Ship tankers, used as floating oil storage and export units, are very sensitive to environmental conditions, and their position keeping during a loading operation may be a critical point. For relatively deep wells and harsh environments, the ship is moored at an Articulated Loading Column with a single point mooring system. Nevertheless, for assuring a better positioning and/or uninterrupted loading operations, tankers with additional dynamic positioning capability are employed. In this paper a hybrid control strategy for the ship dynamic positioning is proposed. The control aim is to reduce the plant capital and running costs, trying to pursue a tradeoff between the maximization of a safety index and the minimization of the control effort. For limiting the power peaks and the total energy spent during each loading operation, it is also planned to utilize the main propulsion system to counteract too severe weather conditions. Starting from almost realistic data for the tanker, the mooring, the flexible articulated loading column and the thrusters, and from a statistical characterization of the disturbances and the uncertain dynamics, some simulation results are produced to prove the energy efficiency and show the good performance of the designed controller.
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