Safety evaluation method for defective PE pipeline under point load

Abstract

The invention discloses a safety evaluation method for a defective PE (Poly Ethylene) pipeline under a point load, which comprises the following steps of: determining a defect form of the PE pipeline under the action of the point load by acquiring constitutive model parameters, fracture toughness and other mechanical parameters of a PE pipe material; selecting a failure evaluation curve used by the failure evaluation diagram; the fracture ratio Kr is calculated based on the M integral, the load ratio Lr is calculated based on the elastic-plastic J integral, and parameters are corrected through experiments and simulation verification. Establishing a three-dimensional simulation model of the PE pipe under the point load according to different analysis parameters; extracting a crack front edge stress intensity factor KI and a J integral maximum value Jmax, and calculating a failure evaluation point; and finally, drawing according to the failure evaluation curve and the failure evaluation points, and judging whether the pipeline is safe or not. According to the method, the actual working condition can be comprehensively considered, the safety of the defective PE pipe under the action of the point load is scientifically evaluated, and the method also has reference significance for safety evaluation of pipelines made of other thermoplastic polymer materials.

Description

Technical Field

[0001] The present invention relates to the field of gas pipeline strength, and particularly to a safety evaluation method for defective PE gas pipelines under point load action. Background Art

[0002] Compared with traditional metal pipelines, plastic pipelines represented by polyethylene (PE) pipelines have been gradually widely used in the field of gas transportation due to their characteristics such as light weight, corrosion resistance, hygienic safety, and convenient installation. Since the implementation of the "replacing steel with plastic" strategy, PE pipelines have gradually replaced metal pipelines and become the main mode of urban gas transportation, significantly reducing the problem of corrosion damage of metal pipelines.

[0003] Most PE gas pipelines are buried. The soil environments and temperature differences in different regions vary greatly, and they are also affected by different external loads. Therefore, they are easily affected by environmental factors, resulting in problems such as pipeline deformation, rupture, and fracture, causing pipeline failure. Gas pipeline failure may cause losses such as local oil and gas pipeline fires and damage to railway power supply equipment. Although such accidents have not caused casualties for the time being, gas pipeline accidents occur from time to time, and it is even more necessary to use pipelines reasonably and safely.

[0004] In PE pipe failure accidents, the phenomenon of point load is an important cause of PE pipe failure. Point load refers to the extrusion caused by hard objects in the soil, such as rigid objects like stones and tree roots, on the pipeline surface, usually caused by irregular laying of gas pipelines or the movement of hard objects to the pipeline surface due to soil changes. This extrusion will cause stress concentration on the pipeline surface. Compared with the situation of only being subjected to internal pressure, the stress on the inner surface of the pipeline will increase significantly. The increase in the stress on the inner surface of the pipeline may cause axial cracks earlier due to small production defects or scratches, leading to major safety accidents. In densely populated urban areas with dense pipelines, to ensure the safety of people's lives and property, it is necessary to ensure the safe use of urban gas pipelines. However, the stress of point load pipelines is complex, and there is little existing research on them and a lack of scientific safety assessment methods. Therefore, it is necessary to study the phenomenon of gas pipeline failure caused by point load, conduct a safety evaluation of buried PE gas pipelines under point load action, and evaluate whether the gas pipeline can continue to be used. Summary of the Invention

[0005] The present invention provides a safety evaluation method for defective gas pipelines under point load action to solve problems such as the limited applicability and accuracy of existing evaluation calculation methods for point load PE pipelines and the calculation difficulty of multi-factor coupling of buried pipelines, and to achieve the purpose of evaluating the safety of buried PE gas pipelines under point load action and providing reference for the preliminary laying of PE gas pipelines and subsequent maintenance and replacement.

[0006] The present invention is achieved through the following technical solutions:

[0007] A safety evaluation method for defective PE pipes under point load, comprising:

[0008] Obtaining the constitutive model parameters and mechanical parameters of the PE pipe material based on the formation settlement rate; the mechanical parameters include fracture toughness parameters;

[0009] Conducting the first point load experiment to determine the defect form of the PE pipe under the action of the point load;

[0010] Selecting the failure assessment curve FAC used in the failure assessment diagram;

[0011] Determining the calculation method of the failure assessment point (L r , K r ), calculating the parameter fracture ratio K r based on the M integral, and calculating the parameter load ratio L r based on the elastic-plastic J integral;

[0012] Verifying and correcting the parameter load ratio L r , including verifying and correcting the parameter load ratio L r through the first point load experiment and simulation data, and then conducting the second point load experiment and simulation to verify the corrected parameter load ratio L r ;

[0013] Establishing several three-dimensional simulation models of PE pipes under different point loads according to different analysis parameters;

[0014] Respectively conducting simulation analysis on the three-dimensional simulation models of PE pipes under point loads with different parameters, extracting the stress intensity factor K I of the crack front and the maximum value of the elastic-plastic J integral, and calculating the failure assessment point according to the calculation methods of the parameter fracture ratio K r and the parameter load ratio L r ;

[0015] Drawing a graph according to the failure assessment curve FAC and the failure assessment point (L r , K r ), and judging whether the failure assessment point is within the failure assessment curve FAC. If it is, it proves that the pipeline is safe. If it is not within the curve, it proves that the pipeline has failed and can no longer be used continuously.

[0016] Aiming at the problem of the lack of safety evaluation methods for PE gas pipelines under point load in the existing technology, the present invention first proposes a safety evaluation method for defective PE pipelines under point load. This method first determines the constitutive model parameters of the PE pipeline material used for gas transmission under the formation settlement rate and mechanical parameters such as fracture toughness, fully understands its material characteristics, then determines the defect form of the PE pipe used to characterize the point load, and then selects the appropriate failure assessment curve FAC) and failure assessment point (L according to the failure assessment diagram FAD). r ,K r ), and verifies and corrects the parameters through experiments and simulations. According to different analysis parameters, a three-dimensional simulation model of the PE pipe under point load is established and simulated respectively. Finally, according to the simulation results, calculate (L r ,K r ) and the curve FAC to form an evaluation diagram, and judge whether (L r ,K r ) is within the curve FAC. If it is within the curve, it proves that the pipeline is safe. If it is not within the curve, it proves that the pipeline is unsafe.

[0017] In addition, the gas PE pipeline safety evaluation method obtained by the present invention is proposed based on fracture mechanics and the constitutive parameters of PE materials being related to the strain rate, and it is also of reference significance in the safety evaluation of other thermoplastic pipelines.

