Research On The Control Allocation For Over-actuated Marine Vessels

The requirements on the performance of propulsion system for ships and ocean structures become increasingly stringent as the oil and gas exploration and exploitation moving into deeper waters and harsh-environment locations. The majority of the submersible drilling rigs, drilling ships, supply vessels, and shuttles are equipped with redundant thrusters and propellers making them full-actuated or over-actuated, which is the best option for the deepwater operations. Hence, it is of vital importance to make a reasonable, efficient use of the redundant propulsion device, and establish a comprehensive allocation management system of control allocation algorithm, diversity operation modes, the variation of marine environment condtions and thruster faults. It can be provided with theoretical research significance and practical value for the safety, economy and reliability to marine operations for over-actuated vessels.The over-actuated control allocation problem of marine vessels and its solving model for optimal solution are formulated. Both the actuator constraints and power constraints of control allocation for over-actuated marine vessels are analyzed, and the feasible regions for different types of actuators are presented, which is the theory fundamental for the following chapters.Due to the fact that the iterative computation of math programming methods is so large that it may fail to guarantee the real-time implementation of engineering applications, hence, the thesis focuses on the generalized inverse solutions for control allocation of over-actuated marine vessels. Different generalized inverse solutions are proposed based on specified thruster configuration matrix accounting for the various thruster configurations of marine vessels. To overcome the difficulty that the generalized inverse method can not handle the actuator constraints, several re-optimization strategies are discussed for generalized inverse solution. Since the solutions to power optimal control allocation problems tends to singular configurations, the generalized inverse solution based singular value decomposition was proposed, which detect and avoid the happening of singularity configuration by monitoring the minimum singular values of thruster configuration matrix under the optimal thrust orientation. Inspired by the singular value decomposition, design considerations on thruster configuration of the marine vessel and thruster planning on the power bus by use of minimum singular value are presented, which is validated on the thrusters system of an over-actuated drilling rig.Traditional thrust allocation always adopts one fixed solution during the long-time operation without considering the changes in the operational and environmental conditions as well as the actuator state. Multi-mode switch control with supervisor is adopted to establish the switched control allocation architecture, where the supervisor make a decision about the automatic switch among different allocators after taking into account the operational mode of system, the ocean environmental condition and the actuator state. Marine vessels operating in different speed regimes and different environmental conditions necessitate different performance criterion of the propulsion system, fixed and variable azimuth allocation solutions are both designed for the marine vessels that equipped with rotated thrusters, which makes the rotated thrusters can operate within specified regime according to the changing of system modes and sea states.To enhance the fault tolerant capability of marine control system in case of faults in propulsion system, the analysis of fault modes and effects of ship propulsion system are conducted. Several thrust re-distributed allocation strategies are presented based the generalized inverse solution according to the different degrees of fault on the actuators by punishing weights of fault actuators, changing the thruster configuration matrix and adjusting the actuator constraints. Fault reconfiguration simulations are carried out with thrust and azimuth fault on the actuators. Fault Modes and Effects Analysis (FMEA) are implemented on the thrusters system of a model drilling rig. Since it is difficult to accurately assess the risk rating of fault modes for a actual system, fuzzy sets are used to depict the occurrence of the fault mode, the severity of the fault model and the detection response of the system to the fault, moreover, fuzzy weighted factor is introduced to assess the risk priority number of the specified fault mode. And the application of grey related theory for the risk assessment of fault mode is discussed.