Offshore operations have been performed successfully during the last decades, in which time period it was established that semi-submersibles are superior to conventional ship-shaped barges in view of minimum down-time requirements. To meet these requirements it is necessary to reduce the motions, which can be attained by optimization of the dimensions or the shape of the underwater hulls. For optimization purposes there is a need for theoretical methods in order to reduce costly and time-consuming model experiments. Such a method has been developed at N.S.M.B., which makes it possible to calculate wave forces, moments and the resulting motions of semi submersible platforms of arbitrary shape. The paper outlines the fundamentals of the method.A comparison has been made between computed heave and model heave measurements for four cross-section types of lower hulls of a simplified platform in beam waves. Both results show a very good agreement. The results indicate that heave will be strongly influenced by the cross-section of the lower hulls in particular with respect to wave period. The paper concludes with a motion comparison for an actual platform, which will be optimized for two operational areas where prevailing weather conditions differ substantially.