The turbine is an upgrade of the company's existing 6.2MW offshore machine but with a 152-metre rotor. The current model's rotor is 126-metres.
Senvion said greater blade length means an increase in the swept area of about 50% and a 20% increase in yield at wind speeds of 9.5m/s.
Senvion expects the prototype to be commissioned by the end of the year. The manufacturer said it is ready to enter commercial production of the new model in 2015 if orders are forthcoming.
The prototype has already been sold to German developer EWE, which also operates a 5MW Senvion prototype nearby.
Speaking to Windpower Monthly prior to the announcement of the new machine, Senvion vice-president for offshore development Norbert Giese the company was planning to stick with the 6M platform until the end of the decade.
He said: "We firmly believe the 6MW platform will be the platform for this decade, We believe we can improve it to make it more efficient, but this will still be the right turbine for us for the next few years."