A theoretical investigation is carried out to study the interactions between waves and large offshore structures at small forward speed. First order added mass, radiation damping and exciting forces as well as dynamic responses of rigid structures at small forward speed are presented. Second order mean drift forces at forward speed and low frequency wave drift damping are also presented. An asymptotic analysis of the mean drift force on a vertical cylinder at small forward speed in long waves is first carried out. The mean drift force is obtained analytically by using a far field method, and the wave drift damping is estimated and discussed. For arbitrary bodies, a perturbation theory on the basis of a small forward speed parameter is proposed and developed. The fluid flow around the body is solved in terms of a zero forward speed problem and a forward speed correction problem. The latter is linearly proportional to the forward speed. A novel multipole expansion is presented for the velocity potential of zero forward speed motion. The expressions are valid for both completely submerged bodies and surface piercing bodies. A computer program is implemented, based on the small forward speed theory. Two coupled numerical methods are utilized. Solutions of the first order dynamic motions, second order mean drift forces and wave drift damping are presented for several floating and submerged horizontal cylinders. It is observed that the influence of the small forward speed on the second order hydrodynamic forces is larger than that on the first order hydrodynamic forces; and that among the first order forces, the influence on the exciting forces is larger than that on the added mass and radiation damping
