OWA releases export cable design recommendations to prevent failure

The Carbon Trust’s Offshore Wind Accelerator (OWA) has published new recommendations for fibre optic cable design to mitigate the risk of export cable failure in offshore wind.

The five key recommendations are:

1. The optical fibres should be encompassed by a stainless steel tube and stainless-steel wire armour, with a combined resistance of no less than 30Ω/km.
2. The fibre optic cable sheath should be semi-conductive, with a material resistivity of no greater than 1000Ω∙m, and dimensions such that the radial resistance through the sheath is not more than 75Ω∙m. 
3. There are to be no insulating binder tapes, adhesives or other insulating material between the armour wires and sheath of the fibre optic cables.
4. The three-core cable should have extruded fillers that are made from semi-conducting material, or have a semi-conducting section around the channel for the fibre optic cable.
5. The position of the fibre optic cable within the three-core cable remains unchanged from present practice.

According to the UK’s Offshore Wind Programme Board’s review from 2017, issues in the fibre optic cable were responsible for six of the seven post-commissioning failures the UK offshore wind industry had experienced. Each failure was estimated to have cost just under GBP 23 million (around EUR 26 million) in both the cost of repairs and loss of wind farm energy production.

Following the industry identifying cable failure as a high-profile issue, Ørsted and Nexans each produced modelling to investigate the mechanism of failure, identifying the fibre optic core as the cause. The OWA’s Cables Working Group then commissioned a project called Fibre Optic Cable Protection Assessment to validate this modelling, and suggest design recommendations to mitigate risk. This was undertaken by engineering experts at RINA Tech UK.

The recommendations published now are a result of the Fibre Optic Cable Protection Assessment project, managed by the Carbon Trust and funded by OWA partners EnBW, Equinor, Ørsted, RWE, ScottishPower Renewables, Shell, SSE Renewables, and Vattenfall Wind Power.

The recommendations are likely to increase the cost of the export cable between 0-0.5 per cent, depending on the design that it replaces, and will not affect the installation cost, with overall savings remaining significant based on the costs incurred through cable failure.