Photo: The 72 offshore wind turbines type SWT-4.0-130 for the Sandbank offshore wind farm each deliver 4 megawatts.
The wind turbines
Once the Sandbank wind farm is completed, 72 wind turbines spread out over an area of 60 square kilometres will be using their vast rotor blades to turn the powerful North Sea wind into green energy.
Each of these units, built by Siemens, stand at a maximum overall height of 150 metres above the water surface and deliver an output of 4 megawatts. The rotors sweep over an area equal to almost two football fields and produce a total of 1.4 million kilowatt hours of electricity per year.
The nacelles themselves are about the size of a bus and house the turbine's main elements, such as the gear box, generator and the automatic controller which ensures the rotor is positioned at the correct angle in relation to the wind.
For the purposes of transporting and installing the wind turbines, the installation vessel "MPI Adventure" is used.
- Rotor diameter: 130 m
- Rotor blade length: 63.45 m
- Hub height: 94.75 m
- Rated output: 4.0 MW
- Nacelle dimensions without rotor and hub: approx. 140 t
- Nacelle dimensions with rotor and hub: approx. 240 t
- Tower: approx. 290 t
- Rotation speed: Rotor: 5-14 rpm
- Cut-in wind speed: 3-5 m/s (= force 2-3 - about 10 mph)
- Nominal speed: approx. 11-12 m/s (= force 6 – about 30 mph)
- Cut-out wind speed: 25 m/s (32 m/s with HWRT) (= force 10 - about 56 mph)
- Maximum blade tip speed: approx. 340 km/h
The wind turbines stand on monopile foundations, a pillar-type structure with a length of up to 70 metres, depending on their location in the wind farm, and a diameter of up to 6.80 metres. Each individual monopile weighs up to 886 tonnes and consists of up to 25 individual cylindrical components, or sections, that are welded together.
These sections are rolled out of flat steel plates and shaped into cylinders under a pressure of 5,700 tonnes. Approximately 52,000 tonnes of steel will be needed to produce all 72 monopiles. The company EEW SPC in Rostock has been awarded the contract to make the foundations for "Sandbank" and the transition pieces will be made by Bladt Industries in Aalborg (Denmark).
For each installation run, four sets of foundations will be loaded onto the installation vessel Pacific Orca in Esbjerg. First, the monopiles are anchored in the sea bed by means of a special pile driver, then the yellow transition piece is screwed in place with the aid of a flange joint.
This joint is a refinement of the previous DanTysk project, where the components were cemented together. Bolting the components together offers many advantages, such as shortening the installation time, since the work can be carried out regardless of the weather. This means that the use of expensive installation vessels can be reduced.
Length: up to 70 metres
Diameter: up to 6.80 metres
Weight: up to 886 tonnes
Height: 25 metres
The offshore substation
A central element of the DanTysk wind farm is the 155/33 kilovolt offshore substation. It forms the gateway for the onward transmission of energy to the mainland via submarine cables. This is where the electricity generated by all of the wind turbines is converted from 33 kilovolts into 155 kilovolts before being transmitted to a converter station. There the current is converted from alternating current to direct current in order to guarantee as far as possible the lossless transmission of electricity to the mainland.
The cables and the grid connections
The cables form the lifeline of the Sandbank wind farm. They ensure that the energy produced at sea by the wind turbines can be transmitted to the substation. It is what the experts call inter-array cabling.
Each cable harness connects nine wind turbines to the substation. Since the cable harnesses can be connected in twos, the turbines can continue to operate even in the event of damage to a cable.
A total of 96 km of cable will be laid within the wind farm, consisting half each of two different diameters: 630 mm² and 185 mm².
The cables incorporate an optical fibre for the data exchange between the turbines and the substation, as well as for remote control and monitoring of the wind farm from the control centre in Esbjerg, Denmark.
The cables are illuviated into the sea bed at a depth of at least 60 cm and in most cases of more than a metre.
The energy is transmitted from the offshore substation to the grid operator's SylWin alpha converter station. The SylWin1 offshore connection conducts the power via the approx. 205 kilometre long sea and land cable to Büttel near Brunsbüttel, where it is fed into the German transmission grid at the substation there.
Sandbank wind farm area
The Sandbank offshore wind farm project area is located 90 kilometres off the coast of Sylt in the German North Sea and is therefore not visible either from land or from the islands just off the coast.
More about the wind farm area
Just as with any other infrastructure project, a responsible attitude towards the environment and the natural surroundings is a key aspect of offshore projects like the Sandbank project.
More about the environment
News about Sandbank
The construction of Vattenfall and Stadtwerke München’s 288 MW Sandbank offshore wind farm started in June 2015. Read more about what's happening in the project on our news page.
Read the latest Sandbank news
The ships in action
Five specially-selected ships will be used during the various construction phases of the project. At peak times up to ten vessels are in action in the wind farm at the same time.
Read more about the ships