Response-based Metocean Criteria for OptimisingDesign and Operation of FPSOs
- 1. Shell Exploration & Production Response-based Metocean Criteria for Optimising Design and Operation of FPSOs Copyright: Shell Exploration & Production Ltd. Hermione van Zutphen Marios Christou File Title Kevin Ewans 9/7/2009
- 2. Shell Exploration & Production Outline • Metocean design conditions & LSM • Description of the metocean environment • Response analysis • Extremes, short term and long term variability
- 3. Shell Exploration & Production Why metocean is so important • Meteorology and Oceanography • Understanding the environment – Extremes: • Winter storms • Tropical cyclones (hurricanes, typhoons) • Currents – Operational • Operability of processing equipment and offloading • Wind waves and swells • Internal currents • VIV • Squalls • Tides
- 4. Shell Exploration & Production Metocean conditions for FPSO design • Independent extremes (guidelines) – For example DNV: • For spread-moored systems, loading from wind, waves and current in the same direction • At least head, quartering and beam load directions as well as in-line conditions for symmetrical anchor patterns • Weather-vaning: include directional spread of wind, waves and current • Without site specific data: colinear and non-colinear environment – API: extreme independent with associated conditions (API) • Response based conditions – Based on extreme response – Long-term environmental dataset (30 years) – Vessel model
- 5. Shell Exploration & Production Response Based Analysis Metocean Environment Extreme Response Based Responses Value Metocean Design Analysis on Conditions Responses Offshore System Operational N-year values for Design Cases for Behaviour responses N-year response
- 6. Shell Exploration & Production Environment “perspective” Response “perspective” Wind Waves Structural Current Model 10-4 Directionality Governing response Wind Waves 10-4 Current Directionality Governing response Simple Jacket Wind Waves (sea /swell) Current 10-4 Directionality! Governing response Turret-moored floating system
- 7. Shell Exploration & Production Shell’s method for response based analysis: LSM Available for: • Fixed structures • Ship-shaped structures, either turret- or spread-moored • Semi-submersibles • TLPs • Pipelines
- 8. Shell Exploration & Production Metocean Data The easy bit for the metocean engineer! Metocean Environment Extreme Response Based Responses Value Metocean Design Analysis on Conditions Responses Offshore System Operational N-year values for Design Cases for Behaviour responses N-year response
- 9. Shell Exploration & Production Ocean Environment Description Requires a long-term database of: Waves • wave height, period, and direction or • directional wave spectrum or • wind-sea and swell Winds • speed and direction • wind spectrum • wind profile Currents • current speed and direction • current profile For 15 years of 3 hour intervals: 44070 records!
- 10. Shell Exploration & Production Waves
- 11. Shell Exploration & Production What is a sea state? • Random superposition of regular waves – Many amplitudes – Many frequencies – Many directions • Statistical representation – Hs: significant wave height • Wind seas & swells – Tp: peak period – Direction – … spectrum
- 12. Shell Exploration & Production Frequency-direction wave spectrum: wave systems Swell Wind sea
- 13. Shell Exploration & Production Wind • Steady wind + wind gusts • Statistical description of random nature of winds – API wind spectrum 3 API wind spectrum 10 2 10 S(f,z) (m2/s) 1 10 0 10 -3 -2 -1 0 10 10 10 10 f (Hz)
- 14. Shell Exploration & Production Currents • Periods ranging from seconds to days Courtesy of Dr. Cort Cooper - Chevron • Geostrophic currents and Ekman Transports • Wind-induced currents • Density-driven currents • Tidal currents • Deepwater currents
- 15. Shell Exploration & Production The easy bit for the Offshore System Modelstructures floating engineer Metocean Environment Extreme Response Based Responses Value Metocean Design Analysis on Conditions Responses Offshore System Operational N-year values for Design Cases for Behaviour responses N-year response
- 16. Shell Exploration & Production Floater model • Hydrodynamics • Hydrostatic coefficients • First order wave forces • Second order wave forces • Viscous damping (incl. roll) • Wind and current drag • Wind and current loading • Relevant areas for wind and current coefficients • Wind and current coefficients / forces • Mooring / Tendons and risers • Horizontal restoring force - deflection curve • Number of lines and orientation • Line length and orientation • Tendon and riser mass and stiffness • Hull inertia
- 17. Shell Exploration & Production Responses Metocean Environment Extreme Response Based Responses Value Metocean Design Analysis on Conditions Responses Offshore System Operational N-year values for Design Cases for Behaviour responses N-year response
- 18. Shell Exploration & Production Floaters: Solving equations of motion • Frequency domain – mostly linear • Time domain – slow with sampling variability • Probability domain – Spectral Response Surface method • Equations of motion are transformed into probability domain • Probabilities of response to a sea state (most probable maximum) • Fast and includes non-linearity in forcing
- 19. Shell Exploration & Production Spectral Response Surface Method • Basic variables for surface elevation and wind gust: – Stochastic variables characterised by a normal Gaussian distribution N(0,1) – Normalized by standard deviation – Normal random variables of unit-variance and mean zero + their Hilbert transforms (to include phase information) • Response equations in terms of standard normal variables solved by a FORM–type (First Order Reliability Method) analysis
- 20. Shell Exploration & Production All methods begin by treating the ocean surface as the sum of many frequency components. Then………..
