Fracture assessment method for girth welds of high-grade steel pipelines considering strength matching

The actual strength under-matching of pipeline girth welds is a principal cause for the fracture failure of welds. The current Failure Assessment Diagram (FAD) method for the applicability assessment of pipeline welds cannot accurately consider the influence of weld strength matching. Thus, accurate inclusion of strength matching becomes critical to broaden the FAD method to apply to the crack defect assessment for the girth welds of high-grade steel pipelines. Based on the nonlinear finite element method, a numerical simulation model of the crack driving forces for the girth welds of high-grade steel pipelines was established, and the general failure assessment curve of girth weld cracks was plotted with the equivalent stress-strain relationship method. Then, a series of finite element assessment points under different load levels were determined with the finite element calculation results of the crack driving forces in combination with the recommended methods in Guide to methods for assessing the acceptability of flaws in metallic structures (BS 7910-2019). Further, the assessment accuracy of BS 7910-2019 to the crack defects of girth welds was determined, and the critical causes affecting the accuracy of the assessment results were investigated. On this basis, the optimized ultimate load calculation model of the pipeline girth welds and the improved FAD method were put forward with the crack depth, the crack length and the weld strength matching coefficient considered, and the weld strength matching was accurately included into the FAD method. Thereby, the assessment accuracy of the girth weld crack defects of the high-grade steel pipelines was further enhanced. Generally, the method could provide reference for the applicability assessment of the girth weld cracks of in-service high-grade steel pipelines.