Stochastic response of a tension leg platform to wind and wave fields

The dynamic behavior of tension leg platforms (TLPs) under the simultaneous action of random wind and wave fields was investigated in this study. Computationally efficient time and frequency domain analysis procedures were developed to analyze wind-wave-current-structure interaction problems.; The aerodynamic load effects were described by the space-time description of the random wind field. The hydrodynamic loads were 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) was developed for evaluating diffraction and radiation forces. In the time domain, ARMA (autoregressive and moving average) and other recursive models were 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 was extended to describe the multi-directional drag forces in terms of bivariate and trivariate expansions up to the quadratic terms. A stochastic decomposition technique was developed which significantly enhances the efficiency of the frequency domain analysis of complex system. Central to this technique is the decomposition of a set of correlated random processes into a number of component random processes. Statistically any two decomposed processes are either fully coherent or noncoherent which reduces the effort involved in computing the cross-spectral density matrices. Depending on the system, a random component process may be expressed in terms of a decomposed spectrum or a bispectrum. A numerical scheme involving iterative and perturbation techniques was utilized to evaluate the second-order response statistics. The response of a typical TLP in six degrees of freedom showed excellent agreement between the time and frequency domain analyses. The influence of the various loading components on the TLP response was delineated. The sensitivity of the platform response to environmental loading conditions and the mechanical and hydrodynamic characteristics of the platform was 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.