A dent damage evaluation method for long-distance oil and gas pipelines

Abstract

The present invention discloses a dent damage evaluation method for long-distance oil and gas pipelines. Based on the pipe material and internal inspection data of the pipeline to be evaluated, a finite element model is established to determine the maximum cumulative damage position of the pipeline dent; determine the damage failure criterion and damage fracture criterion of the pipeline dent; calculate the damage failure threshold and damage fracture threshold; convert the dent data of the pipeline to be evaluated into actual dent defect parameters; compare the actual dent defect parameters with the damage failure threshold and damage fracture threshold to obtain an evaluation result. The present invention provides a dent damage evaluation method for long-distance oil and gas pipelines to solve the problem of insufficient accuracy of dent damage evaluation for long-distance oil and gas pipelines in the prior art, and achieve the purpose of improving the accuracy of dent damage evaluation for long-distance oil and gas pipelines and facilitating engineering application.

Description

Technical Field

[0001] The invention relates to the field of oil and gas storage and transportation, and in particular to a dent damage evaluation method for a long-distance oil and gas pipeline. Background Art

[0002] In terms of the transportation of oil and gas, pipeline transportation has the advantages of low cost, less land use, high efficiency, strong continuity and not easily affected by the surrounding environment, so pipeline transportation has become one of the most important methods in the oil and gas industry. While increasing efforts to build pipelines, only safe and reliable pipelines can provide guarantees for the vigorous development of the oil and gas industry. However, with the increase in the service time of oil and gas pipelines, changes in the natural environment around the pipelines, and the impact of a series of problems such as pipeline corrosion and mechanical damage on the pipelines, the safe operation of the pipelines has been threatened. In the process of pipeline laying and operation, due to various factors, there will be defects such as dents and scratches of varying degrees on the pipelines, which bring safety hazards to the operation of the pipelines. Among them, the dent defect is a common geometric defect in pipeline operation, which is a deformation of the cross section of the pipeline caused by permanent plastic deformation of the pipe wall.

[0003] Pipeline dents mainly occur during the pipeline laying and construction period, caused by collisions and rock obstacles during the transportation or backfilling of soil and rocks; they may also occur during the operation of the pipeline, caused by the crushing of objects such as rocks and excavation equipment, or damage by tugboats and trawls. Dents will affect the local stress strain, residual strength and fatigue life of the pipeline, and will also affect the smooth passage of the pig, hinder the cleaning and pipe wall inspection, and bring difficulties to the monitoring and management of the pipeline; when the depth of some dents is too large or when dents act simultaneously with some other types of defects (such as cracks, welds, corrosion, scratches), it often causes the pipeline to fail and cause serious consequences.

[0004] Therefore, in order to ensure the safe operation of oil and gas pipelines, protect the safety of life and property along the pipelines, and improve management efficiency, it is necessary to quickly evaluate dent defects. However, the existing technology for the study of dent defects in long-distance oil and gas pipelines has the following problems and shortcomings: (1) The traditional metal ductile fracture criterion in the field of dents in oil and gas pipelines was proposed a long time ago, and the criterion itself has the problem of inaccurate prediction, and the existing method of measuring and calculating material constants is not accurate enough, which will lead to the results of analyzing dent damage based on the ductile fracture criterion being unreliable; (2) The existing technology rarely considers the problem of cumulative damage. When using damage values ​​to characterize the degree of dent damage, there will be errors with the actual situation, resulting in insufficient accuracy in evaluating dents based on damage values. (3) The existing evaluation method has the problem of difficulty in obtaining actual evaluation parameters, which is not convenient for actual engineering applications. Summary of the invention

[0005] The present invention provides a dent damage evaluation method for a long-distance oil and gas pipeline, so as to solve the problem of insufficient accuracy in the dent damage evaluation of a long-distance oil and gas pipeline in the prior art, and achieve the purpose of improving the accuracy of the dent damage evaluation of a long-distance oil and gas pipeline and facilitating engineering application.

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

[0007] A dent damage evaluation method for a long-distance oil and gas pipeline comprises:

[0008] S1. Based on the pipe material and internal inspection data of the pipeline to be evaluated, a finite element model is established to determine the maximum cumulative damage position of the pipeline dent;

[0009] S2. Determine the damage failure criterion and damage fracture criterion of pipeline dents;

[0010] S3, calculating the damage failure threshold and the damage fracture threshold;

[0011] S4, converting the dent data of the pipeline to be evaluated into actual dent defect parameters;

[0012] S5. Compare the actual dent defect parameters with the damage failure threshold and the damage fracture threshold to obtain an evaluation result.

[0013] In view of the problem of insufficient accuracy in the evaluation of dent damage in long-distance oil and gas pipelines in the prior art, the present invention proposes a method for evaluating dent damage in long-distance oil and gas pipelines. The method first establishes a finite element model of the pipeline to be evaluated and determines its maximum cumulative damage position. The subsequent evaluation can be performed on the maximum cumulative damage position. The internal detection of the pipeline can be implemented using existing technologies, and the internal detection data includes data such as the shape and size of the dents on the inner wall of the pipeline. Afterwards, the method determines the damage failure criterion and damage fracture criterion of the pipeline dent, and calculates the damage failure threshold based on the damage failure criterion and the damage fracture threshold based on the damage fracture criterion. The dent data of the pipeline to be evaluated is then converted into actual dent defect parameters, and then compared with the obtained damage failure threshold and damage fracture threshold to obtain the final evaluation result.

