A calculation method for limit load of the gas pipelines with girth weld surface cracks

Abstract

In this paper, a finite element model with girth weld defects considering crack tip singularity was established in order to facilitate the accurate and rapid safety assessment on girth weld defects of long distance gas Transmission pipeline. Then the J-integral of girth weld cracks in pipes with different defect sizes was calculated, and the influence of weld matching coefficient and material hardening index on J-integral is analyzed. Furthermore, the influence factors of limit load of pipeline with girth weld defects based on J-integral theory are studied. Based on this, the engineering calculation formula which can calculate J-integral and limit load rapidly in the situations with specific matching coefficient and material hardening index was proposed. And the following research results were obtained. First, the engineering calculation formula based on the fitting of finite element calculation results has high accuracy, which can calculate the J-integral of defects in a large geometric range and the limit load of pipelines with girth weld cracks. Second, the J-integral of cracks in pipes with girth weld defects is closely related to defect size, material properties and weld matching coefficient, and the limit load of pipes with defects decreases as the crack size increases and increases as the material hardening index increases. Third, for the X80 pipes with surface cracks at the centerline of the girth weld, the load carrying capacity of the pipes with low-strength matched weld are lower than that with high-strength or equivalent-matched weld. In conclusion, the research results can be used as reference for the safety evaluation and integrity management of in-service gas pipelines. Keywords: X80 gas pipeline, Girth weld defect, J-integral, Limit load, Safety assessment