Strain Capacity Analysis Of Girth Welds In High-grade Pipelines

Girth welds are normally deemed as the weakest link in the pipeline because of the probable presence of geometrical and material discontinuities as well as welding-related defects.The fracture behavior of pipeline girth weld is affected by multiple parameters.At present,the influence mechanism of material,defect,load and other factors on the fracture limit state of weld remain elusive.There are many deficiencies in the evaluation method and fracture criterion of girth weld,which result in difficulty to guarantee the intrinsic safety of the pipeline system.As an underground lifeline project,high-grade steel pipeline undertakes the important task of transporting oil and gas resources.The research on the fracture behavior and evaluation method of pipeline girth weld are key issues to improve the essential safety technology and pipeline protection level of pipeline.This paper focuses on the following aspects:Based on the digital image correlation method,the spatial distribution law of the stressstrain relationship of the girth welded joint material was verified by the experiment.The curved wide plate tensile test of 1422-mm diameter cracked pipeline girth weld was designed.The axial strain distribution characteristics in the vicinity of the crack surface were verified by using the full-field strain measurement method.The variation of axial strain near the crack surface and remote strain was studied,and the final failure behavior and failure mode of the wide plate structure were discussed.Based on the wide plate tensile test,a numerical simulation model of the wide plate tensile test of girth weld was established.The experimental results and finite element results were compared from three aspects: the change of the overall mechanical response of the structure,the axial strain distribution near the crack surface,and the evolution of the crack mouth opening displacement with remote strain.The rationality and effectiveness of the finite element simulation method and the simulation technology of girth weld fracture were fully verified.Based on the micro-damage mechanics theory and finite element method,a series of finite element models considering multi-scale structures including single-edge notched tension and single-edge notched bending specimens were developed to calculate the crack propagation resistance of girth weld under different in-plane and out-of-plane constraint conditions.The accuracy of the numerical model was verified by the resistance curve test based on the physical replication of silicone compound method.The influence of material properties,specimen and crack geometry on the resistance curve of girth weld was studied.Results show that the yield strength has little effect on fracture toughness,while the toughness increases with decreasing yield to tensile strength ratio and initial void volume fraction,and also decreases with decreasing the strength matching factor.In view of the influence of in-plane and out-of-plane constraints on the fracture toughness of girth welds,the rationality of describing the relationship between fracture toughness and constraint by different crack tip constraint parameters was discussed.A precise constraint characterization parameter which describe the crack tip constraint difference by using the area enclosed by the equivalent plastic strain isoline of the crack tip was proposed.The unified correlation between the fracture toughness and the in-plane and out-of-plane constraints of girth weld was established.The constraint-toughness correlation model was successfully used to predict the toughness of full-scale pipelines girth weld under different crack tip constraint levels.The relative error of the prediction results is less than 10 %,the average relative error is only 3.9 %,and the average absolute error is only0.03 mm.Owing to the fact that the failure assessment diagram cannot accurately consider the strength matching characteristics of the pipeline girth weld,the assessment accuracy of the current method for the girth weld crack was proved based on the failure assessment diagram theory and the equivalent stress-strain relationship method.The influence of crack depth and crack length,pipe diameter-thickness ratio,material yield strength and strain hardening exponent,and weld strength matching factor on the assessment accuracy of girth weld fracture was investigated.The calculation formula of optimized limit load and reference stress were established.The improved assessment procedure of pipeline girth weld fracture was proposed,which makes up for the lack of conservative assessment results of traditional methods and improves the accuracy of girth weld fracture evaluation under high stress conditions.In view of the shortcomings that the strain capacity of pipeline girth weld cannot be accurately quantified under the coupling effect of multiple factors,the most thereat position of cracks in girth weld joint was discussed based on structural stress analysis.Taking the girth weld of pipeline with circumferential root cracks as the research object,the numerical simulation models of the crack driving force of girth weld were established based on parametric programming via Python scripts.Through parallel computing method,6600 sets of girth weld strain capacity database were established.The influence of weld strength matching factor,crack depth,crack length,yield to tensile strength ratio of pipe,high-low misalignment,wall thickness,internal pressure and fracture toughness on the strain capacity of girth weld were discussed in detail.Based on the data-driven theory,artificial neural network and nonlinear regression methods were used to propose the tensile strain capacity model of girth welds of high-grade pipelines respectively.Compared with the experimental results of fullscale pipelines girth weld tests,the proposed prediction model has been proved to have high accuracy and was more accurate than the commonly used strain capacity prediction model.The average absolute error of the proposed strain capacity prediction model is 0.29 %,and the average relative error is 15 %.The shortcomings of the current requirements for the strength of girth weld in pipeline industry were discussed,and the design method of the weld strength matching factor based on reliability theory was proposed.Based on the reliability method,the crack initiation-based limit state function of the pipeline girth weld was established.The Monte Carlo method was used to investigate the evolution of the failure probability of the pipeline girth weld with the strength matching factor under different misalignments,girth weld fracture toughness,and standard deviation of the tensile strength distribution of the pipe material.The calculation method of the strength matching factor meeting the target reliability was proposed,and the parameter sensitivity affecting the strength matching factor was verified,which provided guidance for the strength design of high-grade pipeline girth weld.