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.