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Subsea lifting operationsStavanger - Dec. 1-2 2010

Peter Chr. SandvikMARINTEK

Theory related to subsea lifting operations

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Content Introduction, main criteria for safe lifting Lifting dynamics, simple equation of motion

Static and dynamic forces, wave forces on small objects Mass, stiffness and damping Response (motion) calculation, resonance

Large structures, 6 degrees of freedom Wave forces on long structures (e.g. pipes)

Snatch and impact loads Statistics used for estimation of extremes Stability

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The installation often gives the highest life-time forces on subsea equipment

General

Templates, protection structures

Suction anchor

Snatch at lift-off or after slack Impact after uncontrolled

pendulous motion Local loads from wave impact

Wave forces in the splash zone

Wave forces in the splash zone Soil penetration forces (and soil

failure)

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Lift in general

Is the structure designed for the loads occurring during lifting and deployment?

Hydrodynamic forces

Limited lifting height may give large compressive forces from the slings

Measures:

Lifting frame

Spreader beam

Reinforcement (compression bar)

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Operation criterion:Ensure safe operation Avoid:

Excessive pendulum motion in air

Slack wire (when not intended)

Overload (in any lifting equipment)

Too hard landing (and second lift-off)

Do: Ensure acceptable stability

Have ability to handle unexpected changes

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Static forces may be unknown Example: Drainage when H-frame is taken on board

W = mgB = gVWs = mg - gV

W = Weight in airB = BuoyancyWs = Submerged weight

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Wreck recovery

Unknown weight,weight distributionand stability

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Lift in air

Steel structure with GRP coversDimensions: 18 x 18 x 7 mMass: 180 tonnes

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Wave forces in the splash zone

1

218 x 18 x 7 m, 180 tonnes

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Wave forces in the splash zoneExample: Template (3- 8)

Large dynamic forces ( 140 T)

3 4 5

6 7 8

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Details should be checkedExample: Template structure

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Damaged cover

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-15.00

-10.00

-5.00

0.00

5.00

10.00

15.00

0 500 1000 1500 2000 2500

Tension (kN)

Ver

tical

pos

ition

(m)

!

!

Wire tension when lowering a body through the splash zoneExample

}Splash zone dynamicsReducing

Weight in air

Weightin water

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Forces on the lifted object (Newtons, or kN)

Dynamic forces:

Damping

Inertia, moving object

Inertia, wave force

Slamming force

rrrr

rd vvcvcv

vcF 210 ++=

( ) ( )xcVmxmmF aai +=+=( ) ( ) aaiw cVmVF +=+= 1

2221

ra

rss vdhdcVvAcF ==

x = body motion = wave particle motioncs = slamming coefficientca = added mass coefficient

(depth dependent)

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Wave kinematics (1)

Profile of regular waves propagating in x direction

Wave number (deep water)

Wave length

Propagation speed

Max. wave slope

kx)t( = sin0

2

2 42Tg

k ==

256.12 Tk

==

TT

vw 56.1==

020max

4 T

kdxd

=

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Heave 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:

dxcAf a 2)1( +=

lengthwaveLLF

F ==

sin

0

02

0 )1( acLAF +=

Harmonic wave:amplitude frequency

0

0.2

0.4

0.6

0.8

1

0 0.5 1 1.5 2 2.5 3L / Wavelength

F / F

0,

M /

F0 L

ForceMoment

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Wave kinematics (2)Reduction with depth

Reduction of wave kinematics with depth

-100-90-80-70-60-50-40-30-20-10

0

0 0.2 0.4 0.6 0.8 1Depth reduction

Z (m

)

T = 4sT = 6sT = 10sT = 14sT = 20s

=2

4T

zkz eeR

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Shielding-effect from the crane vesselExample of analysis results

1500

1750

2000

2250

2500

2750

3000

6 7 8 9 10 11 12Tp (s)

Max

liftw

ire fo

rce

(kN

)

Wavedir. 180 deg.Wavedir. 165 deg.Wavedir. 150 deg.

x

y21

3

45

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Hydrodynamic data:Added mass, ma and drag , Fd 6 values - (for motion in 3 directions and rotation about 3 axis)

Plate

Box Suction anchor

Added mass coefficient:Ca = ma / V

= water densityV = reference volume

Drag coefficient: cdFd = cdAv2

v = velocityA = area

Fd

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Added mass - simple structures

ca Vm =

b4aV

2c

=

a

b

Cylinder volume:

