3 - dnv rules for marine operations

DNV Marine Operations’ Rulesfor Subsea Lift Operations

New Simplified Method for Prediction of Hydrodynamic Forces

Tormod BøeDNV Marine Operations2nd December 2009

DNV Marine Operations' Rules for Subsea Lift Operations Slide 22 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 of hydrodynamic forces, DNV-RP-H103 Ch.4

CFD Analyses – Test Cases


Relevant DNV Publications

Lifting- and subsea operations :

DNV-OS-E402Offshore Standard for Diving Systems January 2004(Amendments October 2009)

DNV Rules for Planning and Execution of Marine Operations – 1996

’Special planned, non-routine operations of limited durations, at sea. Marine operations arenormally related to temporary phases as e.g.load transfer, transportation and installation.’

DNV Standard for CertificationNo.2.22 Lifting AppliancesOctober 2008

DNV Standard for CertificationNo. 2.7-1 Offshore ContainersApril 2006

Special planned non-routine operations Routine operations


Relevant DNV Publications - Other

DNV-RP-C205 Environmental Conditions and Environmental Loads April 2007

DNV-RP-H101 Risk Management in Marine and Subsea Operations, January 2003

DNV-RP-H102 Marine Operations during Removal of Offshore Installations, April 2004

DNV-RP-H103 Modelling and Analysis of Marine Operations, April 2009

Standard for Certification No. 2.7-3 Portable Offshore Units, June 2006 (a new revision will be issued 2010 which will include subsea units)

DNV-OS-J-101 and -201 Design of Offshore Wind Turbine Structures and Substations for Wind Farms, October 2007 / 2009

DNV-OS-E303 and -RP-E304 Offshore Mooring Fibre Ropes, Certification (2008) and Damage Assessment (2005)


Relevant DNV Publications - Purchase

DNV publications can be purchased at:

http://webshop.dnv.com/global/


Content


DNV Rules for Marine Operations, 1996, Lifting – Capacity Checks

New Simplified Method for calculation of hydrodynamic forces



Capacity Checks - DNV 1996 Rules

Rules for Planning and Execution of Marine Operations, 1996

Part 1 - General

Pt.1 Ch.1 - Warranty SurveysPt.1 Ch.2 - Planning of

OperationsPt.1 Ch.3 - Design LoadsPt.1 Ch.4 - Structural Design

Part 2 - Operation Specific Requirements

Pt.2 Ch.1 - Load Transfer OperationsPt.2 Ch.2 - TowingPt.2 Ch.3 - Special Sea TransportsPt.2 Ch.4 - Offshore InstallationPt.2 Ch.5 - LiftingPt.2 Ch.6 - Sub Sea OperationsPt.2 Ch.7 - Transit and Positioning

of Mobile Offshore Units


Capacity Checks - DNV 1996 Rules

Part 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 6Dynamic loads, subsea lifts (capacity checks as in Chapter 5 applying dynamic loads from Chapter 6)


Capacity Checks – DAF for Lift in Air

Dynamic loads are accounted for by using a Dynamic Amplification Factor (DAF).

DAF in air may be caused by e.g. variation in hoisting speeds or motions of crane vessel and lifted object.

The given table is applicable for offshore lift in air in minor sea states, typically Hs < 2-2.5m.

DAF must be estimated separately for lifts in air at higher seastates and for subsea lifts !

Table 2.1 Pt.2 Ch.5 Sec.2.2.4.4


Capacity Checks - Crane Capacity

The dynamic hook load, DHL, is given 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 against available crane capacity

The crane capacity decrease when the lifting radius increase.


Capacity Checks - Sling Loads

The maximum dynamic sling load, Fsling, can be calculated by:

Fsling = DHL·SKL·kCoG·DW / sin φ

ref. Pt.2 Ch.5 Sec.2.4.2.3-6

where:

SKL = Skew load factor → extra loading caused by equipment and fabrication tolerances.

kCoG = CoG factor → inaccuracies in estimated position 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.

Example :


Capacity Checks - Slings and Shackles

The sling capacity ”Minimum breaking load”, MBL, is checked by:

The safety factor is minimum γsf ≥ 3.0. (Pt.2 Ch.5 Sec.3.1.2)

sf

slingsling γ

MBLF <

”Safe working load”, SWL, and ” MBL, of the shackle are checked by :

a) Fsling < SWL· DAF

and b) Fsling < MBL / 3.3

Both criteria shall be fulfilled (Pt.2 Ch.5 Sec.3.2.1.2)


Capacity Checks – Structural SteelOther lifting equipment:A consequence factor of γC = 1.3should be applied on lifting yokes, spreader bars, plateshackles, etc.

