DNV - Subsea Structure

50
DNV Marine Operations’ Rules for Subsea Lift Operations Simplified Methods for Prediction of Hydrodynamic Forces Tormod Bøe DNV Marine Operations 29th November 2011

Transcript of DNV - Subsea Structure

Page 1: DNV - Subsea Structure

DNV Marine Operations’ Rules for Subsea Lift Operations

Simplified Methods for Prediction of Hydrodynamic Forces

Tormod Bøe

DNV Marine Operations

29th November 2011

Page 2: DNV - Subsea Structure

DNV Marine Operations' Rules for Subsea Lift Operations Slide 2 29. November 2011

Content

Brief overview of relevant DNV publications

DNV Rules for Marine Operations, 1996,

Pt.2 Ch.5 Lifting – Capacity Checks

Simplified Methods for prediction of Hydrodynamic Forces

o in Splash Zone, DNV-RP-H103 Ch.4

o in Deepwater, DNV-RP-H103 Ch.5

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DNV Marine Operations' Rules for Subsea Lift Operations Slide 3 29. November 2011

Relevant DNV Publications

Lifting- and subsea operations :

Specially planned non-routine operations Routine operations

DNV Standard for Certification No.2.22 Lifting Appliances

October 2011

DNV Rules for Planning and Execution of

Marine Operations – 1996 and

DNV-OS-H101 Marine Operations, General - 2011

’Specially planned, non-routine operations of

limited durations, at sea. Marine operations are

normally related to temporary phases as e.g.

load transfer, transportation and installation.’

DNV-OS-E402 Offshore

Standard for Diving Systems

October 2010

DNV Standard for Certification

No. 2.7-3 Portable Offshore Units

May 2011

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DNV Marine Operations' Rules for Subsea Lift Operations Slide 4 29. November 2011

DNV-RP-C205 Environmental Conditions and Environmental Loads October 2010

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 2011

Relevant DNV Publications - Other

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DNV Marine Operations' Rules for Subsea Lift Operations Slide 5 29. November 2011

Relevant DNV Publications - WebSite

Most DNV publications can be downloaded for free at:

http://www.dnv.com

The 1996 DNV Rules for Marine Operations is not in the DNV intranet site.

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DNV Marine Operations' Rules for Subsea Lift Operations Slide 6 29. November 2011

Content

Brief overview of relevant DNV publications

DNV Rules for Marine Operations, 1996,

Pt.2 Ch.5 Lifting – Capacity Checks

Simplified Methods for prediction of Hydrodynamic Forces

o in Splash Zone, DNV-RP-H103 Ch.4

o in Deepwater, DNV-RP-H103 Ch.5

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DNV Marine Operations' Rules for Subsea Lift Operations Slide 7 29. November 2011

Capacity Checks - DNV 1996 Rules

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 5

Dynamic loads for subsea lifts are estimated according to DNV-RP-H103

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DNV Marine Operations' Rules for Subsea Lift Operations Slide 8 29. November 2011

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

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DNV Marine Operations' Rules for Subsea Lift Operations Slide 9 29. November 2011

The dynamic hook load, DHL, is given by:

DHL = DAF*(W+Wrig) + F(SPL)

ref. Pt.2 Ch.5 Sec.2.4.2.1

Capacity Checks - Crane Capacity

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.

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DNV Marine Operations' Rules for Subsea Lift Operations Slide 10 29. November 2011

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:

Capacity Checks - Sling Loads

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 :

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DNV Marine Operations' Rules for Subsea Lift Operations Slide 11 29. November 2011

”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)

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

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

Capacity Checks - Slings and Shackles

sf

slingsling

γ

MBLF

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DNV Marine Operations' Rules for Subsea Lift Operations Slide 12 29. November 2011

Lifting points:

The load factor f = 1.3, is increased by a

consequence factor, C = 1.3, so that total

design faktor, design , becomes:

design = c∙ f = 1.3 ∙ 1.3 = 1.7

The design load acting on the lift point becomes:

Fdesign = design∙ Fsling = 1.7∙ Fsling

Capacity Checks – Structural Steel

Other lifting equipment:

A consequence factor of C = 1.3 should be applied on lifting yokes, spreader bars, plateshackles, etc.

