Scour Protection around Offshore Wind Turbine Foundations, Full-Scale Measurements

摘要

The major challenge of ensuring the stability of the seabed around offshore wind turbine foundations is addressed in the present paper. Full-scale, high-resolution measurements of the seabed and the scour protection around the monopile foundations in Horns Rev offshore wind farm are analysed and presented. The measured developments in the scour protection are related to wave and current conditions. The wind turbines at Horns Rev wind farm are each placed on circular monopiles at water depths ranging from 6-13 m at MSL. The seabed consists of fine to coarse sand (D_(50) = 0.15-1 mm). The wave conditions are severe - the largest waves are in the order of 8 m high. The max current in the wind farm area is approximately 0.88 m/s (50 year return period). The stability of the seabed around the wind turbine foundations is one of the major challenges in offshore wind farm design. Each foundation at Horns Rev is protected by a 50 cm thick filter layer (D_(50) = 10 cm) and a 1 m thick armour layer (D_(50) = 40 cm) placed within a radius of 9.5 m from the centre of the monopole. 5 high-resolution surveys of the scour protection around all the wind turbine units have been carried out before and after installation of the turbine foundations. The paper presents the development of the surrounding seabed and the scour protection themselves. The surveys around all 80 wind turbines consistently show a lowering of the scour protection and a net transport away from the scour protection area. The measurements also show that up to 1.5 m local lowering of the armour layer have taken place during the time period from 2002 to 2005. The largest lowerings have occurred close to the wind turbines. The history of the wave current climate in the wind farm area has been established from a combination of numerical simulations and measurements. From the wave current climate the movement of the underlying sand bed has been assessed. Simple flow analysis and the surveys indicate that the sand has been transported upwards through the filter and armour layers at most of the wind turbine locations. This has resulted in subsidence of the filter and armour layers into the sand bed. The top level of the scour protection layer is, however, still above the surrounding seabed level, which the wind turbines have been designed for. It is further concluded that some erosion up to 0.5 m (0.1 tower diameter) deep has occurred outside the scour protection in the distance 15 m away from centre of the wind turbines on the east side of the turbines (opposite the main wave direction (coming from)).