A result of preliminary study of statistical estimation of slow drift force of a vessel in a unidirectional deep random sea is presented. The statistical parameters of the slow drift force are estimated using theoretically simulated time series and the results are compared with theoretical prediction. The time domain simulation procedure is described, since it is relatively unknown and different from the others. A number of sample functions for the drift force are produced and the mean of the means, variances and mean up-crossing frequencies for peak drift forces are estimated and compared with theoretical data.


Recently, increased attention has been focused on the prediction of statistics of the slowly varying second-order nonlinear response of large floating structures in the irregular seas. Theoretical researches have been carried on the problem by Neal (1974), Roberts (1981), Vinje (1983), Stansberg (1983), Langley (1984), Naess (1985, 1986) and Kinoshita, et al. (1989). Most works are based on Kac-Siegert Method (1947), except for the papers by Roberts, Langley and Kinoshita et al. Kac-Siegert method is to find the eigen values and eigen functions from an integral equation. Statistics of slow drift motion were also investigated by Pinkster, et al. (1987) using theory and experiment/time domain simulation. This paper is concerned of statistical estimates of lateral slow drifting force acting on a ship at zero speed of advance in a quartering random sea, based on a theoretically simulated time series. Time domain simulation of slowly varying added resistance of a ship running in random seas was made by Dalzell (1976) using experimentally determined quadratic frequency response function (QFRF). The same approach was used by Kim and Breslin (1976) for slow drift force and motion using theoretically determined QFRF for slow drift force in head seas.

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