Handling Unknown External Forces with Highly Responsive DP Controller

Abstract

Rising marine activity in the arctic has introduced many ice related challenges for Dynamically Positioned (DP) marine vessels. Current motion control systems for DP vessels are not designed to handle ice-related disturbances, and will thus need to be improved to be able to operate in these challenging conditions.

The main goals of this master thesis have been to evaluate opportunities for handling large and sudden unknown external disturbances, and compare these with existing solutions.

There will be presented a predictive feedforward controller to minimize the impact of an external disturbance. There will also be presented a selective wave filter with a notch frequency that is adapted by a wave-frequency tracker. A modified highly responsive PID controller with nonlinear stiffness and damping terms has been derived, and proven globally asymptotically stable using Lyapunov based stability analysis. There will also be presented a Disturbance Rejection by Acceleration Feedforward controller that utilizes acceleration feedback from inertial measurements.

All of the proposed improvements have been implemented and tested in Matlab/ Simulink, while some of the methods have been implemented and tested hardware-in- the-loop in a simulator provided by Marine Technologies. The proposed methods were shown both theoretically and through simulations to yield improved stationkeeping performances.