Decontamination of Wind Measurements from Buoys Subject to Motions in a Seaway

Abstract

The motions of buoys in a seaway produce velocities at onboard anemometers that contaminate the measurements of wind, which typically is a severe problem for small, easily deplorable buoys. The severity of this problem is quantified for a MINIMET buoy having a Gill and a propeller/vane anemometer at 3-m height, and a decontamination algorithm is derived and used to correct the Gill wind data. The model for the motions of the anemometer utilizes measurements of all six components of motion of the buoy. A gimballed compass is used to measure the yaw; a vertically stabilized platform measures heave, pitch, and roll; and horizontal linear accelerometers are used for surge and sway. The buoy surge and pitch produce the worst wind speed errors when the waves are in line with the wind. Of these two, the errors produced by the surge motions are larger for the case considered in detail here. The 3D velocity vector at the anemometer location as calculated from the motion measurements is coherently subtracted from the measured wind vector in the anemometer coordinate frame, and the result is transformed back to earth coordinates. The motion subtraction significantly affects the measured wind in a frequency band from roughly 0.1 to 0.6 Hz. The corrected wind fluctuations agree in general form with other measurements in a wind/wave tank and from a tower.