Feasibility assessment of novel on-site installation methods for offshore Ultra Large Wind Turbines

Abstract

Wind energy has seen a transition from onshore to offshore in the last decades. The lack of space on land and up-scaling of turbine sizes led to the need of exploiting further away and harsher environments, where more space is available and there is arguably less of an impact on society. This together with a higher supply of wind resources has led to the development of large numbers of offshore wind farms, of which many are still to be built. Taking Europe as an example, currently the goal is to achieve at least 450[GW] of installed offshore wind power by the year 2050, of which only 25[GW] has been installed up until 2020. This sketches out the enormous challenge that is being tackled by governments and the public market as we speak.
The market is a highly competitive one, where small increases in efficiency of one of the many complex steps within the development of offshore wind farms can make or brake companies and projects. The eventual goal of increasing the efficiency of the overall process, is to decrease the levelized cost of energy of a wind farm. One of the main approaches of increasing this efficiency, has been to up-scale wind turbines, in the last decades the size and power output of wind turbines has already more than doubled from an average of 3MW of installed power per wind turbine 15 years ago to an average of 8.2MW in 2020. This trend does not seem to stop any time soon, as wind turbines of 12-15MW are in active development. The industry has already expressed interest into turbines of 20-25MW and even beyond this. The turbines beyond these sizes will be referred to as Ultra Large Wind Turbines(ULWTs). Due to the size and weight of components of ULWTs traditional top-down installation techniques cannot be used anymore due to relative motion related difficulties during the mating of components. Thus, the following research question will be answered during the defence:

"What would be feasible novel on-site installation methods for a bottom founded Ultra Large Wind Turbines(ULWT) using floating vessels?"

During the research, the base case of, floating installation on a pre-installed jacket type foundation is used as a starting point for the generation of concepts. A total of 15 concepts has been generated and pooled with concepts proposed in the industry and academia. All these concepts were screened to find the most promising ones. These concepts were sequentially tested on their technical feasibility for use with ULWTs, to identify which concepts can possibly be used. The final step was to perform a comparative analysis of the economic feasibility of the technically feasible concepts, to identify the differences in performance and usability of the concepts. From the technically feasible concepts, two main approaches could be identified which are promising for the installation of ULWTs. Further work must be performed to identify the best option from these two and describe their overall economic feasibility.
Self-climbers: Devices which climb the tower without assistance of an installation vessel. Especially interesting due to the fact that they can be used with existing mono-hull lifting vessels. Main downsides are the relatively low flexibility w.r.t. wind turbine size, usability and have a long installation time. They are promising, as they decouple the size of the installation vessel and size of the turbine entirely.
Large semi-submersible vessels: Either using lifting equipment, so that existing vessels can be used. Or specifically designed vessels which provide large flexibility w.r.t. size of turbine and usability. Main downside are the size and costs of these vessels. They are promising, as they provide a platform on which the needed height can be reached effectively.