[0018] Furthermore, the method for determining the constitutive model parameters of the PE pipe material includes:[[]]

[0019] Cut dumbbell-shaped specimens from the PE pipeline. The thickness of the specimens is the pipe wall thickness, and uniaxial tensile experiments are carried out at different rates. According to the experimental results, obtain the true stress-strain relationship curve related to the tensile rate;

[0020] Based on the true stress-true strain relationship curve, fit m and n at different tensile rates through the following formula:[[]]

[0021]

[0022] In the formula, σ t is the true stress, ε t is the true strain, and m and n are fitting constants;

[0023] Determine the stress-strain relationship formula of the PE material according to the rate of the formation settlement to be evaluated, and calculate the elastic parameters and plastic parameters of the material.

[0024] This solution takes into account the obvious non-linear behavior of PE materials, with significant differences in the stress-strain curves at different rates, and the inconvenience of processing the specimen thickness. Therefore, the specimen thickness is set to the wall thickness, parameters are fitted at different rates, and the stress-strain relationship formula of the PE material is calculated according to the actual formation settlement rate.

[0025] Those skilled in the art should understand that the elastic and plastic parameters in this solution are the constitutive model parameters required for subsequent finite element modeling, while m and n are intermediate parameters in the calculation process. m and n are fitted at each tensile rate, and the m and n values fitted at different rates are different. Therefore, the m and n values at different rates need to be logarithmically fitted respectively to determine the specific functional relationship between the m and n values and the tensile rate.

[0026] Furthermore, the method for determining the fracture toughness parameters of the PE pipe material includes:

[0027] Two types of CT specimens are intercepted from the PE pipe, namely notched and unnotched CT specimens. The notched CT specimen is experimentally loaded to the maximum load, while the unnotched CT specimen is experimentally loaded to a load greater than 10% of the load during the experiment of the notched CT specimen, and the displacement-load relationship curves are obtained respectively;

[0028] Based on the displacement-load relationship curve, the J integral of the material and the corresponding crack propagation displacement are calculated through the following formula:

[0029]

[0030] U = U T -U I

[0031] η = 2 + 0.522·(b 0 / W)

[0032] y = 2·σ y ·(Δa - 0.2)

[0033] In the formula: J represents the elastic-plastic J integral; U T is the total energy during crack propagation, obtained by integrating the area enclosed by the load and opening displacement curve of the notched CT specimen; U I is the energy required for plastic deformation during crack propagation, obtained by integrating the area enclosed by the load and displacement curve of the corrected CT specimen; U is the energy required for crack propagation; η is a dimensionless function; B is the thickness of the notched CT specimen; W is the width from the center of the clamping hole of the notched CT specimen to one side surface of the specimen; a 0 is the distance from the center of the clamping hole of the notched CT specimen to the leading edge of the prefabricated crack; b 0 is the width from the sharp corner of the notched CT specimen to one side surface of the specimen; σ yis the yield stress; Δa is the crack propagation displacement; y is the passivation curve;

[0034] The J-R curve is calculated by the following formula

[0035] In the formula: C 1 and C 2 are experimental fitting constants;

[0036] The J integral fracture toughness J of the material is obtained through the J-R curve of the material IC .

[0037] In this solution, considering that the PE materials produced by the same manufacturer in the same batch have different properties and the manufacturer may not have experimental data related to the fracture parameters, the fracture toughness parameters need to be measured through experiments.

[0038] In addition, the fracture toughness J of the PE pipe in this solution IC , is obtained through the J-R curve of the material, and the J-R curve of the plastic material is determined by the compact tension (CT) test of the material; two types of specimens need to be prepared for the CT experiment, namely the notched CT specimen and the unnotched CT specimen; the curve is fitted according to the fitting formula of the J-R curve, and the corresponding (Δa, J) of each CT experiment can be obtained according to the J integral calculation formula. Fitting all the (Δa, J) can obtain the J-R curve, and the intersection point of the 0.2 offset passivation curve and the J-R curve is used to determine the fracture toughness J of the thermoplastic material IC .

[0039] In this solution, the material properties and specific parameters of the evaluated PE pipe are obtained through material parameter experiments, laying a prerequisite for subsequent point load experiments and the study of fracture behavior.

[0040] The measured constitutive parameters and fracture toughness parameters need to be verified through the simulation of the notched CT specimen, and the simulation results are compared with the experimental results to verify the reliability of the parameters.

[0041] Furthermore, the methods for determining the defect form of the PE pipe under the action of point load include:[[]]

[0042] Conduct point load experiments on the intact PE pipe and the defective PE pipe respectively;

[0043] Use a utility knife to make an axially oriented defect inside the intact PE pipe to prepare a defective PE pipe;

[0044] Fix the intact PE pipe and the defective PE pipe on the backing plate to ensure that the defect is at the highest point;

[0045] Add pads around the pipe to fix the pipe and prevent the pipe from rolling during loading;

[0046] Using a constant rate of loading, the spherical indenter has a loading rate of 1 mm / min and a maximum loading displacement of 50 mm;

[0047] By comparing and analyzing the surface morphologies of the intact PE pipes and defective PE pipes after the experiment, the cross-sectional morphologies, corresponding crack propagation mechanisms and morphologies of the PE pipes under point load are analyzed.

[0048] The inventors of this case found in the research that the crack morphologies of PE pipes caused by different loads will be different. Therefore, this solution needs to observe the fracture surface morphologies after the experiment from both macroscopic and microscopic perspectives to determine the crack propagation forms of PE pipes under point load.

[0049] Furthermore, the method for selecting the failure assessment curve FAC used in the failure assessment diagram includes:

[0050] According to the methods for establishing the failure assessment curve FAC in different standards, the corresponding failure assessment curve of the PE material is plotted;

[0051] Determine the cut-off value of the parameter load ratio L r ;

[0052] Compare the failure assessment curves FAC under different standards;

[0053] Select the failure assessment curve FAC that is significantly lower than other standards.

[0054] The failure assessment diagram FAD includes a failure assessment curve FAC and failure assessment points (L r , K r ). Different failure assessment curves FAC are generally similar, but are different according to different situations. A more conservative curve can be selected, and the results evaluated by the conservative curve are safer.

[0055] The method of using the failure assessment diagram FAD in this solution to conduct a safety assessment of the defective structure is the current mainstream method and is applicable to the range from brittle failure to plastic instability of materials.

[0056] Among them, those skilled in the art should understand that the failure assessment curves FAC in this solution have been revised many times by national standards and are relatively perfect. Therefore, only the comparison and selection of the failure assessment curve FAC are carried out in this solution.

