Volterra Models of Ocean Structures: Extreme and Fatigue Reliability

Abstract

A new method is developed to estimate the reliability of nonlinear structural systems, modeled with second‐order Volterra series, against both extreme and fatigue failures. It includes non‐Gaussian response effects for a given input spectrum as well as long‐term variations in the parameters of this input spectrum. Moment influence coefficients are introduced to give response moments for arbitrarily scaled input spectra. Rates of extremes and fatigue damage are then estimated analytically from a non‐Gaussian (Hermite) random process model. The method is applied to study the pitch and heave “springing” motions of a tension leg platform (TLP) under random wave loads. State‐of‐the‐art nonlinear diffraction results are used to model the second‐order loads on the TLP. These forces are then applied to a coupled six‐degree‐of‐freedom (6DOF) linear structural model of the TLP. Statistics of tether tensions, platform motions, and accelerations are estimated. These results are weighted over probabilistic models of the long‐term wave climate to estimate reliability against extreme and fatigue limit states.