Abstract
As a step to further develop the share of renewable energies, the first German offshore test site alpha ventus has been installed in the North Sea in 2009 in water depths of 30 m, where experience shall be gained and made available for
 future offshore wind farms. Regarding converter foundations in deep water, it is well known that in most cases scour phenomena occur around the structures. Due to the complexity of the tripod foundations, significant knowledge gaps in scour progression in general and especially in detail as well as its probable effects on the stability and durability are given. Therefore, investigations on scouring phenomena around complex foundation structures like the tripod are
 carried out within the research project. The investigation method consists of a unique combination of local scour monitoring as well as physical and numerical modeling, whereas the physical modeling part was carried by means of 1:40 laboratory tests and 1:12 large-scale physical model tests in wave flumes. The results show that scours around the tripod foundation do not only occur directly around the foundation piles, but also in the near-field of the structure. Compared to first in-situ measured scours in the test site, at least a good qualitative agreement of the modeled scour depths and evolutions could be shown.
Highlights
AND RESEARCH BACKGROUNDDue to ambitious climate protection targets, thousands of offshore wind energy converters with an overall rated power of 20-25 GW will be constructed in the North- and Baltic Sea to be completed by 2030, with the overall goal to cover 15 percent of Germany’s electricity supply by wind energy alone
The project pursues the overall objective of developing a holistic design concept for offshore wind energy converters (OWEC) in order to reduce costs for the support structures, i.e. tower, substructure and foundation
The characteristics of the local and global scour development described by depths and extent are directly influenced by the wave parameters and the orientation, i.e. turning angles of the tripod in reference to the main wave direction
Summary
Due to ambitious climate protection targets, thousands of offshore wind energy converters with an overall rated power of 20-25 GW will be constructed in the North- and Baltic Sea to be completed by 2030, with the overall goal to cover 15 percent of Germany’s electricity supply by wind energy alone. In summer 2009, the birth for the further development of the German offshore wind energy has been given by the final construction and commissioning of the offshore wind farm alpha ventus, located 45 km off the coast of the island of Borkum in the North Sea (Figure 1). Due to the construction of the twelve wind turbines with a rated power of 5 MW each in water depths of around 30 meters, alpha ventus is the first German offshore test site that has been installed and is being operated under real offshore conditions. In order to gain experience for future offshore wind warms, the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU) has launched and is supporting the research initiative RAVE - Research at alpha ventus - with its overall objective of cost reduction for offshore wind energy deployment in deep water. From measuring campaigns at different locations in the test site undertaken by the German Federal Maritime and Hydrographic Agency (BSH), it can be concluded that the bed material is mainly narrow distributed fine sand with a grain size d50 of about 0.2-0.25 mm
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