The Sheringham Shoal Offshore Windfarm is located off the coast of Weybourne in Norfolk. The windfarm has been developed by Scira Offshore Energy (StatOil and Statkraft) to satisfy the ever increasing need for green energy in the UK.
The total capacity of the windfarm once completed will be in excess of 300MW, supplying over 220,000 homes with clean energy. A total of 88 turbines with a maximum height of 132m including the blades will be placed, and the power generated by the turbines will be collected in two offshore substations from where it will be transported to the shore via the export cables.
VolkerInfra was deployed by the main contractor, sister company Visser & Smit Marine Contracting (VSMC) to design and install the crossing of the primary sea defence by means of 2 Horizontal Directional Drills. In each of these drills, a HDPE duct was installed with an outside diameter of 560mm (PE100, SDR 11). These activities - Phase 1 of the project - were carried out over the winter of 2009/2010. In addition to Phase 1, VolkerInfra also assisted VSMC during the cable pulls (Phase 2 of the project) which was executed in autumn 2010. An important issue during the design and construction were the limitations with regards to the permits and allowable working windows.
For the installation of the ducts, the VolkerInfra-owned 250 ton drill rig was mobilised, as well as the marine vessel MSC Ailsa with a 4-point mooring system and diving facilities including diving tanks.
The rig was positioned onshore, whilst the marine vessel and the divers assisted the drill at the exit point offshore. The HDPE ducts were butt welded onshore by VolkerInfra's own team. Once a drilling was completed the HDPE duct was pulled into the sea connected to the drilling string, ballasted with water and pulled into the borehole.
The HDD works were carried out during the winter months which led to additional requirements to safeguard the health and safety of our employees, Client's representatives and the general public. Special care also had to be taken to minimise the lost time owing to frozen equipment.
Phase 2 of the project was carried out during the autumn of 2010. Once again the weather was extreme for the time of year, and precautions had to be taken to ensure the safety of all components of the project.
For the cable installation, the VolkerInfra-owned 40 ton pulling winch was mobilised. At the offshore side a diving support vessel as well as several other support vessels were mobilised to assist during the 1000m float-in of the HV cable from the cable lay vessel ‘Team Oman'.
Upon completion of the cable installation in the ducts, the ducts were grouted using a special cable grout with a low thermal resistivity to ensure that no hotspots would occur once commissioned.
The scope of works only involved a part of the 20km offshore inter-turbine cabling works as awarded to VSMC, but it was a vital part of the scheme bringing together all the disciplines in a concentrated area.
Thanks to a close collaboration between the VolkerWessels companies (VSMC, Visser & Smit Hanab Marine Department, VolkerStevin Plant Department and VolkerInfra) the weather delays were minimised and the cables installed successfully.