Lifting points:

The load factor γf = 1.3, is increased by a consequence factor, γC = 1.3, so that total design faktor, γdesign , becomes:

γdesign = γc· γf = 1.3 · 1.3 = 1.7

The design load acting on the lift point becomes:

Fdesign = γdesign· Fsling = 1.7· Fsling

Structural strength of Lifted Object:

The following consequence factors should be applied :

A lateral load of minimum 3% of the design load shall be included. This load acts in the shackle bow !(ref. Pt.2.Ch.5 Sec.2.4.3.4)

Table 4.1 Pt.2 Ch.5 Sec.4.1.2


Content






New Simplified Method - DNV-RP-H103

A new Recommended Practice; ”DNV-RP-H103Modelling and Analysis of Marine Operations”was issued april 2009.

A new Simplified Method for calculating hydrodynamic forces on objects lifted through wave zone is included in chapter 4.

This new Simplified Method supersedes the calculation guidelines in DNV Rules for Marine Operations, 1996, Pt.2 Ch.6.


New Simplified Method - Assumptions

The Simplified Method is based upon the following main assumptions:

the horizontal extent of the lifted object is small compared to the wave length

the vertical motion of the object is equal the vertical crane tip motion

vertical motion of object and water dominates → other motions can be disregarded

The intention of the Simplified Method is to give simple conservative estimates of the forces acting on the object.


New Simplified Method - AssumptionsTime-domain analysis:Includes loads and motion responses on both installation vessel and lifted object.

Lifted object modelledapplying correct geometry (not just a point in space)simulation valid for all wave lengths.

Cranewire, lifting slings and tugger lines are included motion response of the lifted object is computed resonance effects are covered in analysis.

Statistical analysis of responses in irregular sea states included.

Coupling effects included (crane tip motions may be influenced by lifted structure).

Non-linear response, as e.g. snap loads in lifting slings, can be computed.

Visualization of lift.


New Simplified Method – Crane Tip Motions

The Simplified Method is unapplicable if the crane tip oscillation period or the wave period is close to the resonance period, Tn , of the hoisting system

KAM

Tn332

+= π

Heave, pitch and roll RAOs for the vessel should be combined with crane tip position to find the vertical motion of the crane tip

If operation reference period is within 30 minutes, the most probable largest responses may be taken as 1.80 times the significant responses

If the vessel heading is not fixed, vessel response should be analysed for wave directions at least ±15° off the applied vessel heading


New Simplified Method – Wave PeriodsThere are two alternative approaches:

139.8 ≤≤⋅ zTg

Hs

A lower limit of Hmax=1.8·Hs=λ/7 with wavelength λ=g·Tz

2/2π is here used.

Alt-1) Wave periods are included:

Analyses should cover the following zero-crossing wave period range:

gH

zTS

⋅≥ 6.10A lower limit of Hmax=1.8·Hs=λ/10 with wavelength λ=g·Tz

2/2π is here used.

Alt-2) Wave periods are disregarded:

Operation procedures should in this case reflect that the calculations are only valid for waves longer than:


New Simplified Method – Wave Kinematics

Alt-1) Wave periods are included:The wave amplitude, wave particle velocity and acceleration can be taken as:

Sa H⋅= 9.0ζ

gT

zaw

z

d

eT

v2

242

ππ

ζ−

⋅⎟⎟⎠

⎞⎜⎜⎝

⎛⋅=

gT

zaw

z

d

eT

a2

242

2π

πζ−

⋅⎟⎟⎠

⎞⎜⎜⎝

⎛⋅=

sHd35.0

v esHg30.0w

−⋅= π

sHd35.0

a eg10.0w

−⋅= π

Alt-2) Wave periods are disregarded:The wave particle velocity and acceleration can be taken as:

d : distance from water plane to CoG of submerged part of object


New Simplified Method – Hydrodynamic Forces

Slamming impact force

Slamming forces are short-term impulse forces that acts when the structure hits the water surface.

AS is the relevant slamming area on the exposed structure part. Cs is slamming coeff.

