In this paper, the second order steady drift forces on the ships and other floating offshore structures are calculated by the far field method. The amplitudes of diffracted waves and radiated waves at infinity are obtained by the two-dimensional source distribution and strip-theory method. For the twin hull structure, the hydrodynamic interaction between the two hulls is taken into account. The drift forces on cross sections of Lewis type as well as on the semi-submersibles are computed. The theoretical results obtained by the present method agree fairly well with experimental results.
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