Collaboration of experienced parties
The wind farm was built by the construction combination Egmond. This is a joint venture between Ballast Nedam and the Danish wind turbine manufacturer Vestas. Both companies have extensive experience in offshore wind energy.
Construction combination Egmond developed an innovative method for the installation of the wind turbines. For the installation of the steel monopile support structures of the wind turbines, the offshore installation vessel the Svanen was used. The masts of the turbines were erected using a floating crane.
The IJmuiden port, the IJmondhaven, which opens directly to the sea was used as the logistical base. Here all the components were collated and transported to the offshore site. Assembly of the components and quality control took place here as well.
Every wind turbine is put on a monopile foundation. The monopile is a large steel pole that is put in the seabed. The diameter of this pole is 4.6 metres and the plate thickness is 5-6 centimetres. The length of these monopiles varies between 40 and 50 metres. This depends on the condition of the soil and water depth where the pole should be placed. The poles weigh about 250 tonnes each. After the pile driving, the poles are about 30 metres in the seabed and the top of each pole is 5 metres below the water level.
For the pile driving of these enormous monopiles, Ballast Nedam’s offshore installation vessel ‘Svanen’ was used. The ‘Svanen’ is a catamaran with a lifting height of 76 metres. This ship was specially designed and built in 1990 for the construction of large concrete bridges in countries such as Denmark. On the back of the ship an additional gantry crane was placed. With this crane, the ship could lift a foundation pile horizontally and then place it vertically on the seabed. After this, the pile-driving hammer of nearly 300 tonnes is placed on the pole to place the pile in the seafloor. Checks are carried out during pile-driving to ensure that the pile is not tilting.
A “transition piece” of around 25 metres high and clearing the water by around 13 metres is placed on the pile. Attached to this transition piece are work platforms, ladders and a berthing facility for boats. The transition piece has a diameter of 4.3 metres and is lowered over the top of the monopile. The space between the monopile and the transition piece is grouted with cement. This results in a very strong joint between the transition piece and the monopile.
Once the transition piece was secured, the ‘Svanen’ weighs anchor and sailed to the next location.
The Svanen took several days to drive a pile and install the transition piece.
A layer of rock 1.5 to 2 metres thick is deposited round each monopile on the sea bed. This layer of rock prevents the sea bed from eroding round the pile as a result of the current.
Installing the tower & turbine
In total 36 wind turbines were installed. A floating crane was used to install the tower and the turbine. The vessel is specially adapted to install offshore wind turbines and can lift more than 100 tons to a height of over 80 metres. By lowering four steel legs the vessel can raise itself a little so that it remains stable in the waves. This way the crane can be used safely. The vessel berths close to a monopile and erects the 53-metre long tower onto the transition piece. This is then secured with 140 bolts.
The top section of the turbine – the nacelle – is then lifted onto the tower. The nacelle contains the generator of the wind turbine and weighs 108 tons. The nacelle too is bolted into place on top of the tower. Two of the three rotor blades have already been attached to the nacelle in the harbour; the third blade is now lifted and attached to the nacelle.
Finally, the electric cables were connected to the nacelle and a comprehensive testing programme was carried out before the turbines started producing electricity.
Electric cables and grid connection
The wind turbines are connected to each other by electric cables laid under the seabed. A sleeve is mounted to the outside of each monopile, enabling the electric cable to be pulled up out of the sea. Underwater cameras and divers were used to ensure that the cables and not damaged during this operation. A special cable laying vessel equipped with an excavator uncoiled the cable and sank it in the sea bed.
The wind farm is subdivided into three sections of 12 turbines. To get the electricity produced by the wind turbines to the Dutch households, there are separate 34 kV cables that run from the three wind turbines closest to the coast to shore. These cables come onshore at Wijk aan Zee and pass under the beach and dunes in special conduits. These cables run to a transformer sub-station where the electricity is converted to high voltage of 150 kV and connected to TenneT’s national electricity grid. A seven-kilometre long underground high voltage cable has been laid for this purpose from the transformer sub-station via Wijk aan Zee through Beverwijk to the coupling transformer at the power plant in Velsen-Noord.
To prevent shipping from being exposed to unsafe situations and damage being caused to the wind turbines, a safety zone is instituted. The safety zone is 500 metres. This is customary for oil and gas extraction platforms as well. The wind farm and safety zone are shown on marine maps. The introduction of this safety zone means that the wind farm and the surrounding zone of 500 metres are closed to all shipping. Only maintenance vessels of NoordzeeWind and ships on government business will be allowed to enter the safety zone, as well as vessels carrying out research for the Monitoring and Evaluation Programme.
Agreements have been reached with the coastguard and the local port authority about the signposting, surveillance and maintaining safety during the operation of the wind farm.