StatoilHydro is planning to install two export cables to connect its Sheringham Shoal Offshore Wind Farm to the UK National Grid. The landfall will be at Weybourne on the North Norfolk coast. This desk study was commissioned to identify the optimum burial depth of the cables at the landfall by investigating the surface geology. Design requirements are: 1) The overburden must never reduce below 1.5 m, which is the safety margin deemed to prevent the cables being exposed; and 2) Subject to requirement 1, the overburden should be kept to a minimum to permit effective dissipation of heat from the cable. The target of the present study is thus to determine the minimum burial depth that will preserve both the above conditions throughout the lifetime of the scheme, taking account of future erosion and climate change deliberations. The storm beach at the landfall is actively mobile and is migrating alongshore in a westward direction; it is also retreating landward due to coastal erosion with an average landward erosion rate of around 1 m per year. Additionally, there is a risk that beach material may be temporarily denuded at times of severe storm surge such as those of 1953 and 1997; events of this severity would be likely to expose the cables if simply buried in the beach material. Therefore, it is strongly advisable to bury the cables in the rock underlying the beach rather than in the beach material itself. Due to the complex geology at this location, it is not possible to prescribe a burial depth profile that will guarantee to satisfy both of the above design requirements. The precise nature of the underlying rock is uncertain; there are several candidate formations ranging from firm chalk to loose glacial material. The Project has provided some geotechnical survey data at locations offshore of the landfall. However, in order to answer the above questions, more geotechnical and geophysical data will be needed at the landfall itself (e.g. subsurface radar and deep boreholes). Also, a site visit is recommended to improve understanding of the beach profile and to help to identify the underlying geology. This desktop study has been based on literature in the public domain; survey charts provided by StatoilHydro; and experience of similar projects in the region. There is potential to progress this work further with follow-up studies as more survey data become available.
2008, Metoc, Sheringham Shoal Offshore Wind Farm, Cable Landfall Erosion Study, Report