Geometry Formula b/a Rectan-gular

a = shortest edge

1.0 0.5791.2 0.6301.25 0.6421.33 0.6601.5 0.6912.0 0.7572.5 0.8013.0 0.8304.0 0.8715.0 0.8978.0 0.93410.0 0.947

(Cf. DnV-RP-H103)

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Added mass of ventilated structuresExample

Added mass

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0 0.5 1 1.5 2

z(1-p)/(2Dp^2)

a/a0 Hatch 20, p=0.15

Hatch 18, p=0.25Roof #1, p=0.267Roof #2, p=0.47Roof #3, p=0.375

a = added massa0 = added mass for solid structurep = perforation ratio = open area / total areaD = dimension

Recommended values arefound in DnV-RP-H103

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Damping

Coulomb damping Friction, hysteresis loss, ....

Linear damping Wave potential damping,

material damping, oscillation damping in wind, ....

Quadratic damping

Hydrodynamic dragMorison's formulaNotice:cd for oscillating objects is larger

than cd for steady flow !

vvcF = 00

vcF = 11

vvcF = 22

rrdd vvAcF 21=

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Stiffness General

Axial wire stiffness E = modulus of elasticity A = section area L = length

Transverse stiffness

Hydrostatic stiffness

Rotation stiffness(spring k, distance b from

rotation center) Parallel springs

Springs in series

dxFkx

=

== ii

tot kF

k

=

==

itotitot kkkF

kF 11

LEAk =

LFFk

LFF

y

yy

yy =

==

WPAgk =

2bkK =

[Compression: k = 0]

[AWP = waterplane area]

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Simple equation of motion

)()()()( tFxkxcxM =++

)()()()( 0 txkxkxcxM =++

Force excitation

Motion excitationF(t)

x0(t)

Force excitation Motion excitation

x

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Response curve (RAO) of simple oscillating system

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Resonance periods - examples

Vertical oscillation in air:

=

== 22220 gmgm

EAmLT

= elongation of wire due to weight mg

Vertical oscillation in water (long wire, wire mass mwincluded):

( )L

EAmmm

T wa 31

0 2++

=

LLgmg

mLF

mLT 22220 ===

Pendulum oscillation in air:

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Example: Lifting in airAmplitude dependent transverse stiffness

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Wire length: 25 mNatural period : 10.0 sVessel: L=100 mRegular waves H=2m

T = 5 s

T = 7 s T = 10 s

T = 9 s

T = 8 s T = 11 s

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Example: Lifting in air

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Wire length: 25 mNatural period : 10.0 sVessel: L=100 mIrregular waves Hs=2 m

Tp = 6 s

Tp = 8 s

Tp = 10 s

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Example: Installation of suction anchorsSplash zone crossing

Two suction anchors:

Anchor B4 B30

D x H (m) 4 x 4 4 x 30

Mass, steel (t) 26 156

Added mass (t) 86 400

Total mass (t) 111 556

Submerged weight (kN) 213 1330

D

H

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Splash zone - Anchor B30(Hs = 2.5 m, Tp = 8 s)

DAF 1.35

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Hydrodynamic mass for suction anchorsfound from model tests

D

A = amplitudep = perforation ratioa0 = added mass,

without ventilation

Example: p = 1%A/D = 0.5

X =

X = 4950

0

0.2

0.4

0.6

0.8

1

1 10 100 1000 10000

(A/D)*(1-p)/p^2

a / a

0 p = 0.01p = 0.03p = 0.11

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-35

-30

-25

-20

-15

-10

-5

0

0 0.2 0.4 0.6 0.8 1

Depth reduction

Z (m

)

T = 3sT = 4sT = 5sT = 6sT = 7sT = 8 sT = 9 sT = 10 sT = 11 sT = 12 sT = 14 s

Reduction of wave kinematics with depth Reduced dynamic pressure at lower end of anchor

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Numerical modelling of a suction anchorElements with distributed forces

Horizontal added mass and damping

Vertical addedmass and damping,

topEnclosed water ++

All are positiondependent

(zero when element is above water

Anchor(steel)

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

35

Bucket B4 Bucket B30

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Liftwire tension (Hs = 2.0 m)

36

Bucket B4 Bucket B30

DAF = 2.5 / 1.5 DAF = 1.4 / 1.3

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Example: a 30x30 m stiffened foundation plate

37

M = 400 t

M = 30 t

M = 50 t

W = 3420 kNma = 15000 t

ma = 3000 t

Just fully submerged Inclination 79 deg.

