Deepwater oil and gas fields (d ~ 500m - 3000m): Critical metocean design conditions

1. MODEL TESTING FOR DEEPWATER CONCEPTSC.T. Stansberg Norwegian Marine Technology Research Institute A.S (MARINTEK), Trondheim, NorwayOGP Workshop on Technology Requirements for Floating Systems, London, UK, 23-24 April, 2001

2. Contents of presentation- Deepwater metocean conditions - physical modelling - Deepwater floating systems - physical modelling - Particular areas of experimental investigation - Laboratory limitations - and solutions - Areas of uncertainty and further development

4. Deepwater oil and gas fields (d ~ 500m - 3000m): Critical metocean design conditions Waves Current Wind OthersNorwegian SeaHigh Moder./High High Atlantic Margin High High High Gulf of MexicoSteep/High High HighOffshore BrazilModerate High Moder.West of AfricaLow High Moder.(Newfoundland)High High High Ice

5. Physical modelling of deepwater metocean conditions in a laboratory basin

6. MARINTEK’s 50m x 80m x 10m Ocean Basin

7. Modeling of waves - items of particular interest - Nonlinear effects (crests; wave heights; kinematics) - Extreme waves(probability; mechanism; freak waves?) - Multi-directionality - Multiple-peak spectra (in frequency & in direction) - Non-stationary hurricanes (in frequency & in direction) - Repeatability - Minimum scale of reproduction - 1:150 ?

8. Second-order deep-water random wave model (numerical) Modeling of waves - Items of particular interest - Nonlinear effects (crests; wave heights; kinematics) - Extreme waves(probability; mechanism; freak waves?) - Multi-directionality - Multiple-peak spectra (in frequency & in direction) - Non-stationarity (in frequency & in direction) - Repeatability - Minimum scale of reproduction?

9. Measured vs. second-order wave model Modeling of waves - Items of particular interest - Nonlinear effects (crests; wave heights; kinematics) - Extreme waves(probability; mechanism; freak waves?) - Multi-directionality - Multiple-peak spectra (in frequency & in direction) - Non-stationarity (in frequency & in direction) - Repeatability - Minimum scale of reproduction?

10. Modeling of deep-water currents - challenges: - Vertical profile (magnitude & direction) - Homogenous & constant current velocity / turbulence? - Full-depth limitations in available laboratory basins - Combine with equivalent force / numerical models u Example: 3000m depth trunc. at 1000m

11. Modeling combined metocean:Wind waves + swell + current + wind- Collinear & non-collinear - Optimal model scale - Modeling of rapid change in hurricane system?

12. Deepwater floating systems

13. Deepwater floating systems - physical modelling Traditional hydrodynamic verification:- Modeling of “complete” system hull+mooring+risers (+DP) - Scales ~1:50 - 1:100 - Dynamic (& static) coupling between floater & lines/risers - Individual line models - dynamic line tension - Line drag induced slow-drift damping - Complex behaviour of total system / “new” effects? - Extreme nonlinear responses in storm conditions / need for calibration of numerical models - Operations - Measurements: Vessel motions - Line forces - Relative wave - Green sea - Slamming - Video observations

14. FPSO in extreme wave event

15. Semisubmersible in extreme wave event

16. Particular areas of experimental investigationMotions - slow-drift forces in extreme waves with current - viscous damping - motion coupling effectsMooring - Dynamic line tension in extreme wave groups - Dynamic coupling to vessel motionsRisers - Steady drag forces - VIV (model testing of separate components)Relative wave / Green Sea - Probability of green sea / negative air gap - Impact loads & structural responsesExtreme responses - non-Gaussian processesNumerical analysis - combined / integrated with experiments

17. Numerical visualisation (from coupled analysis study)

18. Dynamic line tension: 1:55 model tests vs. coupled analysis

19. Laboratory limitations - and solutions Challenge: Depths ~ 1000m - 3000m: Too deep for testing at “conventional” scales (1:50 - 1:100) in available basins How to keep the benefits from “complete” system - couplings etc.?Possible solutions:-Ultra small scales (1:100 - 1:200) - scale effects?* - Integrated tests & computer analysis (“hybrid techniques”)* - Outdoor testing* - Numerical analysis only - New ultra-deep basin?Not recommended:Truncation without subseq. computer-extrapolation**Studies carried out at MARINTEK: VERIDEEP; NDP; Deepstar

20. Ultra-small scale model testing: Comparing 3 scales

21. Verification tests on the P-26 project, a polyester taut mooring system

22. Testing in scales 1:100 - 1:150 (200) is feasible, depending on floater, condition etc. Practical limitations today (environmental modelling) Scale effects on line tension can be accounted for; smaller effects on slow-drift Particular attention and care in preparation and execution Special limitations: Thruster modelling (> 1:100) Truss structure details Spar models with moonpool

23. Hybrid methods: An “off-line” procedure

25. Design of truncated system:(hybrid verification)- Horizontal restoring force characteristics - Vertical coupling mooring / floater - Quasi-static single-line characteristics - “Representative” damping levels

26. Numerical reconstruction & extrapolation(hybrid verification)- Calibration / check of numerical code - coupled analysis - System identification; in particular: slow-drift excitation & damping - Sea state dependent parameters - Final full-depth simulation with calibrated code (coupled analysis)

27. Coupled analysis (RIFLEX-C) model of an FPSO system

28. Empirical surge drift coefficients (semi in irreg. waves)

29. Example (NDP study): Semi-submersible system in 3000m steel catenary mooring(semi-taut) Scale 1:150, truncated at 1100m (7.3m mod sc) Norwegian sea 100yr: Hs=16m Tp=18s Cu=1.3m/s Wi=48m/s

30. Initial check of method: An 1100m system truncated at 550m Extrapolated results compared to full-depth 1100m measurements

31. Final results: Results from truncated set-up (1100m) numerically extrapolated to 3000m

32. Promising experiences with the “off-line” hybrid procedure Some notes for future applications: - Scale of truncated set-up should be > 1:100 - 1:125 - The method works technically fine, while improvements for efficient use are underway(efficient link between experiments and numerical analysis etc.) - Procedures for design of truncated set-up should be established - Uncertainties of 2-step method should be assessed - Guidelines for hybrid verification have been suggested, but should be further discussed and completed

33. Other hybrid methods: - On-line (active) integration between truncated test set-up and computer simulations Potentially a very interesting method. Sophisticated, power-consuming computer tools required: The need for “intelligent” algorithms should be evaluated (How intelligent should it be to represent a real verification?) Need for very large actuators in 6 DOF? - Verification of parts of the system (e.g. the floater only), or of the computer program itself?

34. Areas of uncertainties and further development: - When do we need to use scales > 1:100?- More standard procedures to be established for hybrid techniques - Uncertainties in hybrid techniques- What is required for software used in deepwater verification - qualification? - On-line hybrid technique: intelligent software & actuator / control - More standard procedures for extreme value estimation from model tests - Viscous drift forces in high waves on currents - Higher-order drift forces on ship in high and steep waves - Metocean input: Multi-directionality / multiple-peaked spectra Deepwater currents: profile / turbulence Rapidly changing weather conditions? - Particular problem arising in ultra deep waters: Operations in connection with intervention etc. / multi-body dynamics / floating pipelines