Offshore and Onshore Cables
There will be two HVDC cables plus a fibre optic cable for system communication processes. High Voltage Direct Current (HVDC) cables allow for efficient transportation of electricity over large distances.
250kV HVDC cable
courtesy of Prysmian]
Approximately 170km of the Greenlink cable route will be laid offshore. The final routes will be selected following a detailed assessment of the marine environment and technical challenges.
Proposed routes can be found in the latest edition of our brochure and will be in Planning documents here.
The cables will predominantly be buried in the seabed. However, where the geology or marine environment does not lend itself to this, a cable may be laid on the seabed with protection added. Protection could be in the form of concrete mattress or rock dumping on top of the cable.
One of the benefits of HVDC cables is the relatively small footprint required to install them underground onshore. They can be buried within the local road networks or within agricultural land, as appropriate. Proposed routes can be found in the latest edition of our brochure and will be in Planning documents here.
The onshore cables will be in a single trench with a typical depth of cover of 850mm. These will be installed in plastic ducts to simplify the construction process. It is usual for the two ducts to be positioned close together (approximately 300mm). A protective cover and warning tape will also be buried along with marker posts at regular intervals at ground level. This arrangement is shown in Figure 1.
It is usual to increase the depth of cover in agricultural land to around 1050mm (from 850mm). The width of the trench may also vary with depth of cover (the deeper the cables are buried the wider the trench may become).
A specific design would need to be engineered for utility crossings, crossing watercourses or other areas where the ordinary depth of cover cannot be achieved.
Installation of Cables at Landfalls
We are proposing to use a Horizontal Directional Drill to install the cables at both Baginbun Beach (Ireland) and Freshwater West (Wales). Using this method of installation will ensure that cables can be installed without any impact on the beaches at both locations and will avoid any impact on the dune system at Freshwater West. The cable will emerge below the low water mark so no work will take place on either beach. While the construction programme for the full project is anticipated to take around three years, construction work around each landfall would last for approximately 3 months and be scheduled to avoid the most popular periods of use. Below is an illustration of how an HDD might work.