Controlling line tension in thruster assisted mooring systems

This paper addresses the potential for energy reduction obtained by using dynamic line tensioning in thruster assisted position mooring systems. Traditionally, mooring systemsd have been designed in such a way that thruster assistance has not been necessary under normal environmental conditions. However, as oil production moves into deeper waters, such overdimensioned mooring systems are no longer feasible. Thus, new "hybrid" solutions must be developed, in which increased thruster action compensates for fewer, and lighter, anchor lines. In this paper, controlling the line tensions dynamically is suggested as an additional means of station keeping, and a control law is derived based on passivity. A model consisting of a rigid-body submodel for the vessel, and a finite element submodel for the mooring system is presented and used for simulations. The simulations show the performance of the proposed control system.