[0057] Furthermore, the method for calculating the parameter fracture ratio K r based on the M integral and calculating the parameter load ratio L r based on the elastic-plastic J integral includes:

[0058] The fracture ratio K r corresponds to the brittle fracture criterion, and the fracture ratio K r is calculated through the following formula:

[0059]

[0060] Among them, — The maximum stress intensity factor at the crack front; K C — The stress intensity factor fracture toughness for evaluation; J IC — The J-integral fracture toughness; f 1 — The safety factor; E represents the elastic modulus; μ represents the Poisson's ratio;

[0061] The load ratio L r Corresponding to the plastic instability criterion, the parameter load ratio L is calculated by the following formula r :

[0062]

[0063] In the formula: J max — The maximum elastic-plastic J-integral at the crack front; J IC — The J-integral fracture toughness of the PE material.

[0064] In this scheme, the coordinates of the failure assessment point include two failure assessment parameters, namely the fracture ratio K r and the load ratio L r . Among them, the fracture ratio K r represents the ordinate of the assessment point, and the load ratio L r represents the abscissa of the assessment point. When conducting a safety assessment on a PE pipe under point load, it is necessary to calculate the corresponding fracture ratio K r and the load ratio L r parameter values to determine the failure assessment point.

[0065] In this scheme, the working conditions of the PE pipe are complex, and the internal pressure of the pipe is only one of the influencing factors. The load ratio L r corresponds to the plastic instability criterion and is the ratio of the externally applied load to the plastic yield limit load of the cracked structure, or the ratio of the applied stress to the plastic instability stress. The calculation method considering only the internal pressure is not comprehensive enough, and it is not appropriate to use the method of selecting the plastic limit internal pressure P 0 . When calculating by choosing the effective net section, it is considered that the structure is under unidirectional stress on the crack surface. However, the defective pipe under point load is affected by internal pressure, the gravity of the soil, the friction of the soil on the pipe, and the point load of hard objects. Therefore, only considering the effective net section method is also not applicable. Therefore, this scheme proposes a new method for calculating L r .

[0066] Those skilled in the art should understand that for ductile fracture, plastic deformation at the crack tip dominates, and the stress intensity factor is only applicable to the linear elastic stage and not to the ductile fracture of materials; while the elastic-plastic J integral has a wider range of applications and is applicable to both linear elastic fracture mechanics and elastic-plastic fracture mechanics. For linear elastic material behavior, the J integral is equal to the crack front energy release rate and is related to the stress intensity factor. For the elastic-plastic material behavior of PE, the J integral is a local measure that combines the fracture energy release rate and the plastic power generated by the virtual crack extension.

[0067] The J integral has path independence, that is, its value only depends on the stress-strain state at the crack tip and does not depend on the choice of the integration path. This property makes the J integral have stronger applicability and the advantage of simplifying calculations in crack problems under complex geometries and loading conditions. When calculating the parameter load ratio L r in the failure assessment diagram, there is no need to consider the specific form of the integration path, thus simplifying the calculation process.

[0068] Furthermore, the method for verifying and correcting the parameter load ratio L r calculated based on the elastic-plastic J integral includes:

[0069] According to the first point load experiment, establish a 1:1 finite element simulation model of a complete PE pipe and a defective PE pipe;

[0070] Among them, the backing plate is set as a three-dimensional solid, and the boundary conditions of the backing plate are set as fixed constraints; the spherical indenter is a three-dimensional solid, and the spherical indenter is set with a downward displacement load along the pipeline direction;

[0071] The PE pipe is set to be in surface contact with the backing plate and the spherical indenter; a crack with the same size as the complete PE pipe and the defective PE pipe described in the first point load experiment is set on the pipeline as a pipeline defect, and the local mesh of the pipeline at the defect is encrypted; the shape of the crack front is the crack shape observed in the experiment used to characterize the crack propagation of the PE pipe under point load;

[0072] Calculate the load ratio L r according to the simulation results, and compare and correct it with the current critical L r = 1;

[0073] To verify the corrected load ratio calculation method based on the J integral, the same pipeline material and experimental scheme as the first point load experiment were used, the defect size of the PE pipe was changed, and the point load experiment and simulation were carried out on the defective PE pipe again, which was defined as the second point load experiment; the data of the second point load experiment and simulation were used to verify the corrected parameter load ratio L r .

[0074] This solution first calculates the load ratio L according to the first point load experiment and simulation resultsr1 , compare with the current critical L r0 = 1, and correct the simulation result at critical crack initiation to 1 by modifying the safety factor. Calculate the corrected load ratio as follows:

[0075]

[0076] In the formula: J max is the maximum J-integral value along the crack front path; J C is the J-integral fracture toughness for evaluation; J IC is the J-integral fracture toughness, measured experimentally, and f 1 is the modified safety factor.

[0077] In this solution, the fracture ratio K based on the M-integral r calculation method and the proposed elastic-plastic J-integral load ratio L r calculation method are selected. At the same time, to verify the reliability of the load ratio calculation method based on the elastic-plastic J-integral, verify according to the results of the first point load experiment, and then obtain the load ratio L at the critical state from the plastic instability criterion r = 1. Establish a 1:1 simulation model of the defective PE pipe under point load based on the first point load experiment, and calculate the load ratio L according to the simulation results r ; the load ratio L calculated from the simulation results r is compared and corrected with the current critical L r .

[0078] To verify the corrected load ratio L r calculation method, conduct the second point load experiment and simulation to verify the corrected load ratio L r calculation method. The reliability of the parameter L r calculation is ensured through the method of verification-correction-re-verification.

[0079] Furthermore, the method of establishing the 3D simulation models of PE pipes under several different point loads includes:

[0080] According to the actual working conditions, determine several pipeline internal pressures, several pipeline dimensions, several defect dimensions, several point load dimensions, and several burial depths;

[0081] For several pipeline internal pressures, several pipeline dimensions, several defect dimensions, several point load dimensions, and several burial depths, change a single factor parameter and fix the values of other parameters to obtain the simulation models of defective pipelines under several point loads.

[0082] The casing is meshed using eight - node linear hexahedral elements, and the mesh size required for the calculation is determined through mesh independence verification; the hard object is simplified as a spherical stone, the defect is on the inner surface of the pipeline directly above the hard object, and the hard object is completely fixed;

[0083] A 1 / 2 three - dimensional point load - pipe - soil model is established. Symmetric constraints are set on the 1 / 2 section and the left and right sides of the model, a settlement displacement is set on the upper surface of the model, and a fixed constraint is set on the lower surface.

