Short-Term Extreme Response Prediction for a Jacket Platform Subjected to Nonlinear Wave Loads

Abstract

Abstract For bottom supported jacket platforms, the wave loads are generally nonlinear due to the drag-term in the Morison equation. As a result, the associated dynamical response for the offshore platform, such as the overturning moment and the base shear force, exhibit pronounced nonlinear characteristics. In this work, the dynamic response of the jacket platform is described by a single-degree-of-freedom (SDOF) model. The nonlinear wave load in the SDOF model is modeled as a polynomial of a filtered white noise. Subsequently, by application of the linear filter technique, the SDOF model is extended into a four-dimensional (4D) Markov dynamical system, whose probabilistic properties are governed by the corresponding Fokker-Planck equation. The path integration (PI) method is applied in order to solve the Fokker-Planck equation by taking advantage of the Markov property of the dynamical system. The response statistics calculated by the PI method are validated by Monte Carlo simulation (MCS). The statistical characteristics of the response for the jacket platform subjected to nonlinear wave loads are determined by means of numerical simulations. For the selected sea state, the mean upcrossing rates of the dynamical responses, i.e. the overturning moment and the base shear force, are employed to obtain the extreme responses of the jacket platform.