PSI - Issue 75

Philippe Thibaux et al. / Procedia Structural Integrity 75 (2025) 546–554 P. Thibaux et al. / Structural Integrity Procedia (2025)

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6. Conclusions Two full-scale fatigue tests were performed on tubular joints with the same geometry, one being loaded by in plane bending and the other by out-of-plane bending. The measured fatigue life was slightly shorter for the joint tested in-plane. The general evolutions of the cracks were different between both tests. During the out-of-plane test, the first indications of cracks were detected at one third of the number of cycles for breakthrough crack, and breakthrough crack was observed through the chord. The fatigue cracks were very large (more than 900 mm length) and led to a decrease of the stiffness of the specimen. The first cracks were detected in the in-plane bending test after 50% of the fatigue at the breakthrough crack, through the brace. The breakthrough crack was limited in width (around 300mm) and no stiffness decrease was observed. From the thickness, one would expect that the fatigue strength measured in the in-plane test would be higher than in the out-of-plane experiment. The experimental evidence is opposite and is related to the crack growth evolution. The difference of cracking mechanism should be taken into account to a more accurate estimation of the fatigue strength of these joints. The lower stiffness loss in the in-plane bending experiment highlight that failure of a brace in a structure is more likely to happen without early notice. On the opposite, failure through the chord is coupled with a significant stiffness loss, leading to a redistribution of the forces towards other joints than the cracking one, and a more robust behaviour. Acknowledgements The authors want to acknowledge of the support of the JaCo consortium (EnBW, Equinor, Ørsted, Scottish Power Renewable, Shell, SiemensGamesa, SSE, Vattenfall), brought together by the Offshore Wind Accelerator of the Carbon Trust and the support of the Scottish government. The authors would also like to thank colleagues and participants in the project for their many valuable discussions. References Santhakumar S., Smart G., Noonan M., Meerman H. and Faaj A. , “Technological progress observed for fixed -bottom offshore wind in the EU and UK,” Technological Forecasting and Social Change, p. 121856 (2022) https://doi.org/10.1016/j.techfore.2022.121856 Stehly, Tyler; Duffy, Patrick;, “2020 Cost of Energy Review,” National Renewable Energy Laboratory (2022) Glienke R., Kalkowsky F., Hobbacher A. F., Holch A., Thiele M., Marten F., Kersten R. & Henkel K.-M. “Evaluation of the fatigue resistance of butt-welded joints in towers of wind turbines — a comparison of experimental studies with small scale and component tests as well as numerical based approaches with local concepts” Welding in the World, 1143 -1168, (2024) https://doi.org/10.1007/s40194-023-01630-3 Schaumann P., Böhm M. and Schürmann K. , “Improvements in the fatigue design of support structures for offshore wind turbines,” Steel Construction, pp. 74-82 (2021) https://doi.org/10.1002/stco.202000060 DNV, DNV-RP-C203 Fatigue Design of offshore steel structures (2019) American Petroleum Institute, “ API ‐ RP ‐ 2A ‐ Recommended Practice for Planning,Designing and Constructing Fixed Offshore Platforms ” (2000) ISO, “ISO 19902 Fixed steel offshore structures” (2020) Health & Safety Executive, “Background to new fatigue guidance for steel Joints and connections in Offshore Structures,” HSE (1999) Hobbacher A.F. “Recommendations for Fatigue Design of Welded Joints and Components” Springer (2016) https://doi.org/10.1007/978-3-319 23757-2 Thibaux P., Thiele M., Van Wittenberghe J., Baessler M. Comparison of resonance and hydraulic testing on large scale fatigue tests of welded tubular joints for offshore wind turbine foundations, International Journal of Fatigue, 108797 April 2025 doi:10.1016/j.ijfatigue.2024.108797 Efthymiou, M. “Development of SCF Formulae and Generalised Influence Functions for use in Fatigue Analysis.” Recent Developme nts in Tubular Joint Technology, OTJ’88, October 1988, London Thibaux, P., Van Wittenberghe, J., Fricke, W., Thiele M., Nielsen L.P., Conti F. Results of the JaCo project: fatigue strength of robot-welded tubular joints for offshore wind energy converters. Weld World 69, 619 – 632 (2025). https://doi.org/10.1007/s40194-024-01903-5