Fracture Assessment Method For Girth Weld Based On Finite Element Method

In order to meet the growing demand for oil and gas,pipelines have been developed in the direction of large diameter,thin-walled,high-strength steel.Pipeline safety issues are extremely important.Due to the difference of failure mechanism of high-strength steel pipe and low-strength steel pipe,the results of applying traditional fracture assessment standards may be inaccurate.In this paper,the finite element analysis of large diameter and thin-walled pipes with circumferential surface cracks in girth weld is carried out to study the fracture assessment methods for high-strength steel with defects in girth weld,and to predict limit load and critical crack sizes.The finite element method?FEM?is used to calculate the crack driving force of the surface crack in girth weld in this work.Then the formula of the h₁ factor with the crack sizes as parameters is established by using the J-integral estimation method provided by EPRI.Therefore,the formula of fully-plastic J-integral for a certain range of crack sizes is obtained.In addition,the limit load is predicted through the reference stress method,finding that overmatching is beneficial for high-strength steel pipe.Furthermore,advanced failure assessment curves?FAC?including weld strength mismatch is obtained by using the above fully-plastic J-integral formula,thus establishing the advanced failure assessment diagram?FAD?based on FEM.The critical crack sizes predicted by the advanced FAC are compared with those predicted by the general curve and material specific curve in BS 7910.The results show that the critical crack sizes are larger based on more refined FAD with strength undermatch,meaning that the simplifier assessment yields more conservative results.