Enhancement of material and processing spectrum in the construction of wind energy plants by applying consistent safety assessment for structural parts and weld technology process improvement
The motivation of the project "Enhancement of material and processing spectrum in the construction of wind energy plants by applying consistent safety assessment for structural parts and weld technology process improvement" originates from the fact that weld seams for wind turbines are currently mainly produced in the submerged arc welding process, although electron beam welding (EB) is considerably more suitable for large wall thicknesses, which are typical for these constructions.
The project therefore aimed at developing the required measures to ensure that EB welding could be used in offshore wind turbines in the future. To achieve this, three main aspects were considered. It was investigated how the toughness of EB welds could be improved by an optimised metallurgy or a better process design. Additionally, new damage mechanics models were developed to better define the necessary toughness requirements. This might be achieved by simulating Charpy impact tests as well as components with the help of damage mechanics.
The third research aspect resulted from the motivation to define the toughness requirements not for the entire service life of a wind turbine, but to combine increased material utilisation with safe operation by defining tailored inspection and repair intervals. This is an attractive, innovative approach to make the production and operation of wind turbines more efficient, especially for offshore wind turbines that are difficult to access.
Investigations to improve the toughness properties showed that the chemical composition of the base materials is irrelevant for seam production during welding when using sufficient energy. Yet, that it has a significant influence on the mechanical properties of the joint. A positive influence on the toughness properties was achieved by adding aluminium oxide or aluminium strip as filler materials.
The damage mechanics investigations have successfully established a local, probabilistic, stress condition-dependent cleavage fracture criterion. The probabilistic description of failure behaviour is based on Weibull distributions. The model was validated on notched bar impact tests in the lower transition area. It can be used in combination with ductile failure criteria to derive toughness requirements.
The fracture mechanics investigations were carried out based on the simulated load of a 3 MW onshore wind turbine considering various situations in the life cycle of the plant. FE calculations to quantify the moment caused by the flange geometry led to plausible results of the stress distribution in the weld cross-section. The crack propagation properties of the EB weld seam were defined based on the properties investigated in large-scale tensile tests. It was shown that the chosen approach is suitable for defining inspection intervals. Initial results indicate good properties of the EB welds for these applications.
The three aspects of investigation have provided the necessary knowledge and methodologies to promote the use of EB-welded joints in wind turbines. However, it was not possible to derive specific toughness requirements and inspection intervals due to a lack of publicly available date on the operating loads of offshore wind turbines. Such requirements should be developed in cooperation with manufacturers, booms and operators of such systems.
Research Association for steel Application (FOSTA)
11 / 2014 - 10 / 2017
The research project IFG 18460 N / FOSTA P1080 " Enhancement of material and processing spectrum in the construction of wind energy plants by applying consistent safety assessment for structural parts and weld technology process improvement" from the Research Association for steel Application (FOSTA), Düsseldorf, was supported by the Federal Ministry of Economic Affairs and Energy through the German Federation of Industrial Research Associations (AiF) as part of the programme for promoting industrial cooperative research (IGF) on the basis of a decision by the German Bundestag. The project was carried out at the Steel Institute, the Welding and Joining Institute and the Institute of Steel Construction of RWTH Aachen University.