The slamming velocity, vs, is :

22wctcs vvvv ++=

vc = lowering speedvct = vertical crane tip velocityvw = vertical water particle velocity

at water surface

gVF ⋅⋅= δρρ

Varying buoyancy force

Varying buoyancy, Fρ , is the change in buoyancy due to the water surface elevation.

δV is the change in volume of displaced water from still water surface to wave crest or wave trough.

22~ctawAV ηζδ +⋅=

gVF ⋅⋅= δρρ

ζa = wave amplitudeηct = crane tip motion amplitudeÃw = mean water line area in the

wave surface zone


New Simplified Method – Hydrodynamic Forces

Drag forceDrag forces are flow resistance on submerged part of the structure. The drag forces are related to relative velocity between object and water particles.

The drag coefficient, CD, in oscillatory flow for complex subsea structures may typically be CD ≥ 2.5.

Relative velocity are found by :

22wctcr vvvv ++=

vc = lowering/hoisting speedvct = vertical crane tip velocityvw = vertical water particle velocity

at water depth , dAp = horizontal projected area

Mass force

“Mass force” is here a combination of inertia force, Froude-Kriloff force and diffraction force.

Crane tip acceleration and water particle acceleration are assumed statistically independent.

( )[ ] ( )[ ]2332

33 wctM aAVaAMF ⋅++⋅+= ρ

M = mass of object in airA33 = heave added mass of objectact = vertical crane tip accelerationV = volume of displaced water relative to

the still water levelaw = vertical water particle acceleration

at water depth, d


New Simplified Method – Hydrodynamic Force

The hydrodynamic force is a time dependent function of slamming impact force, varying buoyancy, hydrodynamic mass forces and drag forces. In the Simplified Method the forces may be combined as follows:

22slamhyd )FF()FF(F MD ρ−++=

The structure may be divided into main items and surfaces contributing to the hydrodynamic force

Water particle velocity and acceleration are related to the vertical centre of gravity for each main item. Mass and drag forces contributions are then summarized :

∑=i

iMM FF ∑=i

iDD FF

FMi and FDi are the individual force contributions from each main item


New Simplified Method – Load Cases Example

Load Case 1

Still water level beneath top of ventilated buckets

Slamming impact force, Fslam, acts on top of buckets.

Varying buoyancy force, Fρ , drag force, FDand mass force, FM are negligible.

The static and hydrodynamic force should be calculated for different stages. Relevant load cases for deployment of a protection structure could be:

Load Case 2

Still water level above top of buckets

Slamming impact force, Fslam, is zero

Varying buoyancy, Fρ , drag force, FD and mass force, FM, are calculated. Velocity and acceleration are related to CoG of submerged part of structure.



Load Case 3Still water level beneath roof cover.

Slamming impact force, Fslam, acts on the roof cover.

Varying buoyancy, Fρ , drag force, FD and mass force, FM are calculated on the rest of the structure. Drag- and mass forces acts mainly on the buckets and is related to a depth, d, down to CoG of submerged part of the structure.

Load Case 4

Still water level above roof cover.

Slamming impact force, Fslam, and varying buoyancy, Fρ, is zero.

Drag force, FD and mass force, FM are calculated individually. The total mass and drag force is the sum of the individual load components, e.g. : FD= FDroof + FDlegs+ FDbuckets applying correct CoGs


New Simplified Method – Static Weight

In addition, the weight inaccuracy factor should be applied


New Simplified Method - DAF

Capacity Checks

The capacities of crane, lifting equipment and lifted object are checked as for lift in air. The following relation should be applied:

where

Mg : weight of object in air [N]

Ftotal : is the characteristic total force on the (partly or fully) submerged object. Taken as the largest of;

Ftotal = Fstatic-max + Fhyd or Ftotal = Fstatic-max + Fsnap

Fstatic-max is the maximum static weight of the submerged objectincluding flooding and weight inaccuracy factor

Fhyd is the hydrodynamic force

Fsnap is the snap load (normally to be avoided)

MgFDAF total=


New Simplified Method – Slack SlingsThe Slack Sling Criterion.

Snap forces shall as far as possible be avoided. Weather crietria should be adjusted to ensure this.

The following criterion should be fulfilled in order to ensure that snap loads are avoided:

minstatichyd F9.0F −⋅≤

Fstatic-min = weight before flooding, including a weight reduction implied by the weight inaccuracy factor.