[0014] It can be seen that the present application proposes different criteria for failure and fracture of dent damage in long-distance oil and gas pipelines, and then calculates two thresholds for dent damage evaluation based on the two criteria, which is conducive to improving the accuracy of dent damage evaluation in long-distance oil and gas pipelines; at the same time, the actual evaluation parameters in the present application are easy to obtain and more suitable for actual engineering applications.

[0015] Further, the method for determining the maximum cumulative damage position of the pipeline dent includes:

[0016] S11, based on the shape of the dents on the pipe to be evaluated, classify them into spherical dents and ellipsoidal dents;

[0017] S12, calculating an initial dent depth, wherein the initial dent depth=actual maximum dent depth / (1-rebound coefficient);

[0018] S13. Determine the location of maximum cumulative damage:

[0019] For the ellipsoidal concave type, the maximum cumulative damage location is located at the center of the dent on the inner wall of the pipe;

[0020] For spherical dents: if the initial dent depth is ≤10%D, the maximum cumulative damage position is located at the center of the dent on the inner wall of the pipe; if the initial dent depth is greater than 10%D, the maximum cumulative damage position is located at a position 1%D to 2%D outward from the center of the dent on the inner wall of the pipe along the circumferential direction of the pipe; where D is the outer diameter of the pipe.

[0021] During the finite element simulation process, since the contact surface between the marking object and the pipeline is not a regular plane, the degree of damage at different positions in the pipeline dent area will be different. Therefore, it is necessary to determine the maximum cumulative damage position in the dent area and use this position as the danger point of the entire dent area to evaluate the damage state of the pipeline dent.

[0022] It should be noted that the spherical indentation and ellipsoidal indentation in this solution do not mean that the indentation shape needs to completely meet the spherical or ellipsoidal shape, but it is classified according to the actual indentation shape: if the actual indentation shape is closer to a sphere, it is classified as a spherical indentation; if the actual indentation shape is closer to an ellipsoid, it is classified as an ellipsoidal indentation. In addition, those skilled in the art should understand that the rebound coefficient in this solution is the ratio of the rebound distance to the initial indentation depth, which can be obtained by actual measurement, simulation or experience based on the pipe material.

[0023] Furthermore, the method for determining the damage failure criterion and the damage fracture criterion of the pipeline dent includes:

[0024] S21. Determine the ductile fracture criterion;

[0025] S22. Based on the tensile test and finite element simulation, determine the material constant in the ductile fracture criterion corresponding to the pipeline damage failure, which is defined as the first material constant;

[0026] S23, substituting the first material constant into the ductile fracture criterion to obtain an expression of the ductile fracture criterion as the damage failure criterion;

[0027] S24, determining the material constant in the ductile fracture criterion corresponding to the pipeline damage fracture, and defining it as the second material constant;

[0028] S25. Substitute the second material constant into the ductile fracture criterion to obtain an expression of the ductile fracture criterion as the damage fracture criterion.

[0029] The determination of material constants in the ductile fracture criterion theoretically requires statistical analysis of test data that are the same or close to the actual pipe forming process and loading conditions. The determination methods in the prior art have the disadvantages of being complex and difficult to implement, and not conducive to actual solutions on the engineering site. In order to overcome this problem, after determining the ductile fracture criterion, this scheme uses a hybrid method of tensile testing and finite element simulation to determine the material constants in the ductile fracture criterion corresponding to pipeline damage failure, and the damage failure criterion can be obtained based on the first material constant. The hybrid method of tensile testing and finite element simulation proposed in this application only requires a limited number of tests combined with simulation to determine the mechanical parameters and solve the material constants in specific situations, and has the advantage of convenient, fast and accurate acquisition of material constants in the ductile fracture criterion.

[0030] In order to further evaluate the fracture state of the pipeline dent damage, this scheme also calculates the material constant again based on the pipeline damage fracture moment and defines it as the second material constant, thereby obtaining the damage fracture criterion.

[0031] This scheme creatively proposed two criteria for pipeline dent damage failure and damage fracture, providing a scientific, reasonable and accurate evaluation standard for the subsequent definition of damage evaluation thresholds.

[0032] Further, in step S21, the ductile fracture criterion is determined as:

[0033]

[0034] Where: ε f is the fracture strain; A is the strength coefficient of the pipeline material; n is the hardening index of the pipeline material; L is the Lode parameter; η is the stress triaxiality; C1, C2 and C3 are all material constants.

[0035] Furthermore, the first material constant is determined by the following method:

[0036] S221. Prepare the following test specimens that are the same as the pipe material of the pipeline to be evaluated: standard tensile test specimen, semicircular hole notch test specimen, plane strain test specimen, plane shear test specimen; perform tensile tests on each test specimen:

[0037] S222. Based on the tensile test results, a displacement-load curve and a true stress-true strain curve are obtained;

[0038] S223, establishing a finite element model of each test piece, and performing simulation of the stretching process respectively;

[0039] S224, taking the moment when the displacement-load curve reaches the maximum load as the damage failure moment of the corresponding sample;

[0040] S225. In the simulation results of each sample, a region with the largest equivalent plastic strain at the corresponding damage failure moment is selected as the damage failure position;

[0041] S226. Based on the simulation results, the first average stress triaxiality and the first average Lord parameter of the three specimens, namely, the semicircular hole notch specimen, the plane strain specimen and the plane shear specimen, at the damage failure position from the occurrence of plastic deformation to the damage failure moment are calculated:

[0042]

[0043] Where: η avg is the first mean stress triaxiality; L avg is the first average Lode parameter; is the equivalent plastic strain;

[0044] S227. Substitute the first average stress triaxiality and the first average Lord parameter of the three specimens, namely, the semicircular hole notch specimen, the plane strain specimen and the plane shear specimen, into the ductile fracture criterion to calculate the first material constant.