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

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DNV Marine Operations' Rules for Subsea Lift Operations Slide 13 29. November 2011

Capacity Checks – Summary

Capacity

of lifting

equipment

Weight of lifted object

and lifting equipment

Skew load, CoG and sling angle

Safety factors

Lift in air: VMO Rules Pt.2 Ch.5

Subsea lift: DNV-RP-H103

Compute Apply

Fsling

Crane

capacity DHL

DAF

Check

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DNV Marine Operations' Rules for Subsea Lift Operations Slide 14 29. November 2011

Content

Brief overview of relevant DNV publications

DNV Rules for Marine Operations, 1996,

Pt.2 Ch.5 Lifting – Capacity Checks

Simplified Methods for prediction of Hydrodynamic Forces

o in Splash Zone, DNV-RP-H103 Ch.4

o in Deepwater, DNV-RP-H103 Ch.5

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DNV Marine Operations' Rules for Subsea Lift Operations Slide 15 29. November 2011

Simplified Method, Splash Zone - DNV-RP-H103

The Recommended Practice; ”DNV-RP-H103 Modelling and Analysis of Marine Operations” was issued april 2009. Latest revision is april 2011.

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

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

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DNV Marine Operations' Rules for Subsea Lift Operations Slide 16 29. November 2011

Simplified Method, Splash Zone - 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.

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DNV Marine Operations' Rules for Subsea Lift Operations Slide 17 29. November 2011

New Simplified Method - Assumptions

Time-domain analysis:

• Coupled multi-body

systems with individual

forces and motions.

• Wind, wave and current

forces.

• Geometry modelled.

• Motions for all degrees

of freedom computed.

• Non-linearities included.

• Coupling effects.

• Continous lowering

simulations.

• Varying added mass.

• Statistical analysis of

responses.

• Visualization of lift.

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DNV Marine Operations' Rules for Subsea Lift Operations Slide 18 29. November 2011

Simplified Method, Splash Zone - 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

K

AMTn

332

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

Unless the vessel heading is fixed, vessel response should be analysed for wave directions at least ±15° off the applied vessel heading

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DNV Marine Operations' Rules for Subsea Lift Operations Slide 19 29. November 2011

Simplified Method, Splash Zone - Wave Periods

139.8 zTg

Hs

A lower limit of Hmax=1.8·Hs=λ/7 with

wavelength λ=g·Tz2/2π is here used.

g

H

zTS

6.10A lower limit of Hmax=1.8·Hs=λ/10 with wavelength

λ=g·Tz2/2π is here used.

There are two alternative approaches:

Alt-1) Wave periods are included:

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

Alt-2) Wave periods are disregarded:

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

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DNV Marine Operations' Rules for Subsea Lift Operations Slide 20 29. November 2011

Alt-1) Wave periods are included:

The wave amplitude, wave particle velocity and acceleration can be taken as:

Simplified Method, Splash Zone - Wave Kinematics

Sa H 9.0

gT

zaw

z

d

eT

v2

24

2

gT

zaw

z

d

eT

a2

24

22

sHd35.0

v esHg30.0w

sHd35.0

a eg10.0w

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

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DNV Marine Operations' Rules for Subsea Lift Operations Slide 21 29. November 2011

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 :

Simplified Method, Splash Zone - Hydrodynamic Forces

22

wctcs vvvv

gVF

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.

vc = lowering speed

vct = vertical crane tip velocity vw = vertical water particle velocity at water surface

22~ctawAV

gVF

ζa = wave amplitude

ηct = crane tip motion amplitude

Ãw = mean water line area in the

wave surface zone

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DNV Marine Operations' Rules for Subsea Lift Operations Slide 22 29. November 2011

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.

