Skipping Dynamic Forbidden Sectors in Thrust Allocation for Marine Vessels

摘要

Thrust allocation logic is one component of dynamic positioning system, which allocates the thrust force generated by the control law among the thrusters. Generally, forbidden sectors are set in thrust allocation to prevent thrusters from entering certain azimuth angles in order to avoid large thrust loss on downstream thrusters. Thus, the azimuth thruster can only be operated in the feasible zone which is set at the first place or can be operated in one direction when the azimuth angle is near the forbidden angle. The reduction of the feasible azimuth thrust region may affect the ability of the whole thrust system. In this paper, a novel forbidden sector skipping strategy is proposed. The strategy is based on dynamic forbidden sector concept. Conventional thrust allocation logic is improved by accounting for skipping strategy and the dynamic forbidden sector. The forbidden angle can be real time adjusted according to the propellers' speed ratio. When the forbidden angle becomes small enough, the thruster may skip the forbidden sector. A numerical simulation for a barge was performed to demonstrate the performance of the skipping strategy. The results demonstrated that the employment of the skipping strategy can give the azimuth thruster more flexible zone and can guarantee the thrust allocation more reasonable. The strategy can also assist DP system in saving fuel and higher safety.