Stochastic response of tension leg platforms to wind and wave fields

Abstract

The dynamic behavior of tension leg platforms (TLPs) under the simultaneous action of random wind and wave fields is investigated in this study. Computationally efficient time and frequency domain analysis procedures are developed to analyze wind-wave-current-structure interaction problems. The aerodynamic load effects are described by the space-time description of the random wind field. The hydrodynamic loads are expressed in terms of a combination of viscous and potential effects. A numerically accurate and computationally efficient computer code based on the boundary element method (BEM) is developed for evaluating diffraction and radiat. forces. In the time domain, ARMA (autoregressive and moving average) and discrete convolution and differentiation, and a hybrid combination of these are utilized to generate the time histories describing wind- and wave-related processes and the resulting response. In the frequency domain, the concept of Hermite polynomial expansion of the nonlinear drag force is extended to describe the multi-directional drag forces in terms of bivariate and trivariate expansions correct up to the quadratic terms. A stochastic decomposition technique is developed which significantly enhances the efficiency of the frequency domain analysis of complex systems. A numerical scheme involving iterative and perturbation techniques is utilized to evaluate the second-order response statistics. The response of a typical TLP in six degrees of freedom shows excellent agreement between the time and frequency domain analyses. The influence of the various loading components on the TLP response is delineated. The sensitivity of the platform response to environmental loading conditions and the mechanical and hydrodynamic characteristics of the platform is studied. The frequency domain approach developed here retains the effects of nonlinear interactions and offers accuracy that is comparable to the time domain approach at a fraction of the computational effort. The utilization of the analysis procedure developed here is not limited to the analysis of compliant offshore structures, rather immediate applications exist for other nonlinear stochastic dynamic systems.