Investigation of slowly-varying drift motion and springing and ringing of tension leg platform system in nonlinear irregular waves

In this dissertation, two of three different response modes of a TLP, namely the low frequency mode (slowly-varying drift motion), wave frequency mode, and high frequency mode (springing and ringing), have been investigated systematically. Statistic analyses of nonlinear wave loads and TLP response are also performed.; The first objective was to compute wave drift damping by higher-order boundary element method and investigate its effect on the response of a TLP. The second objective was to compare different approximate methods for computing low-frequency loads and studying their effects on the low-frequency responses. The third task was to find the characteristics of springing and ringing and determine the kind of waves that create springing and ringing and to evaluate the effect of large asymmetries in the laboratory-generated storm seas. The final objective is to study the nonlinear and non-Gaussian effects on the prediction of mean TLP offset and tendon fatigue life.; The present investigation shows first, there is almost no effect on prediction of mean TLP offset including wave drift damping. Second, Newman's approximation can be applied to compute low-frequency loads on TLP without significant error. Third, springing is due to weak asymmetric waves while ringing is due to strong asymmetric waves. And finally, there are strong effects on tendon fatigue life prediction if considering nonlinear and non-Gaussian factors in certain periods.