[0084] For the simulation model established in this scheme, not only the pipeline size and material properties need to be considered, but also the influence of the buried environment needs to be considered to simulate the PE gas pipeline under environmental conditions, operating parameters, etc. as much as possible. In addition, the mesh at the contact between the pipeline and the hard object is encrypted, and the mesh of the model needs to carry out independence verification to reduce the computational time cost on the premise of ensuring the accuracy of the simulation results.

[0085] Furthermore, the stress intensity factor K at the crack front is extracted I and the method for obtaining the maximum value of the elastic - plastic J integral includes:

[0086] According to the results obtained from the simulation model under the influence of different factors, the stress intensity factor K at the crack front is extracted in the simulation software I and the maximum value J of the elastic - plastic J integral max ;

[0087] According to the calculation methods of the parameter fracture ratio K r and the parameter load ratio L r , the fracture ratio K r , the load ratio L r are calculated respectively, and the failure assessment points (L r , K r ) under different analysis conditions are obtained.

[0088] The fracture ratio K r is usually defined as the ratio of the current stress intensity factor K I to the material fracture toughness K IC , that is, K r = K I / K IC . K IC is the critical stress intensity factor for the material to resist crack propagation under plane - strain conditions; the load ratio L r is defined as the ratio of the current load to the critical load that causes crack propagation. The stress intensity factor K at the crack front is extracted from the simulation results I , which usually involves the accurate calculation and analysis of the stress field near the crack tip. At the same time, the maximum value J of the elastic - plastic J integral needs to be extracted max . The J integral represents the energy required for crack propagation per unit area and is an important indicator for evaluating the crack propagation potential.

[0089] Furthermore, the method for determining whether the failure evaluation point is within the failure evaluation curve FAC includes:

[0090] Based on the obtained failure evaluation point (L r , K r ) and the failure evaluation curve FAC, a failure evaluation diagram FAD is constructed to evaluate the safety of the defective pipeline under point load;

[0091] Determine whether the failure evaluation point (L r , K r ) is within the failure evaluation curve FAC. If it is, it proves that the pipeline is safe; if it is not within the curve, it proves that the pipeline has failed and can no longer be used.

[0092] According to the evaluation rules, if the evaluation point is inside the FAC, it indicates that the PE pipe is in a safe state and can still be used; if the evaluation point is on the FAC, it indicates that the PE pipe is in a failure critical state; if the evaluation point is outside the FAC curve, the failure evaluation point is in an unsafe area and the PE pipe is in a failure state. In addition, the more deviated from the FAC curve outside the FAC curve, the higher the safety risk and the greater the degree of failure.

[0093] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0094] 1. The safety evaluation method for defective PE pipelines under point load of the present invention evaluates the safety and acceptability of defective PE pipes under point load by establishing a three-dimensional simulation model of buried PE gas pipelines under point load, considering the possible situations in the actual pipeline burying process, and also has reference significance for the safety evaluation of pipelines made of other thermoplastic polymer materials.

[0095] 2. The safety evaluation method for defective PE pipelines under point load of the present invention proposes a safety evaluation scheme for PE gas pipelines based on fracture mechanics and material nonlinearity, which is conducive to obtaining a more scientific evaluation of the safety of PE pipelines.

[0096] 3. The safety evaluation method for defective PE pipelines under point load of the present invention experimentally determines the constitutive parameters and fracture property parameters of the PE material required for the simulation, and verifies the reliability of the material parameters by virtue of the simulation model, ensuring the accuracy of the subsequent finite element analysis.

[0097] 4. The safety evaluation method for defective PE pipelines under point load of the present invention takes into account that the working conditions of buried PE pipes are complex, the internal pressure of the pipeline is only one of the influencing factors and the stress is complex, and the calculation methods of the load ratio L r based on the internal pressure or the effective net cross-sectional stress both have limitations. Therefore, it is proposed to use the elastoplastic J integral representing the elastoplastic fracture parameter of the material to calculate the load ratio Lr ; And the improved calculation method of L is verified and corrected through experiments and simulation results. r Considering the three-way stress state of the pipeline, the calculation method is more reasonable. And the parameters are verified and corrected through experiments, so as to improve the reliability of the evaluation results.

[0098] 5. For a safety evaluation method of a defective PE pipeline under point load of the present invention, during the process of establishing a finite element model for pipeline safety evaluation, the coupled effects of multiple factors are considered, such as material properties, environmental conditions, operating parameters, etc., to simulate the possible situations that may occur in real PE gas pipelines. The finite element model is verified by grid independence, the elements of key parts are refined, and a 1 / 2 symmetric model is set, which has both calculation accuracy and calculation efficiency while ensuring the calculation accuracy.

[0099] 6. For a safety evaluation method of a defective PE pipeline under point load of the present invention, considering that current failure models are mostly based on statistical analysis and empirical formulas, for complex pipeline systems and failure modes under the influence of multiple factors, the applicability and accuracy of the models may be limited. However, the present invention can accurately evaluate the safety of gas pipelines under the action of point load through experiments and simulations, which is of great significance for the pre-laid pipelines and the damaged pipelines in the later maintenance and inspection. BRIEF DESCRIPTION OF THE DRAWINGS

[0100] The drawings described herein are used to provide a further understanding of the embodiments of the present invention, form a part of this application, and do not constitute a limitation to the embodiments of the present invention. In the drawings:

[0101] Figure 1 is a schematic diagram of the safety evaluation method of a defective PE pipeline under point load in a specific embodiment of the present invention;

[0102] Figure 2 is the material stress-strain curve obtained according to the formation settlement rate in a specific embodiment of the present invention;

[0103] Figure 3 is the J-R curve and fracture toughness J of the PE material in a specific embodiment of the present invention IC ;

[0104] Figure 4 is a schematic diagram of the point load experiment in a specific embodiment of the present invention;

[0105] Figure 5 is a schematic diagram of the FAD method of the failure assessment diagram adopted in a specific embodiment of the present invention;

[0106] Figure 6 is the J-integral variation curve along the crack front of PE pipes with different defect sizes in a specific embodiment of the present invention;

[0107] Figure 7This is the Failure Assessment Diagram FAD of PE pipes under different internal pressures in the specific embodiments of the present invention. Detailed implementation manners

[0108] To make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below in conjunction with embodiments and drawings. The illustrative embodiments of the present invention and their descriptions are only used to explain the present invention and do not limit the present invention.

[0109] Embodiment 1

[0110] As Figure 1 A safety assessment method for defective PE pipes under point loads includes the following steps.

[0111] Obtain the constitutive model parameters and mechanical parameters of the PE pipe material based on the formation settlement rate; the mechanical parameters include fracture toughness parameters.

