The mooring line dynamics is modelled using the finite element method, and its non-Gaussian response property is investigated using the time domain method. By comparison to the time domain (TD) approach, the application of the hybrid time-frequency domain (HTFD) fatigue analysis approach using correct modelling of tension ranges and counting of tension cycles is then studied. The study leads to the proposition of a modified HTFD approach to improve the reliability of line fatigue life prediction.
Mooring line failures have frequently occurred in the past. The line fatigue could have played an important role, especially for long term mooring operations. In this paper, the aspects of line tension response modelling and fatigue life prediction have been studied. Appropriate analytical techniques for reliable prediction of line's fatigue life. Following reviews of relevant techniques and line nonlinear characteristics, the dynamic response of catenary mooring line is modelled using the finite element method, which accounts the nonlinearities due to the fluid loading, geometry and bottom boundary condition. These nonlinearities are retained in the solution by using the time domain approach. The ocean environment is represented by a number of sea states, and the line tension response statistics in each sea states is studied. The study addresses the non-Gaussian response characteristics. It is shown that, due to the nonlinearity, the line tension response is non-Gaussian and the Rayleigh distribution fails to model the tension ranges accurately. It has also been shown that significant improvement can be made by using the Weibull distribution. Though the fatigue of the nonlinear line can be dealt with by the time domain (TD) approach, it requires large amount of computation time. One possible alternative is the HTFD approach. Through comparative study between the HTFD and TO approaches, the application of the HTFD approach is studied.