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7884 |
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Wednesday, September 13, 2023, Brüssel 3:30 PM Offshore applications |
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Research project confirms superiority of hydraulic yield point controlled tightening |
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Patrick Junkers* / HYTORC
Barbarino & Kilp GmbH, Deutschland |
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In practice, mechanical loosening of bolted joints is a great safety risk and therefore requires relatively short inspection intervals to detect insufficient joints. Especially underwater this implies great expenditures and reduces returns. Costs are particularly high offshore, e.g. for wind turbine generators and structures of the oil and gas industry that are built in the oceans. As this is a problem of high relevance for the entire hydraulic steel engineering industry, the Fraunhofer Association and the Technical University of Dresden investigated the issue. HYTORC supported the research project providing material and manpower. All cyclically loaded bolted joints have the same risky process in common: Despite great inspection and re-tightening efforts, critical bolt failure occurs regularly and causes components to fall. The situation is even more difficult offshore. The adverse conditions when servicing in areas with changing water levels or underwater are very challenging for repair technicians. Furthermore, an influence of seawater on the friction behavior is to be expected, so that tightening techniques that are not adjusted cannot create quality connections with defined preloads Test setup demonstrates problems of torque controlled tightening Neither Eurocode 3 nor DIN EN 1090-2 mention explicit hints for the design and assembly of bolted joints underwater. DIN EN 1090-2 merely indicates that underwater assemblies are difficult, as contamination of contact surfaces cannot be ruled out. So far, there are no specifications for assessing the effect of water on the friction coefficient of the contact surfaces. The required experimental setups were designed and implemented within the project. Different tightening techniques and the use of alternative washers were tested for underwater use with different objectives. Yield point controlled tightening for bolt securing With regards to the unclear friction conditions under water when the sets are lubricated with MoS2, meeting the rated preload value in torque controlled tightening is particularly questionable. Because of better results under regular conditions, the combined technique might be an option for reliable assembly under water as well, as the angle controlled superelastistic tightening has the advantage of low friction influence on the created preload force. Yield point controlled tightening is another possiblity and, in case of unclear friction, also a promising alternative for tightening. Thanks to the process control in hydraulic yield point controlled tightening, the greatest possible assembly preload is created in the bolted joint. As a high and defined assembly preload prevents loosening and mechanical loosening, yield point controlled tightening can be used as a measure for securing the bolt. The research project also demonstrated that yield point controlled tightening allows to react most favorably to imperfections in flanges and to achieve the best possible preload force while also achieving a low preload scattering. Extrapolated measurements show that the achievable minimum preload force is ensured even after 20 years if bolted joints are tightened with the hydraulic yield point controlled technique in a process-reliable way. General Information on these reseach-project The research project took place within the IGF project 19.675 B “Qualification of an assembly method for bolted joints that retains preload underwater” by the research association for welding and related processes of DVS e.V. and was financially supported via the AiF within the program for promoting joint industrial research and development (IGF) by the German Federal Ministry for Economic Affairs and Energy based on a resolution by the German Bundestag. Research institutions involved are Fraunhofer-Gesellschaft e.V., Fraunhofer Institute for Large Structures in Production Engineering IGP and the Institute of Manufacturing Science and Engineering at the Technical University of Dresden, chair of joining technology and assembly. |