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Coupled dynamic analysis
FPSO / moorings / risers
Marcelo Caire, DSc
Rafael Schiller, DSc
03 – December - 2012
Outline
i. Introduction
ii. Coupled vs. De-coupled analysis
iii. Case study
iv. Results
v. Summary and conclusions
Introduction
Background Flexible risers operating for more than 20 years Is it safe to continue operation or should they be replaced ?
Conservative assumptions made during the design phase may overestimate the
accumulated damage at the end of the riser life
Improved numerical methods and analysis procedures may help reducing lifetime assessment uncertainties
Motivation/Objectives What`s the impact of a fully coupled global dynamic analysis in the fatigue assessment of flexible risers ?
Introduction DE-COUPLED SIMULATION
Vessel WF motions are calculated from RAOs
(Response amplitude operators), e.g. WAMIT
Representative offset (mean + LF) is usually
obtained from a freq. domain analysis (MIMOSA)
Offset and WF motions applied as boundary
conditions of a detailed riser FEM
COUPLED SIMULATION
Full interaction is taken into account and accurate
floater motions and dynamic loads in mooring lines
and risers are obtained simultaneously.
Wave frequency (WF) response due to 1st order wave
excitation
Low frequency (LF) response due to wave drift and viscous
drift DnV RP-F205
De-coupled x coupled approach
z
x
Z(t)
X(t)
Step 1: Step 2:
Vessel motion analysis Dynamic mooring and riser analysis
Large volume body Slender structures Main shortcomings of de-coupled approach:
i. Mean current loads on mooring lines and
risers are normally not accounted for
ii. The important damping effect from
moorings and risers on the LF motions has
to be included in a simplified way
iii.The dynamics of moorings and risers will
not influence the WF motions of the floater
Large volume body
Slender structures
Simultaneous analysis of vessel motions and mooring line and riser dynamics
Fully coupled approach (SIMO/RIFLEX analysis)
- Floater is considered as a one-node rigid element with 6 DOF
- Detailed model of the complete slender structure system (bar/beam elements)
- Master-slave approach for connecting mooring lines/risers to the floater
EXTCUWAWAWI qqqqqxxtq
cCCAmM
xxtqKxxfDxDxCxM
)2()1(
21
),,(
)()(),()(
),,()()()(
6 DOF equation for the rigid body motion model
12 DOF equation for the dynamic equilibrium of the FE slender structure
Dynamic equilibrium at every time instant
Case study definition
Spread-moored FPSO in typical Campos Basin environmental conditions (1250m) 20 mooring lines
15 risers
MOORING SYSTEM
• Two chain segments and one
polyester line;
• No bending stiffness;
• Mooring properties from
Wibner et al. (2003).
RISER SYSTEM
• 2.5’’ ID flexible pipe;
• Cross-section
properties from Witz
(1996);
Property Unit Value
Internal diameter mm 63,20
External diameter mm 111,5
Axial stiffness MN 128,00
Bending stiffness Nm2 1190,00
Torsional stiffness kNm2/rad 203,00
Mass in air Kg/m 30,43
FPSO
WAMIT
FINITE ELEMENT MODEL
Moorings and risers are
modelled as bar elements
•161 elements/mooring
2520m
•289 elements/riser;
1900m
7555 bar elements in total
Risers connected to port side
Environmental loading and cases definition
Typical sea states from Campos Basin;
1250m water depth;
Waves and currents: 10y return period;
Direction Hs (m) Tp (s) γ
S 6,1 14,00 1,57
SW 6,9 14,62 1,61
W 4,0 8,14 2,10
Direction Speed (m/s) S 1,58
SW 1,39
Case Wave Current 01 S SW 02 SW SW 03 W SW 04 S S 05 SW S 06 W S http://www.rederemo.org
Surface current
JONSWAP spectrum
Offset estimation for de-coupled analysis
1. Perform a coupled simulation for a 1h
period
2. Compute average offset of the floater
3. Perform a de-coupled simulation with the
average offset
Case x (m) y (m) Distance (m)
