The 600MW Gemini wind power plant is being developed in the Dutch waters.
Approximately 30m of the 65m long monopiles was fixed at the sea bottom.
The 150 Siemens SW4.0-130 wind turbines will generate 600MW of electricity. Credit: Siemens.

The 600MW Gemini offshore wind farm, which is being developed in the Dutch part of the North Sea, approximately 85km off the coast of Groningen, is expected to be one of the world’s biggest offshore wind farms. Northland Power, Siemens, HCV and Van Oord will own the farm, while Northland Power will be the operator.

It will produce approximately 2.6TWh of clean and renewable electricity, which will be used by approximately 785,000 households. It is expected to offset 1.25 million tonnes of carbon dioxide emissions a year as well as create 500 construction and 120 operations and maintenance jobs.

“Construction, which will cost an estimated €2.8bn ($3.8bn), started in 2015 and full operations are likely to begin in 2017.”

Construction, which will cost an estimated €2.8bn ($3.8bn), began in early 2015 and full operations are likely to begin in 2017.

Design and construction details of the Gemini wind farm

The project is being built at two locations, Buitengaats and Zee-energie, and will have a total surface area of 68km². The water depth is expected to range between 28m and 36m.

There are 150 Siemens SW4.0-130 turbines, each with a rotor diameter of 130m and blade length of 65m. The turbines are equipped with high-wind ride through (HWRT) technology, which enables the rotor blades to operate at high-wind speeds with minimal exposure to the wind.

The monopiles have a total length of 65m, with 30m resting on the sea bed. Each has a diameter of 7.5m, overall thickness of 7cm and expected weight between 750t to 850t depending on the position and final design. Support structures were driven into the sea bottom by two brand-new ships specially built for the project. Van Oord installed the first monopile foundation for the project in July 2015.

Installation vessels carried the turbines to the offshore site from the Esbjerg harbour and installation began in February 2016, leading to completion in August.

The cables will connect the windfarm to the shore and will have a diameter of approximately 30cm. They will be buried at a depth of 12m beneath the sea bed.

Once in operation, the project will receive a subsidy from the government to cover the difference between the cost price of the renewable electricity produced and market price of non-renewable electricity. It is eligible for a maximum subsidy of €4.4bn ($6bn) over 15 years.

Transmission and sale of power

The 33kV electricity generated by the wind turbines will be transferred to two offshore substations via a network of cables. It will then be converted into a 200kV alternating current and be transported to the transformer station in Eemshaven by two 120km-long export cables.

“Siemens was contracted to construct, install and commission the 150 wind turbines, as well as provide maintenance services for the turbine generators over a period of 15 years.”

The 200kV AC power section will be stepped up to a high-voltage current of 380kV and transported to Oude Schild, via cables, before transmissionn to the national power grid.

Zeelands-based energy company Delta will purchase 85% of the power produced, while the remaining 15% will be supplied to HVC.

Financing of the Gemini offshore wind farm

Northland Power, Siemens, HVS and Van Oord invested approximately €500m, while Northland Power and PKA, a Danish pension fund, provided subordinated loans of €80m and €120m, respectively. Loans worth more than €2bn were also received from banks, export development agencies and credit insurance companies.

Contractors involved with the Gemini wind farm

The engineering, procurement and construction (EPC) contract worth €1.3bn was awarded to Van Oord. This included the supply and installation of the foundations, construction management and electrical infrastructure for the offshore and onshore high-voltage stations, cables and wind turbines.

Siemens was contracted to construct, install and commission the 150 wind turbines, as well as provide maintenance services for the turbine generators over a period of 15 years.

Energy Solutions (Ensol) was awarded the contract to design the electrical infrastructure, while EEW supplied special pipes and SIF provided the monopiles. NKT Cables signed an agreement to manufacture the transport cables, while Smulders Projects was awarded the contract for the transition pieces.

The contract for manufacturing the onshore and offshore transformer stations went to the FICG consortium.

NRI Energy Technology