DNV Marine Operations' Rules for Subsea Lift Operations

DNV Marine Operations’ Rulesfor Subsea Lift OperationsNew Simplified Method for Prediction of Hydrodynamic ForcesTormod BøeDNV Marine Operations2nd December 2009

Content• Brief overview of relevant DNV publications• DNV Rules for Marine Operations, 1996,Pt.2 Ch.5 Lifting – Capacity Checks• New Simplified Method for calculation ofhydrodynamic forces, DNV-RP-H103 Ch.4• CFD Analyses – Test CasesDNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 2

Relevant DNV PublicationsLifting- and subsea operations :DNV Rules for Planning and Execution ofMarine Operations – 1996’Special planned, non-routine operations oflimited durations, at sea. Marine operations arenormally related to temporary phases as e.g.load transfer, transportation and installation.’DNV-OS-E402Offshore Standard for DivingSystems January 2004(Amendments October 2009)DNV Standard for CertificationNo.2.22 Lifting AppliancesOctober 2008DNV Standard for CertificationNo. 2.7-1 Offshore ContainersApril 2006Special planned non-routine operationsRoutine operationsDNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 3

Relevant DNV Publications - Other• DNV-RP-C205 Environmental Conditions andEnvironmental Loads April 2007• DNV-RP-H101 Risk Management in Marine andSubsea Operations, January 2003• DNV-RP-H102 Marine Operations during Removal ofOffshore Installations, April 2004• DNV-RP-H103 Modelling and Analysis of MarineOperations, April 2009• Standard for Certification No. 2.7-3 Portable OffshoreUnits, June 2006 (a new revision will be issued 2010which will include subsea units)• DNV-OS-J-101 and -201 Design of Offshore WindTurbine Structures and Substations for Wind Farms,October 2007 / 2009• DNV-OS-E303 and -RP-E304 Offshore Mooring FibreRopes, Certification (2008) and Damage Assessment (2005)DNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 4

Relevant DNV Publications - PurchaseDNV publications can be purchased at:http://webshop.dnv.com/global/DNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 5

Content• Brief overview of relevant DNVpublications• DNV Rules for Marine Operations, 1996,Lifting – Capacity Checks• New Simplified Method for calculation ofhydrodynamic forces• CFD Analyses – Test CasesDNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 6

Capacity Checks - DNV 1996 RulesPart 2 Chapter 5• Dynamic loads, lift in air• Crane capacity• Rigging capacity,(slings, shackles, etc.)• Structural steel capacity(lifted object, lifting points,spreader bars, etc.)Part 2 Chapter 6• Dynamic loads, subsea lifts(capacity checks as in Chapter 5 applying dynamic loads from Chapter 6)DNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 8

Capacity Checks - Crane CapacityThe dynamic hook load, DHL, isgiven by:DHL = DAF*(W+Wrig) + F(SPL)ref. Pt.2 Ch.5 Sec.2.4.2.1• W is the weight of the structure,including a weight inaccuracy factor• The DHL should be checked againstavailable crane capacity• The crane capacity decrease whenthe lifting radius increase.DNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 10

Capacity Checks - Sling LoadsThe maximum dynamic sling load, Fsling,can be calculated by:Example :Fsling = DHL·SKL·kCoG·DW / sin φref. Pt.2 Ch.5 Sec.2.4.2.3-6where:• SKL = Skew load factor → extra loadingcaused by equipment and fabrication tolerances.• kCoG = CoG factor → inaccuracies in estimatedposition of centre of gravity.• DW = vertical weight distribution → e.g.DWA = (8/15)·(7/13) in sling A.• φ = sling angle from the horizontal plane.DNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 11

Capacity Checks - Slings and ShacklesThe sling capacity ”Minimum breaking load”,MBL, is checked by:F

Capacity Checks – Structural SteelLifting points:The load factor γ f = 1.3, is increased by aconsequence factor, γ C = 1.3, so that totaldesign faktor, γdesign , becomes:γdesign = γc· γf = 1.3 · 1.3 = 1.7The design load acting on the lift point becomes:Other lifting equipment:A consequence factor of γ C = 1.3should be applied on lifting yokes,spreader bars, plateshackles, etc.Structural strength of Lifted Object:The following consequence factorsshould be applied :Fdesign = γdesign· Fsling = 1.7· FslingA lateral load ofminimum 3% of thedesign load shall beincluded. This loadacts in the shacklebow !(ref. Pt.2.Ch.5 Sec.2.4.3.4)Table 4.1 Pt.2 Ch.5 Sec.4.1.2DNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 13

