REpower Systems: Less is More Offshore

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A new 6 MW wind turbine is going into production alongside REpower’s 5M.

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FUENTE – Renewable Energy – 28/04/09

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 panel, right.

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 panel on page 55.

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.

Author:  E. de Vries

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