibration process, so that the energyof the system gradually accumulates.This causes the structure to be unstableunder a slight disturbance. The accumulationof stored energy in the systemleads to the amplitude of vibrationgradually increasing, finally up to astable limit state.In a vibration analysis, the dynamicequation of equilibrium of the structureis written as follows:[M] {X} + [C] {)O + [K] {X} {F} (0Where [M] , [C] and [K] are the massdamping and stiffness matrices of thestructure; {X} is the vector of structuraldisplacement response: {F} is thevector of loading. "." denotes differentiationwith respect to time. The iceforce is calculated by the followingformula:F = Imk a DHc(2)here 0 is the uniaxial compressivestrengtff of ice; it varies with strainrate (or loading rate). The strength vs.loading rate curve is presented in Fig.1, which was provided by Dalian EnvironmentalProtection Institute in 1985(China and America Cooperation Report,1985). The terms I, m and k are theindentation factor, shape coefficient,and contact coefficient, respectively.Terms D and H are ice thickness anddiameter of the column.For the purpose of numerical analysis,the curve is linearized in severalparts and each part can be described by alinear equation:logo-.(kg/em')30 -zo r-----)( .--14-10--/4../,;0~~'"r----------- ~-r---/~xll~)C "".""'-J("'. x;

0cj = a j + Sj OJ (3)where j is the number of each piecewiseline, a and 8 are respectively the ordinateon the 0c-axis and the slope of eachpiecewise l~ne. The piecewise lineidealization can be seen in Fig. 2.On the surface of the column (r=D/2)the loading rate equation can be deducedby elastic theory written as follows:~ = 8Imk ° (V - X)/Dc 0(4)here V is the ice sheet movement velocity.0 By substitution of Equation (3)and (4) into (2), it can be deduced that:F = ImkDH [a.+a.8./(nD/8Imk(V - X)- 8J.)]J J J 0(5)It is adequate to calculate the ice forcewhen the structure is in contact with theice sheet, or the force is equal to "0".Because of the multi-degree-offreedomproblem and the strong nonlinearice force, a numerical method is usuallyused. In this paper the fourth-orderRunge-Kutta Method is used. The contactfactor K=0.25, and the modal dampingratio of the structure ~ = 0.03 are usedin the analysis.Analytical Models of the PlatformsIn order to see the influence of thestructural arrangement on the selfexcitedvibration, three different typesof the structures are analysed separately(see Fig. 3 and 4). They are simplifiedinto the space frames with spring elementsdescribing the soil-structureinteractions (see Fig. 5 (a) - (c». Thestiffness of each spring element isobtained from the soil-structure interactionanalysis according to the soilbehaviour at different depths. Theircalculation results are presented in theTable 1. The first five natural frequenciesof the three platforms are listed inTable 2.Computer Analysis ResultsIn analysing the ice-induced vibrationof the s truc tures, the typicalparameters are used according to theenvironmental conditions in the BohaiGulf (see Table 3).Some of the results are given inFigs. 6-8. The self-excited vibration ofthe jacket structure occurs. The amplitudeand the maximum deflection of No. 53node in the X-axis direction are 6.3 cmand 7.96 cm respectively under 70 cm ice(J. (kg/ CII')30.DU-~~--4--~--~--~!O--~!Z--~!4---(~6---1~8----(kg/cm' . s)Figure 2.Linearized ° - ° curve.c439

I--------------~Figure 3.platform.Mono-legged well-protectedFigure 4.Four-legged production platform.3.\ I--------~/ // 12 11I H" .._- I ~b ':::~',-.- .i'l27 ... _-] I ..~ :;:; z :::..l](.15a. The mono-legged platform.5b. The four-legged platform.Figure 5.Analysis models of the three platforms.440

Figure Sc.The jacket platform.Table 1 Value of stiffness factorsnumber direction the mono- the four- the jacket platformlegged legged (the stiffness factor ofplatform platf6rm the pile head)I vetical O.80SxlO S l.07SxlO S k11=S.2SxlO S (kg/ern)(kg/cm) (kg/ern)I (low end) horizontal 6.638xlO 4 8.8SlxlO 4 k22=k33=1.16lxlO S(kg/crn) (kg/cm) (kg/cm)rotatory 4.836xlO 9 1.146xlO lO k44=7.SSxlO 9 {kg.cm/rad)(kg.crn/rad) (kg.cm/rad)II horizontal 4.426xlO 4 4.4Z6xlO 4kSS=k66=3.93xlO lO(kg/cm)( kg/em)(kg.ern/rad)III horizontal 2.213xlO 4 k26=k62=-S.29xlO 7(kg/rad)(kg/crn)k3S=kS3=S.29xlO 7(kg/rad)441

Table. 2 Natural frequencies (HZ)modal number 1 2 3 4 5-the jacket platform 0.7163 0.7511 1. 346 3.303 3.356the monoleggedplatform0.4487 0.4924 0.7317 1. 05 1.105the four-legged 0.5769 0.6723 1. 27 1.381 1. 453platfonnTable. 3platfonn pattern jacket platform the monolegged the four-leggedplatformplatformice thickness(cm) 70,120 70, 120 70,120ice velocity(cm/s) 30,45,70 30,45,60 30,50,70ice moving angle(crossing with X-axis)0°, 45° 0° 0°, 45°column diameter(cm) 165.1 600 400mode number for 4 1 4ice action442

thickness, 45 cm/s ice velocity and 45°ice moving angle. When ice thickness is120 cm, the amplitude and the maximumdeflection are up to 32 cm and 24 cm andthe dynamic magnification factor is equalto "3.8" (see Fig. 6). The self-excitedvibration of the monoleg platform doesnot occur (Fig. 7). The self-excitedvibration of the four-legged platformoccurs under 120 cm ice thickness, 45cm/s ice velocity and 45° ice movingangle. The amplitude and the maximumdeflection are 9.7 cm and 21.7 cm respectively,and the dynamic magnificationfactor is equal to "4.0" (see Fig. 8).DiscussionThe analytical results given aboveindicate that the dynamic effect of iceforces is significant even in shallowwater. The jacket platform and fourleggedplatform show serious vibrationsunder 120 cm ice thickness and the appropriateice velocities according to theself-excited oscillation theory. Also,their dynamic magnification factors areapproximately equal to "4.0". The selfexcitedvibration depends on a number offactors such as the 0 -0, curve of ice,the ice velocity and c-lcJ thickness, aswell as the dynamic behaviour of thestructure. While the ice characteristicswhich are dependent on the environmentalconditions can not be modified, theself-excited vibration can be suppressedonly through changing the value anddistribution of the mass stiffness anddamping of the structure. The followingremarks can then be made in this respect:1) The principle of "fewer andthicker column" structures should beutilized in the Bohai Gulf so far aspossible. This is not only for reducingthe ice jamming condition, but also forreduction of the relative sensitivity toself-excited vibration. The reasons forthe improvements are: first, the modalvalue at the ice action point is small;second, the negative slope on the curvedecreases and the particular velocitycorresponding to the start-point of thenegative slope section increases, and itis less likely that the actual ice velocitywill reach a critical value.2) In the case of constraints onconstruction costs, structural stiffnessshould be increased in so far as possible.Generally speaking, the constructioncost increases rapidly when thestiffness increases, but because of theshallow water in the Bohai Sea, the costincrease is not great. In addition, ifthe deeply-embedded structure describedabove is used, it is not necessary todrive long piles such as when a jacketstructure is used.3) The upper portion of the structureshould be as light as possiblethrough adjusting the mass distribution.The stiffnesses of the monoleg and thefour-legged structures are quite closeeach other, but the upper structure ofthe monoleg s true ture occupies lessproportion of the total mass than thefour-legged structure, so it is difficultto induce self-excited vibration in theformer case.ReferencesBlenkarn, K.A.analysis of icestructures, OTC.Bohai SeaChina andReport.1970. Measurement andforces on Cook InletIce Design Criteria. 1985.America Combination ResearchJohansson, P. I. 1981. Ice-inducedvibration of fixed offshore structures,Report No. 81-06/1, Norwegian ResearchProj ect.Maattanen, M. 1977. Stability of selfexcitedice-induced structural vibration.POAC-77 •Maattanen, M. 1978. On conditions forthe rise of self-excited ice-inducedautonomous oscillations in slender marinepile structures. Research Report No. 25.Michel, B.Presses deChapter 4.1978. Icel'University,mechanics, LesLaval, Quebec,Neill, C.R. 1976. Dynamic forces onpiers and piles, An assessment of designguidelines in the light of recentresearch, Canadian Journal of CivilEngineering, 3(2).443

F' (T)V(cm/s)6040 \20o-20- 40-601\ '\ \i41I\ \ ~III,I\ \\1Ifo 2 4 0 8 10 12 14 ]6 18 T (s)(a)V (cm/s)(b)X (em)24! ! \ ,\ AI1812\-IZ0\12 4 6 8 o 12 4 1 J 20 T (s)\ ~ \.(c)(d)(a) force plot(b) veloclty plot(c) displacement plot(d) phase plane plotH = 120cmv =45cm/sFigure 6.Ice-induced vibration process plots of the jacket platform.444

X(cm)~.s.o4.04.03.03.02.02.01.0 ~~---r--~--~~-----------o 2 4 6 8 10 12 14 TeS)1.0 L.. ______________________ _o 2 4 6 8 10 12 T(s)(a) H=70cm(e) H=70cmX(c .. )5.Xlcm)8.04.07.CJ3.02.0;'.0__________o 2 11 6 8 10 12 14 Tl s)1.0~-------__ -(b) H=70cm Vo=30em/s4.0~----------________ _o 2 4 6 8 10 12 T(s)(d) H=120cmFigure 7. Ice-induced vibration process plots of the mono-leggedprotected platform.445

x ( em)21181512\: 1I 3 6 91l 15 18 ZI 24 21 T (s)(a)F (TGilU4003[10200r j 5 9 12 lS 1821 2427 T (s)(b)v (rln/s)ISJ 2~ III3 I I III- .'" /1 II, n1oJ.rI II I1 I I i °II ~rl ~1S;~ IJ Ul'Ii ~'IIT (';v (em / '0)J 831 S-3-6-91 2 15 18(c)(d)(a) displacement plot(c) velocity plot(b) f crce plot(d) phase plane plotH=120cmV=45cm/sFigure 8. Ice-induced vibration process plots of the four-leggedproduction platform.446

Sundararajan, C. and Reddy, D. V. 1973.Stochastic analysis of ice-structureinteraction, POAC-73.Xu, J.Z. 1986. Ice-induced structurevibration, The Ocean Engineering, China,4(2): 42-47.Zhang, F.G. The signi!lCanCe of sea iceforces in the design of ocean engineeringstructures in the Bohai Sea, The OceanEngineering, China, 5(2): 59-65.447

VERIFICATION TESTS OF THE SURFACE INTEGRALMETHOD FOR CALCULATING STRUCTURAL ICE LOADSJerome B. JohnsonDevinder S. SodhiU. S. Army Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire, USAAbstractExperiments were conducted to determinethe accuracy of calculating iceloads on offshore structures using icestress measurements and a surface integralmethod. Biaxially-sensitive stresssensors were installed near an ice sheetedge and a flat plate instrumented indentorwas pushed against the ice edge tosimulate a distributed load on the boundaryof a semi-infinite plate. Two experimentswere conducted. The first determinedthe agreement between stress measurementsand calculated results for thecorresponding analytic solution and examinedthe accuracy of the surface integralmethod. The second examined the influenceof cracks in the ice sheet on theaccuracy of the surface integral method.The measured ice stresses were of thesame order but less than those calculatedusing theory. The calculated indentorloads using the plane surface integrationwere within 8 to 30% of the measuredloads. Calculated loads using a cylindricalintegration surface were onlywithin 40 to 50% of the measured loadsdue to stress sensor resolution limitations.The surface integral method isThis is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987.a viable way to calculate structural iceloads using in-situ stress measurements.Accuracy of the load calculations is limitedby the fidelity of representing thestress along the surface of the integrationusing widely-spaced stress measurements.IntroductionBottom-founded marine structuresin cold offshore regions are often usedfor resource exploration and as navigationaids. These structures must be designedto withstand the lateral forcesgenerated by moving ice. Estimates ofstructural ice loads are obtained fromsmall-ecale model experiments, mathematicalmodels and full-scale field measurements.The full-scale field measurementsare needed to calibrate and verify theresults of the small-scale experimentsand mathematical models. Full-scalefield measurements are also used as asafety measure to monitor stresses whilethe structure is occupied to ensure thatice loads do not exceed allowable limits.Full-scale field measurements ofstructural ice loads can be obtained byeither instrumenting the structure directlyor measuring the stresses in theice around the structure and calculatingthe resulting load. Johnson (1983) de-449

scribed a method that used stress measurementsobtained from stress sensorsto an ice sheet or from sensors attachedto a structure and Euler and Cauchy'sstress principle to calculate structuralice loads (the surface integral method).The surface integral method has been appliedby Johnson et al. (985) and BritishPetroleum, Ltd. (Sanderson, 1984)to estimate structural ice loads. Touse the surface integral method, in principle,requires that the stress distributionaround a structure be representedas a continuous function. In practice,stress measurements are made only at adiscrete number of points around a structureand the stress distribution aroundthe structure can only be approximated.Cracks in the ice sheet, ice rubble andthe complex mechanical properties of salineice can add to the uncertainty ofdetermining the stress distributionaround a structure.A small-scale experiment to examinethe difficulties and reliability of usingthe surface integral method was performedat CRREL's ice test basin. This paperoutlines the surface integral method anddescribes the methods and results of theexperiments and the limitations of usingthe surface integral method to calculatestructural ice loads.The Surface Integral MethodThe basic assumption of the surfaceintegral method is that only the surfaceforces or stresses due to the ice actingagainst a structure are important (bodyforces can be neglected). The surfaceforce acting on an imaginary surface thatencloses a structure is the stress Vector.The total force acting on the regionenclosed by the imaginary surfacecan be calculated by integrating thestress vector along the surface.v0) Is T dsvT VjOij'ei = (vxox+vyoxy)i+(v xOxy+vyoy}jvwhere T is the stress vector acting onthe surface element ds whose outer normalvector is ~ (Figure 1). The unit normalvector~ alo~g the x a~d y axes are eiwhere el = ~ and ~2 = J. The stress ten-sor 0ij represents the stresses in theplane of an ice sheet. Out-of-planestresses 0xz' 0yz and 0z are assumedto be negligible. The stress tensor isdetermined from measurements at discretelocations on the structure or in the icearound the structure. Equation 1 is thenintegrated numerically or the discretestress measurements are fitted to a polynomialalong the surface of interest andthe polynomial is integrated to determinethe structural ice loads.The form of the surface integraldepends on the stress distribution andthe surface of integration that is used.The most common surfaces of integrationare cylindrical and connected plane surfacesthat enclose the structure. In Figure1, the experimental configurationused in our tests and the two surfacesof integration used in our calculationsare shown. The surface integral for thecylindrical surface is given byTf/2F = rt I (Oy cos6+o xy sin6)jd6-Tf /2(2)Tf /22rt I (Oy cos6+o xy sin6)jdtJaand that for the plane surface is givenby(3)00+AF t I 0y j dx 2t I00y j dxwhere t is the ice thickness and r isthe radius of the cylindrical surface.The form of Equations 2 and 3 is due tothe loading symmetry with respect to thechosen integration surfaces that was assumedin this study.Experimental MethodsThe surface integral evaluationtests were conducted in the CRREL icetest basin. Eleven biaxial cylindricalstress sensors were installed in an 8.1-cm-thick smooth, uniform ice sheet thatwas free-floating and had been cut freealong two sides to ensure stress-freeconditions at the lateral edges. A thirdedge was cut in the ice sheet perpendicularto the lateral edges against whicha 50-cm-wide instrumented indentor waspushed to load the ice. The installationpattern for the stress sensors was de-450

signed to allow for testing both planeand cylindrical surfaces of integration.Biaxial stresses were determined by measuringthe diametral strains of a steelcylinder using tensioned wires. The wiresare plucked electromagnetically and theperiod of wire vibrations measured inmicroseconds. The change in vibrationperiod is directly related to the diametralstrain of the sensor. The stresssensor has a nominal resolution of about0.7 kPa and has an extensive history oftesting and use (Cox and Johnson, 1987;Johnson et a1., 1985; Frederking et a1.,1984; Cox, 1984).Two loading experiments were conductedin which the ice sheet was loadedto a nominally-constant load while multiplescans of the stress sensors and indentorload were taken (12 and 11 scansrespectively for loading Test 1 and Test2). Test 1 was conducted on a crack-freeice sheet and Test 2 was conducted onan ice sheet into which two simulatedcracks had been inserted (saw-cuts).The stress sensors were scanned aftereach of the two loading tests to establishthe stability of the sensors andto determine the appropriate zero stressreadings (11 and 8 scans respectivelyafter loading Test 1 and Test 2). A nominalload of 8.9 kN was applied to theice sheet during Test 1 and a load of8 kN was applied during Test 2. The loadingconfiguration was not capable ofholding a constant load and the loadmagnitude gradually decreased duringeach tes t. For Tes t I, the loaddecreased from 9.9 kN to 8.6 kN duringthe 2l-minute loading experiment. Theload magnitude during Test 2 decreasedfrom 8.6 kN to 7.6 kN over 20 minutes.The load magnitude reductions duringthe experiment were linearly relatedto the vibrating wire periods and thisrelationship was used to determine theappropriate sensor output at a givenload level.The zero-load stress sensor scanswere used to determine the resolutionand repeatability limits of the stresssensors. The vibrating wire period forthe three wires in each sensor was averagedand the standard deviation determined.Vibrating wires with standarddeviations in excess of 0.1 microsecondwere considered to be too unstable tobe reliable for this experiment. Unfortunatelythe output for all three wiresin a gauge is needed to resolve the biaxialstress field, and instability inone wire can significantly reduce theaccuracy of the stress calculation. Thestability test indicated that sensors610, 632, 621 and 609 had severely unstablewires and could not be used in theanalysis. Sensor 634 had two wires thatmalfunctioned; however, an attempt wasmade to use this gauge since the onegood wire was oriented collinear to they axis (the axis of maximum loading).The reliability and resolution of thebiaxial sensor have been improved sincethe experiment by installing redundantwires in the gauge to allow for the possibilityof unstable wires and also byimproving the electronic instrumentation.The rema1n1ng functioning sensors wereused to determine the stresses in theice sheet and to calculate the indentorload using the surface integral method.The calculated indentor load was comparedto the measured indentor load (as determinedby the load cell attached to theindentor) to assess the viability ofusing the surface integral method tocalculate structural ice loads.Results and Data AnalysisThe deployment pattern of the stresssensors was designed to test both cylindricaland plane surfaces of integrationand to provide information on the symmetryof the stress distribution in theice sheet. We were not able to reliablyassess the symmetry of the stress distributiondue to the lack of stress measurementsfrom sensors 609, 621, 632 and610. An additional difficulty in theexperiments resulted from the inabilityto greatly exceed the 8.9-kN load limiton the indentor, resulting in ice sheetstress magnitudes near the low end ofthe operating range of the stress sensors.The stresses at the stress sensorlocations are reported as a dimensionlessratio of the ice stress at the positionof interest to the stress acting on theindentor surface. A simple analyticaldescription of indentation loading wasalso used to provide reference stressratios at the sensor locations (Obert451

analytical des­and Duvall, 1967). Thecription is given by(4)Ox 1f 11~ 1f 11(B+sinB cosa) ,(B-sinB cosa) ,~ 1f 11 sinB sina,ex= e~+92.The definitions of 91 and 92 are shownin Figure 1. The stresses in the icesheet are ox' 0 y ' Oxy and the stress actingon the indentor is f (fm for the averageindentor stress determined fromthe total measured load on the indentordivided by the indentor area, and Gym'0xm and 0xym for ice stress measurements).In Table 1, the stress ratios calculatedfrom the analytical model and fromthe measured results of Test 1 and Test2 are given. The ratios of measured icesheet stress to measured indentor stressesare shown for only the 0ym/fm and0xym/fm terms as the 0xm!fm term doesnot contribute to the stress tensor forthe surfaces used in this experiment(Equations 2 and 3). Sensors 619, 620and 601 exhibit large uncertainties dueto the low ice sheet stress at these sensorlocations. The remaining sensorshave stress ratios lower than those predictedby the analytical model, with theexception of the values from sensor 604(Test 2), which occurred after saw cutswere placed in the ice sheet (Fig. 1).The loading configuration used in Test1 is most similar to the conditions usedin the analytical solution. The resultsof Test 2, with the saw cuts in the icesheet, were, as expected, completely dissimilarfrom the analytical results.Ice SheetFigure 1. Plan view of the stress sensor and loading configuration for Test 1 and Test2 (saw cuts to simulate cracks were put in place after Test 1). The geometric definitionof 9 1 and 92 used with the analytical model is also shown.452

Table 1.Dimensionless Ice Sheet Stress.Location(cm)Analx:ticala axyI-Sensor x y r-604 28.6 41 0.390 0.198619 80.1 41 0.041 0.071620 115 41 0.011 0.030601 88.3 17.2 0.003 0.015616 46.3 77 .2 0.229 0.124623 -46.3 77.2 0.229 -0.124634 0 91.5 0.332 0.0Test Test 2a a a axymr-m r;;- ~. ~ m0.302 0.294 0.490 0.379-0.004 0.043 -0.045 0.0130.020 0.009 0.114 0.0450.0 0.023 -0.461 0.0090.098 0.055 0.108 0.0600.181 -0.081 0.181 -0.0850.187 0.0 0.193 0.0Calculated Stress Ratios at x-O for0 41 0.632Method 2 0 41Method 3 0 41the Plane Integration Surface0.489 0.7920.472 0.787The surface integral method was appliedto the stress sensor measurementsalong the cylindrical and plane surfacesshown in Figure 1 to calculate the forceat the indentor. Four different methodswere used to estimate the indentor loadusing the stress sensor measurementsalong the plane surface. The first methodused in the assumption was that thestress in the ice sheet could bedescribed by the analytical model anddid not use the stress integral method.The measured stress magnitude at sensor604 was used to calculate the stress actingon the indentor and the ratio of calculatedindentor force to the measuredindentor force:The second and third methods wereused to estimate the stress value at x= 0 on the plane surface of integrationsince no stress sensor had been installedat that location, yet it was the locationof the maximum ice stress. Method 2 estimatedthe stress at x = 0 on the planesurface by assuming that the stress risefrom sensor 604 to x = 0 was equal tothat predicted by the analytical solutionand is given by(6)(5)f = aym/0.39andMethod 3 use4 the stress gradient betweensensors 604 and 619 along with the measuredstress at sensor 604 to estimatethe stress at x = 0:where aym is the measured ice stress,0.39 is the stress ratio at sensor 604determined from the analytical model,f is the calculated indentor stress, Fcis the calculated indentor force magnitude.Fm is the measured indentor forcemagnitude, and fm is the indentor stress(measured average indentor load dividedby the indentor area).(7)+ aym(604)-aym(6l9)llx604,6l9. llx604,Owhere llx60lf,6l9 and llx604,O are respectivelythe distances between sensors 604and 610 and sensor 604 and the x = 0 positionalong the plane surface.453

Method 4 was applied only to Test2 (the other methods were applied to bothTest 1 and Test 2) and utilized the knowledgethat the saw cuts in the ice sheeteffectively isolated the load betweenthe saw cuts. The stress magnitude determinedfrom sensor 604 was assumed to beconstant between x = 0 and the positionwhere the saw cut intersected the planesurface at y = 41 cm, x = 55.4 cm.Methods 2 and 3 were integratedalong the plane surface using trapezoidalintegration and by fitting the measuredstresses with a polynomial and integratingthe polynomial. The form of the trapezoidaland polynomial stress approximationsalong the plane surface for methods2 and 3 and the constant stress approximationfor method 4 and integration limitsfor methods 2, 3 and 4 are shown inFigure 2. The stress magnitudes for eachmethod are given in Table 1. The loadcalculation along the cylindrical surfaceused the stress measurements at sensors634, 616, 619 and 601 and trapezoidalintegration to estimate the indentorforce ratio. The results of the forcecalculations for Test 1 and Test 2 areshown in Table 2. An indentor force ratioof Fc/Fm = 1 would result if thestress distribution were completely andaccurately described along the surfaceof integration. Our force ratio calcula-IMethod 4I---- Polynomial Fit--- Trapezoid- - Constant Stress3o 150X PosItion (em)Figure 2. The form of the stress distributionfor the trapezoid integration andpolynomial fit and limits of integrationused along the plane integration surfacefor methods 2, 3 and 4. The limits ofintegration are shown by a small arrowon the x axis.tions are within 16 to 30 percent of theexpected result for all methods used onthe plane surface. Method 4 applied toTest 2 is the best result giving a stressratio value that is within 8 percent ofthe expected force ratio result. Method4 should give the best result since itmost accurately represents the physicalloading configuration and the stress magnitudeat sensor 604 is well above thesensor's resolution.Table 2.Surface Integral Calculations of Indentor Force.Fc mMethod Test 1 Test 2 RemarksPlane SurfaceIF1 0.77 1.252 0.77 1.24 Trapezoid integration0.71 1.10 Polynomial fit3 0.77 1.29 Trapezoid integration0.73 1.16 Polynomia 1 fi t4 1.08Cylindrical Surface0.47 0.42 Trapezoid integrationFc - Calculated indentor force magnitude using the surface integralmethod.Fm - Measured indentor force magnitude using the instrumentedindentor plate.454

The results of method 4 imply thatsensor 604 was responding accurately tostresses in the ice sheet. This furtherimplies, using the Test 1 results, thatthe ice stresses may not have been welldescribed by the analytical model andthe stress at x = 0 was underestimated,resulting in the low calculated valuesfor force ratio. The addition of a stresssensor at the x = 0 location would givea better estimate of the stress distributionalong the plane surface.Calculated stress ratio magnitudesfor Test 2 were larger than for Test 1using methods I, 2 and 3 (Table 2). Thisis not surprising since all of the surfaceintegrations except for method 4assumed that the stress distributionalong the plane surface was continuousand smoothly-varying. The polynomial fitused in method 2 gave a good agreementbetween the calculated and measured indentorforce. This was fortuitous becauseof the way that the polynomial fit themeasured values and does not imply thatpolynomial fitting is an inherently-accuratemethod (Figure 2), although thesmoothly-varying magnitude of a polynomialfit can in some situations quiteaccurately represent the actual stressvariations.The force ratio calculations usingthe cylindrical surface were only 40 to50 percent of the expected value (Table2). This gross underestimation was probablydue to the ice sheet stress magnitudesbeing at the low end of the operatingrange of the stress sensors used inthe experiment and the uncertainty associatedwith using sensor 634, which hadtwo malfunctioning wires. There is noconceptual reason why a cylindrical surfaceshould be less accurate than a planesurface.Conclusions and RecommendationsThe results of applying the surfaceintegral method to the plane surface havedemonstrated, within the uncertainty limitsof the experiment, the validity ofusing stress sensor measurements and thesurface integral method to estimatestructural ice loads. The experimentalresults also demonstrate the importanceof making stress measurements in the regionof maximum ice stress as this regionis the primary load-bearing zone. Theindentor load acting on the ice sheetas estimated using the surface integralmethod and stress measurements along aplane surface was within 16 to 30 percentof the measured indentor load. The calculatedload was wi thin 8 percent of themeasured load when the physical loadingconfiguration was known and taken intoaccount (method 4). The force ratio calculationsusing the cylindrical surfacewere only 40 to 50 percent of the expectedvalue. This underestimation was probablydue to the ice sheet stress magnitudesbeing at the low end of the operatingrange of the stress sensors used inthe experiment and the uncertainty associatedwith using sensor 634.The experimental configuration wasnot sufficiently controlled to allow usto fully determine the limitations ofus ing the surface integral method. Icesheet stress magnitudes were at the lowend of the operating range of the stresssensors used in the experiments and theposition of maximum stress for the planesurface was not instrumented. These conditionsincreased the experimental uncertaintiesand require that additionaltests be conducted to ascertain the usefullimits of applying the surface integralmethod to determine structural iceloads.ReferencesCox, G.F.N. and Jornson, J.B. 1987. Verificationtests for a stiff inclusions t re s s sen s or . ~I;.:n:;.t=-.:... --'J:....:...--'R;.:.o:..c:..k:..:.......:Mc:.e:..:..c;.;h~._M=i.::n:..:...Sci., 24(1): 81-88.Cox, G.F.N. 1984. A preliminary investigationof. thermal ice pressures. ColdReg. Sci. Tech., 9: 221-229.Frederking, R.M.W.; Sayed, M.; Wessels,E.; Child, A.J.; and Bradford, D. 1984.Ice interaction with Adams Island, winter1982-1983. ~ "Proceedings of the SeventhInternational Association for HydraulicResearch Symposium on Ice; Hamburg, 27-31August 1984," 187-201.Johnson, J.B. 1983. A surface integralmethod for determining ice loads on offshorestructures from in-situ measurements.Ann. Glaciol., 4: 124-128.455

Johnson, J.B.; Cox, G.F.N. and Tucker,W.B. 1985. Kadluk ice stress measurementprogram. In "Proceedings of the EighthInternational Conference on Port and OceanEngineering Under Arctic Condit ions, Narssarssuaq,Greenland, September 1985," 88-100.Sanderson, T.J.O. 1984. Theoretical andmeasured ice forces on wide structures.In "Proceedings of the Seventh InternationalAssociation for Hydraulic ResearchSymposium on Ice; Hamburg, 27-31 August1984, 151-207.456

MUKLUK ICE STRESS MEASUREMENT PROGRAMO. F. N. CoxJ. B. JohnsonH. W. BosworthT. J. VincentU. S. Army Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire, USAA1:lstractDUring the spring of 1985, 23biaxial icp. stress sensors were deployedat seven sites around Mukluk, a man-madegravel island in Harrison Rav in theReaufort Sea. The maxi.mum measuredcompressive and tensile stresses were240 and 340 kPa, respectively. However,stresses were usually less than 100 kPaand seldom exceeded 200 kPa. There wereno major storms, and net ice motionsvaried from 1.6 to 5.3 m during themeasurement program. While signiticantwarming of the ice sheet occurred duringthe latter part of the study, thermalice s tresses were much lower than thosepreviously measured in Mackenzie Bay.This may be due to the fact tha t the icein Harrison Bay was more saline and hada lower I'IOdulus and yield strength thanthe ice in Mackenzie Bay.In troduc tionIce stress measurements aroundoffshore arctic structures are neededfor the development and verification ofanalytical ice force prediction models.This is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987.A number of ice stress measurementprograms have already been conducted(Croasdale and Frederkinp, 19R6). Ingeneral, measured stress values havebeen lower than anticipated and theresults from these studies have had aconsiderable impact on our understandingof ice-structure interaction andoperating procedures in the arcticoffshore.In an effort to further our understanttinp,of the magnitude and variationof ice stresses around fixed offshorestructures, ice stress sensors weredeployed around Mukluk, an artificialfill island situated in 15 m of water inHarrison Bay, Alaska (Figure 1). Icestress and temperature measurements wereobtained from 23 sensors posi tioned atseven sites around the island (Figure2). Data were collected from the beginningof March to the end of May 1985.This paper summarizes the resul ts of theMukluk ice stress measurement program.Si te Condi tionsIn early March, Mukluk was surroundedby grountted rubble resul tingfrom the movement and pile-up of thinnerice earlier in the winter (Figure 2).The rubble was most extensive on thenorthwest side of the island, extendingabout 300 m from the structure. The457

-__ ~30~' ____________ ~1;5~2· ______________ ~3TO~' ____________ ~1~5~1· ______________ ~3fo_' ____________ ~150·Cape HalkettBeaufortSea~-N-~70· 45'+ Mukluk IslandHarrtsonBayThells Island~\70· 30'5I10I15kmIFigure 1.Location of Mukluk Island in Harrison Bay, Alaska.l'lrger pile-ups were about 15 m high.The first-year ice on the north, south,anel west sides of the island was about1.6 m thick. On the east side of theisland the ice was thinner, 1.1 m, as aresul t of the sou theas t move men t of theice earlier in the win ter. The ice hada characteris tic C-shaped salini typrofile wi th an average salini ty ofabout 4 parts per thousand.EquipmentIce stress measurements wereobtained using the cylindrical biaxials tress sensor developed by Johnson anelCox (1982). A detailed discussion ofthe sensor design and performance can befound in Cox and Johnson (19R3) and Cox(1984a). The sensor has been successfullyused to me'lsure thermal f.cepressures in New Hampshire lakes (Cox198/,b) and ice forces on Esso's caissonretainedisland (eRI) in Mackenzie Bay(Johnson et al. 1985) and Adams Islandin the Canadian Arctic (Frederking etal. 1984 and 1986).Unlike the stress mensurementprogram conelucted by CRREL at Esso' s CRIKadluk loca tion, where the data loggersoperated in a heated building on ACpower and were continuously monitored,the Mukluk program was an unatteneled,ba ttery powered opera tion. Da ta loggerswere placed on the ice and serviced attwo-week intervals. Unfortunately, someproblems were experienced wi th theintern'll batteries in the data loggerswhich resul ted in a number of gaps inthe ela ta record.Sensor PlacementTwo to four sensors were deployedat each of seven sites arounel the island(Figure 2). The number anel elepth of thesensors at each sites are given in Table1. Only two sensors were deployed atSite 2 as the ice in that area was only458

7- ~-N-~, 'Figure 2. Aerial photograph of Mukluk Island showing the ice conditions in the area ofthe location of the ice stress measurement sites.1.1 m thick. Four sensors were placedat Sites 5, 6, and 7. Earlier in thewinter, and in the previous year, majorice motions in this area were towardsthe sou theas t, so sensors were concentratedon the northwest side of theisland-ruhble complex.convenience, the stresses are resolvedinto north and east normal componentsand shear. Net ice motion data for eachof the sites are given in Figure 4. Themovements were determined from surveysof the ice stress sites at the beginningand end of the program.ResultsMaximum and minimum s tress readingsfor each of the s tress sensors du ringthe entire stress measurement programare presented in Table 1. Compressivestresses are taken as positive. Anexample stress-temperature record, Site6, is shown in Fip,ure 3. Here, forDiscussionThe maximum measured compressivestress dring the measurement prograr.t was240 kPa and the maximum tensile stresswas 340 kPa. In general, compressiveand tensile stresses were usually lessthan 100 kPa and seldom exceeded 200kPa. This is not too surpris ing as the459

Table 1.sensors.Maximum (compressi.on) anll minimum(tens ion) stress rea1ilnp,s for eilch of thesite DeQth (ern) Sensor1 38 6081 107 7081 135 7032 38 6092 76 7063 76 7043 107 7123 135 7114 38 6134 76 6174 122 7145 38 6205 76 6215 107 7155 135 7196 38 6326 76 7206 107 7256 135 7267 38 6367 76 7277 107 7287 130 729Minimum (kPa) M~ximum (kPa)-80 180-310 200-230 200-140 220-300 220-160 10-300 150-260 200-210 100-260 190-320 70-200 240-300 220-90 120-100 120-60 80-340 60-110 160-80 80-110 120meteorological data collected by Sohioon the island indicatell that there wereno major storms lIuring the study. Whilethe tempera ture da ta g1 ven in Figure 3do show that there was significant warmingof the ice sheet after 25 April(Julian Date 115), thermal ice pressureswere small. In con tras t, maximum thermalice pressures at Kadluk were about500 kPa (Johnson et al. 19115). At theKallluk location in Mackenzie Bay, theice sheet was relatively fresh. InHarrison Bay, the sheet ice hall asalini ty of abou t 4 parts per thousanll.As the ice in Harrison Bay was considerablymore saline, it hall a lower mollulusand yield strength than the ice inMackenzie Bay and was unable to sustainany significant thermal stress duringwarming.During the Kad1uk ice stressmeasurement program, the direction ofthe principal s tresses in the top,middle, and bottom of the ice sheettended to be alignell when the stressesat all three levels exceeded 100 kPa.As stresses were generally lower than100 kPa a t Mukluk, there were only a fewinstances when the stresses were alignedthrough the full thickness of the icesheet.Analysis of the ice stress data didnot reveal any systematic lateral variationsin the ice stress during the firstpart of the program. For example, thedaily mean magnitude and direction ofthe principal stresses on April 4 ateach of the s ltes are plo t ted in Figure5. Data at 76 cm were chosen as thesensors a t this level provided thecleanest record and were close to thenelltral axis of the ice sheet. Inaddi tion to the s tresses being low, theice stress measurement sites wereprobably too far apart to observe anycontinuum behavior in the ridged icecover.IJarming in la te April and Mayinduced thermal s tresses in the icecover resul ting in an overall increasein stress tn the ice sheet from a lowtensile state to a compressive state.The mean da ily magni tude and d 1 rec tion460

o0-..>

712m .-I•\ 3.8 mof the principal stresses at 76 cm on11ay 29 are illustrated in Figure 6. Thestress at Site 5 at 76 cm is anomalousas nearly all of the sensors, regardlessof depth and location, indicated compressivestress in both principaldirections. In general, compressives tresses were higher on the north sideof the island. This is in agree men twith the southerly net ice movementshown in Figure 4.oL--.J100m4•/2.4m• 3/s.3mIt is interesting to note thatvliriations in the air temperature eachday resul ted in diurnal oscUla tions inthe stress record, not only in the upperportion of the sheet experiencing thegreatest temperature change, hut also atdepth. Frequently when the ice sheetwas in a state of hending, the change instress at the top and bottom of the icesheet were of opposite sign, that is,increased compression in the top fiberscaused increase tension in the bottomfibers.Figure 4. Net ice movement at each ofthe ice stress measurement sites.\1\~7---- II-o--­L--.JIOOkPo100m4-.-----\ ............. -2\Ftgure 5. Mean daily magni tlJde and~irection of principal stresses at 76 cmon April 4 (Julian Da te 94). Convergingarrows indicate tension and divergingarrows indicate compression.o--­L--.JIOOkPo100m~4_rt-3Figure 6. Mean daily magnitude anddirection of principal stresses at 76 cmon May 29 (Julian Date 149).462

ConclusionsThe Mukluk ice stress measurementprogram is yet another example of lowice stresses around an offshore arcticstrllcture. ~lhile from a research anrlanalysis perspective we are disappointerlthat no significant stress events wererecorded, the results of this and recen tstudies should rlelight petroleum operatorsas they sugges t tha t we have beenoverly conservative in estimating iceforces on offshore struc tures, particularlyin areas of limi terl ice movement.During the course of the t1ukluk programthe ice sheet moved about 6 m towardsthe islanrl, yet maximum compressivestresses were only 240 kPa anrl usuallyless that 100 kPa. It is conceivabletha t Mukluk experienced its larp,es t iceloarls earlier in the winter during theformation of the rubble pile on thenortheast side of the island. 1"or thisreason an effort needs to be made tomeasure s tresses in the ice early in thewinter prior to the forma tion of arubble pile around the structure.Unfortunately, all ice stress measurement programs have usually been ini tiated in March when there is ample daylightfor logistic support and ambienttemperatures are not too cold to hamperpersonnel and equipment.AcknowlerlgementsThis work was sponsored by theMinerals Management Service (MMS) of theU.S. Department of the Interior withlogis tic and technical support fromStandard Oil of Ohio (SOHIO). We areparticularly grateful to Terry Walden,Des Anderson, Charlie Smith, and JohnGregory for program planning and assistancein the field.Cox, G.F.N. 1984h. A preliminaryinves tiga tion of thermal ice pressures.Cold Regions Science anrl Technology,Vol. 9: 221-229.Croasdale, ~.R. and Frederking, R.1986. 1"ie1d techniques for ice forcemeasuremen ts. ?roceedings of the IAHRSymposium on Ice, Towa City, Aup,lIst 18-22, 1986, Vol. 2: 443-482.Frederking, R.M.W., Sayed, M., Wessels,E., Child, A.J. and Eradford, n. 1984.Ice interaction with Adams Island,winter 1982-83. ?roceedings of the IAHRSymposium on Ice, Hamburg, August 27-31,1984, Vol. 3: 187-201.Frederking, R.M.W., Wessels, E., Maxwell,.I.E., ?rinsberg, S. and Sayed, M.1986. Ice pressures and behavior atAdams Island, winter 1983/84. CanadianJournal of Civil Engineering, 13(2):140 149.Johnson, J.E., Cox, G.F.N. and Tucker,W.B. 1985. Kadluk ice stress measurementprogram. Proceedings of the 8thInternational Conference on Port anrlOcean Engineering under Arctic Conditions,Narssarssuaq, Greenland, Sept~mber 7-14, 1985, Vol. 1: 88-100.Johnson, J.B.Stress sensorelastic andUnited States31, 1982.and Cox, G.F.N. 1982.particularly suited forvisco-elastic materials.Pa ten t 4,346,600, Allgus tReferencesCox, G.F.N. anrl Johnson, .I.E. 1983.Stress measurements in ice. U.S. ArmyCold Regions Research and EngineeringLaboratory, Research Report 83-23, 31pp.Cox, G.1".N. 1984a. Evaluation of abiaxial ice s tress sensor. Proceedingsof the IAHR Sympos ium on Ice, Hamburg,August 27-31, 1984, Vol. 2: 340-361.463

MEASUREMENTS OF MULTI·YEAR ICE LOADSON HANS ISLAND DURING 1980 AND 1981B. DanielewiczD. BlanchetCanadian Marine Drilling Limited, Calgary, Alberta, CANADAAbstractIce forces during impacts of largemulti-year floes against Hans Islandwere measured during joint industryfield programs in 1980 and 1981. Theice loads during failure over widths ofup to 300 m were calculated frommeasurements of floe mass, decelerationand contact area. Floe masses werecalculated from measurements of floearea and thickness. Decelerations offour floes were measured usingtheodolites in 1980. In 1981,decelerations of four other floes weremeasured using accelerometers andtheodolites.Ice load intensities (per unitwidth) were found to be stronglydependent on the ice failure mode.Highest ice load intensities (310tonnes/m) were associated with theinitial stages of direct impacts(limited crushing) during which theresulting ice rubble consisted mostlyof small fragments (less than a cubicmetre). Further penetration led toalternate failure modes of the floe andThis is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17·22, 1987. © The Geophysical Institute,University of Alaska, 1987.larger block sizes (tens of cubicmetres). With this change in failuremode, effective ice load intensitiesfell to between 50 and 230 tonnes/m.The lowest ice loads were observedduring cushioned impacts. Under theseconditions, load intensities built upgradually over the duration of thecollisions and were not observed toexceed 30 tonnes/m.IntroductionThe highest ice load experienced byan offshore bottom-founded structure inthe Beaufort Sea may be caused by theimpact of a large, thick multi-year icefloe. The risk of this type of eventincreases with the distance of thestructure from shore and its proximityto the polar ice pack.With break-up in July and August,most first-year ice melts and leaddevelopment in the pack is greatlyenhanced. Under these conditions,winds with a shoreward component cancause southward displacements anddivergence of the polar pack. This canresult in an incursion of multi-yearice into the area of hydrocarbonexploration and production.465

In addition, multi-year floes arecapable of reaching higher speeds inopen water and covering more territorythan they could while locked into afirst-year ice matrix. Open waterimpacts of multi-year ice floes aretherefore an important designconsideration for offshore structuresin the Beaufort and Chukchi Seas.A series of three research projectswere conducted at Hans Island in 1980,1981 and 1983 to investigate the loadsgenerated by open water impacts ofmulti-year ice floes against anobstruction. The theory andmethodology of this work has beenpublished earlier (Metge et al., 1980;Danielewicz et al., 1983). The resultsof the 1980 and 1981 projects(Danielewicz and Metge, 1980;Danielewicz and Metge, 1981) have nowbeen released by the participants andare presented in this paper. Theresults of the 1983 project will becomepublic in 1989. The public reports areavailable through the Arctic PetroleumOperators Association (APOA).ObjectivesThe objective of the Hans Islandprojects was to measure thedecelerations and sizes of ice floesimpacting with Hans Island and to usethese data to calculate the effectiveice loads generated.At any instant, the net ice load ona fixed structure during an ice floeimpact is equal to the vector sum ofthe decelerating force, the wind andcurrent drag on the floe and theCoriolis force. It has been shown thatfor the impacts monitored at HansIsland, the effects of the wind andcurrent drags and Coriolis force can beignored. The decelerating force, hencethe ice load, can therefore be obtainedsimply from the product of theeffective ice floe mass and the floedeceleration. The hydrodynamic mass,estimated to be less than 10 percent ofthe total mass, can be ignored and theeffective ice floe mass can bedetermined from its volume (Danielewiczet al., 1983).Field Observations and MeasurementsThis section describes themeasurements of many floe and impactparameters which led to the calculationof ice loads. Observations such asfailure modes and type of impact arealso documented. The correspondingmaximum loads are given in tonnes-force.1980The 1980 Hans Island project wasthe first attempt at the kind of datacollection described above. Little wasknown about the field conditions andthe scale and duration of the impacts.The investigators therefore preparedfour independent methods of datacollection. Accelerometers, anelectro-optical distance measurement(EDM) unit, theodoli tes andphotogrammetry were utilized. Theaccelerometers did not functionproperly until the end of the projectand the EDM proved to be unsuitable forthe working ranges required for thesize of floes encountered. Thedecelerations in 1980 were thereforeestimated from manual tracking ofselected points on impacting floes withtwo theodolites and from measurementsof floe displacements fromphotographs. The photographs weretaken at one minute intervals withseveral Super 8 movie cameras installedon the island. The film was projectedonto a grid which was constructed fromtheodolite measurements of distances toknown ice features which had also beenfilmed.On August 15, a multi-year ice floecollision with the north shore ofisland was monitored. Its trajectoryprior to the impact and its size afterit had come to rest were measured bytheodolite from the island. The floedimensions were approximately 7.4 km by6.0 km with an area of approximately31 km 2 • Ice thickness was estimatedfrom five borehole measurements and a400 m free board survey along a typicaltransect and found to be approximately5.6 m. Ice mass was estimated to be1.7 x lOll kg. The impact velocitywas approximately 0.4 ms- l •Deceleration was complete within 600seconds of first contact. The highestdecelerations measured over one minute466

intervals were approximately 1.1 x10-3 ms- 2 •The corresponding ice force on theisland was therefore estimated to beapproximately 20,000 tonnes. The floedeceleration took place over a distanceof approximately 170 m and resulted ina maximum contact width of 300 m.On August 16, at 06:10, the samefloe unexpectedly reversed directiontoward the north and collided with theeast cape of the island. Thetrajectory was obtained only frommeasurements on the time lapse film.This was a glancing hit in which thefloe decelerated from 0.3 ms-l to 0.1ms- l over about 500 seconds. Themaximum deceleration of the contactingedge was estimated to be 6 x 10- 4ms- 2 • The deceleration of the centrewas estimated to be half that, 3 x10- 4 ms- 2 • These values indicatean effective ice force of approximately5,000 tonnes over a contact width ofprobably 150 m.Three hours later, the floereversed direction toward the south andagain collided with the eastern cape.The velocity dropped from 0.1 ms- l toalmost zero over an interval of 800seconds as a result of this impact.This deceleration indicated an ice loadof approximately 2,000 tonnes over acontact width of between 50 and 100 m.These results are associated with ahigh degree of uncertainty due to themeasuring techniques and the size ofthe data base. The data, however,suggested ice loads much lower than hadbeen anticipated. Consequently, it wasdecided to conduct a second projectduring the following summer, utilizingmore reliable equipment andconcentrating on improved accelerometerinstrumentation.1981The first impact monitored with theimproved instrumentation took place onJuly 29. The multi-year floe wasestimated to be 5 m thick and had adiameter of approximately 2 km. Asmall floe with a diameter ofapproximately 200 m cushioned theimpact. The effective mass of thelarger floe was calculated to beapproximately 1.6 x 10 10 kg.Figure 1 is a trace of one secondaverages of the total ice loaddetermined from accelerometer data.The contact zone width grew fromapproximately 50 to 200 m over theduration of the impact. Ice loadsreached 9,500 tonnes.II .. 00a FAllJrG18 41'" eN£" 100.SMAlL FlO(:L56N"~~RU.Flot:.. .. ,.ICE fA.LIJitGovtR ~ ..E:\.APSEDTM C,.Figure 1. One second averages of force exerted on Hans Island during the collisionof a multi-year ice floe on July 29, 1981.467

o~~~ ______~ ~ 10______~ ~ ~ 00__________~ ~ ~ ~ ~ __On August 4, a multi-year ice floemeasuring approximately 2.7 km by 2.3km collided with the island. The meanfloe thickness was determined to be 8 mfrom boreholes and a freeboard survey.Floe mass was calculated to be 4.1 x10 10 kg. Decelerations were measuredboth with theodolite tracking and withthe on-ice accelerometers. Both datasets were in reasonable agreement.Figure 2 is the force history for thisevent. Contact width grew toapproximately 100 m over an interval of60 seconds. Ice loads reached 27,000tonnes.On August 7, 1981 a multi-year icefloe with a major axis of approximately0.6 km collided with the island. Floearea was approximately 0.2 km 2 andthe mean thickness was estimated to be6 m. Floe mass was approximately 1.2 x10 9 kg. Because of its small size,this floe began to decelerate, due tothe disturbance of the current upstreamof the island, even before firstcontact was made. Its speed haddropped to approximately 0.2 ms- lfrom its undisturbed value of 0.3ms- l • The resultant force history isgiven in Figure 3.Contact was limited to 30 m due to thelow amount of kinetic energy availablefor floe penetration. Ice loadsreached approximately 2,500 tonnes.While the above floe was still incontact with the island, a secondinstrumented floe impacted.FIRST I..-.c:T(SNAU. n.0I)~10 20 10"0 11010 1010 10100110 110110I0I0110_0.,.,..0 T'1flC ISlFigure 3. One second averages of forceexerted on Hans Island during thecollision of two multi-year ice floeson August 7, 1981.-,101010Nole:Numbers Correspond 10Failure Mode Descriptionin Text.20~--____ ~--____ ~o 0 20 ~ ro 10 ~--~~~--__________ ~~_ _ _~lLArKO..... 'SFigure 2. One second averages of force exerted on Hans Island during the collisionof a multi-year ice floe on August 4, 1981.468

This impact was cushioned by thefirst floe. The second floe had anarea of approximately 1.4 km 2 and athickness of 6 m. Its mass wascalculated to be 7.7 x 10 9 kg. Thisimpact was at approximately 0.3ms-l • Resultant loads calculatedfrom the accelerometer responses arealso given in Figure 3. No significanteffect of this collision was noticed atthe contact zone between the first floeand the island. The collision was notdetected by the accelerometers on thesmaller floe. The energy of the impactwas dissipated during an interval ofapproximately 100 seconds and adeceleration length of approximately15 m. The energy was utilized for icefailure at local misalignments betweenthe edges of the two floes and alimited amount of ridge building. Thetotal force reached approximately 2,000to 2,500 tonnes over a poorly definedcontact of several hundred metresbetween the two floes.The results of the 1980 and 1981multi-year ice floe impacts aresummarized in Table 1.TABLE 1SUMMARY OF THE CRITICAL IMPACT PARAMETERSFOR MONITORED MULTI-YEAR ICE FLOE COLLISIONSWITH HANS ISLAND IN 1980 AND 1981DATE/TIMEICETHICKNESS(m)INITIALVELOCITY(ms- l )DECELERATION(ms- 2 xl0- 3 )MAXIMUMCONTACT WIIlTH(m)IMPACT TYPE80/08/15/2100 315.60.4300uncushioned 2080/08/16/0600 315.60.30.6150off centre 580/08/16/0900 315.60.10.150-100off centre 281/07/29 3.550.55.0200cushioned 9.581/08/04 5.780.36.0100uncushioned 2781/08/07/0110 0.260.220.030uncushioned 2.581/08/07/0221 1.460.33.0200cushioned 2.5469

Interpretation of the ResultsMany floe parameters have beenanalysed to determine their effect ondesign criteria for Arctic offshorestructures. A comparison of the dataand observations gathered at HansIsland with theories and small scaleice indentation data is made in thissection. Analyses are presented interms of ice failure mode patterns andinterpretation of the results.Comparisons are made with publishedmodels and data. The interpretation ofthe results includes the followinganalyses:variation of the force per unitwidth with failure modes andwidth;variation of the contact widthwith time;effect of confinement; and,ice type, salinity, temperatureand confined compressi vestrength.The ice floes encountered at HansIsland were relatively warm multi-year'ice, similar to floes whichperiodically invade the Beaufort Sea inthe summer. The ice floes exhibited astructure of many smaller floes bondedwith multi-year ice. This arrangement,together with the deteriorated state ofthe ice, indicated by puddles, streamsand melt holes, led to their highlyvariable local strength. Plate 1represents an aerial view of the highlyvariable structure of a floe.The interpretation of loads at HansIsland is hampered by uncertainties incontact width, ice thickness, floemass, floe dynamics and accelerometersignal. Uncertainties were calculatedfrom the square root of the sum of thesquares of uncertainties in floe mass,deceleration and contact area. Themagnitudes depended on the type of datacollection employed. Typically,independent measurements of floe areawere within 5 to 10 percent. Floethickness uncertainties were taken tobe between 10 and 30 percent, dependingon the amount of boreholes and surfaceprofile data collected. Accelerometerresolution (usually 0.2 x 10-3ms- 2 ) accounted for an additionaluncertainty of 5 to 50 percent. Thecontact width was at times a difficultparameter to estimate. Large scalefailures at the contact zone couldaffect the effective size of contactover which ice load was beingtransferred to the island. This levelof uncertainty was estimated to rangebetween 10 and 30 percent depending onthe scale of failures.When only photogrametric data wereavailable and floe thicknesses andcontact widths were visually estimated,total uncertainties were of comparablemagnitudes as the measured loads. Theuncertainties accounted for errors inexcess of 35 percent even in the bestdata sets.Estimates of compressional wavetravel times in ice floes indicatedthat load derivation from theaccelerometer data is probably notvalid for peaks shorter than 3 to 6seconds.Ice failure mode patternsHead-on impacts were associatedwith the highest ice loads. Of these,two types of interaction scenarios havebeen documented. They are:1) uncushioned impacts; and 2) blockedor cushioned impacts. The blocking andcushioning of large floes resulted in asignificant reduction of the globalload on the island. Two factors aremainly responsible for this reduction:1) Kinetic energy is spent over alonger penetration distance withlow confinement and/or compressionof ice debris at the contact zone.2) The failure of small floes anddebris in the early stage of thepenetration favours "lower energy"failure processes such asmulti-modal ice failure andsplitting. Rotation of the icefloe can also occur.470

""' .r -- Thick ice flo ( _

Uncushioned impactsThe events of August 15, 1980 andAugust 4 and 7, 1981 were uncushionedimpacts. They represent the bestimpacts in terms of reliablemeasurements of the contact width, icethicknesses and calculated ice forcesand pressures. The description offailure modes given below isnumerically referenced to specific loadlevels given in Figure 2. All impactswere characterized by a limited amountof crushing observed during the firstphase of the penetration as localbending, flaking, and cracking began toaccumulate rubble (1) . Plate 2represents some of the failure modes ofthe edge of the ice floe at the earlystage of the interaction. As the floepenetrated further, stronger ice andaddi tional confinement helped to spreadout the load more uniformly over ,theent ire contac t width. Crushingcontinued but larger broken pieces ofice were also formed (2). If thekinetic energy was not enti rely spentat this stage, splitting or rotation,or a change in the failure mode at thecontact zone could occur as the contactarea increased.The loading rate, expressed inforce per unit width per second, duringthis early stage of the interactions,was high. For example, for theAugust 7, 1981 impact, it approached 32tonnes m-l s-l. This is the sameorder of magnitude as indentation testsconducted at high strain rates (Micheland Blanchet, 1983; Timco, 1986). Ifsplitting or rotation of the floe didnot occur, the increase in the contactwidth and confinement combined withlocal weaknesses of the floe could leadto a dramatic change in the failuremode. For example, the thick August 4,1981 ice floe failed by a 50 m width"flake" which prevented the force fromincreasing further (3). A schematicrepresentation of this "flaked" pieceof ice is shown in Figure 4. The totalcontact width in this example was 90 to95 m. It is evident, since the loaddid not drop to zero (4), that contactwas maintained between the ice foot andthe ice floe across the remaining width(40 to 45 m). Possibly, this large'spalled' zone may correspond to thezone of maximumsemi-infinite elasticuniformly over a gi ven1971) .shearplatewidthof aloaded(Jaeger,Figure 4. A schematic representation asemi-circular flaked or spalled zone.At the end of this first phase (3),the load was reduced by almost half (5)before it increased again during ridgebuilding over a larger area. Thelatter was characterized by bending ofthe ice behind and along the sides ofthe "flaked" zone (6). The formationof very large ice blocks (greater than5 m) at the contact zone confirmed thechange in the failure process frompredominant crushing to multi-modalfailure. Assuming that the large"flaked" piece of ice had not beencrea ted, multi-modal ice failure wi ththe accumulation of large blocks of ice(ridge formation) at the floe edgecould have been the final globalfailure mode of the ice floe followingthe crushing phase. Plate 3illustrates some of the failure modesduring the last stage (multi-modalfailure phase) of the interaction.Plate 4 is a close-up of a ridge. Aflexural crack and flaked ice blocksare evident as some of the failuremodes before and during the multi-modalfailure process. The ridge building(second phase) loads varied between 50and 80 percent of the peak crushing472

Plate 3. Depression of the leading edge of a multi-year ice floe by a newridge. Note the large blocks in the 6m high ridge.. Ilexural CrackPlate 4. Large blocks in a ridge from flexural failure and flaking at highpressure points.473

(first phase) load measured in thefirst part of the interaction.Crushing however continued tocontribute significantly to the forcesmeasured during the second phase.For the impact of a small floe,such as that on August 7, 1981,however, the available kinetic energywas not sufficient to bring the floe tomulti-modal failure (second phase).As the contact width increases,multi-modal failure (with somesecondary crushing occurring within therubble) can be the final failure modesbefore all the kinetic energy is spent(7). However, if the ice rubble on topof the edge of the floe is high enough,its weight will cause flexural failureof the floe edge in a semi-circularfashion. Observations verifiedvertical deflections at the edge of theice sheet. Plate 5 shows flexuralcracks at the contact edge of a floecaused by the over-burden of icerubble. The ice rubble at the edge ofthe ice floe had already settled a fewmeters when the picture was taken.During the second phase, the loadingrate was 10 times lower than during thefirst phase (3.2 tonnes/m/sec).At this stage, if the kineticenergy of the floe is not fully spent,splitting, rotating or simply thecontinuation of multi-modal ice failurecould follow. Plate 6 shows thesplitting of a multi-year ice floe.Cushioned or blocked impactsA cushioned impact is defined as animpact of a floe on an intervening floeor ice debris at rest against thestructure. Blocking occurs when thecushioning is sufficient to preventeventual contact of the floe with thestructure. The July 29 and August 7ice impacts in 1981 represent cushionedand blocked impacts, respectively.Plate 7 shows ice debris and smallfloes at re st between the Hans Islandice foot and a large multi-year icefloe.When the small floe is weaker thanthe larger floe, the ice debris of thesmall floe and the random contactgeometries initiate early multi-modalfailure. As shown in Figures 1 and 3(the second impact), the early crushingphase is bypassed. The load rate forthese events was observed to vary from.1 tonnes/m/sec to 2.8 tonnes/m/sec.However, when the small floe isstronger than the large floe, and nosplitting or rotating of the large floehas occurred, the small floe will beenveloped by the large floe. Thelatter will fail against the structureon both side s of the small floe,provided that the structure is largeenough and sufficient kinetic energy isavaila ble. At thi s stage of theinteraction, splitting, rotating orshearing pa st by the st ruc ture of thelarge floe could still occur. If thisdid not happen, multi-modal ice failurearound the small floe and across thewidth of the structure could continue.Variation of the force per uni twith failure modes and widthwidthThe primary purpose of measuringglobal ice loads is the utilization ofthe data for arctic offshore structuredesign. A useful representation ofglobal ice loads for the design of wideArctic offshore structures is a plot ofthe ice force per metre of widthagainst the width for different failuremodes and thicknesses. The force/widthdata were plotted for the two majorphases discussed earlier: limitedcrushing (first pha se ) and multi-modalice failure (second phase).The data points are shown inFigures 5 and 6. In Figure 5, firstphase load intensi ties up to 310tonnes/m were measured during theAugust 4, 1981 impact. As shown by thedata, the first phase forcemeasurements were limited to widths upto 230 m. Not enough first phase datais available to show either thedecrease of the first phase force withthe increase in the contact width orwith the decrease in the ice thickness.474

(1680) A.P.I Sui. 2N, 8m(1435) VI.atrat & Slomski (1983)(1345) Bruen et III (1982)(1200) Kreider (1984) and Blanchet and Metge (1984)(1050) .... A. P.1. Bul. 2N, 5m("0)'5 Croteau.t al (1984)• 1981 (1 sec. a.g.)01980 (80 sec. avg.)Ice Thickness ; 5 to 8mIce Temperature ; -3 to -5'C·ci 350• c 300• •0!:. 250•z:~•~ 200• •• •~01501- •• •II.••• •100 • -• .-I••••• 0-• .. 1 • • - ,,-,050 00000 50 100 150 200Wldth(m)250 300 350Figure 5. Variation of the first phase (limited crushing) force/width withcontact width.fU6ge IkIiki'ng: . 1911 1 NC .• wg..• 1NO10 NC . ... g..ErrOf' On force "' .t 35%· 1M1• t 5Q1l1.· 1MO (Due 10 10 Me: 8Wg.)k:. ThIck,.. •• ; 5.0 to I.Om350350:100300! 250:c! 200Z'li~ 150..e0100. . }~ Im;-: "-: ~:'25020015010050(6m).... :::-: ... :- .::-~5050 100150 200 250 :100WI"'h (m)Figure 6.Variation of the ridge building force with contact width.475

~Radial Crack-"of ice rubble, ~~IrcumferentlalCrackPlate 5. Flexural cracks parallel to floe edge resulting from bending by icerubble overburden at new ice ridge.Plate 6.Massive splitting of a floe upon impact with Hans Island.476

Plate 7. Cushioning of a floe impact by an accumulation of ice rubble andsmall floes .Plate 8 . A horizontal cross section of a multi-year ice core showing granularice refrozen into a new matrix.477

Figure 5 compares the Hans Islandresults with theoretical models usingsmall scale indentation data. Bothcrushing 2nd ridge building forcemodels were standarized for thefollowing conditions: summer multi-yearice, ice temperature of -3°C to -5°C,and thickness between 6 and 8 m. Thesemodels estimated forces up to 3 to 6times those measured at Hans Island(A.P.I. Bul. 2N, 1982; Blanchet andMetge, 1984; Bruen et a1., 1982;Croteau et al., 1984; Kreider, 1984;Vivatrat and Slomski, 1983).Of particular interest is thecomparison with the A.P.I. Bulletin 2Nusing the Korzhavin (1970) indentationformula. The latter is expressed by:or:where:DtFFDI fc C x DtI fc Cxt(1)(2)indentation factorcontact factorunconfined compressivestrength of iceDiameter of the structure orcontact widthice thicknessAs suggested by the A.P.I.,Bulletin 2N, granular ice, which isrepresentative of multi-year ice, for asalinity of 3 ppt, a temperature of-4°C and a strain rate of 1 x 10- 3sec-I, has an average uniaxialcompressive strength of C x 3.5 to4.0 MPa.For fc 0.5, I 1.2 and t8 m, equation (2) gives a force perunit width of F/D 1680 to 1920tonnes/m. These results appear toover-estimate the measured loadintensities by a factor greater than5. For a 5 m thick ice floe, the loadintensity is 1050 to 1200 tonnes/mwhich is more than 3 times higher thanthe measured load intensities for the8 m thick floe on August 4, 1981.Using a contact factor of 0.3 and anindentation factor of 1.0, these valuescould drop as low as 530 and 840tonnes/m for 5 m and 8 m thick icefloes respectively.Figure 6 shows a comparison of thefull scale measurements of ridgebuilding (second phase) loadintensities with theoretical models.The effects of cushioning and lowenergy impacts are evident. The August4, 1981 uncushioned impact producedforces 3 to 5 times higher thancushioned impac ts. Maximum measuredridge building load intensities reached230 tonnes/m. This factor of 3 to 5found between uncushioned and cushionedridge building loads is attributed tothe increase of the amount (orpercentage) of ice failing by crushingat the contact zone during uncushionedimpacts. In theoretical models, thebreaking part of the ridge buildingforces is mainly due to bending. Duringmulti-modal failure, the percentages ofthe ice failing by different failuremodes will vary strongly with the icethickness. Other factors such asconfinement cushioning, the presence ofcracks, and the surface irregularitieswill also strongly affect the ridgebuilding loads. The August 4, 1981results are shown in Figure 6 as anaverage (solid dot) of 35 points havingabout the same contact width (seeFigure 7 between elapsed time 55 and100 sec.) and an error bar showing theminimum and maximum values of the 35points.Eilimated C.lculated...... 2m ~ Penetrallon, Y (m)10010 18_90~ 80cN °11

A review by Croasdale (1984) andthe extrapolation of models byParmerter and Coon (1973) and Mellor(1983) show a variation of loadintensities from 20 to 40 tonnes/m.These loads intensities compare wellwith the (second pha se) ridge buildingload intensities measured duringcushioned impacts at Hans Island. Forthe August 4, 1981 data, confinementand possibly the thicker ice edge andthe higher global strength of the floemay have played an important role inincreasing the amount of crushingfailure observed at the contact zone.The results of the Vivatrat andKreider models (1981) are also shown inFigure 6. For 2 m thick ice, theyproposed ridge building pressuresvarying from 30 to 160 tonnes/m. Thisvariation is mainly due to thedifferent models used in their analysisand the choice of the parameters usedin each model.kinetic energy of the impacting mass isreduced to zero. The sine functionalso applied for other uncushionedimpacts.ConfinementAnother important parameter duringan ice impact on a structure is thedevelopment of the pressure during thepenetration of the structure into theice. The pressure/penetration curvehighlights the effect of confinementand velocity on the failure modes andultimately the global ice force.The pressure/penetration curve forthe August 4, 1981 impact is shown inFigure 8 for the ridge building(second) phase. This phase was chosenbecause of the limited variation of thecontact width.Variation of the contact width with timeThe variation of contact width withtime or penetration is anotherimportant parameter. It depends on therelative geometries of the ice and thestructure. For the Hans Island case,the width estimated using a sinefunction is given by:w(t) =w max sin pt(3)•• .• .• D.I. PoInt._Aeg.Cu,".. •..•-.where:pTtmaximum width at the endof the quarter of thesine functionfT2Tinterval for maximumcontacttime(4)As shown in Figure 7, the sinefunction closely simulates thevariation of the contact width withtime and can be used in predictivemodels. This sine function is similarto the results of drop baIlorindentation tests in which the entirePenelr.tlonlflnal Conlact Width ("')Figure 8. Ridge building pressuremeasured during the August 4, 1981 iceimpact.The measured pressures were dividedby an arbitrary pressure of .1 MPa toobtain a non-dimensional pressureC avg which represents the coefficientof indentation. The penetration wasdivided by the final contact width toobtain a non-dimensional penetration.The data points in Figure 8 representthe actual highs and lows of the479

measured pressures. The data pointswere fitted with a linear regressioncurve. This best fit curve representsthe variation of the average pressure(or coefficient of indentation) withthe penetration (X).The coefficient of indentation is equalto:= .07X + 1.9 (MPa) (5)This increase of the coefficient ofindentation with the penetrationclearly demonstrates an increase ofconfinement as a consequence of abetter conformity between the ice footand the ice floe edge and of a highervolume of broken ice on top andunderneath the ice edge. The resultwill be an increase in the amount ofcrushing towards the end of theinteraction.During impacts by larger and thickerfloes, further penetration could leadto loads equal to or perhaps in excessof the crushing - loads measured duringPhase I of the interaction. Equation(5) is valid until the ice edge failsby bending over a very large portion ofthe contact width and the presentrubble is pushed down, cleared away andoverridden by the advancing ice sheet.The process of ridge building willstart again. If the ridge becomesgrounded, and it is not cleared away,then the advancing ice sheet will startto fail behind this newly formedgrounded ridge. This is the initiationof ice rubble formation. At thispoint, it is not known how muchpenetration is required before globalflexural failure of the edge of the icesheet will occur. Equation (5)therefore, is limited to the range ofpenetrations presented in Figure 8.The increase of the coefficient ofindentation with penetration could alsohave been the result of the decrease ofthe impact velocity from .31 ms- l to.09 ms- l addition to the confinementeffect described above.Ice type, salinity, temperature andconfined compressive strengthFull scale ice pressures and forcesmeasured at Hans Island can be betterunderstood when the intrinsicproperties of the ice such as its type,local confined strength, temperatureand salini ty variations are known.Although full scale pressures and loadsdata are not necessarily determined bythe 'small scale' properties of theice, estimates of design loads requirethe knowledge of local propertiesunless full scale data are available atdesign conditions.Temperature, salinity and the typeof ice will not only affect the localstrength of the ice but may also affectthe local and global failure modes ofthe ice sheet. Colder temperature willrefreeze melt pools and cracks therebystrengthening the entire floe.Because of these factors, very carefulinterpretation or extrapolation ofsummer full scale impact data (such asHans Island data) to design conditionsis required.One temperature, two salinity, andfour borehole jack strength profiles ofthree multi-year ice floes are shown inFigures 9a and 9b. The salinity variesfrom less than 1 ppt at the top of theice sheet to about 4 to 5 ppt at thebottom. The salinity profile istypical of multi-year ice as shown inFigure 10 (Sinha, 1986). Thetemperature profile shows a decreasefrom -2°C at the top of the ice sheetto -rC at -5 m. The temperatureincreases to -5°C at -7 m.Temperatures were not measured at thebottom of the ice but are expected tohave increased to -2°C at that level.The total ice thickness is unknown butthe floe was selected as a particularlycompetent sample in otherwisedeteriorating ice conditions onAugust 12, 1981.480

