The diffraction of waves by two bottom-fixed vertical circular cylinders is investigated by using boundary element method. This method has been successfully applied to the isolated vertical circular cylinder and now is used to study the interaction between waves and two vertical cylinders. In this paper, a numerical analysis by boundary element method is developed by using linear potential theory. The numerical analysis by boundary element method is based on Green's theorem and introduced to an integral equation for the fluid velocity potential around the vertical circular cylinders. To verify this method, the wave-exciting forces on two transverse and tandem cylinders obtained in this study are compared with the results computed by the multiple scattering method. The comparisons show that the results of this study are strong agreement with their results. Also in this paper, several numerical examples are given to illustrate the effects of various parameters on the wave-exciting force such as the separation distance, the relative size of the cylinders and the incident wave angle. This numerical analysis developed by boundary element method will be applied for various offshore structures to be constructed in the future.

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