Foundations
The wind turbines were installed on monopiles with transition pieces. Monopiles are steel pipes that are driven into the seabed. Transition pieces were attached to the monopile foundation to support the secondary steel elements such as jetties, ladders and platforms required to safely access and operate the wind turbines. The towers of the wind turbines were then bolted to the top flange of the transition piece.
This design, fabrication and installation work was intended to create a foundation that would give the wind turbines stability in all weather conditions in the North Sea for 25 years.
Main Points of Design
- Transition piece/monopile connected via a bolted flange connection.
- Upper part of boat landing installed onshore to transition piece.
- Lower part of boat landing installed after monopile installation.
Built in the North Sea, offshore wind power plants (WPPs) must be very powerful and efficient and at the same time easy to maintain. Due to the great distances to the coast, the offshore wind turbines have been professionally equipped to withstand the most severe weather conditions.
There are 54 Senvion SE 6.2M126 Wind Turbine Generators (WTGs) in operation at the Nordsee One offshore wind farm. The WTGs have a height of ~152 metres above the lowest astronomical tide (LAT).
The Senvion 6.2M126 is a variable-speed WTG with a rated output of 6.15 MW. It has a rotor diameter of 126 m and an electric single-blade pitch system.
Offshore Substation
The 155kV offshore substation is the heart of the wind farm. The energy of all 54 wind turbines is transferred via the 33kV subsea cables to the offshore substation. There the voltage is transformed from 33kV up to 155kV via two grid transformers. The energy is transmitted further via two 155kV subsea cables, provided by the transmission system operator, to the Dolwin Beta AC/DC converter station.
The Offshore substation comprises of the upper part - the topside - with a weight of circa 2,200 tons, and the lower part, the jacket foundation, with a weight of circa 1,400 tons. The jacket is held in place by a grouted connection to four piles driven into the seabed.
The topside is a steel building with four levels, and houses the grid transformers, switchgear and auxiliary power supply for lights, safety systems and data and control system. The grid transformers provide a total power transmission capacity of 332MW from the wind turbines to the German electricity grid. The offshore substation is standing approximately 40m above sea level. A helicopter landing platform on the top deck of the offshore substation allows for weather independent access to the platform for operation and maintenance works.
Inter-array cable
The fifty-four turbines are connected by ten strings of aluminum cables, with an overall length of around 70 kilometers. In most cases the strings will be looped in order to maximize power export capability.
Two different cable cross-sections are used: 240 mm² and 800 mm².
