Wave forces on large offshore structures using boundary element methods

Au, Man-Chiu (1982) Wave forces on large offshore structures using boundary element methods. University of Southampton, Doctoral Thesis.

Abstract

This work is concerned with the application of the boundary element method for the determination of wave forces on large offshore structures by the wave diffraction approach. Two and three dimensional structures are considered, and special techniques have been developed in order to account for the geometrical symmetry of the structure. To this end the basic assumptions of the diffraction theory are outlined which leads to a boundary-value problem for the potential. The boundary of the domain includes the structural surface, the ocean floor, the water surface and an arbitrary surface at some distance from the structure. A boundary integral equation can be formulated for the problem by using the weighted residual technique. Two dimensional problems are first considered i.e. those dealing with structures with a constant vertical section. The simple InI=l type fundamental solution is used for the formulation together with all the relevant boundary conditions. Attention is also given to structures with a constant horizontal section throughout the water depth. The field equation becomes a Helmholtz's equation in the horizontal plane. For this case a special fundamental solution in terms of flankel functions has been used. This solution satisfies all conditions except those on the surface of the body which is the only region where boundary integral equations need to be considered. The solution for three dimensional problems can be interpreted as an extention of the two dimensional case and the procedure is straightforward. The main difficulty is the size of the problem and because of this geometrical symmetry of the structure has been considered in order to produce efficient analytical procedures.

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