REpower Systems: Less is More Offshore

When REpower commenced developing the 5M turbine in 2001 the company’s R&D department totaled only around 15-20 engineers, compared with the 200+ found there today. During September 2004, REpower Systems AG successfully erected a 5 MW 5M prototype at Brunsbüttel north of Hamburg, Germany, the largest and most powerful wind turbine in the world at that time.

The company’s R&D manager Peter Quell recalls: ‘This small team was not only responsible for 5M product development, but simultaneously worked at 1.5 MW MD series optimization and developing our new 2 MW MM product range.’ Besides limited R&D capacity, nobody in the wind industry at that time had much experience with developing a turbine of that scale, not to mention the actual turbine loads that would be encountered. Quell continues: ‘The combination of issues to be tackled represented a formidable design challenge. We are proud that our development effort worked out so well. In fact during the past four and a half year period, in the prototype, none of the main components had to be exchanged.’

Its 430 tonne (approximate) top head mass (THM) does not exactly qualify the 5M as a lightweight concept. The other side of the coin is that the turbine incorporates substantial design reserves, and these have been (partly) applied to up-scale the initial 5M by 20% to a 6 MW power rating. Quell says: ‘Re-applying design reserves for up-scaling purposes should not be mistaken with cannibalizing design safety factors. On the contrary, we always work according to stringent safety factors for critical key components like the drive train, which in turn forms an integral part of REpower’s in-house product design rules.’

For the 6M, the design team developed a new gearbox, that despite a 20% increase in (torque) load, weighs just 3% more than the current 5M. The 6M’s THM has slightly increased at 450 tonnes. Also new in the 6M is a 6 MW doubly-fed induction generator with higher voltage rating, and a new matching power converter. Another new main component is an RE 61.5 metre rotor blade developed in house.

Design Features

The main hall at Luneort is 160 metres long and 40 metres wide, and inside stand several 5M/6M main components and assemblies like drive trains, nacelles and rotor hubs lined up in various completion stages. Both from a concept point of view, and visually, the new 6M hardly differs from the initial 5M. Both sister models, for instance, feature an 18-metre long box-type nacelle that is 6 metres high and 6 metres wide. The main chassis is a heavy-duty fabricated welded steel structure split into a bolt-on front section and a rear section. Each of the two sub-structures is pre-assembled along opposing (long) main hall sides. The heavier front section accommodates a non-integrated drive train comprising a hollow main shaft supported by two main bearings and a flange-on gearbox equipped with a torque support at each side. The rotor hub is bolted to the main shaft front flange. Considering the 5M/6M power-ratings, their (near) identical hubs are surprisingly compact. This is due to the fact that the pitch motors are placed outside the main casting, says Quell. He further explains that REpower decided on a twin main bearing arrangement to enable a complete offshore gearbox exchange operation in only one day, and without having to remove the rotor.

Quell further explains that there are several key considerations to justify its decision to choose a fabricated welded steel main chassis with all its turbine models. ‘All REpower wind turbine models feature a non-integrated geared drive train, which by definition requires a rather large main chassis. Foundry capacity for especially large complex castings is still rather limited and can easily turn into supply chain bottlenecks, especially when the components market is tight, as it has been during recent wind boom years. On the other hand, there is generally a much larger supply base of dedicated fabricated welded steel main chassis suppliers to choose from. With our extensive structural design experience we are capable of developing very competitive fabricated steel main chassis, both from a cost price and mass point of view,’ he says.

Quell stresses this point, pointing to two 3.XM prototypes being assembled at separate stations in the large hall. This new IEC WC II turbine (rotor diameter 104 metres) again features the typical main chassis layout known from other REpower models. ‘Two of our specialists spent a full year to design and optimize the 3.XM main chassis, and this effort paid off. We simultaneously put a key emphasis on issues like service friendliness and ergonomic working conditions, cost-effective assembly, transport logistics, and easy erection,’ says Quell. Another 3.XM design objective was to limit individual maximum hoist loads to 70 tonnes, which substantially reduces crane capacity and therefore crane hiring costs. For example, the nacelle without the drive train and rotor weighs 60 tonnes. The complete drive train assembly can be hoisted separately if necessary and weighs 55 tonnes, while the 3.XM’s THM is also competitive at about 160 tonnes.

