2 - Estimation of Hydrodynamic Forces During Subsea Lifting

Estimation of Hydrodynamic Forces during Subsea Lifting

Tormod Bøe DNV Marine Operations 4th 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


4. December 2012

Slide 2

Relevant DNV Publications Lifting- and subsea operations : 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.2.22 Lifting Appliances October 2011

DNV Standard for Certification No. 2.7-3 Portable Offshore Units May 2011

Specially planned non-routine operations


4. December 2012

Routine operations

Slide 3

Relevant DNV Publications - Other 

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

Upcoming DNV publications: 

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


4. December 2012

Slide 4

Relevant DNV Publications - WebSite DNV publications can be downloaded for free at:

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

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


4. December 2012

Slide 5

Content 









4. December 2012

Slide 6

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.)

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


4. December 2012

Slide 7

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


4. December 2012

Slide 8

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.


4. December 2012

Slide 9

Capacity Checks - Sling Loads Example : 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.


4. December 2012

Slide 10

Capacity Checks - Slings and Shackles The sling capacity ”Minimum breaking load”, MBL, is checked by:

Fsling

MBLsling < γ sf

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

”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)


4. December 2012

Slide 11

Capacity Checks – Structural Steel Lifting points:

Other lifting equipment:

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

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

γdesign = γc∙ γf = 1.3 ∙ 1.3 = 1.7

Structural strength of Lifted Object:

The design load acting on the lift point becomes:

The following consequence factors should be applied :

Fdesign = γdesign∙ Fsling = 1.7∙ Fsling

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 Estimation of Hydrodynamic Forces during Subsea Lifting

4. December 2012

Pt.2 Ch.5 Sec.4.1.2 Slide 12

Capacity Checks – Summary Compute

DAF

DHL

Fsling Estimation of Hydrodynamic Forces during Subsea Lifting

Check

Apply 

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



Subsea lift: DNV-RP-H103



Weight of lifted object and lifting equipment



Skew load, CoG and sling angle



Safety factors

4. December 2012

Crane capacity

Capacity of lifting equipment Slide 13

Content 









4. December 2012

Slide 14

Simplified Method, Splash Zone - DNV-RP-H103  The

Recommended Practice; ”DNV-RPH103 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.


4. December 2012

Slide 15

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.


4. December 2012

Slide 16

New Simplified Method - Assumptions Time-domain analysis:


4. December 2012

•

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. Slide 17

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



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


4. December 2012

Tn = 2π

M + A33 K

Slide 18

Simplified Method, Splash Zone - Wave Periods There are two alternative approaches: Alt-1) Wave periods are included: Analyses should cover the following zerocrossing wave period range:

8.9 ⋅

Hs g

≤ Tz ≤ 13

A lower limit of Hmax=1.8·Hs=λ/7 with wavelength λ=g·Tz2/2π 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: Tz ≥ 10.6 ⋅

HS g


A lower limit of Hmax=1.8·Hs=λ/10 with wavelength λ=g·Tz2/2π is here used.

4. December 2012

Slide 19

Simplified Method, Splash Zone - Wave Kinematics Alt-1) Wave periods are included: The wave amplitude, wave particle velocity and acceleration can be taken as: 





ζ a = 0.9 ⋅ HS 4π 2 d

−

 2π v w = ζ a ⋅   Tz

 ⋅e  

 2π aw = ζ a ⋅   Tz

   

2

T z2 g

−

⋅e

4π 2 d T z2 g 

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

Alt-2) Wave periods are disregarded:

 vw = 0.30

The wave particle velocity and acceleration can be taken as: 


4. December 2012

π g Hs ⋅ e

aw = 0.10 π

g ⋅e

−

−

0.35 d Hs

0.35 d Hs

Slide 20

Simplified Method, Splash Zone - 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.

v s = v c + v ct2 + v w2 

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

The slamming velocity, vs, is :

 

Varying buoyancy force

Fρ = ρ ⋅ δV ⋅ g

Varying buoyancy, Fρ , is the change in buoyancy due to the water surface Fρ = ρ ⋅ δV ⋅ g elevation.