New Simplified Method – Added Mass

Hydrodynamic added mass for flat plates

ba4

76.0A 233 ⋅⋅⋅⋅=

πρ

Example:

Flat plate where length, b, above breadth, a, is b/a = 2.0 :


New Simplified Method – Added MassAdded Mass Increase due to Body Height

The following simplified approximation of the added mass in heave for a three-dimensional body with vertical sides may be applied :

o332

233 A

)1(2

11A ⋅⎥⎥⎥

⎦

⎤

⎢⎢⎢

⎣

⎡

+

−+≈

λ

λ

p

p

Ah

A

+=λ

Added Mass Increase due to Body Height

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

0 0.5 1 1.5 2 2.5ln [ 1+ (h/sqrt(A)) ]

A33

/A33

o

1+SQRT((1-lambda^2)/(2*(1+lambda^2)))

and

where

A33o = added mass for a flat plate with a shape equal to the horizontal projected area of the object

h = height of the object

Ap = horizontal projected area of the object



Added Mass from Partly Enclosed Volume

A volume of water partly enlosed within large plated surfaces will also contribute to the added mass, e.g.:

The volume of water inside suction anchors or foundation buckets.

The volume of water between large plated mudmat surfaces and roof structures.



Added Mass Reduction due to Perforation

.

Effect of perforation on added mass

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 10 20 30 40 50Perforation

Add

ed M

ass

Red

uctio

n Fa

ctor

e^-P/28BucketKC0.1-H4D-NiMoBucketKC0.6-H4D-NiMoBucketKC1.2-H4D-NiMoBucketKC0.5-H0.5D-NiMoBucketKC1.5-H0.5D-NiMoBucketKC2.5-H0.5D-NiMoBucketKC3.5-H0.5D-NiMoPLET-KC1-4Roof-A0.5-2.5+Hatch20-KCp0.5-1.8Hatch18-KCp0.3-0.8BucketKC0.1BucketKC0.6BucketKC1.2RoofKCp0.1-0.27RoofKCp0.1-0.37DNV-CurveMudmat CFD

0.1AA

S33

33 =

[ ]34/)5p(cos3.07.0AA

S33

33 −+= π

28p10

S33

33 eAA

−

=

if p< 5

if 5 < p < 34

if 34 < p < 50

Recommended reduction:

A33S = added mass for a non-perforated structure.

No reduction applied in added mass when perforation is small. A significant drop in the added mass for larger perforation rates. Reduction factor applicable for p<50.


New Simplified Method – Example CaseExample: Submerged Foundation Bucket

kg218670.2342A 3

o33 =⋅⋅⋅⋅= ππ

ρ

( )

s332

2

2s33

o332

2'

s33

2

2

3o33

A840.2

4.0100P

6154625.375.129496A

29496A78.012

78.011A

78.00.21

0.2

218670.2342A

ofreduction No :n Perforatio

kg : volumeinside Incl.

kg : increaseHeight

:factor Height

kg : plateFlat

⇒<=⋅

⋅⋅=

=⋅⋅⋅+=

=⋅⎥⎥⎥

⎦

⎤

⎢⎢⎢

⎣

⎡

+⋅

−+=

=⋅+

⋅=

=⋅⋅⋅⋅=

π

π

ρπ

π

πλ

ππ

ρ

Added mass for a thin circular disc:

Added mass increase due to body height:

( ) kg33803A50.012

50.011A50.00.25.3

0.2o332

2'

s332

2=⋅

⎥⎥

⎦

⎤

⎢⎢

⎣

⎡

+⋅

−+=⇒=

⋅+

⋅=

π

πλ

Added mass including partly enclosed volume:

kg6585425.375.133803A 2s33 =⋅⋅⋅+= ρπ

Added mass reduction due to perforation:

s332

2A4

0.2

4.0100P of reduction NoSMALL ⇒≈=⋅

⋅⋅=π

π

Bucket Dimensions:Height = 3.5mDiameter = 4.0mPlate thickness = 0.25mVentilation hole diameter = 0.8m