New Simplified Method - AssumptionsThe Simplified Method is based upon thefollowing main assumptions:• the horizontal extent of the lifted object issmall compared to the wave length• the vertical motion of the object is equal thevertical crane tip motion• vertical motion of object and water dominates→ other motions can be disregardedThe intention of the Simplified Method is togive simple conservative estimates of theforces acting on the object.DNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 16

New Simplified Method - AssumptionsTime-domain analysis:Includes loads and motionresponses on both installationvessel and lifted object.Lifted object modelledapplying correct geometry(not just a point in space) simulation valid for all wavelengths.Cranewire, lifting slings andtugger lines are included motion response of the liftedobject is computed resonance effects arecovered in analysis.Statistical analysis ofresponses in irregular seastates included.Coupling effects included(crane tip motions may beinfluenced by lifted structure).Non-linear response, as e.g.snap loads in lifting slings,can be computed.Visualization of lift.DNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 17

New Simplified Method – Crane Tip Motions• The Simplified Method is unapplicable if the crane tiposcillation period or the wave period is close to theresonance period, Tn , of the hoisting systemM+KAT33n = 2π• Heave, pitch and roll RAOs forthe vessel should be combinedwith crane tip position to findthe vertical motion of the crane tip• If operation reference period iswithin 30 minutes, the mostprobable largest responses maybe taken as 1.80 times thesignificant responses• If the vessel heading is not fixed,vessel response should beanalysed for wave directions atleast ±15° off the applied vesselheadingDNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 18

New Simplified Method – Wave PeriodsThere are two alternative approaches:Alt-1) Wave periods are included:Analyses should cover the following zerocrossingwave period range:H s8.9⋅ ≤ Tz≤g13A lower limit of Hmax=1.8·Hs=λ/7 withwavelength λ=g·T z 2 /2π is here used.Alt-2) Wave periods are disregarded:Operation procedures should in this case reflect that the calculations are only valid forwaves longer than:Tz≥ 10 . 6⋅HgSA lower limit of Hmax=1.8·Hs=λ/10 with wavelengthλ=g·T z 2 /2π is here used.DNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 19

New Simplified Method – Wave KinematicsAlt-1) Wave periods are included:The wave amplitude, wave particlevelocity and acceleration can be taken as:ζ• aH Sv• wa• w= 0. 9 ⋅= ζ= ζaa⎛ 2π⎞⋅⎜⎟eT⋅⎝ z ⎠⎛ 2π⎞⋅ ⎜ ⎟T⎝ z ⎠2⋅−e4π2d−T2zTg4π2d2zg• d :distance from water plane to CoG ofsubmerged part of objectAlt-2) Wave periods are disregarded:The wave particle velocity and acceleration canbe taken as:v• wa• w= 0.30 π g Hs= 0.10πg ⋅e−⋅e0.35dHs−0.35dHsDNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 20

New Simplified Method – Hydrodynamic ForcesSlamming impact forceSlamming forces are short-term impulseforces that acts when the structure hits thewater surface.A S is the relevant slamming area on theexposed structure part. Cs is slamming coeff.The slamming velocity, v s , is :v = v + v +sc2ctv2w• vc = lowering speed• vct = vertical crane tip velocity• vw = vertical water particle velocityat water surfaceVarying buoyancy forceVarying buoyancy, Fρ , is the change inbuoyancy due to the water surface elevation.Fρ= ρ ⋅δV⋅ gδV is the change in volume of displacedwater from still water surface to wavecrest or wave trough.Fρ= ρ ⋅δV⋅ g~δ V = ⋅ ζ + ηA w2a2ct• ζa = wave amplitude• ηct = crane tip motion amplitude• Ãw = mean water line area in thewave surface zoneDNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 21

