Nonlinear frequency domain analysis of flexible offshore structures using volterra series

Abstract

Nonlinear behavior of flexible offshore structures governed by Morison's equation is studied in frequency domain through an application of higher order transfer functions. Nonlinearities included in the analysis are the drag nonlinearity considering relative velocity between wave and a structure and the additional effect of current. Nonlinearity in the equation of motion of the syste em is expanded in polynomial series by using statistical method, the polynomially expanded equation converted to sequentially ordered differential equations using Volterra series and the nonlinear transfer functions acquired by transforming these equations to fequency domain. The characteristics of the nonlinear transfer function models are investigated through the analysis of the nonlinear re sponse of SDOF system subject to harmonic wave input. The response spectrum analysis is also performed using the 3rd, 5th and modified 3rd order transfer function model and the reliability of the modified 3rd order model proposed in this study is presented through numerical examples.