Nonlinear Diffraction Of Water Waves:A Classical Perturbation Method

This paper deals with the classical theory of the perturbation method to determine the solution of wave forces on a circular cylinder in regular waves. Nonlinear diffraction of water waves is considered here to demonstrate the powerful perturbation technique. The solution obtained by this method is compared with the available experimental data. It is found that the analytical solution agrees with the experimental data quite well. When we talk about wave effects on offshore structures, it is necessary to distinguish between small and large dimensions in relation to the characteristic wavelength and the wave amplitude. It is well known that the Morison equation displayed an empirical relationship in terms of the coefficient of mass, CM , and of the coefficient of drag, CD , the two hydrodynamic coefficients used to calculate wave forces on a small submerged cylinder. This relationship involves the inertia force and viscous drag force on the cylinder and assumes that the object is small so as not to disturb the incident wave field. However, as the diameter of the cylinder becomes large compared to the incident wavelength, the Morison equation does not apply and a diffraction theory must be used. In this case, viscous drag forces are assumed to be insignificant for smooth dimension structures (cylinders) and the inertia forces predominate. In this paper we discuss the theoretical formulation of second order wave loads. We have obtained the mathematical expressions for the forces and moments to predict the wave loads on large monolithic offshore structures.