Mean and low frequency wave drift forces on moored structures have been shown to be of importance with respect to low frequency motions and peak mooring loads. This paper is concerned with prediction of these forces on semi-submersible type structures by computations based on three-dimensional potential theory. A discussion is given of a method to compute such forces based on direct integration of pressure on the wetted part of the hull of arbitrarily shaped structures. Results of computations of horizontal drift forces on a six-column semi-submersible are compared with model tests in regular and irregular waves. The mean vertical drift forces on a submerged horizontal cylinder obtained from model tests are also compared to results of computations. On the basis of these comparisons it is concluded that wave drift forces on semi-submersible type structures in conditions of waves without current can be predicted to a reasonable degree of accuracy by means of computations based on potential theory.