Response-based Metocean Criteria for OptimisingDesign and Operation of FPSOs

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Hermione van Zutphen, on the Metocean design conditions & LSM, description of the metocean environment ad the extremes, short term and long term variability of the FPSO sector.

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Response-based Metocean Criteria for OptimisingDesign and Operation of FPSOs

  1. 1. Shell Exploration & Production Response-based Metocean Criteria for Optimising Design and Operation of FPSOsCopyright: Shell Exploration & Production Ltd. Hermione van Zutphen Marios ChristouFile Title Kevin Ewans9/7/2009
  2. 2. Shell Exploration & ProductionOutline• Metocean design conditions & LSM• Description of the metocean environment• Response analysis• Extremes, short term and long term variability
  3. 3. Shell Exploration & ProductionWhy 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. 4. Shell Exploration & ProductionMetocean 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. 5. Shell Exploration & Production Response Based AnalysisMetocean 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. 6. Shell Exploration & ProductionEnvironment “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. 7. Shell Exploration & ProductionShell’s method for response basedanalysis: LSMAvailable for:• Fixed structures• Ship-shaped structures, either turret- or spread-moored• Semi-submersibles• TLPs• Pipelines
  8. 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. 9. Shell Exploration & ProductionOcean Environment DescriptionRequires a long-term database of:Waves • wave height, period, and direction or • directional wave spectrum or • wind-sea and swellWinds • speed and direction • wind spectrum • wind profileCurrents • current speed and direction • current profileFor 15 years of 3 hour intervals: 44070 records!
  10. 10. Shell Exploration & ProductionWaves
  11. 11. Shell Exploration & ProductionWhat 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. 12. Shell Exploration & ProductionFrequency-direction wave spectrum:wave systems Swell Wind sea
  13. 13. Shell Exploration & ProductionWind• 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. 14. Shell Exploration & ProductionCurrents• 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. 15. Shell Exploration & Production The easy bit for the Offshore System Modelstructures floating engineerMetocean 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. 16. Shell Exploration & ProductionFloater 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. 17. Shell Exploration & Production ResponsesMetocean 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. 18. Shell Exploration & ProductionFloaters: 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. 19. Shell Exploration & ProductionSpectral 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. 20. Shell Exploration & Production All methods begin by treating the ocean surface as the sum of many frequency components. Then………..
  21. 21. Shell Exploration & Production xi
  22. 22. Shell Exploration & Production xi x lin diffraction
  23. 23. Shell Exploration & Production xi x lin diffraction x dynamics
  24. 24. Shell Exploration & Production xi x lin diffraction x dynamics responsei Sum over all components
  25. 25. Shell Exploration & Production xi x lin diffraction x dynamics xj  xk  O(2) diff  dynamics responsejk Sum over all components
  26. 26. Shell Exploration & Production xi x lin diffraction x dynamics xj  xk  O(2) diff  dynamics Frequency domain xi ~ i
  27. 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. 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. 29. Shell Exploration & ProductionWhich 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. 30. Shell Exploration & Production ExtremesMetocean 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. 31. Shell Exploration & ProductionResponses 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. 32. Shell Exploration & Production 7 Definition of a storm 6 80% peak of storm 5Hs (m) 4 3 2 threshold 1 m 1
  33. 33. Shell Exploration & ProductionEvaluation of extreme response statistics
  34. 34. Shell Exploration & ProductionShort 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. 35. Shell Exploration & ProductionShort 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. 36. Shell Exploration & ProductionLong 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. 37. Shell Exploration & ProductionLong 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. 38. Shell Exploration & ProductionStorm 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. 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 0Hs (m) 0 5 10 15 Wave height (m) 3 2.5 2 1.5 15-Jul-1998 17-Jul-1998 19-Jul-1998
  40. 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. 41. Shell Exploration & ProductionExceedance distribution of storm Hmp 1 0 2 4 6 8 Q(Hmp) 0.1 0.01
  42. 42. Shell Exploration & Production Hmp 1 0 2 4 6 8 Q(Hmp) 0.1 0.01
  43. 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. 44. Shell Exploration & Production Design ConditionsMetocean 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. 45. Shell Exploration & ProductionFloaters• Not so straightforward for floating systems• Manual process• Good understanding required: – Metocean environment – Structure response
  46. 46. Shell Exploration & Production
  47. 47. Shell Exploration & ProductionHeave design condition
  48. 48. Shell Exploration & ProductionSensitivity study

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