The mean drifting forces have a relation only to first-order motion responses and first-order forces; therefore, the key to calculate the mean drifting forces is to solve the first-order problem. Under the assumptions of potential flow and low moving speed, the problem of a floating body moving with arbitrary oscillating motions and arbitrary wave directions is modified, and a form of Green function of the simplified equation is derived. The Green function is then expanded into an oscillating source Go at zero forward speed and a correction term G1 to take into account low moving speed, and the expressions for GO, G1, VGo and VG1 are then derived. The numerical results of this approach will be compared and discussed. The effect of currents to a freely floating body will also be discussed.
It is important to evaluate the motion responses, first-order wave exciting forces and second-order mean drifting forces acting on a three dimensional body in waves and currents, especially, in the design of a single-point mooring system. Because the mean drifting forces only have a relation to first order motion responses and first order forces, the key to calculate the mean drifting forces is to solve the first order problem. Kobayashi(1981) solved the diffraction and radiation problem for an arbitrary shaped floating body with a constant forward speed, In his method, the singularities satisfying the linear free surface condition are distributed on body surface. It is shown that the 3-D method is more rational than the strip method because the 3-D effects are taken into consideration. Huijsmans and Hermans(1985) have presented a numerical method.
