OVER-WATER GUST FACTORS

This paper examines the characteristics of over-water gust factors and turbulence at three offshore locations, and evaluates several techniques for predicting over-water gusts. Mean gust factors were found to agree well with previously published values: typical average values for G(1,3600) and G(3,3600) at 13.4 m elevation were 1.34 and 1.30, respectively. Gust factors were found to be most sensitive to gust duration (t), elevation (z) and measuring interval (T). Wave height, wind speed, and atmospheric stability were observed to have only slight influences on gust factors. Mean values for sigma/u* ranged from 2.26 to 2.58 in general agreement with published values. sigma/u* was observed to display less sensitivity to wind speed and elevation than sigma/U, suggesting it was a more stable parameter to use in modelling gust factors. The inclusion of stability effects in the calculation of u* was found to decrease the height dependence of sigma/u* which was important for reliable gust factor prediction at higher elevations. The gust factor model evaluation revealed that a number of available models can provide good predictions of average gust factors. However, no model was able to account for the observed variation in individual over-water gust factors. Incorporation of the Geernaert et al. [(1987), Measurements of wind stress, heat flux and turbulence intensity during storm conditions over the North Sea. J. geophys. Res. 92, 13127-13139] drag coefficient/wave age relationship into an existing gust model did little to improve results. Several models were able to provide reasonable estimates of the peak gust occurring over a number of wind samples by adding a 95% confidence limit to the predicted maximum gust, or by increasing T to reflect the combined duration of all wind samples. A modified version of the Forristall [(1987), Wind spectra and gust factors over water, Proceedings of the 20th Annual Offshore Technology Conference, Houston, Texas, 2-5 May 1988, OTC 5735, pp. 449-460] approach was recommended for use offshore where the vertical gust factor profile was important. Further work and better data are required to develop a modelling technique which can replicate the observed variability in over-water gusts.