Function
The network of array cables transfers power from the wind turbines to the offshore substation. It also provides auxiliary power to the turbines when they are not generating and provides fibre communications.
What it costs
About £32 million for a 450 MW floating offshore wind farm.
Who supplies them
Hellenic Cables, JDR Cable Systems, LS Cable & System, Nexans, NKT, Prysmian, Sumitomo Electric and TKF.
There are other cable manufacturers based in China and Japan, but they have yet to be used widely for UK projects.
Dynamic array cable. Image courtesy of JDR. All rights reserved.
Graphic showing cables connected from the floating substructure to the grid showing bend stiffner, pull head, bend restrictor and other accessories strapped to it. Image courtesy of BVG Associates. All rights reserved.
Key facts
Array networks are most often designed as “strings” which connect several turbines to the substation. They can also be designed in loops to increase redundancy.
Each turbine is linked to the next with at least 1.5 km of array cable, assuming a 15 MW turbine with 224 m rotor diameter and seven times diameter spacing between turbines.
Array cables have a dynamic cable length between the sea bed and the floating substructures. The dynamic cable length typically follows a lazy wave configuration to accommodate dynamic movement of the floating substructure, including lateral excursion (the horizontal movement of a floating offshore wind turbine). It must also accommodate the loads resulting from the cable being exposed to the whole water column, as well as withstanding abrasion from the sea bed. At the sea bed, the cable is either buried or sits on the sea bed anchored using rocks or protective matting (see I.2 for further information).
The dynamic section of array cable for floating offshore wind farms is incorporated in one of three ways:
- A single length of dynamic cable between turbines
- Dynamic lengths at each turbine connected to a static length in between using either field joints or connectors, or
- A single cable assembly using dynamic cable at each end with a length of static cable in between, assembled using factory joints (so manufactured and installed as a single length of cable).
The final choice depends on the trade-off between the relative costs of static and dynamic cables, the additional costs of using field joints or connectors, and the introduction of additional potential points of failure at field joints or connectors.
In deep water array cables could be suspended across their whole length. This would put greater loading on the cable due to water current-induced movement of the cable but would reduce the length of cable required. Floating projects to date have not used this approach so the water depth at which this becomes attractive is not well understood, but it is likely to be in water depths of around 500 m.
Array cables are typically rated at 66 kV. In the next few years, array cable voltages are expected to increase to 132 kV. This is to accommodate more efficiently turbines rated at and above 16 MW and to reduce the number of array cable strings required.
Array cables are typically supplied by the manufacturer with cable accessories, although the production of accessories may be outsourced. Cable protection may be included in the array cable supplier’s scope, but it is more often part of the installer’s scope.
Some larger cable manufacturers have cable installation equipment and vessels (see I.2 for further information), but EPCI array cable packages have typically been led by marine contractors.