An original method for simulating the interaction of irregular long crested waves with a 3D body is described. Incident waves are modeled using a recently developed pseudo-spectral method for fully-nonlinear free surface waves in water of finite depth. The wave generation process is based on submerged dipoles. The dipoles strengths and orientations are combined in such a way that waves are generated in a single direction. Furthermore, the wave generation transfer function is predetermined analytically, so that sea states of given energy spectrum are easily generated. The interaction of these waves with a 3D body is modeled using the 3D fully nonlinear BEM solver XWAVE, based on a formulation for the nonlinear diffracted flow. The effective coupling of both models is illustrated by simulating the diffraction of 2D wave packets as well as different realizations of ITTC wave spectra, by a vertical bottom-mounted cylinder.