Offshore wind turbines out at sea are welded together from a number of different individual parts. But when it comes to developing ever larger and in turn more powerful systems with lightweight designs, these weld seams represent a weighty challenge. Therefore, the German Federal Institute for Materials Research and Testing (BAM) is testing out special alloys to stabilise the seams and ensure that the entire system enjoys a long service life.
To make wind turbines work better, they must be equipped with longer rotor blades. At the same time, the towers are getting taller – in part because the wind is stronger and more consistent at higher altitudes. To build the gigantic installations in a commercially and technically feasible manner, switching to lightweight construction with modern high-strength steels is a must in the near term.
Depending on the system’s design, up to 2,000 tonnes of steel may be used in an offshore wind turbine today. A large proportion of this is used in the foundations, which sit below the waterline. If the turbines were consistently built using lightweight construction methods, i.e. high-strength steels, this figure could be reduced by up to 20% (as much as 400 tonnes).
Until now, the weld seams have represented a weak point. One of the consequences has been that on safety grounds, the relevant regulations do not currently allow for these steels to be used in construction.
But high-strength steels are crucial when it comes to further improving turbines’ size and thus performance – especially since designs made with conventional steels would simply be too heavy due to their dead weight.
In particular when located offshore, wind turbines have to withstand enormous pressure from wind and waves, which put the material under cyclically changing levels of stress and strain. “The fatigue behaviour of modern high-strength steels in particular is restricted by the welding process, because this leads to structural changes in the material,” says Arne Kromm, an expert in welding technology at BAM. “At the same time, these seams are subjected to the heaviest levels of strain while the system is in operation.”
There are already methods for manually retouching weld seams in locations of key importance, but these processes are labour-intensive and time-consuming, which by extension makes them expensive.
For this reason, BAM is testing out innovative filler materials as part of a collaborative project, which are intended to strengthen the weld seams in particularly critical locations. “These are special alloys. During cooling, they form a structure that reduces the residual stress in the material, stabilising the weld and increasing its fatigue strength,” explains Kromm.
The aim is to enable the industry to apply these special fillers safely. At the same time, the findings will need to be factored into the relevant regulations so that they can be updated appropriately.
The project results are important not only for wind turbines, but also for mechanical engineering, automotive manufacturing and steel construction in general, with a view to fully unlocking the significant potential of high-strength steels for lightweight construction. Not least for the large, specialised cranes needed to install even more gigantic wind turbines.
The project also includes the Fraunhofer Institute for Mechanics of Materials (IWM) and the research arm of the German Welding Society (DVS). It is supported by the Federal Ministry for Economic Affairs and Climate Action as part of a programme to promote the collaborative ventures known as Industrial Collective Research.