[0045] This scheme defines in detail the process of determining the first material constant by a hybrid method of tensile test and finite element simulation. By conducting a limited number of pipe tensile tests, the stress-strain curve, mechanical property parameters, specimen fracture form and displacement load curve of the pipe under different stress states can be obtained, and then combined with subsequent tensile test finite element simulation, the fracture mechanics parameters under different stress states can be determined for the calculation of material constants.

[0046] The fracture mechanics parameters of materials, such as fracture strain, stress triaxiality, and Lode parameters, are difficult to obtain directly through experiments or calculations. Therefore, a method combining experiments and simulations is used to obtain the fracture mechanics parameters of materials.

[0047] In addition to the standard tensile specimen, the specimens in this scheme also include three types of notched specimens, namely: semicircular hole notch specimen, plane strain specimen and plane shear specimen; real tensile tests and finite element simulations of tensile tests were carried out on the above four specimens to obtain the fracture mechanics parameters under different stress states (semicircular hole notch specimen, plane strain specimen and plane shear specimen correspond to three stress states respectively), and then the three sets of parameters are substituted into the expression of the ductile fracture criterion, and the required first material constant can be obtained by solving the set of equations.

[0048] Another key point in solving the material constants in this solution is to obtain the fracture mechanics parameters of the material's "fracture starting point", that is, it is necessary to determine the damage failure moment. There is no unified standard for judging the damage failure moment of pipes in the prior art: from a macroscopic point of view, after the material reaches the maximum load moment, its bearing capacity gradually decreases, which can be considered to be caused by the damage failure of some units of the material; from a microscopic point of view, the process of ductile fracture of the material is accompanied by the nucleation, growth and agglomeration of microstructured holes. When microscopic holes are nucleated, it can be considered that the material has been damaged and failed.

[0049] This scheme draws the displacement-load curve based on the tensile test data of four specimens and the finite element simulation results to determine the damage failure moment, which has the advantages of simple method and effective results.

[0050] Furthermore, in the process of establishing the finite element model of each specimen, the hardening curve of the pipe is determined by the following method:

[0051] S2231. In the true stress-true strain curve of the standard tensile specimen, the first stage hardening curve is obtained based on the data after the specimen yields; (first divide the true stress by the elastic modulus to obtain the elastic strain, then subtract the total strain from the elastic strain to obtain the plastic strain, and finally draw the first stage hardening curve based on the plastic strain and true stress)

[0052] S2232, fitting the first stage hardening curve by Swift hardening criterion to obtain initial parameters in Swift hardening criterion; inputting the initial parameters in Swift hardening criterion into the finite element model of the plane shear specimen for simulation, and extracting the displacement load curve of the plane shear specimen based on the simulation results;

[0053] S2233. Compare the displacement-load curve of the extracted plane shear specimen with the displacement-load curve of the plane shear specimen obtained from the tensile test:

[0054] If the error is within the set range, the second stage hardening curve is obtained by the finite element inverse verification method based on the Swift hardening criterion;

[0055] If the error exceeds the set range, return to step S2232, correct the initial parameters in the Swift hardening criterion, and continue to execute steps S2232 and S2233;

[0056] S2234. Combine the first-stage hardening curve and the second-stage hardening curve to obtain a complete hardening curve of the pipe, and assign the complete hardening curve of the pipe to the material plastic constitutive model of the finite element model of each specimen.

[0057] During the research process, the inventors found that since the standard tensile specimen will experience severe local deformation after necking, the validity of its stress-strain data will be reduced, resulting in an error between the true stress-strain curve of the material and the actual value after exceeding the strength limit; in addition, when hardening characteristics with greater plastic deformation are required, the hardening characteristics obtained by the standard tensile specimen cannot meet the needs, thereby affecting the accuracy of the subsequent calculation of the first material constant using the test-simulation hybrid method.

[0058] Therefore, in order to improve the calculation accuracy of the first material constant, this solution creatively calibrates the pipe hardening curve. The pipe hardening curve is divided into two stages. The first stage is directly obtained using the test data of the standard tensile specimen before necking; for the hardening curve of the second stage, the finite element inverse verification method is used to determine it. This process requires the hardening curve of the first stage to be preliminarily fitted through the Swift hardening criterion, and then the finite element simulation results of the plane shear tensile specimen are compared with the test data, and the parameters are continuously adjusted for correction until the error between the simulation and the test results is within an acceptable range. Among them, the Swift hardening criterion is a prior art and will not be described here.

[0059] The pipe hardening curve obtained by this scheme can be directly assigned to the material plastic constitutive model of the finite element model of each specimen, which can overcome the defects of insufficient validity of stress-strain data and excessive error of the true stress-strain curve caused by the necking of the standard tensile specimen, thereby giving the pipe a more accurate and realistic hardening characteristic and improving the accuracy of subsequent calculation of material constants.

[0060] Furthermore, the second material constant is determined by the following method:

[0061] S241. Based on the simulation results, the time of macroscopic instability fracture of three specimens, namely, semicircular hole notch specimen, plane strain specimen and plane shear specimen, is taken as the corresponding damage fracture time;

[0062] S242, taking the instability fracture position corresponding to each specimen at the moment of damage and fracture as the damage and fracture position;

[0063] S243. Based on the simulation results, the second average stress triaxiality and the second average Lord parameter of three types of specimens, namely, the semicircular hole notch specimen, the plane strain specimen and the plane shear specimen, are calculated at the damage and fracture position and from the time of plastic deformation to the damage and fracture;

[0064] S244. Substitute the second average stress triaxiality and the second average Lord parameter of the three specimens, namely the semicircular hole notch specimen, the plane strain specimen and the plane shear specimen, into the ductile fracture criterion to calculate the second material constant.

