Technology
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.
Wind turbines
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.
Key design characteristics
The Senvion 6.2M126 has following key design characteristics:
- Yield-improved generator and converter system with enhanced grid properties.
- Fail-safe pitch system with separate control and regulation systems for each rotor blade to pitch out the blades in storm conditions.
- Double bearing of the rotor shaft to ensure, to a large extent, that only the torque is transmitted to the gearbox.
- Tilted-Cone concept (negative cone angle of the rotor) and pre-bent, stiff rotor blades for improved weight balance and safe load transfer.
- Proven and reliable gearbox concept to achieve an optimal ratio between the generator speed and rotor speed.
- Ease of maintenance due to the spacious nacelle design.
All 54 WTGs have been equipped with an 24hrs online monitoring system for control and monitoring to obtain the most efficient output. Equipped with such a SCADA system the maintenance teams can react very quickly, and maintenance repairs can be executed in a timely fashion to minimize down times.
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.
Technical Data:
Voltage Levels: | 155 kV/33 kV (AC) |
Power from the wind turbines: | 332 MW |
Grid Transformers: | 2 x 200 MVA, 155/33 kV air cooled |
Auxiliary Transformers: | 2 x 1,6 MVA, 33/0,4 kV, hermetic sealed |
Emergengy Diesel Generator: | 1 x 220 kVA |
Park Diesel Generators (for Turbines): | 2 x 4,1 MVA, 400 V (AC) |
Dimensions:
Topside (L x W x H): | 32 x 16 x 18 Meter |
Jacket (H): | 51 Meter |
Water Depths: | 28 Meter |
Topside Weight: | 2,293 tons full operational |
Jacket Weight: | 1,683 tons excl. piles |
Topside Decks: | 4 incl. Roof Deck with helicopter landing deck |