Drag force

Drag 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 :

Simplified Method, Splash Zone - Hydrodynamic Forces

22

wctcr vvvv

vc = lowering/hoisting speed vct = vertical crane tip velocity vw = vertical water particle velocity at water depth , d Ap = horizontal projected area

M = mass of object in air A33 = heave added mass of object act = vertical crane tip acceleration V = volume of displaced water relative to the still water level aw = vertical water particle acceleration at water depth, d

2

33

2

33 wctM aAVaAMF

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DNV Marine Operations' Rules for Subsea Lift Operations Slide 23 29. November 2011

Simplified Method, Splash Zone - Basics

Forces:

• Weight [N]

• Buoyancy [N] Weight = M*gmoon

Weight = M*g

Buoyancy = ρ*V*g

Properties:

• Mass, M [kg]

• Volume, V [m3]

• Added mass, A33 [kg]

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DNV Marine Operations' Rules for Subsea Lift Operations Slide 24 29. November 2011

Simplified Method, Splash Zone - Added Mass

Hydrodynamic added mass for flat plates

ba4

76.0A 233

Example:

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

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DNV Marine Operations' Rules for Subsea Lift Operations Slide 25 29. November 2011

Simplified Method, Splash Zone - Added Mass

Added 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

2

33 A)1(2

11A

p

p

Ah

A

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

and

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 2.5

ln [ 1+ (h/sqrt(A)) ]

A33

/A33

o

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

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DNV Marine Operations' Rules for Subsea Lift Operations Slide 26 29. November 2011

Added Mass from Partly Enclosed Volume

Simplified Method, Splash Zone - Added Mass

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.

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DNV Marine Operations' Rules for Subsea Lift Operations Slide 27 29. November 2011

Added Mass Reduction due to Perforation

Simplified Method, Splash Zone - Added Mass

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.

.

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

Ad

ded

Mass R

ed

ucti

on

Facto

r

e^-P/28

BucketKC0.1-H4D-NiMo

BucketKC0.6-H4D-NiMo

BucketKC1.2-H4D-NiMo

BucketKC0.5-H0.5D-NiMo

BucketKC1.5-H0.5D-NiMo

BucketKC2.5-H0.5D-NiMo

BucketKC3.5-H0.5D-NiMo

PLET-KC1-4

Roof-A0.5-2.5+

Hatch20-KCp0.5-1.8

Hatch18-KCp0.3-0.8

BucketKC0.1

BucketKC0.6

BucketKC1.2

RoofKCp0.1-0.27

RoofKCp0.1-0.37

DNV-Curve

Mudmat CFD

0.1A

A

S33

33

34/)5p(cos3.07.0A

A

S33

33

28

p10

S33

33 eA

A

if p< 5

if 5 < p < 34

if 34 < p < 50

Recommended reduction:

A33S = added mass for a non-

perforated structure.

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DNV Marine Operations' Rules for Subsea Lift Operations Slide 28 29. November 2011

Simplified Method, Splash Zone - Hydrodynamic Forces

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:

22 )()( FFFFFMD slamhyd

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

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DNV Marine Operations' Rules for Subsea Lift Operations Slide 29 29. November 2011

Simplified Method, Splash Zone - Load Cases Example

Load Case 1

Still water level beneath top of ventilated buckets

Slamming impact force, Fslam, acts on top of buckets. Inertia force to be included.

Varying buoyancy force, Fρ , drag force, FD and hydrodynamic part of 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.

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DNV Marine Operations' Rules for Subsea Lift Operations Slide 30 29. November 2011

Simplified Method, Splash Zone - Load Cases Example

Load Case 3

Still 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

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DNV Marine Operations' Rules for Subsea Lift Operations Slide 31 29. November 2011

Simplified Method, Splash Zone - Load Cases Example

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DNV Marine Operations' Rules for Subsea Lift Operations Slide 32 29. November 2011

Simplified Method, Splash Zone - Static Weight

In addition, the weight inaccuracy factor should be applied

Page 33: DNV - Subsea Structure

DNV Marine Operations' Rules for Subsea Lift Operations Slide 33 29. November 2011

Simplified Method, Splash Zone - 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 [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 object including flooding and weight inaccuracy factor

Fhyd is the hydrodynamic force

Fsnap is the snap load (normally to be avoided)

Mg

FDAF total

Page 34: DNV - Subsea Structure

DNV Marine Operations' Rules for Subsea Lift Operations Slide 34 29. November 2011

Simplified Method, Splash Zone - Slack Slings

The 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.

