A concentrated force acting on the platform at the mean water level is used to represent the inundation effect of the variable wave surface. Its effects on the total and the maximum wave forces are investigated based on structural modal analysis and linear wave considerations. The second-order transfer function of the Volterra/Wiener representation for the nonlinear wave force, with the inundation effect included, is derived. The power spectral density functions of the responses of two typical jack-up platforms are evaluated numerically to show the significant contribution of the inundation effect on the dynamic responses of the structures.
Considerable works have already been carried out by many researchers on the study of nonlinear wave forces on jack-up platforms. It is commonly accepted that nonlinear dynamics has significant effect on the responses of many jack-up structures, and there are mainly two nonlinear effects attributable to wave forces. One is the nonlinear drag force, which is usually the predominant wave force component for the slender structural members of a jack-up platform and can be evaluated using the well-known Morison formula. The other important nonlinear effect can be related to the variable submerged length of structural members near the free water surface, or the inundation effect. The common practice for including the nonlinear effects in dynamic analysis is to perform time-history simulations of the structural responses. In order to have statistically meaningful results, many such time-consuming simulations are usually required. A more direct approach is, of course, to carry out the stochastic analysis in the frequency domain. However, nonlinear analysis in frequency domain is more involved than the normal approach based on linear assumptions. One well-known method of modelling nonlinear systems in stochastic analysis is the Volterra/Wiener representation based on the Volterra series (Schetzen, 1980; Rugh, 1981)
