The project, which came online in 2009, uses Siemens 3.6MW turbines. Siemens said it was its first 15-year post-warranty service contract, which would run through the lifespan of the turbines.
Rhyl Flats is located off the coast of north Wales. Servicing is run out of the Port of Mostyn.
Speaking about the deal, head of Siemens Power Generation Services, Wind Power and Renewables, Mark Albenze, said the company was currently servicing 12 UK offshore projects.