[0065] It can be seen that the method for determining the second material constant in this scheme is the same as the method for determining the first material constant in the previous text. The difference between the two lies in the different criteria based on them, as well as the differences in the corresponding damage failure time / damage fracture time, damage failure position / damage fracture position. Therefore, the formula form for calculating the second mean stress triaxiality and the second mean Lode parameter in this scheme is also the same as the formula form for calculating the first mean stress triaxiality and the first mean Lode parameter in the above text, so it will not be repeated here.

[0066] Furthermore, the damage failure threshold is calculated by the following method:

[0067] S31, under the condition of the damage failure criterion, calculate the first cumulative damage value I': I'=∑I i1 ;

[0068]

[0069] Where: I i1 represents the cumulative damage in the i-th incremental step; represents the equivalent plastic strain; is the equivalent plastic strain corresponding to the i-th incremental step; is the equivalent plastic strain corresponding to the i-1th incremental step; is the fracture strain corresponding to the ith incremental step under the condition of damage failure criterion;

[0070] S32. Convert different dent depths in the finite element simulation into pipeline dent defect parameters: G = S / D; where: G is the pipeline dent defect parameter; S is the actual dent depth; D is the outer diameter of the pipeline;

[0071] S33, establishing a variation curve of the first cumulative damage value as the pipeline dent defect parameter increases, which is defined as a first variation curve;

[0072] S34. Based on the first variation curve, take the pipeline dent defect parameter corresponding to the first cumulative damage value of 1 as the damage failure threshold.

[0073] The damage fracture threshold is calculated by the following method:

[0074] S35. Under the condition of the damage fracture criterion, calculate the second cumulative damage value I”: I”=∑I i2 ;

[0075]

[0076] Where: I i2 represents the cumulative damage in the i-th incremental step; represents the equivalent plastic strain; is the equivalent plastic strain corresponding to the i-th incremental step; is the equivalent plastic strain corresponding to the i-1th incremental step; is the fracture strain corresponding to the i-th incremental step under the condition of fracture failure criterion;

[0077] S36. Convert different dent depths in the finite element simulation into pipeline dent defect parameters: G = S / D; where: G is the pipeline dent defect parameter; S is the actual dent depth; D is the outer diameter of the pipeline;

[0078] S37, establishing a variation curve of the second cumulative damage value as the pipeline dent defect parameter increases, which is defined as a second variation curve;

[0079] S38. Based on the second variation curve, take the pipeline dent defect parameter corresponding to the second cumulative damage value of 1 as the damage fracture threshold.

[0080] The existing dent damage evaluation of oil and gas pipelines only considers the impact of dent depth on pipeline damage, but does not consider the impact of pipeline parameters and other dent parameters on pipeline damage, which will cause inaccurate pipeline dent evaluation results.

[0081] The calculation of the first cumulative damage value in this scheme is carried out based on the loading steps in the finite element simulation process, and the superposition of the damage in each loading step is used as the calculation result of the cumulative damage value of the pipeline dent. According to the setting of the analysis step time length in the finite element model, each incremental step in the finite element model is calculated as a calculation point. This scheme uses the damage failure criterion for the calculation of the cumulative damage value of the pipeline dent. When the calculated first cumulative damage value reaches 1, it can be determined that the pipeline has failed due to damage, and the bearing capacity of the pipeline begins to decrease. Therefore, the pipeline dent defect parameter corresponding to the first cumulative damage value of 1 is defined as the damage failure threshold of the pipeline dent.

[0082] Similarly, under the damage and fracture criterion, the damage and fracture threshold can be obtained using the same idea.

[0083] Furthermore, the method for converting the dent data of the pipeline to be evaluated into the actual dent defect parameter is: G0 = S / D; where: G0 is the actual dent defect parameter; S is the actual dent depth; D is the outer diameter of the pipeline;

[0084] The method of comparing the dent defect parameter with the damage failure threshold and the damage fracture threshold includes:

[0085] If the actual dent defect parameter is less than the damage failure threshold, the pipeline is in a safe state;

[0086] If the damage failure threshold ≤ actual dent defect parameter < damage fracture threshold, the pipeline is in a damage failure state;

[0087] If the damage fracture threshold is ≤ the actual dent defect parameter, the pipeline will crack.

[0088] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0089] 1. The present invention provides a dent damage evaluation method for a long-distance oil and gas pipeline, proposes different criteria for the failure and fracture states of the dent damage of the long-distance oil and gas pipeline, and then calculates two thresholds for the dent damage evaluation based on the two criteria, which is beneficial to improving the accuracy of the dent damage evaluation of the long-distance oil and gas pipeline; at the same time, the actual evaluation parameters in the present application are easy to obtain and are more suitable for actual engineering applications.

[0090] 2. After determining the ductile fracture criterion, the present invention uses a hybrid method of tensile test and finite element simulation to determine the material constants in the ductile fracture criterion corresponding to the pipeline damage failure, and the damage failure criterion can be obtained based on the first material constant; only through a limited number of tests combined with simulation, the mechanical parameters under specific circumstances can be determined and the material constants can be solved, which has the advantage of conveniently, quickly and accurately obtaining the material constants in the ductile fracture criterion.

