Due to the ever increasing demand of energy and the urgency in which this has to be produced using renewable energy resources the request for offshore wind farms is larger than ever. As a result of the competitive nature of the energy sector it is important to keep the cost of energy low in order to keep an edge over the competition. Therefore, to keep wind energy economically viable in the future the cost of producing energy should be lowered. One way this can be done is by lowering the initial costs of installation, as this is relatively high. In this thesis an installation method of monopile foundations will be introduced, which has potential to reduce costs of the installation of wind farms. More specifically, this thesis will focus on lowering of monopiles to the seabed. Problems arise when the monopile hanging on the crane wire is excited by waves hitting the submerged part of the monopile. This can cause excessivemotion of the monopile. Particularly if resonance occurs, which is the case when the wave frequencies and the natural frequencies of the monopile coincide. This could lead to catastrophic failure if no mitigating measures are taken. The most common method to counteract this phenomenon is to wait until weather conditions allow for further continuation of operations, which can consume lots of valuable time and thus increasing costs. Therefore, this thesis will focus on optimizing the workability of offshore installations of monopiles by using gyroscopicmotion actuators. These actuators, called controlmoment gyroscopes (CMG), will reducemotion of the monopile by applying a moment counteracting the moment created by the waves. This system is modelled in MATLAB. The model consists of the monopile suspended on the crane wire, a gripper and the CMG. The equations of motion are determined using the Lagrangian method and linearized around their equilibrium position. The equations of motion are then written in state space representation in order to study the behaviour of the system and to tune the PD controller. The PD controller was tuned using the loop shaping method. The MATLAB model is used to tune the controller. However, the results will be examined using OrcaFlex. Therefore, the same system is also modelled in OrcaFlex, a commercial time domain simulations package, in which an external function is built using Python to represent the CMG and the corresponding PD controller of the CMG moment. The whole system is then subjected to a sea state with a significant wave height of 2 meters and a wave period range from 5-8 seconds. The 3 hour simulations are run for each period step of 0.5 seconds from 5 to 8 seconds. From each simulation the most probable maximum is calculated. This is done for four variants of the system…

The demand for renewable energy is growing, due to a growing awareness among people about the effect of burning fossil fuels on the environment. Governments have signed climate agreements and are obliged to decrease their level of exhaust gasses. This change in attitude regarding the environment led to an upcoming market for renewables. A significant part of the demand for renewable energy is covered by the wind industry, both on- and offshore. Especially the offshore wind market holds huge potential, due to stable wind conditions and growth opportunities. A side effect of the evolving industry is that turbines are increasing in size and are being installed in deeper water depths. It is expected that the methods of installation, currently used in the offshore wind market, will no longer be suitable for the installation of new generation turbines. In this research, installation of offshore wind turbine towers using a heavy lift vessel (HLV) is opted as an alternative to the current installation methods (jack-up installation). The major benefits of floating installation, when compared to jack-up installation, are the independency on both soil conditions and jacking operations. A challenge of floating installation, however, is the introduction of vessel motions resulting in excitation of the tower. The main objective of this research is to find the most efficient way to control the motions of the tower during installation using a HLV. To fulfill this objective, first, a conceptual design study has been conducted, which focused on developing concepts for the safe installation of offshore wind turbine towers using a HLV. Safe installation was assumed if the displacements of the tower bottom maintain within limits, as specified in a list of design requirements and criteria. Before examining any concept in more detail, the response of the tower and the effect of applying a form of motion compensation has been examined in a general way. To test the response of the tower in the crane in a general way, with and without motion compensation, the tower in the crane has been represented by a double pendulum system with a moving support point. Then, using the numeric model, the response of the tower in the crane has been computed for certain sea-states in both frequency- and time domain and for both the free hanging- and the motion compensated case. The motion compensation system has been represented by a virtual spring damper system, of which, by changing the simulation parameters, the optimal configuration (position, stiffness and damping) has been determined that minimizes the tower response. From the results, it can be concluded that a significant reduction of the tower bottom displacements can be achieved under the application of an active motion compensation system, the requirements for safe installation, however, are not yet fulfilled.