Page 35: DNV - Subsea Structure

Simplified Method, Splash Zone - Results

Tables can be computed giving an overview of operable seastates

Maximum allowable Fhyd is derived from max allowable DAF and the slack sling criterion

Red results are above installation limit

”Outside” means non-existent seastates

29. November 2011 DNV Marine Operations' Rules for Subsea Lift Operations Slide 35

Hydrodynamic force on object, FhydTz\Hs 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

2.0 12.24 Outside Outside Outside Outside Outside Outside Outside

2.5 8.33 Outside Outside Outside Outside Outside Outside Outside

3.0 6.14 20.45 Outside Outside Outside Outside Outside Outside

3.5 4.79 15.54 32.45 Outside Outside Outside Outside Outside

4.0 3.89 12.34 25.53 Outside Outside Outside Outside Outside

4.5 3.29 10.19 20.89 35.40 53.71 Outside Outside Outside

5.0 2.87 8.73 17.76 29.97 45.35 63.92 Outside Outside

5.5 2.57 7.70 15.57 26.17 39.52 55.61 74.44 Outside

6.0 2.35 6.92 13.90 23.30 35.10 49.32 65.96 85.00

6.5 2.16 6.27 12.53 20.94 31.49 44.18 59.02 76.01

7.0 2.00 5.72 11.36 18.92 28.40 39.79 53.10 68.33

7.5 1.85 5.24 10.34 17.17 25.72 35.98 47.97 61.68

8.0 1.73 4.82 9.46 15.65 23.39 32.68 43.52 55.91

8.5 1.62 4.45 8.68 14.32 21.36 29.81 39.66 50.91

9.0 1.52 4.13 8.01 13.17 19.60 27.31 36.30 46.56

9.5 1.43 3.84 7.42 12.16 18.06 25.13 33.37 42.76

10.0 1.36 3.59 6.90 11.27 16.71 23.22 30.79 39.44

10.5 1.29 3.37 6.43 10.48 15.51 21.53 28.52 36.50

11.0 1.23 3.17 6.02 9.78 14.45 20.03 26.51 33.90

11.5 1.17 2.99 5.66 9.16 13.50 18.69 24.71 31.58

12.0 1.12 2.83 5.33 8.60 12.65 17.49 23.10 29.50

12.5 1.07 2.69 5.03 8.09 11.89 16.41 21.65 27.62

13.0 1.03 2.55 4.75 7.63 11.19 15.42 20.34 25.93

Page 36: DNV - Subsea Structure

DNV Marine Operations' Rules for Subsea Lift Operations Slide 36 29. November 2011

Simplified Method, Splash Zone - Summary

DAF within

capacity

requirements

Object motion equal crane tip

Wave kinematics dependent on

assumed Hs,Tz seastate

Different deployment levels

Structure divided in main items

Compute Apply

No slack

slings Fhyd

Check

DAF

Fd, Fm,

Fslam and Fρ

Page 37: DNV - Subsea Structure

DNV Marine Operations' Rules for Subsea Lift Operations Slide 37 29. November 2011

Simplified Method, Splash Zone - Summary

The simplified method

assumes that:

• Vertical motion of

structure is equal

to the crane tip

motion.

• The horizontal

extension of the

structure is small.

• Only vertical

motion is present.

More accurate

calculations can be

performed applying:

• Regular design

wave approach

(Ch. 3.4.2)

• Time domain

analyses

• CFD analyses

Page 38: DNV - Subsea Structure

DNV Marine Operations' Rules for Subsea Lift Operations Slide 38 29. November 2011

Content

Brief overview of relevant DNV publications

DNV Rules for Marine Operations, 1996,

Pt.2 Ch.5 Lifting – Capacity Checks

Simplified Methods for prediction of Hydrodynamic Forces

o in Splash Zone, DNV-RP-H103 Ch.4

o in Deepwater, DNV-RP-H103 Ch.5

Page 39: DNV - Subsea Structure

Deepwater Operations - Challenges

Challenges :

Static weight at crane tip increases linearly with cable length.