[0112] Conduct the first point load experiment to determine the defect form of the PE pipe under the action of the point load.

[0113] Select the failure assessment curve FAC used in the failure assessment diagram.

[0114] Determine the calculation method of the failure assessment point (L r , K r ). Calculate the parameter fracture ratio K r based on the M integral, and calculate the parameter load ratio L r based on the elastic-plastic J integral.

[0115] Verify and correct the parameter load ratio L r , including verifying and correcting the parameter load ratio L r through the data of the first point load experiment and simulation, and then conducting the second point load experiment and simulation to verify the corrected parameter load ratio L r .

[0116] Establish several three-dimensional simulation models of PE pipes under different point loads according to different analysis parameters.

[0117] Conduct simulation analysis on the three-dimensional simulation models of PE pipes under point loads with different parameters respectively, extract the stress intensity factor K I at the crack front and the maximum value of the elastic-plastic J integral, and calculate the failure assessment point according to the calculation methods of the parameter fracture ratio K r and the parameter load ratio L r .

[0118] According to the failure assessment curve FAC and the failure assessment point (L r , K r)Perform plotting to determine whether the failure evaluation point is within the failure evaluation curve FAC. If it is, it proves that the pipeline is safe; if it is not within the curve, it proves that the pipeline has failed and can no longer be in service.

[0119] Among them, the method for determining the constitutive model parameters of the PE pipe material includes:

[0120] Cut dumbbell-shaped specimens from the PE pipeline. The thickness of the specimens is the wall thickness of the pipe. Conduct uniaxial tensile experiments at different rates, and obtain the true stress-strain relationship curve related to the elongation rate according to the experimental results;

[0121] Based on the true stress-true strain relationship curve, fit m and n at different tensile rates through the following formula:

[0122]

[0123] In the formula, σ t is the true stress, ε t is the true strain, and m and n are fitting constants;

[0124] Determine the stress-strain relationship formula of the PE material according to the rate during the settlement of the formation to be evaluated, and calculate the elastic parameters and plastic parameters of the material.

[0125] The settlement of the urban formation is equivalent to a quasi-static loading process, and its settlement rate is between 1×10 -6 s -1 ~1×10 -5 s -1 In this example, the settlement rate of 1×10 -6 s -1 is selected to calculate the constitutive parameters of PE, such as Figure 2 is the stress-strain curve for calculating the constitutive parameters of PE at the formation settlement rate of 1×10 -6 s -1 Calculate the stress-strain curve of the constitutive parameters of PE.

[0126] In this example, the method for determining the fracture toughness parameters of the PE pipe material includes:

[0127] Cut two types of CT specimens from the PE pipeline, namely notched and unnotched CT specimens. The notched CT specimen is loaded to the maximum load during the experiment, while the unnotched CT specimen is loaded to a load 10% greater than the load during the experiment of the notched CT specimen during the experiment, and the displacement-load relationship curves are obtained respectively;

[0128] Based on the displacement-load relationship curve, calculate the J integral of the material and the corresponding crack propagation displacement through the following formula:

[0129]

[0130] U = UT -U I

[0131] η=2+0.522·(b 0 / W)

[0132] y=2·σ y (Δa-0.2)

[0133] Where: J represents the elastic-plastic J integral; U T is the total energy during crack propagation, obtained by integrating the area enclosed by the load and opening displacement curve of the notched CT specimen; U I is the energy required for plastic deformation during crack propagation, which is obtained by integrating the area enclosed by the load and displacement curve of the calibrated CT specimen; U is the energy required for crack propagation; η is a dimensionless function; B is the thickness of the notched CT specimen; W is the width from the center of the notched CT specimen clamping hole to the surface of one side of the specimen; a 0 b is the distance from the center of the notched CT specimen clamping hole to the front edge of the prefabricated crack; 0 is the width from the sharp corner of the notched CT specimen to the surface of one side of the specimen; σ y is the yield stress; Δa is the crack extension displacement; y is the passivation curve;

[0134] The JR curve is calculated by the following formula

[0135] Where: C 1 and C 2 is the experimental fitting constant;

[0136] The J-integrated fracture toughness J of the material is obtained through the JR curve IC .

[0137] Preferably, the (Δa, J) corresponding to each CT experiment is obtained, and all (Δa, J) are fitted to obtain the JR curve. Figure 3 The intersection point of the passivation curve with an offset of 0.2 and the JR curve is given in the figure, and the intersection value is 13.23 kJ / m 2 , that is, the fracture toughness of PE material is J IC =13.23kJ / m 2 .

[0138] In order to verify the constitutive parameters and fracture toughness parameters of the PE material measured in this embodiment, the applicant simulated and verified the scheme of the notched CT experiment. The simulation process includes:

[0139] Establish a model with a 1:1 ratio to the specimen used in the notched CT experiment;

[0140] Set coupling constraints for the center hole of the notched CT specimen;

[0141] Set the displacement in the Y direction at the upper central hole constraint point and restrict the degree of freedom of its movement at the lower central hole constraint point;

[0142] Set a 2-mm crack at the sharp corner of the notch CT, locally refine the mesh at the prefabricated crack during mesh generation, and use a 5-ring element ring mesh for the mesh at the crack front.

[0143] Compare the simulation results with the experimental results of the same notch CT experiment. The comparison results show that: during the process of expanding 0.2 mm from the 2-mm prefabricated crack, the reaction force of the notch CT specimen increases continuously with the increase of displacement, and the reaction force after the CT specimen expands decreases. The change trend of the simulation result curve is consistent with that of the experimental result curve, and the maximum error is 6.7%, which is within the acceptable range. Therefore, the obtained PE constitutive parameters and fracture toughness parameters can better simulate the notch CT experiment results, indicating that the obtained PE constitutive parameters and fracture toughness parameters are reliable.

[0144] After fully understanding the material properties of the PE pipe, the methods for characterizing the defect form of the PE pipe under point load include:

[0145] Conduct point load experiments on intact PE pipes and defective PE pipes respectively;

[0146] Use a utility knife to create an axial defect inside the intact PE pipe to prepare a defective PE pipe;

[0147] Fix the intact PE pipe and the defective PE pipe on the backing plate to ensure that the defect is at the highest position;

[0148] Add pads around the pipe to fix the pipe and prevent it from rolling during loading;

[0149] Use constant rate loading, with the loading rate of the spherical indenter being 1 mm / min and the maximum loading displacement being 50 mm;

[0150] By comparing and analyzing the surface morphologies of the intact PE pipe and the defective PE pipe after the experiment, analyze the cross-sectional morphology, corresponding crack propagation mechanism and morphology of the PE pipe under point load.

