Storegga Slide - NGI

ScandpowerRisk Management ASP.O. Box 3N-2027 Kjeller, NorwaySlide Risk Assessment in theOrmen Lange Field Development Area

Jan K. Lund, Scandpower Risk ManagementJørn Olsen, Scandpower Risk ManagementPetter Bryn, Norsk HydroGunnar Brekke, Norsk HydroTom Guttormsen, Norsk HydroTore Kvalstad, NGIFarrokh Nadim, NGI

Location Map showing the Storegga Submarine Slide onthe Mid-Norway Margin

Main Issues• Is this area still prone to submarine slides?• How were slides triggered in this area?• Is there still a possibility for new slides in the area,which can threaten the Ormen Lange installations?• Can the ormen Lange activities alone or in combinationwith natural causes, trigger a new submarine slide with3rd party consequences?

Risk Analysis Framework (NORSOK Standard Z-013)Risk analysis planningRisk acceptance criteriaSystem definitionHazard identificationRisk reduction measuresConsequenceanal ysisFrequencyanalysisRisk estimationRiskanalysisRisk pictureRisk evaluationunacceptableRisk assessmentacceptableFurther risk reduction measuresPart of safety management and risk control

Risk Analysis PlanningSTOREGGA MODELUnderstanding of/mostlikely explanation of theStoregga SlideHAZARD IDENTIFICATION(trigger mechanisms):Shear stress increasingfactors---Shear strength decreasingfactors---ACCIDENT MODELLINGParametric studies to identifymost likely combination oftrigger mechanismsIDENTIFICATION OFSEDIMENT SOURCEAREAS FOR MAJOR &MEDIUM SLIDES IN THEORMEN LANGEINFLUENCE AREA.Influence areas determinedby potential effect ofOrmen Lange activities:- Reservoir subsidence- Underground blowout- Hydrate melting- Anchor forces- Rock fills- TrenchingPOTENTIAL TRIGGERMECHANISMS FORSLIDES CAUSED BYORMEN LANGEACTIVITIES.- Reservoir subsidence- Underground blowout- Hydrate melting- Anchor forces- Rock fills- TrenchingThe Storegga slideMajor slides todaySmall slidesCONSEQUENCE MODEL(3rd PARTY):RISKofSUB SEA SLIDESTSUNAMISize and mass/velocityrelationships and impactdistance from slide. Findminimum parameters tocause 3rd party fatalities.Major slides(Tsunami)MediumslidesSmall slides

Extent of Potential Influence Areas

RAC (1st Party)• Conventional RAC applied by operators in the NorthSea area– Risk to personnelGIR < 10 -3 fat/yr– Risk to environmentDuration of environmental damage to be insignificantcompared to the expected time between such damages

RAC (3rd Party Risk)• Normally not an issue for offshore activities• Known formats– Risk to most exposed person– Society risknot suited

RACChosen Criterion, 3rd PartyRisk is intolerable if the frequency of a slide with "significantdamage potential" generated by OL activities exceeds10 -7 per year.Significant damage potential:A tsunami wave must be generatedVertical run-up must exceed 1.5 m in representativecoastal areas

HAZID: Storegga SlideTrigger candidates/destabilisingfactors for theStoregga SlideRestoration ofIsostacy EustacythermohalineReservoir gas Diapirism/fluid Rapid sedimentcirculationleakageventsdepositionHydraulicheadOsmoticpressureEarthquakesSeawaterwarmingDownslopeerosionWater expulsion/syneresisPore fluidredistributionGravitationalloadGas hydratemeltingExcess porepressures

Storegga Slide - Explanation Model

Storegga Slide - Explanation Model

Storegga Slide - Explanation Model

Storegga Slide - Explanation Model

Risk under Present Conditions

HAZID: Present Situation with Ormen LangeTrigger mechanismspotential new slidesNatural triggersOrmen LangetriggersEarthquakesGravity(alwayspresent)GashydratemeltingReservoirgasleakageFluid redistributionBlowoutTemperature(wells)ReservoirsubsidenceAnchoringrock fillstrenchingGasGashydratesIncreasedslopeSeismicityMay a combination ofnatural triggers and OrmenLange activities generate aslide with 1st or 3rd partyconsequences?Increased porepressure?Reduced shearstrength?Increasedshear stress?Reduced shearstrength?NoYesDocumentationof consensusQuantitative riskanalyses required

Consequence Model

Type of Slides

Calculation of Frequencies - Main StepsDefinition ofdeep & shallowfailure surfacesReliabilitymodelStaticprobabilitiesPostearthquakeprobabilitiesUpdate ( slopestable for 8000years)Annualprobabilities(frequencies)UncertaintyanalysesSystemfrequencies

Risk EstimationDescription oftrigger mechanismsSoil conditionsStrongearthquakeonly realistictriggermechanismMajor slabslide?Sediment volumes notavailableNoYesDeep failuresurfacesSystemfrequencyRemoulded strength lowenough for failed volumeInstability< 4⋅10 -6 to flow away?Shallow failure Frequency Remoulded strength notSmallTsunami-generating? MajorconsequencesP < 10-2consequences10 -5 tsunami < 4 ⋅10 -8surfacelow enough for failedvolume to flow away?10 -5 No failureNo Ormen Lange activity affects stabilitywithin influence area 1⇒ By definition no major consequencesRetrogres-siveslide developsSufficient runout toreach fieldinstallations?P runout < 5 ⋅10 -3Mediumconsequences< 2 ⋅10 -8

Risk SummarySlide consequence class Description of slide Risk results(frequency per year)Comments1st party orenvironment3rd partyMajor:Slide generates tsunami thatcauses damage along thecoast.(3rd party, 1st party risk andenvironmental risk)Large regional slides related tothe glacial/ interglacial cycles.(Volume range 100 - 3000 km 3 )Slides from the back walls of theStoregga slide scar;retrogressive processNot relevantNotrelevantThe prehistoric Storegga slideremoved all soft sediments< 4·10 -8 Only natural causes (extremelystrong earthquakes).No project generated risk

Risk SummarySlide consequence class Description of slide Risk results(frequency per year)Comments1st party orenvironment3rd partyMedium:Global threat to fieldinstallations1st party risk andenvironmental riskSlides from the back walls ofthe Storegga slide scar;retrogressive process< 2·10 -8 No risk Far below Ormen Lange riskacceptance criteria

Risk SummarySlide consequence class Description of slide Risk results(frequency per year)Comments1st party orenviron-ment3rd partySmall:Local threat to pipe-lines andumbilicals1st party risk andenvironmental riskSlide from back wall shallowslip surface10 -5 No risk Far below Ormen Lange riskacceptance criteriaSurficial slide in back wall < 2·10 -2 No risk Risk is acceptable, but detailedNo riskevalua-tions of technical solutionsare ongoingClay drape failures (in steepareas)To beeva-luated

Summary• General risk analysis framework can be applied• Ormen Lange field development activities havenegligible effects on stability (deep failure) and will nottrigger Tsunami-generating slides• The annual probability of a slide with run out to the fielddevelopment area is almost zero and negligiblecompared with RAC• Shallow slide events and surficial slides can threatenpipelines, but the risk is acceptable