A method for integrity assessment of high-pressure natural gas pipelines under jet fire

Abstract

The present invention relates to the technical field of pipeline material performance evaluation, and specifically, to a method for evaluating the integrity of a high-pressure natural gas pipeline under a jet fire. The method comprises the following steps: establishing a heat balance equation to calculate the jet flame temperature; calculating the net heat absorption intensity of the pipeline under the jet fire; iteratively calculating the pipeline temperature after the pipeline absorbs heat; calculating the allowable tensile strength in combination with the safety factor, and considering the fatigue accumulation effect of the pipeline material at high temperature, calculating the dynamic allowable tensile strength; calculating the principal stress, and calculating the equivalent stress of the pipeline based on the temperature gradient and the plastic deformation of the material; comparing the equivalent stress with the dynamic allowable tensile strength, and determining the rupture failure risk of the pipeline by establishing a progressive damage model of the pipeline material. The method for evaluating the integrity of a high-pressure natural gas pipeline under a jet fire introduces the dynamic fatigue accumulation effect and the progressive damage model to evaluate the strength attenuation of the pipeline material, and dynamically evaluates the pipeline rupture risk by considering the temperature gradient, plastic deformation and equivalent stress.

Description

Technical Field

[0001] The invention relates to the technical field of pipeline material performance evaluation, and in particular to a method for evaluating the integrity of a high-pressure natural gas pipeline under jet fire. Background Art

[0002] A method for integrity assessment of high-pressure natural gas pipelines under jet fire aims to improve the safety of pipelines in high-temperature environments and predict pipeline failure risks in real time. By considering the dynamic fatigue accumulation effect and temperature gradient stress of pipeline materials, the stress level and damage accumulation of materials are controlled, and dynamic and accurate failure assessment of pipelines under jet fire environments is achieved.

[0003] Existing integrity assessment methods for high-pressure natural gas pipelines under jet fires are usually difficult to effectively capture material fatigue damage and temperature gradient stress effects under high temperatures. In addition, since pipeline materials are subjected to multi-directional stress, plastic deformation and temperature difference effects in high-temperature environments, it is impossible to accurately assess pipeline rupture and failure problems in different time periods. Therefore, a method for integrity assessment of high-pressure natural gas pipelines under jet fires is provided. Summary of the invention

[0004] The purpose of the present invention is to provide a method for evaluating the integrity of a high-pressure natural gas pipeline under jet fire, so as to solve the problem raised in the above background technology that the pipeline material is subjected to multi-directional stress, plastic deformation and temperature difference effect in a high temperature environment, and it is impossible to accurately evaluate the pipeline rupture failure in different time periods.

[0005] To achieve the above object, the present invention provides a method for evaluating the integrity of a high-pressure natural gas pipeline under jet fire, comprising the following steps:

[0006] S1. Collect total heat flux and ambient temperature data, and calculate the jet flame temperature by establishing a heat balance equation;

[0007] S2. Calculate the net heat absorption intensity of the pipeline under the jet fire according to the jet flame temperature;

[0008] S3. Iteratively calculate the pipeline temperature after the pipeline absorbs heat according to the net heat absorption intensity;

[0009] S4. Calculate the allowable tensile strength based on the ultimate tensile strength of the pipeline material at different temperatures and the safety factor, and calculate the dynamic allowable tensile strength by considering the fatigue accumulation effect of the pipeline material at high temperature;

[0010] S5. Calculate the principal stress and the equivalent stress of the pipeline based on the temperature gradient and the plastic deformation of the material;

[0011] S6. Compare the equivalent stress with the dynamic allowable tensile strength, and determine the rupture failure risk of the pipeline by establishing a progressive damage model of the pipeline material.

[0012] As a further improvement of the technical solution, in S1, the total heat flux and ambient temperature data are collected, and the jet flame temperature is calculated by establishing a heat balance equation. The specific method is as follows:

[0013] S1.1. Collect total heat flux q net And ambient temperature data T ∞ ;

[0014] S1.2. Establish the heat balance equation to calculate the jet flame temperature T f :

[0015]

[0016] Among them, q total is the total heat flux; h conv is the convective heat transfer coefficient; T f is the jet flame temperature; T ∞ is the ambient temperature; σ is the Stefan-Boltzmann constant.