~ δV = Aw ⋅

δV is the change in volume of displaced water from still water surface to wave crest or wave trough. Estimation of Hydrodynamic Forces during Subsea Lifting

4. December 2012

  

ζ a 2 + ηct2

ζa = wave amplitude ηct = crane tip motion amplitude Ãw = mean water line area in the wave surface zone Slide 21

Simplified Method, Splash Zone - Hydrodynamic Forces 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.

v r = vc +

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

  

Relative velocity are found by :



Mass force

Crane tip acceleration and water particle acceleration are assumed statistically independent. Estimation of Hydrodynamic Forces during Subsea Lifting

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

FM =

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

4. December 2012

    

vct2 + v w2

[(M + A )⋅ a ] + [(ρV + A )⋅ a ] 2

33

ct

33

2

w

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 Slide 22

Simplified Method, Splash Zone - Basics Properties:

Forces:

•

Mass, M [kg]

•

Weight [N]

•

Volume, V [m3]

•

Buoyancy [N]

•

Added mass, A33 [kg]

Weight = M*gmoon

Buoyancy = ρ*V*g

Weight = M*g


4. December 2012

Slide 23

Simplified Method, Splash Zone - Added Mass Hydrodynamic added mass for flat plates Example: Flat plate where length, b, above breadth, a, is b/a = 2.0 :

A33 = ρ ⋅ 0.76 ⋅


4. December 2012

π 2 ⋅a ⋅b 4

Slide 24

Simplified Method, Splash Zone - Added Mass Added Mass Increase due to Body Height

 A33 ≈ 1 +  

 ⋅ A 2  33o 2( 1 + λ )   1 − λ2

1.8 1.7

A33/A33o

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

Added Mass Increase due to Body Height

1.6 1.5 1.4 1.3 1+SQRT((1-lambda^2)/(2*(1+lambda^2)))

1.2

and

λ=

Ap h + Ap

1.1 1 0

0.5

1

1.5

2

2.5

ln [ 1+ (h/sqrt(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


4. December 2012

Slide 25

Simplified Method, Splash Zone - Added Mass 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.


4. December 2012

Slide 26

Simplified Method, Splash Zone - Added Mass Added Mass Reduction due to Perforation Recommended reduction:

Effect of perforation on added mass 1

if p< 5

A33 = 0.7 + 0.3 cos[π ( p − 5 ) / 34 ] A33 S

if 5 < p < 34

A33 =e A33 S

10 − p 28

if 34 < p < 50

0.9 Added Mass Reduction Factor

A33 = 1.0 A33 S

0.8 0.7

.

0.6 0.5 0.4

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

0.3 0.2 0.1

A33S = added mass for a nonperforated structure. 

0 0

10

Perforation 20 30

40

50

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.


4. December 2012

Slide 27

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:

Fhyd = ( FD + Fslam ) 2 + ( FM − Fρ ) 2 

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 : FM =

∑F

Mi

FD =

∑F

i

Di

i

FMi and FDi are the individual force contributions from each main item Estimation of Hydrodynamic Forces during Subsea Lifting

4. December 2012

Slide 28

Simplified Method, Splash Zone - Load Cases Example The static and hydrodynamic force should be calculated for different stages. Relevant load cases for deployment of a protection structure could be: 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.

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.


4. December 2012

Slide 29

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


4. December 2012

Slide 30

Simplified Method, Splash Zone - Load Cases Example


4. December 2012

Slide 31

Simplified Method, Splash Zone - Static Weight



In addition, the weight inaccuracy factor should be applied


4. December 2012

Slide 32

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:

Ftotal DAF = Mg where Mg : weight of object [N]



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)

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 Estimation of Hydrodynamic Forces during Subsea Lifting

4. December 2012

Slide 33

Simplified Method, Splash Zone - DAF

DAF < 1.0 Possible ?


4. December 2012

Slide 34

Simplified Method, Splash Zone - DAF The DAF factor is given by:

𝐷𝐷𝐷 =

𝐹𝑡𝑡𝑡𝑡𝑡 𝑀𝑀

=

𝐹𝑠𝑠𝑠𝑠𝑠𝑠 +𝐹ℎ𝑦𝑦 𝑀𝑀

=

𝑀𝑀−𝜌𝜌𝜌+𝐹ℎ𝑦𝑦 𝑀𝑀

𝐹ℎ𝑦𝑦 − 𝜌𝜌𝜌 𝐷𝐷𝐷 = 1 + 𝑀𝑀

Hence, if the buoyancy is larger than the hydrodynamic forces DAF becomes less than 1.0 Estimation of Hydrodynamic Forces during Subsea Lifting

4. December 2012

Slide 35

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:

Fhyd ≤ 0.9 ⋅ Fstatic − min  Fstatic-min

= weight before flooding, including a weight reduction applied by the weight inaccuracy factor.


