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Projektinfo 11/2013

Low-noise anchoring of offshore wind turbines Ocean-going drilling rigs can replace the previously standard pile driving methods used for foundation piles

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 suit-able 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.

This research project is funded by the

Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU)

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, ventila-tion shafts as well as launch and target shafts for tunnel boring machines.

Firm anchoring in deep waterIn 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 ven-tus offshore test field, which was constructed in 2009. The current wind turbine technology shows a trend to even greater output classes. Fig. 1 shows possible foundations for offshore wind tur-bines. 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 di-ameters 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. Depend-ing on the location, this pile is currently sufficient for wind turbines up to 5 MW in size. Jacket or tripod struc-tures are predominantly used for larger wind turbines. These foundation structures are anchored to the sea-bed with several thinner piles (diameters ranging be-tween 1.5 and 3 metres).When constructing offshore wind turbines, the ground is examined prior to the pile driving using seismic tech-niques 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 Bal-tic seas where offshore wind farms are planned.

The OFD machine in useThe new machine (Fig. 2) can be flexibly adjusted to meet desired bore diameters between 5.5 and 7.5 metres. This makes it possible to insert the machine into tapered piles. It is intended to achieve diameters of up to 10 me-tres at a later point.

The machine used for the drilling operation is lowered inside the pile. It is clamped to the inner wall with a variable system consisting of grippers and stabilisers. Using a pivot boom that swivels around the pile axis, the cutter head rotates concentrically as it moves downwards. This allows considera-ble flexibility in terms of the diameter. This system and the thrust cylinder also enable the machine to drill below the lower edge of the pile (overcutting). The cutter head (Fig. 3) acts as a universal tool holder. If there are sandy layers, the cutter head is equipped with so-called cutting knives, whereby round shaft chisels are used with solid bedrock. Large work platforms are required for installing offshore wind turbines. These are installation vessels or jack-up rigs that can rest on the seabed at the respective installation site and lift themselves out of the water using telescopic hydraulic stilts. Since installer units are designed quite dif-ferently, the sequence of drilling work described below only provides a general outline by way of example: as soon as the wave height and wind conditions permit the erection of wind turbines, the drilling machine is transported to the installation site onboard a jack-up platform. Using air-filled buoyancy tanks, a second ship tows the pile to the construction site

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Fig. 1 Foundation types: Monopile, tripod and jacket (from left to right). Source: Hochtief Solutions AG

Fig. 2 The OFD® rig. Source: Herrenknecht AG

Slurry pump

Gripper

Thrust cylinder

Stabiliser

Cutting boom

Cutter head

where it is placed upright and fixed. Its own weight causes it to sink several metres into the seabed. The OFD rig operates inside the pile and cuts a hole into the seabed, into which the pile sinks even further. As its sinks, the gap between the pile and the ground is continuously filled with a special mor-tar. The mortar has not only excellent feed and flow characteristics but also excellent lubricating properties. Without further pumping, it can be piped across the required distances. A laboratory on the installation vessel tests and inspects the mortar quality prior to installation. Since the drill machine operates under water, the excavated ground material, which is mixed with water, is pumped to the surface and separated on a ship. The separated water is then returned to the inside of the pile. As soon as the pile has reached the target depth, the OFD rig is recovered and the inside of the pile is filled with the excavated material that has been temporarily stored. Because of the short annual construction season caused by the weather conditions, it is intended that the offshore foundation drilling machine should be optimally utilised and remain continuously at sea. The machine is mounted on the deck of the jack-up platform so that maintenance and repair work can be carried out while moving to the next installation site.

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Round shaft chisel

Cutting knifes

Harbour porpoises

The harbour porpoises that can be found in the North and Baltic seas are small toothed whales that are dependent on the use of echolocation to orientate themselves. This makes them particularly sensitive to noise pollution. It is the only species of whale native to German waters. These whales are included among the strictly protected species defined by the German Federal Nature Conservation Act and several interna-tional conservation treaties, and are also protected by the terms of the EC Habitats Directive. The harbour porpoises give birth to their young in the summer half-year. There are estimated to be more than 220,000 specimens in the North Sea (including 54,000 in Ger-man waters) and about 40,000 in the Baltic Sea (with 1,000 in German waters). In the Baltic Sea, it can be differentiated between at least two sub-populations („Kattegat-Belt Sea“ and „Central Baltic Sea“), which hardly have any genetic interaction. With less than 250 specimens, the population in the central Baltic Sea is particularly threatened. Dangers include being killed as fisheries bycatch, the chemical pollution of the oceans, overfishing of their food sources and noise exposure.