[0151] As Figure 4 shown in the schematic diagram of the point load experiment. In the embodiment, the defect is located inside the pipe, distributed axially, with a depth of 1 mm and a length of 44 mm; the pipe will deform during the continuous loading process of the spherical indenter, and the surface of the pipe in contact with the spherical indenter is in a compressive state, while the inner surface of the pipe opposite to it is in a tensile state.

[0152] Crack propagation occurred in the defect inside the defective pipe. The defect expansion led to the thinning of the pipe wall, which aggravated the stress concentration phenomenon of the pipe under the action of the spherical indenter, causing further expansion of the pipe defect.

[0153] Example 2

[0154] A safety assessment method for defective PE pipes under point load. On the basis of Example 1, the method for selecting the failure assessment curve FAC used in the failure assessment diagram includes:

[0155] According to the method for establishing the failure assessment curve FAC in different standards, draw the corresponding failure assessment curve for the PE material;

[0156] Determine the cut-off value of the parameter load ratio L r ;

[0157] Compare the failure assessment curves FAC under different standards;

[0158] Select the failure assessment curve FAC that is significantly lower than other standards.

[0159] The failure assessment diagram FAD contains a failure assessment curve FAC and failure assessment points (L r , K r ). Different FACs are generally similar but vary according to different situations; the failure assessment diagram FAD is a two-criterion assessment method that combines the brittle fracture criterion and the plastic instability criterion. Among them, the fracture ratio K r represents the degree to which the defective structure approaches brittle fracture, and the load ratio L r represents the degree to which the defective structure approaches plastic instability. As Figure 5 shown, it is a schematic diagram of the FAD failure assessment method.

[0160] After the applicant compared the FAC curves of multiple standards, it was found that the FAC curve of the API 579 standard was significantly lower than other standard curves; therefore, the FAC curve of API 579 was selected. This curve is on the conservative side, and the results evaluated by this curve are safer.

[0161] In this example, the method for calculating the parameter fracture ratio K r based on the M integral and the parameter load ratio L r based on the elastic-plastic J integral includes:

[0162] The fracture ratio K r corresponds to the brittle fracture criterion, and the fracture ratio K r is calculated through the following formula:

[0163]

[0164] where, —The maximum stress intensity factor at the crack front; K C —The stress intensity factor fracture toughness for evaluation; J IC —The J integral fracture toughness; f1 — safety factor; E represents the elastic modulus; μ represents the Poisson's ratio;

[0165] Load ratio L r Corresponding to the plastic instability criterion, the parameter load ratio L is calculated by the following formula r :

[0166]

[0167] In the formula: J max — the maximum value of the elastic-plastic J integral at the crack front; J IC — the J integral fracture toughness of the PE material.

[0168] To ensure the reliability of the calculation method, the method for verifying and correcting the parameter load ratio L calculated based on the elastic-plastic J integral r includes:

[0169] According to the first point load experiment, a 1:1 finite element simulation model of a complete PE pipe and a defective PE pipe is established;

[0170] Among them, the backing plate is set as a three-dimensional solid, and the boundary conditions of the backing plate are set as fixed constraints; the spherical indenter is a three-dimensional solid, and the spherical indenter is set with a downward displacement load along the pipeline direction;

[0171] The PE pipe is set to be in surface contact with the backing plate and the spherical indenter; a crack with the same size as the complete PE pipe and the defective PE pipe described in the first point load experiment is set on the pipeline as a pipeline defect, and the local mesh of the pipeline at the defect is encrypted; the shape of the crack front is the crack shape observed in the experiment for characterizing the crack propagation of the PE pipe under the action of the point load;

[0172] Calculate the load ratio L according to the simulation results r , and compare and correct it with the current critical L r = 1;

[0173] To verify the corrected load ratio calculation method based on the J integral, the same pipeline material and experimental scheme as in the first point load experiment are used, the defect size of the PE pipe is changed, and the point load experiment and simulation are carried out on the defective PE pipe again, which is defined as the second point load experiment; the data of the second point load experiment and simulation are used to verify the corrected parameter load ratio L r .

[0174] Set a crack with the same size as in the first point load experiment on the pipeline model as a pipeline defect, with a depth of 1 mm and a length of 44 mm, and the local mesh of the pipeline at the defect is encrypted;

[0175] According to the load ratio calculation method based on the elastic-plastic J integral, the load ratio L r = J max / J IC = 0.79. However, under the same load conditions, the defective PE pipe under experimental conditions is in a critical state, and its load ratio L r = 1. Therefore, the simulation result is less than the experimental result, and the calculation method of the load ratio based on the J integral needs to be corrected.

[0176] Referring to the calculation method of the fracture ratio K r to calculate the J integral fracture toughness J of PE IC Set the correction safety factor to obtain the J integral fracture toughness J for evaluation C . By correcting the safety factor to correct the simulation result at critical crack initiation to 1, the correction safety factor can be obtained as f 2 = 1.26; to make the evaluation result safer and more reliable, the correction safety factor is taken as f 2 = 1.3. Finally, the corrected load ratio calculation formula is obtained as follows:

[0177]

[0178] where J max is the maximum J integral value along the crack front path; J C is the J integral fracture toughness for evaluation, J C = 10.18 kJ / m 2 ; J IC is the J integral fracture toughness; f 2 is the correction safety factor.

[0179] In the embodiment, to verify the corrected load ratio based on the J integral, two defective pipes need to be prepared for the second point load experiment. The defect depth of the defective PE pipe 1 is 1 mm and the length is 54 mm; the defect depth of the defective PE pipe 2 is 1 mm and the length is 80 mm. The rest of the operations are the same as those in the first point load experiment.

[0180] According to the defective pipes 1 and 2, establish simulation models respectively, and calculate the load ratio L r and compare it with the critical state value.

[0181] Figure 6 is the J integral variation curve along the crack front of the PE pipe with different defect sizes. The J integral of the defective PE pipe along the crack front shows a concave fluctuating change, and the J integral values at both ends are the largest. The J integral value along the crack front shows an overall increasing trend with the increase of the defect size of the PE pipe. The change of the J integral is related to the stress field at the crack tip, indicating that the stress at both ends of the crack front is the largest, which is the dangerous position for crack initiation and propagation.

