The near field method is a numerical method for calculation of the wave drift forces and moment, by the way of integrating the pressure over the wetted body surface. Therefore, this method is available not only for the single floating body but also for the each floating body in multiple. Computing the velocity potential on the floating body, the boundary element method (BEM) is used to solve the boundary integral equation which is derived by Green function and the linearized boundary conditions. The higher order BEM and the constant panel method are available to discretize the boundary integral equation. The constant panel method is relatively easy to compute the velocity potential. However, when we obtain the velocity potential by means of the constant panel, near field method cannot evaluate the wave drift forces and moment accurately.

In the present paper, a new numerical technic of the constant panel method is proposed, in order to take an advantage of the near field method to calculate the wave drift forces and moment. In the new technic, we introduce the extrapolation function and the isoparametric elements. The velocity potential calculated by constant panel method on the floating body can be extrapolated to the new evaluated points. These new evaluated points can be used for near field method to calculate the wave drift forces and moment. As one example of the proposed technic results, we use an ellipsoid model. The results of the wave exciting forces and the wave drift forces between following three methods will be compared. One is the new numerical method, the others are higher order boundary element method and the conventional constant panel method.