The methods of structural reliability, taking uncertainties related to both loading and strength into account, have been applied to mooring system design. Both the ultimate line tension and the fatigue failure mode are considered. One of the major concerns of the methodology is the combination of different response components, such as those due to low-frequency wind and wave motions, and wave-frequency wave motions. Different combination strategies have been compared. Efforts have also been devoted to obtaining a systems formulation for a single mooring line which accounts for correlation between individual components properly. The example application is concerned with a turret-moored vessel with 12 mooring lines.
Mooring systems for floating production systems can either be single point mooting or spread mooting. Single point mooring tends to be used more frequently for tankers and turret-positioned ships, while spread mooring is used mostly for semi-submersibles. A third type of station-keeping system is dynamic positioning, which often is used to assist catenary mooring. In this paper only a passive turret-positioned vessel is considered. Mooring lines for permanently moored vessels may be made up of chain links, steeI wire ropes, synthetic ropes, or a combination. In addition, clump weights or buoys may be added to achieve given mooring performance requirements. Connecting hardware such as shackles, swivels, triangular plates, sockets, and detachable links are used to connect together the main mooring line components. These components are however not considered to be critical for mooting systems, and is thus disregarded in the present study. Mooring lines and mooring systems are commonly designed according to design codes, where uncertainties in applied loads and mooting line capacities are taken into account through safety factors or utilisation factors. A drawback with these codes is however that they may give different failure probability levels for different mooting systems.