0501980100150E 200~.. 250u.!!:; 300en350it0.. 400CDz: 450"&... 5000550600 !Temperatur650 :700 .. lEnd 01750Measurements0 1 2 3 4 5 6 7 8 9 10Salinity (ppt)Temperature (-'C)Figure 9a. Temperature andprofiles in multi-year iceIsland, August 1980 and 1981.salinityat HansConfined Strength (MPA)0 4 8 12 16 20 24 28 32050100150E 200~.. 250u~ 300en " 350it0.. 400CDz: 450"&... 5000550600650700750Figure 9b. Borehole jack profile inmulti-year ice at Hans Island, August1980 and 1981.E.:"&...0..!:!00.51.01.52.02.54.50.351.522.28Mould BayNorland Floe(Site 3)29 March 1984Ii!IiII! Columnar~ Interlace3.835.0 L-..l.._L-...L_.l-...-LI~lL-____ ---'2 3 4 5Salinity. 0/00Figure 10. Vertical salinity andtexture profile in a large multi-yearice floe (Sinha, 1986).Typically , multi-year ice is a mixtureof granular, columnar and snow ice.Multi-year ice studied in 1980indicated a granular P-3 (Michel, 1978)ice of crystal sizes varying from 0.5to 3 mm. This ice is formed at anagitated surface and nucleated fromfrazil. The grain shape is tubular.The crystal orientation is random.Brine channels accounted for 1 to 2percent of the volume of upper samplesand 5 percent of the ice volume towardsthe base. Plate 8 represents ahorizontal thin section of a multi-yearice sample.Some of the multi-year floes whichimpacted Hans Island were characterizedby a random crystal structure made ofgranular, columnar and snow ice.Columnar ice grains were oriented inall di rections over sections of 10 to20 cm of depth.481

Small granular and large columnar icecrystals less than 1 mm and more than 5cm in diameter were observed. Frazilice could also be seen in the weakmulti-year ice floes.Other multi-year floes werecharacterized by a more uniform crystalstructure. The ice was mainly columnarwi th some layers (less than 10 cm) ofvery fine grained ice. The columns ofthe ice were sometimes oblique to thevertical axis by less than ~O~. Thistype of structure has also been foundby Sinha (1986). No optical analysisto study the orientation of the C-axiswas done on the 1980 and 1981 icesamples.The results of borehole jackmeasurements are presented inFigure 9b. The vertical strengthprofiles show a strong relationshipwith the temperature and crystalstructure of the ice. Confinedstrength varies between 9 MPa at thetop of the ice sheet and 24.5 MPa atthe middle of the ice sheet. Theconfined strength at -2°C varies onlyfrom 9 to 11.5 MPa. At -5°C, it variesfrom 14 to 24 MPa. Sinha [17] foundthat the borehole jack strength ofmulti-year ridge and columnar grainedice at -20°C varied from 30 to 35 MPa.This variation of the localstrength throughout the multi-year icefloe depth is important when settingdesign criteria. The non-uniformity instrength due to the ice type, crystalsizes and orientations, the salinityand the temperature will result in anon-uniform failure pressure throughthe entire ice thickness. Theconversion of ice strength to failurepressure has been presented by Blanchet(1986). This effect, combined with theeffect of the state of the ice, itsedge shape and thickness variation andthe presence of cracks will contributeto a reduction in the amount of purecrushing.Conclusions1. During an impact, initial (firstphase) failure was seen to occur aslocal crushing. This was followedby (second phase) multi-modal icefailure with a tendency toincreasing ice block size.2. The highest ice loads were found tooccur with direct uncushionedimpacts. Loads as high as 27,000tonnes over a contact width of100 m were measured during animpact of a floe 8 m thick. Thecorresponding pressures aresignificantly lower than thosemeasured at smaller scales orpredicted by theoretical models.3. For uncushioned direct impacts,loads were found to increase duringthe early stages of failure until alarge semi-circular wedge (orflake) was created. Sufficient icerubble was then generated resultingin the initiation of flexuralfailure and continued ridgebuilding with large blocks. Atthis point the ice loads dropped byalmost a half for the same contactwidth.4. During multi-modal ice failure,ridge building loads increased withwidth of contact and ridge heightor confinement.5. Impacts that were cushioned orblocked by an intervening floe orice debris resulted a limitedamount of crushing. These impactswere associated with relatively lowloads due to an early initiation of(second phase) multi-modal failure.6. Once multi-modal failure wasinitiated, the failure process wasnot observed to revert tosignificant amounts of crushing.7. The end of an impact occurred whenthe floe either rotated around theisland, came to rest as its kineticenergy was dissipated or split andslid past in fragments.482

8. A sine function closely simulatesthe observed variation of contactwidth with time.9. In measured floes, the salinity ofthe multi-year ice floes variesfrom less than 1 ppt at the top ofthe ice sheet to about 4 to 5 pptat the bottom. The temperatureprofile is parabolic and shows adecrease from -2°C at the top ofthe ice sheet to -7"c at -5m thenan increase to -2°C at the bottom.10. The sampled ice was mainly granular(P-3) with crystal sizes varyingbetween 0.5 and 3 mm. The boreholejack strength profile closelyfollows the temperature profile.Confined strength varies between 4MPa at the top of the ice sheet and24.5 MPa at the middle.AcknowledgmentsThe authors gratefully acknowledge thesupport of all APOA participants,namely; Amoco Production Company, ArcoOil and Gas Company, BP PetroleumDevelopment Limited, Dome PetroleumLimited, Exxon Production ResearchCompany, Gulf Canada Resources, MobilResearch and Development, Petro-CanadaExploration Inc., Shell DevelopmentCompany, Standard Oil Company andTenneco Oil Company. The Hans IslandProjects. were conceived by Mr. C.O'Rourke of Canmar and Dr. M. Metge ofISE Consultants Ltd. Dr. Metge managedthe 1980 field program.ReferencesA.P.I. Bulletin 2N, 1982, "Planning,Designing and Constructing FixedOffshore Structures in IceEnvironments," Washington D.C., Firsted., 50p.Blanchet, D., 1986, "Variations of theLocal Failure Pressure with DepthThrough First-Year and Multi-YearIce." The Fifth OMAE Int. Symposium,Tokyo, Japan, Vol. IV, pp. 310-319.Blanchet, D., and M. Metge, 1984,"Model for Predicting Global Ice Loadson Wide Arctic Offshore StructuresDuring Impacts of Summer Multi-year IceFloes." The 7th Int. Symposium on Ice,IAHR, Hamburg, Vol. III, pp. 139-149.Bruen, F.J., R.C. Byrd, V. Vivatrat andB.J. Watt, 1982, "Selection of LocalDesign Ice Pressures For ArcticSystems." Proc. of the 14th OTCConference, Paper 04334, Houston,Texas, pp. 417-435.Croasdale, K.R., 1984, "The LimitingDriving Force Approach to Ice Loads."The 16th Annual OTC Conference,Houston, Texas, OTC Paper 04716,pp. 57-64.Croteau, P., M. Rojansky and B.C.Gerwick, 1984, "Summer Ice Floe ImpactsAgainst Caisson-Type Exploratory andProduction Platforms." Third OMAE Int.Symposium, New Orleans, Feb. 12-16,Vol. III, pp. 228-237.Danielewicz, B.W., M. Metge and A.B.Dunwoody, 1983, "On Estimating LargeScale Ice Forces from Deceleration ofIce Floes." Proc. of the 7th Int. POACConference, Helsinki, Finland, Vol. IV,pp. 537-546.Danielewicz, B.W. and M. Metge, 1981,"Ice Forces on Hans Island 1981." APOANO. 181, Dome Petroleum Limited, 65p.Danielewicz, B.W. and M. Metge, 1980,"Ice Forces on Hans Island 1980." APOANO. 180, Dome Petroleum Limited, l30p.Jaeger, J .C., 1971, "Elasticity,Fracture and Flow: With Engineering andGeological Applications." Third Ed.,Chapman & Hull.Korzhavin, K.N., 1971, "Action of Iceon Engineering Structures." 1962 U.S.Army CRREL Translation TL 260.Kreider, J .R. , 1984, "Summer ImpactLoads From Multi-year Floes." Proc. ofthe 7th Int. IAHR Symposium, Hamburg,W. Germany, Vol. II, pp. 55-65.Mellor, M., 1983, "Mechanical Behaviorof Sea Ice." U.S. Army CRREL Monograph83-1, pp. 85-97.483

Metge, M., B.W. Danielewicz and R.Hoare, 1981, "On Measuring Large ScaleIce Forces; Hans Island 1980." Proc.of the 6th Int. POAC Conference, QuebecCity, Quebec, Canada, Vol. II, pp.629-642.Michel, B., 1978, "Ice Mechanics." LesPresses De L'Universite Laval, Quebec,pp. 62-75.Michel, B. and Blanchet, D., 1983,"Indentation of a S2 Floating IceSheet in the Brittle Range." Annals ofGlaciology, Vol. 4, Hanover N.H., pp.180-187.Parmerter, R.R. and M.D. Coon, 1973,"On the Mechanics of Pressure RidgeFormation in Sea Ice." Proc. of theOffshore Technology Conference, OTCPaper #1810, pp. 1-735 to 1-740.Sinha, N .K., 1986, "The Borehole Jack.Is it a Useful Arctic Tool?" Proc. ofthe Fifth OMAE Symposium, Tokyo, Japan,Vol. IV, pp. 328-335.Timco, G.W., 1986, "Indentation andPenetration of Edge-loaded FreshwaterIce Sheets in the Brittle Range."Proc. 5th Int. OMAE Symposium, Japan,Vol. IV, pp. 444-452.Vivatrat, V. and J.R. Kreider, 1981,"Ice Force Prediction Using a LimitedDriving Force Approach." Proc. of the13th Offshore Technology Conference,OTC Paper #4115, pp. 471-485.DiscussionL. FRANSSON: What was the weather likeduring the load events, and how would youdescribe the ice surface?D. BLANCHET: The Hans Island fieldprograms were conducted in July andAugust. During this period temperatureswere consistently within two or threedegrees of freezing. Winds at this sitewere generally aligned with Kennedychannel. Winds from the NNW predominatedbut SSW winds probably occurred for 30%of the time. In a three-week program wecould expect 2 to 4 days of light winds(below 10 knots), 10 to 15 days ofmoderate winds (10 to 30 knots), and 4 to5 days of high winds (exceeding 30knots) . Precipitation occurred in theform of drizzle or snow in negligibleamounts. Visibility was limited by fogor low (200 m) cloud for perhaps 30% ofthe time. The ice surface showed noticeableablation. The upper 5 to 10 cm weregenerally soft and resembled old warmsnow in large (1 to 2 mm) granules. Thetopography varied from floe to floe, butundulations with a 10 to 15 m wave lengthand 0.5 m amplitude were typical. Ridgeswere oblated and less than 2 m in height.Interconnected puddles were found on manyfloes and accounted for anywhere between10 and 50% of the surface area. Meltpools probably averaged 20 to 50 cm indepth. Thaw holes were infrequent.Vivatrat, V. and S. Slomski, 1983, "AProbabilistic Basis for SelectingDesign Ice Pressures and Ice Loads forArctic Structures." Proc. of the 15thOTC Conference, Paper 84457, H,ouston,Texas, pp. 121-131.484

IMPACT ICE LOADS ON OFFSHORE STRUCTURESA. TunikAmerican Bureau of Shipping, Paramus, New Jersey, USAAbstractThe impact of a freely floatingice feature against a grounded offshorestructure is analyzed to determineglobal forces and contact pressures inthe time domain. The ice-induceddynamic loads on structures depend onice failure mode, mass and velocity ofthe ice feature, the mechanicalproperties of the ice and shapes of thestructure and ice feature. All of thesefactors are analyzed. The analysisincludes a wide range of ice features(icebergs of various shapes, multi-yearice floes and ridges, bergy-bits, etc)and offshore structures (massivecaisson- type platforms with vertical orsloped walls, conical and prismaticcolumns, wedge-shaped piers, etc).Practical methods and formulas are givento calculate dynamic ice loads forparticular loading scenarios, as well asto select design ice loads for astructure.ANALYTICAL HODELFreely floating ice featuresdriven by environmental forces cangenerate high impact loads on fixedoffshore structures. At slow speeds ofThis is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987. © The Geophysical Institute,University of Alaska, 1987.the advancing ice, the loads are limitedby the contact strength of the ice as asolid. At higher velocities, thecrushed ice being forcibly squeezed outfrom the contact zone continues totransfer pressure from the soliduncrushed ice to the structure. Theimpact pressures generated at thestructure's surface can significantlyexceed the contact strength of ice.Based on experimental studies, thesolid/ice impact was simulated byKurdyumov & Kheisin (1976) as a viscoplaticflow of the ice crushed at thecontact zone. For moderate velocitiesof the order of 1 m/s the problem wasreduced to simplified Reynolds'equations for a thin viscous filmbetween two rigid surfaces approachingeach other. One of the surfaces is thestructure's while another is the surfaceof uncrushed solid ice which has beendistinctly observed in experiments.Assuming linear pressure distributionthrough the thickness of the crushed icelayer, the authors solved the problemfor two cases: 1) axially symmetricice/solid impact (a steel ball droppedon a flat ice surface) (1976), and; 2) aship side impact against the edge of around floe (1974). The first solutionwas intended to verify the validity ofthe mathematical model because the dropball test is easy to perform andreproduce, with all impact parameters485

well controlled and recorded. Thesecond case represents one of thetypical and most dangerous scenarios ofship/ice interaction. Other cases havebeen considered by Tunik (1984),including the general case of axiallysymmetric impact, repeated rams intoimprints of previous rams, rammingicebreaking by icebreakers withconventional and spoon-shaped bows, etc.Whatever scenario is considered,the mathematical model yields thesame differential equation for pressuredistribution over the solid's surface,which is written here for theaxisymmetric impact,d 2 P + 1 (d~dr 2 P dr(1)a structure, a ball, etc) on the icefeature. Using certain geometricidealizations of the ice and the solid,a number of practical cases have beenanalyzed by Kurdyumov & Kheisin (1974)and Tunik (1984, 1985). The shapes ofthe solids (ships or engineeringstructures) and the ice features vary sogreatly that a closed-form solution of(3) in general is hardly feasible.However, for many practically importantidealizations, the closed-form solutionsyield the same structure of theexpressions for impact load parameters,which can be written as follows:P(t)= K MmPVvpaap S P(t)pp(Sa)F(t)- KfMmfVvpaaf SF F(t) (5b)A(t)= KAMmaVvaaaa SA A(t)(5c)whose solution isT= KTMV/F max(5d)P _ aV1/4 ro 1/2 (1_,[,2)1/4(2)Z(t)- KzMmzVvzaaz Sz Z(t)(5e)where a -k(1.5 J.'k3)1/4Pis(2a)(2b)s-thickness of the crushed ice masslayer,p-pressure on the surface of the solidr-cylindrical coordinate~o-contact radiusr - r/roV-penetration velocity of the solid intoiceJ.'-viscosity of the crushed ice massThe impact

critical impact scenario can occur whena relatively small block of ice (bergybit, ice floe or growler) driven by windand waves hits the structure. In eithercase the shapes and sizes of advancingice can vary greatly as well as those ofthe structures. The latter are veryspecific and known in advance while theformer vary randomly.The shapes of the structure andthe ice influence the components ofequations (5) in two ways. Firs t, thesurface integration in equation (4) overthe contact zone area (which is afunction of the penetration depth) andthe following integration of equation(3) with respect to the penetrationdepth yield certain exponents and shapefactors in equations (5). The exponentsand shape factor are, therefore,dependent on the local shapes of thecolliding bodies at the contactlocations. The local shapes also affectthe relationship between the horizontalvelocity of drifting ice and thepenetration velocity component normal tothe structure's surface at the contactlocation. Secondly, the reduced mass ofthe impact "M" depends, via the addedwater masses, on the global shape of thefloating ice. Thus, to use theequations (5) for designing offshorestructures, the exponents in (5) shouldbe specified considering only the shapesof the local contacting surfaces.The grounded gravity structuresdesigned to withstand the impacts ofdrifting icebergs or multi-year icefloes may include vertical walls,slopes, conical or prismatic columns,wedged piers, or some scallopedfeatures. In general, the structure'slocal shapes can be distinguished asflat, cylindrical surfaces, wedges andscallops.The ice at the contact zone canhave any imaginable shape. When the iceshape is somewhat sharp like an apex,the global impact force is significantlyless than that for a flat ice surface,while local pressures for these casesare rather comparable. Therefore, inderiving a design formula for ice loadsthe local contact surfaces of icefeatures are assumed to be flattened.Since the widths of Arctic offshorestructures are considerably smaller thanthe width of most icebergs and multiyearice floes, the local ice shape isidealized either as spherical (forinteractions with flat structures) orflat otherwise. Thus, three principalcombinations of the contactinggeometries are distinguished here (Fig.1):- flat structure versus spherical ice;- cylindrical or conical structureversus flat ice;- vertical or inclined wedge versus flatice.The first group is equivalent tothe ball-drop-test scenario (as shown byTunik, 1984) . The case has beenrecently re-analyzed by Nevel (1986) toestimate the iceberg/structureinteraction forces. The Kurdyumov-Kheisin's model for a drop ball testwas also used by Brown et al (1986) forestimating the iceberg force. For thesecond and third groups, the elementarycontact areas in equation (4) are:dA-hdx (6)x_(2RZ)1/2 for second group(7a)x-Z/cos a for third group (7b.)As a result, equation (3) can be writtenas follows:MV3/4dV __ iah(2R) 1/2 Z 3/4 dZfor second groupMV3/4dV __ iah( cosa) -3/2 Z 3/2dZfor third group(8a)(8b)The resulting ice load parametersare also expressed by equations (5)whose exponents, coefficients and shapefactors are given in Table 1. The datain Table 1 are also applicable forsloped walls (column 2), cones (column3) and inclined wedges (column 4), withappropriate corrections for added watermasses, velocities and shape factors.The latter are to be as given in Table 1where:Dc-Do/cos~(column 3 only)h~=h/sin~ (column 3 & 4)(9a)(9b)Do-cross-section diameter of the conicalstructure at the waterline level, m,h -ice thickness to be used instead of~ h;487

iI9 1a,z,d/ ~J jkb~/:l(~II\e.Ds(?k".~}hcJzFig. 1Ice/StructureImpact Scenariosa,b - vertical wall against an icebergc - a slope against an icebergd,e - cylindrical column against an ice floef - cone against a floeg - vertical wedge against a floe488

The velocity "V" in equations (5)is the initial velocity of thestructure's penetration into advancingice during impact. For verticalstructures this is the horizontalvelocity of the advancing ice, Vo. Forinclined structures:V -Vo sinrp(10)The mass in equations (5) is thereduced mass of collision. Since thestructure is grounded, the mass ofcollision is reduced to the mass of thefloating ice with added water masses.Equations (5), with details givenin Table I, can be used for estimatingthe impact forces and pressures generatedby ice features whose sizes, shapes andproperties are statistically selectedfrom available observation data. Thesemaximum impact loads can be used as abasis for selecting design loads, eitherfactored or not.EFFECT OF THE VARIABLES ON THE IMPACTLOADSThe effect of a particular variableon the impact loads is chdracterized bothby the appropriate exponent in formula(5) and by the variation range of thevariable in question.Velocity7he velocity of massive icefeatures (icebergs, ice floes) can hardlyexceed 3 knots (l.5 m/s). On the otherexPONENTS,hand, for all areas where drifting icefeatures appear, the probability of a 0.5mls drift speed is .very high, i.e.,slower speeds should be ignored. Thus,for Arctic gravity structures, the rangeof velocity variations is relativelynarrow, approximately from 0.5 to 1.5 mlsor even less. For such a narrow velocityrange, a variation in the value ofvelocity exponents "vf" and "vp" by 0.1or 0.2 changes the global force (F) andpressure (P) by less than 5 or 10%. Thismeans that in predicting the impactloads, the velocity effect can beapproximated so that a "standard" valueis used for each of the velocityexponents regardless of particularidealizations (scenarios), e.g. vf - 1.2and vp - 0.5.For pile-supported structures thevelocity of small ice blocks driven bystorm waves can reach up to 3 m/s.However, the velocity variation rangeremains narrow, because velocities slowerthan approximately 1.5 2m/s may bedisregarded for this type of criticalscenarios. Therefore, the same"standard" velocity exponents are alsoapplicable for tubular and platingmembers of the pile supported structures.Shape FactorsFor the considered idealizations,the shape factors are functions of handD whose meanings depend on the scenarioin question. For the first-typeTABLE 1COEFFICIENTS AND SHAPE FACTORSFOR THREE GROUPS OF CRITICAL SCENARIOSCOLUMN 1234ScenarioFlat Wall(caisson face)vs sphericalice (iceberg)51 9 _0.555 . . .11/9_ 1 . 222 .. .i~~-o .444 .. .4/9- 0 . 111 .. .8/9- 0 . 444 .. .-0.888 .. .-l. 3-0.8-1 5D5i9D1/9Cylindricalstructure vsflat ice3/7_0 . 428 ...t/71/7 -0. 57l. ..-0.142 ...1/2_ 0 . 56/7_0 . 857 ...-l.28-0.77-1 °D3/7h4/7(D/h) 1/7Wedged-shapedpier vs flat ice0.6l.30.40.20.60.8l.10.760.8(hSinz)0.4 (Cosz)_0.6(hSinz)-0.2 (Cosz)0.8489

scenarios, the parameter Di in column 2of Table 1 is a double radius ofcurvature of the idealized ice surface.Since any sharp and high-curvaturesurfaces of ice are disregarded, thediameter Di may be statisticallyassociated with a principal dimension ofthe ice feature in question. Therefore,Di can be assumed to be proportionalei ther to bc/d (where b, c and d - icefeature's dimensions of which b is thegreatest one and d is the smallest one)or, what is essentially the same, to thecube root of the ice feature's volume,and consequently, to the cube root ofthe ice mass. When substituted into(Sa, b) and, accounting for the valuesgiven in column 2 of Table 1, the latterproportionality yields:F M20/ 27 _ MO. 741 ... (lla)P _ M4/27 _ MO. 148 ...(llb)For the cylindrical and conicalstructures, parameter D is the diameterof the cylindrical structure, while h isan averaged thickness of the ice edge.For a level ice floe, the ice thicknessis specific and the shape factors aredetermined by the expressions of column3 of Table 1. For an iceberg, the iceedge thickness is somewhat indefinite.Again, it can be assumed proportional tothe cube root of the mass, yielding:F _ M16/ 21 _ MO. 762 .. .(12a)P _ M1/7 _ MO.133 .. .(12b)By analogy, for the wedge-shapedstructures acted upon by an iceberg wehaveF _ MIl/IS _ MO.7333 .. .(13a)P _ M2/ ls _ MO. 133 .. .(13b)Thus, whatever idealization isconsidered, the global impact force isproportional to the ice mass to thepower of 0.733 to 0.762, while for themaximum pressure the exponent variesfrom ° .133 to ° .148. For such narrowranges of the exponent variations, some"standard" values (mf = 0.75 and mp =0.14) can be selected regardless ofparticular shapes and scenarios and evenregardless of the great range ofvariations in ice masses.Dynamic Crushing Strength of IceThe parameter "a" characterizingthe resistance of the crushed ice massto squeezing dynamically out from thecontact zone can be called the dynamiccrushing strength of ice. It is definedby the equation (2a) and has a dimensionof MPa(s/m3)1/4. As seen from (2a) itdoes not have a direct relationship withthe compressive strength of ice. Nordoes it have a direct relationship withother ice strength characteristics instatic loadings. However, a certaincorrelation can supposedly be found.Values of "a" can be approximatelyestimated from available data on iceimpact tests (Glen & Comfort (1983),Likhomanov & Keisin (1971), Khrapaty &Tsuprik (1976), Sodhi & Morris (1984) inwhich the testing parameters werecontrolled, though not all of themreported. A preliminary estimate yieldsa variation range from a 1-4MPa(s/m 3 ) 1/4 for freshwater spring(deteriorating) ice to as-10MPa(s/m 3 ) 1/4 for winter freshwater lakeice. Analogous to the mass and velocityeffects, but with a higher margin oferrors, some simple "standard" valuescan also be selected for exponents "af"and "ap"; for example, af - 0.5 and ap -0.87 (or ap - 0.9).Ice Mass with Added Water MassesThese considerations given to theeffects of the variables on impact loadsmake it possible to derive simpleapproximate equations similar to (5),but with the "standard" exponentsregardless of specific ice features:F '/MO.7SV1.2Ao·sS/ (14a)max-·'"'FFP _K f M O . 14 V O . S AO. 9 Sf (14b)mu P Pwhere only dimensionless shape factors Sfand coefficients Kf depend on particulartypes of structures and scenarios.It should be noted that formulae(14) are dimensionally inconsistent andmay be used only with the given units(MN, MPa, MNs 2 /m, m/s and MPa(s/m3)1/4 forF, P, M, V and "a", correspondingly) andwithin the given narrow ranges of speedand dynamic crushing strengths of ice.Equations (5) are dimensionally correctand can be used within any consistentunit system.490

The mass "M" in equations (5) andthrough (14) is the ice mass "M/ withadded water masses reduced towards thenormal to the structure's contactsurface. Added water mass coefficientsfor icebergs can be obtained; forexample, from Bass & Sen (1986), whilefor ice floes the simple Popovrecommendations (Popov et al, 1967) canbe used. In general, the reduced icemass can be expressed following Popov etal:M = Mi C (15)C- 1 - z.:.[l./(l+A.) +J J J+ (MiR2./I )/(l+B.)](16)J J Jwhere: j - x;y;z;-global coordinateaxes;lj-directional cosines of the normal;Aj and Bj-added water mass coefficientsfor translational and rotationalmovements relative to axis j;I j and R. -moments of inertia and arms ofcentrJid of the ice feature relativeto axis j;Mi-ice massFor the central impact against avertical structure, only the surge addedwater mass may be taken intoconsideration. For icebergs, the surgeadded water coefficient can be taken asAy - 0.5 ± 0.1 (see Fig. 5 of Bass & Sen(I986) , while for level ice floes, A S-0.1. As a result, the reduced mass is,M - 1.5M for icebergs and M - (1 to 1.1)Mi for floes. For the central impactagainst an inclined structure thesurging, pitching and heaving motionsshould be included in considerations.For a central impact of a level ice floeagainst inclined structures, the heaveand pitch added water mass coefficientscan be taken A =1,B =0.25, with ratioz yMiR 2/1 =2. As a result, M=M.~1+1.lCos2a). The heave and pitchco~fficients for icebergs can also betaken from Bass & Sen (1986) (e.g. Az=l,B -0.05 - see Fig. 7,8 and 9 of Bass &S~n (1986), yielding M = M (2/3 + l. 74Cos 2 a) . 1EXAMPLETo illustrate the use of equations(5) or (14) for estimating the impactice loads, the following "typical"examples of summer Arctic events arebriefly considered.A.B.C.D.E.F.A "moderate" size icebergat a very high velocityvertical wall.A large iceberg driventypical current velocityvertical wall.drivenhits aat ahits aThe same as B, but the s truc tureis a sloped wall.A very thick multi-year ice floeof moderate size driven at a highspeed hits a cylindricalstructure.The same as D, but the structureis a cone.The same as D, but the s truc tureis a vertical right-angled wedge(a caisson-type structure impactedsymmetrically at an edge).All data assumed are presented inTable 2. The added water masses aretaken into account using equations (15)& (16). The iceberg's masses areselected as 1 million tons for the"moderate" one and 10 million tons forthe large one (in accordance with Lewiset al (1981) their exceedanceprobabilities for the Grand Banks are0.33 and -0.005 consequently). The massof 1 million tons is also selected forthe ice floe.The quantitative values of theloads given in Table 2 do notnecessarily represent "typical" impactload predictions for the structures inquestion. Rather, they should beregarded as comparative characteristicsdepicting the effects of the variables.It should be noted that the impact loadsgiven in Table 2 present only the impactphase of ice/structure interaction.Post-impact, quasi-static indentationcan produce comparable ice forces, butlower pressures. The examples of impactloads given in Table 2 are selfexplanatory.CONCLUSIONSAn analytical model for predictingice impact forces on structures has beenpresented. As seen both from basicequations (5) (with Table 1) and fromthe examples, the maximum impact forcedepends significantly on the structure'sshape at the impact locations and to alesser degree, on the ice feature'sshape. A similar conclusion has beenreached by Johnson & Nevel (1985).491

ScenarioIce mass, M: tons x 10 6MNs 2 /m x 10 3Velocity, V, m/sDynamic crushing stre~gf? ofice, a, MPa(s/m) 4Dice' mDstructure' mStructure's inclination angle,Z, deg.Added water mass coefficients:~ (surging)A z (heaving)By (pitching)Ice thickness, h. mTABLE 2A111.58100900.5ooB C10 1010 100.5 0.58 8100 10090 450.5 0.5o 1o 0.05D E1 11 11.5 1.58 830 3090 45o 0o 1o 0.258 8F111.583090oo8Impact loads given byF max ' MNP max ' MPaDuration, T, secReduction mass coefficient,Cequations (5)4038 379022.9 18.10.8 3.01.5 1.5and Table 12940 1740 166417.2 18.2 14.32.4 0.9 0.80.95 1 0.966510.91.81The maximum pressure is also dependenton the shapes, though not as much as theforce. In addition to the shape effect,the speed and mass of the ice featureare of primary importance for the globalimpact force and consequently for impactduration, while the maximum pressuredepends mostly on the dynamic crushingstrength of ice.ACKNOWLEDGEMENTThe author gratefully acknowledgesthe American Bureau of Shipping for theopportunity to publish this work.REFERENCESBass, D.W., Sen, D. (1986) Added Massand Damping Coefficient for Certain"Realistic" Iceberg Models. .QQ1QRegions Science and Technology, Vol. 12,No.2, pp 163-174.Brown, T.G., Kocaman, A., Punj, V.,Bercha, F.G. (198?) Iceberg-StructureInteraction Global and Local Loads.Proc. OHAE-86, April 1986, Tokyo, Japan,Vol. 4, pp 374-378.Glen, I.F., Comfort, G. (1983) IceImpact Pressure and Load: Investigationby Laboratory Experiments and ShipTrials. Proc. POAC-83, Helsinki,Finland, April 1985, Vol. I, pp 516-533.Johnson, R.C., Nevel, D.E. (1985)Impact Structural Design Loads.POAC - 85, Narssarssuaq, Greenland,Sept 1985, Vol. 2, pp 569-578.Ice~7-14Khrapaty, N.G., Tsuprick, V.G. (1976)Experimental Investigation of Solid BodyImpact on Ice. Proc. CoordinatingMeeting on Hydraulic Engineering, issueIll, Leningrad, "Energy", pp 166-169,(in Russian).Kurdyumov, V.A., Kheisin, D.E. (1976) AHydrodynamic Model of Solid/Ice Impact.Applied Mechanics, Kiev, Vol. XII, #10,pp 103-109.Kurdyumov, V.A., Kheisin,Determination of Ice LoadsIcebreaker Hull Due toTrans.Institute,Russian).Lengingradissue 90, ppD.E. (1974)Acting uponan Impact.Shipbuilding95-100 (inLewis, J., Lewis, B., Peter, C. (1981)Icebergs on the Grand Banks: Oil and GasConsiderations". World Oil, January1981, pp 109-114.Likhomanov, V.A., Kheisin, D.E. (1971)Experimental Investigation of Solid BodyImpact on Ice. Problems of the Arctic &Antarctic, issue 38, pp 105-111, (inRussian) Translated by AmericanPublishing Co. Pvt. Ltd., 1973.492

Nevel, D.E. (1986) Iceberg ImpactForces. Proc. IAHR Ice Symposium-86,Iowa City, Iowa, August 1986, Vol. 3.Popov, Y.N. Faddeav, O.V., Kheisin,D.E., Yakov1ev, A.A. (1967) Strength ofShips Navigating on Ice. Leningrad,Sudostroenie, (in Russian). TranslationNo. FSTC-HT-23-96-68 by US Army Scienceand Technology Center, Washington, D.C.,1969.Sodhi, D.S., Morris, C.E. (1984)Dependence of Crushing Specific Energyon the Aspect Ratio and the StructureVelocity. Proc. OTC 1984, Paper OTC4688, May 1984, Houston, TX, Vol. 1 pp363-375.Tunik, A.L. (1984) Dynamic Ice Loads ona Ship. Proc. IAHR Ice Symposium 1984,Hamburg, F.R. Germany, August 25-31,Vol. 3, pp 227-313.Tunik, A.L. (1985) Hull Girder BendingForces due to Ramming Icebreaking. ProcPOAC-85, Narssarssuaq, Greenland, Sept.1985 Vol. 2, pp 873-881.493

LOADS ON RESEARCH VESSEL POLARSTERNUNDER ARCTIC CONDITIONSL. MullerH. G. PayerGermanischer Lloyd, Hamburg, F. R. GERMANYAbstractDetailed measurements were made duringtwo ice breaking expeditions with the R. V.POLAR STERN with the aim to check theactual magnitude of the ice loads in comparisonwith the design loads. After a brief descriptionof the instrumentation and the evaluationof the data some more interesting resultsare presented.From the results it is concluded that inreatity the loads act more as concentratedloads than distributed loads, as has been thebasis for the design of ice-going vessels. Thisshould lead to comparable plate thicknesses asused today, with the possibility for a reductionof the supporting structure. Permanent deformationsof the shell plating in the bilge andbottom region of the ship clearly indicate thatconsiderable forces are exerted by ice floes,that are dynamically displaced beneath theship. The magnitude of these forces is determinedanalytically by means of nonlinearfinite element calculations in relation to permanentset observed. Finally the static anddynamic behavior of the propeller nozzles indiffering ice conditions are described.This is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987. © The Geophysical Institute,University of Alaska, 1987.1. IntroductionThe prediction of forces acting on vesselsbreaking arctic ice, whether based on theoreticalconsiderations or on model studies, stillcontains a large number of uncertainties.Therefore, it was agreed at an early stage inthe planning phase for the German Polar ResearchVessel POLAR STERN to make fuUscale measurements upon completion of theship. Apart from a critical re-examination ofmodel test results it was possible here to studyaspects of the ice breaking process which cannot be investigated satisfactorily in modeltests, such as the interaction between ice andthe ship structure as well as the behavior ofthe propulsion plant during a voyage in ice.The first ice breaking expedition with R. V.POLAR STERN was carried out off the coastof Labrador in 1984. Originally Frobisher Bayat the south end of Baffin Island was selectedfor the experiments, as the ice was expectedto be sufficiently thick and hard there for theharshness tests of POLARSTERN. Permissionfor the ice breaking experiments in this region,however, was not granted by the CanadianCoast Guard, on the grounds that thevessel was not adequately reinforced in thestern to fully comply with the Canadian ArcticShipping Poltution Prevention Regulations(CASPPR). The winter of 1983/84 turned outto be very severe in Eastern Canada and sufficientlythick ice developed e\l en off thecoast of Labrador, which is outside the juris-495

diction of CASPPR. Therefore, it was expectedthat the experiments could also be performedsuccessfully in this region. Unfortuna~tely, the cold winter was followed by a relativelywarm spring so that at the time of thePOLAR STERN experiments the ice in the regionhad already started to thaw and had onlyabout half the winter strength. The resultsconsequently could not be generalized. Thiscalled for further experiments in ice in orderto confirm the results of the first voyage andto supplement these findings by results fromwinter ice. The second expedition was conductedin May 1985 in the waters aroundSpitzbergen, where the desired ice conditionswere encountered.Length over all L 118.00 moaLength betweenperpendiculars L 102.20 mBreadth at main deck BPP 25.00 mDraft d 10.50 mDisplacement D 15,690 tSpeed v 17.00 knFour main enginestotal powerN tot= 14,000 kWTwo controlJ.able pitch propellers withtype Becker-nozzles:diameterbladesspeed4,200 mm4182.4 RPMKort2. R. V. POLAR STERNThe R. V. POLARSTERN was planned as aversatile polar research and supply vessel,(Fig. 1). Her principal dimensions are as follows:Figure 1:R. V. POLAR STERN496

3. Objective of Experiments - DataAcquisition SystemDuring both R. V. POLARSTERN icebreaking expeditions the following investigationwere performed predominantly byGermanischer Lloyd:- Determination of loads on the ship hullfor varying ice conditions.- Analysis of stresses in the propeller nozzlesin ice.- Determination of the stresses in deck andsuperstructure under the influence of ambienttemperatures.- Examination of the propulsion plant understationary and non-stationary conditionsin ice.The efforts for the extensili e preparationsactually exceeded those for the expeditionsthemselves. They included the layout and procurementof the instrumentation and the dataacquisition systems as well as the extensiveinstrumentation and installation of cabling. AUsignals, both from the marine engineering aswell as the structural side, were fed into acentral registration unit where they were recordedsimultaneously. It was necessary to beable to relate signals from different transducersto each other for any desired event.For example, the correlation of signals fromthe propeller nozzles and the propulsion plantwas studied for the event of ice rubble passingthrough the propellers or an ice block cloggingthe nozzles.The large amount of data was recordedsimultaneously in digital form by a PCM (PulseCoded Modulation) system. The units used byGermanischer Lloyd recorded up to 120 channelssimultaneously on one track of a magnetictape. For the evaluation and analysis the datais fed into a decoder, from which it is transmittedto a converter unit via a computerinterface. The converter unit sorts the datafrom each signal into data blocks and storesthem consecutively in digital form on a taperecorder. The resulting digital tape finaUycontains the input for the data analysis programson the computer.In the foHowing the topics of local loads onthe hull in ice as well as stressing of thepropeller nozzles win be treated. A detaileddescription of the complete investigation canbe found in MOller et al. (1986). Parallel to theinvestigations on the hull structure, the Hamburgship towing tank HSVA made directmeasurements of ice strength and ice loadsduring the expeditions.4. Ice Load on the Ship HuHThe overall scantlings of the hull as well asthe dimensioning of local structural details onice-going vessels are based largely on empiricaldata, mostly obtained from ships operatingin the Baltic Sea. Load assumptions forArctic service contain extrapolations fromthese data, taking into account the particularitiesof the Arctic, and are supported byvery limited experience. Only very recentlyhas research into the Arctic environment andthe behali ior of ships in extreme ice conditionsprogressed considerably, see for example Glenet a l. (I985).The scantlings of the R. V. POLAR STERNare based on the rules of Germanischer Lloydof 1982. The Arctic ice classes ofGermanischer Lloyd correspond largely tothose of the CASPPR Regulations. There areonly slight differences in the design ice pressures.The hull structure of the vessel is laid outfor Arctic class Arc 7 whereas for reasons ofeconomy the machinery has only class Arc 3.This reflects the reasoning that R. V. POLAR­STERN was to be designed to survive a polarwinter possibly in a frozen-in situation. Themachinery on the other hand is laid out forpolar summer only. The class designation ofR. V. POLAR STERN consequently readsGL + 100 A4 Arc 3, reinforced to 9.5 MPa inthe bow and 6 MPa amidships.Until· then no ice loads were to be consideredfor the bottom and bilge region. It wasassumed that these regions, if at aU, wereexposed only to small ice loads. This assumptionis valid for a voyage in homogeneous levelice. Recent experience with ships in Arcticconditions, however, shows that even the bilgeand bottom regions of the vessels may beexposed to considerable ice loads when travellingthrough drift ice, rubble and pack ice. Theobservations with R. V. POLAR STERN confirmthis, as the nonreinforced areas wereobviously over1.oaded, white the load assumptionsfor the reinforced regions are sufficient497

or conservative as no damage has been observedeven after repeated Arctic service.4.1 Local Ice Loads on the BowIn the bow of the vessel a horizontallongitudinalframe at the port side, slightly belowthe water line, was instrumented with a largenumber of strain gauges (Fig. 2). For the 1984voyage the instrumentation was laid out in away to allow the determination of global bendingand shear stress distributions as well aslocal stress peaks. The region instrumented isbetween frames ll2 and ll4, about 12 metersfrom the forward perpendicular of the ship.This is exactly the location where HSVA madedirect load measurements with special triaxialforce transducers in a recess of the side shellat the starboard side of the vessel, Hoffmann,L. (1985). This way it is possible to comparethe results of these two largely-different recordingdevices.instrumentedlongitudinal frameon port sideOnly a very small fraction of the bowsection of the vessel was covered by thisinstrumentation. It was reasoned that with thelarge number of ice impacts expected, eventhis limited area would give sufficient informationto allow a generally valid prediction ofthe magnitude of ice loads. The 1984 expeditionshowed, however, that due to rolling andtrim of the ship during ice breaking and ramming,the instrumented section frequently e­merged and experienced impacts only occasionally.For the 1985 expedition, therefore, amuch larger area of the bow, of about 2 by 8meters, was instrumented for the determinationof global loads, (Fig. 3). Loca! stresspeaks were no longer a topic for this in\! estigation.D+ ____ ___~=~;;i=' ~L~ __ ~I ,------------------4----~- -------IIi____ , __________ ----t-------- -I~ instrumented area 1985~ instrumented area 1984Fig. 3:Location of Instrumented Areas inBow SectionFig. 2:Typical Arrangement of StrainGauges on Bow FrameThe evaluation of the results from thesecond expedition confirmed that the longitudinalframe selected does in fact belong tothe most highly-loaded region. The ice loadswere determined from the strain results withthe help of comparative theoretical calculations.A rather detailed finite element modelof the extensively-instrumented longitudinalframe was set up for this purpose. The calculatedstress results were fitted to the measurementsby a systematic variation of loading,leading to the information on magnitude anddistribution of ice forces. The results wereexpressed in terms of total force per frame aswell as contact length with the ice.It is interesting to note that contrary toinitial expectations, the tangential componentresulting from the friction of the ice along theshell is signi ficant and may not be neglected inthis analysis. The best agreement between498

cal.culations and measured results was obtainedfor this experiment using a frictioncoefficient of 0.1.Another important result from the analysisof the measurements is the fact that the iceloads act as localty-confined impact loads forall ice conditions, i. e. also for the voyage inhomogeneous sheet ice. In Fig. 4, the calculatedextreme fibre stress distribution in theframe, as a result of the uniform load of9.5 MPa used for the dimensioning of thestructure, is shown. Typical experimentallydeterminedstresses are shown for comparisonin Fig. 5. Where in the first case maximumstresses are found at the support points of theframe, the experimentally-determined stressdistribution exhibits mainly bending near midspan,resulting from a load concentrated midwaybetween supports.ha If that of the design load.100 N/mm2Fig. 5: Measured Extreme Fibre Stress due toImpact with Multiyear Drift Ice. Total Forceper Frame = 1. 7 MNTable 1IceConditionForce/FramekNIce PressureMax. Equiv.MPa MPaMultiyearice Hoecompressiveridge/pack3,6001,260homogeneoussheet ice 870design icepressure 6,0809.5 6.63.4 2.33.0 2.09.5Fig. 4: Calculated Extreme Fibre Stress inFrame due to Design Load of 9.5 MPaThis indicates that the experimental resultscan not be directly compared wi th thedesign loads. Therefore, an "equivalent pressure"load is used in the following, defined as afictitious uniform pressure which results instresses at the support points of equal magnitudeas that caused by the ac tualload near thecenter. The following table lists the maximumvalues which were determined from the 1984measurements in Labrador. The values fromthe Spitzbergen measure ments of 1985 turnedout to be slightly lower.The table indicates that local ice l.oads of amagnitude equal to the design ice pressuremay in fact be encountered under extremeconditions. However, the total transverseloading of a frame is found to be onlyaboutIt was surprtsIng in the evaluation of themeasurements that the maximum loads werelargely independent of the ship's speed andappeared to be independent of the ice thickness.This, however, must be qualified by thefact that the measurements availabl.e weremade for ice thicknesses between 0.9 and 1.3meters. Results for thinner ice may be different.Based on theory one would expec t the iceloads to increase with the ship's speed. Thestrength of ice depends on the deformationrate and has a maximum at the transitionrange from ductile to britt1.e behavior. Theeval.uation of the measurements, however,shows only a very slight increase of loads withspeed, if at all.499

to the spacing of the longitudinal frames, andthe contact length was varied. The assumptionof a rectangular ice raft is of course improbable,but one may assume that after firstcontact of the raft with the' sheU and afterlocal yie1.ding and breaking of the ice, a continuouscontact area will deve1.op for the transmissionof the total impact l.oad., III 1III I\ \I \ \\ \idealizedice floeFig. 6:Calculated Stresses in Drift Iceand FrameDeal ice pressureFig. 7: Calculated Distribution of Ice LoadIn the derivation of the loads from thetests, the question arises whether "ice-bridging"has to be considered. This is the effect inwhich the main portion of the load is appliedto the stiffer parts of the structure at thesupport points. Ice-bridging can be of particularimportance for the layout of the side shell,with its flexible plate bending regions awayfrom the boundary supports at frames. Toinvestigate the relevance of this effect, thecomputer model of the frame mentioned abovewas loaded by a drift ice raft, which was alsodescribed by finite elements (Fig. 6). The icethickness was taken to be 0.4 m, correspondingFig. 8: Damage in the Shell Plating due to IceLoads. Starboard BilgeThe calculated f1.ow of forces in ice andframe is shown together with the extremefibre stresses in the frame for an assumedcontact l.ength of half the spacing of transverseweb frames in Fig. 6. The resulting distributionof pressure on the frame can be seenin Fig. 7. It should be noted that these areresults from a rather simple linear calculation.A more accurate analysis would have to takeaccount of non-linear effects and the rathercomplicated mater.ial. laws for sea ice.500

However, the distribution of pressure loadingobtained here agrees quite well with thoseload distributions derived in this investigationwhich have led to the best agreement betweenthe measured and calculated stress distributionsin the frame as described above. Comparableload distributions have also been obtainedwith non-linear calculation methods,Varsta (1983). A similar calculation for impactof large ice bits with a structure is reportedby Lindberg et at. (1987).4.2 Ice Loads for the Bilge and BottomRegionAt the time of the design and constructionof R. V. POLAR STERN Germanischer Lloyd'srules for the dimensioning of ships for Arcticservice did not include the bilge and bottomregion of the shell in the ice strengthening.The same is true for the rul.es of CASPPR. Itwas assumed that this area, which is far fromthe water line and ice cover, is only occasionallyexposed to small ice loads. However,recent experience from Arctic operations, includingobservations with R. V. POLAR­STERN, indicate clearly that higher ice loadsmust be considered also for the bilge andbottom region when operating in severe icerubble and pack ice. This is due to the factthat large pieces of ice are dynamically displacedbelow the ship by the bow and then hitthe bottom and bilge on their way back to thesurface.Typical damage to the bilge and propellerbossings of R. V. POLAR STERN after operationunder severe conditions in the Arctic isshown in Figs. 8 and 9. Damage like this, withpermanent deformations of the plating, is notconfined to the POLAR-STERN, but has beenexperienced by several other ships for Arcticservice.As the structure had been deformed beforethe experimental investigations, there was littlesense in instrumenting the regions concerned.Rather, it was decided to carefuUymeasure the permanent set in the plating andagain determine the loads which lead to suchdeformations on the basis of theoretical investigations.Non-linear methods are required for suchinvestigations, as the yield strength of thematerial is exceeded and large deformationsare involved. The ADINA program (AutomaticDynamic Incremental Non-linear Analysis) wasused for this part of the study. The theory ofthis finite element program is described byBathe (1975).Finite element models were set up forthose typical shell panel.s from the ship's bottomand bilge which were found to be stronglydeformed. Geometric nonJinearities have asignificant influence here; the membraneaction of the shell plating increases with theFigure 9:Damage in the Starboard Propeller Shaft Bossing due to Ice Impact501

deformalion and, for the given dimensions, theload-carrying capacity grows despite extendedyielding of the plate material.The results from this study are again expressedin the form of equivalent uniform icepressure, in order to facilitate comparison ofthe loads to which the structure must havebeen exposed with those from the rules. Someresults are shown in Figs. 10 and 11, whereequivalent ice loads are found for the bottomand side of R. V. POLAR STERN. In reality,the loads act as concentrated forces, just ashas been described in Section 4.1 above for thebow region. It would, therefore, be more realisticto dimension plate fields for ice loadsin the future by specifying locally-acting icepressures and to analyze and dimension theplating in a way similar to the decks of carcarriers exposed to wheel loads.equivalent pressureacc. to deformationso 1 2 3 N/mm 2bearable pressure I~"t---_ acc. to ~plate thickness0.2 N/mm2Fig. 11: Distribulion of Design Loads between.25L from Stern and Amidships for R.V.POLAR STERN. Modified to Account for BottomDamage Observedo2 3 N/mm2Fig. 10: Distribution of Design Loads betweenAmidships and .2L from Bow for R.V.POLARSTERN. Modified to Account for BottomDamage Observed5. Ice Loads on Propeller NozzlesThe propellers of R. V. POLAR STERNoperate in Kort-type nozzles (Fig. 12). So far,this arrangement is not very common for shipsin polar waters. The typical propulsion plantfor vessels of the size of R. V. POLARSTERNconsists of Diesel.-electric power without propellernozzles. DieseJ -electric transmission allowsflexibility in manoeuvering, but at theexpense of reduced efficiency compared todirectly-driven propellers. Directly-drivenpropellers, on the other hand, have to beprotected from ice bits, as sudden blocking ofa propeller could result in mechanical andthermal overloading of the propulsion plant.The main purpose of the nozztes, therefore, isthe protection of the propellers. Additionally,the nozzles improve the propul.sion at lowspeed, for instance, under severe ice conditions,comparable with a boUard test. Themain problem with nozzles in ice is theirtendency to clog up in pack ice.The nozzles were designed by the engineeringfirm Willi Becker, Hamburg. As there wasno experience with nozzles of the given size inice at hand at the time, the design loads werederived in agreement with Germanischer Lloydfrom the rules used for the hull. Considerablesafety factors were employed taking accountof the consequence of the failure of a nozzleon the safety of the ship.502

It was assumed that during a voyage in ice,the nozzle would be exposed principally toimpacts in the longitudinal direction of theship. Pure compression in the transverse directionwas expected only when the ship wasenclosed by ice, for instance, when stayingthrough an Arctic winter.The nozzles were designed to withstand atransverse compression of 6 MPa. The maximumlongitudinal force was derived to be3.4 MN. Local yielding was accepted for theseloads. Recent studies by Laskow et a1. (1986)would actually suggest a design longitudinalforce of about 7 MN for a ship like R. V.POLARSTERN.To examine and check the theoretical assumptions,the starboard nozzle of POLAR­STERN was fitted with strain gauges. Thestrain gauges were placed to enable the determinationof the support shear and bendingmoments at the connection to the hull, mainlydue to longitudinal forces, and also to thebending stresses in the top of the nozzlecaused by transverse compression. The exac tlocations for the gauges were selected on thebasis of the results from an extensive finiteelementanalysis of the nozzle. It is obviousthat gauges, which had to be externally appliedto the nozzle, as well. as the cablingrequired thorough protection from possible iceimpact. The gauges were covered with a conicalJy-shapedprotective shield (Fig. 13) and thecables were led through heavy tubing.The total transducer and recording systemof the nozzl.e was calibrated experimentaJ1yafter installation, while the vessel was stil.l indry dock, in order to assure that the gaugesfurnished signals well-correlated with theloads. A loading system was attached to thenozzle, conSisting of two horizontal frames,which introduced line loads at the top andbottom of the front end of the nozzle. The twoframes were interconnected aft of the nozzleby a vertical beam. The loading was appliedwith a hydraulic jack fitted between this beamand the end of the propeUer shaft. This way itwas possible to calibrate the propeller thrusttransducers of the propeller shaft at the sametime.The analysis of the ice loading on thenozzle was again performed by comparison ofthe strain signals with the results from detailedcalculations. The finite-element modelof the starboard propeller nozzle and sup-Fig. 12:Arrangement of Propellers, Rudder and Nozzles503

Fig. 13:Protection of Strain Gauges on Propeller Nozztethe nozzle the calibration signals were ratherweak.In the next step of the analysis, differentinstances with large strain signals were simulated.The !.oading of the model was carefullyvaried in magnitude, direction and exposedarea and adjusted so as to obtain thebest agreement between calculation andmeasurements.Three distinct !.oad situations for the nozzleswere observed:Fig. 14:Finite Etement Mode!. of StarboardPropeller Nozzleporting stern structure is shown in Fig. 14. Atfirst, the calibration was simulated in order tocheck and adjust the boundary conditions ofthe computer model and to investigate theinfluence of the protec tion dev ices mentioned.As a result of the very heavy construction ofpure impact toads, predominantly in thelongitudinal direction, during ahead andparticularly for astern motion,dynamic loading resulting from hydrodynamicinteractions between propel1er andnozzle when the nozzle was clogged,dynamic l.oading when ice passed throughthe propeller.Sizable strains were recorded in the nozzlesolely for impacts with solid ice, frequentlywhen going astern, and for ice rubble passingthrough the propeller.504

As they are welt below the water tine, thenozzles had contact with ice only in severerubble and pack ice in regular forward motion.When passing through homogeneous sheet iceonly smaller ice blocks came to the propellersand were drawn through the nozzles withoutexerting noticeable forces.'"" E.......=enenQ)'-+'en302010II ~0-10 2024II1tL~.-=r-,j"0---

the order of 0.5 to 1.0 second.The dynamic loads due to clogging of thenozzle or a breaking ice block passing throughthe prope1J.er can be significant. The comparisonof relevant measured data with resutLsfrom dynamic calculations showed that loadampliludes of up to 1.3 MN may develop. Themaximum thrust from the nozzle under bollardtest conditions is only 370 kN for comparison.2cClE '"OIN/mm2]20~~~: ]1 2 3 t.II me[sec]The frequency content of the recordeddynamic signal from the nozzle, as processedby a Fourier analyzer, can be seen fromFigs. 18 and 19 for two gauges on the nozzle,one at the support and the other one in thecenter of the nozzle as indicated in thesketch. Both signals display a peak at the12 Hz (cps) blade frequency corresponding to180 RPM. Additionally, two natural frequenciesof the nozzle are detectable for thesupport point (Fig. 18) at 14 and 24 Hz. Theseare global vibration modes which do not causesignificant stresses in the nozzle structure(Fig. 19). The measured frequencies of thesemodes correspond well with the results fromthe dynamic finite-element analysis. Thelowest transverse bending mode of the nozzlewas calculated at 14 Hz, the longitudinal modeat 23.5 Hz.20N E"- E:z: 100'" '-'" -10-20~II"-"" ~'lllrt'. '!\iII ""IV'It~--,...."1-'11CLI12 13 14 15 16 17time [sec]Fig. 17: Recorded Stresses in Nozzle due toImpact with an Ice Bit During Ahead ManoeuvreFollowed by Passing of Ice RubbleThrough the Nozzle. Peak Load: 0.9 MNIIIoFig. 18:1015 20 25frequency [cps]Auto Spectrum of Stresses in Nozzleat Support0.08,..----------,----------,0.06 N E~='" 0.04 ~0.02cCl'" E0.00 -1-""':""o [N/mm 2 ]10,--:-----,_1~t:t1Hl""~\~-: --{1 2 3 4time[sec]k....p;.""r>-

It appears sensible to change the designcriteria for ice-going vessels in the futurefrom distributed global loads to concentratedloads, possibly with higher ice pressures actingonly 10caUy. This would lead to the same or,depending on the ice class, slightly increasedplate thicknesses with a reduction of the supportingelements such as ice frames, webframesand stringers.The situation for the shell plating may becompared with that of decks dimensioned forlarge wheel loads, such as due to fork-lifts ortrailers. Contrary to plates under uniformlydistributed load, maximum plate bending occurshere in the center of the plate field. Theextent of the contact area depends on themagnitude of the ice load as well as theimpact velocity. Large ice loads are connectedwith larger contact areas, where a considerablepart of the total load is directly transmittedto the stiffer supporting structure, acondition called "ice-bridging".Until very recently, no ice loads had to beconsidered in the dimensioning of the bottomof ships even for polar service. Experiencewith R. V. POLAR STERN and other vesselsoperating in ice clearly indicate that ice isdisplaced below the ship and can cause damagethere. The analysis of the permanent deformationsobserved on the POLAR STERN showsthat the bottom structure has to be included inthe ice strengthening up to amidships. Equivalentdistributed loads of about 2 MPa weredetermined. Additionally, the bilge has to bestrengthened. Many ships show damage in thisregion after operation in se"ere Arctic conditions.An equivalent load of 2.6 MPa was derivedfor the POLAR STERN in this region.There was no damage in the ice strengthenedregions, indicating that the load assumptionsare correct or perhaps excessive.Propeller nozzles are an effective protectionfor the propellers against ice impacts.While ice reaches the nozzles and the propellersonly rarely when travelling in homogeneoussheet ice, considerable impacts withload peaks of about 2.9 MN were observed onR. V. POLAR STERN in heavy pack ice, predominantlywhen going astern.Aside from the static strength, the vibrationbehavior of the nozzles is also important.Where hydrodynamic loads of negligible magnitudeoccur under normal operating conditions,the nozzles are exposed to consider"able excitation under the non-stationary conditionswhich often occur during a voyage inice. This may be due to clogging of a nozzle orice passing through th~ propeller. In the firstcase, the dynamic forces are a result of thedisturbance of the flow to the propeller, in thesecond case due to direct contact between iceand propelter or nozzle. The dynamic stressescaused by these loads reached a magnitudecomparable to that of ice impact. This underlinesthe importance of a theoretical analysisof the natural vibrations of the nozzle at thedesign stage. Resonance with the blade frequencyat the principal operating speed mustbe avoided, in order to avert damage as aresult of dynamic ampli fication.Propeller nozzles on ships serving in ice sofar were dimensioned generally under the considerationof static or quasi-static loadingonly. The signals from R. V. POLAR STERNevaluated so far show clearly that the responseof the nozzle to ice loads is alwaysaccompanied by vibrations. Fatigue considerationsare therefore pertinent, leading to lowerallowable stresses than those used for the bowstructure for instance. There, local yieldingmay even be acceptable as it does not impairthe safety of the vessel.Generally it must be considered that thepropellers, propeller nozzl.es, rudder or propellershaft bossing are essential to the safety ofship and crew, as the failure of anyone ofthese components may well lead to the inabilityof the vessel to manoeuvre.AcknowledgementsBoth voyages of R. V. POLAR STERN describedhere were supported by the FederalMinister of Research and Technology (BMFT)of Germany and administered by the AlfredWegener Institute for Polar Research. Thissupport is gratefully acknowledged. The expeditionswere conducted in a cooperationbetween the Hamburg Ship Towing Tank(HsVA) and Germanischer lloyd. Representativesfrom German shipyards and shippingcompanies, the German Hydrographic Institute,the Alfred Wegener Institute and severalGerman universities as well as scientists fromCanada, the USA, Denmark and Norway alsoparticipated in the expeditions.507

7. ReferencesBalhe, K. J.: "ADINA. A Finite Element Programfor Automatic Dynamic IncrementalNonlinear Analysis", report No. 82448-1.430,Massachusetts Institute of Technology (MIT),Cambridge, Massachusetts, 1975.Glen, I. F.; Daley, C. G.: "Analysis of lheStructure of lhe Proposed CCG Polar Class 8Icebreaker under Exlreme Ice Loads", Societyof Naval Archilecls and Marine Engineers,SNAME, Transactions vol. 93, 1985, pages283-301.Hoffmann, L.: "Impact Forces and FrictionCoefficients on the Forebody of the GermanPolar Research Vessel POLAR STERN", 8thPOAC, Narssarssuag, Greenland, 1985.Laskow, V.; Bayly, I. M.; Ghoneim,G. A.: "Study of Strength Requirements forNozzles of Ice Transiting Ships", vol. 4. OMAEConference, Tokyo, 1986, pages 630-637.Lindberg, K.; Andersson, L.: "Ice Impact onSemisubmersibles", vol. 4. OMAE Conference,Houston, 1987, pages 313-320.MUller, L. et.al.: "First Ice Breaking Expeditionwith R.V. POLAR STERN" (in German),Final report for the Minister of Research andTEchnology of F.R. Germany (BMFT), reportno. MTK 312-313, Germanischer Uoyd, Hamburg,April 1986.Varsla, P.: "On the Mechanics of Ice Load onships in Level Ice in the Baltic Sea", publicationno. 11, Technical Research Center of Finland,1983."Rules for the Classification and Constructionof Seagoing Steel Ships", Vol. 1, GermanischerLLoyd, Hamburg, 1982.508

ICEBREAKING PERFORMANCE OF RV POLARSTERN IN BROKEN ICE­FULL SCALE TRIALS IN THE WEDDELL SEA, ANTARCTICAFranz Ulrich H"auslerHamburgische Schiffbau- Versuchsanstalt GmbH., Hamburg, F. R. GERMANYAbstractIn 1983, a series of 37 tests wascarried out with the RV POLARSTERN inthe Weddell Sea to evaluate the vessel'sperformance in broken ice of variouscoverage and thickness. The results arepresented in a speed vs. average icefloe thickness diagram, with the shafthorse power as parameter. The averageice floe thickness was determined in athree step procedure: first, thethickness of the various (up to 3) icefloe types encountered was estimated byobservation; second, the portion of eachice floe type of the total ice coveragewas evaluated by analyzing videorecordings; third, the average ice floethickness was calculated by weightingthe individual ice floe thicknesses byits portion. The ship's speed wasdetermined by different methods.IntroductionIn December 1982, the Germanpolar research and supply vessel,POLARSTERN, was commissioned and sailedin the same month on her maiden voyageto Antarctica. During this expedition, acomprehensive research program wascarried out, and the Georg von NeumayerStation was supplied. As a part of thesecond leg of tnis expedition (ANTARKTIS1/2), trials were performed to determinethe icebreaking performance of RVPOLARSTERN in broken ice and in thickfast ice. In the present paper, theresults of the icebreaking trials inbroken ice are presented. Fig. I shows amap of the Weddell Sea with the testsites marked on the route of the ship.Description of the VesselRV POLARSTERN is a multidisciplinaryresearch vessel designed foroperation in polar regions. Its secondtask is to supply the German Antarcticresearch stations.This is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987. © The Geophysical Institute,University of Alaska, 1987.The main data of RV POLAR STERN are:length overalllength between perpendicularsbeam (moulded)depth (main deck)117.91 m110.51 m25.07 m13.60 m509

30 ?O!RV POLARS TERN_---+-E-X-P-ED_IT_ION ANTARKTIS 1/2 198300Fig.Map of the Weddell Sea, Antarctica with marked testsites (map after N.N., 1980).VIDEO CAMERAICE OBSERVATIONSLABORATORY CONTAINERHOLD, DATA RECORDIrlGBRIDGEEMPTY CELL NO. 79V I DEO CAMERASPROPELLER OBSERVATIONSCONTROLLABLE PITCHSETT I NG MECHAtH SM,R.P.M" TORQUE, PITCHTHREAD LOGFig. 2Measuring and observation stations aboard RV POLARSTERN(general arrangement plan from Albert et al., 1983)510

draught (freeb o ~rd ,displacement (emptydeadweight (max)hor se powe rspeed (max)class:tonnageGermanischerseawater)ship)10.71 mI 1300 t4850 t14116 kW16.5 knLloyd 100 A4 ARC 310879 BRTA detailed description of the vesselcan be found in Albert et a!. (1983).Therefore, only some informationrelevant for icebreaking is presentedhere.The bow of RV POLARSTERN exhibitsconcave, v-shaped frames. The flat flareangles 1n the waterline range aredesigned for icebreaking by bending. Thesteep flare angles below serve forsideward clearance of broken ice cusps.The inclination of the stem is 22.5° andthe angle of entrance 27°. The sidewalls1n the parallel midship section areinclined by 8° against the vertical toreduce sine pressure ice force. The shiphas two ducted controllable pitchpropellers of 4.2 m diameter. Thenozzles are attached to the sides of theskeg stern. The clearance between thetop of the nozzles and the ship's hullis 2.35 m. This general ship hull designhas been developed at HSVA in ice modeltests with the aim of combining easyicebreaking with minimal propeller-iceinteraction under level ice conditions.Data Acquisition and ProcessingTo establish the icebreakingperformance of the vessel in broken ice,the following data had to be acquired:- shaft horsepower (from measurements oftorques and rates of revolution);- ship's speed;- ice conditions (thicknesses,coverage);- propeller-ice interaction.Time-dependent physical values(torques, revolution rates, speed) weredigitized on-line and stored onfloppy-discs using a Hewlett PackardHP 21 MX-E computer. Data processing wascarried out off-line. The ice conditionsas well as the propeller-iceinteractions were observed and recordedby means of several video cameras andtwo video recorders. Visual observationssuch as readings of ship-borneinstruments or additional ice conditiondata were recorded manually . Fig. 2shows the var10US measuring andobservation stations aboard RVPOLARSTERN. For approximate ice strengthinformation, the temperature-salinityprofile method was employed .The ship's speed is difficult toacquire in broken ice. This problem canonly be solved sufficiently by employinga remote sensing system (e.g., radardoppler; ct., Hellmann, 1985). Such asystem was not available in the presentstudy. Instead, up to three differentsensors for ship's speed measurementwere employed simultaneously: threadlog, electromagnetic (EM-)log and videorecording plus stop watch.The thread log consists of a threador wire running off from a spool andslung around a free running wheel. Thefore end of the thread is bound to ananchor fixed on the surrounding ice. Therunning of the thread turns the wheel.The revolution rate of the wheel ismeasured and converted into the thread'sspeed. This speed is to a fir s tapproximation equal to the speed of theship. Measuring errors may arise fromelastic oscillations and from driftingof the thread caused by wind as well asdrifting of the ice floes caused by thewake. The latter regularly occurs inbroken ice.·Thus, the speed measured bymeans of the thread log is normallyhigher than, at best equal to, theship's true speed relative to thesurrounding ice.With the EM-log, the ship's speedis. electromagnetically measured relativeto the water. Based on the experience ofthe present study, the output signaldoes not seem to be much affected by icefloes gliding along the ship's hull. TheEM-log signal was read twice: by tappingand recording the analogue output signalof the log and by visually samplingreadings from the speed indicatorinstalled on the bridge. The indicatoron the bridge shows speed values whichare smoothened by integration.The video recordingsconditions observed rightof the icein front of511

the ship, tapped from the ship-borne icecamera, provided an additional speedinformation. By means of the netdeveloped to evaluate the degree of icecoverage in broken ice (see Fig. 3), thetransit times of marked points on thevideo screen (e.g., points on individualice floes) through a distance given bythe net (32 m) were stopped (stop watch)and an average speed was calculated.Possible sources of error in this methodare: apparent speed changes resultingfrom pitch and roll motions of the ship(the ice camera is fixed to the ship),inaccuracies of the net, time stoppingerrors and finally, tape runirregularities of the video recorder.A comparison of the speed readingsobtained simultaneously from all foursources showed the speeds read from theEM-log indicator on the bridge to besurprisingly well correlated with thespeeds determined by means of the videostop watch method. The thread logconsistently yielded higher speeds.Because of this, the thread log speedshave been corrected by -3% and anadditonal zero offset of -0.02 1m. Thetapped analogue signal of the EM-log wasthen calibrated and offset correctedusing the EM-log bridge indicatorreadings, the video stop watch speedsand the corrected thread log values asreferences. For use in the shafthorsepower-speed analysis, the meanvalue and standard deviation of all fourspeed measurements over the duration ofa test run were evaluated.The rates of revolution of thepropeller shafts were measured usingcapacitance relays as pulsetransmitters, and converting the pulsefrequency into r.p.m.-values. Inaddition, visual readings from theship-borne r.p.m.-indicator on thebridge have been sampled.The torques were measured by meansof strain gauges applied to theintermediate shafts sternward from thecontrollable pitch setting mechanisms.For the conversion of strains intotorques, a rigidity modulus ofG = 82.2 GPa was employed for the shaftmaterial (Sprunk, unpublished).The shaft horsepower P S [kNms -1=kW] was calculated from the measuredtorques Qi and rates of revolution n i(i-th shaft) using the equationP 21T (min) '\ Q. n.S - 60 s ~ 1 1(i = 1,2) (I)1The ice conditions encountered bythe ship during each individual test runhave been established by manuallyrecording visual observations and byrecording the video signal of theship-borne ice camera on a video tapecassette. The visual observationsconsisted of an estimation of the degreeof ice coverage, a classification of icetypes encountered and an estimation oftheir ice thicknesses. In addition, iceobservations of the ship-borne weatherbureau were available (N.N.,Eisbeobachtungen, unpublished).Fig. 3Video record of ice conditionswith 2 m x 2 m net projection.For the qualitative analysis of thevideo recordings, a net was drawn on atransparency which then was attached tothe monitor screen. The net correspondedto a 2 m x 2 m grid projected on theplane of the ice surface in front of theship (Fig. 3). The construction of thenet was based on the static trim of thevessel during the test series on March10, 1983. Dynamic changes of the trimhave not been taken into account. Theice observed was sub-divided into threeclasses of ice thickness and floe sizeand a frequency table for the area512

