3.2 Offshore cables

All offshore cable installation activities are preceded with a survey to define the route and identify any unexplored ordnance (UXO). This is followed by a pre-lay grapnel run (or alternative method) to clear debris from the cable route. All offshore cable installation (export and array cables) involves the following activities:

• Cable lay
• Cable burial
• Cable pull-in (to turbine, substation or shore), and
• Electrical testing and termination.

There are different strategies involving one or two vessels, and the chosen approach depends on seabed conditions and equipment available to the contractor. Pre-trenching and simultaneous lay and burial using a cable plough is often preferred if the soil is suitable as immediate burial and protection is obtained in a single pass which reduces costs.

Export cable installation starts with the shore pull-in (first-end pull-in). The installation vessel then moves off, laying the cable as it goes. Export cables are laid in as long sections as possible, of up to 70 km in length, to avoid expensive subsea joints. To avoid unnecessary handling, it is preferred that subsea cables are loaded directly onto an installation vessel from the factory. Lengths may be pre-cut.

Array cable installation starts with the first-end pull-in at the substation (subsequent first-end pull-ins are done at each turbine). Array cables are usually installed in a spider arrangement with a series of strings of turbines connected to the substation or in a series of loops (strings connected together away from the substation). The cables may be carried as a single length then cut offshore or pre-cut from the factory. Using pre-cut lengths can save time offshore but because turbine spacing is not uniform, it limits the order in which the cables are installed. This can lead to important delays if there is a problem at one of the turbine locations.

Cables are typically buried to 1-4 m below seabed to ensure long-term cable integrity and to prevent damage, for example by fishing vessels, ship anchors or seabed movement.

Cable protection typically falls within the installer’s scope of work. This consists of bend restrictors or stiffeners to limit fatigue loading on the cables and cable entry systems that lock and seal the cable as it enters the foundation. Other techniques like rock dumping and mattresses are also used to ensure burial and protection on cable crossings.

Notable differences for floating

There are no major differences between floating offshore wind farms and fixed offshore wind farms.