.

Projektinfo – Detailed information on energy research

read short description

During the construction of foundations of offshore wind turbines the piles are rammed into the seabed. The noise disturbs marine mammals. But there are low-noise alternatives.
© Herrenknecht AG
Offshore wind energy
Projektinfo 11/2013

Foundation types: Monopile, tripod and jacket (from left to right)
© Hochtief Solutions AG
1 / 3

Low-noise anchoring of offshore wind turbines

The intention is to develop offshore wind energy in the German parts of the North and Baltic seas in an environmentally compatible manner. An important provision when constructing the foundations is to mitigate the construction noise in order to protect marine mammals. In comparison with currently standard pile driving methods, a newly developed drilling process for foundation piles enables noise-reduced installation. Engineers are developing an ocean-going drilling rig and are devising the subsequent construction sequence.

The rotor blade tips on wind turbines installed at sea almost reach the height of Cologne Cathedral. These powerful wind turbines need to be installed on solid foundations so that they can withstand the forces from the wind and waves. For this purpose the market offers various types of foundations, known as support structures, which depending on the geological substratum and the water depth are anchored to the seabed with a single large pile or several smaller piles to a depth of about 20 – 40 metres. With most wind turbines up to now, the piles have been driven into the seabed with considerable noise. There are also foundation types without piles. Heavyweight foundations, for example, are particularly suitable for geological conditions where pile driving is difficult (such as when there are boulders in the substratum), but are less common in Germany due to the greater water depths. With these foundations, heavy concrete blocks resting on the seabed provide the necessary support. To be able to use them, the seabed must first of all be levelled, which means encroaching on the natural seabed fauna. Floating foundations and suction bucket foundations are other forms that are still under development.

arrow
arrow

Using a new drilling process that is currently undergoing development, the intention is to launch a noise-reduced alternative for pile driving to the market. The new method protects the hearing of porpoises and seals: according to the current state of planning, the developers predict a sound level (continual sound) of around 120 dB (re 1 μPa) at a distance of 750 metres. The limit value for piling noise is 160 dB at a distance of 750 metres. The method works with different geological conditions and enables larger pile diameters with a minimal impact on seabed fauna.

In a research project, a vertical shaft sinking machine (VSM) is being further developed for offshore use. The concept is known as offshore foundation drilling (OFD®). In this project the companies Herrenknecht AG and Hochtief Solutions AG are working closely together. Shaft sinking machines have long been used onshore to create, for example, access shafts for subway systems, ventilation shafts as well as launch and target shafts for tunnel boring machines.

Firm anchoring in deep water

In Germany offshore wind farms are not allowed to be erected in the environmentally sensitive Wadden Sea for nature conservation reasons. They are therefore almost entirely located more than 30 kilometres off the coast at a depth of 20 to 40 metres. In most other countries, the wind farms are located in shallower water and closer to land. These requirements mean that the wind turbines in Germany have to be as powerful as possible – and therefore large – for economic reasons. For example, only 5-MW turbines are used in the German alpha ventus offshore test field, which was constructed in 2009. The current wind turbine technology shows a trend to even greater output classes.

The figure shows possible foundations for offshore wind turbines. The cost of foundations makes up around 25% of the investment costs for offshore wind farms. Usually the most cost-effective form is the monopile – a long steel pipe with a diameter of several metres. With the currently standard pile driving methods, piles with diameters of up to 6.5 metres are driven into the seabed. With a water depth of 30 metres, a typical monopile is 65 metres long and weighs around 700 tonnes. With this type of pile, around two-thirds of the foundation costs are incurred by the manufacture on land. Depending on the location, this pile is currently sufficient for wind turbines up to 5 MW in size. Jacket or tripod structures are predominantly used for larger wind turbines. These foundation structures are anchored to the seabed with several thinner piles (diameters ranging between 1.5 and 3 metres).

When constructing offshore wind turbines, the ground is examined prior to the pile driving using seismic techniques and core drillings. Rocky geologies, boulders and very densely bedded sands can complicate the pile driving or make it impossible. However, such geological conditions prevail at many places in the North and Baltic seas where offshore wind farms are planned.

Projektinfo 11/2013:
1 / 3

notepad

BINE subscription

Subscribe to publication

Addresses

Overall coordination and drilling technology
Herrenknecht AG

Service

BINE-Projektinfo 11/2013
(PDF, 4 pages, 1.6 MB)