[0182] The load ratios obtained for the defective PE pipe 1 were 10.75 / 10.18 = 1.056, and for the defective PE pipe 2 were 11.12 / 10.18 = 1.092. The load ratios obtained using the corrected calculation method were all greater than the critical value of 1, but all close to the critical value, with a maximum error of 9.2%, within the allowable error range; through the method of verification - correction - re - verification to ensure L r the reliability of the calculation method. Therefore, in the embodiment, the load ratio of the parameter load L is calculated based on the elastic - plastic J integral r The method is reliable.

[0183] Embodiment 3

[0184] A safety assessment method for defective PE pipes under point loads. Based on Example 1 or 2, the method for establishing 3D simulation models of PE pipes under several different point loads includes:

[0185] According to the actual working conditions of buried gas pipelines, determine several pipeline internal pressures, several pipeline dimensions, several defect dimensions, several point load dimensions, and several burial depths;

[0186] For several pipeline internal pressures, several pipeline dimensions, several defect dimensions, several point load dimensions, and several burial depths, change a single - factor parameter and fix the values of other parameters to obtain simulation models of defective pipelines under several point loads;

[0187] For mesh generation, use eight - node linear hexahedral elements to divide the casing, and through mesh independence verification, determine the mesh size required for calculation; simplify the hard object as a spherical stone, with the defect on the inner surface of the pipeline directly above the hard object, and the hard object is completely fixed;

[0188] Establish a 1 / 2 three - dimensional point load - pipe - soil model. Set symmetric constraints on the 1 / 2 section and the left and right sides of the model, set settlement displacement on the upper surface of the model, and set fixed constraints on the lower surface.

[0189] In this embodiment, the method for extracting the stress intensity factor K I at the crack front and the maximum value J max of the J integral includes:

[0190] According to the results obtained from the simulation models under different influencing factors, respectively extract the stress intensity factor K I at the crack front and the maximum value J max of the elastic - plastic J integral;

[0191] According to the results obtained from the simulation models under different influencing factors, extract the stress intensity factor K I at the crack front and the maximum value J max of the elastic - plastic J integral in the simulation software;

[0192] According to the parameter fracture ratio Kr and the parameter load ratio L r Calculation methods are used to calculate the fracture ratio K respectively r , the load ratio L r , and failure assessment points (L r , K r ) under different analysis conditions are obtained.

[0193] The method for judging whether the failure assessment point is within the failure assessment curve FAC includes:

[0194] Based on the calculated failure assessment point (L r , K r ) and the selected failure assessment curve (FAC), a failure assessment diagram (FAD) is constructed to evaluate the safety of the defective pipeline under point load.

[0195] The applicant takes a PE pipeline with a buried depth of 5 m, an outer diameter of 110 mm, a wall thickness of 10 m, and a hard object size of 60 mm causing point load as the simulation object, and it withstands internal pressures of 0, 0.2, 0.4, 0.6, 0.8, and 1.0 MPa to explore the safety of the pipeline when the internal pressure of the pipeline changes. The following table shows the situation of the failure assessment parameter values of the defective PE pipe calculated according to the finite element results when the internal pressure of the pipeline changes, calculates the assessment points, and draws the failure assessment diagram.

[0196]

[0197] Figure 7 For the failure assessment diagram obtained from the simulation results of different internal pressures of the pipeline in this embodiment, the fracture ratio K calculated by the safety evaluation method of the PE pipeline in this example r， The values are all greater than 1, and the assessment points are all outside the assessment curve. This is because the plastic deformation at the crack tip is relatively large, resulting in a relatively large stress intensity factor value and a relatively large parameter value of the fracture ratio K r . Therefore, when installing the PE gas pipeline in the early stage, it is necessary to ensure that there are no hard objects around the pipeline and in the backfill soil, and appropriately reduce the buried depth to ensure the safety of gas transmission. After the pipeline can no longer be used, it is necessary to repair or replace the damaged PE pipe in time. This example can not only evaluate whether the buried PE pipeline is safe, but also reflect the failure degree and safety degree of the pipeline.

[0198] The above specific implementation manners further elaborate on the purpose, technical solutions, and beneficial effects of the present invention. It should be understood that the above is only the specific implementation manners of the present invention and is not used to limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included in the protection scope of the present invention.

[0199] It should be noted that in this article, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article or device.

Claims

1. A safety evaluation method for defective PE pipelines under point load, characterized in that: include: Obtaining constitutive model parameters and mechanical parameters of PE pipe materials based on formation settlement rate; the mechanical parameters include fracture toughness parameters; Carry out the first point load test to determine the defect form used to characterize the PE pipe under point load; Select the failure assessment curve FAC used in the failure assessment diagram; Determine the failure assessment point (L r , K r ) is calculated based on the M integral to calculate the parameter fracture ratio K r , based on the elastic-plastic J integral, the parameter load ratio L is calculated r ; Verification and correction of parameter load ratio L r , including verification and correction of the parameter load ratio L through the first point load test and simulation data r Then conduct a second point load test and simulation to verify the corrected parameter load ratio L r ; According to different analysis parameters, a number of PE pipe three-dimensional simulation models under different point loads are established; The three-dimensional simulation model of PE pipe under point loads with different parameters was simulated and analyzed respectively, and the stress intensity factor K of the crack front was extracted. I and the maximum value of the elastic-plastic J integral, and according to the parameter fracture ratio K r and parameter load ratio L r The calculation method calculates the failure assessment point; According to the failure assessment curve FAC and failure assessment point (L r , K r ) to draw a graph and determine whether the failure assessment point is within the failure assessment curve FAC. If so, it proves that the pipeline is safe. If not, it proves that the pipeline has failed and can no longer continue to serve.

2. A safety assessment method for defective PE pipes under point load according to claim 1, characterized in that: The method for determining the constitutive model parameters of the PE pipe material includes: Dumbbell-shaped specimens were cut from PE pipes. The specimen thickness was the pipe wall thickness. Uniaxial tensile tests were performed at different rates. The true stress-strain relationship curve related to the elongation rate was obtained based on the experimental results. Based on the true stress-true strain relationship curve, m and n at different stretching rates are fitted by the following formula: In the formula, σ t is the true stress, ε t is the true strain, m and n are fitting constants; The stress-strain relationship of the PE material is determined according to the settlement rate of the stratum to be evaluated, and the elastic parameters and plastic parameters of the material are calculated.

