The nacelles of these two machines, and possibly of all six Areva machines , will be transported to shore for repair, probably in August. The gearboxes were supplied by Renk of Augsburg.
As a result, Areva said it plans to invest in a test facility at the German port of Bremerhaven to ensure there is not a repeat of the problem.
Alpha Ventus, which first came online in 2009, was Germany's first operating offshore wind farm.
Alpha Ventus wind station company spokesman Lutz Wiese told Windpower Monthly there is a problem in the slide bearings used in the gears.
A different material was used for the slide bearings in the gears used for the Alpha Ventus Areva Wind machines when compared with the Areva Wind (formerly known as Multibrid) prototype in which no such problems have occurred.
Wiese said: "The problem is irritating, but however much you try to take care of all possible potential problems, it's the practical test that counts. And we mustn't forget how fast developments are going and what time pressure companies are under." The costs of rectifying the problems will be shouldered by Areva and Renk, he added.
Alpha Ventus, comprising six 5MW Areva Wind turbines and six 5MW Repower Systems turbines, was installed last year and is owned by German companies EWE and E.ON Energy Projects and the Swedish Vattenfall Europe Windkraft.