The wind turbines turned for the first time in August 2005
A wind farm off the coast of Kent has suffered persistent mechanical failure in the four years since it began operating, BBC South East has revealed.
Gearboxes in all 30 turbines on the Kentish Flats have been replaced with newer, improved parts.
The cost of the repairs is unknown, but the amount of electricity they have produced has been reduced as a result.
Vattenfall, the site's Swedish owner, said lessons learnt were being applied to its wind farm off the Thanet coast.
The turbines have been operating since August 2005 and provide power to houses in Whitstable, Herne Bay, and Canterbury.
In 2007, the gearboxes - which transfer the power from the blades to the generator - were all replaced because of bearing failures.
In 2008, 20 of them were changed again with an upgraded version, and now the final 10 are being upgraded too.
The Danish firm Vestas, which makes the turbines, said the new gearboxes were an improved design.
Andrew Dever, site manager, said: "This is one of the first offshore wind farms in the UK and we weren't sure of the effects the offshore wind would have on the turbine.
"Now we realise that, we've engineered our way through a solution, so we understand more now what the effects are onto the bearings, onto the gearbox, onto the whole of the turbine actually."
The final 10 gearbox replacements will get under way once there is an improvement in the weather.
Gaynor Hartnell, director of policy at the Renewable Energy Association, said any new technology "has teething problems".
"This doesn't pose any threat for the future development of wind energy in the UK, or indeed anywhere else in the world," she said.
Julian Brazier, the MP for Canterbury and Whitstable, says it is "depressing" that the wind turbines have got off to such a bad start