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Mast stroke

Foundations

Out in the Thames estuary a mammoth piling operation is nearing completion. Danish power companies Elsam and Vestas are installing 30 wind turbines on the Kentish Flats sand banks, 8km offshore.

The array will be one of the largest yet built, with the 3MW machines driven by 22.5m long rotor blades. Their hubs will stand a lofty 70m above mean sea level.

To support the turbine masts Danish marine works contractor MT Hojhaard is putting in steel monopile foundations, working in 6.6m to 7.7m of water from a purpose-built 14,850t jackup barge, the Resolution.

Ground consists of roughly 500mm of silt and sand over 10m to 12m of London Clay. The 4.5m diameter monopiles are up to 44m long. They are driven in to a toe of 35m in 10 to 12 hours.

Piles are positioned using GPS;Resolution is equipped with thrusters which can manoeuvre its bow and stern from side to side, as well as forward and aft. When in position the hull is jacked clear of the water. Monopiles are lifted using a 300t capacity luffing jib crane and placed in a pivoting tail stock on Resolution's stern.

Lowering of the pile to the sea bed is guided by a banksman, but is closely monitored by computerised sensors to ensure pinpoint accuracy.

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