Wind energy is one of the most promising sources of renewable energy worldwide. Its utilization has substantially increased for the last decades, both onshore and offshore. Offshore wind energy will have a lot to offer within the following decades, thus their foundations need to be prepared. Some of the current wind farms and wind turbines are now reaching their lifespan and, the turbines’ market is developing itself so rapidly that current turbines are getting behind the times with tremendous ease. It is here where the scope of this Master Thesis comes: what shall we do? Should we dismantle wind farms when they reach their lifespan, or should we maybe try to give them a further use? Accommodating for a new a larger wind turbine will need to account for new and higher climate actions and loads, namely winds, waves, ocean currents, the water level variation and the always difficult to predict ice actions. What is aimed in this Master Thesis is to set the basis for a procedure to dimension and define feasible solutions for the offshore wind turbines Gravity-Based Foundations to be expanded, avoiding the necessity of replacing them completely, with the environmental and economic benefits this would have. As this could turn to be an unmanageable problem to be solved, a Case Study where details can be set is performed at the Lillgrund Wind Farm site, in the south-west coast of Sweden, in the Öresund that separates Copenhagen and Malmö. A thorough description of the climatic actions and surrounding aspects is performed, while always dealing with uncertainties. With all that information, an analytical stability analysis is performed to account for three failure modes, namely: sliding, tilting andground failure. Additionally, a numerical FE-model is carried out in ANSYS in the aim of assessing the stresses and deformations that this kind of structure will suffer. Four alternatives are evaluated, and their behaviour is assessed based on the new external design actions. Analytical results show stability difficulties in two of the geometries inspected, while assure it in the other two. The FE-analyses show high concentrations of stresses on the GBS shaft, while model affordable deformations under the load combinations inspected. These results are also compared and contrasted in between them, and sensitivity analyses for the FE-models are performed in order to assure their good behaviour and development, and the trustworthiness ofthe results found. Based on these results, some conclusions are drawn from the developed processes. The main finding is the width and weight dependence of the solution, as well as the shape and dimensions. Future research needs such as scouring effects are finally accounted for necessary inspection to be made as continuation of the work here presented.

Ocean wave energy is considered to be one of the important potential renewable energy resources for sustainable development. Various wave energy converter technologies have been proposed to harvest the energy from ocean waves. This thesis is based on the linear generator wave energy converter developed at Uppsala University. The research in this thesis focuses on the foundation optimization and the power absorption optimization of the wave energy converters and on the wave climate modelling at the Lysekil wave converter test site. The foundation optimization study of the gravity-based foundation of the linear wave energy converter is based on statistical analysis of wave climate data measured at the Lysekil test site. The 25 years return extreme significant wave height and its associated mean zero-crossing period are chosen as the maximum wave for the maximum heave and surge forces evaluation. The power absorption optimization study on the linear generator wave energy converter is based on the wave climate at the Lysekil test site. A frequency-domain simplified numerical model is used with the power take-off damping coefficient chosen as the control parameter for optimizing the power absorption. The results show a large improvement with an optimized power take-off damping coefficient adjusted to the characteristics of the wave climate at the test site. The wave climate modelling studies are based on the wave climate data measured at the Lysekil test site. A new mixed distribution method is proposed for modelling the significant wave height. This method gives impressive goodness of fit with the measured wave data. A copula method is applied to the bivariate joint distribution of the significant wave height and the wave period. The results show an excellent goodness of fit for the Gumbel model. The general applicability of the proposed mixed-distribution method and the copula method are illustrated with wave climate data from four other sites. The results confirm the good performance of the mixed-distribution and the Gumbel copula model for the modelling of significant wave height and bivariate wave climate.