09/04/2014 ADVANCES IN MODELLING METHODS FOR …

DNV GL © 2013 SAFER, SMARTER, GREENER DNV GL © 2013

ADVANCES IN MODELLING METHODS FOR ULTRA-DEEPWATER MOORINGS AND RISERS

Alex Argyros

09/04/2014

9 March 2014 1

Madrid, Spain

DNV GL © 2013

CONTENT

9 March 2014 2

MCEDD 2014 Theme

Demand – Deepwater field development

Incidents/integrity

Need for testing

Innovation

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DEEPWATER FIELD DEVELOPMENT - DEMAND

9 March 2014 3

World energy Outlook 2013 – International Energy Agency (IEA)

Deepwater contribution –

1990: 60 thousand barrels per day

2012: 6% of conventional crude

Projection in Brazil by 2035: 11% of conventional crude

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MOORING INCIDENTS - INTEGRITY

9 March 2014 4

Mooring Statistics (Mooring Integrity JIP – Noble Denton)

1980 to 2001

Cost of single line failure

Mooring integrity management (failure detection, inspection –

corrosion / wear assessment, dealing with marine growth…)

Design, analysis and testing

HSE Research Report 047, “Analysis of Accident Statistics for Floating

Monohull and Fixed Installations” 2003.

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ULTRA-DEEPWATER MODEL TESTING

9 March 2014 5

The need for Model Testing

Proof of concept

Confirm assessment of hydrodynamic loads on vessel, verify design

assumptions

Validate numerical models – Global design verification of

loads/motions/tensions

Calibrate numerical models

Unexpected and highly nonlinear phenomena – Wave steepness,

wave impact/run-up, VIM

High importance assets

Innovation

Testing new floater/technology/territory

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MOTIVATION

9 March 2014 6

Challenges

Numerical modelling – Excessive computational times

Impossible to model test complete system using conventional scales

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TRUNCATION

9 March 2014 7

Industry has used line truncation techniques to tackle limited depth problem

If done Correctly it Can:

Improve efficiency of numerical modelling for deepwater line dynamics (shorter line = less elements)

Enable model testing at more than 1:100 scale

Traditionally passive truncation preferred

Equivalent

Numerical Model

at Reduced

Depth

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PASSIVE TRUNCATION

9 March 2014 8

Setting up Truncated System

Replicate static characteristics (stiffness) of full depth system

Truncated water depth limited by lab capability

Optimization function to maintain line geometric properties

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PASSIVE TRUNCATION

9 March 2014 9

Setting up Truncated System

Replicate static characteristics (stiffness) of full depth system

Truncated water depth limited by lab capability

Optimization function to maintain line geometric properties

Performance

Complex truncated line configuration (clumps/buoys)

Vessel motions – increased water depth = increased importance

of line dynamics

Poor estimation of dynamic tensions – fatigue

Complex segmentation corrupts modelling physical coupling

between vessel and lines

DNV GL © 2013

MOORING DAMPING

9 March 2014 10

5 10 15 20 25 30 35 40 45 500

5

10

15

20

25

30

35

40

45

50

Amplitude of LF motion (m)

Da

mp

ing

ra

tio o

f mo

ori

ng

s

%

LF only

Superimposed WF A2mT10s





Ref – Zhengqiang Xu, Strathclyde University

Turret moored FPSO – LF and superimposed WF motions

DNV recommendation for CD

WF motions will dramatically increase mooring damping

Riser damping comparable to mooring damping. Riser damping less

sensitive to WF motions

DNV GL © 2013

MOORING DAMPING

9 March 2014 11


Energy dissipation due to transverse motions highest at:

Fairlead, but motions decay quickly

Touchdown due to geometric coupling axial and transverse

vibrations

CD at touchdown area – monitoring/marine growth

Base line: (CD)

