Second-Order Superharmonic Wave Loading on Jack-Up Platforms

Abstract

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 that a concentrated force acting at the mean water level of the structure can be used to represent the effect of the variable wave surface. The nonlinear transfer function of the modal wave force, with the inundation effect included, is derived. Frequency-domain analyses of the nonlinear wave force and structural response of a jack-up platform, modeled as a linear structure, are carried out to show the effect of inundation. The response spectral density functions obtained from the frequency-domain analysis are also compared with the spectra of time-domain simulations based on wave kinematics predicted by the Wheeler stretching and vertical uniform extrapolation methods.