Development, testing, and validation of an energy-efficient dynamic positioning controller

Alagili, Osama (2024) Development, testing, and validation of an energy-efficient dynamic positioning controller. Doctoral (PhD) thesis, Memorial University of Newfoundland.

[English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (27MB)

Abstract

Exploration of the Arctic and Sub-Arctic regions is a daunting task due to the harsh environmental conditions, including low temperatures, ice floes, wind, and waves. Dynamic positioning (DP) is a crucial technology for ships operating in harsh weather conditions. The goal of this research is to develop energy efficient DP controllers that can withstand harsh environmental conditions. The scope of the thesis includes modelling and simulation of environmental disturbances on DP vessel; performance evaluation of a suite of DP controllers for moderate to harsh environmental conditions; development and simulation of an energy efficient DP system; and finally experimental implementation and evaluation of the proposed DP system. Numerical modelling with real-time simulation capability helps design, test, and validate dynamically positioned and autonomous ships/platforms in harsh environments. However, advanced simulation technology is needed to predict the expected loads on these systems due to complex interactions with environmental disturbances. The first work presents wave, wind, current, and ice models that comply with realtime simulation requirements and capture the dynamic characteristics of relevant physical processes. A 3D dispersive numerical model was deployed to predict the wave parameters to compute the wave loads on a ship with known Response Amplitude Operators (RAO). A uniform current load was incorporated in a superposition manner by using a combined wave-current field dispersion relation capable of expressing the wave-number of an interactive wave-current field. The proposed models can be used to design, develop, and evaluate dynamic positioning and autonomous ship controllers' performance and train conventional, DP, and autonomous ship operators. Next, the work evaluates the performance of several control schemes for DP application in moderate and extreme sea conditions. The controllers include nonlinear PID, sliding mode controller (SMC),multi-resolution PID controllers, model predictive controller (MPC) and nonlinear model predictive controller. Matlab/Simulink models of a full-scale ship and its scaled version are used to compare the efficacy of the controllers. An Unscented Kalman Filter is utilized to filter out wave-frequency motions. While all controllers were effective for moderate sea state, only nonlinear model predictive controller (NMPC) and multi-resolution proportional-integral derivative (MRPID) controllers could stabilize the ship under extreme sea states. The NMPC demonstrates the best ability to handle extreme disturbances. Besides maintaining the position, one of the other goals of DP controller is to minimize the energy requirements and high frequency movements of the thrusters. An energy-efficient controller, called Green-NMPC was developed for the dynamic positioning of marine vessels. The Green-NMPC is motivated by the control goal of minimizing thruster demand. It is based upon the theoretical framework of the economic NMPC (ENMPC). Green-NMPC uses dynamic weights in the cost function depending on the vessel position in contrast to the constant weights in conventional NMPC. The proposed controller was tested for moderate to high sea wave conditions and reduced up to 50% thruster demand in sway direction compared to NMPC while maintaining the vessel positioning objectives. The Green-NMPC showed less thruster demand which was further verified quantitatively from the variance and the spectral strength of the thruster demand. Finally, this research presents the experimental implementation of the proposed Green NMPC to a scaled version of a supply vessel named Magne Viking. Experiments were carried out at the National Research Council's wave basin. This is one of the very few experiments where the NMPCs were tested in a controlled environment with varying wave conditions, and the first experimental implementation of the Green NMPC. The experimental results on the station keeping tests validated that on average, the Green NMPC is the most energy efficient controller for the application. The Green NMPC minimizes the thruster movement by implementing a relaxed control policy. The power spectrum of the thrusters further confirmed that Green NMPC has less high-frequency movements compared to the rest of the controllers. This implies that Green NMPC will cause less wear and tear on the thruster system.

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