Fatigue life characterisation of API X65 steel pipeline for internal vibrational loads under sea water condition

Abstract

The objective of this study is to characterise the fatigue life of API X65 steel based on the effects of corrosion and internal loads fluctuations using experimental and numerical analysis. The structural integrity of offshore in-service pipes is influenced by the existence of cracks resulting from many factors, such as random loads, material characteristics, welding imperfections, and corrosive influences. The internal pressure fluctuations for in-service oil and gas pipeline were experimentally and numerically analysed to determine the fatigue life based on stress life-based models from four test conditions. Mechanical testing was conducted on ASTM E8 specimens which were submerged in sea water for 48 h with pH: 6.8 and salinity: 28. 48ppt.From the scanning electron microscopy (SEM) the localised corrosion characteristics with cavities were observed. In addition, based on the cyclic test based on ASTM 466 were carried out, the endurance limit is found to be 276.3 MPa. The random operational stresses applied to the pipeline's material were used to predict the stress- based fatigue life of in-service pipeline by considering the mean stress correction models. The fatigue life was ranging from 1.15x${10}^9$to 1.1x${10}^{11}$cycles to failure, among them Goodman model showed the least fatigue life. Hence, the study provides an alternative approach to estimate fatigue life of in-service offshore pipelines using loading history and experimental results as a function of corrosion fatigue mechanism.