The cables at the wave site at Billia Croo and the tidal site at the Fall of Warness are broadly similar, with an 11kv cable running from an onshore substation out to each open-sea berth.
The cables are wet-type composite cables consisting of three EPR-insulated stranded copper power cores designed for alternating current, three 2.5mm2 copper signal/pilot trip cables and a 12-core single-mode fibre-optic bundle. The cable is then armoured with two layers of galvanised steel wire. Cables were provided by AEI (wave test site cables) & Pirelli (tidal test site cables).
In 2010, a UK Department for Energy and Climate Change (DECC) funded project enabled us to increase our berth capacity by installing new cables. The cables, built by Draka in Norway, are of a similar specification to the first cables described above with the inclusion of an additional 4 core 4mm2 auxiliary power cable. Two additional cables were installed at the tidal site and one at the wave site using a total of 12km of newly procured cable.
The conductors on the Billia Croo cables are 50mm², giving a nominal rating of 2.2MW, while the conductors for the Fall of Warness cables are 120mm², giving a 5MW rating.
The cables were laid as standard sub-sea cables on the sea bed. As the cables approached the shore, in 15m of water, ductile iron cable protectors were attached. At the low water spring tide mark, each passes into a trench dug 12m into the seabed and beach. On shore, the cables are fed into a manhole and then into the substation.
In the substation, each cable terminates at an 11kV circuit breaker, along with the tripping cable. The fibres are terminated in the communications area of the substation.
At the seaward end, each cable, when not occupied by a developer, is terminated using a specially designed connector provided by J+S Ltd, which allows us to carry out planned condition monitoring of any cables not in use by developers. These terminators can, if required, be converted into splices to enable developers to use umbilical cables to attach their devices to our cables.
Cable lay 2010:
While a great deal of experience has been built up in hostile marine environments within the oil and gas industry, the hardware, operational aspects, maintenance and people do not have the experience of operating in fast tidal flow areas. Therefore the boundaries of marine renewables were pushed to the outermost limits when two new tidal cables were laid in aggressively high tidal velocity for the very first time by a Dynamic Positioning (DP) vessel (by Global Marine Systems Ltd). The vessel, crew, hardware and procedures all worked at the very ultimate limits of their knowledge and ability.
An additional cable was also installed at the wave test site and all the cables laid were fitted with new industry leading cable end terminations, allowing developers faster, safer and easier connections of their devices to the Grid, thereby cutting down developer costs. These terminations were designed by Barnstaple based J+S Ltd from a specification drawn up by us.
With this success, we have provided feedback to the industry on what can be another solution for developers wishing to utilise new techniques and procedures for getting their device in the water.