A potential flow code is extended to include dipole elements to handle potential flow problems for offshore structures consist of thin shell geometry with both sides exposed to ocean waves. The solution on dipole elements can be represented by the drop of fluid potential across the thin shell surface. By adjusting the porous effect parameter, such a dipole element model can be applied for the solid impervious thin shell structures, porous surfaces and even the imaginary sheets attached to the sharp edges where the local flow dissipation occurs. Special attention is paid on different approaches to evaluate the drift forces on floating bodies with porous surface, including the traditional far field and pressure integration methods as well as the control surface integration approach. The solutions from the BEM model on excitation forces, off-body kinematics and drift forces are verified against the analytical solutions for a concentric porous cylinder system fixed at the seabed. The numerical performance of a two-layer source/sink model with very small gaps is investigated, taking the results from the corresponding dipole element model as the baseline. It is suggested that the mesh size requirement from DNV-RP-C205 is too strict for the two-layer panel model in the case that the exact fluid velocity on both sides of the wall is not needed. The proposed model is demonstrated in two case studies which involve the internal fluid dynamics inside tanks with interior web frame structures of a floating barge for one case and the damping effects from the sharp edge of a box barge model for the other case.