Nonlinear Dynamic Analysis of Offshore Towers in Frequency Domain

Abstract

A frequency domain iterative method is proposed to solve the non‐linear dynamic equation of motion encountered in the dynamic analysis of offshore structures under regular and random waves. The nonlinearities due to the relative velocity‐squared drag term and variable submergence are considered and treated using an approximate Newton‐Raphson method. In order to facilitate the computation, the formulation makes use of the normal mode theory. The resulting iterative scheme, which introduces a modal hydrodynamic damping on both sides of a decoupled equation of motion in modal coordinates, is shown to be highly efficient for the analysis of fixed base offshore structures. The numerical study shows that only 3–5 iterations are required to obtain the converged solutions for hydrodynamic loadings on the structure produced by random and regular waves. The computational time is marginally greater than that required for the linearized methods which in certain cases may introduce considerable error in the results.