Research On Radial Buckling Criteria Of HDPE Interspersed Pipe Based On Gas Penetration In Oil And Gas Environment

In recent years,with the further development of oil and gas resources in our country,the corrosion problem of steel pipelines used for oil and gas gathering and transportation has become increasingly prominent.By using the trenchless method,the non-metallic pipe is inserted into the metal pipe to be repaired,which can prolong the service life of the original pipe.High density polyethylene（HDPE）,a non-metallic material,plays an important role in the corrosion protection of traditional steel pipes because of its excellent corrosion resistance.However,with the extension of the service time of the pipeline,due to the penetration of the gas transported in the pipeline,when the pipeline maintenance or sudden pressure relief causes transportation disruption,more and more radial buckling problems with consistent characteristics occur in the HDPE interpenetration repair pipeline,resulting in a decrease in the transportation flux of the pipeline,and in serious cases,it will be forced to stop or even cause safety accidents.Therefore,in order to assess the buckling and instability risk of HDPE lined pipe during service and avoid buckling and instability of the lined pipe,this paper takes HDPE interpenetration repair pipeline under oil and gas gathering and transportation environment as the research object.Firstly,molecular simulation software is used to determine the gas osmotic pressure in the annular of the interpenetration repair pipeline under different oil and gas gathering and transportation environment（temperature and pressure）.Secondly,the buckling and collapse performance of HDPE lined pipe was investigated based on finite element simulation.Finally,the radial buckling criterion of HDPE interpenetrating pipe based on gas penetration in oil and gas gathering and transportation environment is established and verified.The main research contents and results are as follows:The HDPE cell model was established by using molecular simulation software.The relative error of the simulated permeability coefficient was 0.896%compared with the experimental average value,indicating that the molecular model was reasonable.The permeation behavior of monomeric gas CH₄,binary gas CH₄/CO₂ and ternary gas CH₄/CO₂/H₂S in HDPE at different temperatures and pressures was studied.It was found that the permeability coefficient of different components of gas molecules in HDPE and the osmotic pressure in the annulus increased with the increase of pressure.However,with the increase of temperature,the permeability coefficient of different components of gas molecules decreases continuously,and the osmotic pressure at the annulus changes little.The osmotic pressure of mixed gas in HDPE is H₂S>CO₂>CH₄,which is consistent with the permeability of gas molecules in HDPE.The finite element model of HDPE lined pipe was established by using finite element simulation software.The relative error of the ellipticity corresponding to the simulated buckling mode was 7.77%compared with the average ellipticity of the test site,which verified the rationality of the model.The mechanical behavior of lined pipe under a single external pressure load was analyzed,and the influence of relevant parameters on its critical buckling pressure was determined.It is found that under the premise of determining the length of the pipeline（L=2000 mm）,a single external pressure load should be applied to the pipeline at the position of 1/8*100 mm outer ring above the middle of the pipeline;The greater the thickness of the pipeline and the lower the transport temperature of the oil and gas medium,the greater the critical pressure when the radial buckling occurs.When the temperature of oil and gas medium is constant,the critical buckling pressure of lining pipe decreases with the increase of pipeline ellipticity and initial defect.On the basis of single load analysis,the buckling modes and critical buckling pressures of HDPE lined pipe under combined load were studied.It is found that the critical buckling pressure of the lining pipe decreases slightly with the increase of axial tensile displacement and then tends to be flat under the action of the combined tensile and external pressure successively.Under the combined action of internal pressure and external pressure,the critical buckling pressure of the lining pipe shows a nonlinear upward trend with the increase of internal pressure,and the change amplitude is large.In addition,under the action of these two combined loads,the mode of buckling of HDPE lined pipe is"single lobe"buckling.Based on the influence of air permeation pressure and different factors on radial buckling behavior of lined pipe under oil and gas gathering and transportation conditions,the critical time for radial buckling failure of HDPE lined pipe under 2.5 MPa and 30℃is 344.46 days by taking the permeation process of CH₄ as an example.By fitting the relationship between critical buckling pressure and the relationship between annulus pressure and pipe thickness,the radial buckling criterion of HDPE lining pipe based on gas penetration is established.Taking the working condition of 60℃and 2.5 MPa as an example,the calculated value of buckling criterion is compared with that of computer simulation.The relative error is 7.66%,indicating that the radial buckling criterion model is reasonable.