The 30 turbines of the Kentish Flats Offshore Windfarm, 8.5km north of the Kent coast, generate around 280m KWh of ‘green' electricity a year, enough to power over 100,000 homes. Their construction was a major undertaking, costing £105m and taking just under two years to complete, ending in September 2005. Called in by sister company VolkerFitzpatrick, VolkerInfra helped establish a reliable connection to the grid.
The task involved establishing a link between three 33kV "three-phase" circuits, each made up of three cables (red, yellow and blue) from where they met the land at Hampton Pier in Herne Bay to a substation 2.3km away.
For each of these three trefoil transmission bundles there needed to be a fibre-optic cable to transmit vital information such as the temperature of the transmission cables and, potentially, signals sent to the wind turbines from an onshore control centre.
The first step was for VolkerFitzpatrick to cut a trench for three 100mm cable ducts, backfilling with sand and specialist recycled material, and resurfacing as they went to minimise disruption for local people.
Laying the ducts took just over three months ending in mid-March 2005, at which point VolkerInfra could move in and install the cables. But work did not begin until it had suggested a valuable improvement to the original design.
"Our expertise in installing cables meant we could reduce the number of joints in the design from the seven proposed to just four, and also reduce the width of the ducts to a diameter of 100mm," says VolkerInfra Contracts Manager Fred Mastop.
The insight almost halved the risk of faults. "Joints are by far the most vulnerable part of the connection. When faults occur it is almost always at joints," Mastop says. The cost fell dramatically too, with lower material costs and, with only 36 connections needed rather than 63, the job connecting the cables fell to 24 man-days from 42.
This typical example of VolkerInfra ‘value engineering' was achieved thanks to its push and pull installation technique: while winching from the far end, cables were also pushed by half a dozen linear rollers every 400 metres or so, thereby minimising strain. These points were carefully chosen to help ease the cable round right-angle bends.
In a far more delicate operation the fibre-optic cable was installed in a 40mm diameter sub-duct by floating it on a stream of air. It starts vibrating on the airflow, reducing friction to almost nothing.
On top of the technical challenges was the winter weather. Bad weather is always a problem, but VolkerInfra kept working. The cables would have become too brittle to install if it had fallen below minus five, but it did not get quite that cold, which made the installation less difficult than it might have been.
"All of the VolkerInfra staff were very accommodating in extreme weather conditions," according to one Vestas representative. "The installation crews' work on site was very impressive. The work was performed professionally and to a high quality."