Fluctuating Wind Induced Response of Double Hinged Articulated Loading Platform

Abstract

Wind and wave loadings have a predominant role in the design of articulated loading platforms (ALP) for its successful service and survival. Such platforms are very sensitive to the dynamic effects of wind, waves and currents. The compliant nature of these platforms with environmental loads introduces geometric nonlinearity due to large displacements, which becomes an important consideration in the analysis of these structures. In this study, dynamic behaviour of the tower under different wind spectra along with varying platform sizes is carried out. The exposed portion of the platform is subjected to the action of mean and turbulent wind, while the submerged portion is acted upon by random wave forces. The fluctuating component of the wind velocity is modeled using Emil Simiu’s wind spectrum while the sea state is characterized by Pierson-Moskowitz spectrum. Both correlated and uncorrelated wind and waves are considered in the analysis. Random wind and waves are simulated by Monte Carlo simulation technique. The drag force, either due to wind or wave is obtained by transforming the latticed portion of the platform into its equivalent drag diameters. For comparative studies of the ALP, responses under different wind spectra suggested by Kareem, Davenport and API-RP2A are employed. Furthermore, the analysis of the same structure under wind alone with buoyancy as a restoring force is also investigated to establish the severity among the events. The nonlinear dynamic equation of motion is derived by Lagrangian approach. The analysis includes the nonlinearities due to non linear drag force, fluctuating buoyancy, variable added mass and instantaneous tower orientation. The equation of motion is solved in time domain for incorporating the nonlinearities involved in the system by using Newmark-β integration scheme. The response study in terms of time histories of deck displacement, hinge rotation and hinge shear are presented. Also, their power spectral density functions (PSDFs) are plotted to highlight the wind induced dynamic response of the platform. Response time histories are further analyzed by statistical process under various parametric combinations. The outcome of the analyses establishes that the contribution of wind force in the platform responses is mainly governed by the size of the wind generated waves.