In this paper a study is made for the mean drift forces acting on a moored ship with a moon pool. Drift forces are calculated using the near-field method. Improved Green integral equation is employed to solve the three dimensional radiation-diffraction problem. It is found that the moon pool affects significantly the hydrodynamic characteristic of the ship at certain frequencies.


The wave drift forces and moments are known as the most important factor for the optimum design of position keeping system of floating offshore structures such as drill ships and semi-submersibles. Maruo(1960) applied the momentum conservation principle to the estimation of wave drift forces, The usefulness of his method, so called far-field method was verified by Kudou(I977). Pinkster(I977) obtained wave drift forces on a floating body by the near-field method which consists in the direct integration of the water pressure over the wetted surface of the body. In this paper, drift forces are calculated by the near-field method. And the three dimensional radiation-diffraction problem is solved by making use of the improved Green integral equation(Hong, 1987a).


The fluid is assumed to occupy a space D bounded by the wetted surface S of a rigid body and by the free surface F of deep water under gravity. The free surface is assumed to extend in all directions. The body performs simple harmonic oscillations of small amplitude with circular frequency w. Cartesian coordinates(x, y, z) are employed with the origin 0 in the horizontal plane inside the body at the level of the undisturbed free surface and the z axis vertically upwards(see Fig. 1). From these, the motion responses of the body, the relative wave height at waterline, the pressure and water particle velocity on Scan be obtained after some well known formulae.

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