Analysis of subsea lifting operations Introduction

Subsea lifting operationsStavanger - Nov. 29-30 2011Analysis of subsea liftingoperationsPeter Chr. SandvikMARINTEKMARINTEK 1

Analysis of subsea lifting operationsIntroduction• DNV RP-H103 gives recommendations on:• Method(s) for simplified quasi-static analysis• Hydrodynamic data for simple objects• More advanced modelling and analysis• This presentation will focus on modelling and dynamicanalysisMARINTEK2

Good reasons for doing accurate simulations:• Confident estimates of extreme forces to be expected• Support for development of plans and procedures• Good basis for defining limiting environmental conditions• Good understanding of dynamic behaviour• Identification of 'strange' effects and critical stages• Input to adjustment of method and equipment• 3D animations useful / vital for HAZID / Safe Job AnalysisMARINTEK 3

Analysis of subsea lifting operationsDifferent systems or situationsSimpleStationary conditions• No system changes• Small changes in geometry• Unchanged hydrodynamic forcemechanismsComplexChanges during the operation:• Impacts, slack/snatch• Soft impacts• Lift-off• Varying geometry• Water entry / exit• Soil forces at landingLinear methodsFrequency domain analysisNon-linear methodsTime domain simulationMARINTEK4

Wave forces in the splash zoneExample: Template1 2 34 5 6Large dynamic forces (± 140 T)MARINTEK 6

Analysis of subsea lifting operationsStructure sizeSimple• Small horizontal dimensionscompared to wave length• Small vertical dimensions• Structure does not influence onvessel motionComplex• Wave forces vary over thestructure (L > λ / 4)• Wave forces vary with depth• Dynamic forces from thestructure may influence rollmotion• Changing hook load will changethe ship's heel angleObject considered as a pointVessel's motion RAOs can be usedDistributed hydrodynamic forcesSimultaneous solving of allequations of motionMARINTEK7

F / F0, M / F0 LHeave force and pitch moment on a long member(It is conservative to assume small body, concentrated loads)Wave force on an element dx:Maximum total wave force:FF0f A( 1 ca ) dx L sin L F0 AL 1 ca)2 wavelength2( 010.8ForceMoment0.60.4Harmonic wave:amplitude ζfrequency 0.200 0.5 1 1.5 2 2.5 3L / WavelengthMARINTEK 8

Z (m)Wave kinematicsReduction with depthRekze 4z2 TReduction of wave kinematics with depth0-10-20-30-40-50-60-70T = 4sT = 6sT = 10sT = 14sT = 20s-80-90-1000 0.2 0.4 0.6 0.8 1Depth reductionMARINTEK 9

Analysis of subsea lifting operationsNatural periods, resonanceSimpleThe frequency range of dynamicforces is far from the naturalfrequencies of the system(NB! Up to 6 natural frequenciesper body, depending on systemconfiguration)ComplexDynamic forces excite resonantmotion.• Excitation frequency close tonatural frequency• Light or moderate systemdamping• Time to build up resonance maybe importantQuasi-static analysisSystem dynamics consideredTime domain if not linearMARINTEK10

Response curve (RAO)of a simple oscillating systemMass Damping StiffnessMARINTEK 11

Resonance periods - examplesPendulum oscillation in air:mL mL 2T0 2 2 L 2F mg gLVertical oscillation in air:T0mL m 2 2 2 2 EA mg g = elongation of wire due to weight mgVertical oscillation in water(long wire, wire mass m wincluded):T0 2m m EAL1a 3mwMARINTEK 12

Analysis of subsea lifting operationsLinear or non-linear force mechanismsSimpleMass forces dominate over(quadratic) dragHydrodynamic mass can beassumed constantThe stiffness is constantNo interaction between differentnatural modes of oscillationComplexDrag forces significant, responseRAO dependent on excitation level(roll RAO vary with sea state)Ventilated structures showamplitude dependent added massBy large amplitudes also nonimpactsystems can have variablestiffnessParametric excitation, MathieuinstabilityMARINTEK13

a/a0Added mass of ventilated structuresExample0.6Added mass0.50.40.30.20.10.0Hatch 20, p=0.15Hatch 18, p=0.25Roof #1, p=0.267Roof #2, p=0.47Roof #3, p=0.3750 0.5 1 1.5 2z(1-p)/(2Dp^2)Recommended values arefound in DnV-RP-H103a = added massa0 = added mass for solid structurep = perforation ratio = open area / total areaD = dimensionMARINTEK 14

