Floating Installation of Windturbine Towers

Abstract

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.