AT-SEA EXPERIMENT OF A FLOATING OFFSHORE STRUCTURE - WIND CHARACTERISTICS AT A TEST FIELD

The at-sea experiment of the moored floating structure POSEIDON, was carried out in Japan from September 1986 to June 1990. This paper deals with the characteristics of wind speeds measured at the test field. Especially the wind spectrum, 1/nth highest mean values, and gust factors. A sonic anemometer was used and it was installed at the top of the mast of POSEIDON, the height of which is 19.5m above sea level. The main results are as follows: (i) A new formula of wind spectrum was developed under physical considerations. The spectrum can be determined by two parameters as the peak frequency and the power, which are prescribed by the friction coefficient of the sea surface. An expression form of the friction coefficient to mean wind velocity is presented. A comparison between the measured spectra, the spectrum estimated from the formula and the other spectrum forms which have been proposed was carried out. 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-Higgins formula) and the developed spectrum.