In the design and analysis of multi-leg mooring systems, it is significant, but difficult, to determine the tensions in the mooring legs. The analysis is complicated primarily because of the nonlinear behavior of the cables. The equilibrium equations of the moored body are indeterminate if the number of mooring legs is more than the number of unconstrained degrees of freedom being considered at the moored body. In the presence of spatially variable sub-surface currents, it is generally not appropriate to approximate the cable behavior by catenary equation since the current-induced drag forces are both position- and orientation-dependent. A method based on direct spatial integration will be demonstrated for the nonlinear static analysis of three-dimensional multi-leg mooring system response to steady currents.
A multi-leg mooring system, which is comprised of cables, anchors and the moored body, provides efficient restrain in all directions to the moored body by transmitting the forces on the cables and moored body to the anchors. Such a system has been employed in a wide variety of applications in the ocean (Nordell and Meggitt, 1981; Knapp, 1987). A few examples of multileg mooring systems include the systems used to restrain tension leg platforms and guyed towers for deepwater oil operations. A state-of-the-art review regarding the behavior of cables as mooring system components, the types and selection of cables, and the various classes of anchors and their applications was presented by Skop (1988). In the design and analysis of multi-leg mooring systems, it is significant, but difficult, to determine the tensions in the mooring legs. The analysis is complicated primarily because of the nonlinear behavior of the cables (Leonard, 1988). Due to this reason, some analyses of multi-leg mooring systems are based on the mathematical programming, i.e. optimization schemes.