For many years the wind energy industry has focussed on reliable solutions for applications in the offshore sector. In order to fully utilise the potential of the natural forces available approximately 30 km offshore, installations have to be adapted to the arduous prevailing conditions. For the efficient use of these wind parks, offshore wind power stations must have higher performance than onshore installations. Furthermore, special logistics and maintenance concepts have to be developed.
REpower Systems AG, the third largest manufacturer of wind power installations in Germany, already focussed on the expansion of the offshore market at an early date. The REpower group headquartered in Hamburg employs approx. 1700 people who work in the development, production and maintenance of wind power installations. Besides its own three production facilities in Germany, REpower operates two additional factories for turbines and rotor blades via joint ventures. REpower has developed the 5M turbine with a rated output of 5 MW for the offshore technology regarded as the market of the future. With a diameter of 126 m, the rotor blades of an installation cover an area of approximately 12,500 m², which enables them to generate electric current for more than 5,000 households.
The dimensions which REpower entered with the construction of its 5M turbines posed new challenges for the company in terms of production and logistics. A new quay-side assembly site had to be found in Germany, since road transport of the machinery components to the loading terminal was no longer possible. The assembly of the wind power installations has also set new standards. In Bremerhaven, where the machine house (nacelle) and the hub are manufactured, an assembly shop measuring 160 x 40 m has been built for the planned production of 80 offshore installations per year. The efficient use of a multi-level crane system is essential for smooth production processes. Besides staging the components to be assembled, the cranes are used for the majority of the assembly steps and to prepare for shipping.
For this purpose, the entire length of the building is served by two identical double-girder overhead travelling cranes, each of which is equipped with two 80 t open winch units. The MPW Multi-Purpose Winches feature a high level of standardisation and flexibility. Based on the serial type, the winches can be adapted to meet customer requirements. The open winch units operating in the REpower production facility feature load-dependent hoist speeds. Weights up to 45 t are lifted at up to 5.6 m/min, while the speed is reduced to 3.5 m/min at higher loads. Furthermore, the open winches can be operated in tandem. The two 160 t cranes provide for the in-house transportation of large components to the workplaces. They are also used to unload the incoming trucks and to lift the fully assembled hubs onto the heavy-duty vehicles.
The double-girder overhead travelling cranes with a span of 38.42 m travel on a runway at a height of 14.11 m. Beneath them, REpower installed a second crane runway level which is used by a total of five semi-portal cranes in three production zones. This logistics concept ensures that the individual workplaces are supplied independent of any operation on the upper crane level. At the same time, the two overhead travelling cranes can transport their loads over a sufficiently large transport zone in the centre of the bay. The second crane level is subdivided into three sections. At the beginning of the assembly shop, a 12.5 t semi-portal crane with a span of 15 m serves the production of the rotor shafts.
Parallel to this, REpower installed a crane runway measuring 38 m in length along the long side of the building. A semi-portal crane with 5 t load capacity and 18 m span travels on this runway and conveys electrical components, such as control cabinets, from the storage areas to assembly.
Further along the assembly shop, three 5 t semi-portal cranes each with a span of 8 m travel on a 40 m-long runway. These cranes are equipped with a DR-Pro rope hoist and support the workers in the hub assembly operation. The bottom rails of the three semi-portal cranes are embedded flush with the top edge of the floor. For the nacelles, specially equipped rotor shaft and gearbox workplaces are arranged according to the production flow. The overhead travelling cranes link the assembly stations.
A multi-stage function test is carried out when the nacelles have been fully assembled. The cladding is then fitted and the installation is prepared for shipping. The rotor blades come from the production building which is located at a distance of only 300 m and is operated by a REpower joint venture.
While planning the production processes and the resulting crane concept, REpower benefited from experience gained in its other production facilities. The manufacturer of wind power installations has already been using Demag cranes at its main factory in Husum for many years. For this reason, REpower placed great emphasis on standard crane equipment. All overhead travelling and semi-portal cranes can be radio-controlled to enable the workers to maintain a safe and convenient distance to the load. Large displays indicate the weight of a given load suspended on the overhead travelling cranes. The new building in Bremerhaven also offers additional reserves in terms of productivity and capacity. REpower already set a further milestone a short time after production started in late summer 2008, when the assembly of its first three 6 MW turbines marked a further step towards the future of offshore solutions.