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Deltares | R&D Highlights 2015 Delta Infrastructure
Increasing demand for offshore renewable energy is currently leading to the development of large numbers of offshore wind farms. These wind farms are sometimes planned in areas with highly morphodynamic seabeds. It is essential to keep variations in the pile-fixation level within limits for wind turbine foundations so that the resonance frequencies of the structure are kept within an acceptable bandwidth, making the cost-efficient design of the support structure possible. Scour protection is therefore often used to fixate the seabed. The Nordergründe offshore wind farm will be built in 2016 in the Jade-Weser estuary (German Wadden Sea). This area is characterised by a complex system of migrating tidal channels and flats. Variations in the seabed level of up to 10m are predicted. Given these predictions, Deltares proposed several strategies for scour mitigation. The most promising strategy comprised of a combination of first allowing scour development to a pre-defined level and then fixating the seabed with scour protection. Physical model testing was used to develop and validate a scour protection layout for this strategy.
The physical model tests were performed in the Atlantic Basin at Deltares. The morphological scenarios were modelled in an innovative way: the drop in the level of the seabed was simulated by using an artificial island - with the scour protection situated on the top - which was eroded by a tidal current. After several tidal cycles, the sides of the island were fully eroded and the scour protection now covered the slopes of the island, resulting in stabilisation. This falling apron concept had never been tested for extreme seabed-level variations or in offshore conditions with strong currents in combination with high waves. The unique characteristics of Deltares’ Atlantic Basin, where the combined effect of currents and waves can be modelled, were essential for the successful completion of these tests.
Stereo photography was used to determine the performance of the scour protection, resulting in very accurate pictures of the bathymetry before and after each test. In combination with underwater cameras and calibrated cameras inside the scale models, the observations provided a comprehensive picture of the performance of the scour protection at different stages of each test. These measurements allowed for the further optimisation of the scour protection layouts.
Moreover, the physical modelling test programme proved to be a safe and cost-efficient instrument for the development of the Nordergründe wind farm. With this project, Deltares and the offshore wind industry have made a positive contribution to Europe’s transition to renewable energy. In addition, the scientific results of the physical modelling can be applied to other offshore wind farms in the future.
Physical model tests for
Nordergründe offshore
wind farm
Further reading:
Riezebos et al. (2016). Scour protection design in highly morphodynamic environments. 8th International Conference on Scour and Erosion, Oxford, UK
hendrikjan.riezebos@deltares.nl T +31(0)88 335 7647
tim.raaijmakers@deltares.nl T +31(0)88 335 8218
Stereo photography was used to determine the performance of the scour protection, resulting in very accurate pictures of the bathymetry before and after each test. In combination with underwater cameras and calibrated cameras inside the scale
