TABLE OF CONTENTS One main challenge in offshore engineering is the ability to precisely and efficiently predict the wave forces acting on the structures and the resulting structural responses, accounting for inherent and environmental variabilities. Fixed platforms such as jacket and jack-up rigs are mainly subjected to Morison-type wave loads acting on their slender members. The difficulty in prediction of forces and responses, and estimating their stochasticity lies in nonlinearities due to the nonlinear force-kinematics relationship and/or the nonlinear nature of waves.
In practical engineering analysis and design, linear wave theory has been popularly accepted as a basis for modeling the short-term random wave storm, where the wave elevation can be adequately modeled as Gaussian processes. Despite this, the structural response shows deviations from Gaussianity, which can be attributed to three main nonlinear effects, two associated with drag forces and the third with wave inundation. The non-Gaussian response behavior will be more evident if nonlinearity of waves is considered. Nonlinear random wave based on Stokes theory is non-Gaussian in nature.
The first nonlinear effect of wave loading arises from the distributed nonlinear drag force which can be evaluated using the well-known Morison equation. This is usually the predominant wave force component for the slender structural members of a fixed platform and is the main cause of the so-called superharmonic phenomenon.1 In the absence of current, the power spectrum of the wave force exhibits sharp peaks at odd-multiples of the peak wave frequency ? p. If current...
