Use of anchor pattern modelling in the control of floating production vessels

Floating production, storage and offload (FPSO) vessels are increasingly being used instead of conventional oil platforms. These are tanker sized vessels moored permanently on station using large anchor patterns with typically eight anchors. Oil is drawn up from a well head on the sea bed via a flexible riser and rotating turret. FPSOs use their anchor pattern as the primary location system, but additionally they are fitted with thrusters, which in conjunction with a thruster assisted mooring (TAM) control system are used to provide: increased operational time by improving vessel stability in extreme weather conditions; increased safety of operations; rig acceptance approval by Maritime authorities; and control for shuttle tanker and supply vessel operations. The TAM control system operates on the basis that the anchors, as a rule, act as a tight proportional controller. The relative sizes of the vessel and the thrusters are such that the control system cannot provide full dynamic position control. The paper looks at the development of a TAM controller from a standard ship positioning system