Study On Safety Assessment Of Gas Pipeline Containing Cracks

Along with the continuous development of the national strategy of "one belt and one road" and energy revolution,the safety problems of China’s oil and gas pipelines have been highlighting.Among them,the crack defects caused by manufacturing.installation and service environment bring severe challenges to the safe operation of pipelines.Therefore,it is necessary to study the safety assessment of gas pipelines with cracks.The existing standards are not only complicated and tedious in solving the safety assessment parameters of semi-elliptical cracked gas pipelines,but also unclear in which side of semi-elliptical crack should be solved in safety assessment,lacking pertinence and practicability.At the same time,envelope merging criterion is currently used for multi-crack safety assessment of gas pipelines,but it ignores the interference between cracks.The evaluation results are conservative.In view of the above shortcomings,this paper mainly completes the following work:(1)To study the theory and criterion of safety assessment for gas pipelines with cracks,and to clarify the failure criteria and basic parameters for safety assessment of gas pipelines with cracks.(2)Establishment and validation of finite element models for gas pipelines with cracks,including three-dimensional surface crack modeling based on 3D-VCCT method and three-dimensional surface crack modeling based on circumferential integration method(seam method).In order to verify the validity of the finite element model,the 3D-VCCT method is selected as the basis of the subsequent linear elastic fracture analysis by comparing the calculation accuracy,calculation efficiency and modeling difficulty of the two methods on the basis of the analytical solution provided by the Stress Intensity Factor Manual.(3)Elastic analysis of gas pipelines with cracks is carried out.Firstly,a single crack finite element model of gas pipelines is established based on 3D-VCCT method,and the influence of various factors on stress intensity factor K is explored.On this basis,the formula for calculating the maximum stress intensity factor K of semi-elliptical cracks of gas pipelines is fitted by using 1STOPT software.Secondly,a multi-crack finite element model of gas pipelines is established.Based on the analysis of the influence of the auxiliary cracks on the main cracks in different locations,the location of the "high-risk points" of the multi-cracks in gas pipelines is determined,and on the basis of the study of the critical shape ratio of the cracks,the determination process of the "most critical points" of the safety assessment for multi-cracks in gas pipelines is further proposed.(4)Elastic-plastic analysis of gas pipelines with cracks is carried out.Based on the basic theory of elastic-plastic,the method of contour integral is used to calculate the J integral of safety evaluation parameter.By designing orthogonal experiments,the influence degree of each parameter on the elastic-plastic J integral of semi-elliptic cracks in gas pipelines is determined,and the formula for calculating the maximum value of J integral of semi-elliptic cracks in gas pipelines under certain conditions is fitted.The effect of multi-crack interference on gas pipeline under elastic-plastic condition is compared with the position of "high-risk point" and the magnitude of interference effect under linear elastic condition.(5)The application research results are used to analyze the engineering case.The failure assessment diagram(FAD)and elastic-plastic J integral are used as failure criteria,and the fracture parameters K,J fitting formula and multi-crack "most critical point" determination process are used to complete the safety assessment of the engineering case.The research results can provide technical reference for safety assessment and pipeline integrity management of cracked gas pipelines,and provide theoretical support for improving safety assessment of cracked gas pipelines in China.