The mean and low-frequency horizontal wave drift forces on 2 types of semi-submersible structures in regular and in irregular waves are determined from model tests and calculations. For the measurement of the low-frequency drift forces in irregular waves use is made of a special dynamic system of restraint. Comparison of measured and computed drift forces in irregular waves show increasing divergence between predictions based on 3-dimensional potential theory and results of experiments with increasing severity of the irregular sea conditions. Comparison between computed and measured mean drift forces in regular waves show increasing divergence at lower wave frequencies. A simple model for approximating viscous contributions to the drift forces in irregular waves is applied to some test results and it is shown that the correlation between measurements and predictions is improved. In order to gain more detailed insight in the mechanisms of the viscous contribution to the drift force tests were carried out with single fixed vertical cylinder in regular waves. The results of tests confirm that in conditions of waves without current the major part of the viscous contribution to the drift force is confined to the splash zone of the cylinder.
The motions and mooring forces of Semi-Submersibles and TLP' s moored in exposed locations are often dominated by wave effects. These may be subdivided in first order wave frequency forces with frequencies corresponding to the individual waves and mean and low-frequency second order wave drift forces related to wave groups. From the point of view of the design of mooring systems both first and second order wave loads and the motion and mooring load response need to be taken into account. At the design stage predictions of these quantities for a particular design can be based on computational methods, model tests or a rational combination of both. Computational methods for wave frequency loads and motion response for semi-submersibles have been in development since the early 70's. See reference [1]. Such methods are based on linear hydrodynamic theory and have proved their worth on many occasions. Non-linear, mean wave drift forces on semi-submersible type structures can be computed based on the application of linear, 3-dimensional diffraction theory computational methods combined with either a far-field method or a near-field method for the evaluation of the second order wave loads on the structure. In case a far-field method is applied, generally only the mean second order horizontal drift forces can be calculated. See reference [2] and reference [3]. If a near-field or pressure integration method is applied the mean and low-frequency components of the drift forces can be computed for 6 degrees of freedom. See reference [4]. This type of computational method assumes the flow to be inviscid thus excluding any effects which might arise from separated flow around the structure. In the past efforts have been made to verify the computational methods for the mean and low-frequency or slowly varying wave drift forces on semisubmersible type structures. See reference [5].
