Optimal multi-model control for a Motion Compensated Gripper Frame

Abstract

Due to an increase demand for renewable and sustainable forms of energy, the offshore wind industry is growing rapidly. To make wind energy more cost-effective, new wind turbines are becoming larger in size and are placed further offshore. This results in deeper waters, making the installation more difficult. The current methodology uses a jack-up vessel, butt will not be capable of installing the next generation of XXL wind turbines. The legs will simply be too short and the monopile too heavy. Floating vessels with a dynamic positioning system are believed to be the solution since they are able to operate in deeper waters and are assumed to be more time efficient. The foundation of a windturbine needs to be installed within an angle of tolerance of 0.25°. In order to control the position of the monopile during the installation, a Motion Compensated Gripper Frame can be used. This research focuses on the control design of such a gripper frame. The goal is to design a robust multi-model controller for the gripper frame which will guide the monopile during the installation. The controller should minimise the angle output of the system, the used control force and the resulting force which is experienced by the vessel. One of the main challenges is the unknown soil stiffness. This model parameter has large effects on the system dynamics and will show large uncertainties during installation. The robust stability property is shown for the designed cascade controllers. The implementation of these controllers in a multi-model controller show stable results. The fast inner-loop measures the position of the gripper and is able to reject any vessel motion disturbance. The slower outer-loop compensates for angle deviations of the monopile. The optimal controller is activated for the different operating points using gain-scheduling. Compared to gain-scheduling with two controllers, three controllers reduce the mean resulting force on the vessel with 50% and the total required power by 0.06% during the installation. But the deviations of the angle of the monopile increases with 5% but does not violate the set constraints. The results show a stable system for all the changing degrees of soil stiffness. The developed method makes it possible to install the next generation XXL wind turbines in deeper waters, and replaces the current limited method which uses jack-up vessels. Using the designed cascade multi-model controller will result in a robustly stable controller which minimises the used control forces and power. Which in the end minimises the costs of this gripper construction and making the wind-industry even more cost-effective.