Propulsion System Control of Ships Sailing in Waves

Abstract

The need for advanced ship propulsion control systems is constantly increasing as the demand for high vessel performance is growing at the same time. Speed trials are considered as an operation during which the vessel has to achieve the highest performance by attaining the agreed maximum ship speed. However, sometimes vessels have to sail in waves during speed trials. In such case, resistance and wake field disturbances act on the vessel’s propulsion plant causing fluctuations of the engine’s operating point in the engine operating envelope. When these fluctuations are large enough to reach the engine’s operating envelope limits, the propeller pitch control is activated to effectively protect the engine by reducing the propeller pitch. This decrease results in reduced average delivered power, reduced average generated thrust and finally in a drop of maximum average ship speed. This work focuses on investigating the possibilities to refine the existing control system by means of gain scheduling the Diesel engine speed governor. The ultimate goal is the re-sizing and re-orientating of the engine’s operating point fluctuations in the engine’s operating envelope, keeping the propeller pitch control deactivated and thus, the maximum average speed retained. First of all a linearised model of the ship propulsion system is derived which is used for the analysis of the Diesel engine’s operating point dynamic response in terms of engine torque and engine speed fluctuations in case of regular waves. More specifically, this thesis investigates the impact of the direction of the ship with respect to the waves, the Sea State and the system operating point on the dynamic response of the engine’s operating point in the engine’s operating envelope. Based on the analysis of the dynamic response of the engine’s operating point with respect to the three above mentioned factors and by making use of the linearised model, the refinement of the Diesel engine’s dynamic behavior is achieved. In the first place, a static solution is given with the derivation of suitable plots. The derived plots provide the opportunity for manual governor gains scheduling, depending on the wave induced disturbance. In the second place, a gains scheduling algorithm is obtained by combining the linearised ship propulsion model with an optimization algorithm. The developed algorithm, after being integrated in the non-linear simulation model of the ship propulsion system, is capable of dynamically scheduling the governor gains in case of regular and irregular waves. Both obtained solutions achieve the ultimate objective of this work. The fluctuations of the Diesel engine’s operating in the engine operating envelope are effectively re-sized and re-orientated. As a result the activation of the propeller pitch control is prevented and the average ship speed is retained.