Stress concentration analysis of typical imperfections in girth welds of pure hydrogen long-distance transmission pipelines and exploration of applicability evaluation criteria

Objective Hydrogen transmission pipelines offer an important solution for the long-distance and efficient utilization of hydrogen energy. Numerous girth welds exist along these pipelines, so weld imperfections are regarded as high-risk points that can lead to pipeline failure. However, there is currently a lack of risk rating and acceptance criteria for girth weld imperfections that may arise in long-distance pure hydrogen pipelines as a result of construction operations.

Methods Chinese and foreign classification and ranking standards regarding girth weld imperfections in long-distance pipelines were reviewed and compared. Based on characteristic dimensions extracted from typical imperfections consistent with relevant Chinese standards, a finite element model of hydrogen transmission pipelines constructed with L245 pipeline steel and featuring imperfections was developed. This model was employed to analyze the influence of various factors, such as the sizes and locations of imperfections, on the stress concentration factor associated with weld imperfections in these pipelines.

Results The review found that Chinese standards generally impose stricter provisions on the tolerance dimensions of weld imperfections. Within the study range, the maximum stress concentration factor was found in concave imperfections located at the root of the welds. Specifically, the maximum stress concentration factor for circular imperfections was 2.74 at a size of 3 mm; for strip imperfections, it was 1.98 at a length of 25 mm; for concave imperfections at the root, it reached 3.58 with a width of 0.5 mm and a depth of 1 mm; and for misalignment imperfections, it was 2.62 with an offset of 3 mm. Furthermore, the stress concentration factor for 4 mm circular imperfections increased to 3.97 in the presence of any additional bending moment.

Conclusion Buried circular and strip imperfections that are closer to the wall surface, particularly the inner wall surface, exhibit more significant stress concentration. Larger circular imperfections demonstrate more pronounced stress concentration; however, the length of strip imperfections does not significantly influence stress concentration. There is an approximately linear relationship between increases in misalignment and the growth of the stress concentration factor. Stress concentration becomes more pronounced in concave imperfections at the root as they become narrower and deeper. Additionally, bending moments are considered a contributing factor to the substantial increase in stress concentration at these imperfections. The presence of weld imperfections with significant stress concentration heightens the risk for pipelines operating in high-pressure gaseous hydrogen environments. Based on the quantitative analysis of stress concentration in imperfections of pure hydrogen pipelines, this study offers valuable insights into the applicability evaluation criterion of imperfections, which incorporates experimental data reflecting changes in hydrogen embrittlement sensitivity with varying stress concentration factors in imperfections, along with practical experience from engineering operations.