Assessing Fatigue Life Characteristics of API X65 Steel under the Effects of Corrosion in Deep-Sea Environment M.A. Khan1, S.S.K. Singh1,∗, S. Abdullah1, M. Bashir2 1 Department of Mechanical and Manufacturing Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia 2 School of Engineering, Faculty of Engineering and Technology, Liverpool John Moores University, Liverpool, United Kingdom ∗ salvinder@ukm.edu.my Keywords: Fatigue, Stress-Life, Durability The aim of this study is to assess the fatigue life characteristics based on the effectsof corrosion for API X65 steel under deep-sea environment. In the oil and gas industry,assessing the pipeline failure caused by corrosion is important especially when dealing withcrack growth that is induced by internal and external cyclic loads. Likewise, the strength ofoffshore pipelines is affected by the presence of cracks due to the variation of loads, materialproperties and the corrosive effects due to salinity and pH values. During the service life ofsubsea pipelines, corrosion fatigue crack growth is a common occurrence that often results ina decrease in their strength and loss of asset integrity which frequently leads to pipelinecracking [1,2]. Therefore, evaluating the durability of pipelines is essential because of theinterdependence between corrosion fatigue, pipeline material properties, crack dimensions,geometric design and load ratios [3]. Tensile test is carried out for welded and non-weldedspecimens that were not submerged and submerged for 48 hours in sea water condition. Thisis based on the salinity and pH test values of the seawater condition sourced from the coastalarea of Port Dickson, Malaysia. In addition, the microscopic features and chemical compositionwere examined through FESEM and EDX on the fractured surface of the pipeline material. Afatigue stress-life (S-N) curve is plotted using the Basquin equation based on values obtainedfrom the UTS/mechanical testing. These results calculated using the Basquin equation, it wasfound that the endurance limit of API X65 steel is 276.3MPa. Finite element modelling for thecompact tension was carried out for three different load ratios (0.1, 0.4, and 0.7). The finiteelement analysis of the pre-cracked CT specimen shows that the stress intensity factor isproportionally linear with the length of crack and load ratio. Hence, this study confirms thatthe FE analysis provides a relevant alternative approach for fatigue life estimation of API X65steel using crack growth as a function of corrosion fatigue mechanism. References [1] Cheng, A., & Chen, N. Z. (2022). Structural integrity assessment for deep-water subseapipelines. International Journal of Pressure Vessels and Piping, 199, 104711. [2] Guo, Y., Shao, Y., Gao, X., Li, T., Zhong, Y., & Luo, X. (2022). Corrosion fatigue crack growthof serviced API 5L X56 submarine pipeline. Ocean Engineering, 256, 111502. [3] Lyathakula, K. R., & Yuan, F. G. (2021). A probabilistic fatigue life prediction for adhesivelybonded joints via ANNs-based hybrid model. International Journal of Fatigue, 151, 106352. 28
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