The installation of export cables enables the connection between the offshore substation and the onshore export cable.
The site definition assumes the use of a fixed substation, so the export cable installation processes described in this section is the same as that for a fixed offshore wind farm.
What it costs
About £20 million for a 450 MW floating offshore wind farm. This includes the CLV, and cable lay and burial. It also includes survey works, route clearance and the installation of cable protection systems.
Who supplies them
Marine contractors: Boskalis, DEME, DeepOcean, Global Marine, Global Offshore, Huisman, Jan de Nul, Oceanteam, Seaway 7 and Van Oord.
Cable manufacturers with installation capabilities: Nexans, NKT and Prysmian.
Offshore array cables installed on the sea bed with ROV undertaking the cable burial. Image courtesy of Jan de Nul. All rights reserved.
Export cable installation starts with the shore pull-in (first-end pull-in) (see B.1.2 for further information). The CLV then moves off, laying the cable as it goes.
Export cables are laid in sections which are as long as possible to avoid expensive subsea joints.
Export cables are typically buried 1 to 4 m below the sea bed. Burial usually takes place simultaneously as the cable is laid using a cable plough, as immediate burial and protection is obtained in a single pass which reduces costs. A two-stage process may also be used where the cable is first laid on the sea bed, after which a vessel with a trenching remotely operated vehicle (ROV), and a vertical injector and jetting sled, undertakes the burial. The approach taken depends on a number of factors including the availability of equipment, cost and ground conditions.
At the offshore substation, the cable is either set down and wet-stored for subsequent pull-in to the substation, or immediately pulled-in which is preferred. Electrical terminations are made after pull-in. Static export cables are used from shore up to the fixed offshore substation and pulled-in through J-tubes.
Floating substructures may be used in the future to support offshore substations and will require dynamic lengths of export cable. These will be installed in a similar way to dynamic array cables.
Most export cable manufacturers have CLVs and install export cables themselves.