The resonance period of the lifting system increases with cable length. Dynamic forces may increase due to resonant amplification induced by the vertical crane tip motion.

29. November 2011 DNV Marine Operations' Rules for Subsea Lift Operations Slide 39

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Dynamic Forces – Vertical resonance

29. November 2011 DNV Marine Operations' Rules for Subsea Lift Operations Slide 40

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Simplified Method, Deepwater - Assumptions

The following main assumptions are applied:

the subsea structure is lowered into deepwater and is unaffected by wave forces

the vertical motion of crane tip and subsea structure dominates → other motions can be disregarded

Offset due to current forces is disregarded

Heave compensation systems are not taken into account

DNV-RP-H103 Chapter 5 includes a simplified method for estimating dynamic response of lowered object.

29. November 2011 DNV Marine Operations' Rules for Subsea Lift Operations Slide 41

Page 42: DNV - Subsea Structure

Case Study – Main Data

The subsea structure mass is 97 tonnes

Water depth is 3000 m

The crane cable is a conventional steel wire

No heave compensation system

29. November 2011 DNV Marine Operations' Rules for Subsea Lift Operations Slide 42

Page 43: DNV - Subsea Structure

Case Study – Crane Tip Motion

Lift at side of crane vessel

Wave heading 15° off bow

RAO in heave, pitch and roll are combined in order to find the vertical motion at the crane tip

Vessel’s natural period in roll at T=9s dominates

29. November 2011 DNV Marine Operations' Rules for Subsea Lift Operations Slide 43

Page 44: DNV - Subsea Structure

Case Study – Dynamic Load at Lifted Object

Comparison with a non-linear time-domain FE analysis

Dynamic amplification 20% higher at natural period T0=9s

Dynamic amp. at T=1.5s due to longitudinal pressure waves

No wave energy at T=1.5s, hence deviation is acceptable

29. November 2011 DNV Marine Operations' Rules for Subsea Lift Operations Slide 44

Cable length L=2750m

Page 45: DNV - Subsea Structure

Case Study – Dynamic Load at Lifted Object

Transfer functions for dynamic load in cable and crane tip motion are combined with a wave spectrum S(ω)

Jonswap wave spectrum with Hs=2.0m and Tp=9s is applied

Most probable largest response for dynamic force in cable is found by:

A duration time t =30 minutes gives Fd=530kN in this case

29. November 2011 DNV Marine Operations' Rules for Subsea Lift Operations Slide 45

Page 46: DNV - Subsea Structure

Case Study – Dynamic Load at Lifted Object

Calculations are repeated for a range of seastates

Hs=2.0m gives acceptable dynamic loads for all wave periods

Natural period of the lifting system is T0=9s

29. November 2011 DNV Marine Operations' Rules for Subsea Lift Operations Slide 46

Page 47: DNV - Subsea Structure

Case Study – Dynamic Load at Lifted Object

Calculations are repeated for a range of cable lengths

Max Fd for all Tz values

Fd<0.9*Fstatic in order to avoid risk of snap loads due to slack slings; Fd < 68t

Capacity requirement of crane and cable governs for cable lengths above L>2250m due to weight of cable

Non-operable seastates

29. November 2011 DNV Marine Operations' Rules for Subsea Lift Operations Slide 47

Page 48: DNV - Subsea Structure

DNV Marine Operations' Rules for Subsea Lift Operations Slide 48 29. November 2011

Simplified Method, Deepwater - Summary

DNV-RP-H103 chapter 5 contains a simplified method for

establishing dynamic loads and limiting weather criteria during

deepwater lifting operations

Most probable largest dynamic load in the lifting line is computed

taking into account dynamic amplification due to resonance

effects

The simplified method is well suited for common spreadsheet

programs or other computer software for engineering

calculations.

Page 49: DNV - Subsea Structure

DNV Marine Operations' Rules for Subsea Lift Operations Slide 49 29. November 2011

.. Questions ??

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DNV Marine Operations' Rules for Subsea Lift Operations Slide 50 29. November 2011