1. Coupled dynamic analysis
FPSO / moorings / risers
Marcelo Caire, DSc
Rafael Schiller, DSc
03 – December - 2012

2. Outline
i. Introduction
ii. Coupled vs. De-coupled analysis
iii. Case study
iv. Results
v. Summary and conclusions

3. 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 ?

4. DE-COUPLED SIMULATION
Introduction
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
DnV RP-F205
drift

5. De-coupled x coupled approach
Large volume body Slender structures Main shortcomings of de-coupled approach:
z Z(t)
X(t)
x i. Mean current loads on mooring lines and
risers are normally not accounted for
Step 1: Step 2:
Vessel motion analysis Dynamic mooring and riser analysis
ii. The important damping effect from
Large volume body moorings and risers on the LF motions has
to be included in a simplified way
Slender structures
iii.The dynamics of moorings and risers will
not influence the WF motions of the floater
Simultaneous analysis of vessel motions and mooring line and riser dynamics

6. Fully coupled approach (SIMO/RIFLEX analysis)
6 DOF equation for the rigid body motion
model
M ( )x  C( )x  D1x  D2f ( x )  Kx  q(t , x, x )
     
M ( )  m  A( ),C( )  C  c( )
q(t , x, x )  qWI  qWA  qWA  qCU  qEXT
  (1) ( 2)
12 DOF equation for the dynamic
equilibrium of the FE slender structure
- 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 Dynamic equilibrium at every time instant

7. Case study definition
Spread-moored FPSO in typical Campos Basin environmental conditions (1250m)
20 mooring lines
15 risers

8. MOORING SYSTEM RISER SYSTEM
• Two chain segments and one • 2.5’’ ID flexible pipe;
polyester line;
• No bending stiffness; • Cross-section
properties from Witz
• Mooring properties from
(1996);
Wibner et al. (2003).
FPSO
Property Unit Value
Internal diameter mm 63,20
External diameter mm 111,5
WAMIT
Axial stiffness MN 128,00
Bending stiffness Nm2 1190,00
Torsional stiffness kNm2/rad 203,00
Mass in air Kg/m 30,43

9. 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

10. Environmental loading and cases definition

JONSWAP spectrum
Typical sea states from Campos Basin; Direction Hs (m) Tp (s) γ

S 6,1 14,00 1,57
1250m water depth; SW 6,9 14,62 1,61

W 4,0 8,14 2,10
Waves and currents: 10y return period; Direction Speed (m/s)
S 1,58
SW 1,39
Surface current
Case Wave Current
01 S SW
02 SW SW
03 W SW
04 S S
05 SW S
http://www.rederemo.org 06 W S

11. Offset estimation for de-coupled analysis
1. Perform a coupled simulation for a 1h Mean representative offset
period Case x (m) y (m) Distance (m)
01 7,4 -3,7 8,27
02 7,5 0,35 7,50
2. Compute average offset of the floater 03 8,0 3,5 8,73
04 3,2 18,6 18,87
05 3,4 22,0 22,26
3. Perform a de-coupled simulation with the
06 3,9 24,8 25,10
average offset
Current dominates FPSO displacement

12. Sway (deriva)
Heave (afundamento)
CASE 06 – highest offset

13. Sway (deriva)

14. Free decay numerical roll response
- Moorings/risers
increase vessel
damping response

15. Coupled x de-coupled response
Case 06 (6h simulations)
Mooring line response
CASE06 - 6h CASE06 - 6h
4200
Coupled 70 Coupled
4000 Decoupled Decoupled
60
3800
3600
MEAN top tension [kN]
50
Std. Dev. - Top tension
3400
40
3200
3000 30
2800
20
2600
2400 10
2200
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Mooring line ID Mooring line ID
Mean top tension is not significantly Correct inclusion of LF motions in the
different for both cases coupled approach
↓
3-5 %
↑ Higher std. dev. for all mooring lines

16. Bow-starboard Stern-starboard
↑ 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
CASE 06 – head change to starboard (BE) side

17. De-coupled Coupled
Peak value Peak value
~ 12 s ~ 300 s
The mooring line top tension is highly dependent on LF floating unit motions

18. Riser system response
CASE 06 - 6h CASE 06 - 6h
370 2,6
Coupled Coupled
365 Decoupled 2,4 Decoupled
2,2
360
2,0
MEAN Top tension [kN]
Std. Dev. - Top tension
355
1,8
350
1,6
345 1,4
340 1,2
335 1,0
0,8
330
0,6
325
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Riser ID Riser ID
Close correlation due to good estimation Floating unit motion (WF) may be reduced
of the offset in the coupled simulation due to
increased damping of moorings and risers
Riser response is less dependent on the ↓
Lf than the mooring lines ↑ Higher std. dev. for de-coupled
simulation

19. 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

20. Wave energy spreading
Wind wave and swell combined Directional spectrum
cos-2s spreading function
s = 50
2,0
Spreading function D()
1,6
20
1,2 15
0,8
5
4
3
2
0,4 1
0,0
-180 -120 -60 0 60 120 180
Directional angle  [deg]

21. Wind-wave case study spreading definition
s=2 .... s=25
↓ spreading parameter ↑ energy preading

22. Vessel sensitivity response due to spreading
↑ higher spreading parameter ↑ higher standard deviation
(energy less spread)

23. Wave energy spreading effect on the mooring system
Higher sp parameters (wave energy more concentrated) leads to higher standard deviation

24. Wave energy spreading effect on the riser system
Less dependent than mooring lines

26. 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.

27. Thanks !!!
Any questions ?
Marcelo Caire
marcelo.caire@marintek.com.br
(21) 2025-1811