The Influence of Directional Spreading of Waves on Mooring Forces
In this paper the basic expressions for the computation of mean and low frequency second order wave drift forces on floating structures in directionally spread seas as can be derived from potential theory are discussed. The resultant expressions for the mean and slowly varying drift forces are applied to a specific form of the directional seas, i.e. cross seas which consist of two irregular long-crested wave trains from different directions. The analysis shows under which conditions the drift forces in irregular cross seas may be calculated based on the superposition of the drift forces from the component long-crested irregular wave trains. Model tests were carried out with the model of a 200,000 DWT tanker moored in regular and irregular cross seas. The results of the model tests confirm theoretical predictions regarding the superposition of mean drift forces and the interaction effects present in slowly varying components of the drift forces in directional seas.