A solid foundation: Technological developments from the DEWEK conference

In late November 2006, members of the wind industry gathered in Germany to discuss the latest technical developments in the field. Eize de Vries attended the meeting reports on some of the latest research and innovations that were unveiled at the DEWEK conference.

First organized in 1992 by the Deutsches Windenergie-Institut GmbH (DEWI), the 8th international bi-annual DEWEK technical wind conference was held from 22-23 November 2006 in the German city of Bremen. Over 600 participants from 29 countries attended the two-day conference, which was supplemented by a technical excursion that toured three of the latest 5-6 MW wind turbines which operate in Bremerhaven and Cuxhaven.

Since opening in 2003, the DEWI-Offshore and Certification Centre GmbH in Cuxhaven (DEWI-OCC), has operated a test site for offshore wind devices. This demanding coastal North Sea location has space for five offshore prototypes, and is characterized by strong winds and marine-type climate conditions. Once in Cuxhaven, the conference tour-groups visited a 6 MW Enercon E-112 and one of the two 5 MW REpower 5M turbines. A few weeks later a 2 MW DeWind D8.2 prototype nacelle, featuring a novel Voith WinDrive® hydrodynamic power train was transported from Lübeck to Cuxhaven, where it joined the other test turbines. (Renewable Energy World will be covering the new DeWind turbine in greater detail later in 2007.)

Concrete monopile foundations being prepared for installation at Lillegrund in Sweden rsb foundations

The DEWEK conference programme was made up of a mix of technical and scientific contributions, on topics such as multi-megawatt turbines, new concepts, design loads, technical and economic optimization, and condition monitoring. In addition, wind resource assessment, wind farm effects, grid integration, reliability issues, cold climate operation, and many offshore wind power related issues were discussed. The majority of presentations came from German government institutes, engineering consultancy firms, higher education bodies and research institutions, while the contribution from international wind turbine suppliers appeared under presented. A participant from one of the top three wind manufacturers explained this imbalance by saying that there is an increasing fear among equipment suppliers that their strategic intellectual property (IP) interests can leak away through conferences to competitors. On the other hand, he believed that the summary of presentations by universities and research institutes indirectly reflects current technological and scientific advancement levels within the wind industry.

Conference themes

The challenges associated with offshore wind development were one of the key themes of the conference, and accounted for a great many papers and presentations. The German government’s aim is that renewable energy sources should contribute 20% of total electricity use by 2020. Today it is at about 6% in Germany, of which nearly half (42.4%) is provided by wind power (hydro comes second with 34.4%). To help the country reach its targets, the fifth German Energy Research programme laid out the following wind energy R& D priorities – cost reduction, offshore wind energy development, grid integration, and environmental issues. Research and development projects that receive federal support include the offshore research platform (called FINO 1, 2 and 3), gearbox development for multi-megawatt class wind turbines, and offshore foundations. In addition, the establishment of a new competence centre for rotor blade development and testing in Bremerhaven (lengths up to 90 metres), compressed air energy storage, and automated rotor blade production.

The second prototype of the M5000 prototype being installed, showing innovative tripod foundation multibrid

In terms of manufacturing, the German wind industry has clearly taken the lead by developing three different offshore wind turbines in the 4.5-6 MW range, with another under development (see section on BARD below). In total, fifteen prototypes and 0-series turbines in this ‘super’ class have come online since 2002. Two of the wind companies – REpower and Multibrid Entwicklungsgesellschaft – have announced that they will commence series manufacture of their 5 MW turbines at new production locations in Bremerhaven in 2007. The third company, Enercon, announced a new second-generation 6 MW+ E-126 prototype featuring a 127-metre rotor diameter that is expected during 2007. Meanwhile Siemens Wind Power holds the record for installing the largest number of commercial wind turbines with a rotor diameter over 100 metres, the 3.6 MW SWT-3.6-107.

