by Martin Jefferies

An offshore wind farm off the north Kent coast set to be the world's largest is starting to take shape.

The first of 177 foundations have been installed at the London Array site, around 20km out to sea in the Thames Estuary.

Developers say it marks a "significant milestone" for the project.

A monopile weighing the equivalent of around 35 double deckers buses was driven more than 20 metres into the sea bed and a transition piece - on which the giant wind turbines will eventually sit - was lifted into place.

London Array will include 175 wind turbines and two offshore substations, dwarfing the existing Thanet Offshore Wind Farm, located further along the coast towards Ramsgate.

Anders Eldrup, chief executive of DONG Energy, which has teamed up with E.ON and Masdar for the project, said: "This is a significant milestone for London Array and we'll soon be seeing the turbines going up and the wind farm starting to generate low carbon electricity."

Building the wind farm is a complex operation.

The monopiles and transition pieces are brought to the Thames Estuary from Germany and Demark on giant transportation badges.

The piles are lifted off the deck of the vessel using a 60-metre crane and driven into the seabed using one of two powerful hydraulic rams.

They must be installed in a pre-determined pattern, as each foundation has been individually designed and built.

Construction of the first phase of London Array is due to finish at the end of 2012, when the wind farm will have a capacity of 630MW.

If a second phase is approved, the wind farm's capacity could top the 1GW mark, providing enough electricity to power up to 750,000 homes.