[0091] 3. The present invention draws a displacement-load curve based on the tensile test data of four specimens and the finite element simulation results to determine the damage failure moment, which has the advantages of simple method and effective results.

[0092] 4. The complete hardening curve of the pipe obtained by the present invention can be directly assigned to the material plastic constitutive model of the finite element model of each specimen, which can overcome the defects of insufficient validity of stress-strain data and excessive error of the true stress-strain curve caused by the necking of the standard tensile specimen, thereby giving the pipe a more accurate and true hardening characteristic and improving the accuracy of subsequent calculation of material constants.

[0093] 5. The present invention proposes a detailed calculation method for the damage failure threshold and the damage fracture threshold, fully considering the effect of cumulative damage and significantly improving the accuracy of the dent damage evaluation of long-distance oil and gas pipelines. BRIEF DESCRIPTION OF THE DRAWINGS

[0094] The drawings described herein are used to provide a further understanding of the embodiments of the present invention, constitute a part of this application, and do not constitute a limitation of the embodiments of the present invention. In the drawings:

[0095] Figure 1 It is a schematic diagram of a flow chart of a specific embodiment of the present invention;

[0096] Figure 2 It is a schematic diagram of the structure of four test pieces in a specific embodiment of the present invention;

[0097] Figure 3 is a schematic diagram of a first variation curve in a specific embodiment of the present invention;

[0098] Figure 4 It is a schematic diagram of the second change curve in a specific embodiment of the present invention. DETAILED DESCRIPTION

[0099] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with embodiments and drawings. The exemplary embodiments of the present invention and their description are only used to explain the present invention and are not intended to limit the present invention.

[0100] Embodiment 1:

[0101] like Figure 1 A dent damage evaluation method for a long-distance oil and gas pipeline is shown, comprising the following steps:

[0102] S1. Based on the pipe material and internal inspection data of the pipeline to be evaluated, a finite element model is established to determine the maximum cumulative damage position of the pipeline dent; wherein the internal inspection data includes the shape and size data of the pipeline dent;

[0103] In this embodiment, the method for determining the maximum cumulative damage position of the pipeline dent includes:

[0104] S11, based on the shape of the dents on the pipe to be evaluated, classify them into spherical dents and ellipsoidal dents;

[0105] S12, calculating an initial dent depth, wherein the initial dent depth=actual maximum dent depth / (1-rebound coefficient);

[0106] S13. Determine the location of maximum cumulative damage:

[0107] For the ellipsoidal concave type, the maximum cumulative damage location is located at the center of the dent on the inner wall of the pipe;

[0108] For spherical dents: if the initial dent depth is ≤10%D, the maximum cumulative damage position is located at the center of the dent on the inner wall of the pipe; if the initial dent depth is greater than 10%D, the maximum cumulative damage position is located at a position 1%D to 2%D outward from the center of the dent on the inner wall of the pipe along the circumferential direction of the pipe; where D is the outer diameter of the pipe.

[0109] S2. Determine the damage failure criterion and damage fracture criterion of pipeline dents;

[0110] S3, calculating the damage failure threshold and the damage fracture threshold;

[0111] S4. Convert the dent data of the pipeline to be evaluated into actual dent defect parameters; the conversion method is: G0 = S / D; where: G0 is the actual dent defect parameter; S is the actual dent depth; D is the outer diameter of the pipeline;

[0112] S5. Compare the actual dent defect parameters with the damage failure threshold and the damage fracture threshold to obtain an evaluation result; the specific comparison and evaluation method is:

[0113] If the actual dent defect parameter is less than the damage failure threshold, the pipeline is in a safe state;

[0114] If the damage failure threshold ≤ actual dent defect parameter < damage fracture threshold, the pipeline is in a damage failure state and the pipeline should be monitored or replaced;

[0115] If the damage fracture threshold is ≤ the actual dent defect parameter, the pipeline is cracked and should be replaced immediately.

[0116] Embodiment 2:

[0117] A method for evaluating dent damage of a long-distance oil and gas pipeline, based on Example 1, a method for determining the damage failure criterion and the damage fracture criterion of the pipeline dent is as follows:

[0118] S21. Determine the ductile fracture criterion:

[0119]

[0120] Where: ε f is the fracture strain; A is the strength coefficient of the pipeline material; n is the hardening index of the pipeline material; L is the Lord parameter; η is the stress triaxiality; C1, C2 and C3 are all material constants.

[0121] S22. Based on the tensile test and finite element simulation, the material constant in the ductile fracture criterion corresponding to the pipeline damage failure is determined and defined as the first material constant. The specific process is as follows:

[0122] Prepare the following four test specimens that are the same as the pipe material to be evaluated: standard tensile test specimen, semicircular hole notch test specimen, plane strain test specimen, plane shear test specimen; the structures of the four test specimens are as follows: Figure 2 shown.

[0123] It should be noted that Figure 2 (a), (b), (c), and (d) are top views of standard tensile specimens, semicircular hole notch specimens, plane strain specimens, and plane shear specimens, respectively. The plane strain specimen has a thinned middle area, and its thickness is 1 / 2 of the specimen body thickness.

