Monopiles: Firmly anchored
In order to install the wind turbines safely and securely in the 21 to 32-metre-deep North Sea, the foundations need to anchored to a depth of up to 32 metres to the sea floor. Factors such as turbine type, soil composition, wave height and water depth determine which of the various possible foundation methods are chosen. Monopile foundations are best suited to the conditions found at the DanTysk wind farm. During installation the individual foundation piles are driven into the sea bed and then capped with the transition piece. The lowest segment of the tower is attached to this transition piece before the rest of the wind turbine components are subsequently installed.
70,000 tonnes of steel
The Per Aarsleff/Bilfinger Berger joint venture is responsible for the planning, production and installation of the foundations. A total of approximately 70,000 tonnes of steel will be processed to produce the tubes, which can reach up to 63 metres in length. This is the equivalent in weight of roughly 50,000 medium-sized cars. Construction work on the first foundation piles will begin at the end of 2012.
A focus on noise mitigation
One of the main missions during construction is to reduce the amount of noise caused during the installation of foundation piles. A multi-stage procedure is applied to ensure that the noise generated during installation of the foundation piles does not impair or permanently damage the hearing of marine mammals. Before installation work is commenced, pingers and seal scarers are used to produce noises which warn marine mammals and cause them to stay away from the construction site. A soft start procedure is conducted during the first minutes of the following piling operation to signal the start of the actual piling phase.
At the heart of the measures is the “dual big bubble curtain” from Weyres. This is a system with two hoses which are laid by a support vessel over 100 m apart on the seabed around the site of the piling operation. An air compressor on a support vessel fills the hoses with compressed air so that bubbles are emitted from the specially designed openings. When produced on mass, these form a closed bubble curtain that stretches all the way up to the surface. This reduces noise and protects the hearing of marine mammals. The underwater noise levels and the cetacean population are continuously measured at various distances.
As a responsible developer and constructor of wind farms Vattenfall wants to learn more about the environmental impacts associated with the construction phase and has therefore initiated a research project. The Partners are the R&D and the DanTysk departments within Vattenfall, the agency BioConsult SH and the University of Aarhus in Denmark - which will be leading the research. This project aims to gain a greater understanding of the effect of piling on harbour porpoise during the course of the piling period, as well as the period before and after. In order to do so, twelve measurement buoys located in the outer area of the construction site will monitor the underwater noise as well as the presence of porpoises. The results will feed into a larger research programme initiated by Vattenfall aiming to develop a better understanding of any potential consequences of piling noise disturbance to the population of harbour porpoises.
In 2011, Vattenfall and Stadtwerke München also took part in the ESRA research project, an initiative which examines various noise reduction techniques with a view to improve future offshore wind farms and which has managed to increase knowledge on the distribution of sound under water.