The Offshore Wind Accelerator (OWA), a collaboration of eight developers and the Carbon Trust has published new recommendations for fibre optic cable design to mitigate the risk of export cable failure. The design changes are the outcome 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.
In 2017, the UK’s Offshore Wind Programme Board published a review of high voltage export cable reliability, showing that of the seven post-commissioning failures the UK offshore wind industry had experienced, issues in the fibre optic cable were responsible for six. Each failure was estimated to have cost just under £23 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 the Fibre Optic Cable Protection Assessment project to validate this modelling, and suggest design recommendations to mitigate risk. This was undertaken by engineering experts at RINA Tech UK.
The five key recommendations are:
- 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.
- 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.
- There are to be no insulating binder tapes, adhesives or other insulating material between the armour wires and sheath of the fibre optic cables.
- 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.
- The position of the fibre optic cable within the three-core cable remains unchanged from present practice.
The recommendations are likely to increase the cost of the export cable between 0-0.5% depending on the design that it replaces, and will not affect the installation cost. Therefore, overall savings remain significant based on the costs incurred through cable failure.