Fig. 4RV POLARSTERN cruising throughbroken ice in the southernWeddell Sea(tests on 19 February 1983).Fig. 5Ice conditions in the pack iceeast of Larsen Ice Shelf (testson 10 March 1983, run no. 33. I).inside the net was established. This wasdone for several (usually 3) samplesalong a test run. From the average ofthe frequency tables of a test run, thepartial coverages C of the three iceiclasses were calculated and rounded tofull tenths. Based on these coveragesand the ice thicknesses, h., estimatedfor the ice classes, a m~an ice floethickness, h*, was calculatedlater use in the power-speed analy­forsis.h * = -11 0L h. C. (i = 1,2,3) (2 ). ~ ~~For visual observation of thepropeller-ice interaction, RV POLARSTERNis equipped with six underwater windows.Only four of the six windows wereemployed in the present study. Thesefour windows are installed in tubes in aspecial empty cell (#79) located rightin front of the stern thruster andbeneath the stern thruster machineryroom. The windows are located in pairson each side of the ship, arranged oneabove the other. In the two lower tubes,B/W-video cameras were installed whichwere synchronized with the ship-bornecameras. The video signals of bothcameras were mixed on a monitoringfor simultaneous observation ofpropellers on the same monitor.Test Performance and ResultsdeskbothIcebreaking trials In broken icewere carried out at three locations Inthe Weddell Sea, In total, a series of37 tes t runs;- 3 test runs on 19 February 1983 in apack ice field at 77°19'S 39°32'Wabout 10 nm north of the Sovietstation Druzhnaya (Fig. 4)6 test runs on 26 February 1983 in apack ice field at 72°23'S 16° 15' W inthe eastern Weddell Sea. The polyniatypical for the east border of theWeddell Sea during the summer seasonhere was closed due to tidal currents.- 28 test runs on 10 March 1983 in anarea east of Larsen Ice Shelf (westernWeddel Sea at 66°S 5 lOW, (Fig . 5».The various ice conditions encounteredon this test site allowed for asystematic variation of both shafthorsepower and ice conditions.All tests were run employing thecombinator running mode. In this mode513

the rate of rf"/o~\J~ ion, the pitch of thepropellers and the degree of feeding ofthe main engines is set automatically bymeans of an electronic control dependingon the position of the control lever.All tests were run with the shaft drivengenerators clutched-in and withoutsupport from the auxiliary dieselgenerators. In 28 of the test runs, allfour main engines were running, in theremaining 9 test runs only two. Prior toeach test series zero readings wererecorded with clutched-out shafts.The ice parameter employed in thevaluation of measured powers and speedswas the average ice floe thicknes~ h*,evaluated from the partial coverages andthicknesses of the ice types encountered(eq. 2). The choice of this iceparameter takes into account theobservation that the ice floes in mostcases were cleared and not much broken.The use of the average ice floethickness as the ice parameter seemspermissible as long as the individualice floe's diameter does notsignificantly exceed the beam of thevessel (here 25 m) and as long as thefloes have free space to move. Bothconditions were fulfilled in the testsreported here.The three ice type classes used inthe determination of the average icefloe thickness were:- thick ice, ice floe diameter > 10 m(index i = I)- thinner ice, includes larger fragmentsof thick ice and thinner icefloes of 3 to 10 m diameter (indexi = 2)- thinnest ice, including mush ice,debris of thicker ice and thin youngice (index i = 3).Table shows the partialco~centrations C i' and the estimated icethicknesses hi' and the calculatedaverage ice floe thickness h* for eachtest run. The partial concentrations aregiven in 1/10 units.The measured values of torques,rates of revolution, pitch angle of thepropellers, shaft horsepower, ice floethickness and speed are compiled foreach individual test run. In addition,the numberthe controlare given.of main engines running andlever position (10 = max.)For the establishment of the speedover ice floe thickness diagram, withthe shaft horsepower as a parameter, themeasured speeds have been reduced tofive typical power levels (750 kW, 1500kW, 2000 kW, 5000 kW and 11000 kW). Thereduction was performed upon thesimpl ified correlation P - v'. Asupplementary data set was taken fromthe open water trials (Sprunk,unpublished) .For data set of each power level, alinear regression was carried out. Inorder to account for the varyinguncertainty of the measured speeds theindividual speed values were weighted bymeans of the inverse of half the 907-confidence interval. For the open waterspeeds this interval was taken as+ 0.01 kn.The speed over ice floe thicknessdiagram with the regression lines forthe five power levels and reducedmeasured data is shown in Fig. 6. Thisdiagram characterizes the icebreakingperformance of RV POLARSTERN in brokenor pack ice.Also as result from the test inbroken ice and from observations duringthe whole expedition, it was establishedthat propeller-ice interaction occurredonly infrequently. The frequency ofpropeller-ice interaction increased withincreasing delivered power. At controllever positions of 8 and higher,corresponding to a total delivered powerhigher than 5000 kW, one regularly hadto expect single ice cusps or floesinteracting with the propellers. Fig. 7shows a typical time history plot oftorques, thrusts and rates of revolutionduring a propeller-ice interaction atthe starbord propeller. When passingthrough ridges embedded in ice floes oneregularly had to expect larger pieces ofice interacting with the propellers.For the remainder of the time spentnavigating through broken ice, there wasonly a layer of snow, air bubbles andsmall ice particles moving along the514

RunNo.nateTime Part.Concentr.ll/l0] Ice Thickness 1m]C 1C 2C 3 hI h2 h3h*1m]21.1 19.2.8321.2 19.2.8322.1 19.2.8314:01 2 3 5 0.80 0.30 0.1014:06 10 0 0 0.8014:29 4 3 3 0.80 0.30 0.100.300.800.4428.1 26.2.8329.1 26.2.8330.1 26.2.8330.2 26.2.8331.1 26.2.8331. 2 26.2.8306:08 7. I I I. 75 0.50 0.1006:23 3 2 4 I. 75 0.50 0.1006:39 7 0 1 1. 75 0.1006:41 5 1 4 1. 75 0.50 0.1006:48 4 2 3 1. 75 0.50 0.1006:52 3 2 5 1. 75 0.50 0.101.290.671.240.970.830.6833.1 10.3.8334.1 10.3.8335.1 10.3.8335.2 10.3.8335.4 10.3.8336.1 10.3.8337.1 10.3.8338.1 10.3.8339.1 10.3.8340.1 10.3.8340.2 10.3.8341.1 10.3.8342.1 10.3.8343.1 10.3.8344.1 10.3.8345.1 10.3.8346.1 10.3.8347.1 10.3.8348.1 10.3.8349.1 10.3.8350.1 10.3.8351.1 10.3.8352.1 10.3.8353.1 10.3.8354.1 10.3.8354.3 10.3.8355.1 10.3.8356.1 10.3.8317:21 5 2 3 0.75 0.35 0.1517:32 4 4 2 1.15 0.80 0.3018:00 6 2 2 1.15 0.80 0.3018:0218:09 7 2 1 1.15 0.80 0.3018:18 7 2 1 1.15 0.80 0.3018:28 7 2 1 1.30 0.95 0.2018:40 7 2 1 1.55 0.95 0.2518:56 7 2 1 1.60 0.95 0.3019:07 7 2 1 1.60 0.95 0.3019:09 8 1 1 1.70 0.95 0.3019:25 6 3 1 1.60 0.95 0.3020:30 5 3 2 1.60 0.95 0.3020:40 4 3 3 1.30 0.80 0.3020:57 4 3 3 1.25 0.80 0.3021:07 3 3 4 1.30 0.80 0.3021:16 3 3 4 1.30 0.80 0.3021:28 1 4 5 1.30 0.80 0.3021:37 1 3 6 1.30 0.80 0.3021:52 2 4 4 1.30 0.80 0.3022:02 1 2 7 1.30 0.80 0.3022:30 2 2 6 1.30 0.80 0.3022:44 0 1 9 0.60 0.1522:49 0 1 9 0.60 0.151 1 8 1.30 0.60 0.150 1 9 0.60 0.1523:09 0 1 9 0.60 0.1523:15 0 1 9 0.60 0.150.490.840.910.911.001.001.121.301.341.341.491.281.150.850.830.750.750.600.550.700.500.600.200.200.310.200.200.20Table IIce conditions during icebreaking trials withRV POLARSTERN in broken ice515

RunNo.No.ofMainEngs.Contr. Q pS npS ~PS Q SS nSS ~SS h* PsLeverPosit. (kNml (min -II ( °1 (kNml [min -II [ °1 [ml [kwlV ± I[knl21.1 421.2 422.1 428.1 429.1 430.1 430.2 431.1 431.2 433.1 434.1 435.1 435.2 435.4 436.1 437.1 438.1 439.1 240.1 240.2 241.1 242.1 443.1 444.1 445.1 446.1 447.1 248.1 249.1 250.1 251.1 252.1 453.1 454.1 454.3 455.1 456.1 46 90.5 118.6 18.0 81.0 118.6 17.1 0.30 2 1308 172.4 145.1 19.9 166.4 150.9 20.5 0.80 5 24910 2~1.9 173.9 23.9 301.2 175.0 25.B 0.44 10 B366 7B.7 l1B.1 17.9 75.5 11B.0 14.B 1.29 1 906B 169.4 146.5 20.0 149.7 148.3 16.B 0.67 4 93410 2B2.6 lB3.2 20.9 272.5 183.7 18.2 1.24 10 66410 295.1 183.0 21.9 278.9 182.3 19.7 0.79 10 9794 52.2 118.1 14.3 59.5 117.9 17.1 0.83 1 3802 26.9 118.4 8.6 33.6 l1B.4 11.7 0.68 7506 83.2 117.8 16.2 70.6 117.6 10.9 0.49 I 8978 178.2 145.8 19.1 141.8 147.8 14.5 0.84 4 916-10 327.5 180.6 21.6 319.9 181.1 18.6 0.91 12 2619 205.3 159.7 19.7 211.2 161.9 18.5 1.00 7 0144 64.1 118.0 11.4 64.5 117.6 14.1 1.00 1 5862 31.8 117.6 6.5 31.7 117.7 7.6 1.00 7826 81.6 117.6 14.9 66.0 l1B.3 11.1 1.12 1 8238 177.8 145.7 10.5 180.2 147.1 15.8 1.30 5 4B96 91.6 158.3 10.9 89.7 159.7 6.5 1.34 3 0187 136.7 179.5 11.4 117.0 181.2 6.8 1.34 4 8057 137.7 179.2 11.3 119.1 179.2 6.9 1.49 4 B194 63.9 135.1 9.1 57.1 135.4 6.1 1.28 1 7146 84.7 117.0 16.0 67.3 117.5 11.6 1.15 1 8668 182.6 147.7 18.4 170.9 149.4 15.9 0.85 5 479-10 296.0 178.4 21.7 264.8 lB2.3 18.4 0.83 10 5854 44.5 117.3 12.0 61.2 118.3 14.2 0.75 1 3052 19.6 117.5 6.7 33.4 118.1 7.5 0.75 6542 25.5 133.5 6.1 39.7 133.7 6.5 0.60 9124 48.1 133.9 9.2 57.3 136.2 7.1 0.55 1 4927 115.7 175.0 11.7 102.7 172.5 8.9 0.70 3 9766 62.2 161.5 11.3 84,(1 162.9 8.9 0.50 2 8374 45.4 134.0 9.6 57.7 135.0 8.0 0.60 1 4536 64.3 117.5 17.0 57.5 118.1 12.4 0.20 1 5028 169.7 147.0 21.4 114 .1 149.4 14.9 0.20 4 397-10 254.7 177 .9 22.1 234.3 182.0 17.5 0.31 9 211_10 236.1 176.3 21.5 284.2 182.8 18.9 0.20 9 7994 36.0 117.5 11.3 45.6 118.3 9.3 0.20 1 0082 17 .9 117.6 6.6 32.0 IIIl.l 7.4 0.20 6162.68 ! 0.493.21 1 1.015.0B ! 0.733.16 t 0.B85.57 ! 0.673.52 ! 0.955.57 ! 0.383.01 ! 0.172.43 ! 0.514.59 ! 0.158.59 ! 0.3211.46 ! 0.4310.01 ! 0.180.60 ! 0.10-0.02 i 0.040.53 ! 0.145.B6 ± 0.261 .95 ! 0.123.73 ! 0.133.25 ! 0.230.01 ! 0.012.85 ! 0.177.£2 ! 0.1512.30 ! 0.072.74 ! 0.140.92 ! 0.050.77 1 0.043.09 1. 0.155.58 ! 0.165.35 ! 0.174.34 ! 0.117.52 ! 0.1310.51 1 0.1813.48 ! 0.2313.86 ! 0.344.69 ! 0.073.58 t 0.05Table 2Results of icebreaking trials with RV POLARSTERNin broken ice516

~~• •.... '"0:z'">~~~+---~~------~~~ __----------~r---~~--------+-~..1.60H* [tilFig. 6 Icebreaking performance of RV POLARSTERN inbroken ice: diagram of speed vs. mean icefloe thickness for various power levels.~ .. '320.066.5229.5aBB [KNMI160.0400.0~".4B.OTBB [KNI 84.130.0IB4.0176.0177.9168.0NBB [I/MINI160.0 NSTB2 .0 26.0 30.0 34.0 311.02 .0T [51Fig. 7Time history plots of torques, thrusts and rates ofrevolution during propeller-ice interaction at thestarbord propeller (run. no. 54. I, shaft horsepowerP s= 9200 kW, speed v = 10.5 kn).517

ship's hull. A vitiation of thepropeller performance due to thisparticulate flow was not been observed.ConclusionsThe trials in broken ice with RVPOLARSTERN during the Antarctic summer1982/83 have shown that the vessel canbe operated freely in pack ice with atotal coverage of 10/10 and with anaverage ice floe thickness of 1.5 m. Forthis, it is assumed that ice floediameters are up to 25 m and sufficientspace (open water or mush ice) existsbetween the ice floes, such that thevessel can clear them without major icebreaking.With increasing size and decreasingmobility of the floes, the icebreakingprocess and, correspondingly, thevessel's performance in ice approach theconditions given in fast ice. So, duringthe expedition, (but not during thetrials), pack ice conditions wereencountercd, through which the vesselwas unable to travel in continuousmotion, but could proceed through byramming.Under the ice conditionsencountered it has been found thatpropeller-ice interaction occurred onlyincidentally. So that aim of the hullshape design has been achieved.AcknowledgementsThe present paper is a contributionfrom the Hamburg Ship Model Basin(HSVA). The investigations have beencarried out for the German Ministry forResearch and Technology (BMFT) underorder POL 0027- 4. The author isresponsible for the content of thispaper.The Alfred Wegener Institute forPolar Research, Bremerhaven (AWl),represented by its director, Prof. G.Hempel, who was also the expeditionleader, supported this study by makinghelicopter ice reconnaissance flightspossible and by granting personnel andlogistic assistance. This support isgratefully acknowledged. Thanks are dueto the master of RV POLARSTERN, Capt. L.Suhrmeyer and his crew. Theircooperation was a basic prerequisite forthe success of this project. The authoris indebted also to Mr. L. Hoffmann(HSVA) who was responsible for dataacquisition.ReferencesAlbert, H.P., et aI., 1983: Das deutschePolarforschungs- und Versorgungsschiff"Polarstern" (The German polar researchand supply vessel "Polarstern"). By:H.P. Albert, H. Dobinsky, K. Hoffmann,B. Linke, U. Nitzschmann, C. Boie, K.Lederer, B. Pruin, o. Krappinger and J.Schwarz. Hansa, Vol. 120, No.6 (1983)pp. 465-50-6-.-Hellmann, J.-H., 1985: Leistungsmessungenim Eis an Modell und GroBausfUhrungvon "FS POLARSTERN" (Power measurementsin ice with model and full scale versionof RV "Polarstern"). Jahrbuch derSchiffbautechnischen Gesellschaft, Vol.79 (1985), pp. 411-425, Springer,Berlin/ Heidelberg/New York.N.N. , 1980: Knaurs GroBer Weltatlas(Atlas of the World), Droemer Knaur,MUnchen.N.N., unpublished. Eisbeobachtungen fUrdas Deutsche Hydrographische Institut(Ice observations for the GermanHydrographic Institute), FS "POLARSTERN"Route: Kapstadt-GvN-Halley Bay-WeddellSea, Fahrt Nr. I, p. 1-3, 1983.Sprunk, B. , unpublished: LeistungsmesdemDeutschen Polar-sungen aufForschungs- und Versorgungsschiff"POLARSTERN" (Power measurements withthe German polar research vessel"POLARSTERN"). Hamburgische Schiffbau­Versuchsanstalt GmbH., Hamburg, HSVAReport Fm 16/82, November 1982.518

DiscussionR. HAYES: Have you extended your An t­arctic ice-breaking analysis to includethe POLARSTERN sea trials off Spitzbergenand elsewhere in the Arctic to includeconsolidated floes and multiyear ice? Ifso, how do the results compare?F. U. HAUSLER: Such an extension wouldhave been beyond the scope of the presentstudy, and so has not been performed.K. RISKA: In the analysis of ramming theauthor assumed that v is constant. Thismeans that the applied force is constantaccording to Newton's third law. Inramming, the force is not, however,constant; rather of triangular shape.Could the author comment on this.F. U. HAUSLER: Certainly the forcedecelerating the ship during rammingindentation decreases in the course ofthis process because most of the componentsof ship resistance in ice are speeddependent. But the pure ice breakingcomponent of resistance can be taken asspeed independent, if brittle failure isassumed. This applies also to thefriction component of resistance, atleast at a first approximation. Icebreaking and friction components becomeby far the dominant components of resistancein ice, when the ship's speedapproaches zero, e.g., at the end of aramming cycle or when transiting throughan ice cover of an ice thickness justbelow the limit. Thus, the force historyin ramming cannot be of triangular shape.The rough assumption of a constantdeceleration or applied force respectivelyis identical to the assumption ofnegligibly small components of resistanceother than due to ice breaking andfriction. With respect to the scope ofthis study, which was to establish thelimit breakable ice thickness with aminimum of effort, this neglect isallowed as long as the speeds are low andas the systematic error disappears in thenoise of inaccuracies inherent in themeasuring procedures applied. It isworth noting that the neglect of thespeed dependency of the total resistancein ice leads to conservative estimates ofthe limit breakable ice thickness.519

EVALUATION OF THE MAXIMUM BREAKABLE ICE THICKNESSOF AN ICEBREAKING VESSEL FROM RAMMING TESTS IN LEVEL ICEFranz Ulrich If

in thick, first year sea ice with theGerman polar research and supply vessel"POLARSTERN" on the second leg of hermaiden voyage to Antarctica (expeditionANTARKTIS 1/2) in early 1983. Tes tprocedures and test results arepresented in the second part of thispaper.AnalysisThe estimation of the limit icethickness which can be broken by anicebreaking vessel in continuous motionis based on the knowledge of thefollowing data to be measured orestimated in ramming tests:v speed of the ship at theinstant of indentation [ms- Ipropeller thrust[kNm( I-r)shaft horsepower [kNms- 1 = kWindentation depth [mmass of the ship [Mg = kNs 2 m- l _~~:u~~i~~~~~~ion coeffiCient[mmjsnow thickness [ice bending strength [kNm- 2The basic assumption of the analysis isthat the sum of measured shaft horsepowerand equivalent power set free bydeceleration of the vessel duringindentation in an ice cover is equal tothe power required for continuous motionin the same ice cover at a speed closeto zero.In the analysis some crude simplificationsare made:I. The ship's speed decreases linearlyduring the indentation phase.2. The ship's added mass is neglected.3. The ratio of thrust to shaft horsepoweris constant.4. The net thrust TN is the sum of propellerthrust measured at the shaftor gear Tp minus thrust deductiont· Tp plus (if applicable) thrust ofthe nozzle Tp or: TN = a Tp. In thecase of a tWln screw ship wlth ductedpropellers a = 1.3 can be assumed.S. The effective ice thickness hI ff isthe measured thickness of plal~ lceh plus 2/3 of the thickness of theoverlying snow h .6. The required power Sduring icebreakingis proportional to the product of ef-fective ice thickness squaredmultiplied by ice flexural strength:2P - h Ieffa f.With these simplifications, theeffective shaft horsepower duringindentation into an ice cover P Seffcanbe estimated as follows:The inertia forcesmass during decelerationadditional "thrust"of the ship'sresult in anE . 2T = ~ k m v (I)M s 2SEkin is the kinetic energy at the instantof indentation. Thus the effectivenet thrust isT Neff = TN + TM = a Tp + TM (2)The e ffec t i ve shaft horsepower then is2m vT +---Neff Tp 2 a sp Seff = PS(3)---r;- = PS TpEmploying a standard ice flexuralstrength a fO (first year winter sea iceafO = 500 kPa), the effective ice thicknessbreff is reduced to a standard effectiveice thickness ~effh~eff = h Ieff~~a-f/-a-f-O- (4)Introducing now the crude approximation* 2PSeff = C h Ieff(5)which neglects the effects of speed, thecoefficient C (and its standard deviation)can be determined using measuredor estimated data from the rammingtests.If it is now assumed that themaximum shaft horsepower availableduring icebreaking PSI is 90% of theshaft horsepower avalT~~le during openwater trials P s ' the limit standardeffective ice tg~ckness which can bebroken by the vessel in continuousmotion can be calculated as:h~eff(limit) = /0.9 PSojC (6)This limit ice thickness is a conservativevalue because the speed effectsduring indentation have been neglected,522

except for the inertia terms.Ramming Tests with RV POLARSTERNDescription of the vesselThe German polar research andsupply vessel "POLARSTERN" has beendesigned for a wide variety ofscientific operations in polar waters aswell as for supply of the GermanAntarctic research stations. The vesselis 118 m long, 25 m wide and has amaximum displacement of 16150 t. Twoducted controllable pitch propellers of4.2 m diameter provide propulsion. Thesum of installed horsepower (4 dieselengines) is 14116 kW, of which 13840 kWwere available at the shafts during openwater trials (Sprunk, unpublished). Adetailed description of RV POLARSTERN isgiven in Albert et a!. (1983).Data acquisition and recordingDuring the ramming tests with RVPOLARSTERN, the following quantitieswere measured or estimated:- shaft horsepower (torques, rates ofrevolution) ,propeller thrusts,speed,distance of indentation,ice and snow thicknesses,ice flexural strength.FortorquesmeasuredHausler,shaft horsepower evaluation,and rates of revolution wereat the propeller shafts (cf.1987) .The propeller thrusts were measuredby means of the ship-borne system, RENKChecker, supplied by the manufacturer ofthe reduction gears (RENK, Augsburg).This system measures the thrust by loadcells installed in the thrust blockincorporated in each reduction gear . TheRENK Checker was calibrated mechanicallyduring subsequent docking days .Torques, rates of revolution andthrusts were recorded on-line in digitalform on floppy discs using a HewlettPackard HP 21 MX-E computer.The speed at the instant of shipindentation into the fast ice cover wasread visually from the electromagnetic(EM-)log indicator installed on thebridge of RV POLARSTERN. The signals ofthe thread log (cf. Hausler, 1987)recorded simultaneously with the abovementioned quantities (0, T, n) gaveinaccurate speeds. This was due todrifting of the ice floes to which theanchor bound to the thread's free endwas fixed in the first four tests and todrifting of the thread itself caused bystrong winds in the remaining fivetests. For this reason, the thread log'sspeed readings have been discarded.For the same reason, the distanceof indentation had to be estimated byobservation from the vessel's bridge.Due to discarding of the thread logreadings no continuous speed vs. timerecords were available for integration.Other continuous sp'eed signals (e. g.,from the EM-log) were not recordedduring the ramming tests.The ice and snow thicknesses alongthe track of the ship were measured bydrilling holes through the ice cover atintervals of 20 m prior to the rammingtests (see Figs . 2 and 4).The ice flexural strength wasestablished by employing a modifiedversion of the method described byFrankenstein (1970). The temperature andsalinity (T-S) profiles of the ice coverinvestigated were determined on icecores sampled from the ice cover. The Tand S profiles are converted into brinevolume (Vb) profiles using an equationby Frankenstein and Ga rner (1967) andthen into relative strength and elasticmodulus (0/0 or E/EO) profilesaccording to thQ relations given bySchwarz and Weeks (1977). Finally, aflexural strength was computed in whichthe profiles were considered.Ramming test performance and testresultsAfter an appropriate site forramming tests had been found, the trackfor the tests was marked on the ice byflags. The start mark for the first testrun was placed 2 to 3 ship lengths awayfrom the ice edge. Then an ice and snowthickness profile was recorded along thetrack. Finally, at least one ice core523

was drillec for ice strengthdetermination. In preparation for t heproper tests the vessel then r ammed intothe fast ice cover until the bow ha dreached the start mark.In the foll owi ng r ammi ng tests , theship backed from the last position wh e reit had become s tuck for a prescribeda cceleration distance. The accelerationdistance was varied between 1/4 a nd 3/2ship lengths and likewi se the speed atthe ins t ant of indentation.During th e tests, the 4 mainengines were driven under "Combinator"control, with the shaft generatorsclutched-in. The "Combinator" controlautomatically sets the rate ofrevolution and the propeller pitchdepending on the control lever'sposition. All test runs were driven inthe "full ahead" position (10). Tounburden the main engines, bothauxiliary diese l generators wereoperated.Fig.Tes t site at the west side ofCape Druzhnaya (Antarc tica)after performance of rammingtests with RV POLARSTERN.•· 1 I·~~7TH I CKNESS OF SNOW r-----...COVER....../""" ~CORE ~O . 12 CORE NO. 13S I GMAF = qq6 KPA~S I Gf1AF = q29 KPA~ ~.~N~ (NTGA~I.VE)-./ v~TOP SURFACE OFICE COVER-.--/ t----'· 60x (I1JFi g. 2Ice and snow thickness pro file of the ramming test site( 17 February 1983) in a sea ice plate on the west sideof Cape Druzhnaya (Antarctica). The x = 0 positioni s 250 m away from the ice edge .524

The first s~ ~ies of four rammingtests was perforn,e J on 17 February 1983,1n a landfast ice plate situated in abay on the western side of CapeDruzhnaya (77°19' S 40°45' W, southernWeddell Sea). Fig . I shows the test siteafter completion of the tests. The iceplate consisted of first year, level seaice of 1.48 m to 1.77 m thicknesscovered by 0.04 m up to 0.19 m of snow.The ice plate's diameter was about500 m. Embedded in the ice stripadjacent on the seaward side were twopressure ridges of about I to 1.5 m sailheight. The space between the ice plateand the shelf ice was filled with a I kmwide belt of ridged ice. The two coressampled at the positions x = 0 m andx = 150 m of the test track yieldedcalculated flexural strengths of 446 kPaand 429 kPa respectively.The sequence of accelerationdistances of the four test runs was 5/4,1/4, I and 3/2 ship lengths. The resultsare compiled in Table I (run numbers13. I to 16. I). During the third test run(No. 15. I) the vessel crossed throughthe thinnest part of the ice plate(x 160 m, hI = 1.48 m, hS = 0.19 m).Here, the limit for continuousicebreaking was reached; after havingbeen almost stopped, the shipaccelerated again and continued toproceed for some few additional breakingcycles at a speed of about 0.5 kn.wind had to be taken into account. Arough estimate yielded additionalresistance due to wind dr8g 0:R = 90 kN 8nd an additional friclionalresistance (transverse wind force,friction between hull and ice) ofR 55 kN. The estimate was based uponthe following assumptions and inputdata:angle of attack a = 45°,lateral area above water S = 1850 m' ,air density (T = -10°C, P = 985 hPa)p A= 1.307 kg m- J ,drag coefficient CD = 0.66,lift coefficient C L= 0.33,wind speed v A = 18 ms- 1friction coefficient snow-steel c f 0.2An additional estimate based on theresults of the open water tests with themodel propellers (Jacob, unpublished)and the hull model (Fritsch,unpublished) of RV POLARSTERN and on anassumed thrust deduction coefficient of0 . 19 (after Henschke, 1966) gave arequired power of LIP '" 1300 kW forsurmounting the additional resistance.The measured shaft horsepower valueshave been reduced by this amount inorder to evaluate the new propulsionpower available for icebreaking purposesPScorr'The second series of ramming tests,with 5 test runs, was performed on 20February 1983 in a large (> 10 kmdiameter) plate of first year, level seaice freely floating in the Weddell Sea(position 77° S 38° W). Fig. 3 shows RVPOLARSTERN at the end of a rammingcycle. The ice thicknesses along thetrack in the range broken varied between1.36 m and 1.85 m, while the snowthicknesses measured were between 0.07 mand 0.49 m (Fig. 4). The ice coresampled near the start position for thefirst test run (x 15 m) gave acalculated flexural strength of 321 kPa.During the tests, strong winds ofabout 35 kn (Beaufort 8) came from 4points port side ahead (Fig. 5). Thus,1n the valuation of the test resultsthe additional resistance caused by theFig. 3RV POLARSTERN at the end of aramming cycle in 1.5 m thick seaice with 0.3 m snow cover525

• 0~ ~ ~ ~~CORE NO. 20" f--SIGMF = 321 KPA~~FrTOP SURFACE OFI CE COVER"" ~ - ICE THICKNESS (NEGATIVE) ~ /" ~ /r--\ r"- ~·50 Q", 00 '50 '00x OtlFig. 4Ice and snow thickness profile of the ramming test site(20 February 19 83) in a large free floating ice platein the southern Weddell Sea.Fig. 5Broken channel in front of RVPOLARSTERN during ramming test(No. 27. I on 20 February 1983).Video record of the shipborneice camera. Th e wind directionis discernible from the driftingsnow. The shape of the end ofthe broken channel reflects thewaterline contour of the vessel.The sequence of accelerationdistances in this test series Has I,1/2, I, 3/2 and ship length. Theresults are also compiled in Table 1(Run numbers 23.1 to 27. I).Limit ice thicknessThe above test results wereprocessed according to the method ofcalculation described earlier in the"Analysis" section. The data relevant tothe calculation are compiled in Table 2.The mean value and standard deviation ofthe power-ice thickness coefficient Cdetermined from the measured data isC 6796 + 2045 kWm- 2 (n~9) resultingin a 907. confidence interval ofC 6796 + 1268 kWm- 2 for the standardice flexural strength of Of 0= 500 kPa(cold winter sea ice).It is now assumed that duringicebreaking only 907. of the shafthorsepowe r measured during open watertrials (13840 kW; Sprunk, unpublished)is available, i.e., maximallyP . 12460 kW. Based on thisassumption, Sow the maximum ice thickness526

Run sPScorr TPcorrh PvE~~leff Seff TM h· eIeffNo. [kll) [kN) [ms ) [m) [m) [kll) [kN) [m) [kllm- 2 )13.1 10 134 816 3.1 80 1.77 19 200 730 1.65 7 05214.1 9 308 798 1.0 40 1. 78 11 080 152 1.66 4 02115.1 10 009 798 1.8 100 1. 75 12 479 196 1.64 4 64016.1 9 236 775 3.0 80 1.80 17 383 684 1.68 6 15923.1 6 865 607 1.9 50 1. 71 11 827 439 1.37 6 30124.1 7 423 643 1.3 30 1. 73 11 375 342 1.38 5 97325.1 8 719 728 1.8 28 1.87 17 141 703 1.50 7 61826.1 6 829 622 2.2 38 1.66 15 327 774 1.33 8 66527.1 8 722 742 1.9 30 1.58 17 318 731 1.27 10 737Table 1Ramming tests in thick level ice, resultsRun v EMBs P sNo. Ikn) 1m) Ikll)Tp hIIkN) 1m)hs1m)h leff of PscorrIml IkPal IkWIT reorrIkN)13.1 6.0 80 10 13414.1 2.0 40 9 30815.1 3.5 100 10 00916.1 5.9 80 9 236816 1.73798 1. 73798 1.67775 1.730.070.070.130.101.77 4371. 78 4371. 75 4371.80 43723.1 3.7 50 8 16524.1 2.5 30 8 72325.1 3.5 28 10 01926.1 4.3 38 8 12927.1 3.6 30 10 022719 1.50755 1.56840 1.74734 1.57854 1.390.310.260.190.130.281.71 321 6 8651.73 321 7 4231.87 321 8 7191.66 321 6 8291.58 321 8 722607643728622742speed at the moment of impactindentation depthtotal shaft horse powertotal propeller thrust at thrust bearingsice thickness, mean over indentation depthsnow thickness, mean over indentation deptheffective ice thicknessice flexural strengthshaft horse power, corrected w.r.t. headwindand additional frictionpropeller thrust. corrected v.r.t. headwindand additional frictionTable 2Data set for limit ice thickness evaluationwhich can be broken by RV POLARSTERN incontinuous motion in cold winter ice ofa fO = 500 kPa is computed to be 1.35 m(90% confidence interval: 1.24 m to1.50 m). This ice thickness includes asnow depth of 2/3 of the ice thickness.In the summer season, where flexuralstrengths of Of 350 kPa can beexpected (as observed in thesoutheastern Weddell Sea in February1983) the limit values are about 20%higher (mean limit ice thickness 1.61 mL527

VerificationThe observation that RV POLARSTERNoperated right at the transition fromramming to continuous icebreaking duringthe third test run (No. 15. I) of thefirst test series on 17 February 1983,allows the above calculation to beverified. Re-analysis of the situationgiven at the thinnest stretch in thistest run yields a limit effective icethickness of hreff 1.45 m. Themeasured effective ice thickness wash Ieff= 1.61 m. This value lies sligh~lyabove the upper boundary of the 90%confidence interval at 1.59 m.Using the results of the 1985expedition of RV POLARSTERN to Spitsbergen,where extensive icebreakingtrials were carried out (Hellmann, 1985),another verification is possible. Here,at a net ice thickness of hI = 1.40 m, aspeed of I kn was observed at a shafthorsepower of 11380 kW. The snow thicknesswas 0.23 cm (Hellmann, personalcommunication). In terms of the presentanalysis, this results in an effectiveice thickness of h leff = 1.55 m. Withthe power-ice thickness coefficient C ofthe present study and the flexuralstrength Of 325 kPa reported byHellmann (1985) a limit ice thickness ofh leff1.61 m (90% confidence interval1.47 m to 1.78 m) can be computed. Forcomparison, the limit ice thickness ofRV POLARSTERN as predicted by ice modeltests was 1.42 mat P s = 12840 kW andof = 500 kPa (Schwarz, et aI., 1981).Estimation of General IcebreakingPerformance of a Vessel from the KnownLimit Breakable Ice ThicknessThe limit ice thickness establishedin the present study also allows a firstrough estimate of the icebreakingperformance of a vessel in level ice atvarious thicknesses. This estimate isperformed by linearly connecting thelimit ice thickness for a certain powerand ice strength (zero speed) with theopen water speed at the same power (zeroice thickness) in a speed vs. effectiveice thickness diagram.In Fig. 6 this has been done for theP 12000 kW power level of RVPOLARSTERN for a flexural ice strengthof of = 325 kPa. This allows a directcomparison with the 1985 Spitsbergenresults (Hellmann, 1985). The open waterspeeu of 15.9 kn has been taken fromSprunk (unpublished). It should be notedhere, that the effective ice thicknesshI ff used in this study includes thesn~w with 2/3 of the snow depth, whereasHellmann (1985) presents graphs for boththe net ice thickness and a correctedice thickness, which includes the snowdepth in full. The shaded area in Fig. 6represents the 90% confidence intervalsupposing the open water speed to beexact.ConclusionsThe present study shows a rathersimple procedure for establishing, fromramming tests in thick ice, the limitthickness of level ice which can besurmounted by an icebreaking vessel incontinuous motion at a speed close tozero. Keeping in mind the simplicity ofthe calculation and the crudity ofassumptions on which it is based, theverification of the results issurprisingly successful.This is also true for the roughestimate of the overall icebreakingperfoxmance of a vessel based on theknowledge of the open water performanceand the limit ice thickness. But, itmust be emphasized here that whenever amore or less detailed knowledge of theicebreaking performance of a vessel isrequested, this can only be establishedin an analysis with input fromcorrespondingly detailed trials with thepower levels and ice thicknesses variedsystematically. Such an analysis cannotbe replaced by ramming tests inconnection with the present method ofcalculation.Nevertheless, it should be kept inmind that the methods presented havebeen verified for one case, only.Further applications seem worthwhile.528

20v[KN)RV POLARSTERNPS = 12 000 KW105SIGMAF = 325 KPA1.0 1.5 20HIEFF [M)Fig. 6Icebreaking performance of RV POLARSTERN estimatedfrom the limit breakable ice thickness and the openwater speed compared with measured values.AcknowledgementsThe present paper is a contributionfrom the Hamburg Ship Model Basin(HSVA). The investigations reported havebeen carried out for the German Ministryfor Research and Technology (BMFT) underorder POL 0027-4. The author isresponsible for the content of thispublication.The Alfred Wegener Institute forPolar Research, Bremerhaven (AWl),represented by its director Professor G.Hempel, supported the experimental partof this study by making helicopter icereconnaissance flights possible and bygranting personnel and logisticassistance. This support is gratefullyacknowledged. Thanks are due also thethe master of RV POLAR STERN , Capt. L.Suhrmeyer and his crew for theircooperation during the tests. Last, butnot least, the author expresses hisgratitude to Mr. O. Reinwarth of theBavarian Academy of Science, Munich, whounselfishly assisted the preparationalmeasurements on the ice, and to Mr. L.Hoffmann (HSVA), who was responsible forthe data acquisition.ReferencesHellmann,messungenfiihrungJ.-H., 1985. LeistungsimEis an Modell und GroBausvon"FS POLARSTERN" (Powermeasurements in ice with model and fullscale version of RV "POLARSTERN").Jahrbuch der SchiffbautechnischenGesellschaft, Vol. 79 (1985) pp.411-425, Springer, Berlin-Heidelberg-NewYork.529

Sprunk, B. , llnputlished. Leistungsautdem Deutschen Polar-messungenForschungs- und Versorgungsschiff"POLARSTERN" (Power measurements withthe German polar research vessel"POLARSTERN") . Hamburgische-Schiffbau­Versuchsanstalt GmbH., Hamburg, HSVAReport Fm 16/82, November 1982.Hausler, F.U., 1987. Icebreakingperformance of RV POLARSTERN in brokenice. Proceedings, 9th InternationalConference on Port and Ocean Engineeringunder Arctic Conditions (POAC-87), Universityof Alaska, Fairbanks, USA. 17 -21 August 1987.Frankenstein, G.E. , 1970. The flexuralstrength of sea ice as determined fromsalinity and temperature profiles.National Research Council Canada,Associate Committee on GeotechnicalResearch, Technical Memorandum No. 98,pp. 66-73.Frankenstein, G.E. and Garner, R., 1967.Equations for determining the brinevolume of sea ice from -0.5 to 22.9 °c.Journal of Glaciology, Vol. 6, No. 48(1967), pp. 943-944.Schwarz, J. and Weeks, W.F., 1977.Engineering properties of sea ice .Journal of Glaciology, Vol. 19, No. 81(1977) pp. 499-531.Jacob, M.B, unpublished. PropellerFreifahrten. Propeller No. 2072 mit undohne Dllse (Propeller open water tests.Propeller no. 2072 with and withoutnozzle). Hamburgische Schiffbau-VersuchsanstaltGmbH., Hamburg, HSVA ReportPF 12/81, October 1981.Fritsch, M., unpublished. ModellversuchefUr das Deutsche Polarforschungs- undVersorgungsschiff, (Model tests for theGerman polar research and supply vessel)Hamburgische Schiffbau-VersuchsanstaltGmbH., Hamburg, HSVA Report WP 19/81,May 1981.lIenschke, W., 1966. Sch i ffbautechni schesHandbuch, Band I, Schiffstheorie, Widerstandund Propulsion, Schiffsfestigkeit(Handbook of Naval Technology, Vol.-1-,-- Ship Theory, Resistance andPropulsion, Structural Strength). 2ndedition, corrected reprint, VEB VerlagTechnik, Berlin.Hellmann, J.H., personal communication.Schwarz, J., Jochmann, P. and Hoffmann,L., 1981. Prediction of the icebreakingperformance of the German polar researchvessel. SNAME Spring Meeting/StarSymposium, Ottawa, Ontario, June 17-19,1981, pp. 239-248.530

FINITE-ELEMENT ANALYSIS OF THE ELASTO-PLASTIC MODELLINGOF THE INDENTATION PROBLEM IN SHIP-ICE INTERACTIONC. JebarajA. S. J. SwamidasMemorial University of Newfoundland, St. John's, Newfoundland, CANADASteven J. JonesNational Research Council, St. John's, Newfoundland, CANADAK. MunaswamyMemorial University of Newfoundland, St. John's, Newfoundland, CANADAAbstractThe static indentation of sea icefields has been investigated using theidealized form of icebreaker bows andshell frames. The analytical model usesthe finite element analysis and Tsai-Wufailure criterion; a 20-noded, threedimensional formulation is compared with8-noded Serendipity plate element and 4-noded plate bending element with in-planedeformation. Three-dimensional elementsare found to give better results thanplate elements. The results are found tobe comparable to those given by earlierresearchers and those obtained from fieldexperiments _ The presence of frictionincreases the failure loads.IntroductionIce in the marine environment hasbeen recognized as a serious hazard andan impediment to navigation and otheroffshore activities in the Arctic andsub-Arctic regions. Ship designersdesign the polar icebreakers with thecapability of forcing a passage throughvarying concentrations of ice-filledwater. To date, the design concepts areThis is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987. © The Geophysical Institute,University of Alaska, 1987.partly rigorous and partly empirical.This is due to the complex interactionsbetween a ship's hull and an ice-sheet.In this paper, the deformation andfailure characteristics of floating icecovers have been determined as the hullof an icebreaker indents level ice fieldsof different thicknesses. The finiteelement method is used to analyticallymodel the icefie1d with three-dimensionalquadratic isoparametric elements. Theice is modelled as a transverselyisotropicmaterial having elastic andperfectly-plastic behaviour. Thenumerical values are compared with fieldtest results and the published results ofother researchers.Review of LiteratureThe earliest mathematical modellingof the mechanics of icebreaking wasdeveloped and discussed by researcherssuch as Popov et al. (1967), Kasteljan etal. (1968) and Enkvist (1972). Later, amore rigorous mathematical modelling ofthe continuous motion of ship in ice wasproposed by Milano (1973, 1975).Recently Daley (1984) modelled thedynamic problem of ship-ice interactionby simulating the behaviour in the timedomain using the normal modes of the shipand the ice.The finite element method has beenused in the study of ship-ice interaction531

y Pulkkinen (1983), Varsta (1983) andscientists at VTT, Finland (1985).Pulkkinen (1983) used the finite elementmethod in a study of ice sheets failingby crushing against a pier. Pulkkinenused: 1) three-dimensional finiteelements possessing visco-elasto-plasticand orthotropic material properties; 2)three-parameter yield functions and theTsai-Wu criterion in defining the yield;and, 3) also considered friction at theice structure interface. Varsta (1983)considered the ramming impact between theship's hull and the ice sheet anddeveloped finite element procedures tohandle the same. The results werecompared with model tests and theagreement found to be good. In a recentreport, scientists at VTT, Finland (1985)used beam elements to represent the shipand empirical relationships to representthe ice-forces exerted on the bow of theship. Using modal super-positionprocedures, the dynamic response of anicebreaker due to ramming impact wasdetermined.In a comprehensive paper presentedat the Working group Session of IAHR IceSymposium 1986, Jordaan (1986) emphasizedthe need for proper numerical modellingconsidering: 1) the complex behaviour ofice, part icularly its disintegration andsubsequent "flow" resulting in spatiallyvarying mechanical properties; 2) therandom distribution of flaws in sea iceand icebergs; and 3) the fact that flowof crushed ice and its clearing implies aflow (Eulerian) approach as against aLagrangian approach for "solid" icemechanics. Hausler et al. (1987) statedthat the elastic-plastic low strain ratedeformation behaviour of floatingcolumnar-grained ice covers can bepredicted to some extent by the use ofnon-linear finite element analysis incombination with an anisotropic materialmodel. Arctec Canada Limited, in their1986 report to the Canadian Coast Guard(1986), characterized the ramminginteraction as a transfer of energybetween the components of a dynamicsystem which included the ship, icefloes,water and the free surface. Theprocedure is extremely complex andconsiders elastic, plastic and rigid bodyresponses.literature by Keinonen (1983) and Ghoneimet al. (1984) for the vessels MV CANMARand MV LeMEUR. The results of a seriesof tests performed using the MV ARCTIC,while ramming icefloes in the easternCanadian Arctic, were reported byMel ville Marine Consultancy (1983). Inall these field tests many hundreds oframming tests were made at differentspeeds and these data are quitecomprehensive in scope. Vaughan (1986)commented that these real test data arecostly and available only commercially;consequently, he suggested that theimpact forces developed on theice-transiting vessel should bedetermined analytically.Problem Definition and DevelopmentThe static indentation of a floatingice-cover by a ship has been modelled asthe problem of a plate on an elas ticfoundation subjected to end loads. Theship hull is idealized as a rigid bodypenetrating the ice-sheet. Since impactis a short-duration phenomenon and thestrain rates generated during impact arehigh, ice is modelled as an elastoplasticmaterial rather than an elastovisco-plasticmaterial. The ice-sheet ismodelled analytically using 20-nodedisoparametric elements and the fluidfoundation by one-dimensional Winklersprings. The transversely-isotropicnature of the colummar sea ice isconsidered in the material characterizationand the elastic constituentmatrix suggested by Jones (1975) has beenadopted.The constituent relationship betweenstress and strain for a transverseisotropic material is given by{a} = [D]{e} (1)where {a} is the 6 x 1 stress vector, [D]is the 6 x 6 material properties matrix(symmetric) and {e} is the 6 x 1 strainvector. The constituents of [D] matrixare given as,Dl1 (1 - v 13 v31)! El E3 6D12 (v12 + v31 v13)/ E1 E3 6Field measurements made on ship-iceinteraction were reported in the532

D22D33D44D55Dll(1 - vt2)/Gl3D44E2 t:.1D66 (D ll- D 12)/2and t:. (1 -2v12 - 2vl3 v31 -2v 12 v31 vl3)/2E1 E3 (2)The five independent materialconstants Le., E 1 , E 3 , v12' v13' v31(assuming the material to be isotropic inthe 1-2 plane) for the constituent matrixare to be obtained experimentally.Tsai-Wu (1971) yield criterion is used tomodel the failure of the ice field and isgiven byf(o) = Fll (01 + 02) + F33 03 +Gllll (0'1. 2 + al-) + G3333 oj +°1°2 + 2G 1133 (°1 + °2) °3 +2 2 2(~13 + ~23) + 4G 1212 ~12 - 1.0(3)The surface f(o) o is the failuresurface, Le., the stress states insidethis surface do not break the material.The components of the Tsai-Wu criterion,Fij and G ijkl , are determined using theexperimentally-obtained strength valuesof sea ice (Varsta, 1983). To visualizeand improve the applicability of thecriterion, a reference number ~ suggestedby Varsta (1983) is used and is givenby(4 )where AF includes the first two and BGthe next six terms of the Tsai-Wucriterion given in Eqn. (3). When thereference number ~ = 1, the stress stateis on the failure envelope (f (0) = 0)and when ~ < 1.0, then the stress stateis within the failure envelope.The analysis is carried out by usingincremental forces as the input at knownnodes where the ship's hull impinges onthe ice-sheet; since the ice behaviour ismodelled in an elasto-plastic manner, thefirst few load increments give onlyelastic deformations. At the end of eachincremental load analysis, the totalstress field is checked for yieldingusing the Tsai-Wu criterion. When aparticular element starts yielding at itsGaussian points, the correspondingelasto-plastic constituent matrix isdetermined. During the general stage ofthe iterative solution of a finiteelement elasto-plastic problem, theequilibrium equations will not be exactlysatisfied and a system of residual forceswill exist. The solution process isrepeated a number of times until theresidual forces are reduced sufficientlyclose to zero (Munaswamy et al., 1986).The convergence at each iteration ischecked by stipulating tolerances ondisplacements, forces and energy; usuallythe convergence is governed by thetolerance stipulated for energy (Bathe,1985).For each incremental loading, thefailure is checked using the parameter ~at every Gaussian point, and the failureof a Gaussian point (~ = 1. 0) away fromloading surface indicates the initiationof failure. At the initiation of failurein the ice-sheet (when ~=1), the normalload acting on the interacting surface isconsidered as the failure load. It isobserved that the failure occurs in thetensile bending region. Failure loadsare obtained as a 'function of ice coverthicknesses and hull angles. Theinfluence of friction of the interactingsurface on the failure load is alsoinvestigated.Preliminary Studies onChoice of· the Finite ElementThe choice between two-dimensionalplate elements and three-dimensionalisoparametric elements depends on howaccurate the modelling should be toobtain good results. The assumption intwo-dimensional plate elements that thestress and strain are zero (e z= 0z = 0)in the through thickness direction maynot be valid when the ice-sheet is thick.In addition, the load at the interactingedges produces large 0z stresses whichare neglected in the plate modelling.The three-dimensional element models thetrue state of stress in the three533

dimensions (both normal and shearingstresses) and hence it may be closer tothe existing real situation in thefield.Varsta (1983) used a two-dimensionalplate bending element to model thetransverse loading effect and superposedthe in-plane membrane stresses to obtainthe total stress field. In the presentstudy a two-dimensional eight-nodedquadratic isoparametric element of theSerendipity family was chosen to modelthe flexural and in-plane degrees offreedom. This element was developed byZienkiewicz (1983) with a reducedintegration order of 2x2x2 to avoid theshear locking. In addition, anotherfour-noded cubic polynomial plate elementwas used to check the plate performance.Tables I and II show the validity ofdifferent element formulations comparingthese with the classical plate theoryresults. For the case of a plate withthree fixed edges and one free edge(similar to the case of an icefieldsubjected to ship impact) the results ofthe 20-noded and 8-noded elements agreedwell for a thicker plate, while shearlocking phenomenon was observed as theplate became thinner. Shear locking inthinner plates stiffens the plates andreduces the displacement. This is due tothe fact that the force boundarycondition at the free edge of the plate,viz., bending moment and shear force atthe free edge reduce to zero, could notbe explicity incorporated in theformulation. Hence, in our further studyonly 20-noded isoparametric elements wereused for modelling the ice indentation.Trial runs were made to decide onthe size of the plate to be used in theanalysis using 18m x 36m, 40m x 80m and70m x 140m size ice-sheets; only half theplate was used in the analysis takingadvantage of the symmetry in the plate.The variations of 0y stresses along thesymmetry line of the plate are shown inFig. 1. It is observed that the 70m x140m plate better simulates the conditionof an infinite plate on elasticfoundation since the boundary stressesalmost tend to zero. It is also observedthat when the in-plane displacements ofthe fixed boundary are not restrained inthe 18m x 36m plate, the boundarystresses reduce to zero. However, whiledetermining the zone of maximum stresseswhere the failure would initiate, it isobserved that for all the three cases, itoccurs at a distance of 1 to 4m from theloaded end. Further, the 18m x 36m plategives a more accurate state of stress inthe failure zone due to properdiscretization in that zone. Hence, forour further study, only the 18m x 36m(with all the boundary degree of freedomfixed) plate was used.h = 0.6 m, y = 50°......~-(/)18m x 36m Icesheet - BOUNDARY INPLANE D.Q.F. ARE FREE0.6 r.' ~ 18m x 36m Icesheet-' " FIXED/ 40m x 80m Ice sheet _. " •0.4 .~. 70m x 140m Ice sheet - • " "0.232.0 70.0b>' -0.2Distance along x- axis (m)-0.4Fig. I Variation of Stresses along the Symmetry Line of the Icesheets withDifferent Dimensions534

Table IClamped square plate subjected to a uniformly-distributed loadThickness'h' (m)0.10.050.0250.0125160 0.1609 x 10- 3 0.1544 x 10- 3 0.1614 x 10- 3 0.1668 x 10- 3Max.a = 2.0 m (side)E = 2.1 X 1011 N/m 2 , v = 0.3Intensity of loading = 300 N/m 2deflection at the centre of the platea/hClassical theory 20 noded solid 8 noded plate 4 noded plateelement element element20 0.3144 x 10- 6 0.3163 x 10- 6 0.3319 x 10- 6 0.3258 x 10-640 0.2515 x 10- 5 0.2451 x 10- 5 0.2562 x 10- 50.2606 x 10-580 0.2012 x 10-4 0.1935 x 10- 4 0.2029 x 10- 4 0.2085 x 10-4Table IIThree edges clamped, one edge free square plate subjected to auniformly-distributed load.a = 0.8 mE = 2.1 X 1011 N/m 2 , v 0.16666Intensity of loading60 N/m2Thickness'h' (m)0.10.050.0250.0125a/h8163264Max. deflection at the midpoint of free edgeClassical theory 20 noded solid 8 noded plate 4 noded plateelement element element0.4546 x 10-8 0.4377 x 10-8 0.4522 x 10-8 0.4137 x 10-80.3637 x 10- 7 0.3127 x 10- 7 0.3183 x 10- 70.3309 x 10-70.2909 x 10- 6 0.2422 x 10- 6 0.2454 x 10-6 0.2648 x 10-60.2327 x 10- 5 0.1919 x 10- 5 0.1942 x 10- 5 0.2118 x 10- 5535

Data For the AnalysisThe elastic constants for the icesheet are taken as, E1 = 7.28 x 10 9 N/m 2 ,E3 = 10.16 x 10 9 N/m 2 , v 12 = 0.59, v130.34, v 31= 0.48. These elasticconstants are taken from the experimentalresults reported by Varsta (1983). Thestrength constants are S1C = 3.7 MN/m 2 ,S3C5.16 MN/m 2 , SIT = 0.55 MN/m 2 andS3T 1.0 MN/m 2 • The density of thewater is taken as 10045 N/m 3 •The 'bow' indentation and 'sideframe' indentation were considered in thestudy, and for the bow indentation aconventional bow of V-notch configurationwith sharp edges and apex angles wasassumed. The relationships between theinteracting forces and the shipconfiguration angles are presented inAppendix I (Matsuishi, 1984).Results and DiscussionIn Fig. 2, the failure loads for a0.5 m thick ice plate are given and it isseen that the 20-noded solid elementconsistently gives a lower load than the8-noded plate element. This could be dueto the approximation that Oz= 0 in theplate element.The presence of ozin thesolid element tends to increase thetensile stress in the plate and thusgives a lower failure load, Fn. Hence,for further analysis, only 20-nodedthree-dimensional elements were used.The study investigated the effect offrame angle 'y' and the ice-sheetthickness 'h' on the ice resistance loads'Fn', both for "bow" indentation and"side frame" indentation. Whileinvestigating the effect of ice-sheetthickness on the ice resistance loads,the frame angle of the ship was kept at50 0 and the stem angle at 30 0 • In Fig.3, the variation of ice load isapproximately proportional to the squareof thickness, as reported by Riska(1980).The stem angle of the ship was takenas 30 0 and the ice thickness as 0.5m forthe study of ice resistance loads vs.frame angle of the ship. In Fig. 4, theice resistance loads are decreasedconsiderably when the frame angleincreases from 30 0 to 40 0 • For furtherincrease of the frame angle, the curveflattens considerably.In Fig. 5, the contours of theTsai-Wu reference number ~ are presentedat the instant of failure of theice sheet • The frame angle 'y' was 50 0and the ice sheet thickness was 0.35m.In Figure 5, it is shown clearly that thefailure of the icesheet is initiated atabout 2m from the loading edge; the sameQehaviour was also observed by Varsta(1983). At first, local failure of theGaussian points of the face of theelement on which the load is appliedoccurs; then the failure progresses asper the ,~, reference number and at thetime of failure ,~, becomes unity.To study the influence of frictionof ice on the failure load, a staticfriction coefficient of ~ 0.1 wasassumed in the interacting surface. Thefrictional forces, together with thenormal forces, were applied on theinteracting surface and the analysiscarried out. In Fig. 6, it is seen thatthere is an increase of failureresistance loads due to the presence offriction.ConclusionsIt is observed that for relativelythicker plates, the 20-noded formulationgives better results than the plateelements. While the plate elementformulation gives higher failure loads,the three-dimensional 20-noded elementsgive lower loads since the actualbehaviour is better simulated by thatmodel. As expected, the bow indentationcauses the ice to fail at much lowerloads than the side frame indentation.Presence of friction at the interactingedges increases the failure loadssignif icant ly.AcknowledgementsThe authors would like to thank Prof.G.R. Peters and Prof. T.R. Chari, Deanand Associate Dean of the Faculty ofEngineering and Applied Science and Dr.I. Rusted, Vice President of the536

180 140160h = 0.5m~ Varsta 1983 (with Friction)z~z~~140 ~c"C Element ~00 "C...J1200Element ...JE0~0 ~ Ez 100Ul C1l Zw ~...... :::J C1l0 :::J~1201000800~60080 400~~Frame Loadingy= 50°Bow Loading6020 30 40 50 60 7020Fig. 2Frame Angle 00.2 0.3 0.4 0.5 0.6 0.7Effect of Different Finite ElementModels on the Failure Load (FrameThickness (m)Loading) Fig. 3 Variation of Failure Load VS. Thicknessof the Icesheet

140hz:0.5mz.lI:lL. c120100"000 80....JFrame LoadingF n =40 kNh = 0 35 mr = 50·UltH000E...0z 60CIIBow Loading...:::I0 40lL./k'20OL---~L---~----~----~----~20 30 40 50 60 70Frame AngleFig 5 ~ - Reference Number ContoursFig. 4Variation of Failure Load vs. Frame Angleof the Ship

180h= 0.5mZ.JO:c:lJ..'000...J1601400 120E...0ZQ)100 ...:J0lJ..8060~--~~--~----~----~----~20 30 40 50 60 70Frame AngleFig. 6Effect of Friction on the Failure Load(Frame Loading)Memorial University of Newfoundland fortheir continued interest andencouragement. Grateful acknowledgementis due to Ms. Barbara Flynn, for the careand patience exercised in typing themanuscript. The financial support ofthis investigation by the Institute forMarine Dynamics, St. John's,Newfoundland is gratefully acknowledged.REFERENCESArctec Canada Limited, 1986, "ParametricModel Study of MV ARCTIC Hydro ElasticResponse During Multiyear Ice Impacts"Report submitted to Canadian CoastGuard.Bathe, K.J.,Procedures inPrentice Hall,New Jersey.1982, Finite ElementEngineering Analysis,Inc., Englewood Cliffs,Daley, C.G., 1984, "BAFFIN-A DynamicShip/Ice Interaction Model", Paper No.F, Ice Tech-84 Sympos ium, Calgary, Maypp. F1-F8.Enkvist, E., 1972, "On the Ice ResistanceEncountered by Ships Operating in theContinous Mode of Icebreaking", ReportNo. 24, Wartsilla's Icebreaking ModelBasin, Helsinki Shipyard, Finland, pp.1-181.Ghoneim, G.A.M., Johansson, B.M., Smyth,M.W., and Grinstead, J., 1984, "GlobalShip Ice Impact Forces Determined fromFull-Scale Tests and Analytic Modellingof Icebreakers CANMAR KIGORIAK and ROBERTLeMEUR", Transact ions of the Society ofNaval Architects and Marine Engineers,November, Vol. 92, pp. 253-282.539

Hausler, F.U., and Matthies, H.G., 1987,"Elastic-Plastic Deformation of FloatingColumnar Grained Ice ComputerImplementation and Ice Tank Test Results"Paper submitted to InternationalConference on Plasticity, Barcelona,Spain.Jones, R.M., 1975, Mechanics of CompositeMaterials, McGraw-Hill Book Company,Washington, D.C.Jordaan, 1.J., 1986, "Numerical andFinite Element Techniques in Calculationof Ice-Structure Interaction problems",Proceedings of the IAHR Symposium, Iowa,Vol. II, pp. 405-441.Keinonen, A.J., 1983, "Ice Loads on Shipsin the Canadian Arctic", Proceedings ofWest European Graduate Education MarineTechnology, Finland, March.Kasteljan, V. I., Pozrijax, 1. I., andRynlin, A.J., 1968, "Ice Resistance toMotion of a Ship", Sudostryenize,Leningrad.Matsuishi, M., Ikeda, J., Kawakami,H., and Hirago, M., 1984, "Ship-IceFloe Collision Analysis Consideringthe Elastic Deflection of HullGirder", Paper No. E, ICETECH-84Symposium, Calgary, May, pp. E1-E13.Melville Marine Consultancy, 1983,"Measurement of Hull Girder Strains andVibration during Ramming of Multi-yearIce Floes, Lancaster Sound, November1981", Reports P82-01, P82-03 toTransport Canada, Department of Suppliesand Service, Hull, Quebec, Harch.Pulkkinen, E., 1983, "Large DisplacementVisco-Elastic Plastic Finite ElementAnalysis of Ice Forces During Ice Failureby Crushing Agains t a Pier", Proceedingsof the 7th International conference onPort and Ocean engineering under ArcticConditions, Helsinki, Finland, Vol. III,pp. 802-811.Riska, K., 1980, "On the Role of FailureCriterion of Ice in Determining IceLoads", VTT, Espoo, Finland, Report No.7, March, 30p.Tsai, S. and Wu, E., 1971, "A GeneralTheory of Strength for AnisotropicMaterials", Journal of CompositeMaterials, Vol. 1, No.5, pp. 58-80.Varsta, P., 1983, "On the Mechanics ofIce Load on Ships in Level Ice in theBaltic Sea", VTT, Espoo, Finland,Publication No. 11, August, 91p.Vaughan, H., 1986, "Flexural Response ofIce-Breaking Ships to Impact Loads",Proceedings of the Royal Institution ofNaval Architects, Vo!. 128, pp.259-267.VTT Technical Research Centre of Finland,Ship Laboratory, Research Report No.LAI-548/85, 1985.Zienkiewicz, O.C.,Element Method,McGraw-Hill BookLondon.1983, The FiniteThird Edition,Co. (U.K.) Ltd.,Milano, V.R., 1975. "Variation ofShip/Ice Parameters on Ship Resistance toContinuous Motion in Ice", Paper No. B1,ICETECH Symposium, Montreal, April, 26p.Munaswamy, K., Jebaraj, C. and Swamidas,A.S.J., 1986, "The Indentation Problem inShip-Ice Interaction", Report preparedfor the Institute for Marine Dynamics,St. John's, Newfoundland, Department ofSupplies and Services, Government ofCanada, November, 103 p.Popov, Y.N.,Sailing inLeningrad.1967,Ice" ,"Strength of ShipsSudostroyenize,540

Appendix -ISide Frame Indentation of the Ice-FieldBow Indentation of the Ice-FieldIn Figure A-I (i), the interactiongeometry of the ice-sheet is given as itis indented by the bow of the ship. Theinteraction force components acting on anelement dA, shown in the figure, may berepresented as follows:In Figure A-I (ii), the interactionbetween the shell frame of the ship withthe ice-sheet is illustrated. From thegeometry it is evident that theinteraction forces will be in twodirections only, viz., 1; and zdirections.Normal Force component, dFn= -n.oc.dA(AI)Frictional force component,dF~ = -f.~.dFnwhere nand f are the unit vectorcomponents. The force components overthe shell plating (bow) are obtained byintegrating eqn. (AI)Fn f dF n AnF = dF~~fAn(A2)where An is the effective contact area.Let the unit vectors nand f ben(A3)Assuming the shell plating at the area ofcontact to be a plane as shown in Fig.A-l(i), the components of the unit vectorin the three directions are given as:nn - C/tan awhere C = 1//"""1-+--1--+--1--tan 2 4> tan 2 aa = stem angle of the ship4> entrance angley frame angletan ¢ tan a tan y(A4)Knowing the components of the unitvector, the components of the total iceforces in the three directions areevaluated.541

yD'O~B'-XA .y.­PlanZ --',B'Oo._2Section(a) Crushing of Ice/./AIL._(b) Ship-IcexInteraction ForreFig. A-1 (i) Bow Loading. (Matsuishi et at, 1984)BOWOFVESSELFig. A-1 (ill Side Loading542

BARGEv-ICE SHEETICE-RUBBLEACCU MULATIONCHANNEL BED(0)....... CHANN EL BED(b)Fig . 1 .Ice rubble accumulation beneath a barge moving throughan ice sheet, (a); and beneath a barge moving throughbrash ice, (b).Fig. 2.A barge and tow approaching a lock on theMississippi River, Iowa.544

ICE·RUBBLE BENEATH BARGES IN ICE·COVERED WATERSRobert EttemaHung·Pin HuangUniversity of Iowa, Iowa City, Iowa, USAAbstractThis experimentally based study wasdirected to determine the equilibriumthicknesses and forms of ice-rubbleaccumulation beneath barges movingthrough ice-covered channels. It isshown that significant amounts of icemay accumulate beneath a barge moving atcreeping speed through a navigationchannel covered by a sheet of ice andthat the thickness of the accumulationdecreases with increasing barge speed.For a barge moving slowly through acover of broken ice, a false-bow mayform ahead of the barge and deflect icerubble around the barge such that littleice accumulates beneath it. However,with increasing speed, more ice is sweptbeneath the barge as the false-bowdiminishes in size and eventually awave-form accumulation develops beneathit. Wi th further increases in speed,the accumulation diminishes in thickness.The relationship between accumulationthickness and barge speed issomewhat similar to that between theheight of sedimentary bed-forms andwater or air flows.This is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17·22,1987. © The Geophysical Institute,University of Alaska, 1987.IntroductionBarges and other vessels, withrelatively blunt, shallow-draft andflat-bottomed hull forms, moving throughice~cover ed waters may entrap significantquantities of ice rubble beneaththei r hull and ahead of thei r bow asillus t ra ted in Figu res 1a, b. The resultingaccumulations of ice rubble notonly increase the resistance experiencedin pushing a barge, but, for the r e lativelyshallow waters of many navigableri ve rs and es tuari ne harbors, may causethem to ground on the channel bottom.Accumulations of ice rubble maycause especially severe problems for abarge attempting to pass through a lockduring winter. Ice rubble accumulatedahead of a ba rge can be pushed into thelock creating problems for the operationof the lock's miter gates. Ice rubbleaccumulated beneath the barge may causeit to become stuck on the exit sill ofthe lock. Figure 2 illustrates theamount of ice that a barge can shoveahead of itself, especially when movingthrough a narrow or confined channel.As a barge moves through an icesheet, the resulting broken ice isshoved beneath the barge's bow such thatit accumulates as a wave-like pile atsome location along the bottom of thebarge. If the ice cover is formed of alayer of broken ice, ice rubble may543

develop as a false bow ahead of thebarge and then pass beneath the barge,accumulating along its bottom. Operatorsof tow-barges in the waterways ofthe Upper Mississippi River report thataccumulation problems can be particularlysevere for empty, open barges infrigid conditions because ice rubble mayadfreeze to them.The objective of this study was todetermine the equilibrium thicknessesand forms of ice-rubble accumulationslikely to develop beneath flat-bottomedbarges moving through an ice-coveredwaterway. The study is limited to theconditions of barge movement through icecovers comprised of either a sheet ofice or a relatively thin layer of brokenice.For fairly detailed descriptions ofthe problems associated with barge movementthrough ice-covered rivers refer toHuang and Ettema (1987), Ashton et ale(1973) and Mellor et ale (1978).Factors Influencing Ice-RubbleAccumulationIce rubble accumulates beneathbarges in a manner analogous to sedimentarywave-form development; in particular,like large-scale ripples or aeoliandunes (notably through similarityof flow field causing wave-form accumulation).The equilibrium thickness and formof ice-rubble accumulations beneath andahead of a moving barge are influencedby several factors: barge speed relativeto water; form, draft, beam androughness of barge hull; water depth;and the dimensions and properties (e.g.,strength, roughness, porosity) of icerubble. Other factors, including temperaturesof water, air and ice, mayaffect accumulation thickness.If the development and equili briumforms of ice-rubble accumulations aretreated as wave-like formations, thenthe following functional relationshipshould be considered:h1:(1)where h and A = height and wavelength ofwave, respectively; u* and u*c = shearvelocity of water flow, and its criticalvalue, respectively; ~i = a representativelength dimension of ice rubble; v =kinematic viscosity of water; and, Yo =flow depth. Wave forms in particulatematerials are appropriately described interms of their wave-length, A. Waveheight, h, (or steepness h/A) is a morecomplex property than A because it isdetermined by a larger number of variables.However, for ice-rubble accumulationbeneath a barge, certain complicationsarise when attempting to predictequilibrium accumulation form; Le.,A and h. Namely; the rate of icerubbledischarge beneath the barge isinfluenced by barge (notably bow) geometryand draft, as well as ice characteristics;relatively small area ofaccumulation boundary (bottom of barge(s»;relatively large size of icerubble compared to barge size; and, forri vers and many harbors, the shallownessof water depth. These factors mi tigateagainst directly using a purely analyticalapproach to determi ning equi li briumthickness of ice-rubble accumulationbeneath a barge. Therefore, herein, wego directly to dimensional analysis toarrive at a set of pertinent parametersfor describing accumulation thickness.Usingyieldsh Af (-X-' Y -D'0L ~ tB' Y -D' "[,0Buckingham'sPi-theoremV Vt Yo B, - , Y -D' "[,Ig(1-j)/p)t v 0B'~i'~b,ice properties) = 0(2)in which V = barge speed relative towater; Yo= channel depth p ,p = densitiesaf ice rubble and wa~er,wrespectively;B = stem angle of barge bow;~b' ~i = coefficient of static frictionbetween ice and hull, and ice and ice,respectively; B, D and L = barge beam,draft and length, respectively, t and~ = thi ckness and maj or length of icerubble; and ice properties refer to thestrength, temperature, shape, etc., ofice rubble. Strictly speaking, fordescribing wave topography on extensiveplanes, the parameter V II g( 1-j) i /p w)t =F should be replaced by u*/u*c. However,as u* is not readily determinedfor ice rubble beneath barges, it ismore convenient to use F. Additionally,545

the second term in (1), a Reynolds number,need not be explicitly included in(2) as the flow is fully turbulent. Forthe operators of barges who wish to knowthe thickness of accumulation beneath abarge, the following functional relationshipis of interest:creases with increasing length of arubble piece. Tatinclaux and Gogus(1982), for example, propose the followingequation for assessing the stabilityof ice blocks lodged beneath an icecover:The wave-steepness parameter, hi>", canbe replaced by hit, as it can be arguedfor ripple-like wave-forms, or for waveformsin infinitely deep water, that>.. a t. In (2), the parameter yo/Yo-D isa blockage term, and is included in (3)as F* (= FYo/Yo-D) to account for theeffects on h of water depth relative tobarge draft; t/y -D is a measure ofwave-form roughnes~ relative to flowdepth; and Bit relates barge beam torubble size and, thereby, to potentialwave size.A similar formulation exercise asled to (2) can be followed to identify aparametric relationship for accumulationslope, a: viz.aR. t B L¢j (F*,y -D' T' T' B' ~b' ~i' 8oand ice properties) (4)Before discussing the results ofthe experiments, it is helpful to considerbriefly the influence of icerubbleproportions, or the term tl R., onice rubble accumulation beneath a movingbarge. With regard to the submergenceof individual rubble pieces, studies byUzuner (1977), Daly (1985), Ashton(1987) and Huang and Ettema (1986) showthat for an ice piece of given thickness,t, increasing length, t, leads toa decrease in the cri tical veloci ty, orvalue of F, associated wi th bow submergenceof an ice piece. Table 1, whichsummari zes the dimensions of the simulatedice rubble, indicates that thecritical value of F, designated as F s 'associated with the submergence of thesimulated ice pieces beneath the modelbarge decreased with increasing lengthof ice piece. However, the criticalvalue of F* associated with the entrainmentof ice-rubble pieces lodged againstthe flat bottom of a barge, F~, in-+ C ]-0.53here, 11 mean velocity of water flow;C 1 , C2 and C3 are empirical coefficients.Table 1 : Properties of simulated iceblocksForm Length Width Thick. Fs(mm) (mm) (mm)Large block 39 32 10 2.2Medium block 32 32 3 3.6Small block 13 13 3 5.0beads 3 4.5 3 6.4urea ice 10(ave) 10(ave) 4.5urea ice 20(ave) 15(ave) 10critical value ofentrainment Isubmergence beneath 8 = 21 0 bow ofmodel barge.546