3. The safety evaluation method for defective PE pipes under point load according to claim 1 is characterized in that: Methods for determining the fracture toughness parameters of PE pipe materials include: Two types of CT specimens were cut from the PE pipe, namely, notched and unnotched CT specimens. The notched CT specimens were loaded to the maximum load, while the unnotched CT specimens were loaded to a load greater than 10% of the load in the notched CT specimens. Displacement-load relationship curves were obtained respectively. Based on the displacement-load relationship curve, the J integral of the material and the corresponding crack extension displacement are calculated by the following formula: U=U T -IN I η=2+0.522·(b0 / W) y=2·σ y (Δa-0.2) Where: J represents the elastic-plastic J integral; U T is the total energy during crack propagation, obtained by integrating the area enclosed by the load and opening displacement curve of the notched CT specimen; U I is the energy required for plastic deformation during crack propagation, which is obtained by integrating the area enclosed by the load and displacement curve of the calibrated CT specimen; U is the energy required for crack propagation; η is a dimensionless function; B is the thickness of the notched CT specimen; W is the width from the center of the notched CT specimen clamping hole to the surface of one side of the specimen; a0 is the distance from the center of the notched CT specimen clamping hole to the front edge of the prefabricated crack; b0 is the width from the sharp corner of the notched CT specimen to the surface of one side of the specimen; σ y is the yield stress; Δa is the crack extension displacement; y is the passivation curve; The JR curve is calculated by the following formula Where: C1 and C2 are experimental fitting constants; The J-integrated fracture toughness J of the material is obtained through the JR curve IC .

4. The safety evaluation method for defective PE pipeline under point load according to claim 1 is characterized in that: The method for determining the defect form for characterizing the PE pipe under the action of point load comprises: Point load tests were conducted on intact PE pipes and defective PE pipes respectively; A defect along the axial direction is made inside the complete PE pipe using a utility knife to prepare a defective PE pipe; Fix the intact PE pipe and the defective PE pipe on the pad, making sure the defect is at the highest point; Add blocks around the pipe to secure it and prevent it from rolling when loaded; Use constant rate loading, the spherical indenter loading rate is 1mm / min, and the maximum loading displacement is 50mm; By comparing and analyzing the surface morphologies of intact PE pipes and defective PE pipes after the experiment, the cross-sectional morphology of PE pipes under point loads, the corresponding crack propagation mechanism and morphology are analyzed.

5. The safety evaluation method for defective PE pipeline under point load according to claim 1 is characterized in that: The method for selecting the failure assessment curve FAC used in the failure assessment diagram includes: According to the failure assessment curve FAC establishment method in different standards, draw the corresponding failure assessment curve of PE material; Determine the parameter load ratio L r The cut-off value of Compare the failure assessment curves FAC under different standards; Select the failure assessment curve FAC which is significantly lower than other standards.

6. The safety evaluation method for defective PE pipeline under point load according to claim 1 is characterized in that: The fracture ratio K calculated based on the M integral r , based on the elastic-plastic J integral, the parameter load ratio L is calculated r The methods include: Parameter fracture ratio K r Corresponding to the brittle fracture criterion, the parameter fracture ratio K is calculated by the following formula: r : in, — Maximum value of stress intensity factor at crack front; K C —Evaluation of fracture toughness using stress intensity factor; J IC —J is the integral fracture toughness; f1 is the safety factor; E represents the elastic modulus; μ represents the Poisson’s ratio; Load ratio L r Corresponding to the plastic instability criterion, the parameter load ratio L is calculated by the following formula: r : Where: J max —maximum value of elastic-plastic J integral at crack front; J IC —J integrated fracture toughness.

7. The safety evaluation method for defective PE pipes under point load according to claim 5 is characterized in that: The verification and correction parameter load ratio L r The methods include: Based on the first point load test, a 1:1 finite element simulation model of the complete PE pipe and the defective PE pipe was established; Among them, the pad is set as a three-dimensional entity, and the boundary condition of the pad is set as a fixed constraint; the spherical pressure head is a three-dimensional entity, and the spherical pressure head is set with a downward displacement load along the pipeline direction; The PE pipe is arranged in surface contact with the backing plate and the spherical pressure head; a crack with the same size as the intact PE pipe and the defective PE pipe in the first point load test is arranged on the pipe as a pipe defect, and the local mesh of the pipe at the defect is encrypted; the shape of the crack front is the crack shape observed in the experiment for characterizing the crack extension of the PE pipe under the point load; Calculate the load ratio L based on the simulation results r , will be in the critical state of L r =1 for comparison correction; In order to verify the modified J-integral-based load ratio calculation method, the same pipe material and experimental scheme as the first point load experiment were used, the defect size of the PE pipe was changed, and the point load experiment and simulation were performed on the defective PE pipe again, which was defined as the second point load experiment. The second point load experiment and simulation data were used to verify the modified parameter load ratio L r .

8. The safety evaluation method for defective PE pipeline under point load according to claim 1 is characterized in that: The method for establishing a 3D simulation model of a PE pipe under several different point loads according to different analysis parameters includes: According to the actual working conditions of the buried gas pipeline, determine the internal pressure of the pipeline, the pipeline dimensions, the defect dimensions, the point load dimensions and the buried depth; For several pipeline internal pressures, several pipeline sizes, several defect sizes, several point load sizes, and several burial depths, a single factor parameter is changed, and the values ​​of other parameters are fixed to obtain a simulation model of defective pipelines under several point loads; The mesh division uses eight-node linear hexahedral units to divide the casing, and the mesh independence verification is carried out to determine the mesh size required for calculation; the hard object is simplified to a spherical stone, the defect is on the inner surface of the pipe just above the hard object, and the hard object is completely fixed; A 1 / 2 three-dimensional point load-pipe-soil model is established, symmetric constraints are set on the 1 / 2 section and left and right sides of the model, settlement displacement is set on the upper surface of the model, and fixed constraints are set on the lower surface.

9. The method for safety assessment of defective PE pipelines under point load according to claim 1, characterized in that: The extracted crack front stress intensity factor K I The methods for maximizing the elastic-plastic J integral include: According to the results of the simulation model under the influence of different factors, the stress intensity factor K of the crack front is extracted in the simulation software. I and the maximum value of elastic-plastic J integral J max ; According to the parameter fracture ratio K r and parameter load ratio L r Calculation method, respectively calculate the fracture ratio K r , load ratio L r , and obtain the failure assessment point (L r , K r ).

10. The method for safety assessment of defective PE pipelines under point load according to claim 1, characterized in that: Methods for determining whether a failure assessment point is within the failure assessment curve FAC include: According to the obtained failure assessment point (L r , K r ) and the failure assessment curve FAC, constitute the failure assessment diagram FAD, which is used to evaluate the safety of defective pipelines under point loads; Determine the failure assessment point (L r , K r ) is within the failure assessment curve FAC. If so, it proves that the pipeline is safe. If not, it proves that the pipeline has failed and can no longer continue to serve.

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