ABSTRACT

A numerical scheme was developed to predict nonlinear diffractions acting on a floating structure by weakly nonlinear irregular waves. The Stokes weakly nonlinear irregular waves were firstly fed into the wave tank through the inflow boundary and then the scattered wave fields due to the structure were computed by solving the boundary integral equation in the time stepping procedure. The integral equation was approximated using a quadratic-order boundary element method. The instantaneous free surfaces was integrated in time using the fourth-order multi-step method. Open boundary condition was treated by combining two schemes of absorbing beach and stretching. The proposed numerical scheme was verified by comparing linear results with theory and then applied to compute the diffractions acting on a truncated vertical circular cylinder by inputting the linear and nonlinear irregular waves. Additionally, the numerical scheme was examined by the balance check of the momentum-impulse.

INTRODUCTION

An improved prediction technique of the extreme wave load gives a reliable data for design of the offshore structures. Linear irregular waves are commonly used in statistically computing the wave load. A method for estimation of the kinematics of the irregular waves was developed by a kinematic boundary condition fit (Forristall 1985). However, natural ocean waves are known to be nonlinear in the storm seas so that their crest heights are bigger than trough depths. Thus, weakly nonlinear irregular waves may exist in the severe seas due to the interaction of the waves. Longuet- Higgins(1963) and Dean&Sharma(1981) developed the Stokes second-order irregular wave model for deep and finite water depth, respectively. Pierson (1993) proposed a theory for the Stokes third-order nonlinear irregular waves. The third order model was developed using third-order perturbation scheme and can treat the long-crest seas in the deep water.

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