ProspectsThe developers are primarily focussing on the monopile as a foundation type because this provides the most ad-vantages in economic terms. When there is sandy ground and a pile diameter of 7.5 metres is used, the plant needs to burrow through the seabed at a speed of 3 m/h if the OFD method is to compete economically with pile driv-ing. In future even larger pile diameters will be needed as offshore wind turbines become larger and areas are de-veloped with even greater water depths. The strengths of the new method: it produces lower noise emissions, can be flexibly used even with difficult ground conditions and can vary the size of the hole. Until now no competi-tive developments have been available on the internation-al market that would meet all of these criteria. A proto-type is currently being developed, tested and optimised in economic and technical terms as part of a follow-up project that continues until March 2014.

Fig. 4 Inserting the monopile in the lowering frame. Source: Herrenknecht AG

Fig. 3 Cutter head with combined tool assembly. Cutting knives are used for sandy layers and round-shaft chisels for solid bedrock. Source: Herrenknecht AG

Slurry pump

Gripper

Thrust cylinder

Stabiliser

Cutting boom

Cutter head Harbour porpoises depend on the use of echolocation. Source: Fjord&Baelt / F. Graner

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Limiting offshore construction noise Up to 5,000 wind turbines are planned in the German parts of the North and Baltic seas during the next few years. In order to protect the marine environment when constructing offshore wind turbines, the German Federal Maritime and Hydrographic Agency, as the approval authority, has stipulated a sound pressure level of 160 dB re 1 µPa for single events and a peak sound pressure level of 190 dB re 1 µPa, which are to be observed up to a distance of 750 metres from the site. The drilling rig presented here provides a promising method for meeting this requirement. In other research projects, other low-noise foundation types and noise mitigation measures for pile driving are also being researched and developed to market readiness. For example, the German Federal Ministry for the Environment has funded the develop-ment and testing of “little” and “big” bubble curtains as well as a hydro sound damper. With the big bubble curtain, an air hose is laid around the drill site at a distance of about 70 metres. During the pile driving, which with good geological conditions can last several hours, pressurised air is released from this hose that rises in the water. This creates a curtain of air bubbles around the pile, which attenuates the spread of noise. Measurements show that the area exposed to particularly high noise pollution (> 160 dB) is reduced by up to 90% and nearby porpoises are less disturbed. Double bubble curtains are also currently being used to achieve an even higher noise reduction.The little bubble curtain essentially corresponds to its larger brother, with the difference that here several hoses are attached directly to the pile. In a research project, different configurations have also been tested in order to find the optimum sound attenuation relative to the compressed air. The hydro sound damper consists of a net to which numerous sound-attenuating balloons are fixed. The net is laid around the construction site and extends over the entire water column. The three methods could therefore also enable the required noise limit to be achieved with pile driving, however with a greater uncertainty than with drilling. A program simulating the propagation of waterborne sound is currently being developed to limit the need for complex offshore tests. All sound-attenuating methods are being further researched and optimised.

Project participants >> Overall coordination and drilling technology: Herrenknecht AG, Schwanau, Germany, Boris Jung,

Stefan Frey, Jung.Boris@herrenknecht.de>> Logistics and machine handling: HOCHTIEF Solutions AG, Civil Engineering Marine and Offshore (CEM),

Hamburg, Germany, Wolfgang Els, Wolfgang.Els@hochtief.de>> Mortar development: HOCHTIEF Solutions, Civil Engineering and Tunneling (CET), Essen, Germany,

Dr Christoph Budach, Christoph.Budach@hochtief.de

Links and literature>> www.hydroschall.de – This Web portal provides detailed information on the big bubble curtain

and the physical principles behind waterborne sound. www.fino-offshore.de | www.rave-offshore.de | www.erneuerbare-energien.de>> Conducting research in the middle of the ocean. BINE Projektinfo brochure 17/2011>> RAVE – Research on the offshore test field. BINE Themeninfo brochure I/2012>> Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit, Berlin (Hrsg.). Innovation durch

Forschung – Jahresbericht 2012 zur Forschungsförderung im Bereich der erneuerbaren Energien. www.erneuerbare-energien.de

More from BINE Information Service>> This Projektinfo brochure is available as an online document at www.bine.info

under Publications/Projektinfos>> BINE Information Service reports on energy research projects in its brochure series and the newsletter.

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Project organisationFederal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU)11055 Berlin Germany

Project Management Organisation Jülich Research Centre Jülich Christian Schneider 52425 Jülich Germany

Project number 0325064, 0325233, 0325533

ImprintISSN0937 - 8367

Publisher FIZ Karlsruhe · Leibniz Institute for Information InfrastructureHermann-von-Helmholtz-Platz 1 76344 Eggenstein-Leopoldshafen Germany

AuthorUwe Milles

Cover imageHerrenknecht AG

CopyrightText and illustrations from this publication can only be used if permission has been granted by the BINE editorial team. We would be delighted to hear from you.

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