VOLTERRA MODELS OF OCEAN STRUCTURES - EXTREME AND FATIGUE RELIABILITY

A new method is developed to estimate the reliability of nonlinear str uctural systems, modeled with second-order Volterra series, against bo th extreme and fatigue failures. It includes non-Gaussian response eff ects 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 spect ra. Rates of extremes and fatigue damage are then estimated analytical ly from a non-Gaussian (Hermite) random process model. The method is a pplied to study the pitch and heave ''springing'' motions of a tension leg platform (TLP) under random wave loads. State-of-the-art nonlinea r 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 tensio ns, platform motions, and accelerations are estimated. These results a re weighted over probabilistic models of the long-term wave climate to estimate reliability against extreme and fatigue limit states.