DONG Walney (UK) Limited, a wholly owned subsidiary of DONG Energy A/S, is constructing and building an offshore windfarm with a capacity of approximately 183MW. The Walney Offshore Windfarm (WOW) is located in the eastern Irish Sea, approximately 15km west of Walney Island, Cumbria.
Phase 1 of the Walney Offshore Windfarm consists of 51 wind turbines each of 3.6MW, and 1 offshore substation. The turbines will be interconnected to an offshore substation by approximately 6.3km of 33kV internal array cable systems. The offshore transformer platform (substation) will be situated in the middle of the windfarm and will be connected to the 400 / 132kV Heysham grid substation on shore through approximately 45km of 132kV submarine armoured cable and approximately 2.7km of underground cable.
Visser & Smit Marine Contracting (VSMC) was awarded the interturbine cable and export cable installation contract. VolkerInfra and VSH Marine assisted VSMC with the crossing of the sea defence, the pull in of the cable, and burial of the cable in the near shore area.
VolkerInfra installed the cable duct from the onshore joint bay, crossing the sea defence up to about 300m on to the tidal flat. The 560mm HDPE duct was built up in sections onshore, using butt welding technology. The sections were then towed onto the tidal flat, placed in a pre-excavated trench, ballasted with concrete mattresses and backfilled.
One of the complicating factors was the crossing of the Barrow export cable which was in operation. The cable was supported and a concrete tunnel was installed underneath the cable in which the PE duct was installed.
Another challenge was the route of the cable with an almost 900 curve. Two VolkerInfra-owned 40 ton pulling winches were used to pull in the HV cable. Prior to the cable pulling, the VSMC-owned plough was pulled towards the end of the PE duct. During the pull, the friction of the cable was reduced using special VolkerInfra-owned heavy duty cable rollers.
Although the time schedule was tight, VolkerInfra and VSH Marine were able to install the duct and pull the cable with the minimum delay to the cable lay vessel, ‘Stemat Spirit'. Once the cable was pulled in, the PE duct was capped and filled with a cable grout mix with a low thermal resistivity.
Thanks to close co-operation between all parties, the near shore cable works were completed to the satisfaction of all stakeholders.