In September 2015, Max Bögl Wind AG received an order to manufacture and deliver 47 main frames for two offshore wind farms in the North Sea. After their commissioning in autumn 2017, the 47 wind turbines will deliver power to almost 190,000 households with an output capacity for six megawatts each.

Quality process in a weekly cycle

The equipment frame is the core component in a wind turbine's nacelle. It consists of a main frame weighing up to 65 tons and a generator frame with a net weight of some 25 tons. With the azimuth bearing to hold the nacelle on the steel tower located beneath it, the main frame acts as a support structure for the rotor bearing and the gearbox. The transformer and the actual generator for power generation are installed on the generator frame at a later point in time. Both frames are attached to one another as they are screwed and bolted to a platform with a net weight of some 90 tons. Main frames and generator frames are exposed to high dynamic loads due to a required minimum service life of 25 years. Their manufacture is thus subject to quality and safety requirements. The company group has designed and set up its own production line at its headquarters in Sengenthal. This new line delivers short measurement and assembly times for the individual components. A sophisticated logistics concept ensures that the individual workstations receive a new consignment on a weekly basis, ensuring one complete main frame unit is dispatched from production every week.

Extreme impact on people and materials

Strict manufacturing technology standards are ensured by welding the individual components with plate thickness widths up to 200 millimetres and producing complete joints in all seams. The plates must be heated before they are joined together. Great heat is also emitted during the welding process. The welders protect themselves against the heat impact with protective clothing incorporating a water cooling system that we have developed ourselves. Once welding is complete, the large components are subjected to a stress relief annealing treatment in a special annealing furnace at a temperature of around 600 °C for almost ten hours. With the interior stresses in the component removed, the risk of unwanted geometrical deviations is reduced during the continued mechanical processing. The component is further processed in Max Bögl's own CNC lateral drilling and milling centre, designed for large welding assembly groups with components weighing up to 130 tons. Previously processed with a precision down to a tenth of a millimetre, the components are also measured and checked in this centre to ensure that they comply with tolerances.

Up to four paint layers per frame

Before being exposed to salt in the North Sea offshore climate, the steel structures need to be protected with up to four layers of paint in the company's own coating station. Even though it takes up to twelve hours to apply and dry each layer, the state-of-the-art technology ensures that Production is able to maintain its weekly cycle. Finally, the coated components are brought into the final assembly hall on heavy load platforms, where they are then equipped with the walk panels and support structures required to ensure that the main frames can be accessed. After successful final inspection, the main frames and generator frames are packed in splash-proof packaging. Loaded with a maximum of six units each, barges then transport the main frames from the Main-Danube canal quay in Bachhausen to the customer in North Germany.