Research On Optimal Thrust Allocation Method For Ship With Low Speed And Overdrive

With the scarcity of land resources,marine resources have become the main target of competition among countries.The exploration and development of Marine resources put forward higher requirements for ship performance.As the core of Marine thrust system,thrust allocation algorithm is essential for improving the accuracy and stability of the ship’s offshore operations.Optimal thrust allocation in station-keeping has been addressed previously,while simultaneous thrust and rudder allocation for ships at zero speed has received less attention.Since operating a rudder servo is relatively inexpensive compared to operating a propeller,this gives a large fuel saving potential.This solution is well known in practice since experienced captains use a nonzero rudder angle(for positive thrust)in station-keeping and docking to produce additional forces in the transverse direction.In this paper,a constrained rudder/propeller control allocation algorithm is derived in conjuncture with a Lyapunov stable output feedback controller for low-speed tracking,combining ship maneuverability theory and control theory.The overdrive means that the total number of control parameters is greater than the dimension of freedom of ship motion.Due to the existence of overdrive,the conditions for optimal thrust distribution are provided.This paper uses an analytical two-norm optimal algorithm,which combines the matrix theory and the optimization principle to ensure the continuity of the solution and which has certain practical significance.First of all,this paper introduces the principle of ship maneuverability,which including the ship motion coordinate system,the establishment of the mathematical model of ship maneuvering motion,the derivation of ship motion parameters,and the conversion of model between ships with real ships.Secondly,the paper introduces the method of optimal thrust distribution in detailed,which including the ship type coefficientmatrix,the unconstrained solution,the feasible solution domains,the anti-shake,the guaranteed continuity,and the transfer of the optimal solution.The paper next introduces the automatic control system,which including automatic control theory,control stability theory and ship controller design.Finally,according to this method,the ship thrust distribution simulation model is built in the MATLAB/Simulink simulation environment,and the test is carried out for different offshore operations.The results show that the method can effectively complete the thrust distribution of the controller thrust command.