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CABLE PROTECTION FEATURE


CABLE PROTECTION SYSTEMS


AN OVERVIEW BY WOOD GROUP KENNY


Wood Group Kenny has extensive experience in the design of cable systems for wind, wave and tidal projects, and has recently led research on cabling hazards for the Carbon Trust.


ARRAY CABLES AND EXPORT CABLES Offshore wind farms commonly feature two types of cabling: array cables and export cables. The array cables link the wind turbines to one another, in strings or loops, and to an offshore substation. At the substation, the voltage is stepped up to minimise transmission losses and one or more large export cables are used to carry the combined output to land for connection to the onshore grid. Loss of an array cable may result in the loss of output from a string of turbines; perhaps 6 -10 machines depending on the string configuration and the location of the fault. Damage to an export cable, however, may result in the loss of revenue for an entire wind farm.


CABLE PROTECTION


Protection of subsea power cables is therefore critical to the maximisation of revenue of the wind farm. Cable damage may occur during both the installation phase and operational phase of the project. During installation, cables may be bent beyond their design radii or overloaded due to careless handling. Post-installation testing is conducted prior to release of the cable installation contractor to minimise the likelihood of an installation defect remaining undetected. Cables are then at risk of impact from, or entanglement with, anchors, dropped objects, debris and (particularly) fishing equipment throughout the operational lifespan. Damage can also result from cable movement leading to fatigue and abrasion, particularly where amplified by resonance.


CABLE BURIAL


For the operational phase, export and array cables are often buried to avoid subsea hazards, typically either by ploughing, or by post-lay jetting. The burial depth required will depend on the soil characteristics encountered, the mobility of the soil and the site-specific risks to the cable. Cable installation should be preceded by a burial assessment with the objective of recommending a suitable and statistically robust level of protection at minimum cost to the owner. In some areas, burial may not be possible and additional protection measures may be required. Such areas include...


• Where the soil depth is insufficient to provide protection


• Where the soils are too hard to trench economically


• At and approaching crossings with other cables or pipelines


• At the final approach to offshore structures where it is difficult to safely deploy burial equipment


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