A source-dipole distribution is used to solve a variation of Green's theorem for structures with zero wall thickness. Both non-porous and porous structures are modeled. A linear and quadratic relation is used for the pressure difference between the two sides of the walls and the velocity through the walls. Added mass and damping coefficients, the Haskind relation and the energy equation are examined to supplement the description for porous effects. The various checks are numerically verified, which is a nontrivial result. Drift forces due to diffraction are studied. The 3D calculations of the drift force for a very porous geometry of a semi-spherical shape are directly comparable to 2D calculations of a rectangular screen of similar porosity using entirely different methods [Heggen, Master thesis,University of Oslo,2015]. We conclude that the linear and quadratic relations have the same behavior for low porosity. For high porosity, the quadratic model is superior as it reproduces porosity as a physical attribute, where there is no such reproduction in the linear model.