Dynamic Positioning For Shuttle Tankers

As ocean resources are developed continuously in deeper ocean, floating offshore structures are frequently deployed. A typical example is the turret-moored FPSO (Floating Production Storage & Offloading). In such a case, the produced crude oil is normally transported by a fleet of shuttle tankers. For the sake of safety, the shuttle tanker during offloading must maintain its position within a certain limit under the action of external forces with the help of conventional mooring system and/or Dynamic Positioning (DP) system. As the water depth increases, the conventional mooring system becomes ineffective and thus the DP system is recommended. In this paper, two important problems are discussed in relation with the design of the DP system. One is to reduce the excessive thruster modulation caused by wave-induced motions and the other is the thrust allocation. The thruster modulation can be reduced by implementing a Kalman filter for the estimation of the low frequency state vector by removing the wave frequency motion from the measured variables. Herein a static algorithm for thrust allocation is suggested in order to improve the thruster performance. The DP controller is designed based on the optimal control theory and also the robust ∞ H control theory. In order to exemplify the process, the nonlinear behavior of shuttle tankers is simulated, where the hydrodynamic forces are calculated by the Green's function method, while the restoring force of mooring lines is approximated by the catenary equation in a quasi-static fashion. Based on numerical simulations, the capability of the designed DP controller and the thruster modulation are examined. But it is found that the control command contains a certain amount of wave frequency components. Thus a more effective filter needs to be developed to improve the performance of the DP system.