German Offshore development looking brighter

Despite its leading position on the technology front, Germany’s offshore wind market development is still lagging behind, especially when compared to leading offshore wind nations like Denmark and the UK. Even in the Netherlands, the first 108 MW offshore wind farm became operational in the autumn 2006, and a second, 120 MW, project is under construction.

By contrast, in Germany, only one offshore turbine is in operation (a 2.5 MW Nordex N90 which was installed near Rostock in early 2006). However in November 2006, there was an important breakthrough for the stagnating offshore market, when the federal Council of Ministers passed a law which aims to speed up the planning procedure for infrastructural projects. Central to this new legislation is that the grid connection of offshore wind farms in Germany, which should have started construction by the end of 2011, has to be provided by the grid operators. This is highly significant because grid connection costs for offshore turbines can add up to 30% of total investment costs, say experts. Thanks to the new rules, the costs for grid connection of offshore wind farms will – as is the case with all other type power plants – be distributed over the total grid operation. Suddenly, investments in German offshore wind energy projects are becoming very lucrative. According to the German organization VDMA Power Systems, this positive development will have a massive impact on the country’s economy. This will not be limited to the coastal regions, as component suppliers are more or less evenly distributed all over the country. Experts consider the completion of four or five offshore wind farms by 2011 with a total capacity of around 1500 MW as realistic. Such a massive undertaking will require investments in the range of around €3.6 billion throughout Germany, which translates in terms of job creation volume into 25,000 and 40,000 ‘man years’.

BARD Engineering

Founded in 2004 with the backing of a Russian investor who previously worked in the oil and gas industry, Bremen-based BARD Engineering GmbH, is an ambitious new company that has made big strides forward and among others ‘seeks to become one of the biggest offshore wind farm operators by 2010.’ Furthermore, BARD aims to build its ‘own design’ of 5 MW offshore wind turbine in series from 2008. Commercial deployment can either be in company-owned offshore wind farms or contracted to third party (i.e. utilities) owned projects.

The ‘conventional’ pitch-regulated variable-speed-geared BARD VM wind turbine features a 122-metre rotor and a single main bearing (no main shaft). This rotor support solution is also used in the Harokasan Z72 (former Zephyros Z72), Vestas V90-3 MW, Fuhrländer FL 2500, and the (1-5 MW) Multibrid models originating from Finland and Germany. The BARD VM was designed by Rendsburg-based aerodyn Energiesysteme GmbH, a renowned engineering consultancy that is highly experienced in wind turbine and component design. Earlier the German specialists developed the patented 5 MW Multibrid wind technology (fully integrated drive train with slow speed generator), and the small 5 kW Aerosmart. A large share of current aerodyn activity is dedicated to wind turbine designs on behalf of Chinese manufacturers, and ones specially adapted to specific local conditions, said managing director Sönke Siegfriedsen at the DEWEK conference: ‘An example of design adaptations is dealing with large differences in operating temperature between summer and winter, or mechanically coping with sand storms in harsh desert conditions.’ Other local industrial challenges Siegfriedsen mentions include the non-availability or limited availability of high-strength steel and/or high quality cast components in some regions of China. Such limitations require specific wind turbine design solutions and adaptations to meet local conditions and constraints.

Proven technology only
According to Heiko Roß, Managing Director of BARD, his company has already established a manufacturing plant in the port of Emden capable of producing up to 100 units a year. The plant’s location offers direct access to the water. According to Roß: ‘Our aim is to erect two offshore prototypes in 2007 near the port of Emden, and to complete another two units before the end of the year. Series production has to commence by the end of 2008. The first batch of commercial turbines will be available for deployment in our projects by 2009. In order to achieve the required short time-to-market from prototype development to full testing and certification, we deliberately went for proven wind technology only. This means that all 5 MW class key components such as the gearbox, generator with converter, and single main bearing are all practice-proven and – to a certain extent – semi-standard.’

