Control System Design for Vessel Towing System by Activating Rudders of the Towed Vessel

Abstract

In recent years, the development of control and measurement technologies with direct application to the marine field has increased. Since the early 1990s, several control methods have been applied to solve ship motion control-related problems. Among them, the issue of controlling the path of the ship at low or constant speed has got a lot of attention. Moreover, studies on ship motion control systems led to improve dynamic positioning systems and enabled the construction of modern floating offshore installations such as floating production storages and offloadings (FPSOs), drill-ships with multiple azimuth-type propellers, etc. The earliest studies focused mainly on maintaining ship controllability in open sea conditions, however, more sophisticated ship motion control strategies are necessary for confined water conditions during harbor operations. At sea, non-powered ships (such as barges) are used frequently to transport cargos. A barge-type vessel, for instance, is dragged by tugboats to transport large structures from the shipyard to other locations. The combination of the non-powered ship and the tugboats is referred to as the Vessel Towing (VT) system. VT systems are used mainly in canals, rivers, and busy harbor. Towed ships are usually equipped with a propulsion system and active control devices, however, they cannot be used in narrow spaces due to safety issues. Therefore, because of the loss of maneuverability, it is difficult for the towed vessel motion to be controlled - following the tugboat’s movement route. However, severe accidents may occur. Especially, in harsh sea conditions, the uncontrolled vessel may crash into the tugboat, thus safe maneuvering may be impossible. Moreover, the possibility of collision of the towed vessel with other ships may increase if it deviates from the path of the tugboat. The restoring moment and the minimum resistance are conditions that ascertain the towed vessel’s stability. If the restoring moment is larger than the hydrodynamic yawing moment, the towed vessel is hydrodynamically stable. Needless to say, the towline tension corresponding to ship resistance should be larger than a critical value. To preserve the system’s stability and achieve control performance, this dissertation presents the development of positioning and trajectory tracking controllers for the VT system by activating rudders of the towed vessel. By installing rudders at the rear of the towed vessel, the restoring moment is produced due to the lateral hydrodynamic force when the drift angle occurs. Without a doubt, if these rudders are controlled effectively, the stability and performance of the system can be guaranteed. These goals are attained through the following steps. Firstly, the system description and mathematical models of the VT system are presented. The configuration of the VT system consists of mechanical and electrical structures designed accordingly. In the VT system, the towed vessel is a non-powered ship with several rudders installed at the rear to improve the maneuverability. The system modeling of the VT system consists of a kinematic model and a dynamic model. Secondly, several experimental studies are conducted in order to estimate the hydrodynamic coefficients of the vessels. For simplicity, it is assumed that the tugboat and vessel have homogeneous mass distribution, xz and yz are symmetry planes, and the center of gravity coincides with the mass center. Using the experimental results, the unknown parameters in the hydrodynamic inertia matrix and damping matrix are calculated. Besides, the effect of unknown parameters in the inertia matrix and damping matrix is evaluated by simulation tests using the newly defined parameters. Simulation results pointed out that the distance error is tiny and can be ignored. Therefore, the control performance with the presence of unknown parameters is validated through simulation tests. Thirdly, an extended state equation of the towed vessel is developed and presented. Then, a state feedback controller of the VT system based on the servo system configuration is designed and implemented. However, there are a variety of states that cannot be measured directly for the feedback. Therefore, the linear state observer is designed and implemented. The controller and observer gains are obtained by a numerical method, the linear matrix inequality (LMI) optimization technique. The effectiveness of the proposed control system and the state estimator is verified by numerical simulations. Using the proposed configuration, the system stability, the control performance, and the optimization problem are guaranteed. Finally, nonlinear dynamic responses of the VT system are investigated. The motion of the towed vessel is described by a nonlinear model derived from the relationship between the towing tugboat and the towed vessel. Based on the leader-follower system configuration, a nonlinear mathematical model is obtained and a back-stepping controller can be designed. Simulation and experimental studies are compared so that the effectiveness and usefulness of the proposed strategy are validated. The experimental studies are performed in two critical conditions: the first is when the towed vessel motion is tested independently on the flow water, and the second is when the VT system is evaluated in towing operations. The results showed good performance and an undemanding applicability of the proposed control strategy.