[0124] Tensile tests were performed on each specimen:

[0125] Based on the tensile test results, the displacement-load curve and true stress-true strain curve are obtained;

[0126] Establish the finite element model of each specimen and simulate the stretching process respectively;

[0127] The moment when the displacement-load curve reaches the maximum load is taken as the damage failure moment of the corresponding specimen;

[0128] In the simulation results of each specimen, the area with the largest equivalent plastic strain at the corresponding damage failure moment is selected as the damage failure position;

[0129] Based on the simulation results, the first average stress triaxiality and the first average Lord parameter at the damage failure position from the occurrence of plastic deformation to the moment of damage failure are calculated for three types of specimens: semicircular hole notch specimen, plane strain specimen and plane shear specimen:

[0130]

[0131] Where: η avg is the first mean stress triaxiality; L avg is the first average Lode parameter; is the equivalent plastic strain;

[0132] The first average stress triaxiality and the first average Lord parameter of the three specimens, namely the semicircular hole notch specimen, the plane strain specimen and the plane shear specimen, are substituted into the ductile fracture criterion to calculate the first material constant.

[0133] S23, substituting the first material constant into the ductile fracture criterion to obtain an expression of the ductile fracture criterion as the damage failure criterion;

[0134] S24. Determine the material constant in the ductile fracture criterion corresponding to the pipeline damage fracture, and define it as the second material constant. The specific process is as follows:

[0135] Based on the simulation results, the moment of macroscopic instability fracture of three specimens, namely, semicircular hole notch specimen, plane strain specimen and plane shear specimen, is taken as the corresponding damage fracture moment;

[0136] The instability fracture position corresponding to each specimen at the moment of damage and fracture is taken as the damage and fracture position;

[0137] Based on the simulation results, the second average stress triaxiality and the second average Lord parameter at the damage and fracture position and from the occurrence of plastic deformation to the damage and fracture moment of three types of specimens, namely, semicircular hole notch specimen, plane strain specimen and plane shear specimen, are calculated;

[0138] The second average stress triaxiality and the second average Lord parameter of the three specimens, namely the semicircular hole notch specimen, the plane strain specimen and the plane shear specimen, are substituted into the ductile fracture criterion to calculate the second material constant.

[0139] S25. Substitute the second material constant into the ductile fracture criterion to obtain an expression of the ductile fracture criterion as the damage fracture criterion.

[0140] In this embodiment, the damage failure threshold is calculated by the following method:

[0141] S31, under the condition of the damage failure criterion, calculate the first cumulative damage value I': I'=∑I i1 ;

[0142]

[0143] Where: I i1 represents the cumulative damage in the i-th incremental step; represents the equivalent plastic strain; is the equivalent plastic strain corresponding to the i-th incremental step; is the equivalent plastic strain corresponding to the i-1th incremental step; is the fracture strain corresponding to the ith incremental step under the condition of damage failure criterion;

[0144] S32. Convert different dent depths in the finite element simulation into pipeline dent defect parameters: G = S / D; where: G is the pipeline dent defect parameter; S is the actual dent depth; D is the outer diameter of the pipeline;

[0145] S33, establishing a variation curve of the first cumulative damage value as the pipeline dent defect parameter increases, which is defined as a first variation curve;

[0146] S34. Based on the first variation curve, take the pipeline dent defect parameter corresponding to the first cumulative damage value of 1 as the damage failure threshold.

[0147] In this embodiment, the damage fracture threshold is calculated by the following method:

[0148] S35. Under the condition of the damage fracture criterion, calculate the second cumulative damage value I”: I”=∑I i2 ;

[0149]

[0150] Where: I i2 represents the cumulative damage in the i-th incremental step; represents the equivalent plastic strain; is the equivalent plastic strain corresponding to the i-th incremental step; is the equivalent plastic strain corresponding to the i-1th incremental step; is the fracture strain corresponding to the i-th incremental step under the condition of fracture failure criterion;

[0151] S36. Convert different dent depths in the finite element simulation into pipeline dent defect parameters: G = S / D; where: G is the pipeline dent defect parameter; S is the actual dent depth; D is the outer diameter of the pipeline;

[0152] S37, establishing a variation curve of the second cumulative damage value as the pipeline dent defect parameter increases, which is defined as a second variation curve;

[0153] S38. Based on the second variation curve, take the pipeline dent defect parameter corresponding to the second cumulative damage value of 1 as the damage fracture threshold.

[0154] Embodiment 3:

[0155] A method for evaluating dent damage of a long-distance oil and gas pipeline, based on Example 2, in the process of establishing the finite element model of each test piece in step S22, the complete hardening curve of the pipe is determined by the following method:

[0156] S2231. In the true stress-true strain curve of the standard tensile specimen, the first stage hardening curve is obtained based on the data after the specimen yields; specifically: first, the elastic strain is obtained by dividing the true stress by the elastic modulus, then the plastic strain is obtained by subtracting the total strain from the elastic strain, and finally the first stage hardening curve is drawn based on the plastic strain and the true stress;

[0157] S2232, fitting the first stage hardening curve by Swift hardening criterion to obtain the initial parameters in the Swift hardening criterion; firstly, preliminarily fitting the initial Swift criterion parameters A and n by experimental data: assuming that the initial parameters fitted at this time are A1 and n1;

[0158] The initial parameters in the Swift hardening criterion are input into the finite element model of the plane shear specimen for simulation, and the displacement-load curve of the plane shear specimen is extracted based on the simulation results; this step is to input A1 and n1 into the finite element model, run the simulation, and obtain the initial displacement-load curve.