ExperimentsA series of experiments was conductedto determine the equilibriumthicknesses and forms of ice-rubbleaccumulations around barges. In particular,the relationship was sought between(h/t) and F* for varying forms ofice cover. The experiments entailed theuse of a 1:30-scale barge of similarproportions to barges that operate onthe waterways of North America. Themodel barge was 1.8m long, 0.36m wide,of variable draft and fl = 210. It wastowed at one of a range of speeds, 0.03to 0.45m/s. The experimental set-up isshown in Figure 3. Two sets of experimentswere conducted: one entailed useof a glass-sided flume; the other,IIHR's ice towing tank. The flume was20m long, 0.3m deep and 0.76m wide.IIHR's ice towing tank is 21m long, 5mwide and 1.5m deep.For the flume experiments, icecovers were simulated using four sizesof polyethylene blocks/beads. The dimensionsof the blocks are given inTable 1. Polyethylene has the samespecific gravity as ice; 0.92. Thecoefficient of static friction, lib' forsubmerged polyethylene against thepainted finish of the model barge weremeasured to be 0.15, and for urea ice,lib = 0.05. These values were obtainedby pulling blocks over the surface ofthe model barge. We realize the limitationsof using plastic to simulate ice;somewhat higher values of friction coefficient,lack of freezing betweenblocks, non-wetting surface, etc. Tatinc1auxet ale (1978) discuss at lengththe limitations of using plastic tosimulate ice. However, in addition tousing urea ice, polyethylene blocksconstitute an appropriate model icematerial for this study because thestrength properties of ice are not consideredand the accumulation processesmainly involved the movement of fullysubmerged blocks.The 39 x 32 x 10 mm polyethyleneblocks were arrayed in a tight, closepackedformation of 100% areal concentrationsuch that they simulated an icesheet precut into the sizes of brokenice likely to result when a barge movesthrough it. The array of plastic blockssimulated a 0.3m thick ice sheet breakinginto ice blocks with thickness tolength ratio of about 1:3 to 4. Thebeads were used to simulate a cover ofrelatively small-size (O.lm diameterprototype) ice rubble. The areal concentrationof the beads was typicallyabout 75 to 78%. The two smaller sizesof ice block were used to simulate acover of fairly small (0.4 to 0.9msquare x 0.09m thick, prototype dimensions)ice rubble arrayed with an averageareal concentration of about 66%.In the course of experimenting it wasfound that areal concentration of blocksor beads affected the length of bargetransit required for an equilibriumaccumulation to develop beneath themodel barge; lower concentrations ofblocks required longer transits. Thiswas one source of scatter in the data.Another, much less significant, sourcewas the action of buoyancy which, forthick accumulations of ice rubble,slightly altered the mean draft and trimof the barge.Sheets of unseeded urea ice, 4.5and 10mm thick, were used in IIHR's icetank to simulate ice sheets and icerubble. For each experiment, the bargewas towed at constant speed through thesimulated ice covers. Once the accumulationhad attained an equilibriumthickness, which usually occurred afterabout two to three hull lengths of transit,the accumulation was measured bymeans of a small-gauge hook probe. Ascan be appreciated from the photographsaccompanying this paper, accumulationthickness could be measured to an accuracyof about one block thickness forlarge accumulations.A brief set of addi tional experimentswas also performed with two bargesmoving in parallel along the flume. Itwas found that the narrowness of theflume, 7% wider than the beam of thedouble barge configuration, caused thebarges to become occasionally stuck whenthe simulated ice blocks wedged betweenthe barges and the sidewalls of theflume. This is also a problem experiencedby barges moving through locks.ObservationsThe manner inaccumulated beneathmodel barge differedwhether or not thewhich ice rubbleand ahead of thein accordance withbarge transitted a547

GLASS -SIDED FLUME....}PLAN VIEWI·076 mVIEW A-AFig. 3.Experimental set-up.simulated ice sheet or a simulated layerof ice rubble. For both cases, icerubbleaccumulation beneath the bargewas analogous to alluvium wave formation.Figures 4 and 5 illustrate accumulationsobserved for the model bargemoving through simulated ice covers ofsheet ice and an array of floes, respectively.The development of a false-bowand a wave-form accumulation of simulatedbrash ice are depicted in Figure 6and 7, respectively.When the model barge moved throughthe layer of blocks arrayed so as tosimulate an ice sheet breaking into amore-or-less regular train of blocks, itoverrode the blocks and pushed them downbeneath its bow. On rounding the bowand coming to the flat bottom of thebarge, the train of blocks was dislocatedcausing blocks to pile-up. Additionally,due to flow separation (andformation of bilge vortex) associatedwith the change in hull profile, thehydrodynamic forces acting on the blocksdiminished such that, under the actionof buoyancy, the blocks reoriented themselvesinto a jumble lodged beneath theupstream portion of the barge. Theaccumulation thickened until the hydrodynamicforces strengthened so as tobalance the buoyancy force actingthrough blocks along the perimeter ofthe accumulation. With increasing hullspeed, the magnitude of the flow separation,i.e., the diameter of the bilgevortex, diminishes, such that hydrodynamicforces acting on a block are sufficientlystrong to cause the blocks tobe swept along the bottom of the hulland the peak accumulation to be moveddownstream. The maximum thickness ofaccumulation diminished with increasingspeed of barge and the accumulationbecame spread over a greater length ofthe barge. Additionally, the maximumthickness of the accumulation diminishedwith decreasing depth of water beneaththe model barge. Imbrication, overlappingor shingling, of platey ice rubblemay increase the resistance to erosion548

Fig . 4 .Ice rubble accumulated beneath model barge moving throughsimulated ice sheet: large blocks.Fig. 5 .Ice rubble accumulated beneath model bargemoving through simulated cover of floes:medium blocks.549

of an accumulation, as can be seen fromFigure 5.Small or i rregula rly shaped piecesof ice rubble dislocated from one anotherand did not move as a train whenshoved ahead of the test barge. Instead,they gathered to form an accumulationwhen the model barge movedthrough the layer of beads simulating acover of brash ice or pans of frazilice, a false bow developed ahead of thebarge without significant accumulationforming beneath it (Figure 6). Thefalse-bow streamlined the bow, deflectingice rubble around the barge suchthat only a trickle of ice passed beneathit. With increasing speed, thefalse bow shortened and thickened suchthat the beads could resist the fluidforces and a greater discharge of beadspassed beneath the hull causing a dunelikeaccumulation to develop, as illustratedin Figure 7. For very largespeeds, possibly not of practical importancefor conventional barge traffic, afalse bow did not develop and accumulationthickness decreased until only asingle layer of beads flowed beneath thebarge. For the range of speeds associatedwith declining thickness of accumulation,it was not uncommon for rubbleto accumulate as two or more waves beneaththe test barge. It may, therefore,be possible that many wave-formaccumulations may develop beneath theextensive bottom area of full-scales ora tow comprised of several barges.The smaller blocks which simulateda cover of randomly arrayed ice piecesbehaved similarly to the cover comprisedof beads. Lacking the facility to readilyinterlock when shoved, the smallerblocks accumulated as a conical falsebowthat deflected blocks around thebarge. With increasing speed, the falsebow shortened but thickened such thatmore blocks could pass beneath the bargeand collect in a wave-form accumulation.Equilibrium Thickness of RubbleAccumulationThe relationships between hIt andF* differ in accordance with whether ornot the barge transitted simulated icesheets or layers of ice rubble. In eachcase, the maximum value of hIt was about17 to 21, for B/R.) 10. Barge transitthrough ice sheets (simulated using thelargest polyethylene blocks and urea icesheet) is shown in Figure 8. Figures 9and 10 show the cases of the model bargemoving through a broken ice cover (simulatedusing polyethylene blocks andbeads and urea-ice rubble). In thesefigures, normalized accumulation thicknesshIt is plotted against F*, withR. I(y -D) and B/R. indicated as additi8nalo parameters.If the barge were moving at creepingspeed and if the experimental set~upwere truly two-dimensional and thelength of simulated ice sheet transittedwere suitably long, ice rubble wouldtend to accumulate to the full waterdepth beneath the model barge. In otherwords, the maximum value of hIt shouldapproach (yo-D)/t as F* approached zero,provided the barge, or barge-tow, waslarge enough to accomodate the accumulation.This occurred when the test bargewas towed close to the flume wall, orwhen a double barge configuration wasused, because the accumulation was confinedby the flume's sidewalls. However,for a single barge moving slowlythrough the flume of width 2.2 times thebarge's beam, the accumulation becameless 2-dimensional as blocks could beswept, or slide, from its flanks. Consequently,for the condition of a bargemoving through a relatively wide channel,values of hIt are likely to be lessthan (Yo-D)/t, and, for the same reason,they are likely to diminish somewhatwith decreasing value of B/R..Figure 8 shows that hIt diminisheswith increasing value of F*, until F*attains some critical value beyond whichall blocks swept beneath the barge moveas a single layer; hit = 1. Althoughthe data collapse reasonably well, theyvary consistently with R. I(y -D) whichreflects the influence 0 of 0 relativeroughness of rubble size.For barge transit through ice rubble,the development of a false bowcomplicates the relationship of hIt withF* such that a peak thickness of accumulationdevelops, as shown in Figures 9and 10. The relationship is analogousto that between bed-form height and flowvelocity for flow over alluvial beds(e.g., Raudkivi, 1976). In particular,bow and false-bow forms affect the dischargeof ice beneath a hull. As the550

Fig. 6.False-bow formation ahead of model bargemoving through simulated cover of brashice:beads.Fig. 7.Wave-form accumulation of ice rubble beneathmodel barge moving through simulated cover ofbrash ice: beads.551

h II22J 8200 y -0POLYETHYLENE (mm) 0 18LOCKS18 88 0 0.26 II161412108688 6 0.29 II88 0 0.33 II88•0.42 II88 + 0.48 II60 0 0.22 II60 X 0.27 II60 'V 0.32 II-0.05 3.642° °2 3 4 5 6 7 8 9F*Fig. 8.Maximum value of hit versus F ; simulatedice sheets.*20 l\/ ,(022 (11) J B18161412hll10/ \ ~~ (y)f \ 0 I, +f "y-\ 0.67 (36)/ /. \f i \ \ POLYETHYLENE BLOCKS, f i ,.r·· •.

18161412hit1086424 mm OIA BEADS(B/1=90)P-I \I ' ..t, \\00.032\ Y /0 = 0.026. "-.I.J ,..'\j' : '~ \{f ' i ,. \. " '\/l 0.057 \ ,. \, ~.. '0-.I . .!'\ "\ , .Ii' / /... '.. ~\./; / 0.053 0.227....... .• ---~'" ----~--------2 4 6 10 12 14/.1/ '"\,"-....:.. -\.0.084 ~ __Fig. 10.Maximum value of hit versus F : simulatedcover of brash ice.*flow of broken ice beneath the bow increased,so did the height of the accumulationbeneath the barge, until, withfurther increase of both ice and waterflow, the accumulation becomes washedoutand rubble moved as a single layerbeneath the barge.Figure 10 should be consideredtogether with Figure 11 which shows thevariation of false-bow size with F*. Asthe false-bow diminished in size, withincreasing barge speed, more ice wastransported beneath the barge so as toform an accumulation wave. Peak accumulationsgenerally occurred at values ofF* coinciding with the disappearance ofthe false-bow; when a = B.The misalignment of peak values ofhit in Figures 9 and 10 can be attributedto the varying value of F s associatedwith the submergence beneath amoving inclined plate of the differentsizes of simulated ice. The larger peakvalue of hit that was measured for themedium-size block can be ascribed toimbrication of blocks: the medium-sizeblock being longer than the small-sizeblock could readily imbricate (as shownin Figures 4 and 5); and, in accordancewith (5), longer blocks lodged beneath aplanar boundary are more resistant toentrainment. Eventually, due to increasedhydrodynamic forces acting onthe accumulated beads and small blocks,the accumulation washed-out.A typical barge, 9m wide and 2.6mdraft, moving at a speed of 0.5m/sthrough a 0.2m thick ice sheet on a 6mdeep pool of the Mississippi River (F* =2.0, 1/y -D " 0.25), could possiblyaccumula~e about 3m of ice rubble beneathits bottom (Figure 8). If the icecover were comprised of brash ice withpiece thickness of, say, 0.1 m, and thebarge were moving at speed of about 1.0mls (F* ~ 5.3), an accumulation about1.5m thick may develop.The foregoing examples are intendedonly to indicate the potential magnitudesof rubble accumulation. Obviously,many factors affect accumulation andits precise estimation. For example, ifthe barge were moving slowly through aconfined channel, e.g., as an approachto a lock, the accumulations may attainmuch greater thicknesses. These estimatesdo not take into account the effectof ice-rubble pieces freezing toone another, or, on the other hand, thelesser values of friction coefficientthat occur between ice than occur betweenplastic. Freezing would increase553

05Yo04 y(em) o (em) Yo -0 ___ L':~~:AGE STEM ANGLE OF BARGE(218°) 6 21.0 6 140~-.J".0.c 0.2(11.31°)0 18.0 6 1.5003 0 \5.0 6 1.60osr)01(5.7"16000 2 3 4F'"5 S 7 8 9Fig. 11.Variation of a with F : simulated cover ofbrash ice.*the resistance of an accumulation toerosion by fluid forces and likely resultin larger accumulations developing.Lesser values of friction coefficientmay have the opposite influence.Rubble Accumulation as False-BowThe geometry of the false-bowformed at the bow of the model barge canbe desert bed in terms of the angle, asdepicted in Figure 11. With increasingF*, or hull speed, a increased until itwas about the same value as the bow'sstem angle, B. When a exceeded B, thefalse-bow no longer developed and icewas swept beneath the barge. As describedin the foregoing section, thepeak accumulation beneath the modelbarge moving through brash ice more-orlessoccurred when a equalled B.ConclusionsSignificant quantities of ice rubblemay accumulate.beneath flat-bottomedbarges moving through ice-covered channels.The present study shows that themanner of rubble accumulation variesaccording to whether the ice cover is anice sheet or a layer of ice rubble. Ifthe cover is an ice sheet, broken icecan be directly submerged beneath thebarge, and the maximum thickness of theaccumulation decreases with increasingspeed or F*. If the ice-cover is comprisedof comparatively small ice rubble(e.g., brash ice), a false-bow may formahead of a slow moving barge, and actsto deflect much of the rubble around thebarge. With increasing speed or F*, thesize of the false bow diminishes suchthat the flow of rubble beneath thebarge increases and wave-form accumulationsdevelop beneath the barge. Thethickness of the accumulation attains apeak with increasing speed or F*, thereafterdecreasing.The thickness of the rubble accumulationbeneath a barge, expressed ashIt, may attain maximum values of about17 to 21. However, accumulations extendingto the channel bed may formbeneath large, slow-moving barges innarrow or confined channels.554

AcknowledgementsThe work presented here comprisespart of a larger study aimed at determiningtravel frequency effects onbrash-ice accumulation and ice regrowthin ice-covered navigation channels. Thestudy was funded, under grant No. DACA89-85-K-0018, by the U.S. Army Corps ofEngineers Cold Regions Research andEngineering Laboratory (CRREL), Hanover,New Hampshire. Discussions with Dr. J.­C. Tatinclaux and other personnel ofCRREL contributed to the study.ReferencesAshton, G., Den Hartog, S.L. andHanamoto, B. 1972. Icebreaking by Towon the Mississippi River with Renee G.Draft Special Report, U.S. Army Corps ofEngineers, Cold Regions Research andEngineering Laboratory, Hanover, N.H ••Uzuner, M.FloatingAmericanJournal713-722.Discussion1977. Stability Analysis ofand Submerged Ice Floes.Society of Ci viI Engineersof Hydraulics Division, HY7,H. SOININEN: When performing the testswith plastic pieces, did you control thefriction between the hull and the simulatedice pieces and between the piecesthemselves?R. ETTEMA: Other than using two types ofmodel ice--urea ice and smooth polyethyleneplastic--friction between hull andice, or ice and ice, was not systematicallyvaried in this study.Ashton, G. (Ed.) 1986. River and LakeIce Engineering, Published by Water ResourcesEngineering, Littleton,Colorado.Daly, S. 1985. Ice Block Stability.Proc., American Society of CivilEngineers Specialty Conference, Coeurd'Alene, ID, 544-549.Huang, H.P. and Ettema, R. Report inpreparation. Travel Frequency Effectson River Ice Formation. Iowa Instituteof Hydraulic Research, The University ofIowa, Iowa City, IA.Mellor, M. 1980. Ship Resistance inThick Brash Ice. Cold Regions Scienceand Technology, 3, 305-321.Mellor, M., Vance, G.P., Wuebben, J.C.and Frankenstein, G.E. 1978. An Investigationof Ice-Clogged Channels in theSt. Marys River. Report No. CG-D-78,U. S. Coas t Guard Research and DevelopmentOffice, Washington, D.C ••Raudkivi, A.J. 1976. Loose BoundaryHydraulics, Published by Pergamon Press.Tatinclaux, J.-C. and Gogus, M. 1978.Initiation of Ice Jams - A LaboratoryStudy. Canadian Journal of Civil Engineering,Vol. 5, No.2, 202-212.555

MANEUVERING PERFORMANCE IN ICEOF THE UNITED STATES COAST GUARD 140·FOOT ICEBREAKERPekka KannariWdrtsird Arctic Research Centre, Helsinki, FINLANDDavid H. HumphreysU. S. Coast Guard, Washington, D. c., USAAbstractManeuvering tests in level ice, infull and model scale, were conductedwith the USCGC MOBILE BAY (WTGB-103), al40-foot icebreaker. Data on maneuveringand level icebreaking performance weregathered in full scale on Lake Michiganin Green Bay during February 1984.Altogether, 37 turning data points and66 ice breaking performance data pointswere collected. The model test programconsisted of self-propelled maneuveringtests and straight ahead towing andpropulsion tests in ice. The turningperformance of ships in ice has not beenwidely investigated. The tests describedare probably the most comprehensive fullscale investigations and model scalecomparisons of maneuvering performancein ice to date. All data and comparisonsof full scale and model scale resultsare presented.IntroductionBackgroundThe full scale and model icebreakertests described in this paper are partof the work resulting from a ··ProjectAgreement for Cooperation in the Fieldof Icebreaking Technology", which wassigned on July 23, 1981, between theUnited States Department of Transportationand the Ministry of Trade andIndustry of Finland. A working levelexchange of icebreaking technology,based on this agreement, is continuingbetween the United States Coast Guardand Finland's Board of Navigation. Thetests described were conducted jointlyby representatives of the United StatesCoas t Gua rd and Finland's Boa rd ofNavigation. Wartsila Arctic ResearchCentre cooperates with the Board ofNavigation, offering assistance andexpertise in questions related toicebreaking technology.ObjectivesThis is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22,1987. © The Geophysical Institute,University of Alaska, 1987.During the trials of a new icebreakerit is common practice todetermine the maneuvering performance inice by a turning test maneuver. Some ofthese data have been published. Figure 1presents the turning performance of 6different icebreakers (Edwards et a1.557

1976; Hellman and Schwarz 1983; Tue-Fee1985; Goodwin 1981; Vance et al. 1981).The dimensionless turning diameter isplotted versus ice thickness.The great variations in turningperformance can be explained by certaindifferences in rudder arrangement, hullform and some other parameters affectingmaneuverability in ice. The superiorityof twin screw - twin rudder and singlescrew - single rudder over twin screw -single rudder is obvious. It is morecomplicated to determine the influenceof hull form on turning characteristics.The effect of parameters such as iceproperties, ship speed, shaft power,rudder performance and hull lubri-cationhave to be taken into account.In the winter of 1984 a jointresearch program was begun to determinethe influence of these properties onthe maneuvering performance of a shipin ice. The "KATMAI BAY" Class 140 footicebreaker was chosen as the testvessel. Full scale turning tests onLake Michigan and model tests at ascale of 1:9.273 at Wartsila ArcticResearch Centre were conducted.Ship descriptionThe "KATMA! BAY" class of icebreakingtug (WTGB) has provedsuccessful on the Great Lakes. Duringthe winter months these cutters provideicebreaking assistance to navigation inareas that would otherwise be impassablemuch of the time. All Coast Guardcutters are designed with capabilitiesfor multi-mission operations. The KatmaiBay class are responsible for searchand rescue, towing and law enforcement,but are primarily designed to serve asicebreakers on the Great Lakes andAtlantic Coast. The cutters are builtwith an icebreaking hull, an air bubblersystem for hull lubrication and a lowfriction (Inerta 160) bottom coating.Principal characteristics are shown inTable 1. An abbreviated lines drawingis shown in Figure 2.Full Scale TestsPrevious full scale testsIcebreaking tests of the USCGCKATMAI BAY (WTGB-lOl) were conductedduring the winter of 1979 in WhitefishBay, Lake Superior. Tests were performedin level ice of various thicknesses andin brash ice. Bollard pull tests wereconducted both ahead and astern. Themaneuvering performance was tested bytwo turning circle tests, one to theleft and one to the right. These testswere reported by Goodwin (1981), Vanceet a1. (1981), Hunt et a1. (1979),Vance (1980).The main results of these tests andthe level ice breaking performance datacollected during the maneuvering testsR / L1210862R = Pierre RadissonF = FranklinFccC = Canmar KigoriakW Max Waldeck(WAAS)K Katmai BayD DiksonD221112R150Hlce [cm]screwscrewscrewscrewscrewscrew1 rudder1 rudder1 rudder1 rudder1 rudder2 rudderFigure 1. Turning performance of someicebreakers.558

12(0_1.1in Green Bay, Lake Michigan, duringFebruary 1984 are compared in Figure 3.The great difference in the results iscaused by the low flexural strength ofthe ice and the absence of snow duringthe tests in 1984. During the previoustests the flexural strength, obtainedfrom temperature-strength relationships,was found to be 600-700 kPa (87-109 psi)and the snow cover was 8-13 cm (3-5 in).II fOWl.'During the 1979 trials the operationof the air bubbling system led toa decrease of the required shaft horsepowerof approximately 25%. The 1984tests did not show this decrease,probably because the absence of snowprovided a natural low friction surface.The influence of air bubbling was onlyabout 5 - 10 %.The turning test results obtainedin 1979 correspond extremely well withthose of 1984. The points are plottedin Figure 5 for comparison.PlanningFigure 2. Lines drawings of the l40-footicebreaker (West 1975).Full scale tests for maneuveringof a "KATMAI BAY" class icebreaker wereplanned for the winter of 1983-84 onthe Great Lakes. Suitable level, fastice of uniform thickness was found inGreen Bay, Lake Michigan in late winter.The week of 13-17 February 1984 wasTable 1. Principal Characteristics of the 140 Foot WTGB.Length overallLength (DWL)Beam, maxBeam (DWL)Mean DraftMaximum DisplacementBow SlopeEntrance Half AngleFlare AngleHull Condition42.7 m (140 FT)39.6 m (130 FT)11.4 m (37 FT 6 IN)10.4 m (34 FT 2 IN)3.7 m (12 FT)660 tons (650 LongTons)28 degrees28.5 degrees48.6 degreesInerta 160, coated5 years earlierShaft PowerBollard PullPropeller DiameterPitch RatioArea Ratio(Ae/Ao)Rudder AreaGeometrical AspectRatioThickness RatioTaper RatioMaximum Open WaterSpeed1.864 kW (2,500 SHP)250 kN (56,000 lb)2.6 m (8 FT 6 IN)0.720.685.59 m2 (60.2 FT21.90.2317.6 m/s (14.7 knots)559

3000SHAFT POWER(kW]Table 2. Test Program.RUDDER ANGLE Cdeg)200010001984 (53///PROPELLERSPEED3010 20 30 Bubbler onSTBD PORfULL160 RPM 1 te.t 1 teat 2 test. 1 1 --200 RPM 2 teat. 1 teat 2 teats 1 1 --260 RPM 2 tests 2 te.t. 2 teat. -- -- I1984 (43 em)2 3 4VELOCITY (m/s]Figure 3. Comparison of icebreakingtests of 1984 and 1979.chosen for testing and the USCGC MOBILEBAY (WTGB-103) was designated as theprimary test ship, with USCGC BISCAYNEBAY (WTGB-104) assisting. The CoastGuard Aids to Navigation Team inEscanaba, Michigan provided criticalshore support. Commander, Ninth CoastGuard District and Coast Guard AirStations at Traverse City, Michigan andChicago, Illinois also assisted withplanning, reconnaissance and support.Test programAltogether 40 turning tests wereconducted in level ice; 37 resulted inuseful data. The rudder angle and shaftpower were varied. The effect of airbubbling was tested during 10 runs.Three different combinations of bubblerorifice sets were used. The average icethickness was 45 cm (18 in), but sometests could be repeated in ice ofsomewhat greater thickness. The testprogram is listed in Table 2. AppendixA is a tabulation of the turning testdata.In addition to the turning tests,66 ice breaking performance data pointswere collected. Appendix B tabulatesthese results.Instrumentation and measurementThe ship was instrumented byWartsila Arctic Research Centre. Thedata were recorded on a l4-channelanalog tape recorder and analysed witha Hewlett-Packard 9836 computer.The following were measured continuously:1. Speed, using doppler radar2. Propeller speed, using an opticalpulse tachometer3. Shaft torque, using strain gages4. Ship's heading, from the onboardgyro compass5. Rudder angle, using a potentiometerThe ship's path was photographedfrom the radar screen and the turningdiameter was measured with the radar.A LORAN C plotting system was used toback up the radar measurements. Theradar . photographs proved to providemore precise and accurate information.The drift angle was measuredduring the steady stage of the tum.The method is based on two sequentialphotographs, where the position andheading of the ship can be seen againstsome clearly visible mark or a crack inthe ice. The mean drift angle duringthe movement of about half a shiplength is obtained.560

Ice propertiesThe tests took place in Green Bay,Lake Michigan during February 14-16,1984. The air temperature ranged from-10 to +2 degrees C (+17 to +35 deg. F).However, the previous week had beenwarm and the snow layer on top of theice had melted.The ice thickness was ratheruniform compared to Arctic conditions,ranging from 37 cm to 55 cm (14.5 to21.6 in). Little variation in thicknesswas seen in any individual test area.Thickness measurements were taken fromthe side of the broken channel atintervals of some 50 meters.The strength properties of the icewere determined by two cantilever beamtests and uniaxial compression tests inboth horizontal and vertical directions.The ice can be characterized as relativelyweak spring lake ice. Thestrength data are listed in Table 3.Full Scale ResultsExample of recordingsThe minimum turning circle diameterof 105 m (ll5 yds) was obtained with30 degree rudder deflection and lowspeed. Appendix A tabulates the turningtest data measured during the steadyphase of the turn. The decrease in shipspeed and shaft speed is dependent onthe rudder setting and direction of theturn.Figure 4 is an example ofrecordings during a test run. The firstplot shows the rudder deflectionchanging from amidships to 30 degreesto the right, back to amidships, then30 degrees left and finally back toamidships. The ship's heading changesfrom zero to 180 degrees and back tozero. The angular velocity during thesteady phase of the turn is approximately1.2 degrees per second.The ship's speed drops from 2.5 m/s(4.9 knots) during the approach phaseto some 1. 5 m/ s (2.9 knots) during thesteady turn.speed and the increasing propeller-iceinteraction during the maneuver. In thestraight ahead run only light icecontacts could be observed on thepropeller.Table 3. Ice Strength.fLEXURAL STRENGHTBEAM ELASTIC fLEXURALNUMBER MODULUSGPo4.16STRENGHTkPo1 3.83 3533.532 4.26 2603.48COMPRESSIVE STRENGTHTEST SAMPLE COMPRESSIVEDATA TYPE STRENGHTMPoA 1.2514.4.84 B 1.98C 1.280 2.19A 0.9615.4.84 B 1.62C 0.890 1.6216.4.84 B 1.890 1.55SURfACEI~ GJI folBOTTOMAVERAGE COMPRESSIVE STRENGTHHORIZONTAL 1.10 MPo (160 psi)VERTICAL1.81 MPo (263 psi)The propeller speed experiences aslight decrease due to the slower ship561

Turning diameterIt is well known that a vessel witha right-hand single screw tends to turnbetter to port than to starboard. Thisis mainly caused by the influence of thepropeller. The increased wake in theupper part of the propeller disc leadsto a greater tangential flow componentin the upper part of the outflow jet,thus causing a greater lateral forceacting to the right on the ruddercontrol surface.The turning diameters are plottedin Figure 5 versus rudder angle. Thevessel turns approximately 20 percentbetter to port than to starboard. Withfull rudder angle the difference issmaller.The same data are presented inFigure 6 versus rudder lift force.Unfortunately this force was notmeasured, thus it had to be calculated.Difficulties arise in determining theflow velocity in the outflow jet of thepropeller. Secondly, the effectiveaspect ratio of the rudder, which liesonly partly in the outflow jet, had tobe estimated. The calculation procedurewas checked by performing model tests,where the rudder force was measured.Figure 6 shows that the turningdiameter is strongly influenced byspeed. If the rudder force could bekept constant while the ship speed isdoubled, the turning diameter wouldalso be doubled.Angular velocityJltSP . .. .. ... .." h)A linear dependency between turningradius and speed means that with aconstant rudder force the angularvelocity around the vertical axis isconstant, assuming that ice propertiesTurnIng Diameter em]600500¥43fibd. 1_ a.. a.. 4_, ,.1400300200100o o 10 20 30 40Rudder Angle [deg]Figure 4. Example of recordings duringa turning test.Figure 5. Turning diameter vs. rudderangle.Ice thickness = 38 to 54 cm(15 to 21 in)562

are constant. Figure 7 shows the datapoints obtained in ice of 45 cm (18 in)thickness. The angular velocity versusrudder force is linear, wi th a smallscatter.The relation of angular velocityand rudder force is plotted in Figure 8versus ice thickness. Unfortunately,tests in thin ice of 25 to 30 cm (10 to12 in) thickness could not be run.However, the available data give reasonto believe, and the model tests whichare discussed later, show that therudder force needed to produce a certainrate of turn is a function of the icethickness squared.Figure 9 presents the data pointsin dimensionless form as angularvelocity in ice versus rudder force.The non-dimensionalizing is done asfollows:H Hice ice - w 21Tw ' 0.5. --i L (gL) 3602.5w [deg/s]321.5 **.5o o 20 40 60 80 100*Lift [kN]Figure 7. Angular velocity vs. rudderlift force. Ice thickness = 45 cm.Lift' Lift/ (g .p V)TurnIng Diameter em]6005505004504003503002502001501005000 20 40 60 80 100 120Rudder L 1ft [kN]Figure 6. Turning diameter vs. rudderlift force.Lift100908070605040302010o/ w [kN / deg/s]*/.o 10 20 30 40 50 60Hi ee [em]Figure 8. Rudder lift over angularvelocity vs. ice thickness.563

wherewwi'LiftLift'Lg.PVHiceangular velocitydimensionless angular velocityrudder lift forcedimensionless rudder lift forcelength (OWL)acceleration of gravity= water densityvolumetric displacement= ice thicknessThe operation of the bubblersystem has two effects on the turningbehavior of the ship. The increase inspeed, caused by the reduced friction,has a stabilizing effect, as shownearlier. On the other hand, when theair is blown out on only one side ofthe hull, a force acting in the oppositedirection is experienced. The magnitudeof this force is dependent on the airquantity and the flare angle.The dimensionless form is basedonly on the turning test of one hullform and is to be used only whencomparing geometrically similar shipsor for model scale to full scalecorrelation. Attempts to comparedifferent ship types by using thisdimensionless relationship must be madewith caution.Effect of air bubblingThe "KATMAI BAY" class icebreakersare equipped with an air bubblingsystem to reduce the friction betweenice and hull. The air can be directedto any combination of four sets oforifices.Figure 10 presents the test resultswith and without air bubbling. Thepoints are within the scatter, but aslight influence of the direction of airto various orifice sets can be detected.Drift angleIn ice-free waters, during thesteady phase of a turn, a constantdrift angle is established. Accordingto the linear theory of motion, thedrift angle is directly proportional torudder deflection (Mandel 1967). Whenmaneuvering in ice, a constant drift isseldom reached. The stern quarter keepsbouncing against the side of thechannel, occasionally breaking a cusp.The drift angle is changing constantly.UJ i '121086[*10-"(-6)]*lJJi '121086[*10-"(-6)]0 Al r bubbl1ng* No bubb 1 i ng8ubb 1 i ngouter curve*** * **4*422.005 .01 .015 .02LIFT'Figure 9. Dimensionless angularvelocity in ice vs. rudder lift force..005 .01 .015 .02LIFT'Figure 10. The effect of air bubbling.564

The photographic method, which wasused to determine the drift angle, givesa mean drift angle during a movement ofabout half of a ship length. Figure 11presents the drift angle versus rudderlift. The maximum value measured was9 degrees. The location of the pivotpoint is shown in Figure 12.DrIft Angle [degJ15105-5-10,-nrnrnrrMrMrMrnrnrMrMrM~o Left* RIghto-15-120 -80 -40 o 40 80 120Lift [kNJFigure 11. Drift angle vs. rudder force.E...~o "Figure 12. Location of the pivot point.*Model Scale TestsPrevious model scale testsIce-free and icebreaking modeltests were conducted for the "KATMAIBAY" class during design (Lecourt 1975;West 1975; Remmers et al. 1975). Morerecently, Tatinclaux tested two modelsof this class at different scales toidentify scale effects in ice breakingmodel testing (Tatinclaux et al. 1986;Tatinclaux 1984).PlanningModel turning tests in ice with a1:9.273 model of the 140 foot icebreakerwere performed at Wartsila ArcticResearch Centre. The objectives of thetests were to establish the correlationmethod of the model and ship turningperformance in ice, and to complete theparameter study which was started infull scale.The mainWartsila Arcticbasin are:lengthtesting lengthwidthwater depthparticulars ofResearch Centre77.3 m60.0 m6.5 m2.3 mthetestModel tests can be observed fromunderwater windows, 10 on the bottomand 16 on the sides. The carriage speedcan be varied between 0 and 3 mls(9.8 ft/s). A 60 mm (2.4 in) thick icesheet can be produced overnight fortesting.The model ice used is a fine grainsaline ice generated by automatedspraying. It is homogeneous and brittleand cracks without any remnant forces.Fracture occurs at small deflectionswith a ratio of elastic modulus toflexural strength in the range of 1000to 2000. The model ice compressivestrength is strain-rate dependent. Moredetails on the material were presentedby Enkvist and Makinen (1984).The model turning tests wereconducted with a self-propelled model.Straight ahead towing tests were565

conducted to verify the full scale tomodel scale correlation of iceresistance. The model test program inice consisted of 13 turning tests, 21straight ahead towing tests and 3propulsion tests. All tests, except 5towing tests, were performed with themodel lacquered to a low friction. Theinfluence of friction on ice resistancewas tested by towing tests with themodel treated to a higher friction.Instrumentation and measurementThe following parameters from themodel were measured and recorded onmagnetic tape; model speed, shaftspeed, shaft torque, shaft thrust,rudder angle.The following ice characteristicswere measured; ice thickness, flexuralstrength, model track.The run itself was conducted inthe same manner as a zig-zag maneuver,but the steady phase was kept constantas long as the width of the basinallowed. The steady phase was analysedas if it would be just a part of aturning circle. Figure 13 shows theturning path and the rudder settingduring a test. Appendix C is a table ofthe model scale data.Model scale resultsFigure 14 shows the turning testresults as a function of rudder angle.The scatter is caused by the variationsof the ice parameters and shaft speed.A considerably better correlation isachieved when the angular velocity isplotted versus rudder force (Figure 15).The strong influence of ice thicknessis clearly visible. Figure 16 presentsthe results in dimensionless form. Thenon-dimensionalizing method has alreadybeen presented in connection with fullscale test results.The drift angle of the model duringthe steady phase of the maneuver wasmeasured by mounting a camera in themodel and taking photographs of theheat exchangers on the ceiling of thelaboratory for reference. Figure 17shows the drift angle versus ship's55 5550.. ~ ..5045 4540..4035 3530 3025 2520 2015 1510 105 5Figure 13. Model path and ruddersetting during a turning test.TurnIng Diameter [m]605040302010o Left* Right40Rudder Angle [deg]Figure 14. Turning diameter vs. rudderangle, model tests.566

length divided by turning radiusmeasured during the model tests andduring the full scale tests as acomparison.ConclusionsComparison of full and model scaleresultsThe comparison of pure turningdiameters in full and model scale is inmany cases somewhat misleading. All theship and ice parameters have to bemodeled exactly. The minimum turningcircle obtained from model tests liesbetween 111 and 139 meters (101-127yards). The corresponding values,measured in full scale, lie between 105and 142 meters (96 - 130 yards). Withlower rudder angles the scatter of bothfull scale and model scale results wasgreater and the model prediction seemsto be too optimistic.The dimensionless form of angularvelocity in ice versus rudder forcepresents an opportunity to include theinfluence of differing ice thicknesses,shaft speeds etc. in the prediction.wi1210B64*2 * *o o.005*.01 .015 .02LIFT'Figure 16. Model test results indimensionless form. Angular velocity inice vs. rudder lift force.105w Cdeg/sJHieeo30 emDrlft Angle CdegJ1510* Ship0 Model 05 ~a~~ *~*0* * * *-5 I f# 0-108\ff*o50 75 100LIFT CNJ-15 -1 -.5 0 .5L/RFigure 15. Angular velocity of themodel vs. rudder lift force.Figure 17. Drift angle measurements infull and model scale.567

Figure 18 presentsresults together withtest results. Theexcellent.the model testthe full scalecorrelation isThe ice resistance data of the fulland model scale tests are presented inFigure 19. The prediction is based onthe assumption of a dynamic frictioncoefficient of 0.10 between ice andhull.In thinner ice the model test datafall right in the middle of the fullscale points. In thicker ice, however,the model prediction is slightly toosteep, but on average this predictionis also quite satisfactory.WI1210B642[*10"'-(-6)J* Full Scale Tests'If/ r/I* PredictIonIEvaluation of test proceduresSurprisingly good full scale tomodel scale correlation was achievedwith the model test procedures used.These results served to strengthenconfidence in the fine grain model ice.Modeling of ice breaking performance hasalready been verified with about a dozenship types, for which comparisons of,both full scale and model scale testsin ice have been conducted.In turning model tests one of themost important characteristics of themodel ice is the size of the floesbroken by the model. The high elasticityover flexural strength ratio of the finegrain ice secures an adequately smallfloe size.The greatest difficulty in thetests was to produce an ice field thatwas uniform in thickness not only nearthe centerline of the basin, but alsoall the way to the sidewalls. The icefield showed a tendency to be thinnernear the sides. Some tests failedbecause the model ran too far from thecenterline. This problem could bereduced by adjusting the sprayingnozzles which are used to form the icelayer.The greatest inaccuracies in theanalysis of the full scale data areprobably made in estimating the rudderlift force. The rudder lies partly inthe propeller outflow jet, where theflow velocity must be calculated. Theo ~~~~~~~~~~LL~~~ .005 .01 .015 .02LIFT'Figure 18. Comparison of model scaleand full scale turning test results.Ice Resistance [kNJ2502001501005000 F.sc. test . Hice -* F.sc. test Hlce -Prediction50 cm'10 cm*Prediction0 .5 1.5 2 2.5 3 3.5 4Velocity [m/sJFigure 19. Comparison of model scaleand full scale ice resistancemeasurements.568

gap between hull and rudder, which isdependent on the rudder angle, has agreat influence on the effective aspectratio. Therefore it is recommended totry to measure the rudder force notonly in model testing but also duringfull scale maneuvering tests.A shortcoming in the full scaleprogram of measurements is the verysmall number of beam tests for icestrength properties. Problems withscheduling adequate time for these testsand mechanical problems with chain sawslimited the data taking opportunities.For any future work in fresh water icemuch more emphasis must be placed onmaking a large number of beam tests.Both equipment planning and testscheduling must be considered carefully.ReferencesR.Y. Edwards Jr, R.A. Major, J.K. Kim,J.G. German, J.W. Lewis, and D.R.Miller, "Influence of MajorCharacteristics of Icebreaker Hulls onTheir Powering Requirements andManeuverability in Ice", Trans., SNAME,1976, 1-36.E. Enkvist and S. Makinen, "Experiencewith a New Type of Model Ice", ThirdInternational Conference on IcebreakIIigand Related Technologies (Ice Tech'84), SNAME, 1984, 15 pp.M. J. Goodwin, "Icebreaking and OpenWater Tests Performed on USCGC KatmaiBay", USCGRDC Report D-24-81, AD-A099724, 1981, 36 pp.J-H. Hellman and J. Schwarz,"Performance tests of Icebreaker MaxWaldeck in Model and Full Scale", TheSeventh International Conference OnPort and Ocean Engineering Under Arc ticConditions (POAC '83), 1983, 534-549.P. Mandel, "Ship Maneuvering andControl", in Principals of NavalArchitecture, edited by J. P. Comstock,New York, SNAME, 1967, 482-486.K. E. Remmers and R. Heckler, "PoweringPredictions for the United States CoastGuard 140-foot WYTM (Model 5336Propeller 4657)", DTNSRDC ReportSPD-223-18, 1975, 11 pp.J. C. Tatinclaux, "Model Tests on TwoModels of the WTGB 140-footIcebreaker", USACRREL Report 84-3,1984, 25 pp.J. C. Tatinclaux and D. H. Humphreys,"Ice Resistance Tests of Two Models ofthe WTGB Icebreaker", Proceedings ofthe Twentieth General Meeting of theAmerican Towing Tank Conference, 1986,12 pp.K. Tue-Fee, "Full Scale ManeuveringTests in Level Ice of CANMAR KIGORIAKand ROBERT LEMUR", SNAME, 1985, preprint15 pp.G. P. Vance, "Analysis of thePerformance of a 140-foot Greats LakesIcebreaker: USCGC KATMAI BAY", USACRRELReport 80-8, 1980, 28 pp.G. P. Vance, M. J. Goodwin and A. S.Gracewski, "Full Scale Icebreaking Testof the Katmai Bay", Proceedings of theSixth Ship Technology and Research(STAR) Symposium, (Ice Tech '81),SNAME, 1981, 323-345.E. E. West, "Powering Predictions forthe U. S. Coas t Gua rd l40-ft WYTMRepresented by Model 5336" , DTNSRDCReport SPD-23-16, 1975, 15 pp.R. R. Hunt, and L. L. Hundley, "KATMAIBAY (WTGB-104) Speed, Tactical andManeuvering Trials" , DTNSRDC Report79/106, AD-A 07587, 1979, 27 pp.E. J. Lecourt, "Icebreaking Model Testsof the 140-foot WYTM" , Arcted, Inc.Report 202C-2, 1975, 47 pp.569

APPENDIX A 1 (1)USCGC NOBILE BAY OfTGB-I03)Green Bay, February 1984Turning Test DataR - Right Rudder B - Air Bubbler On S - Starboard only1 - 1eft Rudder P - Port onlyTest Rice Prop. Rudder Turn Speed Angular Pshaftspeed angle diam. velocity(em) (rpm) (deg) (m) (m/s) (deg/s) (kw)lR 41 188 32 133 1.30 -1.25 10721L 44 198 31 133 1.40 1.30 10492R 41 200 10 384 1. 90 -0.56 10622Ltest interrupted, overheating3R 38 180 21 206 1.80 -1.15 103831 43 201 22 183 1.50 0.94 10144R 43 237 30 142 2.50 -2.20 195841 45 243 29 142 2.60 1.93 19285R 42 244 20 219 2.81 -1.06 202551 42 255 20 183 3.12 1.88 19886R 44 251 10 475 3.34 -0.88 197161 44 255 11 379 3.23 1.00 19657R 45 158 28 151 0.73 -0.61 5377L 45 160 28 105 0.67 0.80 4728R 45 167 20 201 1.14 -0.69 61481 40 169 21 165 1.04 0.78 5809R 41 162 11 384 1.09 -0.39 51191 test interrupted, rafted ice10RB 40 234 30 137 1. 90 -1.90 1990101B 40 250 30 137 2.62 2.17 1974IlL 53 246 29 123 1.69 1.13 1923llR 51 236 29 164 1.56 -1.06 1921121 49 247 10 357 2.34 0.75 194212R 54 241 11 475 2.12 -0.57 1948131 48 198 28 128 0.87 0.84 104813R 48 192 30 133 0.85 -0.82 1075141 45 207 11 324 1.63 0.65 108114R 47 204 10 398 1.64 -0.58 1115151 45 247 21 178 2.23 1.54 197415R 45 241 20 206 2.08 -1.59 1972161BS 43 204 30 123 1.40 1.47 102216RBS 42 193 30 133 1.08 -1.20 109917LBP 43 201 30 133 1.17 1.24 107517RBP 42 197 29 133 1.18 -1.55 109618RBP 46 168 30 123 0.82 -0.86 630181BP 46 170 30 123 0.73 0.67 64419RBS 44 166 30 123 0.77 -0.52 629191BS 46 168 30 114 0.82 0.65 58820R 47 162 29 142 0.51 -0.55 581201 test interrupted, rafted ice570

APPENDIX B 1 (2)USCGC MOBILE BAY (WTGB-103)Green Bay, February 1984Performance Test DataLevel ice, no snow, Flexural strength = 300 kPaTest Hice Speed Ice Prop. PshaftPoint Resistance speed(cm) (m/s) (kN) (rpm) (kW)1.1 41 0.00 89 144 3621.2 41 1.01 98 165 5061.3 41 1. 78 100 185 7561.4 41 2.28 114 201 9842.1 42 0.99 77 149 3812.2 42 1.18 87 160 5802.3 42 1.88 101 185 7352.4 42 2.17 120 203 10123.1 40 0.00 93 150 3943.2 40 0.82 83 151 3693.3 40 0.96 96 163 4703.4 40 1.46 112 183 68,23.5 40 2.27 117 203 9584.1 40 0.00 93 150 4094.2 40 0.84 85 153 3914.3 40 1.25 85 168 5414.4 40 1. 76 105 185 7064.5 40 2.15 117 200 9424.6 40 2.89 128 221 12634.7 40 3.29 146 240 16534.8 40 3.64 153 252 19035.1 37 0.19 78 138 3035.2 37 1.05 91 161 4865.3 37 1. 78 108 186 7225.4 37 2.04 122 201 8995.5 37 2.75 135 223 12705.6 37 3.32 171 256 19656.1 42 0.00 93 150 3936.2 42 0.91 81 151 3866.3 42 1.28 93 166 5046.4 42 1.60 106 182 6596.5 42 2.19 118 202 9396.6 42 2.77 158 236 14926.7 42 3.51 160 253 1929571

APPENDIX B 2Test Hice Speed Ice Prop. PshaftPoint Resistance speed(cm) (m/s) (kN) (rpm) (kW)7.1 44 0.45 86 149 35311.1 51 0.00 124 171 60011.2 51 0.69 120 175 64511.3 51 1.52 136 202 99311.4 51 1.77 158 221 132911.5 51 2.17 173 235 170711.6 51 0.92 124 183 72112.1 53 0.62 107 167 53312.2 53 0.97 123 184 74212.3 53 1.37 145 206 102112.4 53 1.87 165 225 139212.5 53 2.55 181 247 191113.1 55 0.00 137 179 69313.2 55 0.53 133 184 71613.3 55 0.79 147 198 100014.1 46 0.43 80 llf3 33014.2 46 1.02 101 167 53114.3 46 1.29 123 189 78614.4 46 1.92 133 207 103018B.1 35 0.63 42 102 12918B.2 35 0.57 57 123 22618B.3 35 0.64 65 132 26418B.4 35 0.68 75 142 32518B.5 47 0.96 98 165 48118B.6 47 1.53 110 185 68918B.7 47 1.90 126 203 95619B.1 37 0.57 57 123 22819B.2 37 0.52 69 134 29219B.3 37 0.65 77 144 34619B.4 47 0.86 99 163 48519B.5 47 1.32 116 185 71419B.6 47 1.67 128 203 985572

APPENDIX C 1 (2)Model Test Data - Turning TestsR - Right Rudder1 - 1eft RudderTest Hice Prop. Rudder Turn Speed Angularspeed angle diam. velocity(mm) (rpm) (deg) (m) (m/s) (deg/s)1R 45 450 20 21 0.26 1.4321 48 450 20 22 0.19 1.0031 41 450 30 12 0.24 2.234R 47 450 30 13 0.20 1. 755R 50 550 10 29 0.32 1.2861 43 550 10 38 0.43 1.307R 53 600 10 36 0.46 1.468R 50 600 30 15 0.37 2.8491 47 600 30 14 0.54 4.28101 48 600 30 15 0.60 4.70111 27 400 10 34 1.00 3.33121 28 400 30 12 0.80 7.69131 30 400 20 18 0.90 5.69573

APPENDIX C 2Towed Model Tests in Ice - September 1984Raw DataReduced DataTest V Hice Flex. Rice Rs+v Rb Hice Flex. Rice Rs+v RbStrg.Strg.(m/s) (mm) (kPa) (N) (N) (N) (mm) (kPa) (N) (N) (N)Friction Coefficient = 0.0351 0.3 33.4 29.2 40.0 20.8 19.2 32.4 32.4 39.9 19.5 20.32 1.0 36.3 29.2 102.7 80.9 21.9 32.4 32.4 84.5 64.2 20.33 0.1 33.7 29.2 26.6 7.1 19.5 32.4 32.4 26.5 6.1 20.34 0.1 34.3 0.0 7.5 7.5 0.0 32.4 32.4 6.3 6.3 0.05 0.3 41.4 31.6 67.4 40.5 26.9 43.1 32.4 73.0 43.7 29.36 1.0 43.1 31.6 165.4 136.8 28.6 43.1 32.4 166.3 137.0 29.37 0.1 42.2 31.6 40.5 17.4 23.2 43.1 32.4 42.9 18.4 24.58 0.1 42.1 0.0 17.2 17.2 0.0 43.1 32.4 8.5 18.5 0.09 0.1 43.2 56.4 86.1 18.6 67.5 43.1 32.4 57.2 18.5 38.610 0.6 45.1 56.4 136.9 82.2 54.6 43.1 32.4 105.8 76.5 29.311 0.6 44.9 56.4 137.2 82.9 54.3 43.1 32.4 107.0 77.7 29.312 0.1 43.8 56.4 63.4 19.4 44.0 43.1 32.4 43.3 18.6 24.713 0.3 52.5 31. 9 91.3 49.0 42.3 53.9 32.4 96.2 51.5 44.714 1.0 51.9 31. 9 207.1 165.5 41.6 53.9 32.4 223.3 178.6 44.715 0.1 52.3 31. 9 68.1 26.0 42.1 53.9 32.4 73.0 28.3 44.716 0.1 53.0 0.0 27.1 27.1 0.0 53.9 32.4 28.5 28.5 0.0Friction Coefficient = 0.1717 0.3 40.5 0.0 82.6 82.6 0.0 43.1 32.4 90.8 90.8 0.018 0.3 41.7 29.7 119.4 86.3 33.1 43.1 32.4 129.1 90.8 38.319 1.0 42.4 29.7 236.8 234.7 2.1 43.1 32.4 243.6 242.2 2.420 0.1 41.0 29.7 101.9 61.2 40.7 43.1 32.4 114.4 65.9 48.521 0.1 40.5 0.0 60.1 60.1 0.0 43.1 32.4 66.1 66.1 0.0Raw Data from Model Propulsion Tests in Ice and Ice Free Water (OW)Ice OW Ice Olv Ice OWVelocity (m/s) 0.100 0.100 0.300 0.300 1.000 1.000Force (N) 139.756 94.227 143.892 64.629 201. 559 -1. 676PropellerSpeed (rpm) 450.000 450.000 520.000 520.000 760.000 760.000Thrust (N) 96.904 95.761 125.454 125.154Torque (Nm) 3.109 3.044 4.086 3.985192.7549.747225.1347.399Thickness ofIce (mm) 45.300 0.000 47.300 0.000 48.000 0.000574

MID-WINTER 1983 SHIP TRANSIT IN THE ALASKAN ARCTICBY THE ICEBREAKER POLAR SEAFred SeiboldMaritime Administration, Washington, D.C., USARichard VoelkerNKF Engineering, Inc., Columbia, Maryland, USAAbstractThis paper presents results from thehistoric 1983 mid-winter transit of theU.S. Coast Guard icebreaker POLAR SEA inthe Bering and Chukchi Seas. Operationaland environmental data were collected toprovide a basis for assessing the feasibilityof future, year-round commercialmarine transportation. Data are presentedin pictorial form to show geographicalregions of continuous and ramming icebreakingas well as ship speed and horsepowerrelationships. A description ofother factors which affected the 10-1/2day transit is given. These include propellerthrust imbalances, voyage delaysfor machinery repairs, and stoppingbecause of poor visibility. The resultssuggest that POLAR Class icebreakers andfuture commercial ships will be able tooperate year-round in the Bering Sea. ThePOLAR Class icebreakers are capable ofmid-winter excursions into the Chukchi Seaand, with refueling, into the Beaufort Seaas well. It is expected that commercialicebreaking ships, with improved ice navigationand ice piloting capabilities, mayalso be able to operate year-round in theChukchi Sea.This is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987.IntroductionIn 1979, the MarItIme Administration,in cooperation with the U.S. Coast Guard,began a program designed to assess thefeasibility of year-round marine transportationin the Alaskan Arctic. Thisprogram had three overall objectives:1. to define the environmental conditionsin the Bering, Chukchi,and Beaufort Seas;2. to obtain data to improve designcriteria for ice transiting shipsand offshore structures; and3. to perform an operational assessmentof commercial icebreakingships along possible futuremarine routes.As part of this prog.ram, the USCGC POLARSEA made a mid-winter transit in 1983 fromthe Bering Sea ice edge to Wainwright inthe north Chukchi Sea. This paperdescribes the environmental and operationalperformance of the POLAR SEA duringthis historic voyage.Description of the POLAR Class IcebreakersThe POLAR SEA, along with her sistership the POLAR STAR, is the world's mostpowerful nonnuclear icebreaker. Ofspecial note IS the hull shape of this575

ship which has been designed for maximumicebreaking efficiency. The POLAR SEA isdesigned to ram through ice up to 21 feetthick and to operate continuously through6 feet of level ice at a speed of 3 knots.A powerful propulsion plant has beeninstalled in the ship to provide thiscapability. POLAR SEA's three shafts canbe turned by either a diesel-electric orgas turbine power plant. The dieselelectricplant can deliver a total of18,000 shaft horsepower (SHP). A separategas turbine plant can deliver a total of60,000 SHP. Each shaft is connected to afour-bladed, l6-foot diameter, controllablepitch propeller. The principal featuresof a POLAR Class icebreaker arelisted in Figure 1.Since hull strength is very importantto an icebreaker, the POLAR SEA has beendesigned to absorb high powered rams intothe ice, a feature necessary for higharctic operation. The shell plating andassociated internal support structure arefabricated from steel with an exceptionallygood low temperature strength. Theicebelt is 1-3/4 inches thick in the bowand stern sections, and 1-1/4 inches thickamidships. The POLAR SEA is also equippedwith a heeling system to assist in moresevere ice conditions.Data Collection ActivitiesProject objectives required thatenvironmental and ship performance data becontinuously collected during the midwintervoyage from the ice edge to Wainwright.These data were subsequentlytabulated in half-hour summaries to allowonboard reduction and analysis. By thetime the POLAR SEA arrived in Wainright, aprel iminary, but comprehens i ve, voyageanalysis was completed.As shown in the block diagram ofFigure 2, the primary data recording systemconsisted of a Hewlett-Packard 984STdesktop computer wi th peri phera Is. Theship was instrumented to provide for thecontinuous monitoring and automaticrecording of: ship motion, ship speed,and propeller thrust, torque, rpm, andblade pitch. Concurrently, two pilothouseobservers were manually recording ice andother environmental conditions on datasheet s; a sample is shown in Figure 3.Ice conditions, which were logged at halfhourintervals, included the number andsize of pressure ridges, the level icethickness (average and maximum), the iceconcentration and floe size, the observedice pressure, and the size of rubble icefloes. This data was then telephoned tothe computer operator who keyed it intothe computer to generate the half-hourdata summaries. To augment these datacollection activities, voice annotationwas made on a Honeywell 101 magnetic taperecorder and a time-lapse video recordingwas made of ice conditions in front of theship using a camera in the conning stationaloft. A time code, synchronizing thecomputer and video system, was alsoemployed. The voyage north commenced atDutch Harbor on March 23 and the 0800daily ship position is shown in Figure 4.Ice Transiting PerformanceTwo modes of icebreak-ing, continuousand ramming, are used to define the icetransiting ability of the POLAR SEA. A"continuous mode" is used to describe thecondition where continuous forwardprogress is made, while "ramming" refersto a situation where the ship comes to astop because of difficult ice conditions,backs up, and then rams the ice floe againin an attempt to break through. The frequencywith which a ship needs to back andram is an indication of the severity ofthe ice conditions and the difficulty theship experiences in traversing those conditions.The location of ramming operationsduring the 1983 voyage of the POLARSEA is shown on the route map presented inFigure S. In most areas where ramming wasrequired, it was a result of encounteringlarge expanses of rubble ice rather thanone or more large, discrete pressureridges. Furthermore, when evaluating theperformance of the POLAR SEA along theroute, it is important to understand thatthe ship has a number of engine configurations.When operating entirely ondiesels, 6,000 SHP can be developed withthe maximum two engines per shaft. Atypical operating condition can involvethe substitution of a single gas turbinefor the diesels on any shaft, thusincreasing the available shaft horsepowerfrom 6,000 to 20,000. Therefore, a notationof 6,000/20,000/6,000 in the followingnarrative identifies the specificoperating condition to be two diesels onboth the starboard and port shafts, and agas turbine on the centerline shaft.Engine configu~a~ion has to be considered576

SHIP STATISTICS:LENGTH OVERALL ........................................ 399 FEETLENGTH AT WATERLINE ................................ 352 FEETBEAM ...................................................... 83.5 FEETDRAFT ._ ................................................. 31.8 FEETDISPLACEMENT, CAPACITY LOAD ..................... 13,190 TONSRANGE (MAXiMUM). ....... '" ...................... 28,274 MILESSUSTAINED SPEED (OPEN WATER) .................... 17 KNOTSPROPULSION ...................................... DIESEL ELECTRIC ORGAS TURBINESHP ..... ............... . ..... .... . ......... •. .•....•.. 18.000 D-E80,000 G-TNUMBER OF SCREWS ....................................... 3COMPLEMENT .......................................•..... 14 OFFICERS126 ENLISTEDSCIENTIST ACCOMMODATION ......................... 20HEIGHT OF BRIDGE ABOVE ................................ 55 FEETWATERLINEHEIGHT OF ALOFT CONN ABOVE ...................... 104 FEETWATERLINEFigure 1.Principal features of a POLAR Class icebreaker.in evaluating a POLAR Class lcebreaker'sperformance along any route.Voyage narrativeThe POLAR SEA met the ice edge justnorth of the Pribilof Islands. Fromthere, to a position west of St. MatthewIsland, the ice was generally less than 1foot thick and the ship maneuvered throughbroken ice, leads, and the "shadow" zoneof St. Matthew Island. On this leg, theship was able to move ahead at 12 to 16knots using four main propulsion diesels.At this point in the narrative, it isimportant to mention that the POLAR Classicebreakers have two modes of pitch controlfor the controllable pitch propellers.Selection of the "free-route mode"permits a large blade angle and highpower; this resulted In a 12-knot or577

eHF FIXED ACCELERATIONS (3)SHIP SPEED (1)ROll ANGLE (1)PfTCH ANGLE (1)YAW ANGLE (1)PfTCH RATE (1)PfTCH ACCELERATION (1)THRUST (3)TORQUE (3)PITCH (3)RPM (3)OBSERVEDANOMEA8UR£DICECONDITIONSalGNAlCONDlTiot-IlNGa AIoFL.FER8AID CONVERTERFLEXIBLEDISCDRIVESTAPECARTRDGEDRIVESTAPESEARCHTIoEIDATEGENERATORANALOGTAPERECORDERFigure 2.Block diagram of onboard data acquisition, recording, and reduction system.higher speed through llght ice. On theother hand, when using the "icebreakingmode" the propeller blade anp.,J e waslimi ted in order to prevent damage whenmilling ice. In this mode, the ship wasonly able to make 8 knots in similar iceconditions.Once north of St. Matthew Island, iceconditions became more severe. The icefield was composed of rafted and rubbleice and most of the small level ice floeswere rafted several times. Ice thicknessmeasurements averaged 3 feet of solid icewith broken ice piled beneath that.Sixty-eight rams were needed to transitthis area. Initially, a horsepower distributionof 3,000/6,000/3,000 (D/DD/D)578

DATETIME (HALF HOURLY)SHIP !"081T1ON:latitudeLangItUdeWIND ~ECTION (o.or-. True)WIND IPEED (Knot.)Yl8I8ILITY (Neutlcal .... .,PRECPlTATION"'" TEWERATURE (Dry BuI/-"F)ICE CONCENTRATION (TenIha)MAX LEVEL ICE THlCKNE88 (Ft)ItIG LEVEL ICE THlCKNE88 (Ft)flOE ..zE (A-.ga)aNOW COYER (InaNe)ICE P'PE88URE (None.--...... )ICEBREAKINO MODE (ec. ... ~PITCHRPMNO. of TalES RAMMING (~ Stopped)~NUMBER of M>OEI wtltI:D-aFt"'~.-. Ft ... HaIIht> • Ft ... tWght (8peoIfy)PRE~ ROGE 8AIL HEIGHT (Mall ft)f'RE88URE M>OE SAIL HEIGHT CJwv ft)TOTAL NUroeER of ROGES...aLE ICE flOE TRAN8ITED:DlatMCe T.-.....cI ttwOUGh floe~ ... HeIght eft)COMMENTS:Figure 3.Ice and environmental data sheet.was used. This was later increased to6,000/20,000/6,000 (OOtCT/OO) as the needfor a gas turbine became evident to drivethe ship through pressured rubble icefields.The POLAR SEA subsequently foundleads that led to the lighter ice conditionsin the "shadow" zone south of St.Lawrence Island. This shadow zoneextended south for a distance of about 60579

I Sl i ~0~ 0.,ff172170 168 166 16. 0 162Figure 4. USCGCPOLAR SEA deployment.mi les. Once in the shadow zone, the shi pwas able to run at 17 knots in the lighterice conditions, frequently with the propellerson free-route pitch control mode.Passing to the west of St. LawrenceIsland, difficult ice conditions were onceagain encountered. During the next 2days, the ship backed and rammed 161 times580

Figure 5. POLAR SEA route map showing areas ofcontinuous and ramming mode icebreaking.in order to get through the compactedrubble ice fields which were under pressure.The nature of the ice was such thatdistinct ridges were few and level icefloes were less than 500 feet in diameter.Many floes had freeboards of 1 to 2 feet,indicating a considerable ice thickness.Initially, the ship used 6,000/20,000/6,000 SHP (DD/GT/DD), but later the horsepowerwas increased to 20,000/20,000/6,000(GT/GT/DD). This apparent power imbalanceon the propeller shafts was not by choice.581

The starboard gas turbine was unavailablebecause of damage to the coupling mechanismwhich engages the gas turbine reductiongear system with the propeller shaft.After ramming through this heavy ice, thePOLAR SEA reached a region of light icesouth of the Bering Strait and moved northat about 10 knots using 8,000 to 20,000SHP. Ice conditions were light due westof the Seward Peninsula and the ship wasable to move at a speed of 9 knots withonly 8,000 SHP. This marked the fourthsuccessful winter passage of a POLAR Classicebreaker through the Bering Strait.Following the transit through the Strait,rubble ice fragments were encountered.Available horsepower was increased to6,000/20,000/6,000 as a precaution.Nevertheless, the ship maintained a speedof 12 knots using only 15,000 SHP.Ice conditions in the south ChukchiSea were the most severe of any ice conditionsobserved during the 1983 deployment.Hi le after mile of rafted ice and rubbleice fields were experienced, some of whichwere under pressure. Pressured ice conditionswere so severe that the bow printleft by the icebreaker as she backed for aram, collapsed within 1 minute. Ineffect, before the ship was ready tocharge ahead, the bow print from the priorram was greatly reduced in size. Duringother time periods, the formation ofridges and rafted floes was readilyobserved. In order to get through thi sheavily pressured rubble ice field in thesouth Chukchi Sea, 234 rams were required.Initially, a 20,000/20,000/6,000 (GT/GT/DD) configuration was used, but later allturbines were put on the line so that60,000 SHP would be available. In orderto do this, the starboard gas turbineclutching mechanism had to be hydraulicallyjacked into position. With all threegas turbines on the line, the ship madeexcellent northward progress to the southside of Lisburne Peninsula.Once in the shadow zone of the peninsula,the ship proceeded around PointHope, skirting heavy rubble ice floes.These floes, however, blocked an easypassage from Point Hope to Cape Lisburne.Ice conditions were again best characterizedas mile after mile of heavy rubble,with periodic zones of pressured ice. ThePOLAR SEA began traversing this regionwith a machinery configuration of 6,000/20,000/6,000 SHP (DD/GT/DD) while making aspeed of 7.2 knots. Eventually, all threegas turbines were required as the lceconditions worsened. Throughout thisregion, a total of 50 rams were required.After Cape Lisburne, the ship made aspeed of 12 to 14 knots using 32,000 SHP.The final run to Wainwright was made in a2-foot thick, level ice, flaw lead at aspeed of 6 to 9 knots using about 12,000SHP. However, 32,000 SHP was kept on lineand used during occasional encounters withsmall rubble ice floes. At Wainwright theship encountered blizzard conditions thatincluded blowing snow, a wind chi 11 of -60 0 F, and fog. These conditions persistedfor several days. On one occasion theship shut down its propulsion plant anddrifted southwest at an average speed of1.8 knots for 18 hours.A few observations on pressured iceconditions during the transit north shouldbe noted. Pressured ice conditionsusually occur when wind or current drivenice is restricted from free movement;i.e., the ice is constrained. The pressurewithin the constrained ice fields isthought to ease (or relax) when the windvelocity decreases or changes direction.In both the Bering and Chukchi Seas, theship appeared to move in and out of pressureice conditions over a distance ofabout a mile. Apparently, "pockets" ofpressured ice exist, even in an ice fieldthat appears to have relaxed from a priorpressure event.Weather conditions during the transitwere as expected. In the Bering Sea (iceedge to Bering Strait), winds averaged 22knots and daily air temperatures were 15 0 Fduring the March 24-28 time period. Thiscombination of wind and temperature gave awind chill of about 20 0 F. During theChukchi Sea transit (March 29 to April 4)the daily average weather ranged from awind speed of 10 knots and air temperatureof 13 0 F to 32-knot wind speed and temperaturesof -8 0 F. The latter readings gave awi nd chi 11 of -67 0 F and occurred duringthe planned transfer of personnel from theship. It may al so be of interes t to knowthat during the next 10 days the on-iceprofiling of pressure ridges occurred witha wind chill averaging -40 0 F.In summary, the lce condi t ionsobserved during the transit from the iceedge to Wainwright were either easy (oper-582

ating in the "shadow" zone of land masses)or very difficult (operating in large rubbleice fields with zones of pressuredice). There seemed to be a total lack ofmoderate ice conditions. Overall, excellentspeed was made in the "shadow" zonesof islands and land masses. A 14-knotspeed of advance was frequently achievedin ice conditions where: (1) good visibilityexisted, (2) ice conditions inadvance of the ship were known, and(3) propeller-ice impacts and vibrationswere low. Over 513 ramming cyc les wererequired during the northward transit.Some of these, however, were not trulyrepresentative of ice conditions, as othership-related factors, to be discussedlater, influenced the transiting ability.The voyage was completed in 2 days lessthan the 1981 transit by POLAR SEA inspite of the fact that the 1983 ice conditionsexperienced were more severe than in1981. This faster time is attributable tothe availability of sea ice imagery aboardthe ship, more frequency helicopter icereconnaissance, fewer problems with seachest icing, and the desire of theofficers and crew to better the 1981transit time.Fuel ConsumptionFuel consumption is the major considerationin estimating annual operatingcosts for marine transportation systems inice covered waters. To gain a betterunderstanding of fuel consumption as afunction of general ice conditions bygeographic area, a daily fuel log wasmaintained during the transit. Figure 6shows the quantity of fuel consumed on adaily basis from Dutch Harbor (March 23)to Wainwright (April 3) as well as fuelconsumed during on-ice data collectionactivities (5 to 25) and the return tripto Nome (May 3).A comparison of Figure 4 (daily shipposition), Figure 5 (areas of ramming),and Figure 6 (daily fuel consumption) maybe of interest. Also of note is the factthat fuel consumed from Dutch Harbor toWainwright during the late March transitwas 441,000 gallons while fuel consumed inthe reverse direction transit in early Maywas 234,000 gallons. This equates toabout a 50 percent reduction in fuelconsumption during the later transit.Factors Affecting Ice Transiting PerformanceIce transits are best performed 1nthe cont inuous mode of icebreaking; theramming mode should be avoided as much aspossible. During the 1983 mid-wintertransit to Wainwright, the POLAR SEA hadto perform a considerable amount of ramming,some of which could possibly beavoided in the future. In effect, thetransit could be more efficient. Factorswhich contributed to higher than normalrequirement for ramming were: imbalanceof propeller thrus t on each of the threeshafts; lack of adequate ice informationfor navigation; failure of a hydraulicservo pump; and an assortment of othermachinery malfunctions. These factors donot take away from the success of thevoyage or the special efforts made by thecrew to achieve the project objective.Rather, these factors are attributable toa lack of winter, high arctic marineexperience. They need to be discussed sothat planners of future voyages and futuretransportation systems will have a betterfeeling for the problems that can beencountered and the improved transit timesthat could be realized.Effect of propeller thrust imbalance onicebreaking performanceAs previously mentioned, the POLARSEA is a triple screw icebreaker that usesdiesels and gas turbines as prime powersources. Each diesel can produce about3,000 SHP while each gas turbine can produce20,000 SHP. Normally, the powerplant is balanced in terms of having twodiesels per shaft (DD/DD/DD) or one gasturbine per shaft (GT/GT/GT). The dieselconfiguration gives an on-line capacity ofup to 18,000 SHP and the gas turbinesprovide up to 60,000 SHP. Power sourcescan also be mixed among the shafts. Forexample, a typical combination includesdiesels on the wing shafts and a gas turbineon the center shaft (DD/GT/DD) togive an on-line capability of 32,000 SHP.This configuration works fairly wellexcept that the conning officer has novisual indication of the propeller thrustbeing produced by each shaft. As aresult, there are occasions where the wingshafts, with insufficient ahead pitch, aredragging and not producing useful thrust583

