Marine vessels with dynamic positioning capability are typically equipped with many enough thrusters to make them overactuated, and with satellite navigation and other sensors to determine their position, heading and velocity. An automatic control system is tasked with coordinating the thrusters to move the vessel in any desired direction and to counteract the environmental forces. The design of this control system is usually separated into several levels. First, a dynamic positioning (DP) control algorithm calculates the total force and moment of force that the thruster system should produce. Then, a thrust allocation (TA) algorithm coordinates the thrusters so that the resultant force they produce matches the request from the DP control algorithm. Unless significant heuristic modifications are made, the DP control algorithm has limited information about the thruster effects such as saturations and limited rate of rotation of variable-direction thrusters, as well as systemic effects such as singular thruster configurations. The control output produced with this control architecture is therefore not always optimal, and may result in a position loss that would not have occurred with a more sophisticated control algorithm. Recent advances in computer hardware and algorithms make it possible to consider model-predictive control algorithm (MPC) that combines positioning control and thrust allocation into a single algorithm, which theoretically should yield a near-optimal controller output. The presented work explores advantages and disadvantages of using model predictive control compared to the traditional algorithms.

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Abstract-A dynamic positioning (DP) system on a dieselelectric ship applies electric power to keep the positioning and heading of the ship subject to dynamic disturbances due to the winds, waves and other external forces using electric thrusters. Vice versa, position and heading errors can be allowed in order to implement energy storage in the kinetic and potential energy of the ship motion using the DP control system to convert between mechanical and electrical power. New simple formulas are derived in order to relate the dynamic energy storage capacity to the maximum allowed ship position deviation, as a function of the frequency of the requested dynamic energy storage. The benefits of DP dynamic energy storage are found to be reduced diesel-generator maintenance need, reduced fuel consumption and emissions, reduced risk for blackout, and increased operational flexibility allowing power-consuming operations such as drilling and lifting to be safely prioritized over DP for short periods of time.

Abstract-Modern ships and offshore units built for dynamic positioning are often powered by an electric power plant consisting of two or more diesel-electric generators. Actuation in any desired direction is achieved by placing electrical thrusters at suitable points on the hull. Such ships usually also have other large electrical loads. Operations in the naturally unpredictable marine environment often necessitate large variations in power consumption, both by the thrusters and by the other consumers. This wears down the power plant, and increases the fuel consumption and pollution. This paper introduces a thrust allocation algorithm that facilitates more stable loading on the power plant. This algorithm modulates the power consumption by coordinating the thrusters to introduce load variations that counteract the load variations from the other consumers on the ship. To reduce load variations without increasing overall power consumption it is necessary to deviate from the thrust command given by the dynamic positioning system. The resulting deviations in position and velocity of the vessel are tightly controlled, and the results show that small deviations are sufficient to fulfill the objective of reducing the load variations. The effectiveness of the proposed algorithm has been demonstrated on a simulated vessel with a diesel-electric power plant. A model for simulation of a marine power plant for control design purposes has been developed.

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