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Noise emissions during installation of foundations.

The steel piles are driven into the seabed from special assembly platforms. The time required for pile driving depends on the depth and diameter of the pile. It took 30 to 90 minutes to drive each of the 80 monopile foundations into the seabed in the Danish Horns Rev wind park. Pile driving causes considerable noise emissions, the impact of which on marine mammals and other marine life is currently being researched. Countermeasures could include temporarily repelling the animals from the danger zone or using noise-reducing measures such as bubble curtains. More options for noise reduction, such as a hydro muffler or the drilling of the foundations will be tested.

Foundations for offshore wind turbines.

Foto: Errichtung eines Offshore-Fundaments.

Germany's offshore wind parks are mainly being built outside the 12 nautical mile zone in the North and Baltic Seas. These locations far out to sea take account of nature conservation, the necessity of keeping coastal waters clear and the desire for horizons uncluttered by technology, as well as the requirements of the fishing industry and the safety of shipping traffic. The wind turbines are subjected to extreme conditions out at sea, as they lie between 40 and over 100 kilometres from the coast. They are anchored at depths of 20 to 50 metres and exposed to aggressive salt water, high wind and wave-induced stress, and strong currents for a very long time – their minimum life span is 20 years. These turbines are anchored to the seabed with foundations which have particular need of protection from such pressures, requiring special building materials and designs. Although only one offshore wind park has been built under these conditions to date anywhere in the world, several types of foundation are already available.

Seven foundations for offshore wind turbines.

Monopile.

The construction of the Monopile consists of a cylindrical concaved pile. The Monopile is used in many European near-shore wind farms in a water depth up to 20 metres. Monopiles won’t suit the operation of more efficient wind turbines (6 MW). Operations will only be cost-effective up to a water depth of about 15 meters. They can be installed in an easy and fast way. But heavy pile hammers are needed. Monopiles can easily be protected against the scour phenomenon and collisions. It isn’t applicable at stony sea bed.

Jacket.

The Jacket is a frame construction similar to an electricity pylon. With four extra piles the Jacket is pinned to the sea bed. The Jacket construction is already established in the oil industry. Compared to the Monopile 40 – 50 % of the steel can be saved. Therefore the costs rise slightly with a higher water depth. Since the single components are really small, they can be produced fully automated, easily be transported and installed. A disadvantage is the remote collision protection.

Tripod.

The Tripod is a three-legged steel frame which supports the main pile under water. The Tripod is pinned to the sea bed with three small piles, which have to be hammered. Compared to the Monopile piles with a smaller diameter can be used. It is possible to apply the Tripod up to a water depth of 20 metres. Another advantage is the good scour protection. The Tripod can’t be used on a stony sea bed.

Tripile.

The Tripile consists of three steel piles which sit on a three-legged structure above the water level. Under water it also has to be pinned to the sea bed. The Tripile production is due to its compact construction relatively cost-effective. The first test foundation is located close to the North Sea island Hooksiel. According to the manufacturer Tripiles can be used in water depth between 25 and 50 metres.

Gravity foundation.

Gravity foundations are a big concrete block, which carries the wind turbine. Gravity foundations are already used for bridges and in some European wind farms in a water depth up to 10 metres. The really high costs at the beginning have been reduced due to a different shape. Therefore they can also be installed in deeper water now. In addition gravity foundations aren’t dependent on steel prices.

Bucket foundation.

The bucket foundation is sucked into the seabed by means of a vacuum and is held in the sandy subsoil by suction. It is important that the construction is adjusted evenly and that it builds an upright and safe foundation. No pile driving is needed to install the bucket foundation thus it is especially environmentally friendly.

The scour phenomenon.

Ocean currents and the continuous swell can wash out sediment from under offshore foundations. This process is called scour, and can also be observed on jetties close to beaches and on rocks. Scour causes offshore wind turbines to lose their purchase on the seabed and thus their stability. Layering sandbags and stones around the foundation or increasing the depth can counteract the process. The underwater robots which carry out the annual examination of the alpha ventus test field have recorded scour to a maximum of one metre to date. This degree of scour requires no countermeasures.

Collision protection.

Analysis of the risks to the safety of ships and offshore foundations play a role in the avoidance of personal injury, damage to property or environmental hazards from the collision of a shipping vessel with an offshore turbine. The regulations introduced by the approvals authorities require that the foundations are designed to avoid damage to the ship's hull in case of collision. Research by Hamburg University of Technology has shown that monopiles fulfil this requirement because they bend to the side in the case of collision, and hardly impact with the hull of the ship.

Outlook.

Jacket, tripod and tripile foundations are currently in use in German waters, but other types of structure, particularly gravity foundations, can also be expected to be introduced in future. Floating foundations are being developed for offshore wind turbines in water depths of more than 50 metres, and a prototype floating turbine was anchored off the Norwegian island of Karmøy at a depth of 220 metres in June 2009. Despite use in a large number of facilities at home and abroad, there is a still a need for the new and ongoing development of structures and materials for offshore wind foundations. The newly established test centre for substructures at Leibniz Universität Hannover will address this issue from 2010.


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