60,000NOTES:1. Readings taken at 1800 hn. on the date shown and repreaenttuel conatWnptkm for the prevSQuII 2-4 houri.55,000 2, Fuel consumption Include. main propulsion and •• BUJI. •• ry equipment(2 d .... 1 oenerator. and 1 or 2 bollen).0; 50,000Z0...45,000...~~Z 40,0000;::... 35,000::Ii:;)Ulz 30,00000..Jw 25,000~>- 20,000..J:< W0 15,000 ::Ii0z10,000 w>ita:5,0000I.11111111.1I 111 I J23 25 27 29 31 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 1 J4 6 8 10 12 14 16 16 20 22 24 26 28 30 2.1. 222 24 26 26 30---MARCH APRIL·1· MAYIFigure 6.Daily fuel consumption.as the ship moves through the ice field.Gauges showing thrust on each shaft wouldalert the conning officer to this problem.During most of the transit north, thestarboard gas turbine could not be usedbecause of damage to the coupling mechanismthat engages the gas turbine reductiongear system to the propeller shaft.During times of heavy icebreaking when allavailable power was needed, the propulsionplant configuration was GT/GT/DD (20,000/20,000/6,000). Although this configurationwas the best the ship could achieveunder the circumstances, it did result indirectional instability during icebreakingand caused the starboard shaft to overloadduring propeller-ice milling. The directionalinstability made it difficult toexecute a turn to port. In 4-foot levelice with ridges every 400 to 500 feet, theship could not turn to port with fullrudder. This frequently resulted in moretransits through ridges and rubble icefloes than might have otherwise beennecessary. In addition, any use of therudder to provide directional controlresulted in less ahead thrust for icebreaking.Figure 7 shows the variation inpropeller thrust for different power plantcombinations.Ramming became more difficult becauseof the inability to accurately control thepoint of ridge impact. With the starboardshaft overloading problem, it was necessaryto reduce propeller pitch to avoidtripping the electric propulsion motor.Thi s, in turn, frequent 1 y caused the conningofficer to reduce pitch on the othershafts, which resulted in a reduced speedof advance and therefore less effectiveramming. The point to be made is thatalthough high horsepower may be available,an unbalanced propulsion system canadversely affect the mean speed of advanceof the ship through an ice field.Failure of hydraulic servo pumpNormally, two servo pumps are inoperation to change the propeller pitchfrom full ahead to full astern. 'Cyc1etime is usually under 20 seconds. One of584

..... 600,000~~!II: 400,000~II)~ 300,000...~~ 200,00 •

Lack of adequate ice navigation informationOf the 10.5 days in transit from theice edge to Wainwright, 1.5 days werespent stopped at night. Such nighttimestopping is unnecessary for most icetransits. However, under the most severeof ice conditions, when the use of gasturbines was necessary, the projectmanager and Commanding Officer jointlydecided that the distance which could bemade good during 4 nights did not justifythe high rate of fuel consumption. Nightvisibility, with a clear sky and bow headlights,was limited to about a quartermile for the detection of leads or weakareas of ice. Although real-time satelliteimagery was received aboard the icebreaker,spat ial reso 1 ut ion was about 5miles and only extremely wide leads couldbe detected. So, while the ship was lookingfor 100-foot-wide leads, the bestresolution for the detection of such leadsaboard the POLAR Class was about 25,000feet. With the advent of new satellitesystems with synthetic aperture radar(SAR) and a resolution of under 100 feetduring the next few years, these navigationaldifficulties can be resolved oncethe SAR images can be down-linked to theship.The combined impact of these factorson ice transiting time and fuel consumptionis significant. Controlling them canprobably result in a 30 percent to 50percent reduction in transit time. Itshould also be recognized that developmentof enhanced ice navigation and ice pilotingtechniques alone will be responsiblefor most of the savings.Sl1IIIIIary• On April 3, the POLAR SEA arrivedat Wainwright, Alaska, having madethe second successful wintermarine transit to the northChukchi Sea. The transit from theice edge was completed in 10.5days, 2 days less than the 1981transit, although lce conditionswere more severe.• During the transit north, ice conditionswere either very light orsufficiently severe to require theramming mode of icebreaking. Onlyfirst-year ice was encountered,although multiyear ice floes weresubsequently profiled in thevicinity of Wainwright. Speeds of14 knots were frequently achievedin light ice conditions, especiallyin the "shadow" zones ofislands and land masses. Over 513ramming cycles were needed which,in part, reflects the difficultice conditions transited. It alsoreflects the influence of shiprelatedproblems, such as thepower imbalance on the shafts,which can be corrected.• Excellent fuel economy wasachieved by operating in leads, inthe "shadow" zones near landmasses, and in areas of light iceconcentration. On the other hand,fuel consumption was relativelyhigh when the ship was obliged toram repeatedly through pressuredrubble ice fields. Fuel consumptionfrom Wainwright south duringthe return trip in early May was50 percent less than for the tripnorth in late March. Based on thefive trafficability tests conductedthrough 1983, it appearsthat the mid-winter range for thePOLAR Class icebreakers in thewestern Arctic is the northChukchi Sea. Mid-winter transitsto the North Slope of Alaskaappear to be feasible only byrefueling the ship en-route.• This voyage marked the fourth consecutivewinter passage of a POLARClass icebreaker through theBering Strait without encounteringsignificant ice pressure or icejamming. Prior transits occurredin April 1979, February 1981, andMay 1982.• Ice navigation, rather than icebreaking,will be the key to successfor marine systems in westernAlaskan waters. Marine systemscan be designed with good lcebreakingcapability, but theskills of the ice pilot and theability of the ship to avoid highresistance ice features will ultimatelydetermine the success ofmissions.586

ConclusionsThe two mid-winter transits of theicebreaker POLAR SEA in 1981 and 1983 arepossibly the most meaningful of the POLARClass transits to the northernmost partsof Alaska. The achievements of the "TrafficabilityProgram" have suppressed thebelief held by many people that marineoperations are not feasible and are not aworthy transportation alternative. Bothindustry and government now believe thatyear-round marine transportation throughthe Bering Sea has been proven feasible.In addition, winter transits through theBering Strait, previously consideredimpossible by some experts, have beenregularly accomplished.AcknowledgementsThe authors wish to acknowledge the16 different organizations which sponsoredthe 19B3 POLAR SEA Trafficability Tests.These were the U.S. Maritime Administration,U.S. Coast Guard, Transport CanadaTransportation Development Centre, theState of Alaska Department of Transportationand Public Facilities, the U.S.Interagency Ship Structure Committee, and11 participating companies of the AlaskanOil and Gas Association; Amoco ProductionCompany; Arco Oil and Gas Company; ChevronU.S.A., Inc.; Conoco, Inc.; Exxon CompanyU.S.A.; Gulf Exploration and ProductionCompany; Marathon Oil Company; MobilExploration and Producing Services, Inc.;Phillips Petroleum Company; Shell DevelopmentCompany; and Sohio Alaska PetroleumCompany. Their combined support andinterest were vital to the project.Other related reports from the 1983deployment are:Volume I, Executive Summary, MARADReport No. MA-RD~940-8306l.Volume II, Environmental Data, MARADReport No. MA-RD-940-83062.Volume IV, Instrumentation system andcomputer software, MARAD Report No.MA-RD-940-83064.DiscussionG. VARGES: I noticed with interest Mr.Seibold's concern about the high fuelconsumption of the POLAR STAR. Theques t ion may be asked, "Why doesn't theU.S. Coast Guard adopt for their vesselsa change of the hull shape to ahigh-efficiency hull shape which couldreduce the fuel bill by one half or more,as the Soviets experienced this year withtheir converted Polar icebreaker MUDYUG?F. SEIBOLD: The Maritime Administrationand the other sponsors of the ArcticTrafficability Tests were "users" aboardthe USCGC POLAR STAR. This questionshould be referred to the U.S. CoastGuard for response.In addition, we wish to thank theofficers and crew of the POLAR SEA whomade it happen, in particular, CAPT BruceLittle, CDR Wade Moncrief, LCDR Mark Noll,LT Ed Marmol, LT Wayne Roberts, and CWOTom Cook. Their enthusiasm and cooperationis especially appreciated.ReferencesVoelker, R.P., F.A. Giesel, and K.E. Dane,"Arctic Deployment of USCGC POLAR SEA -Winter 1983, Volume III: TrafficabilityTests," MARAD Report No. MA-RD-940-83063.587

TANKER LOADING AT EXPOSED ARCTIC TERMINALSW. H. JollesCanadian Marine Drilling Limited, Calgary, Alberta, CANADAAbstractThe paper addresses the loadingand mooring of Arctic tankers from eXposedterminals and determines thetechniques, systems and costs required.A number of typicaltanker-terminal combinations areselected from which ext rapo1ations canbe made. Three loading concepts arediscussed; an alongside, a closecoupled and a soft moored concept. Themain parameters influencing the loadingand mooring aspects are deep water,moving ice, 1andfast ice, year-roundloading, tanker size and loading cycle.The feasibility for year-round tankeroperations is evaluated with emphasison the following subjects:environmental criteria, tankerapproach, ship motions and mooringforces, loading and mooring systems,downtime and costs. Results arediscussed and conclusions given.IntroductionExposed tanker loading terminalsin the Canadian Arctic have been theThis is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22,1987. © The Geophysical Institute,University of Alaska, 1987.subject of many studies (Canmar, 1986,Dome, 1984). These studies indicatethat, in moving ice conditions,scena rios in which the tanker comes inbow first toward the terminal arepreferred. Experience by Canadianoperators, gained in the operation ofarctic mobile drilling caissons,indicates that approaches to exposedtype of production structures arefeasible. Other aspects of mooring andloading procedures need to be resolved,however, before the overall feasibilitycan be proven and a preferred systemcan be selected.The following aspec ts need to beaddressed in order to define thefeasibility of mooring and loading.These aspects have been studied indetail (Canmar, 1986) and the resultsare summarized and presented in thispaper. The aspects are:The environmental conditionspertaining to Arctic terminals;The initial approach of the tankersto within 100 m of the terminal,including the manoeuvrability oftankers;The vessel motions and mooringarrangements;589

High speed loading systems includingloading booms;Downtime caused by ice and openwater;The preliminary overall costs.Three basic scenarios are selectedfrom which extrapolations to othercases may be carried out. They aretaken from various productiondevelopment studies (Acres, 1981,Canmar, 1986, Dome, 1982, 1984, 1985)and are assumed to lie in a water depthrange of 20-60 m. They consist of twoyear-round and one summer loadingscenario. The year-round loadingconcepts apply to landfast and movingice conditions respectively. Theformer could apply to developments inthe Beaufort Sea with shuttle tankersto protected bays, while the lattercould apply to the development in themoving ice zone area of the BeaufortSea. The summer, open water loadingcould apply to developments of smalleroil fields in the Arctic Islands or toseasonal production as anticipated byCanadian operators.Three main types of loading systemsare distinguished and are reflected inFigures I, and 2. The first one is aconventional alongside loading andmooring, where the vessel is held bymooring lines against a fixed quaysideor terminal. The second type is aclose coupled system, where the tankeris moored wi th its bow in closeproximi ty (5 - 10m) to the terminal.The third type is a soft moored systemwhere the tanker is moored by bowhawser mooring line(s) at a distance(25 to 125 m) from the terminal.5011 Moored~ 40m Swing RadiusThe design parameters examined,include tanker sizes between 30,000 and175,000 mn and loading times varyingfrom 6 to 24 hours. Typical productionlevels have been selected, varyingbetween 5,000-10,000 B.O.P.D. for thesummer loading, 30,000 B.O.P.D. for thelandfast ice scenario and from 50,000 -100,000 B.O.P.D. for the moving icescenarios.IslandClose Coupled5m/lOm Swing Radlu.200 x 60f II I 1 '" L'" / I

point and are allowed to weathervanewith the variation in ~Yind, waves,current and ice directions. These twoconcepts are referred to as singlepoint mooring systems or variablealignment systems.Environmental DataThree areas in the Canadian Arctichave been investigated, each withdifferent charcteristics. The areas arethe Canadian Beaufort Sea, the SverdrupBasin and Bridport Inlet. The moststringent criteria are found in theBeaufort Sea area; a summary of thedifferent parameters is listed below.Other areas of operation can also beconsidered, for example the U.S.Beaufort Sea. The results of thisstudy can also be applied to suchareas, provided that the specificenvironmental data is generally thesame. The different environmentalparameters are:llind: Storms have two predominantdirections and generally come from theEast or the West. A 20-25 knot windspeed is exceeded about 4%.Waves: Waves follow the same patternas the wind and, therefore, havesimilar predominent directions. Theexceedance probability of 2.5 msignificant waves is 2%.Currents: Currents in the Beaufort Seaare generally wind-driven and thereforeare strongest at the surface. Atypical exceedance probability forcurrents of 0.2 m/sec is 10%.Local I~eather Conditions: Increases ordecreases of the undisturbedenvironmental conditions will occurprimarily due to reflection of wavesand concentration of currents. Thesechanges generally result in a reductionof the environmental parameters.Wind Direction Changes: Downtime iscaused by exceedances of restrictiveenvironmental parameters and by changesin these parameters. The mostimportant one is a change in winddirection, since the tanker may becomeexposed. Therefore the percentage oftime that wind speed and directionchange within given time intervals, hasto be calculated. A typical examplebased on the Beaufort Sea data is:The probability for wind changesgreater than 40 0 and with speedsgreater than 20 knots is 0.25% for aone-hour time span, but is 5% for atime span of 6 hrs.Ice Types: In the Beaufort Sea in thewinter, ice types can be divided in tworegions; the 1andfast ice regionextending to approximately the 20-25 misobath and the pack ice regionencompassing all areas outside the1andfast ice zone. The pack ice regionconsists of mobile ice floes whichdrift in response to wind and current.The most dynamic area is called theshear zone and is located at theinterface of these two regions.Ice Drift Speed: The speed of the packice zone varies with the time of theyear and the amount of ice coverage.An exceedance plot of ice speed at adrilling site with a water depth ofabout 31 m is given in Figure 3.100.. 80uc..'0.. 60u.....~;: 40..Q.201.2 1.6 2.0Ice Drift Speed (knols)Figure 3. Ice Drift Speed (Knots)Percentage ExceedanceIce Direction: The direction of packice drift near the shear zone isdefined by the wind direction and themost common drift direction is alignedwith the edge of the 1andfast ice.591

Drifts normal to the landfast ice edgeare generally of limited magni tude andduration. Figure 4. illustrates thisice drift behaviour.N- 15~- 10~sFigure 4. Ice Direction Frequency roseby Speed & DirectionThe interval over which loadingoperations can continue depends on thechange of ice drift direction. Theprobability of such a change depends onthe angular change as well as theduration of the interval and the driftspeed and is required in the downtimecalculations.Ice Behaviour at a St ructure: Iceconditions around a structure determinewhether the structure is accessible.Two conditions occur, dynamic andstatic.Dynamic ice conditions occur in thelandfast ice region until the icestab! lizes and in the pack ice regionover the entire winter. Due to thedynamic behaviour some ice pile up andridging will occur on the upstreamface, while a wake is created in thedownstream side. The wake generallyconsists of open water or brash ice andis suitable for tanker access. Thisopen water wake has been noted onapproximately 50 percent of winterdays. Refreezing of the wake duringperiods of no ice movement accountedfor the remainder of the observations.The wake can be considered as a veryeffective shelter for the tanker.Static ice conditions occur when theice is sufficiently stabilized and theregion becomes landfast and ice ridgeshave formed. These ridges createextensive semi-permanent ice rubblefields, which are not expec ted toextend beyond the 20 metre water depthat steep-sided structures.The ice may also be repeatedlydisturbed by vessel traffic in thevicinity of a structure in landfastice The resulting increased icethickness around the structure is foundnot to limit the approach of tankersfor arrival frequencies not exceeding12.Open Hater Period: The open waterperiod (summer) is the final parameterrequired in a downtime analysis. Thelength of the open water season ishighly dependent on year and locationand the mean length varies between 100and 120 days. During this period, theSouthern Beaufort Sea is ice freeexcept for occasional polar pack icefloes. These floes may limit thetanker approach and loading operations.The overall findings of the reviewof environmental data (Canmar, 1986) isthat the open water conditions in theBeaufort Sea are less severe than thosefound in the Canadian East Coast or theNorth Sea. Ice conditions prevail formore than two-thirds of a year, withopen water wakes in the moving iceregion providing an effective shelter.Tanker ApproachOne of the quest ions to be resolvedfor the exposed terminal is thefeasibility of the tanker approach.This question has been addressed byusing a simulation program, (A.Churcher et al, 1986) to study theapproach to within 100 m of theterminal.Three typical approach manoeuvreshave been run:592

open \Yater tanker,arctic tanker in open water and,arctic tanker in 1year ice, with anridges per kilometre.m thickaveragefirstof 6The conclusion from the simulationis that each tanker can effectively bebrought to within 50-100 m of a loadingterminal, both in open water and in iceconditions (Canmar, 1986).Ship MotionsThe tanker motions need to beestablished, in order to define themooring procedures and mooringsystems. These motions are a result ofthe normal ship motions, draft changesand tidal changes. The motions will bedifferent for the alongside and thevariable alignment concept. The wavemotions are extrapolated from modeltests (Van Oortmerssen, Pub10 510) andcan be considered as firstapproximations for the design of theloading and mooring systems. Themotions have been calculated for asignificant wave height of about 2.3 m,which has an exceedance probability ofabout 2%. The actual vessel motionswill be lower due to a shelteringeffect of the terminal.The draft changes during loading orunloading depend on tanker design and,in particular, on the designed minimumand maximum operational drafts. Aconstant draft is preferred in orderto improve the icebreaking performancein ballast conditions. This constantdraft requires discharging of theballast when the vessel is at theterminal. A fully segregated ballastsystem has been assumed (Dome, 1984)and clean ballast will be discharged atthe terminal. The results are:The draft change is 5.5 m and the tidedifference is 0.9 m for both concepts.The above motiondeveloped (Canmar, 1986)to be comparable toconventional systems:Mooring Forcescriteria wereand are foun-lcorrespond ingThe mooring forces are a result ofwind, current, waves and ice drift andvary with different directions and varyfor the loading and mooring scenarios.The mooring forces in open waterare selected for conditions when thevessel should disconnect and consist ofa combination of a 50 knot wind,0.4 mls current and 3 m significantwaves. The forces on single pointmoorings are based on an extensivesurvey of the mean wave, wind andcurrent forces on offshore structures,(Remery and Van Oortmerssen, 1973).The mooring forces for the alongsideconfiguration, in open water, areextrapolated from the results of modeltests, where the vessel was mooredalong an open jetty with 4 lines and 2sets of fenders (Van Oortmerssen, Publ510). The assumption was made that thestiffness of the mooring system wasadequate to accommodate the dynamicforces resulting from wave excitation.The ice forces were alsocalculated separately for single pointmoorings and alo'ngside concepts. Theice can exert forces on a single pointmoored tanker in 3 typical ways:Hull FrictionThe frictional type forcesassociated with a tanker in the wake ofa terminal are of the same order ofmagnitude as for open water conditions.Ice DriftLocationVertical Motion*Transverse Motion*Longitudinal Motion*Yaw Motion** Maximum amplitudesAlongsideMooringHidship0.5m0.9m0.15mSinglePointBow0.5 m109m102m0.5 0 593The vessel is moored to theterminal in a stationary ice field andthe ice field begins to move from thebeam direction. The resultingtransverse forces have been based onthe results of model tests with aturret drillship conducted by Canmar in1979. Empirical formulas have been

developed (Canmar, 1986) to calculatethe transvere forces, based on thethickness of ice and the ship's beam.Pressure FieldA mobile ice field closing behindthe terminal causes pressures on thesides, but this condition has not beenobserved to date. On very rareoccasions such a scenario could occurand has been taken as an extreme upperlimit condition.Empirical formulas have beendeveloped (Canmar, 1986) to calculatethe longitudinal forces and they dependon the contact length, the ice load,ice strength, thickness, as well as thetapering of the wake.The ice forces exerted onalongside moored tankers are determinedin a similar fashion, withcorresponding empirical formulas.The results of the calculationsare as follows:The maximum open water mooringforces vary between 100 and 200tonnes. The forces in ice can reachten times these values in extremecircumstances. The mooring forces foralongside systems in moving ice areextremely high and only very thinmoving ice could be accommodated.Alongside mooring in 1andfastconditions is however feasible in allice thicknesses.Tanker VaningThe mooring forces due to icedrift can be extremely high andprotection of the vessel isessential. The forces will be reducedif the vessel is able to vane or rotateand thus lay in the protective shelterof the wake. Parameters influencingthe degree of shelter, while vaningaround a mooring point, are theorientation of the terminal and thenumber of connecting points. The twopredominant directions found in theBeaufort Sea suggests two oppositeloading stations, one in eastern andone in western direction.The vaning capability in openwater is primarily restricted by theminimum acceptable clearance be tweenthe vessel and the island terminal.The vaning in ice is restricted by themaximum allowable ice drift change.The allo'~ab1e drift can be found byplotting the tanker at differentpositions at the terminal and bychecking the exposure to ice as well asthe clearance to the terminal. Themaximum ice drift is found to varybetween 150-180° for the soft mooredand between 70-80° for the closecoupled system. The soft mooring thusgives better protection to the tanker.Loading SystemTanker loading has been studied bylooking at state of the art systems, aswell as new technology. Differentconcepts were reviewed, some withproven operating experience (G.E.C.,1982, F.M.C. Europe, and Canmar, 1986)In the alongside loading conceptthe tanker is berthed as inconventional loading jetties. No newtechnological breakthrough is requiredbecause the existing knowledge andexperience with the conventionalloading arms can be directly applied inthe Arctic.In the close coupled loading, thetanker is moored 5-10 m away from theterminal. The tanker is constrainedwithin a motion envelope with anamplitude about 1.0 m, but is stillfree to roll, pitch, heave, surge andsway. The vessel is restricted in theyaw motion since she rotates around aconnecting point.A typical close coupled loadingsystem is illustrated in Figure 5,which shows a multi-loading armconfiguration. The loading arm isequipped with three swivel joints, sothat the arm can freely track themotions of the tanker. The connectionend of the loading arm is equipped witha quick-connect and disconnectcoupler. Loading arm sizes could be upto 60 em in diameter.594

Figure 6. (G.E.C., 1982).Tanker~ Crane, i /t -' " Loading :Spread BeBm--f /' '/P' ;j I /' 'pes-r-3 I"t yCounter.r! BallastElevationMooring WinchRamrt< Mooring Line-,SwivelScheme of Close Coupling'I~; :~~ BollardFigure 5. Close Coupled Concept: ~ultiLoading ArmsAn alternative close coupledloading concept, consisting of a singleloading arm has been studied as we II.A large pipe of up to 120 cm indiameter is proposed to achieve a 101./system pressure. The vessel will befitted with a special receivingreceptacle on the bow. The loadingreceptacle is flexible to accommodatethe ship's excursions. The advantagesof this latter system are the shortconnection and a very simple and robustloading arm assembly.In the soft moored concept, thetanker is positioned 25 to 125 m fromthe loading terminal and is secured bymooring lines. The advantage of thisconcept is that use can be made ofexisting technology. The soft mooringsystem can be broken down in two types,both found to be feasible. They are:a) A 25 m to 50 m boom. This rangecovers conventional types of offshoreloading booms and is illustrated inIb) 50 m to 125 m boom. This rangecan be selected when larger clearancesare required. The arrangement is shownin Figure 7.The connection between theterminal and the tanker can be made byheavy duty flexible hoses, which areeither suspended in a catenary orsupported along a crane boom. Figure 6shows the hoses for oil transfer. Thehoses are lowered onto the tanker bowby the terminal crane and are pulledand aligned into the oil receiverassembly using deck mounted winches.An oil discharge assembly isfitted to the head of the hose string.This assembly is designed for quickconnection and disconnection to the oilreceipt manifold flange. In the eventof emergency disconnection, the oildischarge assembly is sealed off onboth ends and disengaged from the oilreceipt manifold to drop clear of thetanker's bow. Thus, no uncontrolledoutflow of oil will occur.The connection between loadingboom and tanker can also consist ofhard piping referred to as thearticulated loading arm, or "scissor"arms, where swivel joints provideflexibility. This cotlnection was usedin the concept of Figure 7 (F .M.C.,Europe. Tension cables are attached tothe loading arm assembly and runthrough a motion-compensatingtensioner. The tensioner counterbalancesthe weight of the loading armassembly including the entrained oiland compensa£es the vertical motions ofthe tanker.Mooring SystemMooring system specifications canonly be developed after a review of theope rat ing procedures. These procedureswere reviewed by experienced Arcticmarine operators and included berthingmanoeuvres and connect and disconnectprocedures.Theregardedalongsidepracticalmooring isin shorefastonlyice595

Conlrol CabinSlide Unll & ClampIn haul Winch.-/ "'­/'-- Trac110n Winch?(\(Q,,:' ------., .~~ -/ -Hawswe~ SI~per ~Unll ~., ofH_ Inhaul Winch/)Figure 6.Soft Mooring Concept withTransfer HosesTERMINALPIPELINES LED THROUGH BOOMo~o· •::::- LOADING ARMIII,~:~~IIII"'1lilt!,!.WATERLINEUNITTANKER!lOWFigure 7.Soft Moored Concept With Loading Arm596

egions and harbours where the exposureof the tanker beyond the shelter of theterminal is acceptable. In thelandfast conditions soft moorings mayhe equally attractive however, becausethe berthing area requi res less ruhbleclearing.An alongside mooring layout isillustrated in Figure 8, sholving alimited number of speciallyst reng thened mooring lines connec ted totensioners, in addition to conventionallines. The number of the specialmooring 'strong lines' is dependent onthe tanker size and the design mooringload. The tensioning equipmentconsists of two large diameter sleevebocks fitted over a hydraulic cylinder(Canmar, 1986).The Close Coupled mooring, Figure5, utilizes a mooring mechanismrestraining the tanker's motions, butallowing weathervaning around themooring point. Two methods of mooringare considered, firstly, a system of ahydraulic cylinder, haul-in line andmooring bollards and secondly, a morerigid system, with hydraulic clamps orreceiving cones. These two concepts areconsidered technically feasiblealthough the innovative engineering isstill to be solved.Althoughrotate aroundmotions willforces in thethe vessel is able tothe bollard, sway and yawinduce large transverseconnecting points.Vesse 1 excurs ions, also ref erred to asthe "butterfly motions", are reviewedto determine the order of magnitude ofthe restraining forces resulting fromwave d rtf t forces as Ivell as highfrequency wave motions. The conclusionis that the forces can be accommodatedin the hydraulic couplers.The Soft Moored concepts, as shownin Figure 6 and 7, require the tankerto stand-off at a predetermineddistance from the terminal. Similartypes of offshore loading concepts arein use at several locations around theworld (Canmar, 1986). The vessel isfree to make all vessel motions and isonly held on station by her ownpropulsion. The propulsion should beused to reduce the dynamic halvser loadsand vessel excursions. Calculationswere carried out to assess the impactof astern thrust on the transverseoffset as well as rotation around theloading station. The result is thatastern thrust can limit the excursionsgreatly and thus reduce the exposure.Terminal FacilitiesThe loading terminal facilitiesnecessary for the oil loading operationinclude items such as cargo pumps andpiping and detailed layouts arenecessary to determine the pumppower.In order to calculate the pumppower, certain assumptions have to bemade for oil storage, the type ofCONVENTIONAL LINESSPECIAL MOORING 'STRONG-LINE'@ 400 TTerminal•Fender•--TankerFigure 8.Alongside Mooring Arrangement597

Oeck Elev. (36m)Mechanical RoomProductionPlatform011 toExportMSLVertical Deep-Well PumpsArrangementPump011 toRoomExport~~.--------------h--~Horizontal PumpsArrangementRoomFigure 9. Oil Pumping Arrangementstorage, the type of pumps and theoverall layout of the structure. A wetstorage system has been selected wherethe oil floats on top of the ballastwater. A 'skim off-top' pump suctionarrangement was selected as shown inFigure 9, with horizontal pumps. Thehorizontal pump is preferred over thevert ical pump for ope rat ional reasons.Additional pumps are fitted, to pumpthe ballast water back into the wellwhen the terminal is being filled(Dome, 1984).The piping system must be arrangedsuch that "water hammer" pressures willbe minimised. Such pressures can buildup due to the emergency shutdown ofvalves. However, if the followingprecautions are taken, water hammer canbe prevented:Proper sizing of piping system,Pressure relief components,Controlled and programmed pumpstart-up, shut-down and emergencyshut-down procedures andPump bypass with recirculationlines.The shutdown time should belimited to a practical range of 20 - 30seconds, even though the theoreticalvalve closure time can be in the orderof 2 - 5 second s. This shutdown timewill not have an effect on downtime.Considerations for the pipingsystem should also include the inertgas venting requirements. In theoil-over-ballast water storage system,the amount of inert gas is relativelysmall. It has been assumed that theproduction terminal will be outfittedwith an independent inert gas generatorand that the inert gas from the tankerwill be vented to the atmosphere.Pumping power was determined withthe use of an analytical hydraulicmodel and the results for the softmoored concept are plotted in Figure10. The power requirements for thealongside and close coupled conceptswill be lower due to the shorter pipinglength and reduced head. The pumppower does not include an allowance forstart up or topping-off and the actualloading will thus take longer. Anincrease of 10% of time is expected fora loading terminal with arctic tankers.598

7,0006,0005,000~4,OOO3,0002,0001,00040,000 60,000 80,000 100,000BBL/HRFigure 10. Pumping Power RequirementsCost EstimatesBudget costs were developed foreach mooring and loading concept. Thecost estimates are built up from thecomponent level, with guidance fromequipment vendors on typicalcomponents. The cost estimates aregiven in 1987 U.S. dollars. Separateestimates are made for the alongside,closed coupled and sof t moored syst em,while the loading system costs areseparated from the mooring system costs.Due to the difference in the levelof concept definition and availabletechnical details, the level ofconfidence varies. Consequently, thecontingencies vary and Figure 11displays them for each concept. Theupper and lower bounds represent thecosts wi th and without cont ingencyrespectively.A number of assumptions had to bemade in order to arrive at the cost foreach system. They are:1) Conventionalloading arms areloading.CHIKSAN (FMC) typeused in the alongside2) The close coupled concept is atwin loading arm assembly. Connectionto the tanker is by hydraulic couplers.3) The soft moored concept uses twoboom alternatives; a 50m boom with aconventional type of crane structurewith a flexible hose string and a 125mcrane st ructure, which uti lizes anexisting type of crane boom.4) The cost estimates include amonitoring and alarm system for thetanker berthing and loadingoperations. No provision is made forstructural cost of the installation ofthe loading arms or pumping equipment.No cost is assessed for the terminalspace required for the loading arms orfor the pumping equipment. The costsfor support facilities ego inert gas,power generation, ice management, oilstorage etc. are considered to be partof the production terminal capital cost.5) The alongside mooring usesconstant line tensioners and costs arebased on conventional mooring equipment6) The closed coupled mooring isbuilt up using a key componentapproach, since no systems exist today.Provision is made for tanker bowstructural reinforcement.7) The soft moored concept uses datafrom offshore single point moorings.However, adjustments are made toaccount for the arctic environment.The results of tne loading andmooring cost calculations are displayedin Figure 11 and they apply to ascenario with two loading stations. Thecost for the one and four stat ions areapproximately 0.60 and 1.7 times thecost of the two stations. The mainconclusion is that for the same designparameters the cost for the alongsideand close coupled are comparable inmagnitude and that the soft mooredsystem has the lowest cost.DowntimeOperational experience shows thatsevere weather conditions are the majorcause of tanker downtime in the NorthSea and in other open offshorelocations. In the Arctic, theenvironmental conditions are furthercomplicated by the presence of ice. Thedowntime in open water or in ice is a599

LOADING SYSTEMMOORING SYSTEM50ALONGSIDE(10"10 CONTINGENCY)40ALONGSIDE(20"/. CONTINGENCY)~ 40.-.. 30!:!.:;; 20oU 1030..~ 20l-f/)8 10O+----,----._--~r----r--­o 5 10 15 20TANKER DWT. (OWT x 10')o 2 4 6 8 10 12 14 16 18 20MOORING FORCES (TONNES x 10 2 )CLOSE COUPLED(30"10 CONTINGENCY)40CLOSE COUPLED(50"/0 CONTINGENCY)50g- 40.. 30!:!.:;; 20oU 10~>6hr.d&2~12hr.== _ _ _ _ _ 24 hr.~ 30..!:!. 20l­ f/)oU 10O+----,----._--~r----r--­o 5 10 15 20TANKER DWT. ( OWT x 10')o 2 4 6 8 10 12 14 16 18 20MOORING FORCES (TONNES x 10 2 )SOFT MOORED(20"/0 CONTINGENCY)40SOFT MOORED(25'10 CONTINGENCY)..50~ 40.... 30- l-f/) 20oU10e "" _" ... ' ., .. ...,...~6h~, if- - - ~ 12 hr.~ 8 >.::.=:.. ___ -~ 24 hr.'-a!08 ~2:SZ __ =e~ __._.)6hr. 12hr.-:3 24 hr.Ina!O+-----,-----....---~----._---o 5 10 15 20TANKER DWT. ( OWT x 10')~ 30..e 20f/)oU 10_~I==- C 50 m BOOMo 2 4 6 8 10 12 14 16 18 20MOORING FORCES (TONNES x 10 2 )TWO LOADING & MOORIr.lG STATIONS, 1987 U.S. DOLLARSFigure 11.Loading and Mooring System Cost Estimate600

Tanker Arrive. AtTerminalYetFinalApproachYesI--_~Wait Until ConditionJ-----,ImprovesYesPerform IceManagementBerthingTanker Dock. and Start.LoadingYesLoadingYesWalt Till ConditionImprove. Or Go ToAnother StationLoaded TankerDisconnect. and Leav"Figure 12.Logic Blocks for Downtime601

esult of the tanker being unable toapproach the terminal or due tostopping of the loading operations,possibly followed by a disconnection.The downtime analysis has beenbased on a number of assumptions(Canmar, 1986) and a brief summary ofall the parameters and assumptions isas follows:The downtime is an average foreach tanker loading and is only validfor long operating periods; seasonaland yearly variations may result inlarge variations per tanker loading;Downtime is calculated for thethree main phases of operation, seealso Figure 12: approach, berthing andloading;Each operating phase ischaracterized by a discreet set ofoperating limits for storms, icemovement, wind and ice drift as well asrubble formation;The operating limits are differentfor open Ivater (summer) operations andoperations in ice. They have beendetermined from experience withexist ing terminals as well as Canmar'sarctic experience;The downtime in ice varies withthe following regimes:A. Summer period with isolatedice floes,B. IHnter period in the shearzone,C. Winter period in the 1andfastzone.The downt ime is a func tion of theloading concept, the mooringcapability, the acceptable driftchange, the number of loading stations(!11, 2 and 4) and the loading period(6, 12, 24 hours);The alongside concept is dependenton the mooring forces;The variable alignment isdependent on the drift change;The downtime calculations arebased on a statistical analysis of thedurat ions of unacceptable envi ronmenta1events. This includes the probabilityof changes of the parameters, i.e. windand ice drift changes. The downtime isalso based on Canmar's dril1shipoperations.A) Alongside in 20m Water Depth:: 10C-Olc:'C 8..0:::!..6.Ei:4..01~.. 2~0C

estricted to areasconditions only, withless than 20 m.\~ith landfasta water depthThe downt ime in ice caused by icedrift changes depends on the threetypes of ice regimes given earlier.The winter season in the shear zonenormally extends from November untilMay. The winter season at the outeredges of the landfast ice, inapproximately 15 - 20 m water depth, ischaracterized by moving ice fromNovember to January. The ice can becalled landfast in January or February,indicating that movements are in theorder of metres and thus, in mostcases, small enough not to causedisruptions to loading operations.The results for the averagedowntime per tanker loading in ice areshown in Figure 14. These figures donot show the average time required toclear the ice for berthing procedures.This is estimated to be about 2 hoursfor the alongside and about half thattime for the variable alignment mooring.In comparing the downtime in iceversus open water, the following can beread off from Figures 13 and 14: for a12 hour loading cycle, two loaningstations, 45° drift change and a 500ton mooring design load, the averagedowntime per tanker loading in openwater is 2-3 hours and the averagedowntime in moving ice is 3-5 hours.The main findings from Figures 13and 14 are that a minimum of twoloading points should be provided tominimize downtime and that the softmoored vessel has the lowest downtime.Shorter loading cycles give lowerdowntime but the reduction is verysmall if more than two boom locationsare used. The downtime in moving iceis about 1-1/2 to 2 times the downtimein open water.Results And ConclusionsThe results of the loading andmooring study can be used to developcost and downtime for many differentconcepts. One concept is chosen inthis paper for illustration purposesA) Alongside in Landfast IceEarly Wint.r (Nov. - Jan.)B)C)..: 14:Sg' 12'0ftI 10o-J.....III.586c~4o> 2c(oL--.--.---.--r--...-..--r-.,..-"o 2 4 6 8 10 12 14 16Design Mooring Loads (Tonnes x 10 2 )Variable Alignment in Landfast Ice-..::s 14g- 12:aftI 10o-J.....III 8.EEarly Winter (Nov. - Jan)6c~~ 1 Stallon4o> 2c( o L.: 1~~~~~;:~::~~~!!~2~S~t~.t~io~n~.o NO :1:80 :t120 :t160Ice Drift Change (Deg.)Variable Alignment in Shear ZoneWinter (Nov. - May)-..:.r:. 14- en 12c:a 10 ftI0-J.....IIIa.§ 6c~040> 2} 1 Stationc( 2Stallons00 :t40 :tao !120 !160Ice Drift Change (Deg.)Figure 14. Average Downtime/Loading inIce (llinter)603

only. The parameters are: Tankerdead\veight 100,000 tonnes, with a softmooring suitable for a design load of500 tonnes. The loading time is 12hours and two loading stations areselected. From Figures 11, 13, 14 thefollowing results are read off: Theloading and mooring costs are SUS 17million and US $6 millionrespectively. The average downtime fora 90° ice drift change is 1.5 hours insummer and 1 hour in winter. Thesefigures would mean a total of 1 daydownt ime yearly, based on 4 summer and12 winter trips. The cost reductionfor a single loading station would beapproximately 30%, where the downtimewould increase to about 4 days yearly.Both downtimes can be regarded as avery small percentage of the overalltrip time.The total cost for a close coupledsystem increases to about US $32million, primarily caused by an almost3 times higher mooring system cost.The downtime is higher as well, about 3days yearly, since the vaningcapability is about half that of thesoft mooring.The main objective of the study(Canmar, 1986) was to assees thefeasibility of loading and mooringtankers from exposed terminals. Theconclusions are:1) The alongside, close coupled andsoft moored concepts are all consideredfeasible for year round tankeroperation in the Beaufort Sea.2) The alongside concept does notprotect the tanker effectively in themoving ice zone and is, therefore, onlysuitable for the landfast ice region.3) The close coupled concept has asimple loading system but needs furtherwork on the design of its mooringconnec t ions.4) The cost and downtime estimatessuggest that the soft moored concept isthe most attractive option.5) A minimum of two loading stationsshould be selec ted in the Beaufort Seaarea.AcknowledgmentThe tanker loading and mooringstudy, upon which this paper was based,has been made possible due to thesupport of NO GAP , as a funding source,as well as Coast Guard Northern asManagers of the project.The author wishes to express hisappreciation to both the Canadian CoastGuard and Canmar for their support andpermission to publish this paper. !1anyindividuals have participated in thestudy and their valuable cont ri but ionsare very much appreciated. Thefollowing companies also have to beacknowledged: Petrotech Lavalin Inc.,Arctic Research and Development andArno Keinonen Arctic Consulting.ReferencesAcres Consulting Services Limited,1981. Bridport Inlet LNG Terminal ModelStudy, Arctic Pilot Project.Canmar, 1986. Arctic Tanker Loading andMooring Study. Final Report forTransport Canada, Coast Guard Northern.Churcher A.C. , Ciring J. andFalkenberg, IL, Feb. 1986. Sicesim: AShip/Ice Simulator, The Soc iety ofNaval Architects and 11arine Engineers,Arctic SectionDome Petroleum Limited, 1982.Assessment of Structural ConceptsSuitable for Use as Arctic Marine CrudeOil Loading TerminalsDome Petroleum Limited,Tanker TransportationProject 207.1984. ArcticStudy. APOADome Petroleum Limited, 1984. ArcticOffshore Production PlatformEvaluation. APOA Project 204.Dome Petroleum Limited, 1985, BeaufortPilot Production and TransportationSystem for HavikFMC Europe SA. Sens Cedex, France.Articulated Offshore Loading System.G.E.C. Mechanical Handling Limited,604

Marine Division Kent, England. 1982.Marginal Offshore Fields Tanker Loadingand Storage Systems, Design StudyCapability Document.Remery, G. and Van Oortmerssen, G.1973. The mean wave, wind and currentforces on offshore structures and theirrole on the design of mooring systemsby Netherlands Ship Model Basin. OTCPaper No. 1741.Van Oortmerssen, G., Publication No.510. The motions of a moored ship inwaves, Netherlands Ship Model Bas~n.605

COMPUTER SOFTWARE TO ANALYZEICE INTERACTION WITH MOORED SHIPSJorgito TsengNorm AllynKen CharpentierSandwell Swan Wooster Inc., Vancouver, British Columbia, CANADAAbstractAs activity proceeds lnto deeperwater in sub-arct!c or ~arginal iceareas, the potentIal exists forincreasing use of ship-basea drilling.production and transportatIon systems.To date, the assessment of the abilityof a moored vessel to remain on stationand resist various ice conditions hasrelied on basic calculations and modeltesting. This paper descrIbes thedevelopment of computer software thatis capable of analyzing iceinteractions with moored ships. Thesoftware, named ICESHIP, operates on arIBM microcomputer, is user-friendly andrepresents a major enhancement in theability to analyze ice forces onsingle-point moorings (SPM's) such asarticulated loading towers (ALT's) andcatenary anchor leg moored (CALM)buoys, turret- or spread-moored ships,or combined SPM and ship systems.Ice conditions in the marginal iceareas can range from collisions withindividual ice features that are drivenby wind, wave and current, torelatively steady-state load conditionsThis is a reviewed and edited version of a paper submittedto the Ninth International Conference on Port andOcean Engineering Under Arctic Conditions, Fairbanks,Alaska, USA, August 17-22, 1987. © The GeophysicalInstitute, University of Alaska, 1987.ar IS Ing from slow movIng pack Ice. Thesoftware is capable of analyzIng mooredships in both of these ice conditions.The software is an analytical toolwhich allows assessment of potentialice forces on moored ships, leading toa definition of operation limits ofexisting ships. Alternati vely, thesoftware can be used to establish icedesign criteria for ships to operate ina given ice environment.IntroductionWell established calculationmethods are available for the analysisof the sea-keeping motions of floatingstructures. Similarly, theor les on iceforce estimations, on fixed or floatingstructures, have advanced conslderablyin recent years. The need for a morecomprehensive analytical package forassessing floating structures in icebecomes apparent as hydrocarbonexploration and productlon activitiesproceed into deepwater marginal iceareas.Recently, two joint industryefforts, Alaska Oil and Gas Association(AOGA) Projects 309 and 340, addressedthe development of two computersoftware packages which, together, arecapable of analyzing floating systems607

such as semisubmersibles. SPM's, se~fmoored ship-shaped vesse 15, and SPMmoored vessels operatlng in marginalice. The software package ICESEMI.developed in AOGA Project 309(1986),for analysis of semisubmersibles inice, has been described in a previOuspaper by the au thors (Allyn ana Tseng,1986).The software package ICESHIP,completed recently in AOGA Project 340(AOGA, 1987), is capable of analyzingmoored ship-shaped vessels ln subarcticice conditions and may be consideredthe state-of-the-art in this type ofanalysis. This paper highl ights somesalient features of the softwareincluding:- the computer code- the range of scenar ios that can beanalyzed formoored ship systems. ice types. other environmental forces- the analytical techniques andtheories forice/structure force-motion analysisice forces from collision with adiscrete ice featureice forces from pack ice conditions- the output from analysis.Also noted are several marginal iceareas where assessments of floatingsystems can benefit from the use of ananalytical tool such as ICESHIP.computer CodeICESHIP is a menu-driven programthat operates on IBM personal computers(or compatibles) with a hard disk and640 kBytes of RAM. The program waswr i t ten in Microsoft Fortran 77. ThisFortran compiler allows the 'spawning'of code whereby blocks of executablecode are loaded into memoryindependently as required by a driverprogram. The total size of theexecutable code is thus much largerthan the core memory. ICESHIP containssome two hundred subroutines whichallow for easy implementation of futuretheoretical improvements.analyst to look at the influence ofchanges in a particular tnput parameteron the varlOUS outDuts such as thelocal lce/ship contact force and theglobal moor lng forces. In add 1 t ion totabulated summaries, ICESHIP can outputtime history or parametric plots toeither a monitor, a printer, or apen-plotter.The software package comes in eightdiskettes, containing the source codes,the executable codes and sampleanalyses. The code is divided intofour modules, ICESHIP, the driverprogram, SHIPPRE, the input module,SHIPANS, the analysis module andSHIPPOST, the graphical output module.Moored Ship SystemsIn the 1970's, drilling in earlyfreeze-up and late break-up in theBeaufort Sea was pioneered by icestrengtheneddrillships (Hnatiuk etal., 1984). Several recent studieshave investigated and proposed the useof a variety of floating systems inmarginal ice areas such as the GrandBanks and the Navarin Basin (Pollack,1985; Padron et al., 1985; Loire etal., 1985).Moored vessel systems that arepotentially suited for operating inmarginal ice areas include:- turret moored drillships (Figure 1)- conical drilling unit (Figure 2)catenary anchor leg mooring (Figure3)catenary articulated tower with icebreaking collar (Figure 4)- buoyant articulated loading tower(Figure 5)below water articulated mooringsystem (Figure 6)ICESHIP is capable ofoverall systems as wellindividual SPM structures.analyzingas theTheanalysisprogramoptionhas a parametricwhich allows the608

.' "Figure 1. Turret Moored DrillshLpFigure 2. Conical Drilling Unit609

Figure 3. Catenary Anchor Leg MooringFigure 4. Catenary Articulated Tower with Ice Breaking Collar610

Figure 5. Buoyant Articulated Loading TowerFigure 6. Below-Water Articulated Mooring System (Pollack, 1985)611

Ice Gond lC lonsThe ice scenarlOS 1n a margLnal Lcezone basically belong to one of twotypes, and the program 1S capable ofanalyzing either:- Pack ice, which can be level ice orbroken ice of various coveragesmoving at different speeds.Collision with a discrete icefeature, in which the ice feature maybe first or multi-year sea ice floesor glacial ice pieces such asgrowlers and bergy bits.Local Ice Resisting ConstructlonAs the structure Ltself may beindented if the ice pressure issufficient to cause yielding of thesteel scantlings and/or skin plate,ICESHIP accounts for the local strengthand stiffness of the vessel or thebuoy. The user can input a full set ofstructural element sizes or the programcan generate these according to one ofthe two North American codes containingrules for the design of ice-resistingsteel hulls:The Canadian Arctlc ShippingPollution Preventlon Regulations{CASPPR l, and- The American Bureau of Shipping Rules{ABSl." A comparison of sizes for selectedelements in the mid-body of a vesselfor several CASPPR ice classes is givenin Table 1, as computed by ICESHIP:An assessment of the localstructural effects on the hull of avessel or an SPM is made by consider Lngthe progressive development of plasticstrains in the various structLralelements. Large deforma t ion theory, inconjunction with Yleld line theory forthe hull plate and plastic hinge theoryfor the stiffening elements, lS usedfor the calculation of the plasticdeformation of the hull under lceforces. The size and the shape effectsof the ice contact area are taken intoaccount in the structural resistancecalculation.Table 1Mid-Body Scantling and Shell Plate Dimenslons USlng CASPPR Rules, as Generatedby ICESHIP.Ice class designated by CASPPR:Hull plate thickness* {mml 12Flat bar stiffener*height {mml 119thickness {mml 13Web frame*web depth {mml 1080web thickness {mml 15flange width {mml 287flange thickness {mml 15*The assumed framing layout is:431222241296173261710372502716202038320hull stiffener spaclngweb frame spacingweb frame spanyield strength40012006480350mmmmmmMpa612

Other Environmental ForcesWind:Force and moment coefficIents weredeveloped for ship-shapes from dataobtained in wind model tests for verylarge crude carriers (OCIMF, 1976).Hydrodynamics:[C) =[D](X)[K]matrix of lineardamping coefficientsmatrix of hydrodynamicdrag coefficientsvelocity vectorhydrostaticmatrixstiffnessThe added-mass coefficients forboth the vessel and the buoy can eitherbe calculated by ICESHIP or input bythe user. For the ship, both thelinear damping and the current drag(velocity squared) forces arecalculated, which allows for the slowdrift "fishtailing" oscillations to bemodelled using the methodolgy ofWichers (1986). For the SPM' s, thecurrent drag forces are calculated.Ice/Structure Force-Motion AnalysisDuring a collision, for example,between an ice feature and a vesselmoored to a buoy, the ice feature, theship and the buoy respond to theseforces as in the following equations ofmotion:For the ice feature:[Mi](Xl)+[Ci](Xi)+[Di](Xi)2+ r Ki](Xi) =(Fi,i)+(Fi,w&w)For the ship:. .[Ms](Xs)+[Cs](Xs)+[Ds](Xs)2+[Ks](Xs) =(Fs,i)+(Fs,w&w)+(Fs,m)For the buoy:[Mb)(Xb)+[Cb)(Xb)+[Db)(Xb)2+[Kb)(Xb)(Fb,w&w)+(Fb,m)where [M) virtual mass (mass +added mass) matrix(X ) acceleration vectorfor surge, sway,heave, roll, pitch andyaw(X) =(F,i)displacement vectorice force vector(F ,w&w) = wind and wave forcevectors(F,m)i =s =b == mooring force vector,including hawser orrigid yoke forcessubscript denoting icesubscript denoting theship-shaped vesselsubscriptbuoydenotingEach of the three equations ofmotion describes the six degrees offreedom of the body under theinfluences of external environmentalforces and the interacting ice andhawser forces. The time domainsolution of these 18 degrees of freedomis carried out numerically by applyingthe predictor-corrector techniquesimultaneously to the motion of thethree bodies.Ice Forces From a Collision with aDiscrete Ice FeatureIce impact forces caused bycollisions with growlers or bergy bitsare generally short impu 1 ses that cangenerate large local forces on thecontact area. These large local iceforces, however, do not usuallytranslate into large mooringrestraining forces as they are normallydissipated by the inertias of themoored vessel system and ltS addedmasses, unless the ice feature is verylarge, such as an iceberg.613

The local ice contact force at any timestep during an ice impact analysis iscalculated as a function of:1. the relative positions of the iceand the hull:The relative positions of theice and the hull determine the sizeand the shape of the contact area,which in turn governs theconfinement of the ice and hencewhether the expected ice failuremode is in-plane or out-of-plane(Timco et al., 1986).2. the relative speed between the iceand the hull:The ice crushing strength isdependent on the strain rate in theice, which is a function of therelative speed between the ice andthe hull and the shape of thecontact area (Bohon et al., 1985).3. the relative strengths of the iceand the vessel:The actual ice/structurecontact force that can be developedat any moment is taken to be thesmaller of the forces necessary todeform the hull or to crush theice.4. the fr iction between the hull andthe ice:The angle of impact, togetherwith the friction between the iceand the hull, determines whetherthe collision is a 'glancing blow'.The relationship of theseconsiderations in the calculation ofthe local ice contact force isillustrated in .igure 7.Ice Forces from Pack Ice ConditionsRelatively long-duration ice forceson moored vessels systems arise frompack ice conditions. Slow-moving packice generally generates local iceforces that are much more benign thanthose generated from a collision with afast-moving ice piece. However,because of the longer-duration natureof the ice forces as soc ia ted with packice, large restraining mooring forcesare generally developed.The major aspects of moored shipsin pack ice are:1. the ability of theweathervane in pack ice:shiptoRelativepositionbetween iceand hull}-Size andshape ofcontactareaVelocitiesof ice andhullStrain1-- ratein iceRate of ~deformationin hullHullstrengthpropertiesResistanceofhull+Potentialmode oficefailurePotentialr- iceforceStrength IcePropertiesActualcontactforcedevelopedIFigure 7Calculation of Local Ice Impact Force614

The weathervaning of shipshapes (Muga et al., 1977; Wichers.1986) can now be examined in thepresence of pack ice. Pack iceproduces a damping effect thatretards the fishtailing of thevessel.2. the shielding of the ship by thebuoy or vice versa:The relative post ions of theSPM and the ship may produce theshielding of one body by anotherfrom the oncoming pack ice. Thisshielding phenomenon is significantwhen investigating the effects ofthe relative size of the SPM to theship.3. ice coverage:The pack ice force on a ship isa strong function of the icecoverage. ICESHIP accounts forthis effect using data from the MVArctic cargo vessel.4. ice failure mode on the moored shipsystem:Depending on the flare of thevessel hull, the ice force ondifferent parts of the hull may begoverned by different ice failuremechanisms. On the bow, where thehull may be sufficiently sloped,the ice may fail in flexure as itrides down the hull, or it maybecome unstable and rotate out-ofplanebefore submerging andclearing. On the midbody, wherethe hull is close to beingvertical, the ice may crushinstead. A flaking-type crushingis assumed in ICESHIP. The iceforce on the SPM can likewise begoverned by one of thesemechanisms.Output from AnalysisThe major outputs from ICESHIP are:1. force output:Force outputs include timehistories forlocal impact ice force on thevessel or the SPM, as well ascontact p~essures and contactareaspack ice force distributionaround the vessel and the SPMcatenary mooring line forces onthe vessel and the SPMhawser or rigid yoke forceswind forces on the vessel, theSPM and the ice featurehydrodynamic forces on thevessel, the SPM and the icefeature.2. hull deformation output:Output parameters describing thebehaviour of the hull under ice impactinclude time histories forstrains at critical locationsof the various structuralelementsplastic deformations in thevarious structural elements.Also recorded are whether and whererupture would occur.3. motion output:The motionhistories forfreedom for:output includesthe six degreestimeofthe shipthe buoythe ice feature for theice/ship collision scenario.These data are tabulated in theoutput and are available for timehistory plotting. For the parametr icanalysis option, the maxima of theseoutput parameters can be plottedagainst the variable beingparametrically studied.Geographic Locations for theApplication of ICESHIPThere are a number of ice infestedgeographic locations where there arepotential or proven oil and gasreserves. Exploration using floatingsystems has been carried out in some ofthese ice bound waters and numerous615

studies have investigated their use inothers. Analytical tools such asICESHIP and ICESEMI may proveinvaluable in the determination ofoperation limits of, and in thedevelopment of design criteria for,floating systems to operate in marginalice zones. These areas, with iceseverity ranging from heavy iceintrusions to light seasonal coverage,include:Continental shelf of the BeaufortSea:The ice season in this areaextends typically from the earlyOctober freeze-up to the late Junebreak-up. Ice features whichdrilling platforms may encounterinclude level ice, first-yearridges and rubble, multi-year floesand ridges, hummock fields and iceislands. Even in the summermon ths, pack ice invas ions canbring multi-year ice into theotherwise open water.Navarin Basin in the Bering Sea:Heavy seasonal pack iceintrusions up to 10/10 coverage arenot uncommon in this area. Raftedsea ice and partially consolidatedfirst-year ice ridges are the mostimposing ice features.Grand Banks, offshore Newfoundland:The seasonal pack ice thatinvades this region consists mainlyof discrete floes rather than acontinuous sheet. Drifting glacialice features such as growlers,bergy bits and icebergs, althoughrare, have been observed.Barents Sea, offshore Norway andU.S.S.R. :Ice starts to form in mid­December and remains for sixmonths. Multi-year ice makes upabout half of the dynamic pack.The area is also subject to glacialice hazards.Sea of Okhotsk on the easternof U.S.S.R. and the northernof Japan:First-year ice growthmid-November and remainsconcentration for fivecoastcoaststarts inin heavyto sixmonths. The highly-mobile packresults in rafting and ridging,most notably along the seaward edgeof landfast ice.Bohai Gulf on thecoast of China:north-easternThe ice seasonlate November toextends fromearly March.Annual sea ice in thetidal-current-dr i venimposing raftedpack can formand rubbledfeatures.The Antarctic Seas:The marginal ice areas extendsto as far as 50 0 S. The drift icecan be a combination of first yearice, multi-year ice and tabularicebergs calving off huge iceshelves.Future WorkResearch required for further ingthe understanding of the ice/shipinteraction phenomenon and forimproving an analytical tool likeICESHIP includes the following items:the failure strain of steel at highstrain rates, low temperatures, andof welded construction withpossible inherent imperfections.the crushing strength of ice athigh and variable strain ratesthe hydrodynamics of multiplebodies in close proximitymodel test verification of ICESHIPSummaryA computer software package,ICESHIP, has been developed for theanalysis of interactions between iceand SPM's, ship-shape vessels or SPMmoored vessels operating in an iceenvironment. The software lends itselfto easy use for quick assessments ofthese floating systems in a range ofice conditions.616

AcknowledgmentThis joint industry project wasfunded by CONOCO Inc. , ChevronCorporation and Swan WoosterEngineering Company. The authors wishto thank the participants for thepermission to publish this paper.ReferencesAlaska Oil and Gas Association, 1986Project 309, 'Computer Software toAnalyze Ice Interaction with Semisubmersibles'.Alaska Oil and Gas Association, 1987Project 340, 'Computer Software toAnalyze Ice Interaction with MooredShip-Shaped Vessels'.Albery, Pullerits, Dickson andAssociates, 1984, 'M. V. Arctic IceTransiting Performance ContinuousModel', prepared for Transport Canada.Allyn, N. and J. Tseng, 1986, 'ComputerSoftware to Analyze Ice Interactionwi th Semi-submersibles' , ArcticOffshore Technology Conference.Muga, B.J., 1977, Houston, pp. 185-196,'Computer Simulation of Single PointMoorings' , Offshore TechnologyConference (OTC).Oil Companies International MarineForum (OCIMF), 1977, 'Prediction ofWind and Current Loads on VLCCs'.Padron, D.V., E.H.Y. Han, and M.T.Faeth, 1985, San Francisco, pp. 989-997, ' Ber ing Sea Crude 0 i 1Transportation Systems', ConferenceArctic '85, ASCE.Pollack, J., 1985, San Francisco, pp.309-322, 'Single Point Mooring in IceInfested'85, ASCE.Waters' , Conference ArcticTimco, G. and Frederking, R., 1986,pp.13-28, 'Confined Compression Tests:Outlining the Failure Envelope ofColumnar Sea Ice', Cold Regions Scienceand Technology.Wichers, J.E.W., 1985, Houston, pp.205-228, ' Progress in Compu terSimulation of SPM Moored Vessels' ,Offshore Technology Conference (OTC).Amer ican Bureau of Shipping, 1980,'Rules for Building and Classing MobileOffshore Drilling Units' .Arctic Waters Pollution Prevention Actof Canada, 1972, 'Arctic ShippingPollution Prevention Regulations'.Bohon, W. and Weingarten, J., 1985, SanFrancisco, pp. 456-464, 'TheCalculation of Ice Forces on ArcticStructures', Conference Arctic '85,American Society of Civil Engineering(ASCE) .Hnatiuk, J. and B.D. Wright, 1984,Calgary, pp. 333-345, 'Ice Managementto Support the Kulluk Drilling Vessel',35th Annual Meeting of the PetroleumSociety of the Canadian Institute ofMining (CIM).LOire, R. and W.Y. Chow, 1985, SanFrancisco, pp. 998-1008, 'Single PointMoorings for the Bering Sea',Conference Arctic '85, American Societyof Civil Engineering (ASCE).617

A COMPUTER·AIDED STRATEGIC ROUTE SELECTION SYSTEMN. R. ThomsonJ. F. SykesUniversity of Water/oo, Water/oo, Ontario, CANADAAbstractHistorically arctic marine transportation hasbeen restricted to the short Arctic summer oropen water seasoni however, an extension of thisnavigational season to possibly winter ice conditionsis an important requirement of resourceoperators in order to allow for cost·effective exploition.To aid in the extension of the operatingseason, navigational support has been developedat three distinct levels: stategic, tactical, andclose-range tactical. Of interest in this paper isthe strategic navigational support level which operateson a global spatial scale in the order ofhundreds of kilometres with periodic temporalcoverage from days to months. To assist in theselection of the optimal strategic route at thislevel of navigational support a computer-aidedroute selection system has been developed andinvestigated. This system quantitatively incorporatesa vessel's icebreaking capabilities in conjunctionwith forecasted ice motion informationin order to determine the optimal strategic route.This is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions. Fairbanks. Alaska.USA. August 17-22. 1987. © The Geophysical Institute.University of Alaska. 1987.The developed computer-aided strategicroute selection system is applied to a hypotheticalroute selection problem using data collectedfrom the southern Canadian Beaufort Sea overa route distance of approximately five hundredkilometres and a period of four days.IntroductionFor a vessel to function year-round in the Arcticit is essential that it operate expeditiouslyand safely despite the hazards posed by multiyearice, large ridges, rubble fields, darkness andadverse weather conditions. In order to overcomethese potentially dangerous situations and extenda vessel's operating season, an integration ofvessel design and construction with an advancednavigational support system is required. The primaryconcern of a navigational system is to provideinformation to the decision maker (e.g., thevessel's master) which will aid in the selectionof an optimal route that avoids or minimizes thevessel's interaction with hazardous ice.To aid in the extension of this short operatingseason three distinct levels of navigationalsupport have been delineated: strategic, tactical,and close-range tactical. The strategic naviga-619

tional support system operates on a global spatialscale in the order of hundreds of kilometreswith periodic temporal coverage from days tomonths. This route selection system is based onan integration of ice information gathered fromhistorical records and present conditions. Thecurrent ice conditions are supplied by non-vesselbased sensors such as satellites (e.g., NOAA andLANDSAT) and reconnaissance aircraft (ViatecResource System Inc. and Leigh InstrumentsLtd., 1981). The objective of this system is toselect a long range global corridor in which theoptimal vessel path will lie. From an operationalviewpoint, the selection of this strategic route isthe basis from which tactical navigational supportbegins. The tactical support system is forwardlooking and functions on a localized spatialscale in the range of tens of kilometres alongthe strategic corridor. Ice condition informationat this level is assimilated from reconnaissanceaircraft imagery (e.g., Side Looking AirborneRadar (SLAR) and Synthetic Aperature Radar(SAR)) and vessel based sensors (e.g., marineradar) collected over a temporal period varyingfrom hours to days (Viatec Resource System Inc.,1982). This information allows for selective routingor meandering of the vessel through an icefield, thus avoiding the major ice hazards. The finalnavigational support level, which operates inthe immediate vicinity of the vessel supplying icecondition information on an almost continuousbasis, is the close-range tactical support system.This information is collected from vessel basedsensors such as low light level television, marineradar, sonar and high powered search lights(Sneyd, 1985; and Viatec Resource System Inc.,1982). The ultimate purpose of this system is thedetection and subsequent avoidance of nearby icehazards. A summary of the attributes of each ofthe navigational support systems is presented inFigure 1.Probably the most comprehensive strategicroute selection process based on historical recordsof ice, weather and sea conditions was undertakenby the Arctic Pilot Project (APP), a jointventure undertaken by four Canadian companies.The APP was conceived as a year-round tr~nsportationsystem designed to deliver 7.6 millioncubic metres of liquified natural gas per day usingicebreaking carriers travelling from MelvilleIsland to a terminal location in eastern Canada.An integrated route analysis, which considered,as primary criteria, vessel safety and the avoid-STRATEGIC SUPPORT TACTICAL SUPPORT CLOSE - TACTICALSUPPORT• INFORMATION ACQUISITION• satellite (es. Landsat and NOAA)andreconnaissance aircrart(eg. SLAR and SAR)• INFORMATION ACQUISITION- rt'connaissance aircrart(eg. SLAR and SAR) andship based .ensor.(eg. marine radar)• INFORMATION ACQUISITION- ship ba.ed .en.ors(es. low light Ineltelnision. high inten.ity.earchlight. and .onar)• SPATIAL SCALE- slobal (hundred. orkilometre.)• TEMPORAL COVERAGE- periodic(day. - month.)..o...;:'"o'-'-;;;.0obn• SPATIAL SCALE- ten. or kilometre.• TEMPORAL COVERAGE- continuous to periodic(hours).....9~o ..• SPATIAL SCALE• localized (kilometre.)• TEMPORAL COVERAGE- continuous (minute.)Figure 1Comparison of navigational support systems620

ance of environmentally sensitive areas, was conductedto determine a global optimal corridor(Arctic Pilot Project, 1980). Considering thestrategic route selection problems facing the APP,Dey (1981) applied remote sensing techniques tothree years of satellite information. Feasibleroutes through the Canadian Arctic were selectedbased upon the presence of multiyear ice, formationof polynyas, and a qualitative knowledge ofthe general ice motion.On the tactical level of support, a vessel wouldnaturally follow large leads and areas of openwater, avoiding hazardous ice wherever possible.Norcor (1978) investigated the feasibility of selectedrouting (meandering) through an ice fieldin an attempt to avoid multiyear ice floes. Usinglow level photo-mosaics and enlarged LANDSATimagery of regions in the Queen Elizabeth Islands,they superimposed several arbitrarystraight line routes. Their results indicated a significantreduction in the percentage of multiyearice encountered along the paths that meanderedor avoided zones of multiyear ice as compared tothe percentage of multiyear ice encountered alongthe arbitrary straight line routes. This reductionwas dependent on the average floe diameterand vessel turning radius, and may only be realizedfor a multiyear ice concentration that is lessthan 70 percent. Beyond 70 percent multiyearice coverage, selective routing or deviations fromthe arbitrary routes provided little advantage.Although selective routing is an essential aspectof navigating through an ice field, considerationshould be given to other factors such asice motion, floe size distribution, nature of floepacking and, most important, the ice conditionsthat constitute the remaining portions of the icematrix. It is these ice conditions that will ultimatelydictate the success or failure of selectiverouting.Responding to the ice information needs ofgeneral offshore users (e.g., offshore hydrocarbonexploration sites and marine shipping), TransportDevelopment Centre (TDC) of Montreal,Canada is investigating the feasibility of an integratedapproach for ice information. The pro-posed system called REMSCAN (remote sensing,communications and navigation) is to operateon the strategic and tactical support levels. Airborneimagery and other necessary informationcan be directly down linked to the user from acentral facility (Green et aL, 1985).Since 1983, Canarctic Shipping Company Ltd.of Ottawa, Canada has been following a similarline of inquiry with the development of a shipboardice navigation support system (SINSS).The present objective of SINSS is to extend theoperating season of the M.V. Arctic, an Arcticclass two (recently upgraded to class four) icebreakingbulk carrier. The M.V. Arctic has beentransporting lead/zinc concentrates to ports inEurope from the Nanisivik mines located on thesouth shore of Strathcona Sound since 1978, andfrom the Polaris mine on Little Cornwallis Islandsince 1982.Based on a number of trials conducted duringthe early spring and late fall since 1981, Canarctichas been trying to define an acceptable qualityand reliability of ice information required forthe SINSS. The SINSS approach to navigationinvolves all three levels of navigational support.Strategic route selection is utilized to determinea 50 km wide corridor in which to focus attentionof SLAR/SAR coverage. Ice information at thislevel is gathered from current ice charts, NOAAsatellite imagery, and Atmospheric EnvironmentService (AES) ice reconnaissance flights. In addition,madne weather forecasts are employed toaid in prediction of ice motion. On the tacticalrouting level, the radar image display system(RIDS), a form of marine radar, is used inconjunction with SLAR/SAR imagery collectedalong the strategic route. Sneyd (1985) reportedthat the use of STAR-1 SAR (sea-ice and terrainassessment radar developed by Intera TechnologiesLtd. of Calgary, Canada) yielded excellentimagery for tactical support. With a resolutionof 12 m by 12 m for a 50 km swath width, STAR-1 SAR provided the necessary ice detail for selectiverouting through an ice field. For c1osetacticalsupport, binoculars and high intensitysearchlights were used to detect close-range hazards.The success of the various field trials un-621

dertaken by Canarctic has been very encouragingbut these trials have not been without problems.Aside from mechanical difficulties, the M.V. Arcticexperienced maneuverability problems due toa converging ice field and tactical navigationalsupport lapses from poorly timed SAR coverage(Sneyd, 1985).With the increased attention paid in recentyears by Arctic researchers to navigational supportsystems, it appears that steps in the properdirection have been taken to extend the navigationalseason. Advances in remote sensing techniquesand improvement in information processingand distribution abilities have provided vesselswith improved ice condition information onwhich to base a route selection decision. In orderto handle the large and varied amounts ofinformation that are received by a vessel, InteraTechnologies Ltd. have developed the MIDAS(multi-task ice data analysis system) program.MIDAS is an integrated facility that allows forthe display and analysis of remotely sensed dataon both the strategic and tactical levels (InteraTechnologies Ltd., 1985; and Lowry and Sneyd,1985).The selection of an optimal route at the strategic,tactical and close-range tactical navigationalsupport levels is based on a number of considerationswhich include the icebreaking capabilitiesof the vessel and the spatial and temporalscales that the route selection decision will influence.The vessel's icebreaking capability is likelythe foremost consideration as this will dictate themaximum possible ice thickness that may be traversed.Obvious advantages may be gained byselecting a route that passes through a region ofthick ice in order to reach a region of thinner ice.The spatial and temporal scales involved in theroute selection decision made at each of the navigationallevelsare not identical. For the tacticaland close-range tactical levels, at which ice conditioninformation is acquired in near real-time(minutes to several hours) over generally smallregions (metres to tens of kilometres), there islittle need to estimate changes in the ice conditionsbefore subsequent ice condition informationis acquired. However, at the strategic navi-gational level, where ice information may be receivedevery three to four days over a region thatspans several hundred kilometres, the knowledgeof the temporal evolution of the ice field may beof importance. Depending on the ice regime andenvironmental conditions, large scale ice motionmay significantly change the ice conditions overa three to four day period. For example, in theQueen Elizabeth Islands, ice has been observedto move at a rate of 10 km/day (Norcor, 1978),and under extreme environmental conditions inthe Bering Strait ice displacements in the orderof 100 km/day have been observed (Kovacs et aI.,1982).System Model OverviewThe computer-aided strategic route selection systemthat is described in this paper was developedfor an ice regime characteristic of winterice conditions (Le., 95 to 100 percent ice cover).This system quantitatively incorporates a vessel'sicebreaking capabilities in conjunction withforecasted ice motion information. In addition,attention is focused on the determination of theuncertainty in the vessel's performance along theoptimal strategic route due to uncertainties inthe ice motion forecasts.Normally, a vessel navigating through an icefield begins at a known location with the intentof reaching a specified destination point. Thechoice of a strategic route connecting these twopoints involves the selection of a route (from aninfinite number of possible routes) that is in somesense the best or the optimal route. The discriminationof this optimal route from the others isbased on a criterion which mathematically takesthe form of an objective 'function. Along thisoptimal route the objective function is extremized.The objective function reflects how the vesselperforms as it navigates through an ice field.For illustrative purposes, consider the vessel performanceto be represented by transit or traveltime, which may be derived by integrating theinverse of vessel speed between the two pointsalong a route. The route that minimizes thetravel time between the two points is the optimal622

oute. Some other possible indices of vessel performanceare operating power level and fuel consumption;however, these indices require furtherknowledge of a vessels operating characteristics;knowledge of vessel speed is not sufficient (Germanet aI., 1981). For a vessel performance indexsuch as travel time, the route selection problemmay be represented asminimize J = it/ du(t) t. t,subject to the constraints(1 )dx _ . _ ( ) { cosu(t) }dt -x-vx,t sinu(t) , (2)and boundary conditionsand(3)x(t,) = x' , (4)where J is the functional that represents the totaltime that is to be minimized, x(t) representsthe system state and belongs to the admissiblestate space X, u(t) represents the control or decisionvariable which is defined as the vessel heading(angle) relative to a fixed coordinate systemand belongs to the admissible control region U,v(x, t) is the mean speed of vessel advance, t istime restricted to the interval [to, t ,I, to is theknown initial time, t, is the unknown final time,and Xo and x' are the known initial and final systemstates respectively. Equation (2) describesthe response of the system state, x (t), to changesin the control variable, u(t). Within this context,the selection of the optimal strategic route, givenby (1) to (4), has been formulated as an optimalcontrol problem.To determine a vessel's mean speed of advance,v(x, t), at various points throughout anice field, an understanding of the icebreaking processis required. A number of investigators havestudied this question in detail and have producedvarious and somewhat conflicting theories (e.g.,Kotras et aI., 1983; Naegle, 1980; and Enkvist,1972). Icebreaking is a very complex process,whose basic physics is not yet fully understood.Since no reliable theoretical formulation exists,a number of empirical and semi-empirical relationshipsbased on model and full-scale experimentshave been developed. Most of these experimentshave dealt with vessel resistance in levelor continuous uniform ice. Considering the spatialscales involved in the strategic route selectionprocess, a level ice resistance model seems justified.For smaller spatial scales, where the iceregime could be described in greater detail (e.g.,number of ridges/km), consideration would needto be given to vessel performance in ice ridges.Generally, the total level ice resistance may beexpressed as a function of: vessel speed, variousvessel parameters, material constants, andice thickness. It is the dependence of vessel resistanceto ice thickness that is of importance inthis route selection system. Due to environmentalforcing, ice thickness may vary spatially andtemporally throughout the ice field as a vesselnavigates between the beginning and the terminatingpoint. This variability in ice thickness willinfluence, for example, the vessel's average operatingspeed which in turn will alter travel timeand hence, influence the selection of an optimalstrategic route.Vessel ice resistance models have been utilizedin conjunction with historic ice conditionsby a number of resear~hers in order to investigatetravel delays and economic concerns alongpredetermined routes. Cowley (1976) exploredthe travel delays of supertankers travelling fromthe St. Lawrence Estuary up to Grande lie, Quebec.This investigation developed a relationshipbetween the Bradford ice rank scale and vesselspeed based on a vessel resistance model. Johansson(1977) employed a vessel ice resistancemodel to study the economics of navigation inthe northern part of the Gulf of Bothnia. For avessel route originating in Lake Melville (whichis located on the east coast of Labrador) andterminating just offshore from Labrador, Nordco(1980) simulated average transit times for thejourney using vessel ice resistance theory. Historicice conditions along this route were collectedfrom ten years of Atmospheric EnvironmentService ice charts. Along this same route,623