Example: Lifting in airAmplitude dependent transverse stiffnessWire length: 25 mNatural period : 10.0 sVessel: L=100 mRegular waves H=2mT = 5 sT = 9 sT = 7 sT = 10 sT = 8 sT = 11 sMARINTEK15

Large structuresExample: a 30x30 m stiffened foundation plateM = 50 tJust fully submergedInclination 79 deg.M = 30 tm a = 3000 tM = 400 tW = 3420 kNm a = 15000 tHs = 2.0 mTp = 6 sMARINTEK16

Z (m)Reduction of wave kinematics with depth Reduced dynamic pressure at lower end of anchor0-5-10-15-20-25-30-350 0.2 0.4 0.6 0.8 1Depth reductionT = 3sT = 4sT = 5sT = 6sT = 7sT = 8 sT = 9 sT = 10 sT = 11 sT = 12 sT = 14 sMARINTEK 18

Dynamic forces on a suction anchorAnchor(steel)Vertical anchor motion due to ship motionHorizontal wave forcesWater excitation from dynamic pressure at bottom levelMARINTEK 19

Vertical wave forces on fixed buckets(Hs = 2.0 m)Bucket B4Bucket B30MARINTEK20

Liftwire tension (Hs = 2.0 m)Bucket B4Bucket B30DAF = 2.5 / 1.5 DAF = 1.4 / 1.3MARINTEK21

Numerical modelling of a suction anchorElements with distributed forcesAll are positiondependent(zero whenelement isabove waterAnchor(steel)Horizontaladded massand dampingVertical addedmass and damping,topEnclosedwater ++MARINTEK 22

Slack slings may not be criticalExample: Slack slings due tilting motion, Bucket B30LiftwireDynamic tension range:618- 2102 kNSling 10 - 1341 kNSling 20 - 1244 kNSling 3In this case horizontal hook motionprevent high snatch loads0 - 1186 kNHowever, whencaused byvertical motion:MARINTEK23

Analysis of subsea lifting operationsWave excitation modellingSimpleConstant or harmonic excitation• Design wave analysis (withgiven amplitude and period)• Extreme values calculateddirectlyComplexIrregular excitation• Irregular waves (from spectrum)• Statistic evaluation of results,from time series• (Hydrodynamic damping effectsmay be too small)MARINTEK24

Statistical estimation of maxima, from a time historyMARINTEK 25

ln [-ln (1-P)]Extreme estimation from local peaks, stationary processWEIBULL plotReduced slope due tonon-linear effects, e.g.:• Quadratic damping• Snatch loads”off” points may be dueto statistical spread,or they may have aphysical explanation.P = cumulative probabilityx = local peaks = average value of peaksMARINTEK 26

Vertical position (m)Analysis of transient processes(Splash zone crossing, other non-linear cases)Analysis approachesA few examplesEstimation of extreme forces15.0010.005.000.00-5.00-10.00-15.000 500 1000 1500 2000 2500Tension (kN)MARINTEK 27

Lowering through the splash zoneNumerical analysis - 2 methods• Slow lowering in small, regularwaves• 1 – 2 typical wave periods• Find most onerous object position• Stationary analysis in mostonerous position, in irregularwaves (0.5 - 3 hours)• Calculated max./min. force• Evaluate results• Study time series• Find peaks in time series• Statistical analysis (WEIBULL)• Estimate extreme force• Repeated lowering in irregularwaves• From air to well below surface• Different wave realizations(MonteCarlo random selection)• Find the calculated max./min. forcefrom each lowering case• Evaluate results• Statistical analysis (GUMBEL)• Study time series (explanation ofphysical phenomena)• Estimate extreme forceMARINTEK 28

-ln(-ln(P))Extreme estimation from repeated tests, random processExtremes from 20 lowerings through splash zone, GUMBEL plot43.532.521.510.50-0.5-1-1.5Cumulative probability.95.90900 1000 1100 1200 1300 1400 1500Tension (kN)”off” points may bedue to statistical spread,OR:may represent a differentphysical mechanism(example: impact, snatch).MARINTEK 29

Snatch loads - Example:Lift-off from transport vesselM = 365 tonnes, Hs = 2.5 mSnatch loads can be reduced:• Start in constant-tension mode• Lift when winch stops to pay in’Blind’ lift-off at max.relative velocity:Fmax = approx. 5600 kNP = 0.95General expression:P = 0.80FmaxMgVrelkM V Mg1 grelkMMARINTEK30

Underwater lifting operation(Not intended)(Stability ofthe lifting body)MARINTEK 31

Safe Job AnalysisCan anything gowrong ?Shhh, Zog! ....Here come one now!MARINTEK 32