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Based on boundary element method, a time domain solution for 3D diffraction is developed to study nonlinear interaction of ocean waves and rigid arbitrary shape structures. Solution in time is obtained from integral equation based on Green’s theorem. Free surface boundary conditions are expanded and combined to change moving boundary to constant one in time. Consequently, solving simultaneous equations, resulting from discrete sizing integral equation, is required once instead of at each time step, significantly inducing time increase. The solution is separated to known incident potential and unknown scattered one, developed in time and place and should be determined. Based on these theoretical developments, software entitled “DIFFRACTION” is produced and used in
assessing waves diffraction around arbitrary shape obstacles. The force and free-surface elevation for different cases are presented. Despite less elements and more convenient computations, surface- piercing circular cylinder results are in good agreement with previous studies.