Prediction method for service behavior of oil and gas pipeline under stress corrosion cracking condition

Abstract

The invention discloses a method for predicting the service behavior of an oil and gas pipeline under a stress corrosion cracking condition, and belongs to the field of material performance analysis and prediction. Firstly, based on a convection diffusion equation, a real stress intensity factor of an oil and gas pipeline crack tip is used as an unknown quantity to obtain a stress distribution condition; then, applying boundary conditions and a crack propagation criterion in fracture mechanics, and performing preliminary calculation to obtain crack propagation behavior characteristics of the one-dimensional semi-analytical model; and finally, calibrating and calculating the diffusion coefficient of the material parameter in the one-dimensional semi-analytical model to obtain real full-life service time information of the oil and gas pipeline material. According to the method, the full-life behavior information of stress corrosion cracking can be quickly obtained, and a partial differential equation does not need to be solved; on the basis of fracture mechanics, the mechanism of repeated fracture and reconstruction of the passivation layer in the fracture process can be reproduced; the full-life service time of the oil and gas pipeline can be effectively predicted, and the parameter calibration method can be applied to other conditions and is wide in application range.

Description

Technical Field

[0001] This invention belongs to the field of material performance analysis and prediction, and relates to a method for predicting the service behavior of oil and gas pipelines under stress corrosion cracking conditions. Background Technology

[0002] Despite the current energy transition, oil and natural gas remain the dominant energy source in the global energy system, meeting more than half of the world's energy needs, and their total demand remains at historically high levels. Against this backdrop, researching the corrosion characteristics of API 5L X70 steel, a material used in oil and gas pipelines, is particularly crucial. Among numerous corrosion mechanisms, stress corrosion cracking is considered a significant cause of pipeline steel failure. Therefore, studying the stress corrosion cracking performance of pipeline steel is of great importance for ensuring the long-term stable operation of oil and gas pipelines. While conventional corrosion cracking typically requires extremely long periods of corrosion damage or extremely high stress levels, stress corrosion cracking can occur far below the material's design yield strength and with very slight surface corrosion, posing a significant threat to the safe and stable operation of X70 pipeline steel in oil and gas pipelines.

[0003] The stress corrosion cracking mechanism is as follows: X70 steel material in oil and gas pipelines under tensile stress is placed in a corrosive solution environment, and due to processing or other factors, there are tiny scratches on the surface, which then develop into cracks; the sharp stress gradient at the crack tip intensifies the penetration of metal cations (iron ions), while at the crack tip, due to chemical reactions, a passivation layer is formed on the surface of the X70 steel material to slow down the penetration of ions; at the same time, some factors in the corrosive solution, such as pH and chloride ion concentration, also accelerate the penetration of metal cations through the passivation layer into the corrosive environment, leading to material damage.

[0004] Current experimental studies, despite their significant economic and time costs, can only provide instantaneous measurement data (crack propagation rate) indicating the stress corrosion cracking behavior of X70 steel in pipelines. However, some crucial information, such as the entire service life of stress corrosion-prone materials, cannot be directly measured. Achieving efficient and accurate prediction of the entire service life of X70 steel in oil and gas pipelines is a critical problem that urgently needs to be solved.

[0005] Current research on stress corrosion cracking behavior in pipeline steel is primarily experimental, supplemented by limited simulations. Existing predictive models for stress corrosion cracking behavior in pipeline steel have the following limitations: First, fracture mechanics models mainly address the standard for crack propagation in the presence of pre-cracks. Generally, they cannot be used to predict important stress corrosion cracking parameters such as crack latency (long duration). Moreover, existing models either neglect the dynamic fracturing and reconstruction process of the passivation layer or are computationally too intensive for industrial applications. Therefore, a tool balancing accuracy and efficiency is urgently needed to address these issues. Second, crack propagation under stress corrosion cracking differs from crack propagation under purely mechanical conditions; the corrosion state before the crack tip plays a crucial role in determining crack propagation, and this stress-corrosion interaction is difficult to incorporate into the K... ICSCC (hereinafter referred to as) Among such empirical parameters, developing a physical model to efficiently predict the stress corrosion cracking behavior of X70 pipeline steel is urgently needed; and constructing the mechanism of ion penetration driven by a sharp stress gradient at the crack tip based on physical laws is the key to accurately describing the stress corrosion cracking behavior of pipeline steel in this model.

[0006] To address the above issues, scholars both domestically and internationally have conducted extensive research and proposed several solutions. For example, Chinese invention patent (application number 2024104040336) provides a defect detection technology for oil pipelines, which obtains a two-dimensional spectrum of the dielectric characteristic values ​​of the pipeline under test to determine the length and width of defects, thereby determining whether the pipeline needs repair. Another Chinese invention patent (application number 2025101394297) establishes a finite element model to determine the stress distribution of material elements and displays material damage information based on the stress distribution. However, existing technical solutions still focus on monitoring defects and damage, without quantifying the impact of stress and corrosive environments on material lifespan. Summary of the Invention

[0007] To address the aforementioned problems, this invention provides a method for predicting the service behavior of oil and gas pipelines under stress corrosion cracking conditions. The research object is a single crack developed from an initial scratch in the X70 steel material of the oil and gas pipeline. The pipe wall thickness is negligible compared to the length, and the single crack is equivalent to a line in an infinitely large plate within the pipe thickness plane. This invention focuses on the temporal and spatial variation of the crack tip position, which is one-dimensional in space. Furthermore, it uses a difference method instead of higher-order derivative calculations, thus being semi-analytical. This invention analyzes the influence of stress gradient on ion transport near the crack tip using a one-dimensional semi-analytical model, simulating the mechanism of repeated rupture and reconstruction of the passivation layer film. Because only one parameter needs calibration, this invention can accurately predict stress corrosion cracking-related parameters that are difficult to measure experimentally within seconds, such as the crack initiation latency under various mechanical and chemical environments; this is of great significance for ensuring the safe operation and maintenance of oil and gas pipelines.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0009] A method for predicting the service behavior of oil and gas pipelines under stress corrosion cracking conditions, specifically a method for predicting the service behavior of oil and gas pipelines under stress corrosion cracking conditions for extended periods. The prediction method includes the following steps:

