Second-order superharmonic wave loading on jack-up platforms

Different types of second-order superharmonic wave forces acting on jack-up platforms are studied. The nonlinear effect of variable wave surface (i.e. , the inundation effect) is found to be the most significant. It is shown t hat a concentrated force acting at the mean water level of the structure ca n be used to represent the effect of the variable wave surface. The nonline ar transfer function of the modal wave force, with the inundation effect in cluded, is derived. Frequency-domain analyses of the nonlinear wave force a nd structural response of a jack-up platform, modeled as a linear structure , are carried out to show the effect of inundation. The response spectral d ensity functions obtained from the frequency-domain analysis are also compa red with the spectra of time-domain simulations based on wave kinematics pr edicted by the Wheeler stretching and vertical uniform extrapolation method s.