O.2.2.2 Cable inspection and repair

Function

Identify faults and replace whole or sections of cable.

Who supplies them

Offshore works: Boskalis, Briggs Marine, CWind (Global Marine Group), Flexlife, Leask Marine, Offshore Marine Management, Oceaneering, Power CSL, Pharos Offshore and Seatech Commercial Diving.

Electrical works: Boskalis, Power Cable Services.

Key facts

The frequency of inspections depends on sea bed mobility and results of the initial surveys. Surface surveys can be used to detect substantial cable exposure, but ROV surveys will be required for more accurate burial depth data. Insufficient burial or cable exposure is typically resolved by remedial measures including protective mattressing and rock dumping, normally using a dynamically positioned fall pipe vessel, or occasionally side-dumping vessels. 

Cable damage may come from the mechanical loads of wave and tidal action if 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

The owner is therefore 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. For array cables and export cables before the transfer of transmission assets to the OFTO (UK only, up to 18 months after works completion date), the wind farm owner is responsible. For the export cable the transmission operator or OFTO (UK only) is responsible, although the wind farm owner has a strong interest in ensuring that export cable faults are rapidly fixed to reduce and reduction in transmission capacity. 

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

Cable repair will normally require a full cable laying spread consisting of a cable laying barge with cable plough or jetting equipment, with a quadrant to ensure that the minimum bend radius is not exceeded.

On deck, the cable is cut and a new section inserted with cable joints linking the new and old sections. 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 high interest by transmission 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 will be cut at the bases of the foundations, the internal section of cable removed, and a new cable laid using the same process as installation.