[0010] The first step involves deriving the convection-diffusion equation for stress corrosion cracking using fluid mechanics principles. The stress distribution is then determined by using the actual stress intensity factor at the crack tip in the oil and gas pipeline as an unknown. Boundary conditions are applied, along with crack propagation criteria from fracture mechanics, to preliminarily calculate the crack propagation behavior characteristics of a one-dimensional semi-analytical model. Specifically:

[0011] Step 1.1: Combining the law of conservation of mass in fluid mechanics and the definition of flux in stress corrosion cracking, establish the convection-diffusion equation as the governing equation of the one-dimensional semi-analytical model of stress corrosion cracking.

[0012] First, we introduce the law of conservation of mass and the definition of flux during stress corrosion cracking:

[0013] (1)

[0014] (2)

[0015] in, t is the ion concentration; t is time; j is the flux. For spatial location, Indicates the location of the crack tip. Indicates the passivation layer thickness. The thickness of the oil and gas pipeline material is represented by D, where D is the diffusion coefficient of the X70 steel material used in the oil and gas pipeline. Boltzmann's constant, For volumetric release rate, Indicates temperature. This represents the hydrostatic pressure at the crack tip.

[0016] Let c be the percentage ion concentration per unit thickness. Initially, c=1, and then c gradually decreases as stress corrosion cracking progresses. When c=0, it indicates that the oil and gas pipeline material per unit thickness loses its ability to resist stress corrosion cracking, forming new cracks. Combining formulas (1) and (2), we obtain the information regarding ion concentration. The convection-diffusion equation is used as the governing equation for the one-dimensional semi-analytical model of stress corrosion cracking; as shown in equation (3):

[0017] (3)

[0018] Step 1.2: The one-dimensional semi-analytical model of stress corrosion cracking is uniformly discretized in one-dimensional space, and the stress is represented according to the segmented discretized mesh. The stress distribution in the stress corrosion cracking model is divided into an elastic region, a plastic region, and a stress singularity region at the crack tip, as detailed below:

[0019] First, Erwin's small-range plastic yielding theory is adopted, in the plastic region... Define the stress distribution; then in the elastic region far from the crack tip. Using Saint-Venant's principle, stress is equated to the applied load; however, in the stress singularity region at the crack tip... The stress distribution in the plastic region is linearly extended; it is important to note here that errors generated during the stress approximation process can be offset by subsequent model parameter calibration. The specific piecewise stress representation is as follows:

[0020] (4)

[0021] in, This indicates the stress distribution inside the oil and gas pipeline; The true stress intensity factor at the crack tip; This indicates the load that the pipeline bears;

[0022] Step 1.3, define the boundary conditions for the one-dimensional semi-analytical model of stress corrosion cracking, as shown in formulas (5)-(7):

[0023] Define the initial crack length in a one-dimensional semi-analytical model of stress corrosion cracking. :

[0024] (5)

[0025] in, Indicates the location where the crack tip begins;

[0026] At the free end of the oil and gas pipeline away from the crack tip Set the boundary condition to 0 flux:

[0027] (6)

[0028] in, This indicates the flow rate at the free end of an oil and gas pipeline.

[0029] At the crack tip, where a passivation layer is formed due to corrosion, propagation is suppressed, thus only the convection term remains.

[0030] (7)

[0031] in, This indicates the flux at the passivation layer inside the oil and gas pipeline;

[0032] After determining the boundary conditions of the one-dimensional semi-analytical model, As an unknown quantity, the actual stress intensity factor at the crack tip is solved by using the stress balance equation of the overall one-dimensional semi-analytical model shown in formula (8).

[0033] (8)

[0034] in, This indicates the stress inside an oil and gas pipeline. This indicates the load borne by the oil and gas pipeline;

[0035] Step 1.4: Compare the actual stress intensity factor at the crack tip with the stress intensity factor threshold of the passivation layer. If the actual stress intensity factor at the crack tip is greater than the stress intensity factor threshold of the passivation layer, the crack advances. As shown in formula (9):

[0036] (9)

[0037] in, The true stress intensity factor at the crack tip; The stress intensity factor threshold represents the stress intensity factor threshold of the passivation layer; the stress intensity factor threshold is an inherent property of the generated passivation layer.

[0038] Step 1.5 repeats steps 1.2 to 1.4 to obtain preliminary calculations of crack propagation behavior characteristics. However, the true diffusion coefficient of the material in a specific environment is not determined and needs to be calibrated with experimental data from the crack propagation calculations in step 2.

[0039] The second step, based on the crack propagation behavior characteristics obtained in the first step, involves calculating the actual full-life service time information of the oil and gas pipeline material by calibrating the diffusion coefficient D of specific material parameters in the one-dimensional semi-analytical model. The diffusion coefficient D of these specific material parameters incorporates the corrosive environment and the influence of specific materials on crack propagation. Specifically:

[0040] Step 2.1: First, time, space, and stress are treated as dimensionless. Then, based on the dimensionless time, space, and stress, the stress intensity factor is treated as dimensionless to obtain dimensionless parameters. These dimensionless parameters include dimensionless spatial variables. Dimensionless time variables Dimensionless stress variables Specifically:

[0041] First, the spatial, temporal, and stress variables are treated as dimensionless variables:

[0042] (10)

[0043] in, It is a dimensionless spatial variable. It is spatial location. It refers to the thickness of the oil and gas pipeline.

[0044] Similarly, the dimensionless forms of time and stress are obtained as follows:

[0045] (11)

[0046] in, It is a time variable; It is a dimensionless time variable;

[0047] (12)

[0048] in, It is a dimensionless stress variable;

[0049] dimensionless spatial variables Dimensionless time variables Dimensionless stress variables Substituting into the convection-diffusion equation, we obtain formula (13):

[0050] (13)

[0051] In addition, the stress field is treated dimensionlessly:

[0052] (14)

[0053] in, This represents the dimensionless crack tip stress intensity factor.

