Areva said the decision to sell Adwen means it would maximise the value of its shares, as the troubled nuclear group completes its pull-out of the wind sector.
In June, following months of speculation and negotiations, Siemens and Gamesa announced they had reached a deal to merge their wind businesses.
At the time, Gamesa gave joint-venture partner Areva three options to decide the fate of Adwen: buy Gamesa's share, sell its share to Gamesa or find a third-party buyer.
It had been reported US manufacturer GE had placed a bid to acquire Adwen, but that this was turned down in late August. There were also rumours German manufacturer Senvion was considering a move for the offshore outfit.
Areva said the Gamesa deal would allow it to continue to shift its focus back to nuclear power, as the company struggles with financial difficulties. The transfer of shares from Areva to Gamesa is expected to be complete by January 2017, Gamesa said.
"We are pleased that there is now certainty on the ownership of Adwen. We trust that the decision was made in the best interest of all stakeholders, in particular of Adwen's clients," a spokesman for Siemens said.
The merger between Siemens and Gamesa is still subject to shareholder and regulatory approval in Germany and Spain, but is expected to close in Q1 2017.
"[This] deal would enhance Siemens' leading position in the offshore wind sector. Benefits include an approximate 2GW project pipeline and the addition of one more offshore turbine drivetrain solution," said FTI Consulting senior director Feng Zhao.
"However, our concern is about how Siemens is going to deal with the facilities in France that Adwen agreed to build. And in Germany, both companies have facilities in Bremerhaven, and therefore have a lot of operations there."
According to French media, Siemens-Gamesa is going to uphold the commitment to build two factories (nacelles and blades) at Le Havre, northern France.
It means Siemens will enter the French market after all, after not being selected in the 2011 tender and then deciding not to enter the second bidding round in 2013.
Through Adwen, it will supply turbines to Iberdrola's 500MW Saint Brieuc offshore project, the 496MW Le Treport and 496MW Yeu and Noirmoutier sites for an Engie-led consortium.
Some commentators in France believe Siemens-Gamesa will not hold Adwen for long and it could be sold again.
As the company currently stands, the merged Siemens-Gamesa OEM will own two 8MW turbines, with different gearbox configurations. Siemens' SWT-8.0-154 was revealed in July, as an evolution of its 6MW then 7MW turbines. Testing and type certification is expected in 2017.
Adwen's 8MW, with a world-leading 180-metre rotor, is also set to enter the testing phase, as construction of the first prototype began in late August.
"Adwen made progress on its AD-180 8MW machine, but still at early stage of testing. Does Siemens want to invest more to prove this technology? If they do, which drivetrain (direct drive or medium speed) will be the future mainstream offshore product offered by Siemens-Gamesa?" Zhao wondered.
The result means GE is still lagging behind in terms of turbine availabilty. It owns the 6MW Haliade turbine it acquired as part of its takeover of the Alstom power business in 2015.
Meanwhile, offshore rival MHI Vestas was the first manufacturer to install a commercial 8MW turbine at Dong Energy's Burbo Bank Extension offshore site in the UK.
If all of the deals go ahead, it would leave just two offshore manufacturers with an 8MW-plus machine on offer. This is down from the six companies that offered 5-6MW turbines just a few years ago.