Areva Multibrid's Series Production: An Ambitious Growth Path

Multibrid’s characteristic bright yellow, blue and grey-coated wind turbine assembly hall and office quarters forms part of Bremerhaven’s distinct offshore wind industry cluster, now encompassing four companies. The Multibrid facilities are located almost next door to WeserWind’s new 2010 manufacturing hall, both with close access to a deep-water loading quay. (Left: Tripod substructures for Alpha Ventus M5000s being transported by barge. Credit: Areva MULTIBRID / Jan Oelker)

Outside Multibrid’s main assembly hall is ample space for future expansion, and there is already a substantial handling and storage area for large components, like assembled nacelles and technical units. The latter are distinct grey/yellow-coated structures that fit in between a Tripod substructure and tophead (see WeserWind article on page 80). The main hall is fitted with internal overhead crane facilities and offers room for assembly of several M5000 nacelles.

Fully integrated drive system

The history of the Multibrid M5000 dates back to a major technology development project initiated by the renowned German engineering consultancy aerodyn Energiesysteme during the second half of the 1990s. The novel 5 MW Multibrid wind turbine concept was first presented at the Hannover Industrial Fair during April 1998. Initially, the designers opted for a variable speed system with a seawater-cooled marine generator. Power regulation was by ‘classic’ stall (fixed rotor blade angle) principle, based on a design philosophy focused on combining engineering simplicity with easy replacement and inexpensive upkeep. A 100 metre rotor diameter is, by today’s standards, modest – but it was top of the bill a decade ago. The initial concept evolved into today’s pitch-controlled variable speed M5000 model with its enlarged 116 metre rotor diameter, but the elegant nacelle cover with characteristic lines remains virtually unchanged.

What also remained is the concept of a fully integrated compact drive system that forms the heart of each Multibrid M5000 turbine. (Left: Technical units for offshore installations of the M5000. Credit: Areva MULTIBRID)

Multibrid, as a product, is best described as a cross between a conventional high-speed wind turbine with a multi-stage gearbox and fast running generator – and direct drive with no gearbox.

The Multibrid concept was based on the outcome of ‘extensive comparable studies of various drive train designs,’ focused on critical components and key design variables. In one study the researchers compared design features and operating economics of standardized 1.5 MW (direct drive, geared, and Multibrid) variable speed units, each fitted with a 65 metre rotor diameter. Top Head Mass (THM) and systems reliability were assessed as the two single most important decision criteria for the new drive-train concept.

Besides Multibrid GmbH, a second supplier WinWinD, based in Finland, produces Multibrid-type wind turbines of 1 MW and 3 MW each. Both apply the Multibrid-type drive system in their series produced wind turbine models.

Stiff load-transmitting structure

The compact low-speed geared drive system of the M5000 comprises of a single rotor bearing, a single-stage gearbox with 1:9.92 step-up ratio, and a medium-speed (nmax 148 rpm) permanent magnet (PM)-type synchronous generator. These components are integrated into one compact, stiff, load-transmitting cast structure. The degree of systems integration has been developed such that the rotor bearing, gearbox and generator do not have their own separate housings. Instead, they are fitted directly into the cast iron main chassis, with the generator bearings forming part of the gearbox assembly.

Besides the huge castings, critical Multibrid components proved to include large-diameter internal geared rings for the gearbox and bearing rings. Key engineering challenges were the maximum component width regarding machining, and component diameter, determining hardening limits. (Left: Production of the Areva Multibrid M5000. Credit: Areva MULTIBRID)

According to the designers, compactness combined with modest dimensions suits a PM generator perfectly as a ‘fully enclosed’ solution in which all sensitive components, including the generator windings, are sealed from the aggressive marine environment.

Redundant piping for the pump systems, for gearbox cooling, oil circulation, and water-cooling of the generator stator, are also incorporated within the M5000 concept. The generator rotor is air-cooled. Designing in redundancy – and the clear advantage that a breakdown of a single auxiliary component will not create turbine downtime – must be weighed up against its associated increased investment costs and implementation. However, a consequence of the fully integrated Multibrid drive system philosophy is that individual components cannot be easily exchanged in case of a major mechanical failure.

Nacelle ventilation

M5000 turbines are fitted with an advanced nacelle ventilation system in which air is taken in at the bottom of the tower. The reliability enhancing system cleans the air and removes water droplets and aerosols, and puts it under slight pressure. This is controlled in such a manner that under no condition will unfiltered air be allowed to enter the nacelle and corrode the internals.

The nacelle cover of the ‘original’ four M5000’s operating in Bremerhaven is relatively cramped compared to other multi-megawatt class state-of-the-art (offshore) wind turbines like the Enercon E-126, REpower 5M and Siemens SWT-3.6-107. Consequently there is a rather narrow passage around the drivetrain inside. In order to create more working space during service activities and/or hoisting actions, the nacelle cover can be opened. Two sections split in the middle parallel to the main drive system axis pivot and open sideways.

Niels Erdmann, MD of MULTIBRID, is responsible for offshore wind activities at PROKON Nord, and is closely involved with M5000 product development issues in Bremerhaven. He explains that for certified wind turbine offshore operation in Germany, a number of stringent health, safety and environment demands had to be met. Erdmann explains: ‘Helicopter hoisting is increasingly applied for service activities as well as staff emergency evacuation. Helicopter-based services by themselves are not without risk and require a minimum distance between the landing platform and the rotor blades. Furthermore, there are specific HSE requirements with regard to evacuating sick or injured persons in a horizontal position from the nacelle onto a lower level service platform for further transfer.’