[0054] Thus, all the dimensionless governing equations are obtained: Equation (9), Equation (13), and Equation (14).

[0055] Step 2.2, the dimensionless spatial variables obtained in Step 2.1 Dimensionless time variables Dimensionless stress variables Substituting the data into the one-dimensional semi-analytical model of stress corrosion cracking in the first step, the dimensionless crack propagation behavior characteristics are obtained. Then, the calculated dimensionless crack propagation rate is calibrated with experimental data to obtain the true diffusion coefficient of oil and gas pipeline materials under specific conditions. In the experimental data, experimental scenario T1 is set as the standard operating condition as a control experiment, and other experimental scenarios are set as pH acidification, pH alkalization, or chloride concentration [Cl...]. - The experimental scenario T1 was: pH = 6.8, chlorine concentration [Cl] increased. - =0.004mol / L.

[0056] Step 2.2.1: To reflect the stress corrosion cracking characteristics of X70 steel in oil and gas pipelines under real conditions and the influence of the corrosion environment on stress corrosion cracking, the key parameter D of the one-dimensional semi-analytical model of stress corrosion cracking is calibrated to enable the one-dimensional semi-analytical model to more accurately predict the full service life of X70 steel; at the same time, the parameter calibration of the one-dimensional semi-analytical model offsets the error caused by the stress representation in step 1.2. In dimensionless form, the crack propagation rate of the material is only related to the diffusion coefficient D, and other quantities remain unchanged, as shown in formula (15).

[0057] (15)

[0058] in, This represents the crack propagation rate, obtained through experimental measurement. This represents the dimensionless crack propagation rate, calculated using a dimensionless one-dimensional semi-analytical model.

[0059] By transforming formula (15), we obtain formula (16), which allows us to calculate the actual D of the oil and gas pipeline.

[0060] (16)

[0061] Step 2.2.2: In order to take into account the influence of specific factors in a corrosive environment (such as pH and chlorine concentration), the relationship between the key parameter D and pH and chlorine concentration is explored from the perspective of thermodynamic energy.

[0062] First, the diffusion coefficient is defined from the perspective of thermodynamic energy:

[0063] (17)

[0064] in, It is the total energy of the stress corrosion cracking model; =1.23×10 -23 , is the Boltzmann constant; It represents the system temperature; exp represents exponential operation. It is a constant.

[0065] In the stress corrosion cracking process, in addition to the energy inherent in the fracture model In addition, this invention also considers the pH value and [Cl] in the solution environment. - The chemical energy possessed by (chlorine concentration):

[0066] (18)

[0067] in, This represents the stress corrosion cracking energy provided by pH; [Cl] - The stress corrosion cracking energy provided;

[0068] Combining formulas (17) and (18), we obtain formulas (19) and (20):

[0069] (19)

[0070] (20)

[0071] in, This represents the diffusion coefficient inherent in the fracture model;

[0072] In the dimensionless form of the one-dimensional semi-analytical model, the relationship between crack propagation rate and diffusion coefficient is obtained, as shown in formula (15). By comparing any two experimental scenarios with formula (15), it is found that during stress corrosion cracking, the crack propagation rate increases with pH value and [Cl...]. - The relationship between the changes is shown in Equations (21) to (24). Although any two cases have different crack propagation rates, their dimensionless crack propagation rate values ​​should be the same. The difference in diffusion rate is only due to the difference in diffusion coefficient caused by the different corrosive environments.

[0073] Step 2.2.3, according to formula (15), the ratio of the diffusion coefficients for the second to tenth working conditions to that for the first working condition should be the ratio of the corresponding crack propagation rates:

[0074] (twenty one)

[0075] in, This represents the diffusion coefficient for the second to tenth operating conditions; This represents the diffusion coefficient for the first operating condition (standard operating condition); This indicates the crack propagation rate for the second to tenth operating conditions; This represents the crack propagation rate under the first operating condition;

[0076] Substitute formula (21) into formula (20):

[0077] (twenty two)

[0078] in, This indicates the crack propagation rate for the second to sixth operating conditions; This represents the diffusion coefficient for the second to sixth operating conditions; This represents the stress corrosion cracking energy provided by pH under the second to sixth operating conditions;

[0079] That is to say:

[0080] (twenty three)

[0081] Similarly, for [Cl] - The formula is:

[0082] (twenty four)

[0083] [Cl] represents the seventh to tenth operating conditions. - The stress corrosion cracking energy provided This indicates the crack propagation rate for conditions seven through ten.

[0084] And for It can be calculated using the T1 standard working condition.

[0085] (25)

[0086] For formula (25) This invention selects the slope of the crack propagation curve at the beginning of crack propagation to calibrate with experimental data. Since the passivation layer is not significantly damaged at this stage, the ion penetration rate is stable, which is consistent with the measurement conditions of the 'initial crack propagation rate' in the experiment (the passivation layer is intact), thus avoiding the influence of rate fluctuations in the stable crack propagation stage (the passivation layer repeatedly breaks and reconstructs).

[0087] Finally, the true diffusion coefficient, including D, is obtained through step 2.2. .

[0088] Step 2.3: Substitute the actual diffusion coefficient obtained in Step 2.2 into the one-dimensional semi-analytical model of stress corrosion cracking in Step 1 to preliminarily calculate the behavioral characteristics of oil and gas pipeline materials; then, by converting dimensionless quantities into dimensionless quantities, the precise full-life service time of oil and gas pipeline materials is finally obtained.

[0089] The beneficial effects of this invention are:

[0090] (1) The method of the present invention uses the knowledge of fluid mechanics and fracture mechanics to establish a one-dimensional semi-analytical model. After calibration, the maximum error between the one-dimensional semi-analytical model and the real experimental data is about 10%.

