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Winds (what I know and what I don’t know) John Heideman ExxonMobil Upstream Research Co. OGP Workshop April 2001

Winds (what I know and what I don’t know) John Heideman ExxonMobil Upstream Research Co. OGP Workshop April 2001. Presentation Outline. Importance of winds to floaters Wind data sources and uncertainties Wind models for large-scale storms Wind models for squalls.

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Winds (what I know and what I don’t know) John Heideman ExxonMobil Upstream Research Co. OGP Workshop April 2001

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  1. Winds(what I know and what I don’t know)John HeidemanExxonMobil Upstream Research Co.OGP WorkshopApril 2001

  2. Presentation Outline • Importance of winds to floaters • Wind data sources and uncertainties • Wind models for large-scale storms • Wind models for squalls

  3. Importance of Winds to Floaters

  4. 40 I =  / U0 = O(0.1) U0 Mean 30 U(t)=U0+u(t), u(t)<<U0 20 F(t) = 0.5CdAU2(t)  0.5 CdAU02 +CdAU0u(t) speed (m/s) Mean and fluctuating load 10 SF(f) = (CdAU0 )2 2(f) Su(f) Admittance function 0 1800 0 3600 time (s) Idealized Wind Record

  5. Surge / sway natural frequencies for floaters Natural frequency for fixed platforms Spectral energy density Wave spectrum Wind spectrum 0 0.1 0.2 Frequency (Hz) Turbulent Wind Fluctuations May Be More Important than Waves for Some Floater Responses

  6. Wind Data Sources andUncertainties

  7. Convergence increased Ws deck Wake reduced Ws Stagnation reduced Ws Convergence increased Ws Platform Measurements Structure interferes with flow. Measure sustained wind and gusts at top of derrick and measure air and sea temperature. Adjust winds to reference level, accounting for platform interference.

  8. Increased Ws Reduced Ws Wave form interferes with flow. Anemometer height may be too low. Buoy heave, pitch, and roll may affect measurements. Buoy winds are biased low (5% - 15%??) for speeds above 20 m/s. Buoy Measurements

  9. Satellite Winds • Altimeter and Scatterometer • Agree well with “ground truth” up to about 20 m/s • Biased low (5%-15%??) for speeds above 20 m/s due to calibration with biased buoy winds and “saturation” • Algorithms are being improved; may be good to 35 m/s eventually

  10. Visual Observations • Estimated from the appearance of the sea (Beaufort scale) • Subjective • Require correction for systematic bias (Cardone, for example) • U19.5m = 2.16 (UBeaufort )7/9

  11. Hindcast Winds • Represent one-hour average speed at 10 m (or 20 m) • Accuracy depends on quantity and quality of available wind and pressure data and assimilation methods • Do not account for subgrid-scale features such as “jet streaks”

  12. We lack an absolute standard 1 hr, 10 m reference wind database over the whole dynamic range up to 40 m/s!

  13. Wind Models for Large-Scale Storms

  14. 45 m 100 m Skipheia Sletringen 45 m Froya Wind Measurements Statoil-sponsored JIP on west coast of Norway Froya

  15. Sea temp 45 m mast Wind Tower at Sletringen • Wind speed at 5, 10, 20, 42, & 46 m above land surface (+4 m) • Wind direction at 45 m • Air temperature at 5 & 45 m • Sea temperature at 5 m below sea surface

  16. 100 m mast 100 m mast 45 m mast Wind Towers at Skipheia • Wind speed at 10, 20, 40, 45, 70, &100 m above land surface (+20 m) • Wind direction at 41, 45, & 100 m • Air temperature at 2, 10, 40, 70, & 100 m

  17. Sletringen Wind Database Primary input to NPD wind description • 40-min records @ 0.85 Hz • 1726 complete records

  18. Bulk Richardson Number g 2 (Ta - Ts)/ -  ,  = adiabatic lapse rate  - 0.01 oK/m Ri = Ta Uo2 Ri > 0, stable = 0, neutral < 0, unstable • Atmosphere approaches neutral stability for • small, slightly negative air-sea temperature differences • very high wind speeds Data Analysis Considered Wind Speed and Stability

  19. NPD Wind Relationships (Neutral) Based on Froya wind database and analysis

  20. Shorter gusts have more uniform profiles. Profiles of Wind Speed

  21. Turbulent energy decreases with elevation. Profile of RMS Turbulence

  22. NPD Wind Relationships (Neutral)

  23. Turbulent energy (area beneath spectrum) increases with reference wind speed. NPD spectrum is within the range of API spectra over frequencies of interest for floaters and compliant towers (0.003 - 0.01 Hz) . Spectra

  24. NPD Wind Relationships (Neutral)

  25. Vertical and lateral coherence lengths are an order of magnitude smaller than longitudinal coherence length. Coherence length increases with reference wind speed. Turbulent fluctuations with periods < 2-3 minutes are incoherent across a structure 100 m wide. Coherence Length(squared coherence = e-1)

  26. Uncertainty in Wind Relationships for Large-Scale Storms • Applicability to wind speeds above 25 m/s, beyond the range of the “science quality” calibration data • ***thought to be applicable, since parameterizations are consistent with recognized principles of atmospheric boundary layer theory, and high quality, near-neutral data representative of very high wind speeds were used to derive the parameterizations • Applicability to tropical storms • ***thought to be applicable except in regions of strong convection near the eye • Applicability to frontal passages and squalls • ***not applicable

  27. Wind Models for Squalls

  28. West Africa Wind Measurements • Data collected between December 1996 and July 1998 • 1 Hz measurements at top of derrick, 83 m above mean water level • 10 squalls with 1-second gusts above 20 m/s

  29. Squall of June 1997 • 1-min mean speed is far from constant over a 1-hr period • “Gustiness” about 1-min mean varies over a 1-hr period • Cannot be described with spectral model • Scaled records must be used in time-domain dynamic response calculations Raw and 1-min mean speed Difference between raw and 1-min mean speed Squall Winds are Not Stationary

  30. Jun 97 Mar 97 Oct 97 Maximum structure response can vary considerably for different squall records even if all squall records are scaled to the same peak wind speed. Suite of Scaled Squall Time Series

  31. prevailing 300 m 300 m environment 100 m Squall winds Coherence over length of vessels? Broadside Squall Winds on Tandem Moored Vessels

  32. Squall Wind Unknowns • Vertical, transverse, and longitudinal coherence of gusts • Proper averaging time for “sustained” speed in wind load and response calculations • Vertical profile of speed • Statistics of speed buildup and decay rates • Statistics of direction change rates • Extreme (100-year) speed

  33. Doppler radars + air temp Anemometers on masts water temp Potential Squall Wind Measurement Program • Squall winds affect floating structures offshore West Africa, but little is known of their spatial structure. • Measure spatial and temporal variation of wind fields in squalls using state-of-the-art instruments. • Mount instruments on bridge between platforms offshore Nigeria for a couple of years.

  34. The End

  35. 100-yr Ws & assoc Hs 100-yr Ws > 100-yr response Increasing frequency 100-yr Hs & assoc Ws Most probable point Sustained wind speed 100-yr response Contours of joint occurrence frequency 100-yr Hs Significant wave height Design Points when Winds and WavesAre Both Important

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