Mulcahy and Wright (1983) determined the economicviability of year-round access using thetransit times produced by Nordco (1980). Germanet al. (1981) studied both the economicand the technical feasibility of transporting oiland gas from the Arctic based on results producedfrom a vessel ice resistance model. Thisstudy compared three possible routes, all havingice conditions determined from an ice growthcurve and historical conditions.In the literature, little treatment, if any, hasbeen given to the integration of ice dynamics andvessel ice resistance theory to aid in the selectionof an optimal strategic route. Figure 2 presentsan overview of the computer-aided strategic routeselection system that is the essence of this paper.Notice that this system allows for the finalstrategic route to be selected by either the deci-sion maker (e.g., the vessel's master) or by theoptimization procedure.The data module resides at the front end ofthe system. This module constitutes the acquisitionand preliminary processing stages of theenvironmental driving forces and present ice conditions.The detailed activities in this module aresite dependent and are beyond the scope of thispaper.The ice motion module utilizes the forecastedenvironmental forcing and initial ice characterization(i.e., proportion of constituent ice fractionsand associated ice thickness) to calculateexpected ice motion. The ice motion model attemptsto account for the complex interaction ofthe atmosphere, the ice, and the ocean. The sophisticationof the ice motion model employed inICE MOTIONMODULETRANSITMODULEOPTIMIZATIONMODULE. ice motion. vesselperformance. strategicDATA MODULE. acquisition ofpresent ice conditionsand forecasts ofenvironmental driving forces. uncertainty in vesselperrormance alongstrategic routeUNCERTAINTY. manual selectionof strategic route. vesselperrormanceTRANSITMODl'LEMODULEFigure 2Computer-aided strategic route selection system624

this module may depend on the ice characterization,the environmental forcing, and the particularroute selection domain. However, it is suggestedthat a short term, small scale ice motionmodel be used for this purpose (e.g., Thomsonet al., 1987; Lepparanta, 1981; and Coon et al.,1976). A model of this nature is comprised oftwo basic components: an equation of motioncomponent, and a thickness distribution component.The equation of motion component containsterms relating to the air stress, the waterstress, the Coriolis force, the sea surface tilt, andthe internal ice stress. The internal ice stress is afunction of ice deformation and attempts to describethe intra- and inter-floe forces. The thicknessdistribution component describes changesin the ice thickness distribution due to verticalgrowth (or ablation) of the ice, the advection ofthe ice pack, and the redistribution within the icepack. The redistribution relationship normallydepends on the particular ice thickness and icefraction and describes the creation of open waterand the transfer of ice from one thickness toanother by ridging or rafting.The spatial and temporal distribution of icethickness generated from the ice motion modelis utilized in the transit module to calculate thevessel's mean speed of advance. At each point inspace and time the vessel-ice resistance model isrelated to the available vessel thrust to determinethe ice breaking mode and, hence, the mean speedof advance. The difference between the availablevessel thrust and the resistance of the ice yieldsa net external force which may be expressed byF(h, v) = T(v) - R;(h, v) , (5)where T(v) is the available vessel thrust, R; isthe total ice resistance, h is ice thickness, andv is vessel speed. For a vessel to operate in acontinuous icebreaking mode, given a particularice thickness, a strictly positive solution ofF(h, v) = 0 must exist. If no strictly positive solutionexists then the vessel is required to operatein the ramming mode. In this mode of operation,consideration must be given to all stages of theramming cycle: the backing stage, the accelerationstage, and the penetration stage (Edwardset al., 1979).The optimization module contains the optimizationprocedure that determines the optimalroute between the initial vessel location and thedestination point. The solution to the optimizationproblem, given by (1) to (4), may be obtainedby a synthesis of the maximum principle(Pontryagin et al., 1962) and the dynamicprogramming method. This optimization procedureinvolves a hierarchical process in whichthe result of the dynamic programming methodbecomes the initial condition for the maximumprinciple. This synthesis of methodologies allowsthe optimization procedure to be computationallyfeasible while determining the globaloptimum and maintaining the advantages of themaximum principle. A complete description ofthis optimization procedure is given by Thomsonand Sykes (1987).Once a strategic route has been selected, theuncertainty in the vessel's performance (e.g.,travel time) along such a route to ice motionforecast parameters (e.g., driving forces, materialproperties, boundary conditions and initial conditions)is determined in the uncertainty module.The method that is adopted in this route.selection system is based on a first and secondmoment analyses technique where the requiredsensitivity coefficients are determined by the adjointoperator method (Oblow, 1978).Route Selection ExamplesThe computer-aided strategic route selection systemdescribed in the preceeding section is appliedto a hypothetical route selection problem.This route selection problem utilizes data collectedover a four day period during the winterof 1983 from a portion of the southern CanadianBeaufort Sea. The performance of the M.V. Arctic,in terms of total travel time, was investigatedfor this period.The spatial and temporal distribution of vesselspeed required for the optimization procedurewas determined as a result of the thickness distributiongenerated from an ice motion simulation.The ice motion model used in this sim-625

ulation is outlined by Thomson et al. {1987}.To account for the vessel resistance during thecontinuous mode of icebreaking, the vessel interactionmodel developed by Enkvist {1972} wasemployed. This interaction model is analyticalin nature and considers the following three icebreakingcomponents: resistance due to thebreaking of ice, resistance due to the submersionof ice, and velocity dependent resistance.Two different strategic routes were selectedfor the M.V. Arctic, with the origin for each routelocated at the south-west end of the domain justabove Herschel Island {see Figure 3}. The twodestination points were selected arbitrarily, onelocated near the domain boundary in AmundsenGulf, and the other located slightly northof the first destination point adjacent to BanksIsland. The results of the strategic route selec-oNIo(I')'j'o:!Ioo10...E~o...... ~I...IBuufortSea)//ot::I/om...IooCJ)...IMackenzieDeltamlximum principiidynlmlc programmingI110012001300[ km ]1400 1500Figure 3Strategic routes for the M. V. Arctic626

tion procedure are presented in Figure 3. Shownin this figure are the results of the dynamic programmingmethod (dashed line) which were usedby the maximum principle to obtain the optimalstrategic route (solid line). The routes have beendenoted by MV-1 and MV-2 to identify the routeto the destination point near the domain boundaryin Amundsen Gulf, and the more northerlydestination point respectively. The MV-1 route,which has an associated travel time of 1.19 days,follows a trajectory which initially arcs towardsthe Tuktoyaktuk Peninsula and then runs paraileI with the Peninsula. In contrast, the MV-2route, which has an associated travel time of 1.67days, essentially avoids the thicker multiyear icelocated in the northern portion of the domain byinitially heading southwest, then east, and finallyturning north to reach the destination point. Themean speed of advance for the M.V. Arctic alongeach of the optimal vessel routes is illustrated inFigure 4. For the MV-1 route, the vessel speedremains relatively constant until approximatelythe 350 km mark where there is a sudden increase.This increase reflects the point along theoptimal trajectory where the ice thickness beginsto decrease. Along the MV-2 route the vesselspeed steadily declines as the vessel encountersthicker ice. The M.V. Arctic was able to operatein the continuous icebreaking mode along the entireMV-1 route, and all but the last 15.0 km ofthe MV-2 route where the vessel enters a zone ofthicker ice.::;i-"o< 0Q) '"(1)9-09.enr+Q)t.):::J 0g9_.0:::J"3 ~ooenoFigure 4optimalMean Speed in m/s1.0 2.0 3.0 4.0 5.0Mean speed of advance along thestrategic routes for the M. V. ArcticThe effects of ice pressure on the movementof a vessel through an ice field has been reportedon a number of voyages (e.g., Bradford, 1971;and Sneyd, 1985). Typically, ice pressure acts onthe sides of a vessel and produces an increase infrictional resistance. Under severe circumstancesthis pressure may render the vessel totally immobile(Bradford, 1971). Ice pressure is the result ofenvironmental forcing causing the ice field to converge.To date, no quantitative relationship hasbeen developed to predict vessel resistance fromice pressure; however, a number of qualitative approacheshave been undertaken (e.g., Bradford,1971; and Dickins 1979). The use of continuumice motion models to predict localized ice pressureseems very encouraging. However, the re-lationship between geophysical stress, which isintegrated over several kilometres, and the localizedstress is not yet fully understood (Edwardset aI., 1979). The localized stress field can be asmuch as two orders of magnitude greater thanthe geophysical stress field (Hibler, 1975). Acknowledgingthis disparity, the geophysical stressnormal to each of the optimal routes for the M.V.Arctic are presented in Figure 5. The MV-1 routefollows a trajectory that is initially in compressionfor approximately the first 350 km and thenpasses through a zone of ice tension before enteringanother area of compression. This zoneof tension occurs near the domain boundary locatedin Amundsen Gulf. The MV-2 route, onthe other hand, follows a trajectory that is in627

..g l-e::;fQ) to.)< 0 I-(I) ~-09.(I)-Q):J0(I):JCo)a l-~a~3 ~a~a01g I-aMean Speed in m/sto 2.0 3.0 4.0 5.0I ~ I II,III,r- ,(I) ,r- ,I IN\/IIIIII,\r, (I)I,Ir-II-with forecasted ice motion information. In addition,this system is able to determine the uncertaintyin the vessel's performance function alongthe selected optimal strategic route due to uncertaintiesin the ice motion model parameters .The results of the route selection example illustratethe utility of this route selection system.The two routes selected for the M.V. Arctic seemreasonable considering the spatial and temporaldistribution of ice thickness. Within the frameworkof this system, the utility of the vessel-iceinteraction model is not restricted to a modelthat is accurate or appropriate, but instead tothe model's ability to provide quality vessel performanceinformation that will allow for the delineationamong possible vessel routes.It should be emphasized that the goal of thispaper was not to develop a real time operationalsystem but rather to integrate and investigate itsvarious components. Further, the economic viabilityof the developed strategic route selectionsystem has not been considered as problem specificand system streamlining issues would needto be considered.Figure 5optimalGeophysical ice pressure along thestrategic routes for the M. V. Arcticcompression along the entire length.The results of the vessel performance uncertaintyanalysis performed on both of the optimalroutes produced an uncertainty of (0.019)2 days2for the MV-1 route and an uncertainty of (0.080)2days2 for the MV-2 route. For each of theseroutes the most significant parameters that contributedto this uncertainty were the wind field,and the initial characterization of the ice pack.Summary and ConclusionsA computer-aided strategic route selection systemhas been described. This system, which wasdeveloped for winter ice conditions, incorporatesa vessel's icebreaking capabilities in conjunctionAcknow ledgementsThis work was funded by a Natural Sciences andEngineering Research Council PostgraduateScholarship awarded to the first author. Computingexpenses were mainly borne by the StrategicGrant "Sea Ice Dynamics" from the NaturalSciences and Engineering Research Councilof Canada.REFERENCESArctic Pilot Project, 1980. Integrated Route Analysis,Volumes 1-3.Bradford, J. D. 1971. Sea ice pressure generationand its effect on navigation in the Gulf of St.Lawrence Area, Inst. Nav. J., Vol. 24, No.4,pp. 512-520.628

Coon, M. D., R. Colony, R. S. Pritchard and D.A. Rothrock. 1976. Calculations to test a packice model, Numerical Methods in Geomechanics,Vol. 2, Ed. G. S. Desai, New York, ASCE, pp.1210-1227.Cowley, J. E. 1976. Quantitative application ofice climate data to winter navigation studies, Can.J. Civ. Eng., Vol. 3, No. 229, pp. 229-238.Dey, B. B. 1981. Shipping routes, ice cover andyear-round navigation in the Canadian Arctic,Polar Record, Vol. 20, No. 129, pp. 549-559.Dickins, D.F. 1979. Study of ice conditions alonga year round shipping route from the Bering Straitto the Canadian Beaufort Sea, a report preparedfor Canadian Marine Drilling Ltd., Calgary, Alberta.Edwards, R. Y., A. M. Nawwar, P. G. Nobel andM. Dunne. 1979. Development of a research programfor improving Arctic marine transportationtechnology, prepared for Transport Canada Researchand Development Centre, Montreal, Quebec,by Arctec Canada Ltd., TP 2003, 3 Vols.Enkvist, E. 1972. On the ice resistance encounteredby ships operating in continuous mode of icebreaking, The Swedish Academy of EngineeringSciences in Finland, Report No. 24, Helsinki.German, J. G., M. D. Macpherson, J. Meakinand C. W. Parker. 1981. Marine transportationof oil and gas in the Alaskan Arctic, proceedingsfrom The Society of Navel Architects and MarineEngineers, STAR Symposium, Ottawa, pp. 35-60.Green, D. W., R. S. Routledge, S. O'Connell, A.C. Churcher, M. Doucetter, J. Hughes, S. Hicksand J. B. Mercer. 1985. Remscan feasibilitystudy, Transport Development Centre, TransportCanada, TP 5917E, 26 pp.Hibler, W. D. 1975. Statistical variations in Arcticsea ice ridging and deformation rates, PaperJ., Ice Tech. '75, Symposium on Icebreaking andRelated Technologies, Montreal, April 9-11, pp.J1-J19.Intera Technologies Limited. 1985. A multi-taskice data analysis system, prepared for the TransportDevelopment Centre, Montreal, Quebec, TP6436E, 100 pp.Johansson, B. M. 1977. Economics of winternavigation in the northern part of the Gulf ofBothnia, prepared for the Finnish-Swedish Boardof Winter Navigation Research by Oy WiirtsiliiAb Helsinki Shipyard.Kotras, T. V., A. V. Baird and J. N. Naegle.1983. Predicting ship performance in level ice,paper presented at the Society of Naval Architectsand Marine Engineers Annual Meeting, NewYork, N.Y., Nov. 9-12.Kovacs, A., D. S. Sodhi, and G. F. N. Cox. 1982.Bering Strait sea ice and the Fairway Rock icefoot, CRREL Report 82-31, 44 pp.Leppiiranta, M. 1981 An ice drift model for theBaltic Sea, Tellus, 33, pp. 583-596.Lowry, R.T., and A.R. Sneyd. 1985. A shipboardice navigation system, POAC '85 proceedingsof the Eighth International Conference onPort and Ocean Engineering under Arctic Conditions,Narssarssuaq, Greenland, pp. 838-847.Mulcahy, M. W., and C. Wright. 1983. Determinationof the technical and economic viability ofyear-round access into Lake Melville, POAC '83proceedings of the Seventh International Conferenceon Port and Ocean Engineering under ArcticConditions, Helsinki, Finland, pp. 437-449.Naegle, J. N. 1980. Ice-resistance prediction andmotion simulation for ships operating in the continuousmode of ice breaking, Ph.D. Thesis, NavalArchitecture and Marine Engineering, Universityof Michigan, Ann Arbor, Michigan.Norcor Ltd. 1978. A study of ice conditionsalong marine shipping routes in the ArcticArchipelago, prepared for Transport Canada.Nordco Ltd. 1980. Lake Melville/OffshoreLabrador year-round navigation study, 1978-1979,prepared for Dept. of Industrial Development,Government of Newfoundland and Labrador, 162pp.629

Oblow, E. M. 1978. Sensitivity theory for generalnonlinear algebraic equations with constraints,Nuclear Science and Engineering, 65, pp. 187-191.Pontryagin, L. S., V. G. Boltyanskii, R. V.Gamkrelidze, and E. F. Mishchenko. 1962. TheMathematical Theory of Optimal Process, JohnWiley and Sons Inc., New York, 360 pp.Sneyd, A. 1985. Shipboard ice navigation s~pportsystem: Phase II, report - Canaractic ShippingCompany Ltd., Ottawa, Canada, 151 pp.Thomson, N. R., and J. F. Sykes. 1987. Routeselection through a dymanic ice field using themaximum principle, Transpn Res B, in press.Thomson, N. R., J. F. Sykes, and R. F. McKenna.1987. Short term ice motion modelling with applicationto the Beaufort Sea, J. Geophys. Res.,in press.Viatec Resource Systems Inc., and Leigh InstrumentsLtd. 1981. LNG carrier strategic routingstudy, reported prepared for the Arctic PilotProject.Viatec Resource Systems Inc. 1982. LNG carriertactical routing system study, report prepared forthe Arctic Pilot Project.630

SHIP/ICE PROBABILITIES IN ARCTIC SHIPPINGc. FerregutM. PerchanokC. DaleyArctec Canada Limited, Kanata, Ontario, CANADAAbstractRational damage risk assessment ofstructures is gaining acceptance in themarine engineering field. Recent work inCanada has been directed towards thedevelopment of a risk model that willsupport the design of regulations ofarctic shipping. The basic premise ofthe model is that the amount of damage toship structural elements is a function ofthe number and intensity of collisionswith floating ice. High spatial and timevariability of the environmentalconditions dictate that the number ofship/ice collisions along a given routewill be different each time the shipmakes the trip. Damage risk assessmenttherefore, requires simulation of thenavigation process and estimation of thenumber and intensity of collisionsbetween the ship and any hazardous icepresent. This paper introduces aconceptual model to estimate thecollisions experienced on an arcticshipping route. A Poisson model is usedto estimate uncertainty in the presenceof hazardous ice. Uncertainty in theencounter, detection and avoidance of thehazardous ice are incorporated in theThis is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987. © The Geophysical InstituteUniversity of Alaska, 1987.'model through the use of probabilisticdistributions, or second orderstatistical information of the basic(random) environmental variables such asice thickness, ice concentra~ion andvisibili ty.IntroductionStructural damage due to collisionwith floating ice is an important hazardto shipping and a potential cause ofmarine pollution in the polar regions ofthe world. In Canada, shipping regulationshave been developed in an effort tominimize the likelihood of ships beingdamaged by ice during voyages in CanadianArctic waterways (GC 1972). Theregulations govern the access of ships,according to their structural standards,to 16 arctic zones.Since the regulations were developedin the early 1970's, understandingof the structural implications of ship/ice collisions has advanced dramaticallyand this has been used to advantage byCanadian Coast Guard in recent work aimedat updating the regulations. One aspectof this work has been the development ofa risk model for arctic shipping, theArctic Shipping Probability EvaluationNetwork (ASPEN) (Daley et al.1986).631

The basic premise of the model isthat the amount of damage to shipstructural elements is a function of thenumber and intensity of collisions withfloating ice along a given route. Thiscan be different each time the ship makesa voyage. Risk modelling requires thesimulation of the navigation process andestimation of the number and intensity ofcollisions between the ship and anyhazardous ice present.This paper introduces a probabilisticmodel to estimate the expected numberand the probability distribution of thecollisions experienced on an arctic shippingroute, incorporating uncertaintiesassociated with the navigation processand with the environmental conditions.Conceptual Model of Ice NavigationUseful concepts have previouslybeen developed for models of navigationrisk in non-arctic situations. Lewison(1980) broke the navigation process intothree definable events to model the riskof ship/ship collisions in Dover Strait.He considered a potential encounter asoccurring when two ships pass within .5nautical miles, in the absence ofavoidance manoeuvres, and an actualencounter when they did pass within thisdistance. The probability of a collisionis then related to the geometry of theships' paths during an encounter, and thevisibility distance. Colley and Curtis(1983), Hagart and Crawshaw (1982) andothers added the human element tocollision avoidance models by consideringthe time or distance between ships atwhich avoidance decisions are made andthe behavioural patterns which affectdecision-making. These models provide auseful geometric framework with which toanalyze the ship/ ice collision situation,but are not directly applicablebecause of the differences between targetships and target ice hazards.In the ship/ship situation,knowledge about the presence of 'othership' is deterministic; knowledge aboutthe geometry of possible encounters islimited to a finite range of possibilitiesaccording to the speed and directionof approach and a number of standardmanoeuvres. In the ship/ice situation,knowledge about 'other ship', in thiscase floating sea ice or icebergs, isuncertain. As arctic mariners well know,floating ice may go undetected indarkness, fog or situations of heavyradar clutter or skip. Even when they aredetected, the particular type andcharacteristics of the ice obstacle maybe unknown. For instance, it can bedifficult even under daylight visibilityconditions to distinguish a small floe ofmulti-year ice from a field of rubbledfirst -year ice, or to see a bergy bit inheavy spray conditions. In short, withthe technology in common use on ice transitingvessels, ice obstacles may be presentwithin a ship's navigating domainbut not be detected in time to avoid acollision or if they are detected, thereis an uncertainty associated with themariner's identification of the type ofice obstacle and its potential effects onthe ship.Another important difference is inthe possible outcomes of a navigatingdecision. In the ship/ship case, theobjective is clearly to avoid a collision.This is not always the case in theship/ice situation. Instead, the navigatormay rationally decide to collide withsome ice obstacles in order to avoidother, more hazardous ones. Yhere only afew ice floes are present they can all beavoided with minimal loss of speed ortime, but as ice concentration increasesbeyond about 3 tenths coverage, itbecomes impossible to avoid all floes.Each case is a decision driven equally bysafety and by efficiency considerationsas the mariner must choose a course,either through or around individualobstacles or groups of obstacles, whichwill be most efficient in terms of timeand fuel usage and which will avoidcollisions with potentially damagingtypes of ice.The ice navigation model which wasdeveloped for ASPEN is illustratedconceptually in Figure 1. At the tacticallevel, decisions must be made about ageneral course through or around an icefield, while at the close tactical level,the decisions are about avoidance ofindividual ice obstacles. The sequenceof activities involved in manoeuvringaround an individual obstacle or group ofobstacles we call a navigating event.632

CJCJ111111111111111......Open WaterFYlceMY Iceshortest distance, highestrisk roulelongest distance, lowestrisk route£0,""" "B()~,'~------~~-------I"o Ice obstacle~ Ship's course.A Site at which obstacle i. detected.B Site at which obstacle i8 avoided.C Ice obstacle.A C Detection distanceb Avoidance angleBe Avoidance distanceED Distance of clooest approach to obstacle.Conceptual Approach to Recording a Navigating EventFIGURE 1OVERVIEW OF DECISION·MAKING IN ICE NAVIGATION633

Prediction of the success or failure of anavigating event involves dealing with anumber of uncertain quantities, such asthe ice floe detection distance, actualturning path of the ship and the actualsize of the ice floe detected; thisuncertainty makes prediction onlypossible in probabilistic terms. Forsimplicity and in order to facilitate theintroduction of new concepts, a rationalefor computing the probability ofcollision when only a single obstacle ispresent will be introduced first. Thegeneralization of the rationale to dealwith the presence of multiple obstacleswill come naturally as shown later inthis paper.Probabilistic ModelEach navigating event includes thethree stages; obstacle encounter,detection and avoidance. The possiblesequences of events and outcomes areillustrated as an event tree in Figure 2.In general, the probabilities of subsequentevents are conditional on the priorevents. For example, the probability ofa given path, say path N:4 of the tree,would be given by:Hazard not encounleredI·P(A)P(CBA) P(CIBA)P(BIA)P(A) (1)where A is the event of encountering anice obstacle, B is the event of detectingthe obstacle, C is the event of avoidingthe obstacle, and P(XIY) means the probabilityof occurrence of event X giventhat event Y occurred. From Figure 2 theprobability of collision P(D), is the sumof the probabilities associated withpaths 2 and 3:P(D) = (l-P(BIA»P(A)+(l-P(CIBA»P(BIA)P(A)P(A)-P(CIBA)P(PIA)P(A) (2)The presence of ice obstacles isindependent of their detection, but thesuccess of the avoidance does depend inpart on the distance of detection.Accordingly, equation (2) can berewritten as:P(D) P(A)-P(CIB)P(B)P(A)No encounter1 No colll.IoD(l-P(CIB»P(B)P(A) (3)1 ·P(A)Ship DavigaUng onice covered walenHazard not detectedl·p(BIA)J No detectio. [1 • P(B IA»)P(A)CollisionHazardEncxmntered.PtA)Avoidance manoeuvreunsuccessful1· P(CIBA)3 No avoidanceCollision[1 • P(C IBA)JP(B IA)P(A)HazardDetectedP(BIA)Avoidance manoeuvresuccessfulP(CIBA)... AvoidanceNo collisionP(C I BA)P(B I A)P(A)Event 1Ship NavlgaUngon Ice CoveredWatenEvent AHazardEncounterEvenlBHazordDetectionEvenlCBucce.dul AvoidanceManoeuvreFigure 2 EVENT TREE FOR A SlllPlICE COlLISION634

The analytical procedure which wasdeveloped to evaluate the probabilitiesin equation (3) are described in thefollowing sections.Encounters with Floating IceThe first stage in the icenavigation event is the presence offloating ice within a navigating domain,as illustrated in Figure 3. We definethe area of the domain A' as 15 x 15nautical miles. The bounds of the icenavigation domain represent theapproximate limit at which ice conditionsand motion can be considered essentiallystatic with respect to navigationplanning. This was selected on the basisof extensive discussion with arcticmariners and study of arctic shiptransits (Daley 1984, Perchanok et aL1986) .From a planning perspective, anestimate can be made of how many iceobstacles may be present within anavigating domain, but their exactlocations within the domain are unknown.Figure 3 shows that the intended path ofthe ship is represented by its beam, b,and length of the navigating domain, L.Therefore an ice encounter may be definedas the situation in which an ice obstaclewith diameter, d, lies on the ship'sintended path and would be struck by theship if no avoidance manoeuvres wereundertaken. As shown in Figure 3, animpact could occur if the centroid of theobstacle lies at some point within theinfluence surface shown. Since thecentroid of the obstacle could be at anypoint in region A' with equal probability,the probability that it lies withinthe influence surface can be computed as:P(A) = alA' (4)Ship Beam (b)Ice Target DiameterInfluence Area (a)Sh,d/2bFigure 3SHIP/ICE ENCOUNTER635

where a is the area of the influencesurface. In the particular case of asquare navigating domain, equation (3)takes the form:P(A) = (b+d)/L (5)where L is the length of the sides of theregion.Analytical Model for Detection ofFloating IceAs with ship/ship collision models,the distance at which an obstacle isdetected is of primary importance in theoutcome of a navigating event. Detectionof ice hazards is done either visuallyfrom the wheel house or through the useof a radar. If x is the distance atwhich the ice target is detected, thenthe probability of detecting the targetat a distance greater than x is:P(B) 1 P(B RU By>1 (P(B R) + P(Bv)-P(BR)P(B v » (6)where P(B R) and.P(B v) are. the probabi~itiesof oetectlng the Ice target WIthradar and the probability of detectingthe target visually from the wheel houseat a distance lesser than or equal to x.Extensive studies of the detection of seaice and icebergs in arctic shipping lanesindicate that both probabilities areaffected by the type and size of theobstacle, weather, sea state, surroundingice conditions and visibility (Perchanoket a1. 1986).Making estimates of P(B R) and P(B ) yas a function of all these parameters ISnot possible at this time becauserelationships among them are not yetfully understood. However, some advanceshave been made towards this goal, and atleast the influence of ice type andvisibility can now be taken into account.Visibility and Detectability: Inthis paper, visibility is taken as ameasure of the degree of clearness of theatmosphere as determined by the conditionsof light and weather. Each hasvariability in time and space. Detectabilityis taken as the capacity of anobject to be discerned from the backgroundscene. This definition impliesthat detectability is an inherentproperty of a body which can be comparedamong objects for a given condition ofvisibility.Although they represent differentvariables, both visibility and detectabilityare measured in the same spaceunits, and can be assigned probabilitiesin terms of these units. For example, itis possible to talk about the probabilityof having a visibility of 2.0 km, or thelikelihood of detecting an object layingat 1.5 km from the observer.Detectability Densi ty Fllnctions:To the writer'S knowledge no previousattempt has been made to model thedetectability density functions of icetargets, nor are there published data tojustify any previous approach. However,recent field studies (Perchanok et al.1986) suggest that ice targets aredetected within a region whose boundariesdepend on the type and size of the ice,the surrounding ice conditions and themeans of detection. On the basis ofthese premises, it is the authors'assumption that the detectability of anice target may be modelled inprobabilistic terms using a beta densityfunction:fX. (x)1f(q+r)f(q)f(r)i = v,R(7)and the probability of detection at adistance lesser than or equal to x is:i = v,R(8)in which a,y are the minimum and maximumdetectability limits, q and r are parametersthat depend on the type, size andcondition of ice and f(*) is the gammafunction. Detectability functions asdefined by equation (7) can be definedfor visual detection as well as forradar, sonar or other means of detection.636

As a first attempt to model detectabilities,the parameter, q, will beassigned a constant value of one in thispaper, while the parameter, r, isassigned different values depending onthe ice category. That is, r = 1.0 isassigned to highly-detectable ice such asicebergs, r = 2.0 to moderately-detectableice such as ridges, and r ~ 3 tolevel ice floes. Figure 4 shows theshapes of these functions for a normalizedvalue of the distance parameter.Notice that the higher the parameter, r,the smaller the likelihood of detectionat longer distances. Other values forthe parameter, r, could be used to modelthe detectability of fishing boats,drilling platforms, navigational buoys,etc. The detectability functions inFigure 4 consider only visual detectionand identification. Recent work indetection of objects at sea by radar orsonar (Ryan et al.1985), could be handledin a similar manner to derive functionsfor each sensor used.Influence of Visibility: In thecase of visual detection, the family ofdetectability functions generated usingequation (7) is only valid in thepresence of clear skies; this means thatthe visibility distance in the region isgreater than or equal to the maximumdistance a given object can be detected(Figure 5a). Very frequently, however,visibility distances are shorter than themaximum detectability distances (Figure5b). If the visibility distance isknown, say, y, the conditional probabilitydensity of visual detection,giventhe visibility distance, is obtained bytruncating the original function at theCf)ZoI­oZ:lII.>-I-4.03.0Cf)Z\1,1o 2.0>-I--'III

visibili ty value, y. Thus: y, are cancelled from the original density.The integral in the denominator is afX(x)normalizing constant that assures thatfxly

In general, visibility distances are notknown and uncertainty on their predictionexists. This uncertainty can be takeninto account using any of the followingapproaches. Yhen y is assumed acontinuous random variable withprobability density function fy(y),equation (10) becomes:On the other hand, if Y is assumed to bea discrete random variable withprobability mass function PY(Yi) equation(10) becomes:Q) x'P(B) ( 11)where, n, is the number of discretevisibility values considered.xXRrZDistance at which ice target is detected.Turning radius of ship.Ice target radius.Safety Inargin for avoidance of target.Figure 6 A VOIDANCE EVENT639

Analytical Model for Avoidance ofFloating IceThe probability of avoidance wasmodelled by considering whether it isphysically possible for the ship to turnin a radius smaller than that at which itwill hit an ice obstacle on its path,given the distance at which it wasdetected. It is assumed that as soon asa target is detected the navigator willstart turning the ship in order toprevent collision. Success of thisaction depends on the detection distance,the ship's speed and the ice conditions.Figure 6 shows that avoidance can only bepossible when the ice target sits out ofthe ship's turning path.Now, let x define the distance atwhich a given ice target is detected, Rthe turning radius of the ship, and rthe radius of the detected ice target. Asafety margin, Z, can be defined as theminimum distance between the ice targetand the turning path:(13)Collision can only be possible when Z SO. Then, the conditional probability ofcollision, given that the ice target wasdetected, is:P(CIB) P(Z S 0)p«x 2 _r2)1/2_R_r S 0) (14)According to Daley (1984) the radius ofturning of a ship can be estimated as:RLs2(13.2 TK+3)(1+V/V max)(15)where, Ls is the ship's length, T is the(random) ice thickness, K is the (random)ice concentration, V is the initial velocityof the ship and V is the openwater velocity.maxIn most Arctic regions, the shapeof the probability distribution of theice thickness and ice concentration isunknown and their variability isrepresented by their second momentestimates only (mean and coefficient ofvariation). Accordingly, a second momentrepresentation of the radius of turningcanandare:RLs2be computed.coefficientThus, the mean value Rof variation ~ of R(13.2 TK + 3) (1 + V/V max) (16)(17)where T and ~ are the mean value andcoefficient of variation of the icethickness, and K and 2K are the meanvalue and coefficient of variation of theice concentration.Using equation (16) and (17) andlinearizing equation (13) by means of aTaylor series expansion, the mean value Zand variance a Zof Z can be computed(Benjamin and Cornell 1970). Thus:Z ~ x + R - R - r (18)RlR2a Z(19)~ x + RUsing these two parameters the conditionalprobability of collision can beapproximated by:'(CI8) - • [- a:l(20)where t is the standard Gaussian cumulativedistribution function.Multiple Obstacles PresentPrevious sections introduced arationale to estimate the probability ofa ship colliding with an ice obstaclewhen it is the only obstacle present inthe navigation domain. This section willgeneralize that rationale to deal withthe more realistic assumption of multiplepresence of ice obstacles. The modelalso considers the navigating events from640

a planning orin ..,hich theof obstacleskno..,n. Thisdescription ofclimatological perspectiveexact number and locationsat a point in time are notrequires a probabilistictheir presence.Probability of More than One Ice ObstacleBeing PresentLet M be the (random) number of icetargets of a given type (e.g. multi-yearice) and size (e.g. 25 m) ..,hich arehistorically present during a specificmonth in the navigation region ofinterest. The probability that exactly mtargets ..,ill be present is based on thefollo..,ing assumptions:i) An ice target can be found at randomand at any point ..,ithin the area ofthe navigation domain.ii) The number of ice targets found inany given navigation domain isindependent of the number in anyother (non-overlapping) domain.iii) The probability of finding an icetarget in a small subregion ~, ofthe navigation domain is proportionalto ~, and can be given byv~', ..,here v is the average numberof ice targets found in the navigationdomain during the month athand and the probability of findingt..,o or more ice targets in ~, isnegligible.On the basis of these threeassumptions, the probability of finding acertain number, m, of ice targets in agiven navigation domain can be modelledby a Poisson distribution of the type:m -vv em!(21)Estimation of v for each arcticregion or navigation domain may be donethrough the use of historical data or, incases ..,here the data is scarce, bycombining the available data ..,ithexperts' estimates as presented byFerregut and Perchanok (1986).Distribution of the Number of CollisionsIf the number, M, of ice targets ofthe same type and size present in thenavigation domain ..,ere kno..,n ..,ithcertainty, and considering that theprobability of collision bet..,een a shipand any single ice target is constant orgiven by P(D) (equation 3) (and theprobability of not colliding is I-P(D»,then the probability of exactly mship/ice collision occurrences among th~M targets is given by the binomialdistribution.mM-mP(D) c (I-P(D» c (22)..,here ~) = M!/Im !(M-m )!] is the binomialco~fficient.c HoSever, it ..,asagreed before that the total number ofice targets M found in a given domain isa random quantity ..,ith probability distributiondefined by equation (21). Thusequation (22) should be modified to takeinto account the uncertainty in M,leading to:P(M =m )= \c c LM=OM!(M ) m M-m~ P(D) c(I_P(D» cc(23)..,hich can be sho..,n to be a Poissondistribution ..,ith average number ofoccurrences v = vP(D) (Ang and Tang1975). The di~tribution of the number ofcollisions ..,ith a certain ice type andsize ..,ithin a region can be re..,ritten as:m -vc cv ecm !c(24)This result can be generalized to obtainthe distribution of the number ofcollisions along a series of navigationdomains (ship route) Mas:c R641

References\)C,1P(M =m )_------------------­c Rc R(26)where n is theRdomains crossed.Conclusionsnumberof navigationIce navigation is a complex processinvolving a highly - variable physicalenvironment and also a variabilityassociated with human performance anddecision - making, This process canno t bemodelled in a deterministic fashionwithin present levels of understanding,A conceptual model based in part onship/ship collision studies and in parton the unique ship/ice situation wasdeveloped. This divides the process intodiscrete navigating events, and dividesan event into an ice encounter, iced'etection and ice avoidance component.Approaches were developed to provideprobabilistic models for each of thesecomponents, and to link them with probabilisticmodels of the arctic environmentand of the structural consequencesof ship/ice collisions. It is hoped thatthese models will provide a basis for abetter understanding of the components ofrisk in arctic shipping and an improvedbasis for structural design andregulation in the marine community,AcknowledgementsThis work was supported by CanadianCoast Guard Northern under contractOSZ85-00114. The authors acknowledge thevaluable comments and assistance of Mr,John McCallum, Canadian Coast GuardNorthern and the efficaciouscollaboration of Mr. Rick Brown and Mr.Arthur Karton in building the ASPENcompu ter model.Ang, A.H-S. and Tang, ~.H., 1975.Probability Concepts in Engineering~P~l~a=n=n~i=n2g_a=n~d~D~e~s~i~g~n~, Volume I, John ~iley& Sons.Benjamin, J.R. and Cornell, C.A. 1970.Probability, Statistics, and Decision forCivil Engineers, McGraw-Hill.Colley, B.A., Curtis, R.G. and Stockel,C.T., 1983. "Manoeuvering Times, Domainsand Arenas", Journal of Navigation 36(2),pp. 324-327.Daley, C.G., 1984. "ASPEN (ArcticShipping Probability Evaluation Network)Group III Studies", Volume 1. Report by\rctec Canada Limited to Canadian CoastGuard Northern, Transport Canada.Daley, C., 1984. "Data CollectionStrategies for the Ship/Ice CollisionModel - ASPEN". Report by Arctec CanadaLimited to Canadian Coast Guard Northern,Transport Canada, Ottawa.Daley, C.G., Perchanok, M., Ferregut, C.and Brown, R., 1986. "ASPEN ModelContinued Development", Report by ArctecCanada Limited to Canadian Coast GuardNorthern, Transport Canada.Ferregut, C. and Perchanok, M., 1986. "OnModelling the Arctic Environment: ABayesian Approach", Proceedings ofEnvirosoft '86 Conference. P. Zanneti(Ed.) 1986, pp. 733-743.GC. Government of Canada, Ottawa.Assessment of Airborne Imaging Radars forthe Detection of Icebergs. EnvironmentalStudies Revolving Fund Report No. 016.Hagart, J."PersonalityBehaviour",pp. 202-206and Crawshaw C.M., 1981.Factors and Ship HandlingJournal of Navigation 34(2),Lewison G.R.G., 1980. "The EstimatedCollision Risk for Marine Traffic in U.K.~aters". Journal of Navigation 33(3),pp. 317-328.642

Perchanok, M., Yells, D. and Lovings,M.L., 1986. "Merchant Ship IceNavigation Studies in the CanadianArctic", International PolarTransportation Conferences, Vancouver,B.C., pp. 859-877.Ryan, J., Harvey,"Assessment ofDetection of IceReport #008.DiscussionD. DICKENS:M. and Kent, A., 1985.Marine Radars forand Icebergs". ESRF1. Does the "Columbialand" data mentionedin your paper distinguishbetween multi-year and first-yearcollisions?2. How well does ASPEN predic t damageincidents when compared with actualexperiences in the Canadian Arctic?3. How reliable is ASPEN as a winterplanning tool for high ice classvessel operation when mos t of theverifications currently availableare derived from summer transitswith low ice class vessels?C. FERREGUT:1. In the presentation, data was shownwhich compared observed versusprediction ice collision for theM.S. COLUMBIALAND. The comparisonwas made for collision with alltypes of ice combined.2. On an overall (probabilistic) basis,ASPEN predicts quite well (i.e.,save order of magnitude). Ofcourse, no particular damage incidentcan be predicted. The otherreal value of ASPEN is in comparingrelative damage probabilities ofvarious parts of the ship structure(i.e., plating versus framing).G. VARGES: I am impressed by yourapproach to unders tanding the problem ofnavigational risk. However, I am wonderingwhat the initial purpose of yourstudy was:Do you intend to arrange for betterrouting in ice, orDo you intend to influence theClassification Societies to reducescrubbing requirements because ofthe result of your risk study?If the latter is the case, I doubtyour chance for success. The ships willalways have to have a steel structurewhich allows for survival of the worstcaseice impact.C. FERREGUT: The main objective of ourstudy was to develop a decision-makingtool that would be used to assess therisk levels associated with the shipclasses defined by the Canadian ArcticShipping Pollution Prevention Regulations(CASPPR) and to study the effects on thesafety of ships if the current zone/datesystem is changed. It was also intendedto aid regulatory bodies and classificationsocieties to formulate more rationalcodes and regulations.With respect to the comment thatships should be designed for the "worstcase"ice impact, we agree that would bethe optimal design' approach if we coulddevise all possible ship/ice interactionscenarios, and we had perfect knowledgeabout the magnitude of the loads actingon the ship during each scenario. However,that is not the case, and uncertaintyshould be recognized as aninherent component of any structuraldesign pnilosophy. By using the "worstcase"approach, the resulting design isone that is adequate under the circumstances,and certainly one that works,but it is not necessarily the mostefficient possible.3. Obviously, ASPEN has not beenchecked against actual high ArcticClass experience in winter, becauseno such data exists. Nevertheless,ASPEN is a decision tool which canbe used to compare designs for suchconditions.643

WINTER RELOCATION TECHNIQUES FOR ARCTIC STRUCTURESG. A. N. ThomasStandard Oil Production Company, Dallas, Texas, USAAbstractReducing the cost of arctic offshoreexploration drilling is necessary torevitalize industry interest in theseareas. This paper discusses studiesthat have been targeted at enablingstructures to be moved from one site toanother in ice conditions in the AlaskanBeaufort Sea. By moving quickly betweensites and planning a drilling programaccordingly, the per-well cost of arcticoffshore exploration can besubstantially reduced.Technically feasible winter relocationtechniques have been identified, and twoare considered likely to becost-effective. Firstly by usingicebreakers to extend the summer seasoninto early winter, perhaps intoDecember. Secondly by using a"mechanical" technique for shortdistance moves in late winter,Marchi April, that includes vehiclesdeployed on the ice sheet. Using asummer move and these two windows forwinter relocation, a drilling program ofthree well sites per year can beenvisaged (see Figure 1).This is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987. © The Geophysical Institute,University of Alaska, 1987.IntroductionArctic offshore exploration drillingstructures represent major capitalinvestments and consequently they shouldbe used as efficiently as possible. Inorder to achieve maximum utilization itwould be necessary, on completion ~fdrilling and testing a site, toimmediately relocate the structure tothe next site.By making relocation possible in seasonswhere there is ice coverage, it will bepossible to drill one or two additionalsites in a one year program. Dependingon contract terms, much of the cos t ofdrilling is the fixed yearly cost ofcapital deployment and of structureoperation. Therefore additional wellscan be drilled at a much reducedadditional cost, thereby substantiallyreducing the per-well cost of arcticoffshore exploration.ObjectiveThe objective is to be able to utilizemobile arctic drilling structuresyear-round in the arctic. This wouldgive the operator complete flexibilityin planning an integrated drillingprogram. In practice the studiesperformed have concluded that there are645

two windows in which the winterrelocation techniques are likely to becost-effective. Therefore work wasconcentrated on these two areas with theobjective of establishing technicalfeasibility and reasonable costdefinition.With relocation only in the summer, astructure can drill only one site eachyear. The prize is to drill two orthree sites in that one year, wherefixed annual costs remain the same andtherefore the per-well cost issubstantially reduced. A hypotheticalprogram to drill six sites in two years,including one site with two wells, isshown in Figure 1.Seasonal TechniquesTechnically feasible techniques forrelocating mobile arctic offshoredrilling structures have been definedthat can be implemented virtually yearround. However, there are some times ofyear that the associated equipmentrequirements and costs make relocationuneconomic, except where there may beunusual financial or businessconditions. The techniques are set outbelow and are summarized in Table 1.•Summer: In the summer, structuresare moved using ocean tugs.Icebreakers may be used for icemanagement in case of ice incursions,or may assist because they areavailable in the arctic waters. Atpresent, all movement of mobiledrilling structures has occurred inthe two to three month summer andearly fall period. This can lead toa heavy scheduling demand on theavailable vessels, and if offshorearctic activity were significantmight drive up costs.• October/November: In the AlaskanBeaufort Sea, the first year icecover may reach a thickness of twofeet by the end of November. It ismostly still floating and mobile, andice ridging has not reached itsmaximum. By using a fleet of threeicebreakers drawn from the existingBeauDril (Kimmerly and Jones, 1986)and Canmar fleets, studies show thatmobile drilling structures can bemoved. One icebreaker will prepare awide track in the ice, while theothers will tow the structure throughthe broken ice. Although therelocation in early winter is atslower speed and greater cost than inopen water, it is likely to becost-effective in most circumstances.• December: In December the first yearice sheet thickness may attain athickness of three feet, and ridgingalso will have increased. Icemanagement could be performed by anexisting Class 4 icebreaker, but itis doubtful that existing icebreakerswill have the power to tow thestructure through the broken ice. Itis likely that techniques forreducing the towing resistance of thestructure will be required. Themodel tank towing tests describedbelow have identified components ofthe towing resistance in broken ice,and several alternative techniqueshave been considered. In summary,either new, more powerful icebreakerswould have to be constructed or, morepractically, a fourth icebreakerwould be required to engage inmanagement of the broken ice adjacentto the structure. The later in theyear that this relocation is done,the slower will be the move and themore costly in terms of vessel hire,fuel and demobilization to a wintermooring site.January/February:•The first year iceis now too thick for meaningfulicebreaker progress, yet not thickenough to support heavy equipment.In shallow to medium water depths,the ice may be landfast or stabilizedby grounded rubble piles. Groundedridges and rubble piles also makeicebreaker transit difficult. It isconsidered that the March/April"mechanical" technique describedbelow could be used, but with iceroads having first to be built bothto the departure site for equipmentmobilization, and between thedeparture and arrival sites. Theexpected costs for construction andmaintenance of the ice roads are suchthat relocating a structure at thistime of year is probably uneconomic,even for moves over short distances.646

YEAR 1 YEAR 2 YEAR 3WITH NO RELOCATIONIN WINTER:RELOCATINGOPERATINGSTAND-BYJAS OND JFM AMJ JAS OND JFM A MJ JAS ONDI I I2 1WELLSI//L~~WITH EARLY AND LATEWINTER RELOCATION:RELOCATINGOPERATINGSTAND-BYI I I I 111 I1 1 .- 2 1 1WELLS 1~r21 ~rJJL W~~~~~ V//. '/AI• •FIGURE 1.DRILLING PROGRAMS WITH AND WITHOUT WINTER RELOCATIONPeriod Ice Thickness Technique Relative CostFeet 10 Miles 30 MilesSummer Open Water Tugs 1.0 1.0Early Winter 0-2 Icebreakers 2.4 2.6(Oct/Nov)(3 existing)December 2 - 3 Icebreakers 4.8 5.0Jan/Feb 3+ "Mechanical" 4.5 7.0(with ice roads)Late Winter 6+ "Mechanical" 3.0 4.8(March/April)June 6+, rotten Icebreakers 4.8 5.2(4 existing)TABLE 1.TECHNIQUES AND COSTS FOR RELOCATING ARCTIC STRUCTURES INTHE ALASKAN BEAUFORT SEA647

For a structure in deeper water thatis to be moved between two sites inthe mobile ice to seaward of theshear zone, the use of Class 4icebreakers may still be feasible.• March/April: At this time of year,the ice is at its thickest and is inits most stable condition. A"mechanical" technique has beendevised that includes cutting the iceinto large blocks to form a channelthrough which the structure is towed.A promising technique for the cuttingis to use high pressure water jets.The ice blocks are to be handled bywinch trucks deployed directly ontothe surrounding ice sheet, and are tobe moved around the structure andstacked behind it. The structure isto be hauled forward either by morewinch trucks or by using its ownanchor cables attached to ice anchorsset out ahead. Due to the slow speedof advance, this technique isconsidered cost-effective for shortdistances, perhaps up to 15 miles.The technique is further described asthe late winter technique below.• May/June: With the ice ablating andbecoming soft, it is no longerpractical to use vehicular equipmentdeployed on the ice. However thewarmer ice is much weaker thanearlier in the year, and icebreakersmay again be feasible later in thisperiod. Areas of grounded rubble mayrestrict possible routes, and after achannel is broken the volume ofbroken ice will be such thatadditional ice management adjacent tothe structure will be required. Thecoordination of vessels moving in arestricted area close to thestructure will mean that progress isslow and expensive. It is consideredthat to achieve a summer well, itwill be more cost-effective to movein March/April by mechanical meansover a short distance.From the above, two alternativetechniques have been determined as bothtechnically feasible and cost-effective.These are the early winter(October/November) technique usingexisting icebreakers, and the latewinter (March/April) technique for shortmoves using mechanical methods. Theseare discussed in more detail below.Early Winter TechniqueInvestigation of the feasibility of theearly winter relocation technique hasincluded study for the followingaspects:• Definition of environmentalconditions. Based on both public andproprietary data, the ice conditionsin each month of the year weredefined for two geographical areas ofthe Alaskan Beaufort Sea in whichthere was potential for relocating astructure in winter. Level icethickness, ice ridge size andfrequency and multiyear iceconcentrations were described interms of expected or mean conditions,and of 0.1 annual probability ofexceedence both mild and severe.• Definition of icebreakers. Existingicebreakers that might be availableto support such an operation werecharacterized based largely on publicinformation but also including someproprietary data. For comparativepurposes, a series of new morepowerful icebreakers were alsodefined.• The performance of the icebreakersrelative to the defined environmentalcondi tions was assessed by computersimulations. An ice sheet with adistribution of ridges was modelledand the effect on icebreaker averagespeed was assessed. As the icebecame thicker and the ridges largerand more frequent, the speed wasslower and in larger ridges theicebreaker would have to back and ramseveral times in order to advance.The simulation algorithms, althoughproprietary, had been developed andcalibrated against full scaleicebreaker research and performancedata. Therefore, there is a highdegree of confidence in the results.• The structure must be moved through achannel of broken ice that is widerthan the structure itself, to allowthe ice to flow around it. The studyhas concentrated on a channel 2.5 to648

3 times the maximum structure width.An icebreaker assigned to icemanagement must be able to break achannel of this width in the definedice conditions.Alternative tracks were examined andthe pattern shown in Figure 2 wasselected as the basis for furtherdevelopment. The average speed ofadvance of an icebreaker performingice management depends on how manytimes it must steam up and down andon maneuvering time at each end ofthe track. It was anticipated thatthe channel would be broken inlengths of perhaps a mile at a time.• The resistance of the structure tobeing towed in broken ice wascalculated by examining the componentactions that contribute to the total.Relationships were developed topredict the total resistancedepending on the structure geometry,the forward velocity and the size,thickness and concentration of thebroken ice pieces. Two structureswere considered, one of circularshape and one of rectangular shape.In both cases, there was noexperimental or field data againstwhich the theoretical model could becalibrated and there was felt to beconsiderable uncertainty in theresistance predictions. Therefore,model tests were performed asdescribed below.• From the above, the relocation fleetsnecessary to move a structure fromone site to another can be defined.The minimum acceptable average speedof relocation relative to the icesheet is probably about 5 miles perday, to ensure an adequate speed ofadvance in the face of opposingcurrents and ice movement. Inlandfast ice this could be slower,perhaps 2 miles per day. The averagespeed of advance might reach 20 milesper day in favorable conditions.Therefore in the Alaskan BeaufortSea, the structure could be moved ina matter of days.Additionalconsideredfactorsin orderthat mustto confirmbethefeasibility of winter relocation includemobilization of icebreakers to thedeparture site, breaking the structureout of any rubble field that may haveformed around it, de-ballasting thestructure and lifting off the seabed,checking out the arrival site,reballasting the structure to the seabedand demobilization of the icebreakers.The towing resistance of the structurewas determined to be the area in whichthe desk study was least reliable.Accordingly, a series of model testswere performed in cooperation with NKKat their ice model tank in Japan. Arectangular structure was towed atvarious speeds through ice sheets thathad been cut into blocks representativeof the sizes expected in the brokenchannel. Ice sheet thicknesses rangedfrom 2 to 4 feet (full scale). Theresults had similarities with thoserecently reported for a multi-leggedsemi-submersible in broken ice (Szeto,Rowe and Jones, 1987). An ice prowformed in front of the structure, asillus trated in Figure 3. However, thebehavior varied depending on theoriginal sheet thickness and on theconcentration (defined as the ratio ofice blocks per unit area to what was cutfrom a flat sheet, and intended to modelthe break-up of ice ridges). Theresults allow better definition of fleetrequirements related to the severity ofthe ice conditions along the relocationroute.Late Winter TechniqueInvestigation of the late wintertechnique has included many of the sameconsiderations as outlined above forearly winter. The late winter techniquethat is considered both feasible andcost-effective is for short moves inareas of stable ice. The governingenvironmental considerations are thethick and heavy ice condi tions and thelonger daylight hours than earlier inthe year. In stable ice, the use ofequipment and vehicles based on the iceis therefore feasible.From a number of schemes and variants,including methods for breaking orcut ting the ice, for clearing the iceand for providing motive force, the649

""-'1 MILESTART~-------------------------------------­/... - -----------------c:==J( ~ICEBREAKER~,--------------------------------------('-------MANEUVERINGMANEUVERINGFIGURE 2.BREAKING ICE BY STEAMING ON PARALLEL TRACKS.r SOLID ICE SHEET~ -PROW· OF BROKEN ICE BLOCKS() Q0000 00 0MOBILEARCTIC0 DRILLING0STRUCTURE\)'J\)0(JCHANNEL OFBROKEN ICE\:)BROKEN ICE IS-EXTRUDED' PAST STRUCTUREFIGURE 3.SCHEMATIC OF EARLY WINTER RELOCATION650

technique selected as most promising isillustrated in Figure 4. The principleis to cut the ice into large blocks,that are then moved around to behind thestructure by winch trucks based on theice sheet. The structure is movedforward using its own winches hauling onice anchors pre-installed along theroute.Once identified as promising, this latewinter technique was pursued with amajor Alaskan arctic constructioncontractor in order to improve technicaland cost definition. The principle areaof uncertainty was concluded to be incutting the ice blocks; ditch-witchescoulci be used but are probablyunreliable in thick ice. However highpressure water jets have been usedsuccessfully to cut fresh water ice, andto cut small-scale simulated sea ice(Coveney, 1981). The conceptual designof a framework attached to the structureand supporting a series of water jetssupplied from equipment based on thestructure has been developed. Althoughfeasibility is not in question, a fieldtrial cutting cold sea ice is requiredto define the relationships betweencutting depth, speed of advance, waterpressure, nozzle size and nozzlestand-off.The expected average speed of advance isbetween one half and one mile per day.Therefore it might take a full month tocomplete a 15 mile move and thefeasibility of longer moves would dependon specific conditions and experiencegained. Cost estimates for short movesare favorable and drilling programsplanned accordingly will benefit.Conclusions1. Studies have identified and developedtechniques that will allow mobilearctic drilling structures to berelocated in ice conditionsthroughout the year.2. For most purposes, there are twoscenarios that are concluded to becost-effective in the AlaskanBeaufort Sea and therefore deserveserious consideration in thedevelopment of an arctic offshoreexploration program. These are:TRACKED WINCH VEHICLEDDLARGE ICE~ DSEGMENTSDDDDD£6 FT THICK ICE SHEETANCHOR TUBE DROPPEDINTO HOLE IN ICE BLOCKICE CUTTING USING WATER JETSICE CLEARING USING WINCHESFIGURE 4.SCHEMATIC OF LATE WINTER RELOCATION651

• In Early Winter, as late as November,using three vessels from the existingicebreaker fleets;• In Late Winter, in March/April, usingthe "mechanical" method with somespecial equipment and vehiclesdeployed on the ice, to move shortdistances within stable ice.3. Winter relocation techniques offerthe potential for substantiallyreducing the per-well cost of arcticoffshore exploration, throughutilization of expensive plant thatotherwise stands idle for much of theyear.AcknowledgementsThe author is grateful to the StandardOil Production Company for its supportof the development of this technology.Thanks are also due to other companiesthat have participated in the studiesoutlined, including but not limited toAIC-Martin, Brian Watt Associates,Bugsier Towing, Canmar, Flow Industries,Nippon Kokan K.K., Swan Wooster andWartsila Arctic.ReferencesCoveney, D.B. 1981. "Cutting Ice with"High" Pressure Water Jets". 6thInternational Conference on Port andOcean Engineering under ArcticConditions, POAC 81, Quebec, Canada.Kimmerly, P.C. and Jones, K. 1986."Ice Management Procedures forSpecialized Drilling Structures".International Polar TransportationConference, IPTC 86, Vancouver, Canada.Szeto, K., Rowe, J. and Jones, S.J.1987. "Observation of a Model-ScaleSemisubmersible in Pack Ice". SixthInternational Offshore Mechanics andArctic Engineering Symposium, OMAE 87,Houston, Texas.DiscussionK. CROASDALE: How confidently can youextrapolate the NRC work on the use ofhigh pressure water jet (say to ice 7 ft.thick)? Are you confident that highpressure water jets can be used inambient conditions down to -40°C?G. THOMAS: Our work in this study hasbeen to review previous work on cuttingice and other materials using water jets.NRC performed tests on both fresh waterand artificial saline ice of up to 2-3 ftthickness, using parameters of nozzlesize, water pressure and traverse speedthat would be close to those expected forthe application described in the paper.CRREL have performed tests on fresh waterand saline ice up to 30 inches thick, butusing much smaller nozzles, higherpressures and faster traverse speeds.Out analysis of the data gives us confidencethat a system can be designed tocut 7 ft. thick saline ice, and thereforethe technical feasibility is consideredconfirmed in principle. Nevertheless, wewould propose a field test as a necessaryprerequisite to being able to specify asystem with the actual pump pressures andvolumes, and volumes and nozzle sizesthat would be required.Regarding temperature, it is envisagedthat trace heating to all pipework wouldbe required. On very cold days it mightbe necessary to suspend operations;however, records show the median temperaturein the Alaskan Beaufort Sea inMarch/April to be about -25°C.G. VARGES: Can you explain in moredetail the mechanical transport of iceblocks from the front end of the drillingstructure to the aft end of the structurein the late winter methods?G. THOMAS: The procedure is indicated inFigure 4 of the paper. This Figure showsa particular type of arctic drillingstructure that has a wide base forfounding on the seabed. During relocation,the structure will float with thebase submerged so that ice blocks can bemoved over the top of it. Figure 4indicates that the channel cut in the icesheet is wide enough to allow the largeice blocks to be towed on the open waterto either side of the central core of the652

structure, and floated over the submergedbase.The cutting procedure that will work bestin level first year ice is to cut thesheet into large square blocks that maybe 50 feet on a side. One or more iceanchors will be fixed in the blocks, andtowing wires will be run to winch trucksthat are located on the undisturbed icesheet to either side of the cut channel.The winch trucks tow the ice blocks andstack them behind the structure. Additionalplant will ensure that sufficientspace is maintained behind the structureto allow the process to continue.I might add that these procedures havebeen developed in consul tation withcontractors who are experienced inoffshore arctic operations in winter.653

UTILIZATION OF COMPOSITE DESIGN IN THE ARCTIC AND SUB·ARCTICBen C. GerwickUniversity of California, Berkeley, California, USADale BernerBen C. Gerwick, Inc., San Francisco, California, USAAbstractComposite steel and concretestructural elements are being developedby a number of organizations around thewor ld. The prirrary noti vat ion has J::eento develop a structural element, whichwill serve effectively as a peripheralwall to resist high local pressures ofice floes and icebergs. All of thesestudies demonstrate the efficiency ofconposites in resisting such loads, andtheir excellent ductility and energyabsorption characteristics. Compositeconstruction can also apply to thebulkheads which support the peripheralwall, especially since they also areoften subjected to out--of-plane flexure.Horizontal slabs are structurallyefficient but difficult to construct.For these, corrposi te steel, with trussedreinforcement, can act as a stay-inplace,self-supporting soffit, as wellas participate structurally. This alsoapplies to the base of gravi ty-basestructures, which often are constructedoff the ground due to the need forskirts. During installation andThis is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987. © The Geophysical Institute,University of Alaska, 1987.service, such base slabs are subjectedto "hard points" and ridges. Where longconcrete skirts are to be employed, useof composite steel-concrete constructionshould facilitate construction whilealso providing an airtight membraneduring launching.The use of composite constructionoften produces cost, weight, andschedule advantages as well as offeringsuperior structural performance.Conposite construction, by eliminatingcover, permits lighter overall weight,thus reducing draft, which is ofcritical importance for the transit ofshallow water. Shipyard fabrication andlaunching of the steel shell of the baseand lower walls eliminates the need forlarge graving docks, which is ofparticular importance for remote areas.The double membrane of steel ensures oiland water tightness even after damagefrom irrpact. Also, design for ductilityand energy absorption nay prove to bethe rational approach to resistingintense local loads and irrpacts of lowfrequency of occurrence.IntroductionSteel-concrete composites have hada long and successful history instructures ranging from bridge decks tohigh-rise floor slabs. However, the655

Horizontal diaphragm ribs and slabs arevery efficient in distributing iceloads. Both of these =nfigurations aredifficult, time-=nswning and cootly to=nstruct in =nventional reinforced=ncrete. Prefabricted double-wallsteel sections, properly =nnected byshear elements, can be self-su~rting,thus enabling e=nanical filling with=ncrete by tremie or pumping.Where internal air pressurization isto be errployed, the use of the steelmembrane provides an air-tightenclosure. Conversely, where watertightness or oil tightness is =ncerned,the steel plates will span any localcracks and prevent in-or-out leakage.Indeed there are preliminary indicationsthat the sandwich plates may meetregulatory body requirements for adouble hull.Considering all of the advantages ofcomposite sections it is important togain an understanding of the behavior ofthis relatively new type of=nstruction. In the nwnerous studiesof composite design cited above, a largenwnber of different =nfigurations havebeen examined. An examination of thetest results yields significant insightinto the behavior of composites andfactors that affect their design.However, before examining some of thegeneral details of =nposite design, aswell as areas meriting additionalresearch, it is valuable to brieflyexamine some of the principles of iceloading resistance.Principles of Shear ResistanceThe worst-case impact loading forthe design of a peripheral wall for anoffshore =ncrete structure typicallyoccurs for mid-span loading between thebulkheads or su~rt walls. For thisloading =ndition, "punching shear" isthe =ntrolling failure mode which mustbe resisted. Design by conventionalbuilding code formulae for the levels ofpunching shear en=untered in irrpactloading can result in excessive wallthickness and weight. However, recentresearch indicates that it is possibleto prevent a punching shear failure modein a peripheral wall, while maintainingthe ability to carry high flexuralbending and shear. The followingdiscusses some of the principlesinvolved in the resistance to punchingshear.The failure mode ccmnonly termed"punching shear" is actually acanbination of both flexural and shearstresses. Flexural cracks alter thestress paths in the wall and unless thewall is properly designed to carrystress along the new paths, the wall mayfall in punching shear. Thus in orderto achieve an efficient design, it isessential to balance the design of theflexural and shear steel.Shear is related to flexure by thefollowing equation:V= om/ ox =O(Tjd)/ x = (jd) (OT/ x) +(T) (Ojd/Ox) eq. 1Where: V = ShearM = MunentT = Tensile force in flexuralsteeljd = Moment armx = Distance in the longitudinaldirectionsEquation I separates shear carryingcapacity into its two corrp:ments (1)beam action,(jd) (or/ox) and (2) archaction, (T) (ojd/ x).In pure beam action, for reinforcedand =rnposite concrete members the=ncrete wi thin the web of the memberacts as a series of diagonal =rrpressi vestruts which carry stress from the=rrpressive face of the member down tothe flexural steel. These compressivestruts =ntinuously change the value ofT along the length of the flexuralsteel, thus ac=unting for the term (OT/x) in equation 1, while the IOOIDe11t arm,jd, remains =nstant. As shown inFigure 1, the vertical reaction of thesteel tie, T t , balances the forcetriangle between the diagonal=rrpressi ve strut, C , and the change inforce In the flexuraf steel,AT to form astrut-tie mechanism in the truss analogy.656