[0091] (2) Compared with conventional experimental research, the use of difference form instead of higher-order derivative calculation can reduce time and cost;

[0092] (3) This invention uses Erwin’s small-range plastic yielding theory to approximate the stress calculation, which can quickly obtain the full life behavior information of stress corrosion cracking without the need to solve partial differential equations.

[0093] (4) Based on fracture mechanics, it can reproduce the mechanism of repeated rupture and reconstruction of the passivation layer during the fracture process;

[0094] (5) The stress gradient at the crack tip is considered to have a significant effect on ion transport; the unified model parameter calibration method can be applied to other situations.

[0095] In summary, the one-dimensional semi-analytical model of stress corrosion cracking established in this invention can effectively predict the full service life of oil and gas pipelines, which is of great significance for ensuring energy security and economic benefits. Attached Figure Description

[0096] Figure 1 The key mechanism of stress corrosion cracking in X70 steel pipelines;

[0097] Figure 2 This provides dimensional information for standard CT specimens and simplifies the setting of a one-dimensional semi-analytical model for stress corrosion cracking; Figure 2 (a) in the figure is a standard compact tensile specimen drawing; Figure 2 (b) in the figure is a side view of the compact tensile specimen;

[0098] Figure 3 This is a physical model diagram of the present invention established with reference to a compact tensile specimen;

[0099] Figure 4 An example of using a one-dimensional semi-analytical model to predict the evolution of stress corrosion cracking throughout its entire life cycle;

[0100] Figure 5A schematic diagram of parameter calibration for a stepped curve model of crack propagation;

[0101] Figure 6 The relationship between stress corrosion crack propagation and time;

[0102] Figure 7 The crack initiation time of pipeline X70 steel is different [Cl - The relationship between stress and stress;

[0103] Figure 8 This is a prediction result for the full-life behavior of stress corrosion cracking;

[0104] Figure 9 The relationship between stress corrosion crack initiation time and various factors;

[0105] Figure 10 Error diagram of stress corrosion cracking time for X70 specimens under the condition that the influencing factors do not exceed 5%; Figure 10 (a) describes the effect of stress on the total service life under different pH conditions; Figure 10 (b) describes the effect of stress on the total service life under different chlorine concentrations; Figure 10 (c) describes the effect of stress on the total service life under different crack length conditions; Figure 10 (d) describes the effect of stress on the full service life of materials with different passivation layer thicknesses. Detailed Implementation

[0106] The present invention will be further described below with reference to specific implementation examples.

[0107] This embodiment provides a method for predicting the service behavior of oil and gas pipelines under stress corrosion cracking conditions. The research object is a single crack developed from an initial scratch in the X70 steel material of the oil and gas pipeline. The pipe wall thickness is negligible compared to the length, and the single crack is equivalent to a line in an infinitely large plate within the pipe thickness plane. This invention focuses on the relationship between the position of the crack tip and the changes in time and space, which is one-dimensional in space. Moreover, it uses a difference method instead of higher-order derivative calculations, thus it is semi-analytical. As shown below:

[0108] X70 steel for oil and gas pipelines under tensile stress is placed in a corrosive solution environment. Due to processing or other factors, there are tiny scratches on the surface, which then develop into cracks. The sharp stress gradient at the crack tip intensifies the penetration of metal cations. At the crack tip, due to chemical reactions, a passivation layer is formed on the material surface to slow down the penetration of ions. At the same time, some factors in the corrosive solution, such as pH and chloride ion concentration, also accelerate the penetration of metal cations through the passivation layer into the corrosive environment.

[0109] The model established in this embodiment is one-dimensional and semi-analytical, such as... Figure 1 As shown, a crack is equivalent to a line in an infinitely large plate, and the surface of a pipe with scratches is filled with a corrosive solution. Driven by the composition of the corrosive solution (pH and chlorine concentration) and applied stress, metal ions (iron ions) inside the material will penetrate into the corrosive environment under the sharp hydrostatic pressure gradient at the crack tip, leading to material corrosion and damage. At the same time, due to corrosion, a passivation film will form at the crack tip to inhibit the penetration of metal ions. Our one-dimensional semi-analytical model of stress corrosion cracking ignores the effect of thickness, and all the fields involved are functions of position x and time t. In the calculation model, the left side of the model is the crack tip, which is in contact with the corrosive environment and the surface material forms a passivation film; the right side is the free end.

[0110] The calculation model in this embodiment is a reference. Figure 2 The test specimens were established based on experimental CT scans, and the specimens were samples taken from the cross-section of oil and gas pipelines. Figure 2 (a) in the figure is a schematic diagram of the dimensions of an X70 steel specimen for an oil and gas pipeline. Figure 2 (b) in the figure is a side view of the specimen. Figure 3 This is a schematic diagram of a one-dimensional semi-analytical model derived from the evolution of the specimen; Figure 2 The lengths in (a) are as follows: L1=23 mm, L2=56 mm, L3=70 mm, L4=52 mm, L5=4 mm, L6=10 mm, L7=10 mm; where L1 represents the distance from the tensile position of the specimen to the initial crack tip; L2 represents the position from the tensile position of the specimen to the free end of the specimen; the one-dimensional semi-analytical model requires the above two length parameters for calculation, and other length parameters conform to the design specifications of standard CT specimens.

[0111] The prediction method provided in this embodiment includes the following steps:

[0112] The first step involves deriving the convection-diffusion equation for stress corrosion cracking using fluid mechanics principles. The stress distribution is then determined by using the actual stress intensity factor at the crack tip in the oil and gas pipeline as an unknown. Boundary conditions are applied, along with crack propagation criteria from fracture mechanics, to preliminarily calculate the crack propagation behavior characteristics of a one-dimensional semi-analytical model. Specifically:

[0113] Step 1.1: Combining the law of conservation of mass in fluid mechanics and the definition of flux in stress corrosion cracking, establish the convection-diffusion equation as the governing equation of the one-dimensional semi-analytical model of stress corrosion cracking.

