Numerical Solution of effects of water waves on a large horizontal cylinder in sea Using an improved method in time domain

Cylindrical bodies constitute the main part of the marine structures which are widely used in exploitation of see resources including oil, gas and wave energy. Bulding see reservoirs, harbors, floating breakwaters, see bridges, and wharves are the other examples of their applications. In addition, for achieving safer and more economical design, especially in hostile see environment, it is necessary to do deeper researches on the interactions of water waves with marine structures. Assuming potential and irrotational flow the nonlinear interaction of water waves with a fixed and large semi submerged horizontal cylinder is considered. This problem in nonlinear and two dimensional with moving boundaries. By employing perturbation method, two sets of equations, namely first and second order, are obtained, and so the problem with moving boundaries is reduced to a problem with constant boundaries. Then each set is separated into incidence and diffraction equations and solved sequentially. The diffraction equations are solved numerically using panel method, Green'sintegral equation and time marching. Compared to previous works a simple radiation boundary condition with constant celerity is employed. Also an immproved method is used to calculate numerically the derivatives, which are appeared in the free surface nonlinear boundary conditions. The results show that Comparing to the previous works, using both constant wave speed in radiation condition and using improved method in numerical calculation of the free surface reduce required computer memory and CPU time 54% and 50% respectively . These improvements are due to the fact that only boundary panels are used while internal points in addition to boundary points have been used in the previous works. It is shown that second order effects are significant in hydeodynamic forces, wave profile and wave run up. Also it is good agreement between the results presented here and the previous works.