Study On Mechanical Factors Resulted In Hydrogen Enrichment In X80 Spiral Welded Pipeline In The Hydrogen Environment

The penetration of hydrogen into the pipeline,which works in a high pressure hydrogen environment,can reduce the ductility and toughness of pipeline steel.Most of long distance pipelines by now are spiral welded pipelines produced through submerge-arc welding technique.Affected by the heat effect during the welding process,there are serious microstructure inhomogeneity and welding residual stress in the welded joint.The existence of the residual stress gradient induces hydrogen diffusion from the surface of the joint to the stress concentration region,which facilitates hydrogen induced failure.In addition to the influence of the stress gradient,when there is local plastic deformation in the material,the dislocation will increase,so that the hydrogen trapping site will increase,the hydrogen in the material further accumulate.As known by the public,the current pipeline welding quality acceptance standard is the defect tolerance principle.There are inevitably some welding defects.Corrosion defects will also appear during pipeline service process.Under the influence of internal pressure in the pipeline,large local stress concentration and local plastic deformation will appear in the defect position.Therefore,it is necessary for the hydrogen pipelines to study the effects of welding residual stress and local stress and plastic on the defects on hydrogen accumulation.The method of finite element numerical simulation is used to study the welding residual stress and the stress concentration and local plastic deformation caused by defects on the hydrogen enrichment of pipeline.The main research results are as follows(1)ABAQUS software is applied to calculate the welding temperature field and welding residual stress field of the welded joint of X80 steel pipeline,and the stress state of the pipeline joint area under the pressure of 12 MPa is further calculated.The method of checking the heat source is the way to make the molten pool shape match with the observed results ofthe actual weld fusion line,so as to ensure the accuracy of the welding temperature field and residual stress field.The results showed that the welding residual Von mises stress of weld and heat affected zone are around 580 MPa.The hoop stress is relatively high and there is obvious stress concentration at the root of outer weld.And the hoop stress will increase under working pressure.(2)The hydrogen permeation kinetics parameters of the weld joint obtained by the previous high-pressure hydrogen permeation test are set as boundary conditions and initial conditions.ABAQUS software is used to simulate the hydrogen diffusion process of the pipeline in three conditions which are no stress,residual stress and residual stress coupling working stress.The hydrogen concentration distribution can be obtained when the diffusion process reaches a steady state.The results show that the hydrogen often accumulate in the area where there is stress concentration.And the hydrostatic stress at the center of weld is higher than other regions,the hydrogen enrichment is obvious.The working stress can reduce the hydrostatic stress of the weld root,which can reduce the concentration of hydrogen.When the welding stress is coupled to the working stress,the hydrogen concentration is about 2.1times that of the no stress condition.(3)The influence laws of the axial groove defect depth,length and width on pipe failure pressure are studied without considering the effects of hydrogen.And the pipeline defect size range that failure does not occur is calculated.Results show that the failure pressure of the pipeline will decrease along with the increase of defect depth.When the axial length of defect is in the range of 50 mm-800 mm,the failure pressure will decrease with the increase of the length of defect.In the case the defect scale exceeds 800 mm,the pipeline failure pressure has no change with the axial length change.With the increase of the width of the defect,the pipeline failure pressure will increase,but the impact is not significant.For pipeline with defect of 10 mm in width and less than 7 mm in depth.the defect will not lead to pipeline failure.These defects as follow will not lead to failure: axial length less than 200 mm when depth is 9 mm,axial length is less than 130 mm when depth is 11 mm,the axial length is less than 90 mm when depth is 13 mm.(4)The effect of plastic deformation on the hydrogen diffusion and the rules of hydrogen concentration changes with the defect geometry are studied.The results show that the plasticdeformation will increase the highest concentration of hydrogen,and two peaks hydrogen concentration appear at the defect position.The influence of plastic cannot be ignored when the hydrogen distribution is calculated.The hydrogen concentration will increase with the increase of defect depth,and with the increase of depth,the growth of hydrogen concentration speeds up.The axial length of defect is longer,the hydrogen concentration becomes more obviously when depth increases.When depth is certain,the hydrogen concentration increases with increasing axial length.But when axial length is longer than 800 mm,the hydrogen concentration will not increase.The width has a great influence on the hydrogen concentration.With the increase of the width,the hydrogen concentration will decrease obviously.