Winds (what I know and what I don

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Winds(what I know and what I dont know)John
HeidemanExxonMobil Upstream Research Co.OGP
WorkshopApril 2001
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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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Importance of Winds to Floaters
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Idealized Wind Record
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Turbulent Wind Fluctuations May Be More Important
than Waves for Some Floater Responses
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Wind Data Sources andUncertainties
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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.
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Buoy Measurements
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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

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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

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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

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We lack an absolute standard 1 hr, 10 m reference
wind database over the whole dynamic range up to
40 m/s!
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Wind Models for Large-Scale Storms
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Froya Wind Measurements
Statoil-sponsored JIP on west coast of Norway
Froya
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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

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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

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Sletringen Wind Database
Primary input to NPD wind description

• 40-min records _at_ 0.85 Hz
• 1726 complete records

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Data Analysis Considered Wind Speed and Stability
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NPD Wind Relationships (Neutral)
Based on Froya wind database and analysis
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Profiles of Wind Speed
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Profile of RMS Turbulence
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NPD Wind Relationships (Neutral)
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Spectra
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NPD Wind Relationships (Neutral)
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Coherence Length(squared coherence e-1)
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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

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Wind Models for Squalls
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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

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Squall Winds are Not Stationary
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Suite of Scaled Squall Time Series
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Broadside Squall Winds on Tandem Moored Vessels
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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

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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.

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The End
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Design Points when Winds and WavesAre Both
Important