Function
The installation of the onshore export cable completes the connection between the offshore export cable and the onshore substation.
Who is involved
Onshore export cable installation services are conducted by specialist suppliers contracted by the developer.
Key facts
The subsea cables terminate a short distance inland at the transition joint bay. Onshore cabling is generally underground to address local concerns over the siting of overhead power lines. There are a range of local services used before and during the cable installation. These include wheel washing, road cleaning, traffic management, signage and temporary bridges over rivers and ditches. At least one site compound will be established along the cable route. These sites will provide equipment storage, car parking and welfare facilities for staff. Typically, they will be 1 ha in size.
Before construction, site investigation and environmental work is undertaken to plan the installation and minimise impact on the surroundings. A cable corridor is used during installation, which comprises the cable trench, storage for spools and access road.
Installation can be carried out using open trenches or by placing ducts into the trenches and covering them over more quickly. Ducts which are laid in the trench and the cable pulled through the ducts at a later stage can be up to 1 km lengths. This option allows excavation, duct installation and backfilling to be completed in sections of up to 120 m in a day. This minimises the amount of excavation left open outside working hours, which can help reduce environmental and safety concerns.
Where the cable crosses obstacles such as roads or railways or encounters difficult or highly sensitive conditions, directional drilling may be used to route and pull the cable under the obstacle without the need for trenching. Once drilled, a cable duct is then pulled through and the cable is then pulled through again using specialist equipment.
The cable is tested to ensure a complete circuit is in place. Once fully installed, an energised test is carried out to verify operation at or close to the intended voltage. Care is taken to reduce the impact on endangered species such as newts, bats and dormice which might require specialist environmental monitoring and/or mitigation.