For this onshore turbine concept REpower returned to a proven three-point gearbox support, comprising a single main rotor bearing and a main shaft that is flanged directly to the gearbox input shaft. Quell says: ‘As part of a pre-design concept evaluation we looked into various concepts including direct drive, but our calculations clearly showed that a fast-speed geared system is still the most cost-effective drive solution.’

Low Loaders Beat Crane Performance

5M/6M rear sections are lined up opposite the drive train and main chassis front section assembly stations. These rear sections contain the main electrical components including six-pole doubly-fed induction generator, frequency converter, switchgear, and medium-voltage transformer.

Each 5M/6M front and rear chassis section moves position several times during the assembly process, and these limited-distance heavy transport movements are accomplished with the aid of special multiple-wheel low-bed trailers – supplied by Scheuerle, of Germany. Such special modular expandable vehicles are capable of moving freely along their X-axis as well as Y-axis, explains hub and nacelle assembly manager Rainer Oppermann, who previously worked in the automotive industry. ‘Prior to the design of this new assembly plant we looked closely at the most cost-effective means of medium series assembly. One of the outcomes was that due to the huge masses to be moved – the mass of +5M/6M nacelle without rotor is about 325 tonnes – these vehicles prove a superior alternative to heavy cranes,’ he says.

When a rear section assembly process is ready it is moved to a separate part of the main hall, to be joined with the front section containing key components like the yaw bearing and yaw motors. ‘The last mechanical nacelle assembly step is transporting a pre-assembled 130-tonne drive system – gearbox, main shaft with bearings, and rotor hub – by crane into position and lower it onto the main chassis,’ Oppermann concludes.

Offshore and Onshore Development

About four and a half years after the first 5M was built, REpower now has 17 of its 5M turbines operating onshore and offshore. The company is also a market leader in this 5 MW+ turbine segment where it currently faces few competitors, see sidebar “Some REpower Offshore Challengers,” below.

The first 5M offshore unit was erected during 2006, and a second in 2007. Both units comprise the Beatrice research project off the Scottish coast, and were erected in record 44-metre deep water. Last year REpower delivered six 5M turbines for the first stage (30 MW) of Belgium’s Thornton Bank offshore project. The remaining nine 5M turbines, including the 2004 prototype, all operate onshore. The last of these has been operating in Bremerhaven since 2008, and as a wind industry novelty has been put on a custom-developed jacket-type fabricated steel foundation. The latter solution closely resembles the jacket-type foundation applied earlier at Beatrice and that will be used again for Germany’s 60 MW Alpha Ventus offshore research wind farm. As part of this 12-turbine project REpower will erect six 5M offshore turbines this year.

Additional European offshore wind farms planned with REpower 5M turbines include, among others, Nordergründe (Germany 18 x 5M), but the absolute highlight is a recent framework agreement with RWE Innogy involving the supply of 250 REpower 5M and 6M offshore wind turbines, see sidebar, “RWE Framework Agreement for 250 5M/6M REpower Turbines,” below.

Finally, marking the next step in its rapid offshore wind turbine development era, a first batch of three 6M prototypes were ready to leave the Bremerhaven facilities by the end of January 2009. These test units were shipped to Ellhöft near the German – Danish border and are all expected to be installed as REW goes to press. In the years to come, annual output from the Bremerhaven site will be boosted to 80–100 5M and 6M turbines a year in a two-shift system, serving operators at demanding offshore sites in European waters and beyond.

Eize de Vries is Wind Technology Correspondent for Renewable Energy World.  You can contact him through

Sidebar: Some REpower Offshore Challengers

Siemens Wind Power of Germany and Vestas Wind Systems of Denmark are today’s offshore wind market leaders. The latter offers 2 MW and 3 MW models, and Siemens a 2.3 MW and a 3.6 MW model. Siemens is currently testing a new 3.6 MW direct drive ‘Concept’ turbine. Meanwhile, Vestas is rumoured to be working on a new offshore turbine, perhaps based on a 4 MW+ former NEG Micon model.

