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.

Offshore wind energy is one of the solutions to meet the growing demand for renewable energy. The offshore wind turbines producing this energy keep increasing in size and, as a result, the monopile foundations are becoming larger and heavier. The traditional jack-up installation vessels have limited crane capacity and many of these vessels are unable to install the XXL monopiles. Therefore, the offshore industry is currently investigating a new installation method using a motion-compensated gripper frame on floating vessel with a dynamic positioning system. The gripper frame is attached to the vessel and encloses the monopile with a ring to keep it vertical during the installation. In addition, the gripper frame compensates for the vessel motions such that the vessel motions do not influence the monopile motions. The purpose of this thesis is to investigate the feasibility of such a motion-compensated gripper frame and to determine what control settings minimise the monopile inclination and the force exerted on the monopile. The system is composed of three main bodies: the vessel, the gripper frame and the monopile. The monopile and PID controller, which controls the amount of force exerted on the monopile to keep it vertical, have been modelled in the frequency domain to gain insight in the effect of changing the control parameters. To model the dynamics of the coupled system an OrcaFlex model has been set-up. The system has been tested for various values of proportional and derivative gain, kP and kD respectively, in various wave conditions. First the perfect control system is tested, where the force to keep the monopile vertical is applied instantly and the vessel motions are fully compensated. However, as the real world is never perfect, the system tested for sensor lag and imperfect motion compensation as well. The results are judged based on three criteria regarding the maximum monopile inclination, actuator force and actuator stroke. Resonance is observed in case a value of kP is selected such that the natural frequency of the monopile and controller matches the wave forcing frequency. Adding derivative gain kD limits the monopile motions and force exerted in this case. To limit the monopile motion the proportional gain should be selected such that resonance is avoided. Two different control settings are investigated and it has been found that a relatively high value of kP of 10,000 kN/m in combination with a kD of 11,000 kNs/m is a suitable setting based on the three criteria. Furthermore, bow quartering waves is the favourable wave direction compared to head waves for the system considered in this thesis, as the force on the monopile is more evenly distributed over the actuator in x- and y-direction. Introducing a sensor lag into the system results in higher monopile motions and forces on the monopile. If the sensor lag exceeds 0.3 s it leads to instability of the monopile for both settings. The effect of not fully compensating the vessel motions is found to be limited due to the fact that these motions are slowly varying. The results of this work contribute to a better understanding of the dynamics of the system in various wave conditions. Furthermore, it provides insight in the effect of sensor lag and imperfect motions compensation, contributing to the design of a motion-compensated gripper frame for the installation of XXL monopiles.