For the prototypes, Winergy supplied the gearboxes, doubly-fed induction generator and converter system, while Polymarin Composites of the Netherlands supplies BARD with blade production moulds and additional production technology (although the blades themselves will be manufactured in-house in Emden). With regard to offshore foundation technology Roß says that BARD Engineering conducted a comparative benchmark study, which compared various steel and concrete foundation options on multiple criteria: ‘Gravity-based concrete foundations scored well for instance in terms of total costs, but we have worries about lifetime performance. The study results earmarked a so-called Tri-pile foundation to be the most favourable overall solution for our situation. This foundation comprises three independent steel pipes that are all rammed into the seabed. As a second step the piles are joined together on top and above the seawater surface into a rigid assembly by means of a special “transition bracket”, on which the bottom wind turbine tower flange is bolted.’

So far, Germany’s offshore wind portfolio contains 15 approved offshore projects with a total volume of a 1000 turbines, giving an indication of the market potential for the BARD’s offshore wind technology.

Gravity-based concrete foundation

Hugo Mathis is managing director of Austrian-based RSB Schalungstechnik, a firm specializing in complex concrete ‘formwork’ structures. During his presentation in Bremen, Mathis argued that future 5-7 MW offshore wind turbines erected in 25-40 metre deep water will require new foundation solutions. If such huge foundations were constructed as steel monopiles, the required diameter would be in the range of 8-10 metres and the total length about 50-60 metres. Utilization of jacket type or tripod type foundations with similar capacity and water depth range will, in his view, result into even higher demands with regard to fabrication, welding complexity and corrosion protection. This points to concrete foundations as the solution. However, the construction of gravity-based concrete foundations requires sophisticated formwork systems and new transport logistics methods to deal with component masses between 3000 and 7000 metric tonnes. One commercial vessel with a maximum hoist capacity of 8700 tonnes is the Svanen. This huge custom- built floating barge is owned by Dutch civil engineering contractor Ballast Nedam, and was in 2006 deployed for installing 36 monopiles of a 108 MW Dutch offshore wind farm.

RSB-type concrete foundations are designed for a 30-50 year operational life span, explains Mathis. ‘The design should already consider mass and height requirements of the next generation offshore wind turbines, which enables a foundation mid-life wind turbine repowering exchange. Axis-symmetric reinforced concrete structures will become increasingly important as future offshore foundations, due to the bigger potential for cost savings compared to steel foundation structures. Additional cost savings can be achieved when the foundations are floated to the construction site.’

In 2006, RSB completed formwork activities for the 49 concrete foundations for the Swedish ‘Lillegrund’ project developed by Vattenfall of Sweden. Mathis concludes by telling the audience that offshore wind developers like C-Power from Belgium, E.ON of Germany, and Vattenfall are seriously considering the use of concrete gravity-based offshore foundations in 2007.

Beatrice wind farm

As part of the Deep Offshore Wind Farm with No Visual Impact (DOWNVInD) project, REpower of Germany, along with various international partners, erected a 5 MW wind turbine off the Scottish coast under very demanding conditions. The average wind speed at the site is 10.5 m/s, while at the extreme wave heights hit 15.6 metres. The REpower 5M turbine features a rotor diameter of 126 metres and a Top Head Mass (THM; nacelle + rotor) of 430 tonnes. The machine used in the Beatrice wind farm is a slightly modified version of the prototype that has been operational at Brunsbüttel (Germany) since the autumn of 2004. Among several marine modifications carried out was the installation of dehumidifiers in the tower and nacelle, which keep air humidity constant at 50%. This prevents condensation of water on cold surfaces and thus reduces corrosion risk. A second important marine modification is the switch from a 20 kV transformer cooled by external air, to a fully enclosed 33 kV nacelle step-up transformer. Also new is an onboard crane in the nacelle with a maximum hoist capacity of 3.5 tonnes. This device allows the handling of all equipment for a standard service as well as any components that might need replacement during service visits (except heavy main components).

