Safety Research On Girth Welds Of Pipelines Crossing Rivers Under Complex Stress

At present,countries around the world widely use pipelines to transport oil and gas.This transportation mode has the advantages of safe,efficient,economic and stable transportation,plays an extremely important role in the economic strategies of various countries.China covers a great area and is rich in resources.Long-distance oil and gas transmission pipelines have long construction distances,large spans and complex geographical conditions along the way.It is inevitable to cross railways,rivers,highways and other special geological conditions,and is vulnerable to the impact of natural geological disasters and human social activities,resulting in the destruction of service conditions,which is not conducive to its safe operation.The pipeline girth weld is the vulnerable position of the pipeline,which is prone to fracture failure,and even evolves into large-scale yield fracture,leakage,explosion and other major accidents.Therefore,this dissertation focuses on the girth weld of a pipeline crossing a large river under complex stress,and carries out the following work:Combined with the actual situation of the overhanging section of the gas pipeline crossing the river,the causes and characteristics of the overhanging span of the pipeline are studied.The static load on the pipeline and the lateral load of the water flow are analyzed,and the simplified mechanical model of the local suspended span of the pipeline and the interaction between the pipe and the soil is established.Based on the theory of fracture mechanics,the failure criterion of crack initiation is determined.Uniaxial tensile test and quasi-static fracture toughness test at room temperature were carried out for the X70 pipe base metal and girth weld used in the field.The mechanical properties,resistance curves and critical fracture toughness values of base metal and welding zone were measured.The experimental results show that the tenacity and crack resistance of the weld zone is lower than that of the base metal.Based on the ABAQUS finite element software,a nonlinear finite element analysis model of the girth weld of the pipeline with crack defects across the river was established,and the stress,strain,displacement distribution of the suspended pipeline and the crack driving force represented by J integral were obtained.The model innovatively uses a coupler to establish a nonlinear coupling element of pipe and soil,and based on the Python parametric programming method,the crack form is considered as a canoe type that is more in line with the actual engineering,and a numerical model of the driving force of the pipe crack is established.Based on BS 7910 standard to verify its accuracy.Considering the parameters of pipeline yield strength,weld matching coefficient,internal pressure and soil bearing capacity,the influence law of each parameter condition on pipeline J-integral is identified by using single factor sensitivity analysis method,and the key factors affecting the crack initiation of pipeline girth weld are obtained.This dissertation provides a theoretical basis for the safe operation of girth welds of pipeline crossing rivers under complex stress.