[0114] First, the law of conservation of mass and the definition of flux during stress corrosion cracking are introduced, as shown in formulas (1) and (2). c is set as the percentage ion concentration per unit thickness. Initially, c = 1, and then c gradually decreases as stress corrosion cracking progresses. When c = 0, it indicates that the oil and gas pipeline material per unit thickness loses its ability to resist stress corrosion cracking, forming new cracks. Combining formulas (1) and (2), the following equations are obtained regarding ion concentration. The convection-diffusion equation is used as the governing equation of the one-dimensional semi-analytical model of stress corrosion cracking, as shown in formula (3).

[0115] Step 1.2: The one-dimensional semi-analytical model of stress corrosion cracking is uniformly discretized in one-dimensional space, and the stress is represented according to the segmented discretized mesh. The stress distribution in the stress corrosion cracking model is divided into an elastic region, a plastic region, and a stress singularity region at the crack tip, as detailed below:

[0116] First, Erwin's small-range plastic yielding theory is adopted, in the plastic region... Define the stress distribution; then in the elastic region far from the crack tip. Using Saint-Venant's principle, stress is equated to the applied load; however, in the stress singularity region at the crack tip... The stress distribution in the plastic zone is linearly extended; it is important to note here that the errors generated during the stress approximation process can be offset by subsequent model parameter calibration. The specific segmented stress is represented by formula (4).

[0117] Step 1.3 defines the boundary conditions for the one-dimensional semi-analytical model of stress corrosion cracking, as shown below:

[0118] Define the initial crack length in a one-dimensional semi-analytical model of stress corrosion cracking. As shown in formula (5);

[0119] At the free end of the oil and gas pipeline away from the crack tip Set the boundary condition to 0 flux, as shown in formula (6);

[0120] At the crack tip, where the passivation layer is formed due to corrosion, the propagation is suppressed, so only the convection term is retained, as shown in formula (7);

[0121] After determining the boundary conditions of the one-dimensional semi-analytical model, Using the stress balance equation of the one-dimensional semi-analytical model as shown in formula (8), the true stress intensity factor at the crack tip is solved as stress corrosion cracking progresses.

[0122] Step 1.4: Compare the actual stress intensity factor at the crack tip with the stress intensity factor threshold of the passivation layer. If the actual stress intensity factor at the crack tip is greater than the stress intensity factor threshold of the passivation layer, the crack will advance forward, as shown in formula (9).

[0123] Step 1.5 repeats steps 1.2 to 1.4 to obtain preliminary calculations of crack propagation behavior characteristics. However, the true diffusion coefficient of the material in a specific environment is not determined and needs to be calibrated with experimental data from the crack propagation calculations in step 2.

[0124] Figure 4 This paper demonstrates a complete stress corrosion cracking process of X70 steel in an oil and gas pipeline, providing a one-dimensional semi-analytical model to predict the life-cycle stress corrosion cracking evolution and presenting a three-stage evolution process. Specifically: In the first stage, crack initiation occurs as stress corrosion begins. Material damage is minimal, and the actual stress intensity factor at the crack tip is low, insufficient to exceed the stress intensity factor threshold of the passivation layer. The crack hardly advances in this stage. As stress corrosion progresses, material damage intensifies. Once the actual stress intensity factor at the crack tip exceeds the stress intensity factor threshold of the passivation layer, the crack propagates, entering the second stage. After crack propagation, based on the overall material stress balance, the stress intensity factor at the crack tip decreases, slightly falling below the stress intensity factor threshold of the passivation layer. Continued stress corrosion cracking is required until the material damage reaches a certain level, at which point the crack repeats the above steps to propagate. Therefore, the stress corrosion cracking propagation exhibits a step-like curve, as shown in the diagram. Figure 5 As shown, once stress corrosion cracking reaches a certain level, the X70 steel material of the oil and gas pipeline completely loses its ability to withstand stress corrosion cracking, enters the third stage, and undergoes mechanical failure. Figure 5 The diagram describes a stepped curve for crack propagation, and the slope of the dashed line should correspond to the crack propagation rate when the crack is just activated and then continues to propagate. Figure 6 The image shows the internal damage of the material during stress corrosion cracking, with blue indicating complete destruction and yellow indicating intactness; the transition area in the middle represents damage of different types occurring.

[0125] The second step involves calculating the actual service life information of oil and gas pipeline materials based on the crack propagation behavior characteristics obtained in the first step, through calibration calculation of the diffusion coefficient D of specific material parameters in the one-dimensional semi-analytical model. The diffusion coefficient D of the specific material parameters includes the influence of corrosive environment and specific materials on crack propagation.

[0126] Step 2.1: First, time, space, and stress are treated as dimensionless. Then, based on the dimensionless time, space, and stress, the stress intensity factor is treated as dimensionless to obtain dimensionless parameters. These dimensionless parameters include dimensionless spatial variables. Dimensionless time variables Dimensionless stress variables Specifically:

[0127] First, the spatial variables, time variables, and stress variables are dimensionless, as shown in formula (10); similarly, the dimensionless forms of time and stress are obtained as shown in formulas (11) and (12).

[0128] dimensionless spatial variables Dimensionless time variables Dimensionless stress variables Substituting into the convection-diffusion equation, we get formula (13); in addition, the stress field is dimensionless, as shown in formula (10).

[0129] Thus, all the dimensionless governing equations are obtained: Equation (9), Equation (13), and Equation (14).

[0130] Step 2.2, the dimensionless spatial variables obtained in Step 2.1 Dimensionless time variables Dimensionless stress variables Substituting the data into the one-dimensional semi-analytical model of stress corrosion cracking in the first step, the dimensionless crack propagation behavior characteristics are obtained. Then, the calculated dimensionless crack propagation rate is calibrated with experimental data to obtain the true diffusion coefficient of oil and gas pipeline materials under specific conditions. In the experimental data, experimental scenario T1 is set as the standard operating condition as a control experiment, and other experimental scenarios are set as pH acidification, pH alkalization, or chloride concentration [Cl...]. - The experimental scenario T1 was: pH = 6.8, chlorine concentration [Cl] increased. - =0.004mol / L.