4. December 2012

Slide 36

Simplified Method, Splash Zone - Results Hydrodynamic force on object, Fhyd

 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


Tz\Hs 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0 12.5 13.0

0.5 12.24 8.33 6.14 4.79 3.89 3.29 2.87 2.57 2.35 2.16 2.00 1.85 1.73 1.62 1.52 1.43 1.36 1.29 1.23 1.17 1.12 1.07 1.03

1.0 Outside Outside 20.45 15.54 12.34 10.19 8.73 7.70 6.92 6.27 5.72 5.24 4.82 4.45 4.13 3.84 3.59 3.37 3.17 2.99 2.83 2.69 2.55

1.5 Outside Outside Outside 32.45 25.53 20.89 17.76 15.57 13.90 12.53 11.36 10.34 9.46 8.68 8.01 7.42 6.90 6.43 6.02 5.66 5.33 5.03 4.75

2.0 Outside Outside Outside Outside Outside 35.40 29.97 26.17 23.30 20.94 18.92 17.17 15.65 14.32 13.17 12.16 11.27 10.48 9.78 9.16 8.60 8.09 7.63

2.5 Outside Outside Outside Outside Outside 53.71 45.35 39.52 35.10 31.49 28.40 25.72 23.39 21.36 19.60 18.06 16.71 15.51 14.45 13.50 12.65 11.89 11.19

3.0 Outside Outside Outside Outside Outside Outside 63.92 55.61 49.32 44.18 39.79 35.98 32.68 29.81 27.31 25.13 23.22 21.53 20.03 18.69 17.49 16.41 15.42

3.5 Outside Outside Outside Outside Outside Outside Outside 74.44 65.96 59.02 53.10 47.97 43.52 39.66 36.30 33.37 30.79 28.52 26.51 24.71 23.10 21.65 20.34

4.0 Outside Outside Outside Outside Outside Outside Outside Outside 85.00 76.01 68.33 61.68 55.91 50.91 46.56 42.76 39.44 36.50 33.90 31.58 29.50 27.62 25.93

𝑭𝑯𝑯𝑯 ≤ 𝑴𝒎𝒎𝒎 𝒈 𝑫𝑫𝑫𝒂𝒂𝒂𝒂𝒂𝒂𝒂𝒂𝒂 − 𝟏 + 𝝆𝝆𝝆 𝑭𝑯𝑯𝑯 ≤ 𝟎. 𝟗(𝑴𝒎𝒎𝒎 𝒈 − 𝝆𝝆𝝆)

4. December 2012

Slide 37

Simplified Method, Splash Zone - Summary Compute Fd, Fm, Fslam and Fρ

Apply 

Object motion equal crane tip



Wave kinematics dependent on assumed Hs,Tz seastate



Different deployment levels



Structure divided in main items

Check

Fhyd

No slack slings

DAF

Capacity checks


4. December 2012

Slide 38

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:


4. December 2012

•

Regular design wave approach (Ch. 3.4.2)

•

Time domain analyses

•

CFD analyses Slide 39

Content 









4. December 2012

Slide 40

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.


4. December 2012

Slide 41

Dynamic Forces – Vertical resonance


4. December 2012

Slide 42

Simplified Method, Deepwater - Assumptions DNV-RP-H103 Chapter 5 includes a simplified method for estimating dynamic response of lowered object. 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


4. December 2012

Slide 43

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


4. December 2012

Slide 44

Case Study – Dynamic Load at Lifted Object

Cable length L=2750m Estimation of Hydrodynamic Forces during Subsea Lifting

4. December 2012

Slide 45


 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 Estimation of Hydrodynamic Forces during Subsea Lifting

4. December 2012

Slide 46


Non-operable seastates


4. December 2012

Slide 47

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.


4. December 2012

Slide 48

Finally – One Last Comment:

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


4. December 2012

Slide 49

Easy Handling ..


4. December 2012

Slide 50

.. and Access for Survey !!


4. December 2012

Slide 51


4. December 2012

Slide 52

2 - Estimation of Hydrodynamic Forces During Subsea Lifting

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