London Array Limited, a consortium comprising E-on Group, DONG Energy and Masdar, have been developing the London Array Offshore Windfarm located within the outer Thames Estuary. In total, 341 turbines shall be constructed and, at the time of tender, is considered the largest offshore windfarm in the world, generating enough energy to power 750,000 homes.
The windfarm includes both onshore and offshore substations and associated HV connections. VSMC were appointed to undertake the associated HV cabling works linking the turbines, offshore substation and onshore substation.
VolkerInfra assisted VSMC to undertake the landfall of 4 no. export cables by Horizontal Directional Drilling (HDD). The works comprised installation of HDPE ducts beneath the sea defence wall. This required close liaison with the client and co-ordination with a number of other contractors engaged on the work site. Access to the site was restricted and the project was considered contentious in the local community.
The client advised that the area was deemed sensitive from an archaeological and environmental perspective. As part of the contract, there was a requirement to undertake an Explosive Ordnance Threat Assessment. Initially VolkerInfra instructed a desktop UXO study which was subsequently followed by both non-intrusive and intrusive surveys. The intrusive survey resulted in the controlled explosion on site of four live rounds.
In addition, the client instructed the pre-excavation of 4 no. cable trenches, each 800m in length and 1.5m wide and deep. The excavation works were to determine the presence of archaeological artefacts which resulted in the discovery of a prehistoric fish trap. The soft ground conditions encountered during the excavation works led to further challenges and required a rapid response from VolkerInfra.
4 no. 630mm diameter PE pipe strings of 130m length for the HDD operation were welded by butt fusion within land behind the sea wall. The pipe strings were manoeuvred over a ditch and the sea wall before being pulled through the drilled bore(s). The tidal offshore area made the operation difficult as access to the area was restricted due to the soft ground conditions and the limited and restricted water depth during high tides.
The methodology adopted was determined given the ecological and environmental constraints imposed.
In the summer of 2011 and in 2012 VolkerInfra will assist VSMC, together with VSH Marine, with the cable pull of the 4 export cables into the landfalls and then will bury the cables on land.