[0131] Step 2.2.1: To reflect the stress corrosion cracking characteristics of oil and gas pipeline materials under real conditions and the influence of the corrosion environment on stress corrosion cracking, the key parameter D of the one-dimensional semi-analytical model of stress corrosion cracking is calibrated to enable the one-dimensional semi-analytical model to more accurately predict the full service life of X70 steel; at the same time, the parameter calibration of the one-dimensional semi-analytical model offsets the error caused by the stress representation in step 1.2. In dimensionless form, the crack propagation rate of the material is only related to the diffusion coefficient D, while other quantities remain unchanged, as shown in formula (15).

[0132] By transforming formula (15), we obtain formula (16), and then calculate the actual D of the oil and gas pipeline.

[0133] Step 2.2.2: In order to take into account the influence of specific factors in a corrosive environment (such as pH and chlorine concentration), the relationship between the key parameter D and pH and chlorine concentration is explored from the perspective of thermodynamic energy.

[0134] First, the diffusion coefficient is defined from the perspective of thermodynamic energy, as shown in formula (17). In the stress corrosion cracking process, besides the energy inherent in the fracture model... In addition, the pH value and [Cl] in the solution environment were also considered. - The chemical energy possessed by (chlorine concentration) is shown in formula (18). Formulas (19) and (20) are then obtained.

[0135] In the dimensionless form of the one-dimensional semi-analytical model, the relationship between crack propagation rate and diffusion coefficient is obtained, as shown in formula (15). By comparing any two cases with formula (15), it is found that during stress corrosion cracking, crack propagation rate increases with pH value and [Cl]. - The relationship between the changes is shown in Equations (21) to (24). Although any two cases have different crack propagation rates, their dimensionless crack propagation rate values ​​should be the same. The difference in diffusion rate is only due to the difference in diffusion coefficient caused by the different corrosive environments.

[0136] Table 1: Crack propagation rate data of X70 steel for oil and gas pipelines under different pH and [Cl⁻] conditions.

[0137] Experimental scenario pH [Cl⁻] concentration (mol / L) Crack propagation rate (mm / s) T1 6.8 0.004 <![CDATA[3.19×10⁻ 8 <!-- 10 -->]]> T2 6.0 0.004 <![CDATA[4.35×10⁻ 8 ]]> T3 5.5 0.004 <![CDATA[4.5×10⁻ 8 ]]> T4 5.0 0.004 <![CDATA[5.51×10⁻ 8 ]]> T5 4.5 0.004 <![CDATA[7.81×10⁻ 8 ]]> T6 4.0 0.004 <![CDATA[1.24×10⁻ 7 ]]> T7 6.8 0.04 <![CDATA[9.58×10⁻ 8 ]]> T8 6.8 0.4 <![CDATA[1.48×10⁻ 7 ]]> T9 6.8 1 <![CDATA[1.44×10⁻ 7 ]]> T10 6.8 3 <![CDATA[1.46×10⁻ 7 ]]>

[0138] Step 2.2.3, according to formula (15), the ratio of the diffusion coefficient of the second to the tenth working conditions to that of the first working condition should be the ratio of the corresponding crack propagation rate, as shown in formula (21);

[0139] Substituting formula (20) into formula (19), we obtain formulas (22) and (23); similarly, for [Cl - ], thus obtaining formula (24); and for The solution is obtained through the T1 standard working condition, as shown in formula (25). The final true diffusion coefficient is then obtained, including D, .

[0140] Step 2.3: Substitute the actual diffusion coefficient obtained in Step 2.2 into the one-dimensional semi-analytical model of stress corrosion cracking in Step 1 to preliminarily calculate the behavioral characteristics of oil and gas pipeline materials; then, by converting dimensionless quantities into dimensionless quantities, the precise full-life service time of oil and gas pipeline materials is finally obtained.

[0141] Figure 7 The text shows different [Cl] - Under certain conditions, the service life of stress corrosion cracking decreases with increasing stress; and [Cl] - The impact of stress corrosion cracking on the entire service life is not a monotonically increasing or decreasing phenomenon; in [Cl] - At low concentrations, the service life after stress corrosion cracking decreases with increasing concentration; however, after the concentration reaches a certain level, the service life after stress corrosion cracking increases with increasing concentration. - Excessive concentration inhibits ion penetration and slows down the stress corrosion cracking process.

[0142] Figure 8 The figure shows the time information in the stress corrosion cracking process: as can be seen from the figure, the crack initiation time accounts for most of the stress corrosion cracking, the crack propagation time accounts for a smaller proportion, and then it enters the mechanical failure stage.

[0143] Figure 9 The figure shows the impact of increasing the load by 10% in various scenarios relative to the standard operating condition; as can be seen from the figure, besides Increasing the passivation layer stress intensity factor increases the service life under stress corrosion cracking, while increasing other factors decreases the service life under stress corrosion cracking. This indicates that... Increasing the stress will help improve the material's overall service life; while stress, [Cl] - Increased stress and pH acidification or alkalization are detrimental to the stress corrosion cracking lifespan of materials; stress, in particular, has the most significant impact on the stress corrosion cracking lifespan of materials.

[0144] Figure 10 The text shows different factors (pH, [Cl]). - The impact of a 5% fluctuation in crack length and passivation layer thickness on the service life of stress corrosion cracking. Figure 10 (a) describes the effect of stress on the service life of stress corrosion cracking under different pH conditions; Figure 10 (b) describes different [Cl] - The impact of stress on the service life of stress corrosion cracking under certain environmental conditions; Figure 10 (c) describes the effect of stress on the service life of stress corrosion cracking under different crack length conditions; Figure 10 (d) describes the effect of stress on the stress corrosion cracking full-life service time of materials with different passivation layer thicknesses.

