AXIS Explores
Offshore cables
Overview
Offshore wind farms have two types of cables:
Inter-array cables (IACs)
- IACs connect individual WTGs to each other in a ‘string’, and to the offshore substation. They are medium voltage (typically 66 kV)
- The failure of an IAC results in that string of WTGs going offline until it is repaired/replaced
Export cables
- Export cables connect the offshore substation to the onshore substation and are now 400 kV or higher
- The failure of an export cable may take the whole wind farm offline until it is repaired/replaced
According to the Catapult Offshore Renewable Energy Centre, cable failures account for 75-80% of offshore wind insurance claims* with most occurring in the fabrication and installation (construction) phase.
- Cables may fail due to defects in design or manufacturing, poor installation, or be broken by vessels or anchors
- Cable breaks can typically be repaired where they lay but a defect may require complete replacement of the cable
- Cable protection systems have recently suffered failures on a number of offshore wind farms, requiring costly remediation work
Typical failures
Design
A number of design issues/defects have resulted in offshore cables failing:
- Degradation of, or inadequate insulation between, the three power cores within the cables
- Failure of the fibre optic cable due to induced current or magnetic from the power cores
- Failures at the termination point in the WTG (the T-connector)
- Failures at the joints along the length of the cable. Ideally the export cable has no joints along its length but as offshore wind farms are built further from shore this creates manufacturing challenges
- Excess movement caused by the failure of the Cable Protection System (CPS) surrounding the IAC or export cable
Transport and installation
- The minimum bend radius being exceeded, resulting in structural failure of the cable Improper storage or handling at the factory or mobilization port
- Inadequate burial, leaving the cable exposed to anchors or other fishing activity
- During construction cables may be struck by vessels before they are buried unless guard vessels are used
Operations
- Scour caused by water swirling around the foundation may cause the cables to move and break up unless they are properly protected, such as with layers of rocks
- Vessels breaking cables with trawling nets or anchors is relatively rare
- Condition monitoring systems measure temperature and strain along the cable and can identify problems before a failure occurs. However, no systems have yet been installed which cover the entire cable length including the terminations
Cable protection systems
Issues A number of offshore windfarms have recently suffered failures of the CPS which is designed to stop excess movement in the cables:
- Fixed foundation offshore wind farms all use ‘static’ cable designs for both IAC and export cables
- Static cables can be damaged by excessive movement in the ‘freespan’ between the foundation and the seabed caused by water currents (circled in red in the image)
- The CPS is designed to restrict this movement and also protect cables from anchor strikes, etc. There is evidence that some CPS products have been under-designed leading to failures, particularly on shallower sites
- Remediation may include dumping rocks over the CPS to stabilise it
This material is for general information and education purposes. The advice and recommendations in this document may help to reduce the risk of property loss and business interruption. Implementing these recommendations does not guarantee the avoidance of loss or that all reasonable preventative measures have been taken. By providing this advice and recommendations, AXIS does not relieve the owner or others of their duties and obligations with respect to assessing and implementing loss prevention measures. AXIS disclaims any liability with respect to loss prevention or loss mitigation programs.
