In this study, numerical and experimental assessment of the 2nd order responses of a twin hulled semi-submersible subjected to monochromatic, bi-chromatic and random seas has been attempted. For numerical analysis, the 2nd order wave forces resulting from the 2nd order temporal acceleration and the structural 1st order motions have been obtained. The latter has been estimated by an iterative frequency domain analysis. Experimentally, the semi-submersible was modeled to a scale of 1:100 using Froude's scaling law principles. The measured drift force has been compared with available formulae in the literature and reported. The numerical results have shown good agreement with the experimental results. Copyright © 2011 by the International Society of Offshore and Polar Engineers (ISOPE).