2 - estimation of hydrodynamic forces during subsea lifting

Estimation of Hydrodynamic Forces during Subsea Lifting

Tormod Bøe DNV Marine Operations 4th December 2012

Estimation of Hydrodynamic Forces during Subsea Lifting Slide 2 4. December 2012

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


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


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

DNV-RP-H103 Modelling and Analysis of Marine Operations, April 2011

DNV-OS-E407 Underwater Deployment and Recovery Systems, October 2012

Relevant DNV Publications - Other

Remaning DNV Marine Operation Offshore Standards DNV-OS-H202,H204-H206

Upcoming DNV publications:


Relevant DNV Publications - WebSite

DNV publications can be downloaded for free at:

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

http://www.dnv.com/resources/rules_standards/


Content







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


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


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.


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 :


”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 γsf ≥ 3.0. (Pt.2 Ch.5 Sec.3.1.2)

Capacity Checks - Slings and Shackles

sf

slingsling γ

MBLF <


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

Capacity Checks – Structural Steel Other lifting equipment: A consequence factor of γC = 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


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


Content







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.


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.


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.


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

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

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


Simplified Method, Splash Zone - Wave Periods

139.8 ≤≤⋅ zTg

Hs

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

2/2π is here used.

gH

zTS

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

2/2π 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:


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⋅= 9.0ζ

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


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

22wctcs vvvv ++=

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.

vc = lowering speed vct = vertical crane tip velocity vw = vertical water particle velocity at water surface

22~ctawAV ηζδ +⋅=

gVF ⋅⋅= δρρ

ζa = wave amplitude ηct = crane tip motion amplitude Ãw = mean water line area in the wave surface zone


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 :


22wctcr 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

( )[ ] ( )[ ]233

233 wctM aAVaAMF ⋅++⋅+= ρ


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]


Simplified Method, Splash Zone - 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 :



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

233 A

)1(211A ⋅

+

−+≈

λ

λ

p

p

Ah

A

+=λ

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 2.5ln [ 1+ (h/sqrt(A)) ]

A33

/A33

o

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


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


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

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.



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 )()( ρFFFFF MD 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


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.



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


Simplified Method, Splash Zone - Static Weight

In addition, the weight inaccuracy factor should be applied


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)

MgFDAF total=



Possible ? DAF < 1.0



𝐷𝐷𝐷 = 𝐹𝑡𝑡𝑡𝑡𝑡𝑀𝑀

= 𝐹𝑠𝑡𝑡𝑡𝑠𝑠+𝐹ℎ𝑦𝑦𝑀𝑀

= 𝑀𝑀−𝜌𝜌𝑀+𝐹ℎ𝑦𝑦𝑀𝑀

𝐷𝐷𝐷 = 1 +𝐷ℎ𝑦𝑦 − 𝜌𝜌𝜌

𝑀𝜌

The DAF factor is given by:

Hence, if the buoyancy is larger than the hydrodynamic forces DAF becomes less than 1.0


Simplified Method, Splash Zone - Slack Slings

The Slack Sling Criterion.

Snap forces shall as far as possible be avoided. Weather criteria 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 applied by the weight inaccuracy factor.

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

4. December 2012 Estimation of Hydrodynamic Forces during Subsea Lifting Slide 37

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 Outside2.5 8.33 Outside Outside Outside Outside Outside Outside Outside3.0 6.14 20.45 Outside Outside Outside Outside Outside Outside3.5 4.79 15.54 32.45 Outside Outside Outside Outside Outside4.0 3.89 12.34 25.53 Outside Outside Outside Outside Outside4.5 3.29 10.19 20.89 35.40 53.71 Outside Outside Outside5.0 2.87 8.73 17.76 29.97 45.35 63.92 Outside Outside5.5 2.57 7.70 15.57 26.17 39.52 55.61 74.44 Outside6.0 2.35 6.92 13.90 23.30 35.10 49.32 65.96 85.006.5 2.16 6.27 12.53 20.94 31.49 44.18 59.02 76.017.0 2.00 5.72 11.36 18.92 28.40 39.79 53.10 68.337.5 1.85 5.24 10.34 17.17 25.72 35.98 47.97 61.688.0 1.73 4.82 9.46 15.65 23.39 32.68 43.52 55.918.5 1.62 4.45 8.68 14.32 21.36 29.81 39.66 50.919.0 1.52 4.13 8.01 13.17 19.60 27.31 36.30 46.569.5 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 39.4410.5 1.29 3.37 6.43 10.48 15.51 21.53 28.52 36.5011.0 1.23 3.17 6.02 9.78 14.45 20.03 26.51 33.9011.5 1.17 2.99 5.66 9.16 13.50 18.69 24.71 31.5812.0 1.12 2.83 5.33 8.60 12.65 17.49 23.10 29.5012.5 1.07 2.69 5.03 8.09 11.89 16.41 21.65 27.6213.0 1.03 2.55 4.75 7.63 11.19 15.42 20.34 25.93

𝑭𝑯𝑯𝑯 ≤ 𝑴𝒎𝒎𝒎 𝒈 𝑫𝑫𝑭𝒎𝒂𝒂𝒂𝒂𝒎𝒂𝒂𝒂 − 𝟏 + 𝝆𝝆𝒈

𝑭𝑯𝑯𝑯 ≤ 𝟎.𝟗(𝑴𝒎𝒎𝒎 𝒈 − 𝝆𝝆𝒈)


Simplified Method, Splash Zone - Summary

Capacity checks

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


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


Content






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.


Dynamic Forces – Vertical resonance


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.


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


Case Study – Dynamic Load at Lifted Object


Cable length L=2750m


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

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

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




Non-operable seastates


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.


Finally – One Last Comment:

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


Easy Handling ..


.. and Access for Survey !!


http://www.dnv.com/

2 - estimation of hydrodynamic forces during subsea lifting

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