Response of articulated tower platforms to random wind and wave forces

Abstract

The displacement response of an articulated tower is mainly governed by its rigid body mode of vibration which has a very low frequency. Since the wind velocity spectrum of the fluctuating component has a low frequency energy content, the wind-induced vibration of an articulated tower may be significant. The combined wind-, wave- and current-induced vibration of the tower is an important phenomenon to study because of the complex hydrodynamic interactions. In this paper, the response of a model articulated tower to random wind force alone and to the combined effect of wind, wave and current forces is investigated. Only the first-order forces due to the waves have been considered. The response of the tower is determined by a frequency domain iterative method and is evaluated for a parametric study. Both wind velocity and random sea are characterized by Power Spectral Density Functions. A Monte-Carlo simulation procedure is used to obtain sample records of time histories of both the wind velocity and water particle kinematics. The response analysis considers the nonlinearities due to writing the equilibrium equation based on the instantaneous position of the tower and relative-velocity squared drag force. Using this method of analysis a parametric study is conducted to investigate the behaviour of the tower under different important factors which include the pressure drag effect, the effect of nonlinear drag on both wind and wave forces, the mean wind velocity at the reference height, the current velocity and different combinations of wind, wave and current forces.