Time Development of Scour Around Offshore Monopiles

The seabed around an offshore wind turbine foundation may risk being eroded by the flow. Much of the existing knowledge and design methods are taken from the context of bridge piers in rivers which are exposed to current. However, the offshore wind turbine foundations are in most cases exposed to waves too and this reduces the risk of erosion significantly. Therefore, there is a significant interest in obtaining further understanding of the development of erosion around the foundation during changing sea climates. The project aims at investigating this by simple and advanced computer calculations, model-scale experiments and/or analysis of data from the field.

In recent years, there has been an increased interest in offshore wind turbines compared to the onshore ones. The background for this preference is, among other factors, less turbulence and higher intensity of wind and thus higher power output, lower visual impact and the possibility of occupying greater areas of the offshore wind turbines compared to the onshore ones.

The prospect of such continued interest in offshore wind turbines justifies an optimization of their foundation. One key aspect is the understanding and determination of the scour process of the seabed around the foundation.

Today, the design of the foundation is based upon a vast amount of experimental and field observations from the context of bridge piers which are subjected to steady current. In this case, the scour hole has a depth in the same order of magnitude as the pier or pile diameter. The protection against such scour is usually carried out by covering the seabed around the base of the foundation with rocks at the cost in the order of 1 mill DKK per foundation. Alternatively, a monopile foundation may be intentionally unprotected and designed to allow full development of the scour hole. In this design, a scour hole will both amplify the hydrodynamic loads and reduce the soil support.