New Simplified Method – Example CaseExample: Submerged Foundation Bucket

( ) N5222rPDD 1037.048.125.00.296.00.25.0vAC5.0F ⋅=+⋅⋅⋅⋅=⋅⋅⋅⋅= πρρ

( )[ ] ( )[ ] ( ) N52w33

2ct33M 1033.169.16585413031aAVaAMF ⋅=⋅+=⋅++⋅+= ρ

2m/s and m/s 69.1v5.5

2a48.1e5.5

275.1v ww81.95.5)25.11(4

w2

2

=⋅⎟⎠

⎞⎜⎝

⎛==⋅⎟

⎠

⎞⎜⎝

⎛⋅= ⋅

+⋅− πππ

Regular Wave Data:Wave Height, Hmax = 3.5mWave Period, Tz = 5.5s

Water particle velocity and acceleration:

Drag force:

Mass force:

Hydrodynamic force:

1.0m

1.25mCoG

Other DataBuoyancy, ρV = 13031kgNegligible crane tip motionsLowering speed = 0.25m/s

( ) ( ) ( ) ( ) N525252M

2slamDhyd 104.11033.11037.0FFFFF ⋅=⋅+⋅=−++= ρ


Content






CFD Analyses – Test CasesComputational Fluid Dynamics (CFD) is a numerical method for computing fluid flows based on the Navier Stokes equations.

The CFD-program COMFLOW is able to study complex free surface problems applying the Volume of Fluid method.

The fluid domain consists of a cartesian grid where the fluid cells are defined either as boundary cells, empty cells, surface cells or fluid cells.

Pressure forces are calculated as the integral of the pressure along the boundary of an object.

Motion responses are not included, but the object can be given a prescribed motion.

Structure

Fluid domainInflow boundary,

Airy or Stokes5th wave

Numerical beach at aft end


CFD Analyses – Protection Structure

CFD analysis:Regular Stokes 5th wave: H=3.5m T=5.5s

Domain 95x30x37m 4.4 million fluid cells

Minimum grid size 0.18m near object, stretched elsewhere

8.5x8.5m solid roof and 10x10xØ1.0m top frame

Ø1.0m legs, height 8m and hollow

3.5xØ4.0m buckets at x,y=±8.5m

ventilation holes Ø0.8m

Wall thickness 0.25m

half model

60s simulation time



Highest upwards hydrodynamic force when bucket is fullysubmerged occurs when the object is located in a wave trough.

Fhyd ≈ 1.1·105N

Buoyancy, ρVg



Half wave length is ~23.5m and the distance between buckets is 17m.

Hence, there is a large phase difference between the hydrodynamic forces on forward and aft bucket.



ComFlow results show very high slamming loads on bucket top and the solid roof structure.

These values are most likely too high as compressibility and formation/ collapse of air cushions are not included in the simulation.

Slamming load on aft bucket

Slamming load on roof structure


CFD Analyses – Spool Piece

CFD analysis:

Regular Stokes 5th wave: H=3.5m T=5.5s

The wave length is ~equal spool length

Domain 130x30x31m 2.2 million fluid cells

Minimum grid size 0.25m near object, stretched elsewhere

50m long closed pipe with diameter Ø1.0m

Two simulations; 1) half submerged 2) 2m below surface

22s simulation time

computer time 13-18hrs


CFD Analyses – Spool Piece Half Submerged

N N 552mvertical

522

waddm 104.1106.081.92540.1FVgF106.0

25.3

5.522254

0.10.2a)mV(F ⋅=⋅−⋅⋅⋅⋅=+=⇒⋅−=⋅⎟⎠

⎞⎜⎝

⎛⋅⋅⋅⋅⋅⋅−≈⋅+= πρρπ

ππρρ

The wave length is equal the spool piece length

Vertical force on aft half at time t=5s :

Half of the spool piece is always out of the water.

The total force on each half vary between zero and buoyancy+Fhyd


CFD Analyses – Spool Piece 2m Submerged

Total vertical force

Vertical force, fwd half

Vertical force, aft half

N522

waddm 1045.025.3

5.5277.02254

0.110250.2a)mV(F ⋅=⋅⎟⎠

⎞⎜⎝

⎛⋅⋅⋅⋅⋅⋅≈⋅+=π

ππρBrief approximation of mass force:

Dynamic force amplitude (mainly mass forces) ≈ 0.55·105 kN


And then – One Final Comment:

When planning Marine Operations, remember to take into account ....


Easy Handling ..


.. and Survey Access !!

3 - dnv rules for marine operations

Documents

relevant dnv publications

subsea lift operations

dnv marine operations

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routine operations

capacity checks