Function
What it costs
About £34 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
Cable manufacturers with installation capabilities: Nexans, NKT and Prysmian.
Key facts
Array cables can either be installed after the floating offshore wind turbine has been installed or can be pre-installed. The advantage of pre-installation is that cable laying is removed from the installation’s critical path. There are challenges to store the dynamic section with its jewellery on the sea bed and recovering it without damage, so it has not become established practice yet.
Array cable installation after the floating offshore wind turbine installation:
- This starts with the cable pull-in at the offshore substation, if it is the first connection in the array loop or string, otherwise it starts at a floating offshore wind turbine.
- Cable accessories such as bend stiffeners and buoyancy modules are attached to the cables on the CLV.
- The cable is then laid away using the CLV towards the next turbine. The buoyancy and ballast modules attached to the cable create the lazy wave shape for the dynamic cable section. After the cable touches down, it is either laid on the sea bed or buried using a cable plough or a trenching ROV.
- The cable is pulled-in at the next turbine to complete the single array length.
Array cable installation where array cables are pre-installed prior to floating offshore wind turbine installation:
- Pre-installing array cables allows as much work as possible to be completed ahead of the installation critical path, so that electrical connection of the floating offshore wind turbine can be completed faster.
- The array cables are installed using the method described above but the cable ends are laid on the sea bed for wet storage with a marker buoy attached.
- At the point of hook-up, each cable end is picked up using an ROV and handled by a support vessel. The support vessel attaches accessories, manages the cable pull-in and termination at the turbine or offshore substation.
Array cable installation with joints or connectors (see B.1.3.4 for further information):
- Some array cable designs use different cable for the static part on the sea bed, and for the dynamic part. If factory joints are used offshore connectors can be avoided and the installation process is almost unchanged from laying a continuous section of cable.
- Where designs use a connector between sections of static and dynamic array cable:
- Dry mate connectors require the connection between static and dynamic cable sections to take place on the deck of a vessel.
- Wet mate connectors allow for connections to be completed underwater using ROVs.
Array cables are usually installed by marine contractors.