BARD Engineering of Germany has developed a 5 MW class offshore wind turbine and has so far erected two onshore prototypes and one near-shore turbine late last year. BARD also announced a turbine scaling-up to 6.5 MW, while in the German press the company founder reportedly said his final goal is ‘7+x’ MW.

Multibrid, like REpower, series manufactures a 5 MW offshore wind turbine with among other features, a single-stage gearbox in Bremerhaven. The company, 51% owned by nuclear giant Areva, operates four onshore prototypes in Bremerhaven and will this year deliver six units to the Alpha Ventus offshore wind farm.

Gamesa of Spain is developing a 4.5–5 MW offshore turbine with a likely nameplate of the G-128, in reference to the 128-metre rotor diameter.

DarWinD of the Netherlands is developing a 5 MW direct drive offshore turbine with two prototypes planned for 2009–2010. However, parent company Econcern faces severe financial difficulties, which might have an effect on DarWinD’s future (ownership). Another Dutch/German consortium, according to well-informed wind industry sources, is at an advanced stage in developing a multi-megawatt class offshore wind turbine.

Again from the Netherlands, Blue H plans to develop a 5 MW two-blade floating offshore wind system for deep-water applications.

Meanwhile, Norwegian/Swedish supplier Scanwind is developing a 4 MW+ direct drive offshore wind turbine based on the current 3.5 MW model. According to recent company information, a prototype is planned for 2011.

US-based Clipper Windpower plans develop a 7.5 MW MBE or ‘Britannia-class’ offshore turbine in the UK with local partners. The rotor diameter is 150 metres, whereas a ‘second-stage’ up-scaling to 10 MW has been mentioned.

US-based American Superconductor Corporation (AMSC) has entered into an agreement with the US Department of Energy to validate the economics of a full 10 MW class wind turbine with a so-called superconducting generator.

Finally, Enercon of Germany has developed a second-generation E-126 direct drive turbine with a 127-metre rotor that succeeds the initial E-112 (4.5/6 MW, 2002). The rated capacity is 6 MW, but wind industry insiders expect a substantial up-scaling into the 7–8 MW range. However, officially Enercon has no plans to enter the offshore wind market.


Sidebar: RWE Framework Agreement for 250 5M/6M REpower Turbines

A February 2009 framework agreement between RWE Innogy and REpower. This deal has a potential volume of approximately €2 billion, making it one of the largest contracts ever signed in the wind industry and the largest ever in the area of offshore wind energy. The first 30 5Ms are scheduled for delivery in 2011, while this volume will continue to rise through the years 2012–2015. A majority of the units are destined for Innogy’s planned Nordsee 1 wind farm, located 40 km north of the East Frisian island of Juist in water depths of 26–34 metres. According to planning, the roughly 1 GW offshore wind farm will comprise between 150 and 180 wind turbines of both the 5 MW and 6 MW series. In a statement, RWE Innogy says that the agreement offers them significant flexibility across its full range of offshore projects under development, in markets like the Netherlands and the UK.



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Eize de Vries worked from 2001 to 2010 as Wind Technology Correspondent for Renewable Energy World magazine and rejoined the publication again in 2013. He also works as a Technology and Market Trends Consultant for Windpower Monthly and WindStats Report, is contributing editor to reNews, and provides specialized editorial services to various other international clients. An automotive and agricultural engineer by profession, he among others worked in Botswana, Mauritania and Bangladesh and Eastern Europe as a design engineer and technical consultant. In 1996 Eize established Rotation Consultancy, a sustainable energy consultancy specialised in wind power turbine technology and integrated solutions for onshore and offshore application. His company offers a range of dedicated services including turbine concept and systems analysis and development assistance, turbine and drivetrain technology reviews, turbine technical comparisons, and for older-generation turbines upgrading support. Eize has finally taught courses in sustainable (wind) energy technology, product design, lifecycle-based sustainability issues and related fields. Today he serves as a guest speaker/lecturer/moderator at universities, technology institutes and for other client groups.

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