The prototype BARD VM 5 MW turbine was unveiled at DEWEK bard

Designed for a water depth of 50 metres, ‘simple’ sub-structure solutions like monopile or gravity-based foundations were found not to be feasible at Beatrice. Based on studies performed by specialist firms Atkins, Kellogg Brown & Root, REpower and OWEC Tower, three substructures were considered for the final selection process:

  • centre column tripod (CCT);
  • flat faced tripod (FFT);
  • OWEC jacket quatropod (OJQ), a four-legged jacket solution.

Among several key selection criteria were dynamic characteristics, material fatigue performance, fabrication costs, and environmental performance.

According to the study a CCT design requires cast nodes to improve fatigue performance, bringing the total mass up to 1080. The FFT needs three large 96-inch (243 cm) diameter piles but no cast components, while the substructure mass is 1140 tonnes. Finally the OJQ is based on a design from OWEC Tower A/S, a ‘traditional’ jacket structure adapted for REpower 5M wind turbine use. The mass of the lightweight structure, including three 72-inch piles for fixing the substructure to the seabed, is approximately 600 tonnes.(For more general information on the Beatrice project see Renewable Energy World November-December 2006)

OWEC Jacket Quatropod (OJQ)
With regard to engineering estimates of fabrication costs, the OJQ showed the best result of the three options. Flat-faced tripod fabrication costs were only 14% higher, but the additional centre column tripod fabrication costs amounted to 72%. This huge cost difference is attributed to the need to use an expensive cast central node. The CCT on the other hand proved a low-risk design option in terms of dynamic behaviour, while the FFT design holds some uncertainties. Initial OJQ risks resulted from dynamic excitation induced by the blade passing frequency (3P) at low wind speeds during standard rotational speeds. This problem could be solved by a slight modification of the rotational speeds aimed at increasing the safety margin.

A tripod type foundation bard

Based on the detailed comparative analysis mentioned above, and additional criteria, the OJQ was selected as the most favourable sub-structure solution for the Beatrice demonstrator project. The major challenge of the OJQ design was the transition note, made up of four coning legs and a central tube with tower mounting flange. This rather complex assembly fits on top of the four-legged jacket structure, and the bottom wind turbine tower flange is bolted to its tubular centre section.

After the jacket is put onto the seabed, four 72-inch piles (one at each corner) are put in the so-called pile sleeves and driven in to target depth. The next step is to level the jacket and establish a permanent connection between jacket sleeve and pile by means of swaging. (Swaging is a metal forming technique during which metal tubes are shaped using high pressure.) Before installation the turbine was pre-installed as a complete assembly, comprising tower nacelle and complete rotor. The turbine and tower assembly was then put on a crane barge and floated to the erection site. For the final installation of the turbine on top of the jacket, it was necessary to use a novel ‘soft landing’ installation technique that includes a custom designed ‘tower interface frame’. This device compensates for crane barge movement caused by waves and swells – up to around 1 metre sea level movements.

Steel tripod foundation

On the morning of 24 November, technical staff at WeserWind GmbH Offshore Construction Georgsmarienhütte in Bremerhaven were busy completing the world’s first tripod type land-based foundation for multi-megawatt class offshore wind turbines. The impressive structure is 47 metres high and weighs approximately 600 tonnes, and is installed close to the German North Sea coast near the first 5 MW Multibrid M 5000 wind turbine prototype. The latter features a concrete-steel hybrid tower and has been operational since the end of 2004. The second M5000 prototype nacelle was mounted on the tripod structure in December 2006. This tripod is expanded with additional tubular steel sections, and, like the first M5000 prototype, has 102 metre hub height. Construction of the onshore tripod commenced during the second half of 2006. The operation involved moving about 600 tonnes of prefabricated tubular steel sections to the construction site, where it was put together into the final welded tripod structure. Tubular steel sections with wall thicknesses up to 90 mm have been used, explained a WeserWind representative at the construction site. As part of the tripod design process, which specifically aims at cost-effective high-quality series manufacture, a number of innovative methods and techniques had to be developed and pre-tested. This process included finding new time-saving component handling and positioning techniques, and the introduction of automated welding operations. In the context of this ambitious project a ‘best-fit’ analysis for matching components was developed. Including advanced control systems, the method was executed and validated for the first time during the Bremerhaven project. A ‘best-fit’ analysis itself involves the use of advanced mathematical algorithms to achieve an optimized match between component pairs, for instance with regard to minimizing welding surface manufacturing tolerances. WeserWind aims to use all data and experience gained in Bremerhaven to design a further optimized offshore tripod. A major difference with this first land tripod is that an offshore sub-structure is exposed to a combination of turbine loads, as well as sea current and wave loads. The eventual aim is to achieve cost-effective series production of about 50 tripods a year in Bremerhaven.

