The project consists of 30 Siemens 3.6MW turbines with 120-metre rotors. Riffgat is located 15 kilometres north-west of the island of Borkum.
However, transmission operator Tennet is working on the cable link for the wind farm. The company's work has been held up by the discovery of World War Two munitions on the seabed and the death of a diver last week.
Tennet's work was due to be completed this month. While the wind farm is set to be officially opened by Lower Saxony prime minister Stephan Weil on 10 August.
There have been reports the project would not be completed until the end of the year. Earlier this month, a Tennet spokeswoman declined to comment, stressing it was not possible to definitely say how fast work would progress.
"Work is ongoing around the clock by a 60-strong team clearing the material, including mines and boxes of munitions, but the situation is complicated by strong currents and turbulence, and problems with algae," she said.
The situation was compounded when a diver was killed after becoming trapped under a concrete block designed to hold down cables.