Flume Tank Testing of Offshore Wind Turbine Dynamics with Foundation Scour and Scour Protection

Scour erosion processes can occur at seabed level around offshore wind turbine monopile foundations. These scour processes are often especially severe at sites where mobile sediments, such as sands, are present in the superficial seabed soils. Loss of local soil support to the monopile, caused by scour erosion, can lead to significant changes in the dynamic characteristics of the wind turbine support structure. This can result in accelerated fatigue damage, owing to the applied cyclic loads from the wind turbine generator, especially at the rotor frequency. Although scour erosion can be controlled by appropriate scour protection systems, there is a lack of knowledge to support the design and optimization of these protection measures, to ensure that the dynamic performance of the wind turbine support structure remains within acceptable limits. This paper describes an experimental campaign conducted on a 1:20 scale model of a driven monopile foundation and wind turbine support structure, founded in a prepared sand test bed in the Fast Flow Facility flume (HR Wallingford, UK). Scour processes were induced by applying cycles of flow. Experiments were conducted to investigate the influence that these scour processes, and selected concepts for preventative and remedial scour protection, have on the dynamic characteristics of the monopile–tower system. The paper describes the experimental procedures that were adopted, and provides an assessment of the results.