STOCHASTIC FATIGUE OF NONLINEAR OFFSHORE STRUCTURAL SYSTEMS.

Nonlinearities in the wave loading due to nonlinear wave kinematics and free surface fluctuation are considered for a jacket-type platform. The first four moments of the response are estimated using the mean square estimation technique via conditional distribution. Nonlinear stiffness is considered for a guyed tower system. Approximating the loading by the ARMA process, Ito stochastic differential equations for the response moments are solved up to fourth order where the system of equations is closed by neglecting the fifth and higher order cumulants. The response moments are considered to be a mixture of Gaussian and non-Gaussian distributions. By mapping a Gaussian process into this response process, the expected rate of positive crossings is estimated, leading to the probability density of the peaks. Palmgren-Miner's hypothesis for fatigue damage accumulation is used. It is shown that the conventional method is unconservative when the response distribution is leptokurtic.