Nonlinear Structural Response in Random Waves

Abstract

A simple single degree of freedom model of an offshore structure is used to study the effects of the nonlinear drag force on the structural response in random waves. Both random waves with a zero mean velocity and with a steady velocity component are considered. The fluid force is assumed to be given by the relative velocity formulation of the Morison equation which couples the fluid force to the structural response. A new perturbation method is formulated to obtain the mean square response and the response spectral density. This method is shown to be closely related to the method of equivalent linearization and represents a consistent improvement to the linearized solution. Also, numerical simulations are performed to assess the accuracy of the perturbation method.