Identify faults in the array and export cables themselves, in addition to any cable accessories installed, and replace whole or sections of cable.

Who supplies them

Boskalis, Briggs Marine, CWind, Offshore Marine Management, Pharos Offshore and Power CSL.

Key facts

Cables are monitored and inspected to ensure they are operating as intended and to identify issues that could lead to future faults (see B.1 for further information).

Dynamic and static cables are monitored and inspected using mostly the same methods. The electrical performance of cables, joints and connectors is monitored using techniques including distributed acoustic and temperature sensing, and partial discharge monitoring. Surface or visual inspections are used to monitor the cable exteriors, cable accessories, cable protection system and cable entry/exit points from floating substructures, the sea bed and offshore substation foundations. Subsea visual inspections use ROVs.

The dynamic sections of array cables require additional visual inspection of the buoyancy and ballast elements and where it is tied down to enter the sea bed.

The frequency and number of units inspected depends on the results of the initial surveys, and are varied as results change for subsequent inspections. Surface surveys can be used to detect substantial cable exposure, but ROV surveys are required for more accurate burial depth data.

Cable damage may result from the mobility of dynamic sections in the water column, mechanical loads of wave and tidal action where the cable is exposed, from anchors or fishing gear, or as a result of handling during transport or installation that exceeds the cable’s specification. Although cables typically come with a two-year warranty, none of the main causes of damage is covered by the warranty.

Excessive cable exposure or insufficient cable burial depth is typically resolved by remedial measures including protective mattresses and rock dumping, normally using a dynamically positioned fall pipe vessel, or occasionally side-dumping vessels. In the worst case, sections of cable may need to be reburied.

The owner is responsible for monitoring and surveying the cable and repairing it when required. The survey work and remedial work is likely to be subcontracted to a specialist provider.

  • Array cables are always owned by the wind farm owner.
  • Export cables are normally owned by the transmission system operator, or in the UK they are transferred to an offshore transmission system owner (OFTO) within the first 18 months after works completion date.

Some offshore wind farms have redundant export cables so a fault on one cable does not necessarily lead to loss of wind farm output.

Cable repair normally requires a full cable-laying spread consisting of a CLV with a cable plough or jetting equipment, with a quadrant to ensure that the minimum bend radius is not exceeded. On deck, the cable is cut, a new section inserted with cable joints linking the new and old sections, and any accessories are replaced. Unlike in subsea telecoms, where cables are largely standardised, subsea power cables may differ substantially. In the past, bespoke joints have been used but there is interest from transmission system operators in developing universal joints.

For array cables, shorter cable lengths and challenges in joining shorter cables mean that replacement of the cable may be more cost effective than repair. If so, the cable is cut or disconnected from each floating substructure, the array cable length removed, and a new cable laid using the same process as for its installation.

What’s in it

Guide to a Floating Offshore Wind Farm