- 21. Shell Exploration & Production xi
- 22. Shell Exploration & Production xi x lin diffraction
- 23. Shell Exploration & Production xi x lin diffraction x dynamics
- 24. Shell Exploration & Production xi x lin diffraction x dynamics responsei Sum over all components
- 25. Shell Exploration & Production xi x lin diffraction x dynamics xj xk O(2) diff dynamics responsejk Sum over all components
- 26. Shell Exploration & Production xi x lin diffraction x dynamics xj xk O(2) diff dynamics Frequency domain xi ~ i
- 27. Shell Exploration & Production xi x lin diffraction x dynamics xj xk O(2) diff dynamics Frequency domain xi ~ i 1 Time domain 0 0 12 24 - 1
- 28. Shell Exploration & Production xi x lin diffraction x dynamics xj xk O(2) diff dynamics Frequency domain xi ~ i 1 Time domain 0 0 12 24 - 1 Probability domain
- 29. Shell Exploration & Production Which responses are calculated? FPSOs • Offsets (any and specified direction) • Accelerations • Hull girder bending moment • Heave, Roll, Pitch, Yaw • Heave at turret, Heave at bow • Green water elevation relative to bow
- 30. Shell Exploration & Production Extremes Metocean Environment Extreme Response Based Responses Value Metocean Design Analysis on Conditions Responses Offshore System Operational N-year values for Design Cases for Behaviour responses N-year response
- 31. Shell Exploration & Production Responses per Storm - Storm Generation • Correlation between successive sea states • Uncertainty in the extreme wave of a sea state • Highest maximum wave in a storm not necessarily associated with peak significant wave height • Assumption of independence of sea states avoided by using storms as independent events
- 32. Shell Exploration & Production 7 Definition of a storm 6 80% peak of storm 5 Hs (m) 4 3 2 threshold 1 m 1
- 33. Shell Exploration & Production Evaluation of extreme response statistics
- 34. Shell Exploration & Production Short term variability Realisation # 1 3 Hmax = 4.7 m Wave height (m) 2 1 0 -1 -2 -3 0 200 400 600 800 1000 1200 Realisation # 2 3 Hmax = 5.1 m Wave height (m) 2 1 0 -1 -2 -3 0 200 400 600 800 1000 1200 Realisation # 3 3 Hmax = 3.9 m Wave height (m) 2 1 0 -1 -2 -3 0 200 400 600 800 1000 1200 Time (s)
- 35. Shell Exploration & Production Short term variability 1 H 0.9 Hmax Hmax - density 0.8 0.7 Non-exceedance probability 0.6 0.5 0.4 0.3 0.2 0.1 0 0 1 2 3 4 5 6 7 8 9 10 Wave height (m)
- 36. Shell Exploration & Production Long term variability 8 6 Hs (m) 4 2 0 01-Jul-1998 01-Sep-1998 01-Nov-1998 01-Jan-1999 01-Mar-1999 12 10 Mean period (s) 8 6 4 2 01-Jul-1998 01-Sep-1998 01-Nov-1998 01-Jan-1999 01-Mar-1999
- 37. Shell Exploration & Production Long term variability 8 6 Hs (m) 4 2 0 01-Jul-1998 01-Sep-1998 01-Nov-1998 01-Jan-1999 01-Mar-1999 12 10 Mean period (s) 8 6 4 2 01-Jul-1998 01-Sep-1998 01-Nov-1998 01-Jan-1999 01-Mar-1999
- 38. Shell Exploration & Production Storm history 5 8 4.5 6 Hs (m) 4 4 3.5 2 Hs (m) 0 3 01-Jul-1998 01-Sep-1998 01-Nov-1998 01-Jan-1999 01-Mar-1999 2.5 12 10 2 Mean period (s) 8 1.5 15-Jul-1998 17-Jul-1998 19-Jul-1998 6 4 2 01-Jul-1998 01-Sep-1998 01-Nov-1998 01-Jan-1999 01-Mar-1999
- 39. Shell Exploration & Production 1 0.9 0.8 Non-exceedance probability 0.7 0.6 0.5 5 0.4 4.5 0.3 0.2 4 HmaxMPs 0.1 3.5 0 Hs (m) 0 5 10 15 Wave height (m) 3 2.5 2 1.5 15-Jul-1998 17-Jul-1998 19-Jul-1998
- 40. Shell Exploration & Production Hmps10 8 HmpsN 6 Hmps 3 Hmps4 … Hs (m) 4 2 0 01-Jul-1998 01-Sep-1998 01-Nov-1998 01-Jan-1999 01-Mar-1999 12 10 Mean period (s) 8 6 4 2 01-Jul-1998 01-Sep-1998 01-Nov-1998 01-Jan-1999 01-Mar-1999
- 41. Shell Exploration & Production Exceedance distribution of storm Hmp 1 0 2 4 6 8 Q(Hmp) 0.1 0.01
- 42. Shell Exploration & Production Hmp 1 0 2 4 6 8 Q(Hmp) 0.1 0.01
- 43. Shell Exploration & Production Hmp Hmps100 1 0 2 4 6 8 Q(Hmp) 0.1 Q(Hmps|n-years) Q(Hmps|100-years) 0.01
- 44. Shell Exploration & Production Design Conditions Metocean Environment Extreme Response Based Responses Value Metocean Design Analysis on Conditions Responses Offshore System Operational N-year values for Design Cases for Behaviour responses N-year response
- 45. Shell Exploration & Production Floaters • Not so straightforward for floating systems • Manual process • Good understanding required: – Metocean environment – Structure response
- 46. Shell Exploration & Production
- 47. Shell Exploration & Production Heave design condition
- 48. Shell Exploration & Production Sensitivity study