Frequency-Domain Analysis of Offshore Platform in Non-Gaussian Seas

Abstract

A frequency-domain solution approach for the response of a system whose inputs are nonlinear transformations of non-Gaussian (nonlinear) wave kinematic processes is introduced. Particularly, this paper compares the probabilistic response characteristics of jacket-type platforms in deep water that are subjected to both Gaussian and non-Gaussian random wave loadings. Unlike earlier analytical treatments of this class of system, a statistical description of the wave forces is first developed to reflect nonlinearities and associated non-Gaussianity in the wave field kinematics. The kinematics are derived from Laplace's equation and nonlinear boundary conditions using a second-order Stokes' perturbation expansion. The deck response resulting particularly because of the effects of the second-order contribution to the loads on an idealized platform is computed. Consideration is given to the importance of the spacing of the legs to the response of the structure. The impact of swell in addition to locally wind-generated waves also is assessed. Ignoring the nonlinearity of the waves results in underestimation of the response level for all scenarios considered.