ABSTRACT

The objectives of the present pap.er are to develop a powerful numerical method for computing second-order wave loads on three-dimensional bodies and to clarify the effects of nonlinear wave diffraction. In this paper, second-order wave loads are computed from the radiated wave potential of double frequency and first order scattered wave potential by applying the extended Haskind formula. The infinite integral over the free surface is evaluated by using Fresnel and Gaussian integrations. The second-order oscillating wave loads on circular, rectangular and elliptic vertical cylinders are computed systematically. The second-order effects on wave forces are evaluated from numerical results and summarized in the charts.

INTRODUCTION

It is important for the design of offshore structures to predict the wave loads on the structures in storm. Since the wave loads in the storm and rough weather show remarkable nonlinear effects, it is difficult to estimate accurately the wave loads by using linear theories. The objectives of the present paper are to develop a powerful nummerical method for calculating the second-order wave loads on three-dimensional bodies, and to provide useful charts which can predict the wave forces without numerical computation, with nonlinear effects (second-order effects) taken into account. (1) To directly solve the second-order boundary value problem including inhomogeneous free surface condition and then obtain from the second-order potential(Lee,1966; Kyozuka, 1980; Papanikolau,1987; Shimada,1987). (2) Without solving the second-order boundary value problem, to obtain from the radiated wave potential of double frequency and the first-order scattered wave potential by using the extended Haskind relation(Lighthill,1979; Molin,1979; Masuda,1989; Matsui,1990; Eatock Taylor,1987; Masuda, 1991). It is difficult to obtain an exact second-order diffracted wave potential satisfying the inhomogeneous free surface condition, and the approach (2) is superior when the wave load alone is considered.

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