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.
Stationary vessels floating or submerged in irregular waves are subjected to large, first order, wave forces and moments which are linearly proportional to the wave height and contain the same frequencies as the waves. They are also subjected to small, second order, mean and low frequency wave forces and moments which are proportional to the square of the wave height. The frequencies of the second order low frequency components are associated with the frequencies of wave groups occurring in irregular waves.
The first order wave forces and moments are the cause of the well known first order motions with wave frequencies. Due to the importance of the first order wave forces and motions they have been subject to investigation for several decades. As a result of these investigations, methods have evolved by means of which these may be predicted with a reasonable degree of accuracy for many different vessel shapes.
For semi-submersibles which consist of a number of relatively slender elements such as columns, floaters and bracings, computation methods have been developed which determine the hydrodynamic loads on such elements without taking into account interaction effects between the elements. For the first order wave loads and motion problem this has been shown to give accurate results. See Hooft .
This paper deals with the mean and low frequency second order wave forces acting on stationary vessels in regular and irregular waves in general and, In particular, with a method to predict these forces on basis of computations.
The importance of the mean and low frequency wave drift forces from the point of view of motion behaviour and mooring loads on vessels moored at sea has been recognized only within the last few years. Verhagen and Van Sluijs , Hsu and Blenkarn  and Remery and Hermans  showed that the low frequency components of the wave drift forces in irregular waves could, even though relatively small in magnitude, excite large amplitude low frequency horizontal motions in moored structures.