Studies in fatigue

The University of Hannover has conducted an interesting research project into the fatigue performance of grouted joints. Grout is a high performance concrete (HPC) that is used in monopile offshore wind turbine foundations for the fixed connection between the actual pile and the transition piece. Grouting is also being considered for the pile-sleeves of tripod foundations. However, little is currently known about the fatigue characteristics of the brittle material itself and the entire construction under predominant bending. So far, results indicate the need for so-called shear keys (in the shape of welded steel beads), as tests have shown outstanding fatigue characteristics with limited material deterioration of the HPC near the shear keys.

The wide variety of research projects and dedicated product developments shows how dynamic the emerging offshore wind market is, and it seems finally to be getting established in Germany. With regard to the availability of offshore wind turbines, 2-3 MW installations continue dominate offshore, but larger machines are slowly but surely entering the market. An example of this is the offshore wind farm at Burbo Banks in the UK, which comprises 25 Siemens turbines of 3.6 MW each (due to be in operation by end summer 2007). With the growth of turbine size and increasing offshore water depths, the need to find cost-effective sub-structure solutions that are easy and quick to install increases. Which of the solutions presented in this article will finally turn out to be the best options, including a choice for the most favoured materials (steel, concrete, hybrid designs) is still largely undecided. Finally the fast and unexpected development of the 5 MW BARD VM turbine is a clear indication that the combination of vision and determination is a key factor in wind industry success. Above all, it shows the trust which new investors place in the potential of offshore wind power as a contribution to solving the world’s continuous and growing hunger for clean energy in the decades to come.

Eize de Vries is Wind Technology Correspondent for Renewable Energy World
e-mail: rew@pennwell.com

BAUER ‘Flydrill’

At the DEWEK conference, German company BAUER Maschinen GmbH presented a novel Flydrill bored piling system especially developed to enable offshore monopile foundation installation in difficult soils. The BAUER BFD 5500 Flydrill is a 52 tonne, portable, self-powered drilling assembly, with two 260 kW diesel-hydraulic power packs and a rotating soil drill. This new technology was first put to use in the United Kingdom’s 90 MW Barrow wind farm. For sites with ‘normal’ seabed soil conditions monopiles are rammed to the required depth with the aid of a hydro-hammer. At Barrow, for nine foundations the ‘shaft friction’ with the soil was so large that the huge 452-tonne piles featuring a 4.75-metre diameter could not be rammed far enough into the seabed. This phenomenon is called ramming up to refusal point. In such cases a Flydrill assembly is fixed on top of the monopile casing by means of a hydraulic clamping device and remains clamped during soil drilling. The soil is taken out from inside the casing whereby the casing guidance contributes to overall system stabilization. For emptying the drilling tool, the clamping cylinders are opened and the complete Flydrill assembly is swung sideways towards the open sea where the soil is released into the water. After removing sufficient soil from the seabed inside the casing, the reduced shaft friction enables the hydro-hammer to be reinstated and the pile is rammed the remaining metres into the seabed.

The Flydrill device in operation at the Barrow wind farm bauer machinen gmbh
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