A computational procedure for three-dimensional simulation of nonlinear gravity wave propagation and response of floating structures

The response of floating offshore structures to nonlinear wave loading is one of the most important considerations in design, yet it is also the most challenging to analyze. The nonlinear boundary conditions at the free surface and on the surface of a moving body present mathematical difficulties from both theoretical and computational points of view. A fully nonlinear wave-structure interaction computation requires a technique combining several different numerical methods into one seamless algorithm.; A computational procedure for the time-domain simulation of the interactions between floating structures and nonlinear water waves has been developed in this dissertation. The procedure is based on the Mixed Eulerian-Lagrangian method, in which the evolution of the free surface is described in terms of only the vertical displacement. The boundary element method is used as the framework of the formulation, combined with a semi-explicit predictor-corrector method for the time-integration of the boundary conditions. A significant portion of this work focuses on the development of a multi-directional absorbing boundary condition based on a generalization of existing ideas of modal superposition for two-dimensional computations. The absorbing boundary implementation minimizes the errors in matching the nonlinear interior to the linear far field in a least-squares sense. Numerical studies show that the proposed absorbing boundary performs more favorably compared to another commonly used absorbing boundary, namely the sponge layer, over a wide frequency-range.; The proposed computational procedure is verified and validated against experimental data and other numerical results from second-order diffraction calculations in the frequency domain. It is found that the numerical results from the proposed method are in good agreement with the experimental results in a physical wave basin. It is also shown that no artificial smoothing or regridding is required in the present method. It can, therefore, be concluded that the proposed computational procedure is reasonably efficient in producing accurate predictions of the motions of floating bodies.