F==========*==============~!p Ooad)II,,tI,R (reaction)L\T,IIIII,.Figure 1. Strut-tie mechanism in beam action.relatively recent advent of steelroncrete-steelsandwich ronposi tes hasbeen demonstrated to be particularlywell adapted for use in the offshoreArctic and Sub-Arctic envirol1llE11t. Inearly work reported by Matsuishi et ale(1977) , sandwich carposites were shownto be able to resist high local loadingand it was reported that arch actionthrough the roncrete was primarilyresponsible for the high capacity.Subsequent published research hasronfinned and extended Matsuishi' sfinding. Numerous proprietary studieshave extended this early work onsandwich ronposites. These includeresearch and tests by ABAM Engineers,Brian Watt Associates, Taylor Woodrow,Mitsui-Kajirna, Shimizu, C-FER, and theU.S. National Bureau of Standards.Valuable research and development onsteel-roncrete ronposi tes has also beencarried out at the otto Graflaboratories in Stuttgart. Our own firmhas had the 0HJOrtuni ty to workextensively in this development, sUHJOrtedby Joint-Industry groups.Advantages of Composite ConstructionThe steel hulls of icebreakingships and exploratory drilling platformshave experienced denting of the exteriorplate, buckling of scantlings (stiffeners)and, most serious of all, bucklingof frames, due to high local icepressures. The buckling of scantlingsis, in part, due to the membranecompression forces which are inefficientlycarried by typical steel framing.On the other hand, wi th ronposi teronstruction, shell forces are resistedin la:rge part by arch action andmembrane compression in the roncrete.This enables longer spans betweeninterior frames or bulkheads, which area major cost and weight item. Sincetheir design is controlled by localloads of high intensity, increasing thespan does not significantly change thedesign axial rompression in thebulkhead. The required thickness ofexterior steel plate is reduced, sincelocal stiffness is provided by theroncrete backing. This in turn reducesfabrication costs and reduces thedemands on plate ductility and throughthicknessproperties.Carposite structures are not subjectto brittle modes of failure at lowtemperatures, or under high rates ofiI!p3ct loading. Also, the naturalfrequency of composite sections issignificantly lower than for all-steelstructures, thereby greatly reducing the657

possibility of dynamic resonance undercontinuous ice crushing. Such dynamicresponse in steel structures, forexample, can result in structuralvibration and potential liquefaction ofinterior sand fill. Of specialimportance, properly designed compositestructures can develop the largedeflections required for catenaryaction, thus giving large values ofdeflection ductili ty and energy absorption.Heavily reinforced and prestressedconcrete shell elements require coverover the prinary reinforcement i thus,their total thickness is increased ascanpared with that of the equivalentsteel-concrete corrposite. As the strainin the extreme fiber reaches 0.3%, thecover spalls. Deflection ductility maystill be obtained provided the core ofthe concrete is properly confined, butthere is a temporary drop in capacity.This does not occur to the same extentwith the composite section because allof the concrete is confined.Composite construction permits theprefabrication of standardized doublewallsections in a shipyard and theirjoining and launching by conventionalfacilities, thus making a constructionbasin unnecessary. The double hullsection may be launched and then theconcrete fill placed while the structureis afloat. These aspects, canbined withthe overall lighter weight of compositeconstruction as canpared with steel,make it practical to construct gravitybase structures in relatively shallowharbors.Many of the concepts proposed forArctic structures have sloping exteriorsin order to reduce global ice forces.For relatively deep reinforced andcomposi te concrete members, the shearstress is carried from the point ofloading directly to the sUJ.:POrts alongprincipal stress trajectories forming atied-arch (Figure 2). The change in theheight of the IllOOlent arm, jd, along thearch accounts for the term ( jd/ x) inequation 1 above, while the horizontalcorrpressive force in the arch, H,remains constant. In arch action it ispossible for the horizontal compressiveforce in the arch, H, to exceed thetensile force, T, in the flexural steel,if either bom slip or yielding of theflexural steel occurs and the sUJ.:POrtelements of the wall can sustainhorizontal thrust. In such cases it isappropriate to substitute the term Hinto equation 1, in place of the term T.Figure 3 shows results based on workby Leonhardt (1965) for sirrply supportedconcrete beams without shearreinforcement. The figure shows theinherent strain incorrpatibility of archand beam action. As span lengthincreases, the shear capacity of archaction decreases while the capacity ofbeam action increases, when expressed asa percentage of the theoretical flexuralcapacity of the beam. Figure 3 alsoshows that, within a critical range ofspan to depth ratios, the shear capacityof a member without shear reinforcementmay be inadequate to develop the fullIllOOlent capacity of that member,resulting in an abrupt shear, orICE PRESSUREFigure 2. Tied-arch action (after Fotinos 1985).PRINCIPALSTRESS DIRECTION658

~ 100.'80.-..- rou ......Q);....oQ)..cE---604020o~. .. ,..... 1'..... ,... '-Capacity of....... / "Arch Actiono 1 2 34 5- .. _--"Beam Action"Without Ties6 7 8 9Shear Span/Depth Ratio, aidM/VdFigure 3. Hom. cap. vs. shear span/depth ration (after Leohhardt 1965).punching shear failure. Under theextremely high shear loads developed ina peripheral wall during inpact loadingit is possible to develop the samephenomena even when the members haveshear reinforcement. Ha.vever, wi thproper design it is possible to enhancethe shear capaci ty of a membersufficiently to develop the full momentcapacity of a member by the awropriateuse of transverse and longitudinalsteel.Ductility and OompositesFigure 4 shows an exanple of thelevels of ductility possible for apassively confined beam. In this figurethe yielding of the flexural steel islabeled as the proportional limit, point(a). In conventional design, yieldingof the flexural steel would delineatethe ultimate capacity of the beam.However, due to the ductility providedby passive confinement, the stresses canbe redistributed so that the horizontalcorrpressive farces in arch action, H,can exoeed the tensile forces, T, in theflexural steel, thereby increasing theultimate capacity of the beam asindicated by point (b) in Figure 4.In conventionally-designed deepbeams, the development of arch action isa=npanied by an outward migration ofthe neutral axis due to the deformationof the arch as it picks up load. Thisoutward migration of the neutral axisdecreases the cross-sectional area ofthe arch which reduces the momentcapacity of the beam. This effect isclearly shown in Figure 3 where the archaction decreases rapidly with theincreased deformation of the archassociated with increasing span length.Passive confinement can providesufficient ductility to the concrete sothat when the plastic positive momentcapacity is reached, the negative momentthen increases, alla.ving for additionalload.Passively confined concrete alsoleads to the formation of plastic hingesat mid-span and over the suwart wallsthus permi tting the section to undergolarge deflections. As sha.vn in Figure4, point (c) , these deflections canbecame sufficiently large that659

Member With a Lower Degree ofPassive ConfinementMember With a Higher Degree ofPassive ConfinementPost-Ultimate/ /)":;o',,""7':'I'7'--Catenary Action(c)§ Elastic Energy Absorbed~ Pre-Ultimate PlasticEnergy Absorbedtz2) Post-Ultimate PlasticEnergy AbsorbedDEFLECTIONFigure 4. Load-deflection curve for a passively confined beam (after Fotinos 1985).addi tional load is carried in catenaryaction. Irrleed recent tests have shoNnthat catenary action can sustain loadsequal to or exceeding the proportionallimit of the peripheral wall throughdeflection ductility values as great as30.Figure 4 also indicates thatrelatively large amounts of energydissipation can be achieved by means ofpassive confinement. The energydissipated up to the ultimate limit isrelatively srrall, whereas the very largeamounts of energy dissipated in thepost-ultirrate range provide added safetyagainst a progressive collapse.General Details of Sandwich Corrqx:>si tesSteel/concrete composites offer anatural means of providing passiveconfinement. Composite concepts offerthe ability to prefabricate sections,which can ease construction proceduresespecially in congested areas. As shownin Figure 5, corrposi tes can befabricated in several different ways;however, the steel/concrete/steel/sandwich configurations shown in 5b andc are especially well suited to passiveconfinement. The heads on theoverlawing welded studs shown in Figure5b are very irrportant for the adequatedevelopnent of l::orrl with the concrete.The mid-depth longitudinal steel alsoshown in Figure 5b contributes to thepassive confinement by crossing andresisting the diagonal shear cracks.This mid-depth longitudinal steel alsoserves to distribute the diagonal shearcracks, thus minimizing their width. Asshown in Figure 5c, plate steeldiaphragms and stiffener bars can alsobe utilized effectively for sandwichcomposite construction.660

(a)T············." .." " ". ".....Longitudinal SectionTransverse SectionMid-Depth Reinforcing SteelWelded Headed Studs(b)Longitudinal SectionTransverse Section(c){II II ~fFigure 5.Longitudinal SectionSome typical forms of compositeTransverse Sectionconstruction (after Fotinos 1985).Recent research by Nojiri et ale(1986) on carposite spec:iIrens, withadequate reinforcement to providepassive confinement, has shown that thespecific arrangement of the reinforcingsteel is not as irrp:>rtant as thepercentages of steel. This researchalso showed that changing the spacing,orientation, or kind of reinforcement,within limits, resulted in little or nochange in the ultiIrate capacity of thespecimens tested, provided that thesteel was adequately anchored.Obviously, the steel plates mlst beprovided with adequate protectionagainst corrosion and abrasioncorrosion.The development of denseepoxies and polyurethanes offers pranisein this regard. When steel anror plateshave been attached to the concrete wallsof river locks which were subject towater absorption and freezing, the watervapor has migrated to the cold face andfrozen behind the plate leading tofreeze-thaw disintegration. The use ofthe double~alled steel elements and theuse of ION permeability concrete of IONwater absorption should eliminate thispotential problem.The performance of steel concretecorrposi tes is very dependent on thecompressive strength of the concretecore. The advent of extremely highstrength concrete made possible bysuperplasticizers and condensed silicafume, promises to increase theefficiency of corrposite construction.High strength, lightweight concreteappears especially attractive forcomposite construction.Philosophical ImplicationsThe performance ofsandwich compositesteel-concreteraises sore661

interesting questions regarding thephilosophy of design for concentratedlocal loads and inpact loads, especiallywhere their occurrence on anyone areaof the structure is of very lewfrequency. Such events bear a strongresemblance to earthquakes, for which wehave adopted concepts of energyabsorption and ducility rather thanelastic and elastoplastic strength.Like earthquakes, intense local loadsfrom ice tend to spread and diminish inintensity under large strains anddeflections. The ability of a compositewall section to deflect up to one meter,for example, without reduction inresisting force, indicates that theremay be far better mechanisms forresisting impact loads than juststrength.SUllIllaI)'Composite steel and concretestructural elements have beendemonstrated by numerous researchers tohave excellent potential for use inoffshore Arctic and Sub-Arcticplatforms. Some of the promisingfeatures include: (a) resistance tolocal indention and buckling under iceleading; (b) high load-resistingcapacity; (c) resistance to brittlefailure modes at low temperatures andhigh rates of loading; (d) ability tocarry membrane forces around theperimeter of the platform for optimumdistribution; (e) ability to maintainfull load carrying capacity into thecatenary range, with deflection ducilityfactors of 30 and above; (f) lownatural frequency which inhibitsresonance effects with ice loading; (g)excellent confinement of the coreooncrete; (h) no potential for spallingof cover; (i) self-suHJQrting forrrwork;( j) potential for rapid constructico inshallow draft basins or shipyards; (k)potential for ease in forming horizontalelements; and , (l) a double air, waterand oil-tight membrane.Reccmnended Areas for Future ResearchComposite structures offer newoptions and/or potential problems inboth design and construction over thoseuse in oonventional practice. Howevernot all of these options or potentialproblems have been fully explored ordelineated. The following is a list ofsame future research areas.1. Concepts of self-suHJOrting forrrwork2. Freeze-thaw performance and thepotential for internal ice lensformation.3. Longer span to depth ratios.4. Better delineation of the influenceof boundary conditions and plateaction.5. Use of sandwich composites asinternal bulkheads and diaphragms.6. Peripheral wall-bulkhead joints(nodes) •7. Methods of repair of permanentlydeformed composite elements.ReferencesFotinos, G. C., "CaTpOsite Steel­Concrete Ice Resistant walls for ArcticExploration Structures, " Presented atthe American Society of Civil EngineersArctic Engineering Conference, March1985.Hattori, Y., Ishiharna, T., Yarnanoto, T.,Matsuishi, M., and Iwata, S., "On theUltimate Strength of CaTpOsite Steel­Concrete Structures," Proceedings ofthe 8th International Conference on Portand Ocean Engineering Under Arctic Conditions,Vol. 1, Sept 7-14, 1985,Narssarssuaq, Greenland, pp. 445-454.Leonhardt, F., "Reducing the Shear Reinforcementin Reinforced Concrete Beamsand Slabs," Magazine of ConcreteResearch, Vol. 17, No. 53, December1965, pp. 187-198.Matsuishi, M., Nishirnake, K., Takeshita,H., Iwata, S., and Suhara, T., "On theStrength of New Catposite Steel-COncreteMaterial for Offshore Structure,"Proceedings of the Offshore TechnologyConference, OK: No. 2804, May 2-5, 1977,Houston, Texas, pp. 589-594.Nojiri, Y., Koseki, K., Yoshiki, T., andSawayanagi, M. "Structural Behavior andDesign Method of Steel/Concrete IceWalls for Arctic Offshore Structures,"Proceedings of the Offshore TechnologyConference, Vol. 3, OK: No. 5292, May 5-8, 1986, Houston, Texas, pp. 597-04.662

DESIGN AND BEHAVIOR OF COMPOSITE ICE-RESISTING WALLSP. F. AdamsT. 1. E. Zimmerman1. G. MacGregorCentre for Frontier Engineering Research, Edmonton, Alberta, CANADAAbstractThe exterior walls of arctic oiland gas production structures will besubject to large, concentrated iceforces. Composite steel/concrete wallshave been proposed as a cost-effectivesolution to resisting these forces.The research discussed in thispaper has investigated the behaviourand failure mechanisms of compositewalls, so that recommendations can bemade concerning analysis and designmethods and effective constructiondetails. The research involved thedevelopment of design-oriented ultimatestrength models, as well as thephysical testing of composite wallspecimens.Results are presented infor tests on composite beamThe high strength and greatof this form of constructionstrated. Effective designutilizing empirical designand limit-analysis plasticityare given.this paperspecimens.ductilityis demonmethodsequationssolutionsThis is a reviewed and edited version of a paperpresented at the Ninth International Conference on Portand Ocean Engineering Under Arctic Conditions, Fairbanks,Alaska, USA, August 17-22, 1987.IntroductionOffshore oil and gas productionstructures which operate year round inthe harsh environments of the Arcticand off the Canadian east coast must bedesigned to resist very large, concentratedice loads in a safe andefficient manner. In an effort to findcost-effective solutions to the designof ice-resisting walls for thesestructures, composite steel/concretewalls have been proposed. The generalstructure of a composite ice-resistingwall as shown in Figure 1 consists ofoutside and inside steel plates,fastened together with diaphragm plates(or by some other means), and concretefill in between.Several advantages of compositeice-resisting walls are alreadyrecognized; simplified construction,reduced material and stiffeningrequirements for the steel plates, andimproved load distribution (Gerwick1985; Zinserling and Cichanski 1986;Rojansky and Hsu 1985; Bruce andRoggensack 1984). In addition, recentresearch has shown that composite wallscan exhibit very high shear andflexural strengths, as well as thepost-failure ductility required toprevent a progressive collapse failurein the event of a local overload(Nojiri and Koseki 1986; Shioya et al.663

1986; Hattori et al. 1985; Matsuishi etal. 1977, 1978, 1980).The research discussed here is thefirst phase of a larger researchproject which is investigating thebehaviour and failure mechanisms ofcomposite walls. The goal is todevelop recommendations concerninganalysis and design methods, effectiveconstruction details and optimum sizesof the various wall components. Theresearch involves the development ofdesign-oriented ultimate strength andfinite element models, as well as thephysical testing of representativecomposite wall specimens.Scope and ObjectivesIf a particular composite walldesign is to be considered for use inan offshore structure, it must satisfythree basic requirements:1. Strength: the wall must possesssufficient capacity in both flexureand shear to resist very high iceloads;2. Ductility: following a localfailure, the wall must possesssufficient strength through largedeformations to ensure the preventionof a progressive collapse;compos>,e_BulkheadrWalls. '"'' ".3. Economy: details of the wall mustbe sufficiently simple so as toreduce the fabrication and theconstruction costs by a significantamount below the costs for aconventional steel or reinforcedconcrete wall .In developing composite walldetails, the last of these requirementsmust always be kept in mind. If acomposite wall design satisfies thefirst two criteria, but not the last, aconventional design will undoubtedly beused.Inner Steel PlateFigure 1Composite Ice-Resisting WallConventional, all-steel iceresistingwalls are composed ofheavily-stiffened plate panels, withthe skin plate spanning betweenclosely-spaced T-ribs and the T-ribsspanning between closely-spacedbulkhead frames. An extensive amountof labour-intensive welding isrequired. Conventional, reinforcedconcrete ice-resisting walls arecomposed of thick sections containinghigh percentages of reinforcing steelin all directions (Gerwick 1985). Thisconstruction method is also labourintensive,requiring the difficultplacing of highly-congested reinforcingbars.The philosophy adopted for thisresearch work, then, was to develop acomposite wall scheme which would avoidthe high cost items associated with thetwo conventional wall types and toshow, through a physical testing664

program, thatalso satisfystrength andments.the chosen scheme couldthe appropriate highhigh ductility require-In order to evaluate the largenumber of composite schemes that are'possib1e (see, for example, Nojiri andKoseki, 1986) and to choose thosesimple schemes with a reasonable chanceof success, it was important to developa preliminary analysis and designmethod to estimate the capacity ofthese walls. An initial assessmentshowed that a limit analysis approach,which utilizes lower bound plastictheory, appeared appropriate formembers of this type.With all of the foregoing in mind,the objectives of the research programwere set out as follows:1. Ascertain the present level ofknowledge regarding composite iceresistingwalls specifically, andsteel or concrete ice-resistingwalls in general;2. Ascertain the current state-of-theartregarding plasticity conceptsin reinforced concrete design, as aprelude to using these concepts incomposite design;3. Develop a basic, simple compositewall configuration which avoids, asmuch as possible, welded stiffeningdetails and complicated reinforcingschemes;7. Prepare recommendations concerningoptimum structural configurations,as well as analysis and designmethods.Thisfrom theprogram,items 3,Ice Loadingpaper considers the resultsfirst phase of this overalland deals essentially with4, and 5.The ice load to be used in thedesign of an offshore structure isstill the subject of much debate andresearch. It is known that local icepressures are primarily a function ofice type, temperature and salinity, thegeometry of the ice feature, thegeometry of the structure and the rateof loading (Allyn, 1986). In addition,it is well-recognized that theeffective ice pressure decreases as theloaded area increases (Croasda1e,1984).A reasonable consensus of availableinformation is that ice pressuresappropriate for the design of iceresistingwalls are between 10 MPa and15 MPa for concentrated loads on smallareas (less than 1 m2) and between 3and 5 MPa for loads on larger areas(5-10 m2). For the purposes of thisresearch work, these values areconsidered appropriate.Design Assumptions4.5.6.Test a series of 1/4 scale beam andslab specimens subject to transverseloads;Develop limit analysis methods,which use concrete plasticityconcepts and incorporate thesemethods into an overall designapproach;Evaluate existing finite elementcomputer programs to determinetheir applicability in modelling aproblem of this type and developappropriate modelling techniques onan existing program;As a result of discussions with anumber of engineers familiar witharctic offshore structures, thefollowing assumptions were madeconcerning the design of a compositewall:1.2.The composite wall spanstally between verticalsupports, resulting in aslab system;Limitations on bulkheadrequire a bulkhead spacinggreater than 5 or 6 m;horizonbulkheadone-waycapacityof not3. The wall thickness should be largeenough to allow a worker access tothe inside of the wall during665

4.fabrication; ie. not lessapproximately 1 m;thanSteel plate thickness greater than35 mm would not be considered dueto the resulting welding difficulties;the latter load case. The tests alsoprovide information to be used in thedesign of slab specimens subject tohigh concentrated "punching" loads(where three dimensional effects aremore important), for a second phase ofthe research program.5.6.7.Although in-plane restrainingforces in the wall may increase itscapacity, the magnitude of theseforces are not easily determinedand will conservatively beneglected for the initial work;Under "service" loads, the wallshould be of sufficient stiffnesssuch that cracking is not excessiveand deflections remain small in theelastic range;The "ultimate" capacity of the wallis considered to have been reachedwhen "arch" action breaks down dueto: yielding of the steel tensionplate; shear/compression failurein the concrete, or; bearingfailure. The catenary action ofthe steel plates may provideadditional capacity which will beavailable to help prevent acatastrophic failure.Test ProgramTesting ProcedureThe test frame constructed for thebeam specimen tests is shown in Figure2. The specimen is inverted in thistest frame, with the load being appliedfrom the bottom, with a total ofsixteen, 45-tonne hydraulic jacks (inpairs of two, at eight load points).The total load that can be applied to aspecimen in this frame is 7120 kN,distributed along the length of themember, dr 890 kN applied at anyonelocation. The specimens were allsingle span, with cantilevered endsproviding continuity over the supports.The majority of the specimens weretested with all the load points acting,intending to simulate a uniform load onboth the main span and on the cantileveredends of the beam. A total offourteen beam specimens were tested bythe authors and an additional fifteenspecimens were tested in the same frameby O'Flynn and MacGregor (1987).GeneralThe two ice load cases that cangovern the design of an ice-resistingwall are a concentrated load giving ahigh pressure on a small area, or alower pressure acting on a larger area.In the concentrated-load case, threedimensionaleffects are important,since as the load is transferred tobulkhead supports it will distributelaterally and, thus, a larger"tributary area" of the wall will beavailable to resist the load. For thelower-pressure case, the problem isessentially two-dimensional, with theeffect of the third dimension beinglimited to the imposition of acondition of plane strain.The beam specimen tests describedin this paper are intended to considerHydraulicLoadingJacksConcreteReactionBeamFigure 2Test Frame666

Specimen DetailsThe wall configuration which thepreliminary design work selected forinitial study is the simple configurationshown in Figure 1. The outer andinner steel plates are connectedtogether with continuous, verticaldiaphragm plates at the bulkheadsupports and at the third points. Thediaphragm plates are welded to both theouter and inner plates with continuousfillet welds. Only vertical diaphragmsare used to facilitate concrete placingand worker access. The wall containsno other welded details.The dimensions of the two specimensdiscussed in this paper are shown inFigure 3. The two specimens, CF-2 andCF-3, were identical in all respects,with the exception of the boundaryconditions that were imposed at thebulkhead support locations. SpecimenCF-2 utilized steel rollers at thesupports, which virtually eliminatedany horizontal restraining force.Specimen CF-3 utilized Teflon-coatedsteel plates for this same purpose;however, due to friction, the Teflonplates provided a small horizontalrestraining force (approximately 3% ofthe normal force). As will be shown,this small boundary restraining forcehad a dramatic effect on the failuremode of the specimen.Materialsfollows:f'cFyFuproperties60 MPa270 MPa390 MPa.Test Results for Specimen CF-3wereFigure 4 shows specimen CF-3 atthree different stages of loading.Loads are given in MPa (N/mm2) andrepresent a uniform pressure over thetributary area of each load point (200mm x 375 mm). A load-deflectiondiagram for this specimen may be seenin Figure 5.II+ II lIIT T T T T T II(a) Load - 4.0 MPa(b) Load -6.0 MPaas1000"(c) Conclusion of TestLoad - 9.4 MPaFigure 4Specimen CF-3Figure 3Specimen Dimensions (mm)At an applied load of 4.0 MPa themember was uncracked; very slightseparations had opened at particularlocations between the concrete and thediaphragms. These separations closedagain if the load was removed. Thedeflection at this point was approximately3 mm. By the time a load of 6MPa had been reached, diagonal shearcracks had formed adjacent to eachsupport. A flexural tension crack had667

also formed in the concrete atmid-span. Load cycling at this loadlevel caused no further progression ofthese cracks. At a load of 9.5 MPa,shear failures occurred in the crackedregions at each end of the beam. Theload subsequently dropped to approximately7.5 MPa, where it remainedreasonably constant as the specimen waspushed through significant deformations.As deformations became large,the specimen began to take more load.At the conclusion of the test (thespecimen could be pushed no further inthe test frame), the load had nearlyreturned to what it had been at firstfailure. When the test was terminated,the specimen, in the damaged conditionshown, was still able to carry a loadof almost 9.5 MPa. Both inside andoutside steel plates had yieldedextensively, but were not fractured atany locations.tension plate yielded in the centrethird of the specimen. Both the midpanflexural crack and the gaps at thediaphragms opened up as deflectionsbecame large. Failure of the specimenoccurred at a load of 8.2 MPa when thetension plate fractured in the heataffected zone (or adjacent to it) atthe diaphragm/tension plate weld.Following this failure, the specimenhad no further load capacity. Thefailed specimen is shown in Figure 6.A load-deflection curve for thisspecimen is shown in Figure 7.Test Results for Specimen CF-2Up to a load of approximately 6MPa, specimen CF-2 behaved identicallyto CF-3. At this point however, aflexural failure began to occur, as theFigure 6Specimen CF-2 at Failure111110987

Discussion of ResultsThe two specimens were identical,yet exhibited dramatically-differentmodes of failure. The reason for thisis believed to be the differentboundary restraint conditions imposedat the supports. The horizontalfriction developed at the support inspecimen CF-3, gave it a higherflexural capacity than the steeltension plate alone would provide.Referring to Figure 8:more ductile flexural failure wouldundoubtedly have been exhibited byspecimen CF-2, if not for the presenceof the diaphragm fillet welds on thetension plate (a/Flynn and MacGregor1987) .The following comments can be madeabout the test observations:1. The initial response of the specimenswas stiff, with few cracks atloads below 5 MPA.Muwhere(d-t) (T+JLR)Mu ultimate momentresistanceT tensile force in thesteel plateC compressive forceJL coefficent of frictionR support reactiond - wall thicknesst plate thickness.[1]2.3.4.The crack patterns suggest adefinitemember;"arching" action in theLoad cycling fromresulted in nopropagation;zero to 6 MPaprogressive crackA moderate amount of in-planerestraint can change the failuremode from one of flexure to one ofshear. This mayor may not bedesirable;R-L~R5.The flexural and shear capacitieswere both very high; greater thanthe target load of 5 MPa(corresponding to the load caseappropriate to these beam typespecimens) ;6. The shear failure mode was ductile,indicating a large capacity toabsorb energy.Figure 8Free Body DiagramIncluding Support FrictionThis increase in flexural capacityallowed the specimen to support moreload, until its shear capacity wasreached.On the other hand, specimen CF-2failed in flexure, as the tension plateyielded, strain hardened and thenfractured. While both failures couldbe termed "ductile", the ductility ofthe shear failure is clearly superiorto that of the flexural failure. Itshould be noted, however, that a farDesign MethodsFlexureThe "truss" model shown in Figure 9can be used for the purpose of flexuralcapacity calculations. This modelassumes that it is inappropriate toconsider the shear stresses asuniformly distributed over the depth ofthe cross-section. Instead, loads areassumed to be transferred directly tothe supports (or diaphragms) via aseries of concrete compressive"struts". This method, which is alower bound plasticity method, issimilar to that suggested in theCanadian concrete building code for thedesign of deep beams (CSA 1985; Marti669

1985). The steel plates are not bondedto the concrete fill, so that straincompatibility is not enforced at thisinterface. Therefore, load transfercan only take place between theconcrete "struts" and the steel tensiontie at the location of a diaphragmto-plateconnection. This model givesa reasonable prediction of the stressesin the two longitudinal steel plates aswell as the diaphragm plates.Use of this model gives thefollowing equation for ultimate flexuralcapacity:There was no plateau in the loaddeflectioncurve for specimen CF-2(Figure 7) associated with the tensionplate reaching the material yieldplateau. This appears to be due to thepresence of the diaphragms and theresidual stresses caused by theirwelding. Similar specimens withoutdiaphragms did exhibit a marked yieldplateau (O'Flynn and MacGregor 1987).Despite the increased capacity ofSpecimen CF-2, equation [2] isrecommended for design.ShearwhereMu- AsFy(d-t)As area of tension plateb tFy yield strength.[2]Three methods have been developedto calculate the ultimate shearcapacity of composite walls: 1)empirical equations; 2) a plastic trussmodel; and 3) an energy method.1. Empirical ApproachIce Load~JJ~~~~J~J~~~J~~~~J~J~tJJJJJJThis approach involves the use of anequation derived by applying thetechniques of dimensional analysis andstatistical regression analysis (Zsutty1968) to the data from this researchwork and from the work of O'Flynn andMacGregor (1987). The derived equationis shown below:Composite Ice-Resisting Wall3.39 f,0.8 b d pO.54c(a/d) 1. 29[4]Assumed Flow of ForcesFigure 9Plastic "Truss" ModelThe ultimate load attainedspecimen CF-2 actually correspondeda flexural capacity calculated byfollowing equation:Mu - AsFu(d-t)where Fu - tensile strength.bytothe[ 3]whereVu ultimate shear capacity,Nf~ concrete strength, MPab beam width, mmd beam depth, mmp reinforcement ratio(tension plate)As/bda = shear span, mmThe equation is used to calculatethe shear capacity at a number ofdifferent sections a distance "a" fromthe support and determine which iscritical. Loads which act between thesupport and the section beingconsidered are ignored (i.e., it isassumed that these loads are transferreddirectly to the support andtherefore do not affect the sectionbeing considered).670

This equation can be simplified,with a negligible loss in accuracy, asshown below:1.35 f' b d pO.5c(aid) 1. 3[5]In Figure 10, equation [5] is comparedto the test results.201.B -C·uco 16a.-coU 1.4 -~coQ)12J::(f)"0 1.0Q)t)"0 O.B~a.. 06co~-::Jt)«0.4 f-. r •• I •- •• • ..I• • •average = 1.00.2 f- standard deviation = 0.1100I I I I IO.B 1 2 1 6 2.0 2 4 2.Baid (shear-span/depth ratiO)Figure 10Empirical Solution2. Plastic Truss ModelThis is a lower bound plasticityapproach which uses a plastic trussmodel similar to that shown in Figure 9and is discussed briefly in the sectionon Flexure. This method is discussedby O'Flynn and MacGregor (1987) and isnot discussed further here, except tosay that it appears to give goodresults.3. Energy MethodThe "energy method" is an upper-boundplasticity approach which considers theinternal and the external work expendedwhen the member fails in accordancewith some assumed geometricallypossible displacement field. Thismethod is based on work by Nielsen andBraestrup (1978) and Nielsen (1984).It considers a kinematicallyadmissiblefailure mechanism consistingof a yield line running from the edgeof a load plate to the edge of thesupport plate as shown in Figure 11.hrFigure 11Upper Bound Failure MechanismThe relative displacement ratevector v is inclined at an angle a tothe yield line and the work equation isminimized with respect to the angle a.The design equation which results fromthe failure mechanism shown in Figure11 is as follows:Vu -W f' bd [ J (aid} + K - aid]cwhere K =2a4~(111 - ~)W[ 6]for ~ < 111/21.0 for ~ ~ W/2W- concrete efficiency factorassumed equal to 1.0~ - ( 2As/bd) (Fy/f~)This energy method provides arelatively simple procedure forcalculating the shear failure load andfor including the effects of shearreinforcing and diaphragm members byadding the appropriate energy terms to671

the work equation. In Figure 12, thisequation is compared to the results ofthese tests and the tests by O'Flynnand MacGregor (1987). Since this is anupper-bound solution procedure, itshould give an upper bound to the loadcarrying capacity. However, if thecorrect failure mechanism is chosen, ittheoretically gives the exact solution.Since there are relatively few possiblefailure modes to check, the procedurecan be considered a viable design tool.2.01.8 -C'(3ra 1.S -0.ra() 1.4 -L-raQ)..c:1.2CI)- -"0 1.0 te-Q)•- • • - ••~"0 0.8 • .Q)ct::::, os r-ra~0.40 r-«Conclusions-• • , •average = 0.930.2 f- standard deviation = 0.100.0 I I I I I0.4 0.8 12 1.S 2.0 2.4aid (shear-span/depth ratio)Figure 12Energy Solution•This research work has developed acomposite ice-resisting wall which issimple from a fabrication and constructionpoint of view and which alsosatisfies the high strength andductility requirements for a structureof this type.relatively wide spacing. There are noother welded details in the wall. Thisresults in a significant reduction inlabour-intensive welding, compared to aconventional, all-steel wall. Thesteel frame is self-supporting and,therefore, requires no additionalformwork when concrete is poured.There is no additional reinforcingsteel required, beyond that provided bythe external steel plates and thediaphragm plates.From a repair point of view, thewall appears to possess severaladvantages: 1) in damaged areas, theinternal and external steel plates caneasily be flame cut; 2) with the steelplate cut away, damaged areas ofconcrete can easily be removed, sincethe concrete is not held together by adense maze of reinforcing steel; and 3)the wall can then be made good, withrelative ease.Tests have shown the wall topossess high strength in both flexureand shear and to exhibit a very ductilemode of failure when the ultimatecapacity of the wall is exceeded.Further, the tests have shown that thewall can develop this ductile mode offailure in shear.The shear strengths were adequatelypredicted by: 1) an empirical equationdeveloped from a statistical regressionanalysis; 2) a plastic truss model; and3) an upper bound plasticity approach.Additional research work isrequired to further assess the capacityof the wall to resist high, local"punching" loads. Some of this work isalready underway at the Centre forFrontier Engineering Research; earlyresults appear very prom~s~ng. Morework is now being planned to furtherevaluate punching shear and shearcapacity for combined transverse andin-plane loads, and to further thedevelopment of this particular walldesign.The most notable feature of thewall is its simplicity. The outer andinner steel plates are fastenedtogether with continuous, verticalsteel diaphragms which are at a672

AcknowledgementsThis research work is being fundedand conducted by the Centre forFrontier Engineering Research. Theauthors wish to thank C-FER's Membersfor their support of this project andfor permission to publish. Specialthanks are due to Gulf Canada ResourcesLimited and Sandwell Swan Wooster Inc.for their valuable input and to theUniversity of Alberta for the use ofits laboratory facilities.ReferencesAllyn, N.F.B. 1986. Global and LocalIce Loads Including Dynamic Effects.Canada Oil and Gas LandsAdministration.Ice/StructureInteraction. PERD Task 6.2. ProgramEvaluation Workshops Proceedings,May-June, Calgary, Alberta, Canada.Bruce, J.C. and Roggensack, W.D. 1984.Second Generation Arctic Platforms;Lessons from First Generation DesignExperience. OTC Paper 4798, OffshoreTechnology Conference, Houston.Croasdale, K. R. 1984. Sea IceMechanics: A General Overview. MarineTechnology Society Journal, Vol. 18.Canadian Standards Association. 1985.Standard CAN3-A23.3-M84. Design ofConcrete Structures for Buildings.Canadian Standards Association,Rexdale, Ontario.Gerwick, B.C. 1985. Lessons from anExciting Decade of Concrete SeaStructures. Concrete International,August, pp. 34-37.Hattori, Y., Ishihama, T., Yamamoto,T., Matsuishi, M. and Iwata, S. 1985.On the Ultimate Strength of CompositeSteel-Concrete Structures. POAC 85,8th International Conference on Portand Ocean Engineering Under ArcticConditions, Proceedings, Narssarssuaq,Greenland, pp. 445-454.Marti, P. 1985. Basic Tools ofReinforced Concrete Beam Design. ACIJournal, January-February, pp. 46-56.Matsuishi, M. Takeshita, H., Suhara,T., Nishimaki, K. and Iwata, S. 1977.On Strength of New Composite Steel­Concrete Material for OffshoreStructure. OTC Paper 2804, OffshoreTechnology Conference, Houston, pp.589-594.Matsuishi, M., Takeshita, H., Suhara,T., Nishimaki, K. and Iwata, S. 1977.On the Strength of Composite Steel­Concrete Structure of a Sandwich System(1st Report). Hitachi Zosen TechnicalReview, Vol. 38. Sept, pp. 11-20.Matsuishi, M. Takeshita, H., Suhara,T., Nishimaki, K. and Iwata, S. 1978.On the Strength of Composite Steel­Concrete Structure of a Sandwich System(2nd Report). Hitachi Zosen TechnicalReview, Vol. 39, March, pp. 26-35.Matsuishi, M. Nishimaki, K., Iwata, S.and Suhara, T. 1980. On the Strengthof Composite Steel-Concrete Structureof a Sandwich System (3rd Report).Hitachi Zosen Technical Review, Vol.40, March, pp. 8-12.Matsuishi, M., Nishimaki, K., Iwata,S., and Suhara, T. 1980. On theStrength of Composite Steel-ConcreteStructure of a Sandwich System (4thReport). Hitachi Zosen TechnicalReview, Vol. 41, December, pp. 266-271.Nielsen, M.P. 1984. Limit Analysis~a~n~d~~C~o~n~c~r~e~t~e~_P~l~a~s~t~i~c~i~t~y. Prentice­Hall, Inc., Englewood Cliffs, NewJersey.Nielsen and Braestrup. 1978. ShearStrength of Prestressed Concrete BeamsWithout Web Reinforcement. Magazine ofConcrete Research, Vol. 30, No. 104.September, pp. 119-128.Niwa, J. 1984. Equation for ShearStrength of Reinforced Concrete DeepBeams Based on FEM Analysis. ConcreteLibrary International of Japan Societyof Civil Engineers, No.4, December,pp. 283-295.Nojiri, Y. and Koseki, K. 1986.Structural Behavior and Design Methodof Steel/Concrete Composite Ice Wallsfor Arctic Offshore Structures. OTCPaper 5292, Offshore TechnologyConference, Houston, Texas.673

O'Flynn, B., and MacGregor, J.G. 1987.Tests on Composite Ice-Resisting Walls.Centre for Frontier EngineeringResearch. C-FER Special Publication No.1, October, pp. 71-87Rojansky, M. and Hsu, Y. 1985. An IcyChallenge.~C~o~n~c~r~e~t~e~~I~n~t~e~r~n~a~t~i~o~n~a~l,August, pp. 38-44.Shioya, T., Matsumoto, G., Okada, T.and Ota, T. 1986. Development ofComposite Members for Arctic OffshoreStructures. POLARTECH U86. VTTSymposium 71, Helsinki, Finland, pp.660-677.Zinserling, W. and Cichanski, W. 1986.Economical Arctic Structures UsingConcrete. OMAE. Fifth InternationalOffshore Mechanics and ArcticEngineering Symposium., Tokyo, Japan,pp. 153-159.Zsutty, T. 1968. Beam Shear StrengthPrediction by Analysis of ExistingData. American Concrete InstituteJournal, Vol. 65, November, pp.943-951.Zsutty, T. 1971. Shear StrengthPrediction for Separate Categories ofSimple Beam Tests. American ConcreteInstitute Journal, Vol. 68, February,pp. 138-143.674

THE RESISTANCE OF COMPOSITE STEEL/CONCRETE STRUCTURESTO LOCALIZED ICE LOADINGJ. R. SmithA. McLeishTaywood Engineering Limited, Southall, ENGLANDAbstractRecovery of hydrocarbon resourcesfrom the offshore arctic demandsplatform structures which will surviveextreme gloool and local ice forces.The J;l3.per presents the results ofan experimental programme aimedspecifically at evaluating the potentialbenefits of composite steel/concreteconstruction in resisting highlylocalised loading.Comprehensive testing andinterpretation were undertaken onthree large scale composite shelland slab elements with geometriesrepresentati ve of prolX'sed arcticstructure concepts. ComJ;l3.risons aredrawn both on differing approachesto composite design, and on theperformance of composites relativeto equivalent re-inforced concreteelements.IntroductionThe developnent of cost-effectiveplatform structures to withstand extremelocal ic e forc es is fundamental toThis is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987. © The Geophysical Institute,University of Alaska, 1987.the successful exploitation ofhydrocarbon resourc es in the offshorearctic. As a oosic material, concreteoffers many advantages in an Arcticenvironment. However, the applicationof conventional design practic e toextreme ic e loadings has resultedin concepts with massive structuralsections with high percentages andcomplex arrangements of re-inforcingand pre-stressing steel which leadsto difficult and expensive construction.Alternative construction techniques,which can simplify the constructionand reduce costs without impairingperformanc e, therefore become highlydesirable.There is a growing recognitionthat the economic solution to structuralengineering problems in the arcticcould lie in the use of compositeconstruction. Such structural elementswould comprise external steel platesinfilled with concrete so that thebest characteristics of the componentmaterials are combined, resultingin serviceable, durable structureswhich are relati vely simple toconstruct.While composite constructioncomprising concrete decks connectedto steel beams has been usedsuccessfully in bridges for manydecades, the form of construction675

utilising face plates and concreteinfill has little precedent.Exploitation of this form of compositedesign technology is, therefore, limitedby the lack of an accepted designmethodology and relevant test data.To evaluate the potential of thisform of composite construction, adouble skin shell and two double skinslabs have been designed and testedby Taywood Engineering to providedata relating to their ability toresist concentrated local loadings.The shell model was designedspecifically to enable a validcomparison to be made with theperformance of a re-inforced concreteshell tested in a previous programme(Birdy et a1. 1985) . Both flexuraland shear steel plates were designedto be equivalent in area to the barre-inforcement. For the slabs, however,the steel was designed as the mainstructural member, with the concreteused as a stiffening medium. Theprimary function of the concrete wasto prevent buckling and enable thesteel to operate to optimum capacity.This paper presents details ofthis experimental work, the resultsobtained and, where appropriate,compares the behaviour of models.Preliminary conclusions are drawnon the value of composites and thedirection of future development.Description Of ModelsDouble skin shell (SKSH)The geometry and physicalproperties of the steel skin shellwere chosen to allow comparison ofits behaviour with a data base ofinformation on the behaviour of reinforcedconcrete shells (Birdy eta1. 1985) . The models represent toa scale of l/lOth the geometry ofthe envisaged prototype shown in Figure1, (Byrd et al. 1984).Tests on the conventionally reinforcedshells had shown that thepunching shear capacity was notsensitive to the percentage of flexuralsteel. The steel skin models weredesigned, therefore, on the basisof the lower percentage of flexuralsteel previously adopted for thereinforced shells. The biaxialbehaviour of the steel skin, togetherwith the greater flexural lever armresulted in the percentage of steelbeing reduced to one third of thatin the corresponding conventionalshell.It had also been shown that thestirrup re-inforcement contributedto the shear strength by a greateramount than its yield strength wouldsuggest. Therefore, in an attemptto utilise this strength enhancement,the shear steel for the double skinshell was similar in area to thatof a highly shear re-inforced modelpreviously tested. The shear steelused had a higher yield stress thanthe stirrups which resulted in a highershear steel strength but similarstiffness. Details of the steel skinshell are shown in Figure 2, withthe percentages of steel gi ven inTable 1.Double skin slabs (SLT and SLL)The steel skin slabs were designedto investigate an alternative philosophyto composi te construction aimed atcreating the simplest shipyardfabrication technique. The load testswere used to assess whether thisapproach could achieve satisfactorystrength and serviceabilitycharacteristics.Figure 3 is typical of a geometryproposed for an arctic structure conceptat the time of the test programme.The outer surface is formed of doubleskin concrete slabs acting compositelywi th deep steel I beams. The conceptof this structure relies on the interactionof the slabs with the I beamsand on the ability of the slabs tospan between the beams.In developing the steel arrangementconsideration was given to the mostcosteffective fabrication techniqueslikely to be available at ship yards.It was understood that L-shapedstiffeners could be readily manufacturedfrom plate on site and automaticallywelded in a continuous process.676

-------Figure 1.Artist I s sketch of the ACES.Elevalran Along Span Centre LineI 0000000000000000000.00:0:00:00:00:0 li7 ~L- .--- 1l~_ ----------JElevatIon Along T ronsverse Centre LineFigure 2.Details of steel skin shell.677

·T""":lI'-+-I.,.-L- IlEfP mEL '1. BUMSOO\.I!lE SKIN WLSFigure 3. Conceptual design for a composite offshore structure for the Arctic.W.t»5 01 1016 en Fot'"9 the ~dtrK hon and •• Idtd olt.,.nal.., 10tap ond bottom "llInsf -~ rti~:I J ] 1 ] 11 I j 1 ] 1: 1 ] J ] I ]( ] IL 22 ... _____ • __ :~ ____ jTransverse Section 01 Model Sll~J_ (atltl"-'ll fill., ..."QIDI'It IfId~ pta" .t ..... ort onIr "r __________________________________________________ -,\J~I---------------------------------------~. 21,,&.Span Section 01 Model SUFigure 4.Double skin flat slab model SLL.678

Two steel skin slabs wereconstructed, with the geometry forSLL as shown in Figure 4, and thesteel percentages given in Table 1.Their overall dimensions and thethickness of steel plate were chosento represent to a scale of 1/4 thosesupplied by the designer of the proposedconcept. Slab model SLL had the websteel spanning between supports, whilemodel SLT had the webs parallel tothe supports. This enabled theinfluence of the web stiffeners onthe load distribution efficiency tobe assessed.Material PropertiesFor the purpose of this modelstudy, it was desirable that theproperties of the concrete and steelsshould be typical of those likelyto be employed in arctic construction.The concrete selected for the compositetests repeated that specified andused in the previous re-inforcedconcrete programme. The measuredphysical properties at 28 days ofage are given in Table 2.The 2mm steel used for the shellwas BS 1449 grade 50/35. The 7mmand 3mm steel plate used in the slabswas BS 4360 grade 50B. The materialcontrol tests showed that, althoughthe yield stress of the 7mm platewas clearly defined, the 3mm platedid not have a stress plateau. The0.2% stress was, therefore, takenas the yield stress for the 3mm plate.The steel used on an actualstructure in the arctic is expectedto be similar to grade 50D or 50Eto BS 4360. These have the sameproperties as grade 50B at ambienttemperatures and, therefore, thissteel, which is more freely availableand less costly, was used for themodels.Model No.RCSHShell radius (m) 1.829Flexural steel % oftotal depth (per face) 3.90Shear steel % 0.47Ultimate Capacity (kN) 2791Estimated Level )kN 1560of Shear Crack )Initiation ) % ult. 56Estimated Level )kN 2400of Flexural )Serviceability ) % ult. 86SHELLSSLABSSKSH SLT SLL1.8291.31 4.59 4.590.60 2.59 4.593315 2064+ 2706+1780 1500 1500(Transverse) (s~n)54 73 552670 220 22080 11 8* model unloaded without reaching ultimate loadTable 1 Model ParametersCylinder Compressive StrengthElastic ModulusIndirect Tensile StrengthDensityTable 2Mean Std. Deviat10n48.2 MPa 3.2 MPa37.8 MPa 1.5 MPa4.1 MPa 0.3 MPa2345 kg/m 3 70 kg/m 3Concrete Properties679

Test Rig and Loading SystemThe test rigThe general arrangement of thepurpose designed rig, assembled readyfor testing a shell, is shown in Figure5. To accommodate the double skinslabs, the test rig was modified bywelding restraint plates to the columnsabove the slabs. A level of bendingresistanc e was generated, therefore,by the restraint plates at the topand the support plates beneath theslabs. This did not provide completefi xi ty , and edge rota tions weremonitored in the tests to aidinterpretation of moment-rotationcharacteristics.Loading system.. .."n Ir'1l ~r7"I'" :-i~; it t !~iDue to the relatively poor stateof knowledge of how ice transmitsand distributes load, it was notpossible to simulate the ice loadingthat would be applied to an offshorestructure in the Arctic. The loadingsystem was designed, therefore, toproduce a uniform pressure, whichcould be more readily accommodatedin any analyses.Figure 6 shows the loading systemus ed with the she lls . Load from thehydraulic jacks was applied througha solid steel piston to a mouldednatural rubber p3.d resting on theupper surface of the model. The naturalrubber was selected because itsbehaviour above a pressure ofapproximately 3 MPa simulated thatof a fluid. The rubber was containedwithin a steel cylinder and extrusionat the steel cylinder/model interfacewas inhibited by three 6mm thick leatherdiscs. Friction losses between therubber p3.d and the steel cylinderwere reduced to 1% by silicone grease.51UOOtHG fOf'~~~:C-'::-N -NOT IN US[ ~ lOOT .JACKlOOT JAca ",..,.". .... _. L...~ S",(AO("::~::1.. ~:~ .~~~I ~'~~~~,.,...,. ··n··,." ,:." :.. :, ,.. :: :: ..:'..!.ll-- •• 1. --1 ~ISIO( fLfYATIOH .. , ,Figure 5. General arrangement of test rigThe diameters of the loaded areaswere chosen such that the averageshear stress at the critical perimeter(d/2 from the loaded area) exceededthe antic ip3.ted shear strength ofthe concrete at an applied pressureof the order of 14 MPa. Preliminarycalculations had previously beenundertaken to confirm that this loadingFigure 6.Loading System680

was unlikely to generate a prematurebending failure. To meet these arearequirements, two sep3.rate loadingsystems were manufactured, withdiameters of 457mm for the shell and152mm for the slab. The 457mm diametercylinder and rubber p3.ds were shapedto match the shell radius.Model instrumentationThe following behaviour wasmonitored during each test:i) The total load applied;ii) Deflection profiles along themajor axes;iii) Support edge rotations anddeflections;iv) Steel plate strains at or nearthe critical section for shearcrack development;v) Observations of crack developmentand post failure crackdistributions.Strain gauges were mounted onboth the flexural and shear steelat locations as near as possible tohalf the effecti ve depth from theperimeter of the loaded area alongeach major axis. The flexural straingauges were oriented in the planesof sp3.n and transverse bending, andthe shear strain gauges were similarlyoriented at mid-section depth withinthe concrete. Main deflection profileswere monitored at model soffi ts, andedge rotations and displacements weremoni tored by deflection transduc ersat the ends of the sp3.n main axesand at two of the diagonally oppositecorners.Test ProgrammeThe overall test programmeconducted on the models covered variouscycles of loading in discrete incrementsin the "elastic" and "post-elastic"phases.In addition to obtainingobservations on overall and localresponse, the tests were directedtowards an evaluation of criticalstates achieved, viz; ultimate cap3.city,estimated load for initiation of shearcracks and the likely flexuralserviceability limits.For the purposes of cOrn:p3.rison,the serviceability limits summarisedin Table 1 were based upon the followingcriteria:i) In shear the limit was takenas the load at which a significantincrease in the rate of shearstrain development occurred;ii) In flexure a servic ea bili tylimit of 600 microstrain. Thisis generally in line with typicalcode requirements for controlof crack widths from durabilityconsiderations. For the steelskin models, of course, thislimitation is unduly onerous ascracking will not have durabilityimplications equivalent toconventionally re-inforced members.Within the limits of this p3.per,the discussion which follows coversonly the observations obtained fromthe primary load cycle on each modelthrough to "failure" or maximumsustained load. In each test, thiscondi tion was generally achieved aftermore than one cycle to progressivelyincreasing load limits.Discussion Of ResultsShell testsThe main events occurring duringthe loading of the double skin shell(SKSH) may be cOmp3.red with the resultsobtained during tests on a conventionalreinforced concrete shell (RCSH) (Birdyet aI, 1985) . When comp3.ring theperformances of the two shells itshould be noted that:i) Geometry, support and loadingconditions were identical;ii) The percentage of flexural steelin the re-inforced concrete shellwas three times that in the steelskin shell;iii) The percentage and distributionof shear steel was approximatelythe same in both cases.Figures 7 -10 show centraldeflection, typical global response,flexural and shear strain development.681

4000--SKSH- ---RCSH3000LOAD(kN)20001000-10 -I -7 -6 -5 -4 -3 -2 -1 -0DEFLECTION IN MMsFigure 7.Central vertical deflections at soffit.--- - RCSH. 911kN• 1BI.OkN• 2791kN--SKSH. 919kN• 1B4SkN• 2774kN• 331SkN~L-__ ~~~~--~~~--~~--~~~~~~-1200 1000 auo 600 400 200 000 200 400 600 BOO 1000 1200DISTANCE FROM CENTRE ImmlFigure 8.Sran deflection profiles.682

SKSH3000RCSH2000, ,\\\\\\LOAD IN( kN)1000o~----.---.---+---.----.------1000 -500 o 500 melMICRO STRAINFigure 9. Mean strain in span flexural steel.SKSHRCSH3000LOAD IN( kN)20001000II/I///II",,-, ,,­,,.­,," , , ,,,­O~----~r---~--.---'----.--~---.----~---o 500 1000 1500 2000 2500 3000 3500MICRO STRAINFigure 10.Mean strain in span shear steel.683

The central deflections and thestrains in the flexural steel measuredon the two shells are very similarup to a load of 1800 kN. After this,however, the deformation of the steelskin shell is considerably less thanthe re-inforc ed IIIOdel. This enhanc edperformanc e could be attributed tothe greater restraint against shearcrack developnent offered by the steelwebs acting over the full thicknessof the shell.Strains measured in the shearsteel in both mdels increasesignificantly at applied loads ofapproximately 1560 kN (RCSH) and 1780kN (SKSH), suggesting the onset ofshear cracking through the concrete.At a load of 2400 kN (RCSH) and 2670kN ( SKSH) the strains in the flexuralsteel exceed the flexural serviceabilitylimit of 600 microstrain.The maximum load carried by there-inforced shell was 2791 kN at acentral deflection of 7mm. On itssecond load cycle, the steel skinshell sustained a maximum load of3315 kN at a central deflection of8mm, after which the load droppedoff with further deformation. Ata maximum central deflection of 15mm,the recorded load was 1820 kN. Onunloading from this point, a residualdeflection of llmm remained.Following the load test on thesteel skin shell, it was found thatdeformations on the soffit in thesp:i.n direction were delineated bythe web and also affected by the "T"­beams formed by the shear plates andthe soffit skin. In the transversedirection the deformation was delineatedby the shear plates. On the topsurface, punching-in occurred at theedge of the loaded area. This punchinginoccurred during the second loadcycle at a load between 3030 kN and3315 kN, causing failure of a weldbetween plates forming the skin. Thefailed shells were later sectionedalong their centre lines. The positionsof the diagonal shear cracks throughthe concrete are cOffip:i.red in Figure 11.Slab model testsFigures 12-15 comp:i.re centralvertical deflections, global responseand steel strain developnent in thetwo mdels.Load-deflection relationships werealmost linear up to 1300 kN, althoughconsiderable shear deformation betweenthe web steel and the concrete wasapp:i.rent at the free edge of mdelSLT commenc ing at loads below 1000kN. After two cycles, the maximumloads achieved by the mdels were2706 kN (SLL) and 2064 kN (SLT) atdeformations in excess of 8Omm. Onunloading, 80% of this deformationwas unrecovered. Despite thisconsiderable deformation, nei therof the steel skins was perforated.The tests were terminated for safetyreason\3 and it is believed that bothmdels could have accepted greaterloads.The developnent of flexural andshear strains in the steel on thetransverse axis of mdel SLT cOffip:i.refavourably with the equivalent strainsdeveloped on the sp:i.n axis of mdelSLL. A similar, but less well definedbehaviour was observed on the otheraxes. This would seem to indicatethe strong influence of web orientationon load distribution within thecomposite structure. Rapid increasein web shear strain beyond 1500 kNwould appear to indicate the initiationof shear cracking.It is interesting to note thateach mdel, in its deformed state,was subsequently inverted in the rigand re-tested under simply supportedconditions. SLT failed in globalbending at 1710 kN whilst SLL failedby punch through of the steel skinat 1976 kN.ConclusionsSteel skin shellThe steel skin mdel was designedto have similar characteristics tothe reinforced concrete shell withoptimum mbilisation of steel andconcrete cap:i.cities. Similar "elastic"684

WestE .. tN.E ~ S.E.it~:r:]:r:.:t(lf.f~[~t:'J:t:jlRCSH I I SK5H i'---..\Saw CutFigure 11.Shear crack locations.3000 ---- SLT- 5LL-!IO ~o -70 -60 -so -40 -30 -20 -10 - 0·DEFLECTION IN MM sFigure 12.Central vertical deflections.685

---- 51.T 0 631kN• 1212kN... 2062kN--SLlo102030i e- 40~>-~ SO...JI:;D 60701090-1400 -1200 -1000 ~OO -600 -4OD -200 0 20D 400 600 800 1000 1200 1400DISTANCE FROM C£NTRE Imm)Figure 13_Vertical deflection span profiles_5000--SLLI Span I---- SLT (Transversel4000SLT ( TRANSVERSE ISLL ( SPAN I2000LOAD(kNI1000o 5000 10

esponses were recorded despite thecomposite model containing only onethird of the flexural steel by volume.Serviceability limits, at approximately55% in shear and 85% in flexure, weresimilar for both models and thecomposite shell exhibited a 19% increasein ultimate capacity.Steel skin slabsThe stiffnesses of the slab models,and the development of load distributionwi thin them, were highly dependentupon shear web orientation. Despitethe skins only being welded to alternatewebs, the models exhibited substantialcapacity with a high degree ofductility. Consequent deformationswere large, and these could be reducedby full welding of the webs, but atsome penalty to fabrication costs.Even at very high deflections, whenthe concrete has become extensivelyfragmented, the composite can resisthigh loads through the containmentof the concrete and maintenance ofload distribution paths.GeneralThe results confirm the promiseof composite steel/concrete constructionas a medium for resisting high localloading. Taking into account themany addi tional benefi ts in termsof ease of construction, relaxationsof serviceability limits, reductionin pre-stress and concrete quality,permanent formwork and abrasion/adfreezeresistance, composites therefore offerenormous technical and economicpotential. The realisation of thispotential will depend upon the rationaldevelopment of methodology for thepractical design of such structures.This will demand further analysisof the fundamental behaviour ofcomposite systems, supported by furtherexperimental work, leading to safeand coherent design procedures.ReferencesBirdy et al 1985, "Punching Resistanceof Slabs and Shells used for ArcticConcrete Platforms. " Proceedings ofOffshore TeChn010~ Conference, Houston,May 1985. Paper 855.Byrd R. C • et al 1984, "The ArcticCone Exploration Structure: A MobileOffshore Drilling Unit for Heavy IceCover" Proceedings of OffshoreTechnology Conference, Houston, May1984. Paper 4800.AcknowledgementThe Authors wish to thank TaylorWoodrow Construction for theirsponsorship of the test work and forthe permission to publish, and alsotheir colleagues, particularly Dr.S.J. Wicks, for their substantialefforts in expediting the experimentalprogramme.687

STRENGTH OF COMPOSITE, SANDWICH SYSTEM,ICE-RESISTING STRUCTURESMasakatsu MatsuishiSetsuo IwataHitachi Zosen Technical Research Laboratory, Inc., Osaka, JAPANAbstractRecently, various kinds of Arcticoffshore structures have been constructed.In designing such structures, designershave to pay particular attention as to howto overcome ice loads acting on thestructures. The use of a composite,steel-concrete sandwich system appears tobe viable solution for offshore structuresin the Arctic_Discussed in this paper aretheoretical and experimental studies whichwere carried out to study the elasticplasticbehavior and the ultimate strengthof a composite steel-concrete sandwichsystem structures, where concrete wasplaced between steel plates.IntroductionVarious kinds of offshore structureshave been constructed with the new oilexploration and production developments inthe Arctic and sub-Arctic waters. Theuse of a composite steel-concretestructure (sandwich system) appears to bea viable solution for offshore structures,because it enhances their load-carryingcapacity in the severe Arctic environment.This is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987. © The Geophysical Institute,University of Alaska, 1987.A huge offshore structure, if made ofsteel, necessitates a great deal of deadload to offset its large buoyancy. Concretemay be used as ballast and forstrength in such an offshore structure.Drawbacks of concrete structures areassociated with low tensile strength andlow ductility. It is difficult to securewatertightness, once concrete crackingdevelops in an offshore structure.The authors have developed a newcomposite steel-concrete sandwich systemstructure to eliminate as much as possiblethe various drawbacks of both concrete andsteel (Matsuishi et al 1977, Matsuishi etal 1980). The new composite structureendures large deformation and absorbs agreat deal of energy until failure, becausethe steel plate, having highstrength and ductility, suppresses thedevelopment of surface cracks_ Thus, thesafety of the offshore structure is ensuredif a composite structure is used asan ice-resistant wall, which has to wi t­hstand excessive ice loading.In this paper, experiments and theoreticalanalyses were carried out to studythe ultimate strength of a compositestructure with a small ratio of span todepth which could be used as an iceresistantwall of an offshore structure inthe Arctic. Experiments were carried outto study the ultimate strength of the689

composi te structure under distributedloads, which correspond to ice pressure,and the reserved ultimate strength of thecomposite structure which had been exposedto thermal cycles, i.e. recurrence offreezing and thawing of concrete.the upper steel plate of MB-model is equalto the sum of the sectional areas of thelongitudinal stiffener and upper steelplate of the MA-model. The contributionof the longitudinal stiffeners to theultimate strength of the compositestructure can be examined by comparingtest results of the MA- and MB-models.To study the effect of severe Arcticweather conditions upon the strength ofthe composite structure, MA-2 and MA-3were exposed to twenty and forty cycles offreeze and thaw, respectively.(0) SF-modelFig.1 Ice-resistant wall of Arcticoffshore structuresNodel Tests and ResultsTest modelsFour types of two-dimensional testbeams of the composite steel-concretestructure shown in Fig. 2 were employed inthe model tests. They are approximatelyone-third scaled models of an iceresistantwall of an offshore structurewhich will be used in the Arctic ,as shownin Fig. 1. The ratio of span to depth ofthe SF- and ST-models is 1.67. Theeffective ratio of the MA- and MB-modelsis 1.00 due to the large, attached loadingblocks. Longitudinal stiffeners werefurnished at the outer surface of the MAmodeland the inner surface of the SFmodelto increase shear strength while nostiffeners were furnished to th~ ST-modeland the MB-model. The sectional area of1·~{mi;:~~:i1Ij" I~ 353 I 500 19106 500 I :l53 111 ~(d) MB-modelFig.2 Test modelsAfter placing light-weight concretewi th a 28-day design strength of 44.~N/mm 2 (SF- and ST-models) and 37.8 N/mm(MA- and MB-models), the specimens weremoisture cured at 55 0 C forthree hours. Yield stress and tensilebreaking strength of the steel plates were690

299 and 428 N/mm 2280 and 465 N/mm 2respectively.(SF- and ST-models),(MA- and MB-models),Experimental apparatus and testingprocedureTest models ST-2 and SF-2 were loadedby a uniformly distributed load, whichcorresponds to ice loading, until theirfailure as shown in Fig.J(a). Continuousbeams, MA-1, MA-2, MA-J and MB models,were supported at three sections andloaded at two sections as shown inFig.J(b) so that the bending moment andshearing force distribution would besimilar to those of an actual iceresistingwall of an Arctic offshorestructure exposed to ice loading. Simplebeams, ST-1, SF-1 and MA-4, were loaded bya concentrated load.pday.Table1 Mechanical properties of concreteafter freeze-thaw cycleskqf/cm 2freeze- 0 20 40aw~compressive strenqth 456 332 402splittinq tensile strenqth 34 31 28frexural strenqth 29 31 27initial tanqent modulus 1.51 1.37 1.33shear strenqth 110Test results and discussionMechanical properties of concrete forthe MA- and MB-models are shown in Table1. Due to freeze-thaw cycles, thecompressive strength and tensile strengthdecreased approximately 15 and 45 %,respectively. Flexural strength remainedalmost the same.Ca) siaple be .. under uniforrdy distributed loading(c) siaple be .. undera concentrated loadFig.J Experimental set-up(b) continuous beamTest beams, MA-2 and MA-J, wereexposed to twenty and forty freeze-thawcycles in a cold room. The cold roomtemperature was controlled so that themaximum temperature of the concrete duringthe thaw period was 10 0 C and the minimumduring the freezing period was _20 0 C.The rate of freeze-thaw was one cycle aThe measured load-displacement curvesof the ST- and SF-models are depicted inFig.4, together with their ultimatestrengths. The development of concretecracking in the SF-models are shown inFig.5. Diagonal tension cracks in theconcrete propagated diagonally in thecomposite beam, SF-1, which was acted uponby a concentrated load. Concrete cracksin SF-2, under uniformly distributed load,propagated vertically as bending stressesexceeded shearing stresses. Cracked concretein both beams developed arch actionsand the beams exhibited large deformationsat failure. Their failures occurred whenthe elongation of the tension plates increasedso much and the concrete began tocrush by compression.The measured load-displacement curvesof MA-1, 2, J and MB models are shown inFigs. 6 and 7 together with the ultimatestrengths. It was found that the compositestructure, which experienced freezethawcycles, did not exhibit anappreciable decrease in ultimate strength691

(If )150Cl«o...J100_~~~~~-..u~-----b--~~~./:r----------Pmo,' 122 51t/"'\ST2 ~..u~ ,A"'-- \----~----Pmo,'36.5tt_0------0----0, S T I,'0BEAM THEORYo5 10 15DEFLECTION20 (mm)Fig.4 Measured load-deflection curves10-t-----'lJOSF ISF2Fig.5 Observed cracks in concrete692

150I196tf_1

and the amount of deformation which thecomposite structure exhibited before itsfailure. Its flexural stiffness decreaseddue to the freeze-thaw cycles. A greatadvantage of the composite structure, whenused as structural members for Arcticoffshore structures, is that the freezethawcycles do not decrease its loadcarryingcapacity.The development of concrete cracks inMA-1 and MB models is shown in Fig. 8. Itwas found that concrete cracks start at,or very close to, a shear connector and aplace where load was applied. Theconcrete cracks propagated diagonally.This suggests that the cracked concretetransmits diagonal compressive force andcarries shearing force.Non-linear Analysis by FEMWhen a load, acting upon thecomposite structure of a sandwich system,exceeds the proportional limit of thestructure, the structure exhibits nonlinearbehavior and finally reaches itsultimate strength. The non-linearbehavior of the structure is caused by thefollowing non-linearities:Cracking starts in concrete when thetensile stress reaches the tensilestrength. As a crack forms, stresstransfer across the crack is reduced tozero. When the compressive stress actingon the concrete reaches the yield stress,plastification of the concrete starts.The concrete crushes when the stressreaches its compressive strength. Thesteel plate buckles and loses its loadcarrying capacity when its compressive(a) continuous beamoo(b) simple beamFig.8 Observed cracks of concrete694

stress reaches the buckling stress. Whenthe stress of steel plate reaches theyield stress, the steel exhibits plasticdeformation. Finally, the steel plateloses its load-carrying capacity when itsstress reaches the tensile breakingstrength. A gap occurs between the steelplate and concrete when tensile forces actbetween them. No stresses are transferredthereafter.In this section, the non-linearfinite element method applied to analysethe non-linear behavior of the compositestructure is discussed. The steel platesand concrete are divided into triangularelements. Linkage elements areincorporated into the analysis torepresent the effect of the bond betweenconcrete and steel. The solution of thelinear incremental equation is obtainedunder successive, small increments of loaduntil failure of the composite structureoccurs.Figs. 9 and 10 summarize the measuredand calculated load-deflection curves ofthe test models. The calculated ultimateloads are in good agreement with measuredresults.Figs. 11 and 12 show the principalstress distributions and change of elementproperties in the post-elastic region,i.e. cracking, yielding, crushing ofconcrete and yielding of steel plate. Asshown in the figure, for the ST-model,whose ratio of shear span to depth is1.67, cracking of concrete started at orvery close to the shear connectors andpropagated in a direction perpendicular tothe horizontal axis of the model. Thecalculated orientation and progression ofthe concrete cracks coincided well withthe measured ones. In the post-elasticrange, the tensile stresses in -theconcrete are reduced by the development ofcracking. The uncracked region, whichoccupies the upper part of the test modeland extends horizontally, carriescompressive stresses in the same manner asa concrete arch does.The calculated results of the MA-model,whose effective ratio of shear span todepth is 1.0, showed that a diagonalcompression field developed in theconcrete after diagonal cracking developedin the concrete. Higher stresses wereobserved at the corner of stiffeners,which contributed to suppress localdeformation of the concrete and the gapbetween concrete and steel.Concluding RemarksVarious excellent properties of thenew composite steel-concrete structure,with a small ratio of span to depth, wereclarified through experimental andtheoretical inverstigations. Importantinformation obtained in theseinvestigations is summarized below.(1) Arch action developed in the crackedconcrete of the composite steel-concretestructure under uniformly distributedload. The composite structure exhibitedlarge deformation before its failure.(2) The continuous composite beam, with aratio of shear span to depth, 1.0,developed a diagonal compression field inthe concrete and carried a large diagonalcompressive force.(3) Longitudinal stiffeners furnished atthe outer surface of the compositestructure contributed to spread theconcentrated load and to increase itsultimate strength.(4) Although mechanical properties of theconcrete deteriorated due to freeze-thawcycles, the ultimate strength of thecomposite structure, exposed to freezethawcycles, did not decrease appreciably.This is because the failure was caused bygeneral yielding of steel plates, not byconcrete crushing.ReferencesM. Matsuishi, et al.1977. On The Strengthof New Composite Steel Concrete MaterialFor Offshore Structure, Paper No. 2804,OTC. 1977, 589-594.M. Matsuishi, et ai.1980. On the Strengthof Composite Steel-Concrete Structures ofSandwich System, Naval Architecture andOcean Engineering, Vol.18, The Societyof Naval Architects of Japan, 132-145.695

Pexp max = 122.5 t fPeal max=116tfI--: EXP.--0--: FEMo 5 10 15 20--+ DEFLECTION (mm)Fig. 9Comparison of load-deflection curves (ST-2)Pmox =203tf200_-0...'"~~~~~----------~8~1--~--------~~FEM Cal150-g 100o50Experiment {=:= ~~deflection ~(mm)Fig. 10Comparison of load-deflection curves (MA-1)696

'~'/ .',i', ".P-'008,(a) principal stresscomp_tens.(b) variation ofelement properties-d:)(:concretecrackyieldsteelyieldFig. 11 Calculated principal stresses and variation ofelement properties697

-/- ,// --;/ . I //_ --=Ly /!/ /1-// -' I! / / .I _ I •/ -'I / IJ..'/' II! I / ! 'I-\~ /" ,, \\.. '\'.tI,'.SCALE'-' 22 kgf/mm 2CONCRETEELEMENT~ dl ~CRACK YIELD CRACKa YIELDSTEELELEMENT~ =CRUSHYIELDFig. 12 Calculated principal stresses and variation ofelement properties (MA-1, p = 187tf)698