Chain:1.1;

wire:1.2

Chain 1.11.3

Wire 1.21.4

0 100 200 300 400 500 600 700 800-1

0

1

2

3

4

5

6

7

8

9x 10

5

Arc Length (m)

En

erg

y D

issip

ati

on

Vari

ati

on

E` (C

D)

(N.m

)

HF(T=6s,A=1m)

HF(T=10s,A=4m)

HF(T=15s,A=4m)

LF only 150s40m

Touch down

zone

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LOCALIZED TRUNCATION

9 March 2014 12

Ref – Alex Argyros & Robin Langley, Cambridge University

Method

Computationally* - Upper sections modelled in detail terminating to an approximate analytical

model

Hybrid model testing – Using localised truncation to simulate lower section

Exploit Physical Features of System

Close to water surface: Wave action, strongest currents and coupling

Transverse line dynamics decay rapidly – Bottomless/semi-infinite line

Truncated Water Depth based on Line Dynamic Response

Assessment for decay of transverse line dynamics – Stage 1

Minimum truncation length: Transverse vibrational characteristics are inertia driven – Stage 2

Approximate termination: Local quasi-static line properties – Stage 3

Simple truncation mechanism for model testing

Truncated

Lines

Detailed

Model Approximate

Termination

Reduced Depth

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EXAMPLE TRUNCATION

9 March 2014 13

Numerical truncation using Passive vs Localized Approach

Details

2700m WD

Taut mooring system

Fairlead/seabed segment 200m

Polyester rope 3700m

Pretension 300Te

Hs = 15m; Tp = 14s


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EXAMPLE TRUNCATION

9 March 2014 14



DNV GL © 2013

EXAMPLE TRUNCATION

9 March 2014 15


Passive Truncation

6 segments – 3 line types

2 buoys + 1 clump weight

Localized Truncation

Stages 1 & 2 – Truncation within rope segment with minimum d:dT

~ 1:4.5

Upper section modelled exactly as the full depth line

End termination – 3 non-linear coupled springs in X, Y, Z


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EXAMPLE TRUNCATION

9 March 2014 16


System statics (stiffness)

Passive truncation stiffer system


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EXAMPLE TRUNCATION

9 March 2014 17




Vessel Motions

Passive truncation some disagreement in surge/sway


600 700 800 900 1000 1100 1200 1300 1400-40

-20

0

20

X (

m)

Vessel Motions

600 700 800 900 1000 1100 1200 1300 1400-40

-20

0

20

40

Y (

m)

600 700 800 900 1000 1100 1200 1300 1400-4

-2

0

2

4

Z (

m)

time (sec)

Full Depth

Localised Truncation

Passive Truncation

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EXAMPLE TRUNCATION

9 March 2014 18




Vessel Motions


Line dynamic tension

Passive truncation underestimates LF component

Additional large amplitude higher frequency peak tensions


600 700 800 900 1000 1100 1200 1300 14004500

5000

5500

6000

6500

7000

7500

SeaW

ard

Tensio

n (

kN

)

Dynamic Tension

600 700 800 900 1000 1100 1200 1300 14001400

1600

1800

2000

2200

2400

2600

2800

time (sec)

LeeW

ard

Tensio

n (

kN

)

Full Depth


Passive Truncation

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EXAMPLE TRUNCATION

9 March 2014 19




Vessel Motions





Simulation of only WF motions reveals error in WF tension ~

100%


600 700 800 900 1000 1100 1200 1300 14003100

3200

3300

3400

3500

3600

3700

SeaW

ard

Tensio

n (

kN

)

Dynamic Tension

600 700 800 900 1000 1100 1200 1300 14003000

3100

3200

3300

3400

3500

time (sec)

LeeW

ard

Tensio

n (

kN

)

Passive Truncation

Full Depth


DNV GL © 2013

EXAMPLE TRUNCATION

9 March 2014 20




Vessel Motions





Simulation of only WF motions reveals error in WF tension ~

100%


800 820 840 860 880 900

3350

3400

3450

3500

3550

3600

3650

SeaW

ard

Tensio

n (

kN

)

