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Belwind in a nutshell

The Belwind wind farm is 46 km off the coast of Zeebrugge on the Bligh Bank. It is the furthest offshore wind farm and therefore cannot be seen from land.

Belwind featured 25-04-2012

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Belgian company Belwind is building Belgium's largest renewable power plant : a wind farm in the North Sea with 55 turbines.

It is the furthest offshore wind farm with the deepest foundations in the seabed.

It was developed in just 3.5 years and built in 13 months.

This piece of technical ingenuity will supply green power to 160,000 households.

Thanks to the collaboration of local, regional and national authorities, the project will be completed in record time.

The foundations of the 55 wind turbines and the offshore high-voltage station (OHVS) are ready-made 7 cm thick monopiles . These monopiles are easier and faster to put in place than lattice towers or concrete foundations. The gigantic hollow tubes have a diameter of 5 metres and can be up to 72 metres in length. Instead of transporting these 300 to 550 ton monopiles by ship, we float them. Both openings are sealed airtight with a hydraulic plug. Weather conditions largely determine how quickly the works progress. Tugs will only go out to sea when the waves are less than 1 metre high and it takes about 6 hours to get to the Bligh Bank.

At the Bank, the monopile is taken over by the powerful crane ship Svanen, the largest floating crane in the world with a lifting capacity of 8,700 tonnes. The Svanen's special grab hook lifts the monopile into the air. After removing the airtight seals, the monopile is moved to the desired location. GPS technology allows the technicians to determine the exact position to one centimetre, which requires one hour of extreme concentration. Because the sea at the Bank is 20 to 37 metres deep, the length of the foundation piles also varies, from 50 to 72 metres. It takes about 3 hours to drive a pile 35 metres into the seabed. The last blows are about 12 times more powerful than the first. Before the pile is driven into the seabed, special sound signals are emitted to scare away any marine animals. When it's all quiet again, only the top of the foundation pile is sticking out of the water.

In the second construction phase a ready-made transition piece is fitted on every foundation pile with a mooring area, a 25 metre high ladder, a platform, an entrance door and pipes for the power cables (to protect them against the waves). Every transition piece has a 4.30 metre diameter, is 25 meter high and weighs 167 tonnes. The JB 114 jack-up platform transports the transition pieces 3 at a time from the port to the Bligh Bank, a trip that takes 6 hours. At the Bank, the 70 metre long legs extend and the ship anchors itself to the seabed. Once these legs are firmly on the seabed, the platform elevates itself out of the water. This means it is no longer affected by wave action and it is a stable work surface for very precise crane work.

A crane lifts the transition piece and slides it over the foundation pile so that it overlaps about 7 metres. The transition piece is placed perfectly vertically, and any deviation of the foundation pile is corrected. It can be maximum 0.3 degrees off plumb. Considering that the top of the foundation pile has a smaller diameter than the transition piece, there is a hollow space where it overlaps 7 metres. 6.5 m³ of concrete is poured into this space, to anchor the transition piece on the foundation pile. The foundation pile plus transition piece are now 65 to 90 metres in length, most of which is in the seabed and under water. The platform is raised 17 metres above sea level.  

Because it is more difficult to work at sea, the turbines are assembled mostly on land. The two parts of the tower are put together and the nacelle and the rotor head are assembled. Then, the entire installation is electrically tested on land. Following assembly, the JB 114 jack-up platform transports 2 upright towers, 2 nacelles and 6 loose rotor blades to the wind farm.

At sea, the 55 metre high tower, with the nacelle and rotor head on top, are positioned on the yellow transition piece. This is the size of a bus and weighs 120,000 kg. Then, the three rotor blades, 44 metres in length, are positioned in the rotor head. At the highest point, the ends of the blades stick out 117 metre above the sea.. Including the foundation piles, the total height of the construction amounts to almost 190 metres. In good weather conditions the turbine can be assembled in six hours.

The wind turbines are connected with infield cables (33 kV), in 5 ‘strings’ of 10 turbines and 1 string of 5 turbines. Per string there is also a 33-kV cable to the Offshore High Voltage Station (OHVS ). All infield cables put together have a total length of 50 km. The cables are buried into the seabed for protection. The OHVS converts the voltage from 33kV to 150 kV to reduce any loss during transport. The station is located in the middle of the farm, has five 250 m2 decks and weighs 1,100 tonnes. From the OHVS a 30 cm thick, 52 km long export cable (150 kV) runs to the mainland. The cable is rolled out in one piece and then buried in the seabed. At the beach of Zeebrugge the export cable is connected to a 3 km long land cable. This goes to the second high-voltage station, the Booster Transformer Station. This takes care of the reactive current after which the power is fed into Elia's transmission grid in Zeebrugge, and via the power grid ends up in end users' houses.