Hs = 2.0 mTp = 6, 8, 10 s

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Calculated forces

38

Max.: 5700 kN

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-10.00

-5.00

0.00

5.00

10.00

15.00

0 500 1000 1500 2000 2500

Tension (kN)

Ver

tical

pos

ition

(m)

Analysis of transient processes(Splash zone crossing, other non-linear cases)

Analysis approach

A few examples

Estimation of extreme forces

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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 most onerous 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 of

physical phenomena) Estimate extreme force

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Statistical estimation from a time history

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Extreme estimation from local peaks, stationary processWEIBULL plot

ln [-

ln (1

-P)]

Reduced slope due tonon-linear effects, e.g.: Quadratic damping Snatch loads

P = cumulative probabilityx = local peaks = average value of peaks

off points may be due to statistical spread,or they may have aphysical explanation.

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Extreme estimation from repeated tests, random processExtremes from 20 lowerings through splash zone, GUMBEL plot

Cumulative probability

-1.5-1

-0.50

0.51

1.52

2.53

3.54

900 1000 1100 1200 1300 1400 1500Tension (kN)

-ln(-l

n(P)

)

off points may be due to statistical spread,OR:may represent a differentphysical mechanism(example: impact, snatch).

.95.90

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Snatch loads -Example: Lift-off from transport vessel

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Blind lift-off at max.relative velocity:

Fmax = approx. 5600 kN

Start in constant-tension modeLift when winch stops to pay in

Mass: 365 tonnesHs : 2.5 m

P = 0.95

P = 0.80

+=

+=

Mk

gVMg

MkVMgF

rel

rel

1

max

General expression:

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Example: Personnel transfer to offshore wind turbines

46

Intention:The boat should hang onfender friction during personnel transfer.

All slippages were considered as events. Statistics by use of GUMBEL plots

.90.95

.80

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Assume:Centre of buoyancy, CB, off centre of gravity, CG.

The force centre,

will be vertically below the hookheel angle

If center of vertical added mass is not inline with F:vertical excitation tilting oscillations

Lifting at points below CG should be analysed with care

Stability -Tilt angle and angular oscillations

B

CF CG CB

F = mg-B

mg

BmgCGmgCBBCF

=

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Underwater lifting operation(Not intended)

(Stability of the lifting body)

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Safe Job Analysis

Can anything go wrong ?

Shhh, Zog! ....Here come one now!

Subsea lifting operations Stavanger - Dec. 1-2 2010ContentThe installation often gives the highest life-time forces on subsea equipmentLift in generalOperation criterion:Ensure safe operationStatic forces may be unknown Example: Drainage when H-frame is taken on boardWreck recoveryLift in airWave forces in the splash zone Wave forces in the splash zone Example: Template (3- 8) Details should be checked Example: Template structureDamaged coverWire tension when lowering a body through the splash zoneExampleForces on the lifted object(Newtons, or kN)Wave kinematics(1)Heave force and pitch moment on a long member(It is conservative to assume small body, concentrated loads)Wave kinematics(2)Reduction with depthShielding-effect from the crane vesselExample of analysis resultsHydrodynamic data:Added mass, ma and drag , Fd 6 values - (for motion in 3 directions and rotation about 3 axis)Added mass - simple structuresAdded mass of ventilated structures ExampleDampingStiffnessSimple equation of motionResponse curve (RAO) of simple oscillating systemResonance periods - examplesExample: Lifting in airAmplitude dependent transverse stiffnessExample: Lifting in airExample: Installation of suction anchorsSplash zone crossingSplash zone - Anchor B30 (Hs = 2.5 m, Tp = 8 s)Hydrodynamic mass for suction anchorsfound from model testsReduction of wave kinematics with depth Reduced dynamic pressure at lower end of anchorNumerical modelling of a suction anchorElements with distributed forcesVertical wave forces on fixed buckets (Hs = 2.0 m)Liftwire tension (Hs = 2.0 m)Example: a 30x30 m stiffened foundation plateCalculated forcesAnalysis of transient processes (Splash zone crossing, other non-linear cases)Lowering through the splash zone Numerical analysis - 2 methodsStatistical estimation from a time historyExtreme estimation from local peaks, stationary processWEIBULL plot Extreme estimation from repeated tests, random processExtremes from 20 lowerings through splash zone, GUMBEL plotSnatch loads - Example: Lift-off from transport vesselExample: Personnel transfer to offshore wind turbinesStability - Tilt angle and angular oscillationsUnderwater lifting operation (Not intended)Safe Job AnalysisCan anything go wrong ?