Dynamic Tension

800 820 840 860 880 900

3050

3100

3150

3200

3250

3300

time (sec)

LeeW

ard

Tensio

n (

kN

)

Full Depth


Passive Truncation

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CD VARIATION

9 March 2014 21


Numerical modelling (coupled analysis) of moored FPSO

CD varied by factor of 2.0

Reduction of LF motion near natural frequency

Reduction of LF tension

Increase of WF tension

0 0.02 0.04 0.06 0.08 0.1 0.120

1

2

3

4

5

6x 10

6

Frequency (Hz)

Sp

ectr

al d

en

sity o

f lin

e fo

rce

(kN

2/H

z)

CD1

of moorings

CD2

of moorings

DNV GL © 2013

CD VARIATION

9 March 2014 22

CD Variability due to:

Marine growth – inspection

Motion


),(Re,, kKCfCC MD

400 450 500 550 600 650 700 750 800 850 9000

200

400

600

800

1000

1200

1400

1600

1800

ArcLength (m)

KC

LF motion only

60s39.5m

100s39.5m

150s39.5m

200s39.5m

250s39.5m

400 450 500 550 600 650 700 750 800 850 9000

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5x 10

5

ArcLength (m)

Re

LF motion only

60s39.5m

100s39.5m

150s39.5m

200s39.5m

250s39.5m

LF motion only

Surge WF motion

Range of KC and Re numbers for

R3s chain (D=152mm) around

touch-down zone

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CD DETERMINATION BY CFD

9 March 2014 23

CFD Model Validation – Flow past a smooth cylinder

Turbulence models: LES vs k – w SST (URANS)

Near wall treatment for wall boundary flows

Steady flow and oscillating flow

Steady flow – LES more recommended – 3D vortex in wake

Oscillatory flow – flow in the wake is more 2D (suppression of 3D by

harmonic motion of cylinder)

k – w SST (URANS) uses a coarser mesh and so is selected


1x104

1x105

1x106

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Me

an

dra

g c

oe

ffic

ien

t C

D

Reynolds number (log coordinate)

Experimental CD (Schewe, 1983)

Predicted by 3D LES (Present)

Predicted by 3D SST (Present)


104

105

106

0.0

0.2

0.4

0.6

0.8

1.0

1.2

RM

S o

f lift

co

eff

icie

nt

C

' L

Reynolds number (log coordinate)

Experimental range (Hallam et al., 1977)

Predicted by 3D LES (Present)



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9 March 2014 24

Chain Numerical Models

2 links (1 full & 2 half)

Periodic boundary conditions

No slip wall

Inlet/outflow boundaries

Symmetry in lateral sides

8 million cells


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9 March 2014 25

Results for studless chain

Flow direction

Re and KC number

Considering only Re CD

variation is small

For oscillating chain in

still water the effect of

KC on CD is significant

for flow with low Re


Case Predicted

0 degree flow 2.42

45 degree flow 2.36

90 degree flow 2.41

5x103

104

6x104

105

5x105

1.6

1.8

2.0

2.2

2.4

2.6D

rag

co

eff

icie

nt

Reynolds number

Present LES calculated results

DNV-RP-C205 recommended value range (Re=104~10

7)

Present SST calculated results

0 0.5 1 1.5 2 2.5 3

x 105

1.8

2

2.2

2.4

2.6

2.8

3

3.2

3.4

3.6

Reynolds number

CD

of stu

d-l

ess c

ha

in

Oscillatory-Flow-KC12




Steady-flow-k--SST

Steady-flow-LES

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SUMMARY

9 March 2014 26

Need for deepwater/model testing

Present difficulties and challenges

Innovative method for truncation

Line dynamics

LF damping

Zones of importance

Motions and tensions

CD determination using CFD

09/04/2014 ADVANCES IN MODELLING METHODS FOR …

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