A Study on Vessel Motion Control

Abstract

In the vessel motion control, ship berthing has been known as one of the most complicated and challenging procedures. The difficulties for berthing operation in the harbor could be outlined as the reduction in controllability at low speed, the effect of changes in hydrodynamic coefficients on ship handling, the large effect due to environmental disturbances, etc. Usually, tugboats should be used to assist it by pulling and pushing the vessel. Another difficult task that vessel motion control had to face is the station keeping where constant of slowly-varying disturbances such as mean wind forces, mean currents and tidal currents need to be rejected. This is entirely handled by the positioning mooring (PM) system. Besides some vessels such as barges, disabled ships, semi-submersible rigs, drill ships, etc. could not move by themselves and need to be towed by tugboats. In any case, rope is used as an indispensable facility, and rope tension control is the key point. In order to precisely and safely control the vessel’s motion, the dynamic characteristics of the towing rope should be well examined. This dissertation presents further study on modeling dynamics of towing rope system and applying rope tension control strategy on vessel motion control. A control-oriented dynamic model of a towing rope system with variable length is established. In this simple model, a winch which is driven by a motor torque uses the towing rope to pull a cart. The rope is modeled as a straight and massless segment, and mass of rope is lumped partly to the cart and halfway to the winch. In addition, the changing spring constant and damping constant of towing rope are considered when the winch winds in. Finally, a reduced-order observer-based servomechanism controller is designed, and the overall system is verified by computer simulation. Next, a rope tension control system is designed and implemented. A family of dynamic models of towing rope system are identified by changing the rope length. Among them, a representative model is selected, and the others are considered as uncertain models. Then, this works designs a μ-synthesis based on two-degree-of-freedom (2DOF) robust control system to cope with strong uncertainty and nonlinear property included in the real plant. An experimental comparison is conducted between the proposed control method and existing proportional–integral (PI) control method. The results indicate that the proposed method is more efficient and useful than the conventional one. The problem of designing a PM system for a barge vessel is addressed. A model ship and four mooring systems used in this study are designed and created. The three-degree-of-freedom (3DOF) mathematical model of a system consisting of a barge ship and mooring systems is derived. Hydrodynamic coefficients of the low speed model for PM vessel are identified by suitable experiments. A PI control scheme is implemented to achieve PM for the vessel by using rope tension control strategy. Real time control scheme integrating CompactRIO platform and LabVIEW program language is developed. The proposed strategy is finally tested on station keeping, and desired positions of the model vessel are obtained.