The various excitation sources that have to be considered in design of mooring systems for structures subjected to wind, wave and current loading are reviewed. Furthermore, a state of the art review of mooring system design methods are presented. Wave drift forces and slowly varying wave forces are important excitation sources and are given special attention.
A better understanding of the effect of slowly varying forces in design of mooring systems is of great importance for a safe design. A numerical model for non-linear dynamic time domain analysis of mooring systems capable of handling the real time varying nature of such forces has been developed. This model is used in sample calculations and compared with quasi-static calculations in order to study the effect of slowly varying wave forces in particular. Recommendations on the applicability of the different methods are given.
The response of mooring systems subjected to wind, wave and current loading is particularly complicated because 61" the great influence of nonlinear effects. Wave excitation forces and hydrodynamic reaction forces may be non-linear due to large wave amplitudes, viscous effects and second order terms in the wave pressures (wave drift forces). Other non-linearities are associated with the hydrostatic restoring forces and the load-excursion characteristics of the mooring system itself.
The most commonly used method in analysis of mooring systems is a simple quasi-static procedure not capable accounting for the non-linear effects in a realistic manner. It is neither able to handle the true time-dependent nature of the exciting loads nor the dynamic response of the system. However, the quasi-static method has despite of its shortcomings, served as a useful engineering tool for many years.
When moving into deeper water it becomes increasingly important to be able to predict the excitation loading and the dynamic response of the system more accurately. This can only be achieved by applying a dynamic time-domain solution of the equations of motions. In this manner the effect of the non-linearities mentioned above, particularly the role of the slowly varying wave drift force, can be evaluated.
It should be emphasized, however, that the slowly varying wave drift force is only one of many sources that may excite a moored structure and result in oscillatory motions and high mooring line tensions. A brief review of various exciting sources will therefore be given in order to put this effect in a wider perspective.
The main excitation sources for moored structures at sea are the environmental actions. Each of the environmental factors wind, waves and current may produce motions of the moored object and tensions in the mooring lines. Acting together they may produce large oscillations not predictable by studying the effects of individual components. The behaviour of moored objects is also highly influenced by the characteristics of the mooring system itself.