[0159] S2233. Compare the displacement-load curve of the extracted plane shear specimen with the displacement-load curve of the plane shear specimen obtained from the tensile test:

[0160] If the error is within the set range, the second stage hardening curve is obtained by the finite element inverse verification method based on the Swift hardening criterion;

[0161] If the error exceeds the set range (e.g., the simulation curve is higher or lower than the experimental curve in some areas, or there is an obvious deviation in the plastic deformation area), return to step S2232, correct the initial parameters in the Swift hardening criterion, and re-simulate and compare until the error between the simulation result and the experimental data is within an acceptable range;

[0162] S2234. Combine the first-stage hardening curve and the second-stage hardening curve to obtain a complete hardening curve of the pipe, and assign the complete hardening curve of the pipe to the material plastic constitutive model of the finite element model of each specimen.

[0163] In a more preferred embodiment, the method for correcting the initial parameters in the Swift hardening criterion includes:

[0164] According to the error situation, manually adjust parameters A and n, and the adjustment direction is as follows: if the simulation curve is overall high, reduce parameter A (for example, reduce it by 5%-10%) or increase parameter n; if the simulation curve is overall low, increase parameter A (for example, increase it by 5%-10%) or reduce parameter n; if the simulation curve has large deviations in certain specific areas, adjust A and n at the same time to change the shape of the curve so that it better matches the experimental data.

[0165] Embodiment 4:

[0166] A dent damage evaluation method for a long-distance oil and gas pipeline is disclosed. This embodiment takes a long-distance natural gas pipeline in the southwest region as an example to illustrate the method.

[0167] The pipeline is made of X80 steel with an outer diameter of 914mm and a wall thickness of 18.4mm.

[0168] In the process of establishing the pipeline finite element model, the model pipeline length is set to 6 times the pipeline outer diameter.

[0169] In the process of establishing the finite element model of the four specimens, the maximum incremental step number is set to 1000, and the initial and minimum incremental step sizes are 0.02 and 0.00001 respectively. The mesh adopts C3D8R solid unit, and the reduced integral calculation is performed; the mesh is encrypted and gradually transitioned at the fracture position of the semicircular hole notch specimen, plane strain specimen and plane shear specimen. For the standard tensile specimen, the necking position in the finite element model is first determined, and then the mesh is encrypted at the necking position. The modeling software used in this embodiment is Abaqus software.

[0170] The first material constants calculated in this embodiment are: C1 = 0.14266, C2 = 447.78698, C3 = 0.85383.

[0171] The second material constants calculated in this embodiment are: C1 = 0.07352, C2 = 481.03674, C3 = 0.93617.

[0172] Therefore, the damage failure criterion in this embodiment is:

[0173]

[0174] The damage fracture criterion in this embodiment is:

[0175]

[0176] The first variation curve obtained in this embodiment is as follows Figure 3 As shown, the corresponding damage failure threshold G1 = 7.18%; the second variation curve obtained in this embodiment is as follows Figure 4 As shown, the corresponding damage fracture threshold G2 = 8.96%.

[0177] It should be noted that in Figure 3 and Figure 4 In the figure, the horizontal axes are all pipeline dent defect parameters, unit %; Figure 3 The vertical axis is the first cumulative damage value, Figure 4 The vertical axis is the second cumulative damage value.

[0178] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

[0179] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprises" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device.