Enlarged nacelle

These offshore operational requirements proved hard to meet with the existing M5000 turbine concept. The AREVA MULTIBRID management explains: ‘MULTIBRID engineers therefore developed a solution whereby the nacelle is substantially enlarged but especially expanded towards the rear by means of a bolt-on box-shape space-frame-type steel structure. A patented feature is the roof section that can slide open for service actions or otherwise. The metal floor contains a smaller hatch that allows lowering staff for evacuation in case of emergency.’

Compared with the original M5000 (2003–2004), the new M5000 Offshore nacelle is perhaps the most spacious found in today’s multi-megawatt wind turbine models. The compact drive train takes up surprisingly little space and is now located in the front of the nacelle.(Left: An artist's impression of the M5000 in service offshore. Credit: Areva MULTIBRID)

Service personnel benefit from ample space to carry out their work comfortably inside, especially around the drive system. Apart from an emergency diesel-generator, the remaining floor area remains largely empty. The cooling utilities for the drive train are now located in the nacelle rear for easy access, below the steel walking floor. As before, all power electronics are located in the foot of tower.

The initial M5000’s Top Head Mass (THM) was only about 310 tonnes, a favourable figure when compared to the REpower 5M (~430 tonnes) or Enercon E-112 (~530 tonnes). The newly enlarged nacelle of the M5000 Offshore with its helicopter hoisting platform has resulted in a THM increase of about 30 tonnes to 340 tonnes. A low THM translates directly into substantial cost advantages in terms of a reduced need for construction materials like steel and copper as well as indirect cost savings. For instance a lower THM can enable the application of lighter, and therefore cheaper, towers and foundations.

With regard to the M5000 performance record since 2004, Erdmann said that the four onshore prototypes in Bremerhaven achieved an overall availability in excess of 98%: ‘Trying to achieve the highest possible wind turbine and wind farm availability is perhaps the single most important issue with offshore operations. [The] helicopter hoisting application ensures that installations can be accessed nearly all the time and even under harsh weather conditions,’ he says.

Major offshore player

Pfleiderer of Germany acquired the rights for Multibrid technology (>1 MW) from aerodyn in 2000. Under the wings of its wind subsidiary, Pfleiderer Wind Energy, a prototype was built during 2003. However, the objective to finish and install a first M5000 in the period September/October 2003 near Bremerhaven was not met. Instead Pfleiderer stepped out of the wind business and sold its Multibrid technology package in November 2003 to the German project developer PROKON Nord Energiesysteme GmbH.

Apart from its MULTIBRID company involvement, a second PROKON Nord shared company – Offshore Wind Technology GmbH (OWT) – has specialized in the design of offshore wind turbine foundations. In late 2004, PROKON Nord finished and erected the M5000 prototype in Bremerhaven on top of a concrete/steel hybrid tower. A second Bremerhaven onshore prototype, in late 2006, put on an offshore type steel Tripod which serves as a model for the substructures that MULTIBRID today (semi-standard) applies to its offshore projects.

During October 2007, engineering giant AREVA Group acquired a 51% majority share in MULTIBRID. Minority shareholder PROKON Nord continues to maintain strong links to AREVA MULTIBRID as a main component supplier and major M5000 wind turbine buyer.

In July 2008, AREVA erected MULTIBRID prototypes 3 and 4, again onshore on 130 metre-high concrete/steel hybrid towers. The completion of this project effectively marked the completion of prototype testing and optimizing.

AREVA’s strategy is to develop its MULTIBRID subsidiary within five years into a major offshore wind market player with 15% global market share. As part of this strategy plan annual production has to increase from 10–15 turbines this year to 80–100 units in 2012.

With regard to Alpha Ventus, plans to erect six proposed M5000 turbines in 2008 were postponed to this spring.

The current economic crisis is impacting on some already permitted offshore projects with M5000 turbines, including the postponement of Borkum West II (400 MW) and C√¥te d’Alb√¢tre (105 MW), France’s first offshore project. A recent positive development is that AREVA, through its subsidiary MULTIBRID, will supply eighty M5000 turbines for the German Global Tech 1 offshore wind farm (400 MW). AREVA will also provide commissioning, testing and maintenance services. The order will be worth more than €700 million and delivery is scheduled for 2011–2012. The wind farm will generate an estimated 1.4 TWh annually.

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

SIDEBAR: THE PLAYERS

Since 2000, MULTIBRID has developed, planned and produced the M5000: the first 5MW wind energy converter designed solely for large-scale offshore wind parks. .

AREVA has manufacturing facilities in 43 countries and a sales network in more than 100 countries.

PROKON Nord was founded in 1997 and since 1999 has been part of project developer Enertrag AG. International activities include wind energy projects as well as developing and operating biomass and bio-fuel plants. The company has accumulated a 940 MW offshore project portfolio, all projects planned with M5000 wind turbines.

Now a minority shareholder in Multibrid, PROKON Nord (PN) manufactures the 56.5 metre PN-type rotor blades for M5000 turbines in a former aluminium processing plant with deep-water access at Stade, in northern Germany. At the same location a new PN foundry for M5000 heavy castings has been built, which became operational this year.

Furthermore, a jack-up barge is currently being built, says Erdmann: ‘This 90 metre-long and 40 metre-wide barge will be fitted with a 1400 tonne crane. It is not self-propelled and our plan is that the barge enters active service in 2010.’

WinWinD, based in Finland, produces Multibrid-type wind turbines of 1 MW and 3 MW.