Offshore Cable Burial: How deep is deep enough?

Offshore Wind (OSW) is critical for the UK's economy and energy security. It is also an area of huge investment, for example £14bn has been committed up to the end of 2021 for new OSW sites - the 4th largest construction programme in the UK. Beyond this, the UK's current 2030 OSW installed capacity targets will require £48bn of investment and provide direct employment for 27,000 people.

Despite the growing maturity of the OSW sector, certain elements of the installed infrastructure remain problematic. Principally, problems associated with subsea power cables that transport and distribute the electricity generated offshore in wind turbine generators to the onshore transmission system currently account for 75% of the cost of all insurance claims and faults typically take 100+ days to rectify. This leads to breaks in supply and loss of revenue for the wind farm operator which in the long term can lead to longer payback periods and reduced investment elsewhere in their renewables portfolio. In shallow waters these cables must be protected from anchors and fishing gear and the primary protection method is to bury the cable below the seabed. The cable burial depth is a compromise between economic cost of burial (going deeper takes longer, requires larger ships and may require more complex operations) and risk to the cable being damaged by anchors/fishing gear penetrating the seabed. Within this context, anchor-cable interactions currently account for 85% of power cable failures. The planned rapid expansion of offshore wind around the UK - installed capacity increasing 7.5 times over the next 30 years - will require new cable installations within some of the busiest shipping/fishing waters in the world and it is essential that these new cables are installed at the appropriate depth. However, the industry currently lacks appropriate scientific tools to determine anchor penetration depths in different soil conditions. Instead they use simple look-up tables based on very broad descriptive classifications of the soils on the seabed that basically split the huge spectrum of real soil conditions into two categories - soft or hard. This approach has been shown to be highly conservative in some soils leading to unnecessarily deep (and costly) burial. However, it is clearly non-conservative in other conditions as anchor-cable interactions dominate cable failures.

This proposal will tackle the lack of sound anchor penetration models head on and, through physical testing and computational modelling, develop a toolkit to assess anchor penetration in different soil conditions. This anchor penetration prediction tool will be based on the site investigation data typically available along cabling routes and avoid the use of oversimplistic look-up tables. Its development will be guided by an industrial project steering group made up of key parties from the OSW sector. Crucially, this innovative anchor penetration model will be calibrated and validated using a geodatabase comprising actual site investigation data. Model performance will be assessed against proven, demonstrable ground conditions and therefore will not rely on hypothetical ground conditions which can be oversimplified using current cable burial assessment techniques (e.g. descriptive single-type soils that do not change with burial depths, as opposed to more complex, multi-layered soil types). In addition to the anchor penetration predictive tool, a number of spatial mapping layers (specific to the UK Continental Shelf) will be created, derived from the tool application to known ground conditions across the UK seafloor. These mapping layers will be made openly available, and are anticipated to feed into high-level spatial planning decisions at project concept stage.

In summary, this project will provide an industry usable anchor penetration model allowing the OSW sector to answer the key cable burial question - how deep is deep enough?