Claims

1. A method for evaluating dent damage in long-distance oil and gas pipelines, characterized in that: The following steps are involved: S1. Based on the pipe material and internal inspection data of the pipeline to be evaluated, a finite element model is established to determine the maximum cumulative damage position of the pipeline dent; S2. Determine the damage failure criterion and damage fracture criterion of pipeline dents; S3, calculate the damage failure threshold and the damage fracture threshold; S4, converting the dent data of the pipeline to be evaluated into actual dent defect parameters; S5. Compare the actual dent defect parameters with the damage failure threshold and the damage fracture threshold to obtain an evaluation result; Methods for determining the location of maximum cumulative damage from pipe dents include: S11, based on the shape of the dents on the pipe to be evaluated, classify them into spherical dents and ellipsoidal dents; S12, calculating the initial dent depth, wherein the initial dent depth = actual maximum dent depth / (1-rebound coefficient); the rebound coefficient is the ratio of the rebound distance to the initial dent depth; S13. Determine the location of maximum cumulative damage: For the ellipsoidal concave type, the maximum cumulative damage location is located at the center of the dent on the inner wall of the pipe; For spherical dents: if the initial dent depth is ≤10%D, the maximum cumulative damage position is located at the center of the dent on the inner wall of the pipe; if the initial dent depth is greater than 10%D, the maximum cumulative damage position is located at a position 1%D to 2%D outward from the center of the dent on the inner wall of the pipe along the circumferential direction of the pipe; where D is the outer diameter of the pipe; Methods for determining damage failure criteria and damage fracture criteria of pipeline dents include: S21. Determine the ductile fracture criterion; S22. Based on the tensile test and finite element simulation, determine the material constant in the ductile fracture criterion corresponding to the pipeline damage failure, which is defined as the first material constant; S23, substituting the first material constant into the ductile fracture criterion to obtain an expression of the ductile fracture criterion as the damage failure criterion; S24, determining the material constant in the ductile fracture criterion corresponding to the pipeline damage fracture, and defining it as the second material constant; S25, substituting the second material constant into the ductile fracture criterion to obtain an expression of the ductile fracture criterion as the damage fracture criterion; In step S21, the ductile fracture criterion is determined as: Where: ε f is the fracture strain; A is the strength coefficient of the pipeline material; n is the hardening index of the pipeline material; L is the Lord parameter; η is the stress triaxiality; C1, C2 and C3 are all material constants; The first material constant is determined by the following method: S221. Prepare the following test specimens that are the same as the pipe material of the pipeline to be evaluated: standard tensile test specimen, semicircular hole notch test specimen, plane strain test specimen, plane shear test specimen; perform tensile tests on each test specimen: S222. Based on the tensile test results, a displacement-load curve and a true stress-true strain curve are obtained; S223, establishing a finite element model of each test piece, and performing simulation of the stretching process respectively; S224, taking the moment when the displacement-load curve reaches the maximum load as the damage failure moment of the corresponding sample; S225. In the simulation results of each sample, a region with the largest equivalent plastic strain at the corresponding damage failure moment is selected as the damage failure position; S226. Based on the simulation results, the first average stress triaxiality and the first average Lord parameter of the three specimens, namely, the semicircular hole notch specimen, the plane strain specimen and the plane shear specimen, at the damage failure position from the occurrence of plastic deformation to the damage failure moment are calculated: Where: η avg is the first mean stress triaxiality; L avg is the first average Lode parameter; is the equivalent plastic strain; S227, substituting the first average stress triaxiality and the first average Lord parameter of three kinds of specimens, namely, the semicircular hole notch specimen, the plane strain specimen and the plane shear specimen, into the ductile fracture criterion to calculate and obtain the first material constant; The second material constant is determined by the following method: S241. Based on the simulation results, the time of macroscopic instability fracture of three specimens, namely, semicircular hole notch specimen, plane strain specimen and plane shear specimen, is taken as the corresponding damage fracture time; S242, taking the instability fracture position corresponding to each specimen at the moment of damage and fracture as the damage and fracture position; S243. Based on the simulation results, the second average stress triaxiality and the second average Lord parameter of three types of specimens, namely, the semicircular hole notch specimen, the plane strain specimen and the plane shear specimen, are calculated at the damage and fracture position and from the time of plastic deformation to the damage and fracture; S244, substituting the second average stress triaxiality and the second average Lord parameter of the three specimens, namely, the semicircular hole notch specimen, the plane strain specimen and the plane shear specimen, into the ductile fracture criterion to calculate and obtain the second material constant; The damage failure threshold is calculated by the following method: S31, under the condition of the damage failure criterion, calculate the first cumulative damage value I': I'=∑I i1 ; Where: I i1 represents the cumulative damage in the i-th incremental step; represents the equivalent plastic strain; is the equivalent plastic strain corresponding to the i-th incremental step; is the equivalent plastic strain corresponding to the i-1th incremental step; is the fracture strain corresponding to the ith incremental step under the condition of damage failure criterion; S32. Convert different dent depths in the finite element simulation into pipeline dent defect parameters: G = S / D; where: G is the pipeline dent defect parameter; S is the actual dent depth; D is the outer diameter of the pipeline; S33, establishing a variation curve of the first cumulative damage value as the pipeline dent defect parameter increases, which is defined as a first variation curve; S34, based on the first variation curve, taking the pipeline dent defect parameter corresponding to the first cumulative damage value of 1 as the damage failure threshold; The damage fracture threshold is calculated by the following method: S35. Under the condition of the damage fracture criterion, calculate the second cumulative damage value I”: I”=∑I i2 ; Where: I i2 represents the cumulative damage in the i-th incremental step; represents the equivalent plastic strain; is the equivalent plastic strain corresponding to the i-th incremental step; is the equivalent plastic strain corresponding to the i-1th incremental step; is the fracture strain corresponding to the i-th incremental step under the condition of fracture failure criterion; S36. Convert different dent depths in the finite element simulation into pipeline dent defect parameters: G = S / D; where: G is the pipeline dent defect parameter; S is the actual dent depth; D is the outer diameter of the pipeline; S37, establishing a variation curve of the second cumulative damage value as the pipeline dent defect parameter increases, which is defined as a second variation curve; S38. Based on the second variation curve, taking the pipeline dent defect parameter corresponding to the second cumulative damage value of 1 as the damage fracture threshold; The method for converting the dent data of the pipeline to be evaluated into the actual dent defect parameter is: G0 = S / D; where: G0 is the actual dent defect parameter; S is the actual dent depth; D is the outer diameter of the pipeline; The method of comparing the dent defect parameter with the damage failure threshold and the damage fracture threshold includes: If the actual dent defect parameter is less than the damage failure threshold, the pipeline is in a safe state; If the damage failure threshold ≤ actual dent defect parameter < damage fracture threshold, the pipeline is in a damage failure state; If the damage fracture threshold is ≤ the actual dent defect parameter, the pipeline will crack.

2. The method for evaluating dent damage of a long-distance oil and gas pipeline according to claim 1, characterized in that: In the process of establishing the finite element model of each specimen, the hardening curve of the pipe is determined by the following method: S2231. In the true stress-true strain curve of the standard tensile specimen, the first stage hardening curve is obtained based on the data after the specimen yields; S2232, fitting the first stage hardening curve by Swift hardening criterion to obtain initial parameters in Swift hardening criterion; The initial parameters in the Swift hardening criterion are input into the finite element model of the plane shear specimen for simulation, and the displacement load curve of the plane shear specimen is extracted based on the simulation results; S2233. Compare the displacement-load curve of the extracted plane shear specimen with the displacement-load curve of the plane shear specimen obtained from the tensile test: If the error is within the set range, the second stage hardening curve is obtained by the finite element inverse verification method based on the Swift hardening criterion; If the error exceeds the set range, return to step S2232 to correct the initial parameters in the Swift hardening criterion; S2234. Combine the first-stage hardening curve and the second-stage hardening curve to obtain a complete hardening curve of the pipe, and assign the complete hardening curve of the pipe to the material plastic constitutive model of the finite element model of each specimen.

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