Windfarm in Congested Seabed Requires Cable Management

HR Wallingford carried out rock berm scour tests for Ørsted in its Fast Flow Facility, which is able to re-create realistic marine conditions in the laboratory.

The world’s largest working offshore wind farm, occupying an area of 55 sq. mi., opened in the Irish Sea off Barrow-in-Furness in the North West of England in Sept. 2018. HR Wallingford has been helping Danish operator, Ørsted, to build stable cable crossings for their new Walney Extension windfarm, on what is becoming an increasingly congested seabed.

The U.K. has one of the largest offshore windfarm estates in the world.  Approximately 7,000 turbines across 32 sites contribute up to 30% of the power delivered to the U.K. national grid.  Each new windfarm must be “plugged in” onshore using buried high voltage sub-sea cables. The most recently constructed round three windfarms, which are located in significantly larger development zones than previous projects, are experiencing a new phenomenon: a congested seabed with a complex network of existing cables and pipelines that new ones must cross in order to make it to land.

At Walney, the four existing windfarms in Morecambe Bay, along with gas pipelines and cables, meant that there were eight obstacles in the way. To get over these obstacles, it was necessary to bring the buried cables up out of the seabed, over the existing asset, and then rebury them, securing the exposed cabling with a cover of rock—a rock berm.

However, rock berms can sometimes be problematic, causing large holes to be scoured in to the seabed that can extend hundreds of meters from the structure. This scour can expose the existing assets and can lead to expensive repair bills. This makes it increasingly important to have a crossing design from the outset.

HR Wallingford carried out scour tests for Ørsted in its Fast Flow Facility, which is able to recreate realistic marine conditions in the laboratory. An assessment of the as-built footprints relative to the design was also conducted.

The research work behind new design guidelines that minimize scour is the focus of two joint publications by Ørsted and HR Wallingford that will be presented in November 2018 in Taiwan at the International Conference of Scour and Erosion (ICSE), a platform for sharing advances in research and practice on the scientific and engineering challenges related to scour and erosion. —HR Wallingford

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