The direct-drive permanent-magnet generator turbine has a 150-metre rotor and was first commissioned in March 2012 at Le Carnet in northern France.
Alstom said the installation will help it see how the machine works in the offshore environment, which it was designed for.
A 78-metre tower was assembled on a 61-metre jacket set on top of pillars, which were sunk more than 60 metres deep. The nacelle sits more than 100 metres above sea level.
Alstom claims this is the largest offshore wind turbine ever installed in sea waters due to its 150 metre rotor. Since September, two Siemens 6MW turbines, with 120 metre rotors, have been tested at Gunfleet Sands III in the UK’s North Sea.
Two factories that will produce the turbine's nacelles and generators, in Saint-Nazaire, are due to be completed by summer 2014 and two factories in Cherbourg will build the blades and towers.
Alfonso Faubel, Alstom Wind’s senior vice-president, said their turbine will increase the competitiveness of offshore wind energy.