The department of energy and climate change (Decc) has released Mainstream's supply chain plan as part of the contracts for difference (CfD) process.
In February, Mainstream was awarded a subsidy contract by the government in the first competitive auction for the CfDs.
The plan details the work Mainstream has done in sourcing suppliers and reducing costs for the project and its support for innovative technologies.
Dated 8 October 2014, the document reveals the agreement between Mainstream and Siemens to use its new 7MW turbine.
In February 2014, Mainstream named Siemens as the preferred supplier to manufacture its 6MW offshore turbine for the site.
Siemens unveiled its 7MW to the industry at the EWEA Offshore 2015 event in Copenhagen in March. A prototype has been installed at an onshore test site in Osterild, Denmark this month.
The upgrade in turbine means only 64 will be installed at the site, down from the previously expected 75.
Siemens is also providing its offshore transmission module for the project, which removes the need for a dedicated offshore substation. It will be the first project to use the modules.
Elsewhere in the report, Mainstream said 50% of suppliers that were approached or made bids in the tender process were new entrants to the offshore sector.
The plan reveals how Mainstream attracted business from outside the sector to bid on the project by hosting supplier days and business tours.
One example given showed how Mainstream's foundation installation contractor GeoSea developed floating storage solutions for jacket foundations. The storage could be used at ports without enough "laydown areas" or quay capacity. Mainstream said this would allow more ports to be included in the tendering process, thereby increasing competition with larger ports and reducing costs.
The plan has also revealed the intent to use a three-legged jacket structure for the foundations. The report said GeoSea found using a three-legged structure would cut 5% off manufacturing costs.
Mainstream said it expects to make a final investment decision on the project in Q1 2016, with construction starting later that year and first power in Q1 2018.