[0145] Table 2: Comparison of Simulation Results and Experimental Data

[0146] Experiment number Experimental crack propagation rate (mm / s) Simulated crack propagation rate (mm / s) error(%) T1 <![CDATA[3.19×10⁻ 8 ]]> <![CDATA[3.34×10⁻ 8 ]]> 4.65 T2 <![CDATA[4.35×10⁻ 8 ]]> <![CDATA[3.84×10⁻ 8 ]]> 11.68 T3 <![CDATA[4.5×10⁻ 8 ]]> <![CDATA[4.59×10⁻ 8 ]]> 1.99 T4 <![CDATA[5.51×10⁻ 8 ]]> <![CDATA[5.88×10⁻ 8 ]]> 6.66 T5 <![CDATA[7.81×10⁻ 8 ]]> <![CDATA[8.06×10⁻ 8 ]]> 3.26 T6 <![CDATA[1.24×10⁻ 7 ]]> <![CDATA[1.19×10⁻ 7 ]]> 4.34 T7 <![CDATA[9.58×10⁻ 8 ]]> <![CDATA[9.21×10⁻ 8 ]]> 3.85 T8 <![CDATA[1.48×10⁻ 7 ]]> <![CDATA[1.48×10⁻ 7 ]]> 0.12 T9 <![CDATA[1.44×10⁻ 7 ]]> <![CDATA[1.53×10⁻ 7 ]]> 6.09 T10 <![CDATA[1.46×10⁻ 7 ]]> <![CDATA[1.41×10⁻ 7 ]]> 3.65

[0147] The above embodiments are merely illustrative of the implementation methods of the present invention, but should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the protection scope of the present invention.

Claims

1. A method for predicting the service behavior of oil and gas pipelines under stress corrosion cracking conditions, characterized in that, The prediction method Includes the following steps: The first step is to derive the convection-diffusion equation for stress corrosion cracking by combining knowledge of fluid mechanics; Using the actual stress intensity factor at the crack tip of an oil and gas pipeline as an unknown, the stress distribution is obtained; by applying boundary conditions and crack propagation criteria in fracture mechanics, the crack propagation behavior characteristics of a one-dimensional semi-analytical model are preliminarily calculated. Specifically: Step 1.1: Establish the convection-diffusion equation as the governing equation for the one-dimensional semi-analytical model of stress corrosion cracking; Step 1.2: The one-dimensional semi-analytical model of stress corrosion cracking is uniformly discretized in one-dimensional space, and stress is represented according to the segmented discretized mesh. The stress distribution in the stress corrosion cracking model is divided into an elastic zone, a plastic zone, and a stress singularity zone at the crack tip. Step 1.3: Define the boundary conditions for the one-dimensional semi-analytical model of stress corrosion cracking; Step 1.4: Compare the actual stress intensity factor at the crack tip with the stress intensity factor threshold of the passivation layer. If the actual stress intensity factor at the crack tip is greater than the stress intensity factor threshold of the passivation layer, the crack advances forward. Step 1.5: Repeat steps 1.2 to 1.4 to obtain preliminary calculations of the crack propagation behavior characteristics; The second step, based on the crack propagation behavior characteristics obtained in the first step, involves calculating the actual service life information of the oil and gas pipeline material by calibrating the diffusion coefficient D of the material parameters in the one-dimensional semi-analytical model. The diffusion coefficient D of the material parameters includes the influence of the corrosive environment and specific materials on crack propagation. Specifically: Step 2.1: First, time, space, and stress are treated as dimensionless. Then, based on the dimensionless time, space, and stress, the stress intensity factor is treated as dimensionless to obtain dimensionless parameters. These dimensionless parameters include dimensionless spatial variables. Dimensionless time variables Dimensionless stress variables ; Step 2.2, the dimensionless spatial variables obtained in Step 2.1 Dimensionless time variables Dimensionless stress variables Substituting the data into the one-dimensional semi-analytical model of stress corrosion cracking in the first step, the dimensionless crack propagation behavior characteristics are obtained. Then, the calculated dimensionless crack propagation rate is calibrated with the experimental data to obtain the true diffusion coefficient of the oil and gas pipeline material. Step 2.3: Substitute the actual diffusion coefficient obtained in Step 2.2 into the one-dimensional semi-analytical model of stress corrosion cracking in Step 1 to preliminarily calculate the behavioral characteristics of oil and gas pipeline materials; then, by converting dimensionless quantities into dimensionless quantities, the accurate full-life service time of oil and gas pipeline materials is obtained.

2. The method for predicting the service behavior of oil and gas pipelines under stress corrosion cracking conditions according to claim 1, characterized in that, Step 1.1 is as follows: First, we introduce the law of conservation of mass and the definition of flux during stress corrosion cracking: （1） （2） in, t is the ion concentration; t is time; j is the flux. For spatial location, Indicates the location of the crack tip. Indicates the passivation layer thickness. The thickness of the oil and gas pipeline material is represented by D, where D is the diffusion coefficient of the X70 steel material used in the oil and gas pipeline. Boltzmann's constant, For volumetric release rate, Indicates temperature. The hydrostatic pressure at the crack tip; Let c be the percentage ion concentration per unit thickness. Initially, c=1, and c gradually decreases as stress corrosion cracking progresses. When c=0, it indicates that the oil and gas pipeline material per unit thickness loses its ability to resist stress corrosion cracking and forms new cracks. Combining formulas (1) and (2), we obtain the information regarding ion concentration. The convection-diffusion equation is used as the governing equation for the one-dimensional semi-analytical model of stress corrosion cracking; as shown in equation (3): （3）。 3. The method for predicting the service behavior of oil and gas pipelines under stress corrosion cracking conditions according to claim 2, characterized in that, Step 1.2 is as follows: First, Erwin's small-range plastic yielding theory is adopted, in the plastic region... Define the stress distribution; then in the elastic region far from the crack tip. Using Saint-Venant's principle, stress is equated to the applied load; however, in the stress singularity region at the crack tip... The stress distribution in the plastic region is linearly extended; the specific piecewise stress representation is as follows: （4） in, This indicates the stress distribution inside the oil and gas pipeline; The true stress intensity factor at the crack tip; This indicates the load that the pipeline bears.

