Fatigue analysis and structural integrity of seamless carbon steel pipelines: Insights from surface imperfections to industry standards

The fatigue performance of seamless carbon steel pipelines is essential for maintaining structural integrity under cyclic loading. This study examines the effect of external surface imperfections on fatigue behavior, focusing on their role in crack initiation and propagation. Six full-scale fatigue tests were performed on seamless pipe specimens (276.1 mm outer diameter × 14.27 mm wall thickness), made of grade 450 carbon steel, compliant with DNV-ST-F101. Three specimens developed through-wall fatigue cracks that originated at external surface imperfections and propagated inwards. All three samples exceeded their target fatigue life, indicating that the imperfections were within allowable limits defined by the fatigue design curve. Metallurgical analysis showed that larger imperfections would likely have caused earlier crack initiation due to increased local stress. All pipes passed both mill and third-party inspection before testing. The results indicate that current inspection and quality control methods may not identify small imperfections that influence fatigue performance. A review of industry standards identified differences in surface defect limits, inspection techniques, and repair procedures. Proposed improvements include standardizing grinding limits, adopting automated non-destructive testing methods, and applying risk-based acceptance criteria. These measures aim to improve fatigue resistance and support consistent pipeline integrity management.