Thrust allocation in semi-submersible rig using model predictive control

Abstract

A thrust allocation system is used to determine how the desired forces, computed by a high level control sytem, can be distributed among the thrusters. The main goal of the thrust allocation is to obtain the desired force, but other objectives can also be included. Such secondary goals can be to minimize fuel consumption, keep wear and tear of the thruster to a minimum and avoid overloading the power systems. The thrust allocation should also take forbidden sectors and actuator rate constraints into account. It is essential to safe operation that the allocation system provides a solution, and provides the solution in time. In this thesis MPC (Model predictive control) is suggested as a method to solve the control allocation problem for CyberRig I (a scaled model of a semi-submersible drilling unit). 3 MPC algorithms are simulated in matlab, and the most complete are chosen for on-line implementation. The algorithm is based on an extended thrust formulation, and allows for rotatable thrusters. The cost function penalizes change in thust magnitude and in the azimuth angle. Forbidden sector constraints and rate constrains, both for thrust magnitude and angle, are implemented. It is shown in simulations that the MPC algorithm performs well in comparison with an existing quasi-static method. Its main benefit over the quasi-static method is the ability to handle constraints. The cost of using MPC is increased computational efforts.