Fatigue assessment and S–N curve development for one-sided girth welds in close proximity in tubular structures

Abstract Girth-welded structures, such as tubular trusses, risers, and pipelines, are frequently subjected to cyclic loading. Therefore, conducting a fatigue analysis of girth welds is essential to ensure the structural integrity of these components, as failure could have catastrophic consequences. To ensure adequate quality, fatigue assessments of the most critical welded joints are performed based on S–N curves for welded joints, as specified in standards like DNVGL-RP-C203 and BS 7608. However, girth-welded structures come in various configurations, where welds are often positioned in close proximity to each other. While some international codes and standards specify minimum spacing requirements between welds, many lack technical justification or do not provide specific fatigue design criteria for such configurations. To address this gap, laboratory fatigue testing was carried out on 36 small-scale specimens extracted from butt-welded tubular sections, each with two girth welds in close proximity. The specimens were fatigue tested under cyclic tensile loading to evaluate their fatigue performance. The study includes testing of welds at three different proximity distances of 5 mm, 10 mm, and 15 mm and aims to establish design criteria for proximity welds, including relevant S–N curves and stress concentration factors for one-sided girth welds made in close proximity. The results show similar tendencies to those observed in fatigue testing of single girth-welds were the fatigue fracture occurring on the side of the weld root due to higher stress concentrations. The findings provide experimental validation for the relevant fatigue Class used in assessing the fatigue performance of proximity welds and contribute to the development of improved design guidance for proximity welds.