4. The method for predicting the service behavior of oil and gas pipelines under stress corrosion cracking conditions according to claim 3, characterized in that, Step 1.3 is as follows: Define the initial crack length in a one-dimensional semi-analytical model of stress corrosion cracking. : （5） in, Indicates the location where the crack tip begins; At the free end of the oil and gas pipeline away from the crack tip Set the boundary condition to 0 flux: （6） in, This indicates the flow rate at the free end of an oil and gas pipeline. At the crack tip, only the convection term is retained: （7） in, This indicates the flux at the passivation layer inside the oil and gas pipeline; After determining the boundary conditions of the one-dimensional semi-analytical model, As an unknown quantity, the stress balance equation of the whole one-dimensional semi-analytical model shown in formula (8) is used to solve the true stress intensity factor at the crack tip as stress corrosion cracking proceeds. （8） in, This indicates the stress inside an oil and gas pipeline. This indicates the load borne by the oil and gas pipeline.

5. The method for predicting the service behavior of oil and gas pipelines under stress corrosion cracking conditions according to claim 4, characterized in that, In step 1.4, the formula for comparing the actual stress intensity factor at the crack tip with the stress intensity factor threshold of the passivation layer is as follows: （9） in, The true stress intensity factor at the crack tip; This represents the stress intensity factor threshold of the passivation layer; The stress intensity factor threshold is an inherent property of the generated passivation layer.

6. The method for predicting the service behavior of oil and gas pipelines under stress corrosion cracking conditions according to claim 5, characterized in that, Step 2.1 specifically refers to: First, the spatial, temporal, and stress variables are treated as dimensionless variables: （10） in, It is a dimensionless spatial variable. It is spatial location. It refers to the thickness of the oil and gas pipeline; Similarly, the dimensionless forms of time and stress are obtained as follows: （11） in, It is a time variable; It is a dimensionless time variable; （12） in, It is a dimensionless stress variable; dimensionless spatial variables Dimensionless time variables Dimensionless stress variables Substituting into the convection-diffusion equation, we obtain formula (13): （13） The stress field is then treated dimensionlessly: （14） in, This represents the dimensionless crack tip stress intensity factor. Thus, we obtain all the dimensionless governing equations: Equation (9), Equation (13), and Equation (14).

7. The method for predicting the service behavior of oil and gas pipelines under stress corrosion cracking conditions according to claim 6, characterized in that, In step 2.2: Experimental scenario T1 was set as the standard operating condition, serving as a control experiment. Other experimental scenarios were set to pH acidification, pH alkalization, or chlorine concentration [Cl]. - The concentration of chlorine [Cl] increased; Experimental scenario T1 was: pH=6.8, chlorine concentration [Cl] - =0.004 mol / L; Step 2.2.1: By calibrating the key parameter D of the one-dimensional semi-analytical model of stress corrosion cracking, the one-dimensional semi-analytical model can more accurately predict the full service life of X70 steel; at the same time, by calibrating the parameters of the one-dimensional semi-analytical model, the error caused by the stress representation in step 1.2 is offset; in dimensionless form, the crack propagation rate of the material is only related to the diffusion coefficient D, and other quantities remain unchanged, as shown in formula (15). （15） in, This represents the crack propagation rate, obtained through experimental measurement. This represents the dimensionless crack propagation rate, calculated using a dimensionless one-dimensional semi-analytical model. Transform formula (15) to obtain formula (16), and calculate the actual D of the oil and gas pipeline; （16） Step 2.2.2: Determine the relationship between the key parameter D and pH and chlorine concentration; First, define the diffusion coefficient: （17） in, It is the total energy of the stress corrosion cracking model; =1.23×10 -23 , is the Boltzmann constant; It represents the system temperature; exp represents exponential operation. It is a constant; In the stress corrosion cracking process, in addition to the energy inherent in the fracture model In addition, the pH value and [Cl] in the solution environment are also considered. - The chemical energy it possesses: （18） in, This represents the stress corrosion cracking energy provided by pH; [Cl] - The stress corrosion cracking energy provided; Combining formulas (17) and (18), we obtain formulas (19) and (20): （19） （20） in, This represents the diffusion coefficient inherent in the fracture model; In the dimensionless form of the one-dimensional semi-analytical model, the relationship between crack propagation rate and diffusion coefficient is obtained, as shown in formula (15). By comparing any two experimental scenarios with formula (15), it is found that during stress corrosion cracking, the crack propagation rate increases with pH value and [Cl...]. - The relationship between the changes of ] is shown in formula (21)-formula (24); Step 2.2.3, according to formula (15), the ratio of the diffusion coefficients for the second to tenth working conditions to that for the first working condition should be the ratio of the corresponding crack propagation rates: （21） in, This represents the diffusion coefficient for the second to tenth operating conditions; This represents the diffusion coefficient for the first operating condition; This indicates the crack propagation rate for the second to tenth operating conditions; This represents the crack propagation rate under the first operating condition; Substitute formula (21) into formula (20): （22） in, This indicates the crack propagation rate for the second to sixth operating conditions; This represents the diffusion coefficient for the second to sixth operating conditions; This represents the stress corrosion cracking energy provided by pH under the second to sixth operating conditions; That is to say: （23） Similarly, for [Cl] - The formula is: （24） [Cl] represents the seventh to tenth operating conditions. - The stress corrosion cracking energy provided This indicates the crack propagation rate for conditions seven through ten. And for The calculation is performed using the T1 standard working condition. （25） For formula (25) The slope of the crack propagation curve at the beginning of crack propagation was selected and calibrated with experimental data. Finally, the true diffusion coefficient, including D, is obtained through step 2.

2. .

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