New Simplified Method – Hydrodynamic ForcesDrag forceDrag forces are flow resistance onsubmerged part of the structure. The dragforces are related to relative velocity betweenobject and water particles.The drag coefficient, CD, in oscillatory flow forcomplex subsea structures may typically beCD ≥ 2.5.Relative velocity are found by :v = v + v +rc2ctv2w• vc = lowering/hoisting speed• vct = vertical crane tip velocity• vw = vertical water particle velocityat water depth , d• Ap = horizontal projected areaMass force“Mass force” is here a combination of inertiaforce, Froude-Kriloff force and diffractionforce.Crane tip acceleration and water particleacceleration are assumed statisticallyindependent.FM2[( M + A ) ⋅a] + [( V + A ) ⋅a] 2= ρ33 ct33 w• M = mass of object in air• A33 = heave added mass of object• act = vertical crane tip acceleration• V = volume of displaced water relative tothe still water level• aw = vertical water particle accelerationat water depth, dDNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 22

New Simplified Method – Hydrodynamic ForceThe hydrodynamic force is a time dependent function of slamming impactforce, varying buoyancy, hydrodynamic mass forces and drag forces. In theSimplified Method the forces may be combined as follows:F22hyd = ( FD+ Fslam) + ( FM− Fρ)• The structure may be divided intomain items and surfaces contributingto the hydrodynamic force• Water particle velocity andacceleration are related to thevertical centre of gravity for eachmain item. Mass and drag forcescontributions are then summarized :FM = ∑F M iiF D = ∑F DiiFMi and FDi are the individualforce contributions from eachmain itemDNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 23

New Simplified Method – Load Cases ExampleThe static and hydrodynamic force should be calculated for different stages. Relevantload cases for deployment of a protection structure could be:Load Case 1Still water level beneath top of ventilated buckets• Slamming impact force, Fslam, acts on top ofbuckets.• Varying buoyancy force, Fρ , drag force, FDand mass force, FM are negligible.Load Case 2Still water level above top of buckets• Slamming impact force, Fslam, is zero• Varying buoyancy, Fρ , drag force, FD andmass force, FM, are calculated. Velocity andacceleration are related to CoG of submergedpart of structure.DNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 24

New Simplified Method – Load Cases ExampleLoad Case 3Still water level beneath roof cover.• Slamming impact force, Fslam, acts on the roofcover.• Varying buoyancy, Fρ , drag force, FD and massforce, FM are calculated on the rest of thestructure. Drag- and mass forces acts mainly onthe buckets and is related to a depth, d, down toCoG of submerged part of the structure.Load Case 4Still water level above roof cover.• Slamming impact force, Fslam, and varyingbuoyancy, Fρ, is zero.• Drag force, FD and mass force, FM are calculatedindividually. The total mass and drag force is thesum of the individual load components, e.g. :FD= F Droof + F Dlegs + F Dbuckets applying correct CoGsDNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 25

New Simplified Method – Load Cases ExampleDNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 26

New Simplified Method – Static Weight• In addition, the weight inaccuracy factor should be appliedDNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 27

New Simplified Method - DAFCapacity ChecksThe capacities of crane, lifting equipment andlifted object are checked as for lift in air. Thefollowing relation should be applied:DAF=FtotalMgwhereMg : weight of object in air [N]Ftotal : is the characteristic total force on the(partly or fully) submerged object. Taken as thelargest of;Ftotal = Fstatic-max + FhydFtotal = Fstatic-max + Fsnapor• Fstatic-max is the maximum staticweight of the submerged objectincluding flooding and weightinaccuracy factor• Fhyd is the hydrodynamic force• Fsnap is the snap load (normallyto be avoided)DNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 28

New Simplified Method – Slack SlingsThe Slack Sling Criterion.• Snap forces shall as far as possiblebe avoided. Weather crietria shouldbe adjusted to ensure this.• The following criterion should befulfilled in order to ensure that snaploads are avoided:F≤0.9 ⋅hyd F static −min• Fstatic-min = weight before flooding,including a weight reduction impliedby the weight inaccuracy factor.DNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 29

New Simplified Method – Added MassHydrodynamic added mass for flat platesExample:Flat plate wherelength, b, abovebreadth, a, isb/a = 2.0 :π 2A33 = ρ ⋅0.76⋅ ⋅a⋅b4DNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 30