01 7,4 -3,7 8,27
02 7,5 0,35 7,50
03 8,0 3,5 8,73
04 3,2 18,6 18,87
05 3,4 22,0 22,26
06 3,9 24,8 25,10
Mean representative offset
Current dominates FPSO displacement
Sway (deriva)
Heave (afundamento)
CASE 06 – highest offset
Sway (deriva)
Free decay numerical roll response
- Moorings/risers increase vessel damping response
Coupled x de-coupled response Case 06 (6h simulations)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
2200
2400
2600
2800
3000
3200
3400
3600
3800
4000
4200CASE06 - 6h
ME
AN
to
p te
nsio
n [kN
]
Mooring line ID
Coupled
Decoupled
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
0
10
20
30
40
50
60
70
CASE06 - 6h
Std
. D
ev. -
To
p te
nsio
nMooring line ID
Coupled
Decoupled
Mooring line response
Correct inclusion of LF motions in the coupled approach
↓ ↑ Higher std. dev. for all mooring lines
Mean top tension is not significantly different for both cases
3-5 %
↑ Higher deviation in the coupled approach due to LF motions
Coupled approach leads to floating unit heading deviation (dependent on environmental conditions combination)
↓ Different mean top tensions
Bow-starboard Stern-starboard
CASE 06 – head change to starboard (BE) side
Peak value ~ 12 s
Peak value ~ 300 s
The mooring line top tension is highly dependent on LF floating unit motions
De-coupled Coupled
Riser system response
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
325
330
335
340
345
350
355
360
365
370CASE 06 - 6h
ME
AN
To
p te
nsio
n [kN
]
Riser ID
Coupled
Decoupled
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
0,6
0,8
1,0
1,2
1,4
1,6
1,8
2,0
2,2
2,4
2,6CASE 06 - 6h
Std
. D
ev. -
To
p te
nsio
n
Riser ID
Coupled
Decoupled
Floating unit motion (WF) may be reduced in the coupled simulation due to
increased damping of moorings and risers ↓
↑ Higher std. dev. for de-coupled simulation
Close correlation due to good estimation of the offset
Riser response is less dependent on the
Lf than the mooring lines
For the present case study configuration, WF dominates the riser top tension response...
...but, the coupled simulation leads to lower values of
standard deviation which may impact fatigue assessment
Wave energy spreading Wind wave and swell combined Directional spectrum
cos-2s spreading function
-180 -120 -60 0 60 120 180
0,0
0,4
0,8
1,2
1,6
2,0s = 50
20
15
5432
Spre
adin
g f
unction D
()
Directional angle [deg]
1
s=2 s=25
Wind-wave case study spreading definition
....
↓ spreading parameter ↑ energy preading
Vessel sensitivity response due to spreading
↑ higher spreading parameter ↑ higher standard deviation
(energy less spread)
Wave energy spreading effect on the mooring system
Higher sp parameters (wave energy more concentrated) leads to higher standard deviation
Wave energy spreading effect on the riser system
Less dependent than mooring lines
Main conclusions
The comparison between de-coupled x fully coupled simulations for a spread moored FPSO lead to the following conclusions:
The mooring and riser system increase the floating unit damping
Current acting on moorings/risers may lead to an asymetric response of the floating unit
The vessel heading is correctly taken into account in the coupled approach. There is no need for
a separate heading distribution calculation as would be the case for the de-coupled approach.
For the mooring lines, Higher standard deviations are observed for the coupled approach while the opposite occurs for the riser system, where the de-coupled simulations lead to higher values of standard deviation.
Mooring response is more affected by the spreading parameter variation when compared to the riser response
The coupled approach lower the level of analysis uncertainties with a more physically correct modelling when compared to de-coupled methodologies.
