SUC Brasil 2012 : Coupled Dynamic Analysis FPSO / Mooring / Risers

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Presented at SUC by Marcelo Caire from Marintek.

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SUC Brasil 2012 : Coupled Dynamic Analysis FPSO / Mooring / Risers

  1. 1. Coupled dynamic analysis FPSO / moorings / risers Marcelo Caire, DSc Rafael Schiller, DSc 03 – December - 2012
  2. 2. Outlinei. Introductionii. Coupled vs. De-coupled analysisiii. Case studyiv. Resultsv. Summary and conclusions
  3. 3. IntroductionBackground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 uncertaintiesMotivation/ObjectivesWhat`s the impact of a fully coupled global dynamic analysis in the fatigue assessmentof flexible risers ?
  4. 4. DE-COUPLED SIMULATIONIntroduction 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. 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 floaterSimultaneous analysis of vessel motions and mooring line and riser dynamics
  6. 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. 7. Case study definition Spread-moored FPSO in typical Campos Basin environmental conditions (1250m) 20 mooring lines 15 risers
  8. 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. 9. FINITE ELEMENT MODEL Moorings and risers aremodelled as bar elements •161 elements/mooring 2520m •289 elements/riser; 1900m7555 bar elements in total Risers connected to port side
  10. 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 Shttp://www.rederemo.org 06 W S
  11. 11. Offset estimation for de-coupled analysis1. 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 7,502. 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 22,263. Perform a de-coupled simulation with the 06 3,9 24,8 25,10 average offset Current dominates FPSO displacement
  12. 12. Sway (deriva) Heave (afundamento)CASE 06 – highest offset
  13. 13. Sway (deriva)
  14. 14. Free decay numerical roll response - Moorings/risers increase vessel damping response
  15. 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 3600MEAN 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. 16. Bow-starboard Stern-starboard↑ Higher deviation in the coupled approach due to LF motionsCoupled 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. 17. De-coupled Coupled Peak value Peak value ~ 12 s ~ 300 sThe mooring line top tension is highly dependent on LF floating unit motions
  18. 18. Riser system response CASE 06 - 6h CASE 06 - 6h 370 2,6 Coupled Coupled 365 Decoupled 2,4 Decoupled 2,2 360 2,0MEAN 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. 19. For the present case study configuration, WF dominates the riser top tension response... ...but, the coupled simulation leads to lower values ofstandard deviation which may impact fatigue assessment
  20. 20. Wave energy spreadingWind 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. 21. Wind-wave case study spreading definition s=2 .... s=25 ↓ spreading parameter ↑ energy preading
  22. 22. Vessel sensitivity response due to spreading ↑ higher spreading parameter ↑ higher standard deviation (energy less spread)
  23. 23. Wave energy spreading effect on the mooring systemHigher sp parameters (wave energy more concentrated) leads to higher standard deviation
  24. 24. Wave energy spreading effect on the riser system Less dependent than mooring lines
  25. 25. Main conclusionsThe comparison between de-coupled x fully coupled simulations for a spread moored FPSO lead to the followingconclusions: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 responseThe coupled approach lower the level of analysis uncertainties with a more physically correctmodelling when compared to de-coupled methodologies.
  26. 26. Thanks !!! Any questions ? Marcelo Cairemarcelo.caire@marintek.com.br (21) 2025-1811

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