For the purpose of improving the utilization of semisubmersibles such as oil rigs in random seas, an optimal design method is proposed to seek optimal forms with minimum motion against sea wave. In the optimal design method proposed here, the design objective is chosen to minimize the variance of heave motion amplitude of a semisubmersible oscillated by the sea wave in a working environment. By giving the displacement and the hull length of the semisubmersible as design conditions a priori, diameters of both hulls and each columns are determined optimally by using the quadratic programming technique. Through a numerical study on a semisubmersible design problem, it is ascertained that by the optimal design method a considerable improvement of the motion performance can be attained compared with the result given by the conventional design method.

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