Short-Term Variability of Wind Measurements in South China Sea

Abstract

Good understanding of the wind characteristics is essential in offshore structural design and operational activities. The short-term variability of wind is captured in design codes with prescribed forms for the wind spectrum and for the ratio of mean wind speed maxima for given durations to the one hour mean. Relationships are also prescribed for scaling these wind speeds between different elevations, essentially defining the wind profile in the turbulent boundary layer. All of these are based largely on measurements made in mid-latitude regions during extra-tropical storms or hurricanes; but these measurements will not be representative of the wind field during squalls, which are not stationary, and possibly not representative of the wind field during general monsoonal conditions. Several years of continuous recordings of wind speed and direction, sampled at 1Hz, at several South China Sea locations allow us to examine the short-term variability of wind in squalls and monsoonal conditions, and allow comparison with expressions given in design codes. We investigate the variability of the stationarity of the winds, the wind spectrum, the mean wind speed, and the wind profile, for the South China Sea measurements. The premise for the short-term variability prescriptions is that the wind field is stationary. Accordingly, we first report our evaluation of this assumption, and based on this, our evaluation of the relevance of the various prescriptions in the codes for describing short-term variability is developed.