New Simplified Method – Added MassAdded Mass Increase due to Body HeightAdded Mass Increase due to Body HeightThe following simplified approximation of theadded mass in heave for a three-dimensionalbody with vertical sides may be applied :⎡2 ⎤1A ⎢ − λ33 ≈ 1 +⎥ ⋅ A⎢2 33o2(1 )⎥⎢+ λ⎣⎥⎦andλ =h +ApApA33/A33o1.81.71.61.51.41.31.21.111+SQRT((1-lambda^2)/(2*(1+lambda^2)))0 0.5 1 1.5 2 2.5ln [ 1+ (h/sqrt(A)) ]where• A33o = added mass for a flat plate with ashape equal to the horizontal projectedarea of the object• h = height of the object• Ap = horizontal projected area of the objectDNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 31

New Simplified Method – Added MassAdded Mass from Partly Enclosed VolumeA volume of water partlyenlosed within large platedsurfaces will also contributeto the added mass, e.g.:• The volume of waterinside suction anchorsor foundation buckets.• The volume of waterbetween large platedmudmat surfaces androof structures.DNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 32

New Simplified Method – Example CaseExample: Submerged Foundation Bucket• Added mass for a thin circular disc:2 4 3A33 o = ρ ⋅ ⋅ ⋅π⋅ 2.0 = 21867 kgπ 3• Added mass increase due to body height:π ⋅ 2.0λ =3.5 + π ⋅ 2.022= 0.50 ⇒A'33s⎡= ⎢1+⎢⎣2 ⋅1−0.5022( 1+0.50 )⎤⎥ ⋅ A⎥⎦33o= 33803 kg2 4 3Flat plate: A33o= ρ ⋅ ⋅ ⋅π⋅ 2.0 = 21867 kgπ 32π ⋅ 2.0Height factor : λ =21+π ⋅ 2.0⎡'Height increase: A33s= ⎢1+⎢⎢⎣Incl.inside volume:2π ⋅0.4Perforation : P = 100 ⋅ = 42π ⋅ 2.021−0.782 ⋅= 0.782( 1 + 0.78 )⎤⎥ ⋅ A33o= 29496 kg⎥⎦⎥2A33s= 29496 + π ⋅1.75⋅3.25⋅ ρ = 61546 kg< 8 ⇒ No reduction of A33s• Added mass including partly enclosed volume:A33 s = 33803 + π ⋅1.75⋅3.25⋅ ρ =265854 kg• Added mass reduction due to perforation:2π ⋅0.4P = 100⋅2π ⋅2.0= 4 ≈ SMALL ⇒ No reduction ofA33sBucket Dimensions:• Height = 3.5m• Diameter = 4.0m• Plate thickness = 0.25m• Ventilation hole diameter = 0.8mDNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 34

New Simplified Method – Example CaseExample: Submerged Foundation Bucket1.0m• Water particle velocity and acceleration:24π⋅(1+1.25)⎛ 2π⎞−222v 1.75 e 5.5 9.81⎛ ⎞w = ⋅⎜⎟⋅ ⋅π= 1.48m/s and aw= ⎜ ⎟⋅vw= 1. 69 m/s⎝ 5.5 ⎠⎝ 5.5 ⎠CoG1.25m• Drag force:F( 0.25 + 1.48) 2 = 0.37 ⋅5 ND = 0.5 ⋅ ρ ⋅CD⋅ AP⋅ vr2 = 0.5 ⋅ ρ ⋅ 2.0 ⋅0.96π 2.02 ⋅10• Mass force:2[( M + A ) ⋅ a ] + [( V + A ) ⋅ a ] 2 = ( 13031 + 65854 ) ⋅1.69= 1.33 ⋅5 NFM = ρ1033• Hydrodynamic force:Fct33w225 25 2( F + F ) + ( F − F ) = ( 0.37 ⋅10) + ( 1.33 ⋅10) = 1.4 ⋅5 Nhyd = D slam M ρ10Regular Wave Data:• Wave Height, H max = 3.5m• Wave Period, T z = 5.5sOther Data• Buoyancy, ρV = 13031kg• Negligible crane tip motions• Lowering speed = 0.25m/sDNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 35

