5. At-sea Experiment of a Floating Offshore Structure : Wind Characteristics at a Test Field

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The at-sea experiment of the moored floating structure, which is called POSEIDON, was done at the Japan Sea from September, 1986 to June, 1990. This paper deals with the characteristics of wind speeds measured at the test field. Especially, we concentrate upon wind spectrum, 1/nth highest mean values, and gust factors. As an anemometer, a sonic anemometer was used and it was installed at the top of mast of POSEIDON, the height of which is 19.5m above sea surface. The main results are as follows. i) A new formula of wind spectrum is developed under physical considerations. The present spectrum can be determined by two parameters as the peak frequency and the power, which are prescribed by the friction coefficient of sea surface. And an expression form of the friction coefficient to mean wind velocity is presented. A comparison between the measured spectra, the spectrum estimated from the present formula and the other spectrum forms which have been proposed has been done. The measured spectrum agrees very well with the present spectrum and it is close to the Ochi-Shin's spectrum among the other proposed spectrum forms. ii) 1/nth highest mean values and gust factors can be estimated by using the linear random noise theory (i.e. Cartwright-Longuet-Higgings formula) and the present spectrum.
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