The installation of the onshore export cable completes the connection between the offshore export cable and the onshore substation.

What it costs

About £3 million for a 450 MW floating offshore wind farm, depending on distance and complexity of route.

Who supplies them

Construction companies: Balfour Beatty, J Murphy and Sons, NKT and Nexans.
Marine contractors: DeepOcean and Global Offshore.
Onshore export cable trenching process. Image courtesy of Ørsted. All rights reserved.
Onshore export cable trenching process. Image courtesy of Ørsted. All rights reserved.

Key facts

The subsea cables terminate a short distance inland at the transition joint pit. This could be located on the beach, behind a sea defence, or up to 1 km inland.

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 is established along the cable route. These sites provide equipment storage, car parking and welfare facilities for staff. Typically, they are around 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 is carried out using open trenches, typically around 1 m wide and up to 1 km in length (depending on the cable) or by placing ducts into the trenches and covering them over more quickly. With ducting, it is typical to use medium density polyethylene (MDPE) ducts which are laid in the trench and the cable pulled through the ducts at a later time in 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.

Specialist drilling equipment creates a bore that passes the obstacle and can be up to 1 km in length. Drilling mud is used as lubrication, and this is recycled through a temporary mud lagoon during construction and disposed of after construction. 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 mitigation.

What’s in it

Guide to a Floating Offshore Wind Farm