Content• Brief overview of relevant DNVpublications• DNV Rules for Marine Operations, 1996,Lifting – Capacity Checks• New Simplified Method for calculation ofhydrodynamic forces• CFD Analyses – Test CasesDNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 36

CFD Analyses – Test Cases• Computational Fluid Dynamics(CFD) is a numerical method forcomputing fluid flows based onthe Navier Stokes equations.Structure• The CFD-program COMFLOW isable to study complex freesurface problems applying theVolume of Fluid method.• The fluid domain consists of acartesian grid where the fluidcells are defined either asboundary cells, empty cells,surface cells or fluid cells.• Pressure forces are calculatedas the integral of the pressurealong the boundary of an object.• Motion responses are notincluded, but the object can begiven a prescribed motion.Inflow boundary,Airy or Stokes5 th waveFluiddomainNumericalbeach ataft endDNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 37

CFD Analyses – Protection StructureCFD analysis:Regular Stokes 5thwave: H=3.5m T=5.5sDomain 95x30x37m4.4 million fluid cellsMinimum grid size0.18m near object,stretched elsewhere8.5x8.5m solid roofand 10x10xØ1.0m topframeØ1.0m legs, height 8mand hollow3.5xØ4.0m buckets atx,y=±8.5mventilation holesØ0.8mWall thickness 0.25mhalf model60s simulation timeDNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 38

CFD Analyses – Protection StructureF hyd ≈ 1.1·10 5 NHighest upwardshydrodynamic forcewhen bucket is fullysubmerged occurswhen the object islocated in a wavetrough.Buoyancy, ρVgDNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 39

CFD Analyses – Protection StructureHalf wave lengthis ~23.5m andthe distancebetween bucketsis 17m.Hence, there is alarge phasedifferencebetween thehydrodynamicforces on forwardand aft bucket.DNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 40

CFD Analyses – Protection StructureSlamming loadon roof structureSlamming loadon aft bucketComFlow resultsshow very highslamming loadson bucket topand the solid roofstructure.These values aremost likely toohigh ascompressibilityand formation/collapse of aircushions are notincluded in thesimulation.DNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 41

CFD Analyses – Spool PieceCFD analysis:Regular Stokes 5thwave: H=3.5mT=5.5sThe wave length is~equal spool lengthDomain130x30x31m2.2 million fluid cellsMinimum grid size0.25m near object,stretched elsewhere50m long closedpipe with diameterØ1.0mTwo simulations;1) half submerged2) 2m below surface22s simulation timecomputer time 13-18hrsDNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 42

CFD Analyses – Spool Piece Half SubmergedThe wave length is equalthe spool piece lengthHalf of the spool piece isalways out of the water.The total force on eachhalf vary between zeroand buoyancy+FhydVertical force on aft half at time t=5s :22 2 2 3.5255Fm ( V madd) aw2.0⋅1.0⎛ π ⎞= ρ + ⋅ ≈ − ⋅ ρ ⋅ π ⋅ 25 ⋅ ⋅⎜⎟ ⋅ = − 0.6 ⋅10N ⇒ F1.0vertical = ρVg+ F⋅m = ρ ⋅ π ⋅ 25 ⋅ 9.81 − 0.6 ⋅10= 1.4 ⋅104 π ⎝ 5.5 ⎠ 245NDNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 43

CFD Analyses – Spool Piece 2m SubmergedTotal vertical forceDynamic force amplitude (mainly mass forces)≈ 0.55·10 5 kNVertical force,fwd halfVertical force,aft halfBrief approximation of mass force:222 2 3.55Fm ( V madd) aw2.0 1025⋅1.0⎛ π ⎞= ρ + ⋅ ≈ ⋅ ⋅ π ⋅ 25 ⋅ 0.77 ⋅⎜⎟ ⋅ = 0.45 ⋅104 π ⎝ 5.5 ⎠ 2NDNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 44

And then – One Final Comment:When planningMarine Operations,remember to takeinto account ....DNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 45

Easy Handling ..DNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 46

.. and Survey Access !!DNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 47

DNV Marine Operations' Rules for Subsea Lift Operations 2 December 2009Slide 48