Abstract
Due to an increasing concern and awareness about the effects of fossil fuel energy in the climate change, and an recent increased tension inside the geo-political climate of Europe, a new all-time high interest in alternative renewable energy technologies was reached. Although underdeveloped and understudied, the marine energy harvesting technology sector presents an extremely high potential. Foundation design is a crucial step towards optimization of offshore wind turbines, it accounts for roughly 25% to 34% of the total cost of implementation. Even though scour mechanisms for monopile foundations have been extensively studied, there is considerable knowledge gaps in scour mechanisms around foundations with complex geometry, such as gravity-based foundation solutions. Furthermore, scour research considering complex soils, such as layered sand, are still scant. In order to increase the short database of scour tests in literature to date for GBF foundations and complex soils, two distinct sets of scour tests were performed. The first set of tests using a simple soil configuration of uniform fine sand including two different foundation models, a monopile model and a GBF model. The second set of tests was performed using a layered soil, with a coarse sand layer over a fine sand layer, also for both foundation models. Most studies and literature in scour phenomena in offshore environment commonly consider the effect of currents in combination with waves. In order to simulate the sea state conditions, the wave and current flume at FEUP was used. Optimization can also be sought after through hybridization of technologies, such as the deployment of wave energy converters in the vicinity of the offshore wind farms. Studies and literature regarding this type of solutions are very scarce. The current research will also include an innovative laboratory study on the scour effects of the implementation of a wave oscillating wave surge, such as an oscillating wave surge converter, to the bottom, near the foundation of offshore wind turbines. This innovative study was performed in combination with both sets of tests referred before. The oscillating wave surge converter tests extended to three different distances from the foundation. The present research focuses on three main key areas. Firstly, with the GBF tests, it is intended to expand the current small database of scour tests for complex geometry foundations. Secondly, expansion of existing laboratory scour tests for layered sand soils. Lastly, the present dissertation aims to set a preliminary work and serve as a first step towards further and deeper studies of hybrid solutions towards offshore energy optimization. Results for layered soil indicated an increase in scour depths compared with the uniform fine sand. The test results in the presence of the oscillating wave surge converters indicates an average 60% decrease in scour depth near the foundations.