TESTS ON COMPOSITE ICE-RESISTING WALLSB.O'FlynnBecker EI Zein and Associates, Ltd., Edmonton, Alberta, CANADAJ. G. MacGregorUniversity of Alberta, Edmonton, Alberta, CANADAAbstractComposite steel-concrete-steelsandwich walls have been proposed as aneconomical and structurally efficientsystem for resisting local ice pressureson Arctic offshore structures. It isenvisioned that such a wall would act asa deep concrete member in which '"shear'"behaviour dominates.Seventeen beam-type sandwichspecimens were tes ted in the laboratoryto examine the effec ts of various designdetails on member behaviour. Both plainconcrete and steel-fiber-reinforcedconcrete are seen as potentially attractiveinffll materials. Early distressmay be encountered in the concrete ifstiffened plate bulkheads withexcessively large stiffener spacings areused. Finally, heat-affec ted zones nearthe supports may reduce the postcrushing-ductilityof the wall.An analytical method is outlinedwhich uses the lower bound theorem ofplasticity. The method gives predictedstrengths which are in good agreementwith those which were measured.This is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987. © The Geophysical Institute,University of Alaska, 1987.IntroductionThe discovery of significant oildeposits beneath the Canadian BeaufortSea and off the east coast of Canada hascreated a need for offshore structurescapable of withstanding the rigors ofthese harsh environments. Inparticular, the walls of thesestructures must resist ice pressures ofthe order of 10 MPa (Watt 1984).Existing exploration structures for theArctic have been constructed usingeither structural steel or reinforcedconcrete walls. Composite ice-resistingwalls have been proposed as analternative to these systems (Matsuishiet al. 1977). A composite ice-resistingwall consists of two continuous steelplates separated by a concrete core andsupported by internal bulkheads, asshown in Fig. 1. Composite actionbetween the steel and concrete layers isprovided by a combination of interfacialfriction and mechanical shearconnectors.This composite sandwich systemcombines the advantages of bothmaterials, and eliminates the drawbacksassoc ia ted wi th walls built from ei thersteel or reinforced concrete alone.Since ice impact is localised at thewaterline, the significant wallthickness allows for verticaldistribution of load to the bulkheads,699

~: } E i f=::: ~sl;;g mod_U_I_e __...SeabedTypical gravity platformComposite ice- resisting wall-~- Steel plate~", l front 8 bock)-,.',',, • Concrete infillFigure 1.Composite ice-resisting wall configuration.thereby permitting efficient bulkheaddesign. The large volume and lowelastic modulus of the concrete corepermi t significant energy absorptionwithout visible permanent deformation.Finally, the external steel platesdouble as both formwork and structuralreinforcement, thereby eliminating theneed for expensive temporary works andreinforcement cages.The optimum span-to-depth ratio forthe walls is a function of the strength,serviceability and constructabilityrequirements for both the wall andbulkheads. In a prototype structure, aspacing between the plates of at leastone metre is required to allow access tothe inters titial s pace for welders,concrete placers and inspectors duringconstruction. A spacing of five metresis considered reasonable for thebulkheads. Accordingly, a s pan-to-depthratio of about 5 is prac tical. Such amember is "deep beam or plate" for thepurposes of analysis and design. Thesmallest area over which ice loading canreasonably be exP cted to act is1approximately 0.5 m. On a 5 meterspan, it is therefore proper to considerice-loading as a patch load or a uniformload.Object and ScopeA research program was carried outto fulfill the following objectives.1) To develop a procedure for theanalysis of the strength of compositeice-resisting walls. Such aprocedure should be rational, simpleand versatile.2) To calibrate the analytical procedureby means of a series of tests.700

3) To investigate the effects of variousdesign details on the behaviour ofcomposite ice-resisting walls.4) To proposecompositeconstruction.suitable detailsice-resistingforwallThe effects of the followingparameters on the behaviour of composi teice-resisting walls with span-to-depthratios from four to six, inclusive, wereinvestigated:1) Concrete strength2) Concrete type (plain or steel-fibrereinforced)3) Type and layout of mechanical shearconnectors4) Type of bulkhead (solid or stiffenedplate)5) Lateral6) Type of7) Extentsupports8) Load cycling.concrete confinementloading (uniform or patch)of rotational restraintatExperimental ProgramSeventeen 1/4-scale beam-typespecimens were constructed and tested.The beams were simply supported, andincluded cantilevered ends to modelcontinuity. Five specimens with spanto-depthratio (~/d) 4 were testedfirst, followed by seven specimens with(~/ d) = 6. The results of these twoseries were used to optimise details forfive further specimens with (~/d) = 5.Table 1 summarises the parametersassociated with each beam. Within thecontext of these beam-type specimens,two moitPA of' fail.ure loJere possible;yielding and strain hardening of thesteel plates (ductile behaviour) orfailure of the concrete core (brittlebehaviour). Most of the test specimensin the experimental program were overreinforced,to investigate the effectsof various parameters on concretecapacity. However, some underreinforcedspecimens were also included,cl~-£t .... Iai~~ c- ~E

SpeClmen b L 1tiP tspd/I (mm) (mm) (mm) (mmi (mm) (mm)84/1 375 400 1000 10.28 10.20 25084/2 375 400 1000 10 14 9 45 25084/3 375 400 1000 9.35 9.42 25084/4 375 400 1000 10.09 10.25 25084/5 375 400 1000 10.18 10.22 25086/1 375 500 1500 13 30 13.17 25086/2 375 500 1500 6.43 6.31 25086/3 375 500 1500 13.17 13.26 25086/4 375 500 1500 6.32 6.34 25086/5 375 500 1500 13.35 13.26 25086/6 375 500 1500 13.37 13.30 25086/7 375 500 1500 13.31 13.26 25085/1 375 500 1250 16.22 16 18 25085/2 375 500 1250 16.26 16.21 25085/3 375 500 1250 6.97 6.70 25085/4 375 500 1250 6 98 6.68 25085/5 375 500 1250 6.96 6.70 250Table 2.Dimensions of test specimens.to observe some general characteristicsof duc tile behaviour. Beam dimensionsare given in Table 2. The support widthwas 150 rom throughout. In the B4series, either 12~ x 50 Nelson studs(B4/1, B4/4, B4/5) or 8 x 50 weldedplate connectors (B4/2, B4/3) wereused. The B6 series and B5 series usedNelson studs only. A typical studlayout is shown in Fig. 2.The specimens were loaded by meansof 16 hydraulic rams. Because of thelarge number of point loads, the loadingarrangement could be idealised asdistributed loading. Three specimens(B4/3, B4/4, B4/5) were supported onstiffened plate supports intended torepresent a steel bulkhead withstiffeners (Figure 3), to see if thistype of support affected the loadcarrying capacity of the beam. Theremaining 14 tests were supported onsteel blocks.Figure 2.Typical stud layout.702

Test SetupFigure 4 shows an overall view ofthe setup used to tes t the spec imens •The tension-Ieg/crosshead assemblieswere movable, allowing beams ofdifferent spans to be tested.The specimens were inverted andloaded by 16 450 kN capacity hydraulicrams. Interaction between the testspecimens and the loading frame wasminimised by means of rocker and rollerassemblies. To further reduce theinfluence of the test frame on theload/deflection response of a testspecimen, the frame was very stiff. Themaximum anticipated elongation of thetension legs was approximately 1 mm.Figure 3.Stiffened plate support.Dis placements were recordedelectronically using linear variabledisplacement transducers (LVDTs).Concrete s trains were recorded manuallyusing 50.8 mm gauge length Demecrosettes. Steel strains were measuredusing 5 mm gauge length electricalresistance strain gauges.Four rams at midspan had 325 mmstroke, to accommodate largedeflections. The remaining 12 rams had150 mm stroke. Load was determined frompressures measured at a pressure gaugenear the pump, and by an extra ramreacting against a load cell. Thetension legs in the loading frame werealso instrumented to measure thereactions.(Braces between loading devicesnot shown for clarity ICrossheodR1Ir!!!!!!::::===~=;IIm+---- Steel reactionframeReoction deviceTension leg ---~Specimen~!i49'''l9'='~=l3''''!f''1:f'13::--LoOding deviceRamPedestalReinforcedreacti on beam'. ' .. ',........ . .......: .. , ... ..... . ~~~'., .. ""., .. : ..•.~ Section Side elevationFigure 4.Test setup.703

Load and reaction devices allowedrotational and translational freedom atthe external boundary of the testspecimen. The rollers and rockers atthe jacks were designed to apply loadperpendicular to the loaded surface ofthe specimen. This simulates thesituation where the friction between theice-resisting wall and the crushed iceis small. Corres ponding pairs of jackswere braced together so that the linesof action of the forces in the ramsremained constant through large specimendeforma tions. The static coefficient ofrolling friction for a typical loadpointroller assembly was measured to be0.8 to 1.0% under an applied load of 400kN. The rocker/roller system utilisedat the supports was designed so that thereaction always remained vertical, andthat its line of action always passedthrough the same point on the supportplate of the specimen. This wasintended to simulate a fixed bulkheadlocation.Test ResultsWith the exception of B6/4 (whichwas under-reinforced and used steelfibre-reinforcedconcrete), allspecimens failed by diagonal failure ofthe concrete core. Typically, a seriesof tension cracks formed a fan-typepattern centered at the support as loadwas increased. These tension cracksbecame closely spaced just prior tofailure (Fig. 5).Figure 6 shows the load-midspandeflec tion res ponses for the B4series. It can be seen that all of thespecimens continued to carry adecreasing load after the peak load hadbeen reached. The tests were terminateddue to asymmetry in the loading systemafter one side had failed, and notbecause of any inability of thespecimens to undergo furtherdeformation. In the case of specimenB4/3, crushing developed at both endsand the loading system remainedsufficiently symmetric for the load toeventually increase beyond the value atwhich concrete crushing first occurred(Fig. 6).Specimen B5/4 was tested with theends restrained against rotation. Thisallowed a symmetric failure pattern todevelop, and very large pos t-failuredeformations were obtained (Fig. 7). Itcan be seen that the load carryingcapacity after failure is obtained froma combination of the shallow concretearch and the hanging action of bothsteel pIa tes •Post failure ductility isimportant, since the area under theload-deflection curve is representativeof the energy absorbed from the iceevent. In this context, the failuremode of specimen B5/3 is alarming. Inthis specimen, the support plateruptured inside the face of the support,shortly after the concrete crushed. Thefailure occurred in the heat-affectedzones due to studs along the inside faceof the support. These zones reduced theductility of the plate and initiated theearly rupture.The load was cycled 5 times betweenzero and approximately 70% of themaximum load in specimens B4/1, B4/2,B4/4 and B6/1. B4/4 used a stiffenedplate support (Fig. 4); B4/l, B4/2 andB6/1 reacted against solid supports.For the specimens wi th solid supports,load cycling resulted in only veryslight elongations of some existingcracks. However, B4/4 (with thestiffened plate support) exhibiteddistress during cycling. Existingcracks elongated and joined, and newcracks were formed. Significantinternal noise from the concrete wasalso detected during the unloadingphases of cycling.The load transfer at stiffenedplate supports is shown schematically inFig. 8. Curvatures measured on thesupport plate by the strain gauges shownin Fig. 8 indicated tha t the curvatureincreased rapidly at approximately 60%of the failure load. This curvature canonly be caused by direct transverseloading of the support plate between thestiffeners. It is therefore concludedthat the lateral concrete arches betweenthe stiffeners deteriorate at an earlystage due to the high concretecompressive stress over the stiffeners.704

Figure 5. Typical cracking pattern inan overreinforced specimen at incipientfailure.Figure 7. Failure mode of specimen B5/4.LEGENDCt---l!l B 4/1~B4/2--... B4/3+--+ B4/4-..: B4/5DeflectionB4 Series: Load .vs. Midspan DeflectionFigure 6.Load-midspan deflection plots for the B4 series.705

I'375'jGouge\A-\-,--,lfTI-h-T'rr4t- loco I ion sFigure 9. Failure mode of specimen B6/4.Figure 8. Schematic load transfer at astiffened plate support.Further cycling could cause appreciabledamage within the concrete core, andultimately lead to a reduction in loadcarryingcapacity. Stiffened platebulkheads must be designed toaccommodate the external wall strengthrequirements, as well as their own.Specimens B6/2 and B6/4 differed intha t B6/4 used steel-fi bre-reinforcedconcrete, whereas B6/2 had a plainconcrete core. Both specimens wereunder-reinforced, to promote ductilefailures, and significant strainhardening was indeed obtained in thesupport plates of both specimens.Specimen B6/2 ultimately failed shortlyafter the formation of a major diagonalcrack in the concrete. The steel-fibrereinforcedconcrete in B6/4 did notfail. The specimen continued to carryincreasing load through very largemidspan deflections, and the test wasterminated when the testing frame couldaccommodate no further deflection of thespecimen (Fig. 9). Steel-fibrereinforced concrete, then, is seen as anattractive infill material for compositeice-resisting walls. The addition ofsteel fibres approximately doubles theunit cost of the concrete. However, thetotal cost of the infill material wouldbe small when compared wi th the overallcost of the ice-resisting wall.One cantilever in B6/4 failedprematurely due to the inadequate fusionof one headed stud. Two more studssheared before the "unzipping' wasarrested. Because of this failure, thespecimen was loaded on the span only forthe latter part of the test. This inpart accounts for the very large endrotations seen in Fig. 9.The effect of loading arrangementwas investigated in B6/7, which wasloaded by an eccentric patch. Failureoccurred on the long shear span (Figure10), even though the shear force islarger in the short shear span. Theconcrete strains were of approximatelythe same magnitude in the two shearspans, due to the shallower inclinationsof the principal strains on the longshear span.Figure 10.B6/7 .Failure modeof specimen706

The effects of the type of shearconnector, the presence of diaphragmplates at the supports and the presenceof shear studs on the span of thesupport plate were also investigated.It was found that, even though crackingpatterns are affected at the earlystages of loading, these parameters hadno apparent effect on the load carryingcapacities of the beams.The longitudinal strain profile inthe loaded plate of specimen B4/1 isshown in Figure 11. This figure can beconsidered as representative of thestrain profiles in the otherspecimens. The strain gradient in theloaded plate indicates significant sheartransfer across the steel-concreteinterface, and so composite action isdominant. Despite the lateral restraintoffered to the loaded plate by theembedded studs it has been shown(O'Flynn 1987) that the assumption ofuniaxial stress in the loaded andsupport plates is reasonable. Thestrain in the support plate was almostconstant between the supports,indicating tied-arch behaviour in theconcrete.! §i -IEy= 14911'E t-------------- ---,i 0 1---'----+----+---'-----"-------1.!:.Ii ~1? CS(i; TFigure 11. Strain in loaded plate ofspecimen B4/1.Analytical ModelAn analytical model based on thelower bound theory of plasticity hasbeen developed to define the loadingconditions at which failure of thespecimen will occur. The procedure isdescribed in detail elsewhere (O'Flynn1987), but is summarized below.An appropriate analytical modelmust satisfy the following threecriteria (in accordance with the lowerbound theorem of plasticity).(1) A state of stress which is inequilibrium with the applied loadsmust be described for all pointswithin the specimen.(2) The prescribed state of stress mustnot exceed the material capacity atany point within the specimen.(3) The specimen must be sufficientlyductile to allow the state of stressto occur.Criterion (3) is deemed to be satisfiedby prescribing a stress field which doesnot deviate very far from the elasticstress distribution (Schlaich et al.1987).Rather than checking load effectagainst material capacity at every pointwithin the continuum, a specific failurelocation which was consistently observedin the tests is chosen. This locationis designated as "K" in Figure 12. Theanalytical procedure then reduces tofinding the applied load magnitude atwhichf * c[1]where O"lK* is the principal compressivestress at location K, and fc* is theconcrete capacity at that location.In order that O"lK* can be defined,it is necessary that the entire stressfield in the concrete core bedescribed. This is accomplished usingequilibrium fans (Marti 1985). The fanstress field for a typical beam underuniform load is shown in Fig. 13. It isseen to consist of three zones (I, IIand III). These zones can be compared707

KStress-free areaFigure 12. Failure location uK" which was observed in the tests.papI(a) Fan model(b) Strut and tIe mQcLelFigure 13.Statically admissible stress fields (a) fan-type (b) strut-and-tie.708

with struts I, II and III in thecorresponding strut-and-tie model ofFig. 13(b). In this figure, the strutsare uniaxially stressed, the nodes arehydrostatically stressed, and sheartransfer is necessary between the steelplates and the ends of the struts whichdo not adjoin a hydrostatic node.Figure l3(a) is statically equivalent toFig. 13(b), but the struts are "fannedout" to form the fans in Zones I, II andIII. Since the fans diverge, theuniaxial stress decays along a fan line(Thurlimann et al. 1984). Nevertheless,the uniaxial stress C1 1K* at K can bedetermined graphically from purelystatical considerations.The effective concrete strength fc *can be defined asf c* = vf' c[2]where f~ is the concrete cylinderstrength at the date on which thespecimen was tested, and v is theconcrete effectiveness factor. Once vis quantified, then equation [1] can bechecked. The effectiveness factor v isa measure of the degree of confinementof the concrete, and can be defined as ameasure of the interaction between thegeneric transverse stiffness of theconcrete and the restraint offered bythe adjacent concrete at K, i.e.v = f (f' G)c' K[3]where G Kis a confinementcharacteristic. To define G K, considerFig. 14. It is assumed that the crackAD exists at any and every load level.From geometry, the crack width, w iswherew[1 - (h s /d)]aTsp"1 + (a/d)2 • E tsp sp[4]tension (N) in unit width ofsupport plate at load levelcorresponding to C1 lKheight of studs (rnm)modulus of elasticity of supportplate (MPa)thickness of support plate (mm)dFigure 14. Definition of the crack width w, used in the definition of the confinementcharacteristic G • K709

The confinementis then defined asa 1Kcharacteristic, G K,w[5)and is independent of load level. Tand a 1Kare found from the fan stre~gfield.To find the form of equation [3), aregression analysis was performed on thetest data. The proposed equation is(3.40 + 0.0243G ) K [6)v = --------------~-IT'cEquation [6) is plotted against theexperimentally obtained values for v inFig. 15. This figure can also beconsidered as representative of test-topredictedfailure load ratios which areobtained using [6)."1.61.20.80.40.4Figure 15.effectiveness[6).Conclusions0.8 1.2Predictionfactor v usingof theEquation1. Significant post-failure ductility isavailable in specimens after initial"shear" failures occur.2. This ductility may be reduced bypremature plate fracture at heataffectedzones.3. Stiffened plate bulkheads may reducethe peak load which can be sustainedafter load cycling.4. Diaphragm plates at the supports haveno effect on the load carryingcapacity of the beams.5. Studs on the support plate betweenthe support regions have no effect onthe load carrying capacity of thebeams.6. Rotational restraint at supportsforces a symmetric "shear" failureand allows significant post-failureductility, but does not affect thepeak load sustainable.7. Steel-fibre-reinforced concreteoffers an attractive alternative asan infill material.8. An analytical method has beendeveloped which realistically andsimply describes the load pathswithin the specimens and hence theloads at which failures occur.AcknowledgementsThe research described in thispaper was funded by the Centre forFrontier Engineering Research (C-FER)and the National Research Council ofCanada through operating grant A1673.Steel fibres for the testing phase weredonated by Domecrete Canada Ltd.ReferencesMarti, P., "Zur Plastichen BerechnungVon Stahlbeton", Dissertation Nr. 6602,Eidgenossischen Technischen Hochschule,Zurich, 1980.Matsuishi, M. et a1., "On the Strengthof New Composite Steel-Concrete Materialfor Offshore Structure", Paper No. 2804,Offshore Technology Conference, Houston,Texas, 1977.O'Flynn,Walls",Alberta,1987.B., "Composite Ice-ResistingPh.D. Thesis, University ofEdmonton, Alberta, Canada,Schlaich, J. Schafer, K., and Jennewein,M., "Towards a Consistent Design ofReinforced and Prestressed Concrete" ,Journal of the Prestressed ConcreteInstitute, May-June 1987.Thurlimann, B. et al., "Anwendung derPlastizitatstheorie auf Stahlbeton",Institut fur Baustatik und Konstruktion,Eidgenossische Technische Hochschule,Zurich, 1983.Watt, B.J., "Ice Load Considerations forConcrete Structures", Proceedings of theFIP/CPCI Symposia, Calgary, Canada,1984.710

EXPERIMENTAL STUDIES ON COMPOSITE MEMBERSFOR ARCTIC OFFSHORE STRUCTURESF.OhnoT. ShioyaY. NagasawaG. MatsumotoT. OkadaT.OtaShimizu Construction Co., Ltd., Tokyo, JAPANAbstractPresented are the results frommodel tests of a composite steel/concrete member using a steel/concretesandwiched system for the purpose ofestablishing a strength estimationmethod and studying the characteristicbehavior of the members. The results aresummarized as follows;1) The results of the shear tests showthat the ultimate shear strengthof the composite membersestimated by Niwa's equation andZsutty's equation (both for reinforcedconcrete) exhibited goodagreement with the measuredstrength.2) Flexure tests confirmed that theyield moment of the compositemembers can be calculated by theconventional theory of reinforcedconcrete.3) It has been confirmed by shear testsand flexure tests that the compositemembers have high ductility whencompared with reinforced concretemembers.This is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987. © The Geophysical Institute,University of Alaska, 1987.IntroductionArctic offshore structures areexposed to severe ice conditions. Acomposite member, in which concrete isinjected into a steel encasement, isapplicable for use in such conditionsowing to its excellent ductility andhigh strength. It has the potential toimprove the constructability of astructure which may result in thereduction of construction costs and thetime period needed. However, practicalmethods for the construction and designof such composite members have not yetbeen fully established. Therefore,extensive research work has been carriedout. to develop an applicable constructionprocedure and to verify thestrength characteristics (Shioya et al.1986) .Previously, the authors carried outconstructability tests and establishedan effective concrete injection systemfor composite members.After the constructability tests,the authors conducted a series of modeltests on the composite members. Thispaper reports the results of thefollowing model tests.(1) Shear tests (5 models)The purpose of the tests was toevaluate the shear strength of711

~.t----------~-- ----------Table I-AVarieties of modelsDimensions of model Concrete Steel Inner Stiffener----- ------,---- ---r-------~-----Sphttlng---HainTest Test Loading Hect>Coarse Compo tensile Yield Tennie steelmodel Length span depth Breadth auregat strength strength Th1ckness strength strength ratio Type Helght SpaclOL Q d b G mllx f'" f't t fsy f st P h, 0,(mm) (M) (mm) (M) (00) (kgf/c",l) (kgf/co') ( ..) (kgf/c.- (kglle.- (I) (M) (M)CBR 4000 1800 618 590 15*3 606 26.4 12 I 3100 4500 1. 94 L-shape 550 350~-~}-t~-:~~- r-- ~--CBF 4000 1800 618 590 15*3 579 31.7 4500 1. 94 Lattice 100 200--~---- ----t---~-- f----- ~-- -----. S*' CSR 4000 1800 618 1190 15*' 606 31.1 4500 1. 94 L-shape 550 350--r--- r-~ 1-- --- -----~~-CSF 4000 1800 618 1190 15*3 579 30.612 ~oo 4500 1. 94 Lattice 100 200---- ----- -_.-----~-RCB 4000 1800 600 600 15*3 564 27.9 -1/10-;9 3500 5600 1. 99 - I -CBHO-Al 3900 3300 296 300 25*' 423 28.3 9 3300 5300 3.05 A 10 -1----- ------ ----- -._- -- ------ -~---------CBHO-A2 3900 3300 296 300 25*' 442 29.7 9 3300 5300 3.05 A 25 ----- ---------- --.-._---------- ----CBNO-A3 3900 3300 296 300 25*' 446 29.9 9 3300 5300 3.05 A 50 -CBNO-Bl 3900 3300 296 300 25*' 449 30.2 9 3300 5300 3.05 B 10 250~------- ------------ --- ------------ ----- ----CBNO-B2 3900 3300 296 300 25*' 464 31.3 9 3300 5300 3.05 B 25 250-~---- ~------ ---- ---- I---F*' CBNO-B3 3900 3300 296 300 25*' 470 31. 7 9 3300 5300 3.05 B 50 250----r----- ~----------r--CBOO-B4 3900 3300 296 300 25*' 472 31.9 9 3300 5300 3.05 B 10 500-----r--CBHO-B5 3900 3300 296 300 25*' 477 32.4 9 3300 5300 3.05 B 25 500CBNO-B6 3900 3300 296 300 25*' 478 32.6 9 3300 5300 3.05 B 50 500----- - I~CBNO-B7 3900 3300 296 300 25*' 482 32.9 9 3300 5300 3.05 B 10 750CBNO-B8 3900 3300 296 300 25*' 483 33.1 9 3300 5300--~-3.05 B 25 750-~ --'--CBNO-B9 3900 3300 296 300 25*' 488 33.6 9 3300 5300 3.05 B 50 750-------- ---- --- ----RBMOP-3-1 3950 3300 300 300 25*' 472 35.5 116xl0 3500 5300 3.19 - - -*1) Shear test *2) Flexure test *3) Mlx.l: Llght welght aggregate*4) Mix.2: Normal weight aggregate712

Table 1-BTest results~·---'r------'r------------------=T-e-s~t-r-e-s-u~l~t-s-------------------'TestTestmodelCBRYieldingloadPy(ton)I -Max.load·P mllx !(ton) !iI 221I 329I m:::~tMUlal

Steel plates, 12mm thick, were used asboth tension and compression plates andthe steel ratio of each plate was about2%. Two types of stiffener configurationswere tested:(A)(B)RIB type:L-shaped stiffeners were arrangedtransversely on both tension andcompression plates.FB type:Lattice shaped flat bars werearranged on both tension and compressionplates.At the same time, a RCB model(Reinforced Concrete Beam), having thesame steel ratio as the composite testmodels, was provided to compare theshear strength.(2) MaterialsThe compressive strength ofconcrete was approximately 600kg/cm2.The mix proportions are shown in Table2. The types and properties of the steelare shown in Table 1.Table 2 Mix proportions~lix numberTarget compre~sl ve slrength (kgf/em') f' .. ,Tnrget uni t weight (ton/m') 7·MaXimum Slze of coarse aggregate (mm) G.,."Rnnge of slump (em)Range of air content (%)Water-cement ratio (%)Sand-aggregate ratio (X)UnitWaterCementi t ious ! Cementcontent binder I Sllica fume(kg/m')SandGravel,~ Normal weight aggregate**Light weight aggregateMix number 1: Shear test2: Flexure testJ3) Concrete curing methodwlcslaWCSFSGI 2500 45()2.00 -IS*' 25'>25 24127±2 51229 3538 39146 157502 45050 -589 646598 1042The test models were cured in airuntil the loading tests.(4) Shear test methoda/d0.73The shear span-to-depth ratios(a/d) were 0.73 (CBR,CSR,CBF,CSF) and0.75 (RCB). The tests were carried outusing simply supported models, withfour-point concentrated loadings. Theloading method is shown in Fig. 1.CSF(CBF) model(: :: : ; ~ : : : ~ ; : ~ : ]3RCB modela/d0.73~I (J"u) IWIioFig. 1 Shear test models and loadingconditionsResult of shear test(1) Failure modeFig. 2 shows the test models afterfailure under shear force.(A) CBR: Although the shear loadcarrying mechanism of the CBR modelwas a tied-arch, the maximum shearstrength was smaller than that ofthe FB type models. This isbecause, for the RIB type, a L­shaped stiffener existed betweenthe support and loading point, andwhen a concrete diagonal compressionstrut was compressed to acertain degree the concrete slid onthe L-shaped stiffener. This slidecaused the crushing of the diagonalcompression strut.714

(B) CBF: The shear load carryingmechanism of the CBF model was alsoa tied-arch. Around "t: 82 to 84kgf/cm2 the tension and compressionplates yielded at the supportingpoints and the loading points, thenthe local deformation at thesepoints increased with the gradualincrease of the load. The test wascontinued until L= 90 kgf/cm2.(C) CSR: Although the CSR modelshowed similar cracking behavior tothe CBR model, the shear strengthof the CSR model was larger thanthat of the CBR model.(D) CSF: The cracking behavior andthe shear strength of the CSF modelwere similar to those of the CBFmodel.(E) RCB : The shear load carryingmechanism of the RCB model was atied-arch, and the failure mode wasa concrete shear compressionfailure of the diagonal compressionstrut .(1) CSR model(2) CSF modelNeu;;:;- 100t1>.>

~ ,,, .. : Shear strengrh of deep beam\kgf/cm' )V, Shear force of deep heam(kgf jcJIl ")Pw Steel ratio (tension plate)f' , Compressive strength ofconcrete (kgf / cm ")bw Width of model (cm)d Effective depth (cm)a Shear span length (em)'l Width of the support plate(cm)Z~~!y~e9uationr L"TTY=V,. / (bw·d)= 1 50 · (f ' c . Pw) , I ' • (d / a) • I 3[ Z", 'TTY : Shear s trength of deep beam(psi)V, Shear force of deep beam(psi)f' C'PwbwdaCompressive strength ofconcrete (psi)Steel ratio (tension plate)Width of model (inch)Effective depth (inch)Shear span length (jn ~ h)Flexure t estsOutline of flexure t est(1) Test modelsThe dimensions and basic configurationsare shown in Fig. 4 . The t estmodel is a beam of 300mm in depth and300mm in width. This is regarded as aone half to one third scaled model ofthe existing concept of offshorestructures. This test model consists oftop and bottom plates (~ thickness,each steel ratio was about 3%) withconcrete between the plates . As for theflexure test, the followings areconsidered to be the test parameters .(A) Hei ght of stiffeners (hj )(B) Spacing of stiffeners (11)(C) Direction of stiffenersType A: Stiffeners were setlongi tudinallyType B: Stiffeners were settransverselyAt the same time, a reinforcedconcrete model (RBMo-P-3-1 ), having thesame steel ratio as the composite testmodel, was fabricated to compare theflexural strength .Conc'reteTable-3 Shear strength and absorbed energy of shear test model--Test Model CBR CBF CSR CSI! HCBCompressive strength fo (kgf/cm') 606 579 606 57'} 'jh4Modulus of elasticity Eo (kgf/cm') 217,000 210,000 217,000 210,000 24'1,000Modified steel ratio P'm (%) 2.50 3.31 2.21 3.02. 1. 99Modified effective depth d" (em) 61.3 60 .1 61.5 60.2 60.0Calculated by Niwa's equation r N tWA (J

(2) Materials(1) CBMO-A1The target compressive strength ofthe concrete was approximate l y 450kgf/cm2. The mix proportions are shown~n Table 2 . The steel properties areshown in Table 1.3,900.1300 900 1 ,500130300(2) CBMO- B 1Q,r1: : :RBMO-P-3-1300 9001 ,500Fig.4 Flexure test models andloading conditions(3) :oncrete curing methodncAfter concreting , water curing wasapplied for one week and moisture curingwith water sprinkling followed until theloading t est .(4) Flexure test methodThe tests were carried out usingsimply supported models with two- pointconcentrated loading. The loadingcondition is illustrated in Fig. 4.Result of flexure tests(1) Cracking and failure mode(A) Cracking : The test models,afterflexural failure, are shown inFig . 5. Although transverse innerstiffeners were no t arranged forType- A cracks initiated at aspacing of 0.7d to 1 . Od, as shownin Fig. 5. As for Type- B, theflexural cracks initiated aroundthe stiffeners . For CBMO- B7 toB9, where the spacings of stiffeners(gj' ) were relatively large,Fig. 5 Test models after failureunder flexure forcecracks also developed near thecenter of the model.(B) CBMO-A1 and B1 to B8, afterthe yielding of the tensionplate, the buckling of the compressionplate and the crushingof upper concrete occurred .However, this failure did notconfirm wheth~r buckling orcrushing was the primarymechanism.(C) CBMO-A2 and A3 exhibitedlarge deformation and before theyreached the maximum load thetests were interrupted due to theslip of the support plates, thetests · resumed and confirmed themaximum load.(D) CBMO-B9, the elastic bucklingof the compression plateoccurred prior to yielding of thetension plate and the modelcarried load until failure . Thefailure was similar to flexuraltension failure of a beam withsingle reinforcement. Thisprobably occurred due to theadverse effect of residual strainby welding and inadequatemanufacture of the model .717

(2) Load deflection relationThe measured deflection at mid-spanunder flexural loading is shown in Fig .6. For the composite models, unlesselastic buckling of the compressionplate took place, failure occurred byplastic buckling of the compression plateafter 8 to 10 times the deflection atyield. For the reinforced concretemodel, it failed by the crushing of theupper concrete with small deflectionwhen compared with the compositemodels. In short, the composite modelshave a high ductile capacity underflexural loading.(3) Flexural failure strengthFig. 7 shows ratios between theexperimental yield load and thecalculated value in terms ofconventional RC (Reinforced Concrete)beam theory. It can be said that theexperimental yield moment agreed wellwith that calculated by the RC theory .Even when elastic buckling of thecompression plate takes place, the RCtheory, without respect to the.compression plate, can be applicable.The yield load of Type-A is larger thanthat of Type-B. This i s due to theeffect of the longitudinal stiffeners .Considering the longitudinal stiffenersas a part of tension plate in Type-A,the calculation by the RC theory showsgood agreement with experimentalresults . It was observed, up to theyield load, that in the section ofconstant moment span the "plane sectionsbefore bending remain plane afterbending" assumption was made for thecomposite members . The composite memberexhibited higher increment of strengthfrom yield strength to ultimate strengthin comparison with the correspondingreinforced concrete member . For theestimation of ultimate strength, we mayconsider the effect of strain hardeningstate of steel and enhanced strength ofconcrete by multiaxial confinement.(4) Effect of each parameter(A) Type- A and Type-BThe load carrying capacity of Type-Awas higher than that of Type-B.Regarding the degree of mid-spandeflection, no significant differencebetween Type-A and Type-B was100_--- __ --0,--'-00...",'".3 50SymJ:;olooFailure rrodeureo100 200 300Deflection at midspan (mm)400Fig . 6 Load/deflection relationshipobserved.(B) Height of stiffenersThe models with higher stiffenershow higher flexural strength forboth types. For Type-A, thisprobably means mainly the increaseof tension plate, and for Type-B theincrease of confinement of bucklingsupport-pointsat stiffeners.(C) Spacing of stiffenersWithin a spacing that restrains the718

compression• buckling,between thestrength of•....; 1.2plate from elasticno significant effectspacing and the flexuralmodels was observed.Symbol Fa t e ~ r (mm)• 0o B 250A B 500D B 750u '"~,., 1.1.......•D.AQa- LaQ),., 0 ••D~0.9a I- -.a 10 20 30 40Height of FB : hf (mrn)Fig. 7 Experimental yielding load/calculated load relationSummary of the test resultsThe conclusions from the sheartests are;1) The width of model doesn't affectthe shear strength of the FB typecomposite member (CBF and CSF). Theultimate shear strength of thecomposite members estimated byNiwa's equation and Zsutty'sequation exhibited good agreementwith the measured strength.2) The shear strength of the RIB typeis less than that of FB type.3) Comparing the absorbed energy up tothe maximum load, the CBR model was5 times greater and the others weremore than 20 times greater than theRC model's.The conclusions from the flexuretests are;4) The conventional theory ofreinforced concrete can be appliedto calculate the yield moment of thecomposite member.5) The composite member is regarded asa high ductile structure underflexural loading since the test ofthe composite member resulted in a50post-yield buckling failure afterdeforming 8 to 10 times the steelyield deformation, unless thecompression plate buckles in anelastic mode. As for the RC member,the failure mode occurred, prior tosuch large deformation, by crushingof the concrete in the compressionregion .Future considerationsThrough those tests, the compositemembers have shown high ductility andlarge load carrying capacity, and wewould like to continue the projecttaking ductility, ultimate shearflexuralstrength, and adequatestructural configuration into accountfor the design of the steel/concretecomposite structure.The followings will be consideredas the next step of the research anddevelopment.1) The study of shear behavior,especially considering the shearspan-to-depthratio and the shearreinforcement as test parameters.2) The study of the structuralcharacteristics of the compositemembers with lightweight concretewhich is subject to freeze/thawactions.ReferenceNiwa, J., Maeda, S., and Okamura, H.,:"Proposed Design Methods for ReinforcedConcrete Members such as Deep Beams"Proceedings of Japan Concrete Institute5th Conference 1983, pp.3s7-360.Shioya, T., Matsumoto, G., Okada, T .•and Ota, T.,: ,iDevelopment of CompositeMembers for Arctic Offshore Structures"Proceedings of POLARTECH '86 (InternationalOffshore and NavigationConference and Exhibition), Vol.2October 1986, pp.660-677.Zsutty, T. ,: "Shear Strength Predictionfor Separate Categories of Simple BeamTests" ACI JOURNAL 1971, pp.138-143.719

THE ARCTIC AND OFFSHORE RESEARCH INFORMATION SYSTEMHarold D. ShoemakerU.S. Department of Energy, Morgantown, West Virgina, USADavid L. ChiangScience Applications International Corporation, McLean, Virginia, USAAbstractThe U.S. DOE has established a computerizedinformation system to assistthe technology and planning community inthe development of Arctic oil and gasresources. This paper describes theArctic and Offshore Research InformationSystem (AORIS), how it was developed,which sources were accessed for inventory,and how it can be used. The AORIShas an on-line thesaurus and userfriendlyaids to assist in querying theAORIS. There are three principal components:a directory that listsapproximately 85 other data bases containingArctic energy-related informationand how to access them; abibliography/management information system(B/MIS) containing approximately7,000 references and abstracts onenergy-related research; and a scientificand engineering information system(SEIS) containing quantitative data onsea ice, ice gouging/ scouring, and subseapermafrost characteristics from theB/MIS citations. The AORIS also containsmuch of the so-called grey literature,i.e., data and/or locations ofThis is a reviewed and edited version of a paper presentedat the Ninth International Conference on Port and OceanEngineering Under Arctic Conditions, Fairbanks, Alaska,USA, August 17-22, 1987.Arctic data collected but neverpublished.IntroductionThe unique Arctic environment posessignificant technological and economicbarriers to developing Alaskan oil andgas resources. That is, the technicaluncertainties associated with the developmentof offshore oil and gas resourcesin the Alaskan Arctic created a varietyof engineering problems regarding structures,pipelines, and shipping in theoffshore regions. These include iceforces on structures, ice accretion, thestability of seafloor soils on structures,and potential frost heave andsubsidence problems created by subseapipelines. Each phase of Arctic operationspresents technical challenges thatrequire use of the best available scientific,engineering, and managementinformation. This will enable industryto apply innovative and economical solutions,and assist researchers in defininginformation gaps and research needs.The solution to the engineeringproblems identified above requires awide range of Arctic information anddata from a variety of sources. A largenumber of data bases or data centerscurrently exist that contain Arcticinformation and data. These data bases721

cover a broad range of Arctic informationincluding history and culture, aswell as engineering information. TheU.S. DOE role in developing the AORIS isto provide a single source of easilyaccessible bibliographic and technicalinformation which focuses on the needsof the offshore oil and gas community.Purpose and ObjectivesThe development of the AORIS ispart of the programmatic activities ofthe Arctic and Offshore Research Programat the Morgantown Energy TechnologyCenter (METC). That program is involvedin developing an energy-related knowledgebase that will serve to improve theeconomics of fossil fuel production inthe Alaskan Arctic, and determining withmore confidence how much Alaska can contributein offsetting the known declinein future 10wer-48 oil and gas production.Hence, the purpose of the AORISis to advance the Arctic environment andtechnology knowledge base related tooffshore oil and gas development. Acentralized and computerized bibliographicand scientific information systemis being developed and made available tothe engineering, scientific, policy making,and planning community.The principal objectives of thiseffort are: (1) develop an on-linedirectory that identifies and describesother data bases containing Arcticenergy-related information and how toaccess them; (2) develop a bibliographic/managementinformation system containingreferences and abstracts onArctic energy-related research;(3) develop a scientific and engineeringtechnology information system containingquantitative data on sea ice andseafloor/soils characteristics; and(4) make the AORIS available via computerto the user community on a real-timebasis.AORIS DescriptionThe computerized AORlS is menudriven and contains Arctic energyrelatedbibliographical and technicalinformation, i.e., information that isunique to the development of offshoreoil and gas, and a directory where allrelevant Arctic information (databases/libraries) resides. Figure 1illustrates the structure of the threeprincipal components of the AORIS. Figure2 illustrates the AORIS developmentconcept and depicts the ultimate residenceconcept, that the AORIS be readilyavailable to the user community. Thatis, the ultimate purpose of the AORlS isto promote extensive private use of theArctic technology to accelerate developmentof domestic oil and gas resources.In general, the AORIS will be geographicallydependent and, where possible,it will be site specific. Themajor topics will be sea ice, geotechnology,oceanography, meteorology, andArctic engineering, as they relate tooffshore oil and gas activities; i. e. ,exploration, production, storage, andtransportation. The AORIS is beingdeveloped incrementally, and updates andadditions to the informational componentsof the AORIS will be implementedas appropriate.DirectoryThe directory component of theAORlS serves as a "roadmap" to the variousdata bases of interest to thoseseeking the energy-related informationand how to access these data bases. Thedirectory (roadmap) contains a listingof the major sources of Arctic relevantdata or library centers. The directoryalso contains a summary of what is containedin each data base or librarycenter, search methodology, where thedesired information resides, point ofcontact, and appropriate telephonenumber(s). Figure 3 represents themajor Arctic information areas containedwithin the directory.B/MlS and SElSThe information containing portionsof the AORIS consist of two components,the bibliographic/management informationsystem (B/MIS) and the scientific andengineering information system (SElS).The B/MlS contains references and informationabstracts on energy-relatedresearch and engineering activities inthe Arctic. The B/MIS contains the followinginformation on the variousreports or references cited: title,author(s), language(s) available in,722

AORISBIBLIOGRAPHICDATA• PUBLISHED DOCUMENTS • SEA ICE• POST-1965 • ICE GOUGING• "GREY" LITERATURE • SUBSEA PERMAFROST• THESAURUSROADMAP• 85 DATA BASES• CROSS REFERENCE• HOW TO CONTACT• SEARCH METHODOLOGYFIGURE 1.Principal Components of AORIS.USER'S NEEDSARCTICINFORMATION Im:~==~mBIMISAND SEIS• R&D PLANNING• SCIENTIFICRESEARCH• INNOVATIVEENGINEERINGNON· ENERGY !ENGINEERINGINFORMATION• R&D PLANNING• SCIENTIFIC RESEARCH• ENGINEERING DESIGN!INNOVATIVE ENGINEERINGFIGURE 2.AORIS Development Concept.723

established, and the relative levels ofcurrent and future activity for eachuser survey contact were reviewed. Withthis background, approximately 70 companiesor entities from eight sectors inArctic energy development were surveyed:Federal, state, and local agencies; university researchers; A/E firms; drillingcontractors; consultants; and, operators.Their specific informa'tion needs fellwithin a number of critical areas ofArctic research and development, as outlinedin Table 1. The survey recommendationsare being used to the maximumextent possible in the development ofthe AORIS.FIGURE 3.Directory of Other Sources ofArctic Information.source (where published), publicationdate, keywords, and an abstract. TheB/MIS also contains an on-line thesaurusand other appropriate aids to assist insearching the AORIS. The SEIS will containquantitative data and descriptionsof analytical models on sea ice, icegouging or scouring, and subsea permafrostcharacteristics related to energyrecovery. The SEIS will contain thefollowing information from the B/MIScitations: computational procedures,predictive models, characterization data(both tabular data and graphical representationas available), and descriptiveobservations. An illustration of theabove is presented in Figure 4. Thesetwo hierarchical information systems(B/MIS and SEIS) will be user-friendly,linked with the directory, and installedas AORIS on the DOE/METC computer systemin Morgantown, West Virginia.AORIS Development and StatusUser SurveyThe development of the AORIS wasinitiated during the fall of 1985 andbegan with an Arctic users I needs survey.The roles of each sector in Arcticoffshore energy recovery activities wereThere was agreement among the sectorson a variety of issues related toAORIS format and structure. Theseincluded the need for a focused (energydevelopment) information system and a"roadmap" or directory to other Arcticenergy-related data bases or libraries.This will foster coordination with otherdata base managers to minimize "windows"or gaps in Arctic information crucialto energy resource development. Otherrecommendations from the Arctic sectorsincluded:• Enter the Alaska Oil and Gas Association(AOGA) studies/reports inAORIS. List the titles and researchobjectives initially and enter thereports as they are available.• Increase access to internationalinformation sources.• Provide AORIS users with a feedbackloop to DOE/METC.• Provide hard copy on a quick turnaroundbasis.• Provide timely entries or updatesto the AORIS.• Point to or abstract grey literature(unpublished information).• Design the computerized AORIS to beuser-friendly, i. e., not requiringprior training in data base languagesto use and retrieve informationquickly.724

LOCATION/"//""I­ CI)WIeWI-~LLoCI)

TABLE 1.Arctic Information Needs from Users' Survey.Bibliographic NeedsFederalGovernmentStateLocalGovernment Agencies UniversitIesAlE FirmsDr111iogContractorsOperatorsConsultantsIce and Ocean• Structure andComposl.tl.on ofArctlc Ocean• Water Mass Movementsand Impacton Ice Movements• ReactIon ofMarl.ne Organlsmsto Petroleum orPetroleumProducts• Ports andHarbors Studles• Under Ice AmblentNOl.seGeotechnicalEcologICal EOVI roo- Ecologica 1 Env1- roo- SCIencesment- Upper Atmosphere• Environmental • InternatIonalPhys~csRegulations from Sources- AtmosphericOther ArctIC - Bowhead Wha Ie SCIencesCountries• lnternatl-onalSources of• Arctic HazeO~l Spill Clean Up- Physical andChemIcal OceanographyInforma tion- Mar~ne Life• SeismIC Effects - Status of TechnologySc~enceson Fish, Larvae,• Glaciology andand Mammals - Impact Coastal Hydrology• Subsea PermafrostLocatIon andAreas- Geology andGeophys~csCharacteristIcs• Effects of Causeways• PermafrostResearch• Arct~c Eng~neer-• Subsea Permafrost• Sea So~ls • Dr~lling ThroughPack Ice• Offshore Leak~ngin Ice Congested• Grey L~terature Waters- Subsea P~pelines~n Trans~tionZoneO~l Sp~ll Clean Up• Status of Technology- Fate and Effects- O~l Sp~ll Track->figRegulatory• Env~rorunentalConsideration• Design StandardsAOGA Stud~es- Reports- TitlesTechnologIcalDevelopment- Computer Models- DevelopmentTheorIes- Equ~pmentAOGA Stud1e.- Reports- T~tlesStructure/IceInteractIon- By RegIon- AnalogousSubjectsMar~ne Transportation- Ice Breakers- Pipel~nes- A~r Cush~onVehIcles- SubmarInes10g- TerrestrIal and Geotechn~calFreshwaterB~ology - Geohazards RegulatoryArct~c Engineering - EnvironmentalConsiderations- Ice Features - Marit~me Law• Failure ModeConditions- Equations- lee Crystallography- Local VersusGlobal IceProperties- AOGA StudiesIce- Summer IceDynamICs• Factors ActIngon Structures- Scope Protect~onforArtIfiCIalIs lands- Ice EdgePhenomena- Ice Floes• CoastalProcessesAffectIngEros~onPhysJ.cal Env~ronment• Reg~onal Differencesof Ice- Current~

TABLE 1. Arctic Information Needs from Users' Survey. (Continued)Data NeedsFederalGovernmentState Local DrillingGovernment Agencies Un~versities AlE FirmsContractorsOperatorsConsultants• Sea Ice Extent• Ice Composition• Ice Thl.ckness• Location of PackIce Edge• Sea Ice Velocitl.es• Ice Gouglng• BathymetrlcReadings• DClfting BodyData• Wave EnergyGeotechnl.ca 1• Seisml.cAtmosphenc• Data Collectionsnot Appean.ng inJournals (e.g. tCurrent Data)No Specl.fic Sug- No Specifl.c Sug- Need ExtensIve Data Geotechn1calgestlons gestlons Bank to ConductComparative - Soil Propert1esAnalysis t L0081- - Soil CharacteritudinalStudies, zatlonEtc., Data Needed • Seisml.c10 Following Topl.cS • PermafrostSCJ.ences• Upper AtmospherePhysics- AtmospherlcSciences• Physical andChemical Oceanography• Man.ne LifeSciences• Glaciology andHydrology• Geology andGeophysl.cs• PermafrostResearch• Arctic Englneering• Terrestrla 1 andFreshwaterBiologyIce Features• Islands• Pressure Rldges- Multlyear Floes• Scourlng andGouglng• Stress and Straln• Thickness• Movement VelocltiesHeteorologlcal• Wind/Waves• Temperatures- SAR DataOceanographic• Currents• Ice Cover• BathymetryGeotechnical• Sl.te Specl.fic- Soil Properties- Geothe rma IGradlents- Soil Boring Data- PermafrostIce Features- Concentrations- Strength- Thlckness- MovementVelocityMeteorological- Wind Speed• AtmosphericConditions• Ice Cover- SAR DataOceanographic• Waves/"hnd• Currents• Bathymetry• Ridge Statistics • Material Propertles• Ice Gouging of Ice• Ridge Clearing - Ice Motlon DataLoads Around • Ice ThlcknessStructures - Data from Ice Island• Loads on Struc- Sensorstures for Wlnter • Sateillte Imageryand SwrunerSeasonsOcean- Properties ofIce Rubble • Currents Throughout- Multlyear IcePackWater Column• Wave Helghts• Thickness(H,ndcast)- Temperature andSalini ty Profiles• Bering Str81.tCurrents• Water TemperatureOcean and Atmospheric• Currents- Bathymetric- Water and AlrTemperatures• Wind SpeedsGeotechnical• Sea FloorResistance toOffshore Structures(e.g.,5011 ShearStrength)• Seismic Readings• Sea Floor PermafrostAtmospherlc• Wl.nd Speed (Use toInfer Wave Helght)• FogMeteorologlcal- Hlndcast Data- AtmospheriC Pressure- Al.r Temperature• Humldity• Cloud CoverGeotechnIcal• Data in Wedk Zones(e.g , ~lacKenzleDelta has 0 ShearStrength)

Corporation; and Dr. Harold D. Shoemaker,DOE/METC AORlS Manager. The TRP usuallymeets three to four times a year.RoadmapThe roadmap or directory componentof the AORlS contains a listing of86 data bases containing Arctic topicsof interest to those interested in developingArctic offshore oil and gas. TheB/MlS and SElS components of the AORlSgave priority to Arctic engineering andgeotechnical information and data. Theroadmap complements the B/MlS and SElSby guiding the user to sources of Arcticinformation in these and the major Arctictopic areas (Figure 3). When the variousdata bases contained in the roadmap arelisted in alphabetical form, they aredisplayed in a matrix containing theArctic information resources indexed bythe 10 main topics shown in Figure 3.This cross-referenced matrix is illustratedin Figure 5. The screen inFigure 5 shows the first 12 Arctic databases whose titles begin with the letter"A." This matrix gives a concise pictureof which specific topics of Arctic informationare referenced by the sources.The roadmap then allows the user to viewselected information from these specificdata bases. Selections may be made viaone of the 10 main topics (Figure 3), byvarious subtopics with respect to eachmain topic, from a matrix of data basesby topic (Figure 5), and by a specificdata base title or portion of title.When a particular data base is called up,besides the title, the other informationdisplayed includes: data base type, subjectmatter, data base developer or producer,whether there are on-line servicesor not, whether access through a gateway,whether it is open to the public, whetherhard copies or reports are available,language, time span of literature containedin the data base, how often thedata base is updated, who to contactincluding telephone number and address,and a summary abstract describing thecontents of the data base or librarycenter (see Figure 6).B/MlSThe bibliographic/management informationsystem (designated BlBLlO onscreen in AORlS) currently contains over7,000 Arctic energy-related citations.The B/MlS component of the AORlS providessearches of articles, journals, books,and other publications dealing withArctic topics. The B/MlS will providemuch of the needed information on suchtopics as sea ice, ice gouging or scouring,seafloor/soils, subsea permafrost,seismic activity, pipelines, offshorestructures, icebreakers, and subicehydrocarbon development technology. TheB/MlS is structured as a user-friendly,menu-driven information system to expeditethe search procedure, and to enablethe user to use the AORlS with as littleinstruction as possible. Scientific andnonscientific personnel alike havesearched the AORlS with ease and withoutdocumented assistance. The initial orbaseline B/MlS has been completed andup-loaded on a DOE/METC VAX 11/780computer (November 1986). The bibliographicportion (reference citations)of the B/MlS will become part of the DOERECON data base system at Oak Ridge,Tennessee, during 1987.The AORlS centralizes referencesfrom currently available sources ofArctic energy-related information byincluding information that has alreadybeen catalogued by other sources. Figure7 illustrates how the AORlS pullsinformation from a wide variety ofsources. A computerized data base searchwas conducted to establish a baseline ofcurrently available Arctic energy-focusedreferences. This information is and hasbeen enhanced by adding nonclassifiedmilitary, international, and currentlyunpublished information or grey literature.Computerized information services,as well as libraries, universities, andprivate industry research contribute tothe more focused AORlS objectives.On-line bibliographic informationsystems use a set of keywords to facilitateand expedite searches in the topicdesired by the user. To further facilitatethe user-friendliness of AORlS, theB/MlS contains an on-line thesaurus.Thus, if a descriptive phrase that isfamiliar to the user is not a "legal"AORlS keyword, he/she can query thethesaurus for the appropriate legal keyword.This on-line thesaurus helps theAORlS users to narrow their queries byproviding a hierarchy of terms so they728

******************************************************************************123456789101112AORIS DATA BASE LIST X-REFERENCES WITH TOPICSDATA BASE 1 2 3 4 5 6 7 8 9 10ACTIVE WELL DATA ON-LINE X XAERIAL PHOTOGRAPHY LIBRARYXALASKA CLIMATE CENTERALASKA DEPARTMENT OF FISH AND GAHE LIBR* XALASKA DIVISION OF GEOLOGICAL AND GEOPH* X X XALASKA OIL AND GAS CONSERVATION COHMISS* XALASKA RESOURCES LIBRARYX XAMERICAN PETROLEUH INSTITUTE HONTHLY CO* X XAMERICAN/CANADIAN STRATIGRAPHIC INDEX 0* X XAPILITXAPIPATXAQUACULTURE1 ARCTIC ENGINEERING2 GEOLOGY AND GEOPHYSICS3 GEOTECHNICAL4 GLACIOLOGY AND HYDROLOGY5 MARINE LIFE SCIENCESX6 METEOROLOGY7 PERMAFROST8 PHYSICAL & CHEMICAL OCEANOGRAPHY9 TERRESTRIAL/FRESH WATER BIOLOGY10 UPPER ATMOSPHERE PHYSICSRETURN to continue, or number of Data Base to View:PF1 PF2 PF3 PF4 ESC-HROADMAP BIBLIO DATA EXITKEY HELP******************************************************************************XXXXXXFIGURE 5.Roadmap Cross-Reference Matrix.can use the narrowest appropriate term.The AORIS thesaurus is appropriate tothe B/MIS and is less extensive in scopeand more intensive in detail than mostother technical thesauri. The user mayalso consult the thesaurus at any pointin his/her search of B/MIS citations.The user may search the B/MIS by upto six keywords, one author, a title (orbeginning portion of a title), a datarange, or the unique AORIS identificationnumber. If mul tiple search cri teriaare used, a logical AND is assumed.Keywords used in the search are automaticallychecked for validity and if akeyword is also used in the roadmapcomponent of the AORIS, a message isdisplayed to indicate that the roadmapcould also be referenced. If an invalidkeyword is used that is known as a "donot use term" in the AORIS thesaurus, amessage will be displayed indicating theappropriate "use" term, or the thesauruscan be consulted.After the user's search criteria hasbeen entered, the user is asked if he isready to search or if he/she will needto re-enter the criteria. If the userindicates that he/she is ready, thesearch will be performed. Then, thenumber of records found will be displayedand the user is given the opportunity tolist the citations with or withoutabstracts and to enter the number ofcitations to view. Figure 8 illustratesa completed search that will display thefirst two citations with abstracts.SEISThe scientific and engineeringinformation system (designated DATA onscreen in AORIS) is currently in thedevelopment phase. It will contain quantitativedata on sea ice, ice gouging orscouring, and subsea permafrost characteristicsin both tabular and graphicalformats. The initial prototype SEIS willbe operational in early fall 1987.729

*******************************************************************************TITLE: COLD REGIONS CODE: COLDREGNTYPE: BIBLIOGRAPHIC DATA BASESUBJECT: Arctic and Antarctic Studies, EngineeringPRODUCER: Library of Congress, Science and Technology Division (support fromArmy Corps of Engineers CRREL and NSF)ONLINE SERVICES: YES, SDC Information ServicesGATEHAYS: EasyNet PUBLIC: YesLANGUAGE: Russian, English, French HARDCOPY: YesTIME SPAN: Earliest citations from 1951UPDATING: 5,000 records a year, "Cold Regions Science and Technology"CONTACT: Nancy Liston PHONE: (603) 646-4221ADDRESS:U.S. Army Cold Regions Research andEngineering Laboratory72 Lyme RoadHanover, NH 03755-1290Press RETURN for Abstract, N for Next Data BasePFI PF2 PF3ROADMAP BIBLIO DATAPF4EXITESC-HKEY HELP**************************************************************************************************************************************************************ABSTRACT:The COLD Regions Data Base contains over 90,000 citations, with abstracts, tojournals, monographs, technical reports, conference papers, patents, and mapson temporarily or permanently frozen areas, including the Arctic, Antarctica,the Antarctic Ocean, and the sub-Antarctic islands. The data base covers thepolitical, social, and natural science aspects of these areas (except theArctic), as well as the relationship of snow, ice, glaciers, and permafrost(frozen ground) to civil engineering, navigation, the behavior and operationof materials and equipment, and transportation. It also covers relationshipsof freezing temperatures to such activities as expeditions to cold areas,photography, reconnaissance, remote sensing, and construction. The data basecorresponds to "Antarctic Bibliography," from 1962 to date, and "Bibliographyon Cold Regions Science and Technology," from 1951 to data.Press RETURN for Next Data Base, P for Previous ScreenPFl PF2 PF3 PF4ROADMAP BIBLIO DATA EXITESC-HKEY HELP*******************************************************************************FIGURE 6.Sample Roadmap Listed Data Base.730

COLDFIGURE 7.AORIS Bibliographic Component Information Sources.The sea ice data section will concentrateon identifying data on ice distribution,movement, morphology, andmechanical and physical properties inu.S. waters (Beaufort, Chukchi, and BeringSeas). It will present abstractedinformation to the user in several formats.In cases where the originalreport contained the results of a statisticalanalysis, the information willbe available as an author statistics filewhich provides a tabulated summary ofthese statistics. A histogram plot willbe displayed where grouped frequency datahave been abstracted. Text files will beused to present excerpts from thosereports which provide qualitativedescriptions of results and observations.When movement data are retrieved, trackmaps will display the drift patterns. Ifthe original data from a project areavailable in raw form, i.e., magnetictapes, a pointer will be provided toindicate its location.The ice gouging/scouring data sectionhierarchy categorizes the data byarea; date; gouge type; and gouge characteristics,distribution, and effects.The gouging section will also concentrateon gouge statistics, such as spatialdistribution, frequency of recurrence,and data used to generate statisticalresults. The accumulated ice gougingdata will be structured in data matrices.A matrix is useful for statistical datalike ice gouging, because of the manyfactors associated with describing thisphenomenon. Data may take graphicalform, such as histograms and otherstatistical distribution functions. Topinpoint data locations, a map of the731

**********************************************************************************Sequence II - 1ID Number - 1054Title - IN SITU RECRYSTALLIZATION OF POLYCRYSTALLINE ICEAuthor - WILSON, C. J. L.: MITCHELL, J. C.: BURG, J. P.Doc Type - JLanguage - ENGLISHDate - 1985Source - AUSTRALIAN NATIONAL ANTARCTIC RESEARCH EXPEDITIONS. ANARE RESEARCHNOTES, SEP. 1985-NO. 28, P. 122-129, 27 REFS.; ISSUE: AB VOL. 15ITEH 32561Keywords - ICE PHYSICAL PROPERTIES; lCE DEFORMATION; ICE CRYSTALS; ICE FLOES;ICE CREEP; ICE CRYSTALS; ICE MODELS; ICE DEFORMATIONAbstract - EXPERIMENTAL DEFORMATION OF ICE ABOVE -SoC PRODUCES DYNAMICRECRYSTALLIZATION BY ROTATION OF SUBGRAINS AND/OR BULGING OF NEWHIGH ANGLE OR PRE-EXISTING BOUNDARIES, THROUGH A PROCESS OFMIGRATION RECRYSTALLIZATION. RECRYSTALLIZED GRAINS IN THE BOUNDARYOF AN OLD GRAIN UNDERGO THE GREATEST DEGREE OF ROTATION AND ALSOSHOW THE HIGHEST GRAIN BOUNDARY MOBILITY. SUPERIMPOSED O~ THESEPHENOMENA THERE MAY BE POST-DEFORMATION "RECOVERY ANNEALING" WHICHPRODUCES LOCAL BOUNDARY MIGRATION WITH A FURTHER REDUCTION OF THEINTERNAL STRAIN ENERGY. (AUTHORS)Sequencell - 2ID Number - 1461TitleAuthorDoc TypeLanguageDateSourceKeywordsAbstract- EXPERIMENTS ON ICE RIDE-UP AND PILE-UP- SODHI, D. S.; HIRAYAMA, K.-I.; HAYNES, F. D.; KATO, K.- PA- ENGLISH- 1983- ANNALS OF GLACIOLOGY, REPT. NO. MP 1627, 1983-VOL. 4, P. 266-270,48 REFS.- ICE FLOES; ICE TEMPERATURE; ICE MECHANICAL PROPERTIES; ICE PILEUP- ICE PILE-UP AND RIDE-UP ARE COMMON OCCURRENCES ALONG BEACHES IN THESUB-ARCTIC AND ARCTIC. AN UNDERSTANDING OF THE FACTORS WHICH LEADTO PILE-UP IS IMPORTANT FOR DESIGN OF A DEFENSIVE STRATEGY TOPREVENT DAMAGE TO COASTAL INSTALLATIONS. SINCE ICE ACTION ON ASLOPING BEACH IS COMPLEX, AN EXPERIMENTAL MODEL STUDY WAS UNDERTAKENTO DETERMINE THE FACTORS WHICH PROMOTE ICE PILE-UP. THE FACTORSVARIED IN THIS STUDY WERE THE FREEBOARD, SLOPE, AND ROUGHNESS OF THEBEACH. ONE EXPERIMENT WAS PERFORMED TO OBSERVE THE EFFECTIVENESS OFA SHORE DEFENSE STRUCTURE AGAINST ICE RIDE-UP.**********************************************************************************FIGURE 8.Bibliographic Citations with Abstracts.732

Arctic regions will be overlaid with agrid to indicate the areas for whichice gouging data is contained in theAORIS.The subsea permafrost data sectionwill contain data necessary for characterizingpermafrost in particular offshoreareas. These will be categorizedby geographic location, consisting ofsubdivisions of the Beaufort, Chukchi,and Bering Seas, and other offshore areasin the U.S. Arctic which are in thepermafrost zone; and by date, when theresearch was performed. Generic data forthe various soil types and permafrostdepths will also be available. These maybe input to predictive thaw models. Thesubsea permafrost data will be organizedinto three general data categories:distribution, characteristics, andproperties.AORIS FutureUpon the completion of AORIS in1988, efforts will already be underwayto have a permanent residence for theAORIS for ready public access. The ideais to have some service organization toperiodically update the AORIS and makeit available to the user community on acost reimbursement basis. The serviceorganization(s) may be a university, aprofessional society, and/or a data baseservice company.733

AUTHOR LISTAdams. P.F. . . . . . .... 663Agnew. T. .. ..... ........ .. ...... 205Ahlmis. K ...... ........ . ...... 137; 149Allyn. N .............................. 607Bentley. D.L. .... .... . ... 289Berner. D.. .............. . ....... .655Birdy. J.N. . . . . . . . . . . . .. .. . .. 367Blanchet. D ........................... 465Boaz. LB. ... ... .................. 367Bosworth. H.W.. . .................... 457Bruun. P. ................... . ..... 187Carsey. F.D. . . . . . . . . . .. ... .... . .103Carstens. T.. . ....................... 253Chiang. D.L. ................. . ...... 721Charpentier. K .... . ................. 607Colony. R. ... ..... .. ............... 85Cox. G.F.N. .............. . ......... 457Daley. C ........................... 631Daley. T.W. .................... .. . .. 95Danielewicz. B. . ..................... 465Dean. KG. . . . . . . . . . ... ............ 149Dempsey. J.P. ............ ....... . 289Earle. E.N. . . . . . . . . . . . . . . . . . .. . ....... 1Ettema. R. . . . . . . . . . . . .. . ............ 543Ferregut. C. ...... ...... ........ 631Fingas. M. ...... .... . . .. . ....... 95Foerster. J.W. ............ . . . .. . 227Frederking. R. ........................ 317George. T.H ........................... 149Gerchow. P. . .. .. . . .. . ........... 1Gerwick. B.C. . . . . . . . . . .. . ............ 655Goodman. R.H. ................... . .. 95Gowda. S.S. . . . . . . . . . . . .. .... .. . .. 339Gulati. KC. ...................... . .. 345Hakala. R. ................... . .... 339Hausler. F.U ..................... 1;509;521Hibler. W.O. III.. . ................... 159Hilland. J.E. . . . . . . . . . . . . . .. 103Hirayama. K . . . .. .. . .... . ....... 299Holoboff. A.G. ................ .. ..... 95Huang Hung-Pin.. ....... . .......... 543Hufford. G.. . . ....... .. .... 215Humphreys. D.H. .. ....... 557Ishikawa. S. . . . . . . . . . . . . . . . .. . ...... 427Iwata. S. .. . ......................... 689Jebaraj. C.. .. .. . ..... 531Jeffries. M.a. . . . .. ....... ..... 57;69Johnson. J.B ...................... 449;457Jolles. W.H. . . . . . . . . . . . . . . . . ... ...589Jones. S.J ............................ 531Jordaan. I.J. .. . . . . . . . . . . . . . . 13Kamesaki, K .......... .Kannari, P .................. ... 307..... 557Kato. K .............................. 413Kawasaki, T. . ........................ 427Kej. A. ................. . .... 175Kovacs. A.. .. ... . ............ 111;121Kumakura. Y. ......... ..... .... 413Larsen. J. . . . .. .. . .' .. ......... 279Lehmus. E. ................. . ........ 45Li Fu-cheng ......................... 33;39Li. Zhi-jun .. ................. . .. 33;39Lu Qian-ming . . . . . . . . . . . . . . . . . . 175;279MacGregor. J.G.. .......... ..... 663;699Makkonen. L. .......................... 45Mathews. T.W. . . . . . ............. 205Matsuishi. M. . ........................ 689Matsumoto. G. . . . . . . . . . . . . . .. . ....... 711McLeish. A. . . . . . . . . . . . . . . . .. . . ..675McRoy. C.P. . . . . . . . . . . . . . .. ........ .149Meng Zhao-ying . . . . .. ....... . ....... 437Miller. J ............... ' .. ......... 103Morey. R.M ............,. ........ 121Muller. L. . .. . .. . ................ .495Munaswamy. K .. .. . ............... 531Murdock. L.D. ... . ................... 95Nagasawa. Y. . . . . . . . . . . . . . . . . ..... 711Nasseri. T. . . . .. ................ 353Nessim. M. . . . . .. .................. 353O·Flynn. B. . . . . .... .. ............. 699Ohno. F.............. .. .... 711Okada. T. .................. . ....... 711Olmsted. C. . . ....................... 137Ono. T.. ................ . . ...... . 395Ota. T. .. ......................... 711Pawlowski. J.S ......... ' ............ 239Payer. H.G. ............ . ........... 495Perchanok. M. . .. ... .... . . . . . . . . . . 631Rasmussen. E.B. ........ . ........... 175Riska. K. . ............... . ....... 317R0sdal, Arild ................ ...... . 253Sackinger. W.M .................. 57;69;269Saeki. H. ............... .......... 395Sakamoto. N. . . . . . . . . . . . . . . . . . . . . . .. . 299Sasaki, K . . . .. ..... ............. 395Seibold. F ............. " ..... .... 575Shapiro. L.H.. ..... 137Scheidt. R. ................... 215Shibue. T ............................ 413Shioya. T. ........ .... .. . ..... 711Shoemaker. H.D. .. . . . . . . . . .. .57;69;721Smith. J.R. ........... .... . ...... 675Sodhi. D.S. . ............... . ... 289;449Sui Ji-xue . . . . . . .............. 33;39Swamidas. A.S.J. . . .. ..' ............. 531Sykes. J.F. . . . . . . . . . . . . . . . .. . . .619735

Taguchi. Y .......................... .427Tanaka. S. . .......................... 395Thomas. G.A.N ........................ 645Thomson. N.R. ........................ 619Timco. G.W ............................ 13Tippens. H.R. ......................... 269Toi. Y .............................. .413Tozawa. S ............................ 427Tucker. W.B. III ....................... 159Tuhkuri. J.. . . . . . . . . . . . . . . . . . . . . . . . . . . . 21Tunik. A. . ........................... 485Turner. B.E ........................... 387Tryde. P ............................. 279Tseng. J. . ........................... 607Utt. M.E. . ........................... 387Valleau. N.C .......................... IIIVaudrey. K.D ......................... 387Vincent. T.J. . ........................ 457Vinje. T .............................. 263Voelker. R. ........................... 575Wang Ling-yu ........................ 437Weeks. W.F ........................... 103Weidler. J.B. . ........................ 345Weller. G ............................. 103Winkler. M.M. . ....................... 329Wishahy. M.A ......................... 239Yoshimura. N ......................... 307Yu Yong-hai ........................... 39Zhang Ming-yuan ....................... 39Zimmerman. T.J.E ..................... 663736