On secular terms which appear in perturbation problems of non-linear wave forces acting on offshore structures, and solutions of the problems compared with model tests

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This research analyzes nonlinear wave loads using perturbation method. Higher order nonlinear hydrodynamic forces such as third order wave excitation and wave drift added mass had not been obtained due to secular terms which appear in higher order problems, whilst lower hydrodynamic forces such as second order wave excitation and wave drift damping can be obtained. Therefore even if we calculate full nonlinear problem using pure numerical method such as a finite difference method, we have no bench marks to be compared to assure its accuracy. This research presents new analysis method of solution to these secular term problems, as well as measurement method of experiment to verify the solutions of this new analysis.First year we solved the problem of a vertical circular cylinder in both wave and current by the eigenfunction expansion method, and compared free surface elevation with the measured date by Swan et al. Second year we extended our method to the problem of floating vertical cylinder from the cylinder touching to the bottom. Third and the last year we developed a hybrid method combining near field boundary element method (BEM) and far field eigenfunction expansion method. We obtained wave drift added mass of a spheroids.We developed a measurement method to obtain maneuvering hydrodynamic force derivatives by Planar Motion Mechanism (PMM) in waves. These derivatives verify the solutions of this new analysis.