[0017] As a further improvement of the technical solution, in S2, the net heat absorption intensity of the pipeline under the jet fire is calculated according to the jet flame temperature, and the specific method steps are as follows:

[0018] S2. Calculate the net heat absorption intensity q of the pipeline based on the jet flame temperature net :

[0019]

[0020] Among them, q net is the net heat absorption intensity of the pipeline; a s is the pipeline absorption rate; ∈ f is the flame radiation rate under jet fire; ∈ s is the pipeline emissivity; A is the surface area of ​​the pipeline; T out is the outer wall temperature of the pipeline;

[0021] As a further improvement of the technical solution, in S3, the pipeline temperature after the pipeline absorbs heat is iteratively calculated according to the net heat absorption intensity. The specific method steps are as follows:

[0022] S3.1. Use the linear approximation formula to express the change of pipeline specific heat capacity with temperature, as follows:

[0023] C p (T) = a + bT + cT 2 ;

[0024] Among them, C p (T) is the pipeline specific heat capacity; T is the temperature;

[0025] S3.2, determine the calculation time step Δt, and according to the net heat absorption intensity q ney and pipeline specific heat capacity C p (T), calculate the pipeline temperature change ΔT after time step Δt at time t:

[0026]

[0027] Where m is the pipeline mass; ΔT is the change in pipeline temperature after a time step of Δt at time t; Δt is the time step; C p (T) is the specific heat capacity of the pipeline;

[0028] S3.3. Update the pipeline temperature T of the n+1th iteration n+1 :

[0029] T n+1 =T n +ΔT;

[0030] Among them, T n is the pipeline temperature of the nth iteration; T n+1 The pipeline temperature at the n+1th iteration;

[0031] S3.4. Repeat the iterative calculation of pipeline temperature until the pipeline rupture temperature is reached and then stop the iterative calculation.

[0032] As a further improvement of the technical solution, in S4, the allowable tensile strength is calculated based on the ultimate tensile strength of the pipeline material at different temperatures in combination with the safety factor, and the dynamic allowable tensile strength is calculated taking into account the fatigue accumulation effect of the pipeline material at high temperature. The specific method steps are as follows:

[0033] S4.1. Find the ultimate tensile strength σ of the pipeline material limit (T);

[0034] S4.2, introduce the safety factor SF, at the ultimate tensile strength σ limit (T), calculate the allowable tensile strength σ of the pipeline allow :

[0035]

[0036] Among them, σ allow Allowable tensile strength for pipeline;

[0037] S4.3. Under high temperature environment, considering the influence of fatigue accumulation effect on pipeline material, calculate the dynamic fatigue stress σ of pipeline material fatigue (t);

[0038] S4.4. Calculate the dynamic allowable tensile strength σ over time based on the dynamic fatigue stress of the pipeline material allow(t).

[0039] As a further improvement of this technical solution, in S4.3, the influence of fatigue accumulation effect on pipeline material is considered and the dynamic fatigue stress σ of pipeline material is calculated. fatigue (t), the specific method is as follows:

[0040]

[0041] Among them, σ fatigue (t) is the dynamic fatigue stress; N f is the fatigue life of pipeline material; cyclic (t′) is the cyclic stress, which indicates the cyclic stress borne by the material at time t′;

[0042] S4.4, combined with the dynamic fatigue stress of pipeline materials, calculate the dynamic allowable tensile strength σ over time allow (t), the specific method is as follows:

[0043] σ allow (t) = σ allow -σ fatigue (t);

[0044] Among them, σ allow (t) is the dynamic allowable tensile strength over time; σ allow Allowable tensile strength for the pipeline.

[0045] As a further improvement of the technical solution, in S5, the principal stress is calculated, and the equivalent stress of the pipeline is calculated based on the temperature gradient and the plastic deformation of the material. The specific method steps are as follows:

[0046] S5.1. Calculate the principal stresses, which include the hoop stress σ hoop , axial stress σ axial and radial stress σ radial :

[0047]

[0048] Among them, σ hoop is the hoop stress; axial is the axial stress; σ radial is the radial stress; P is the pressure inside the pipe; D is the outer diameter of the pipe; d is the inner diameter of the pipe; r is the inner radius of the pipe; σ long is the longitudinal stress caused by the internal pressure of the pipeline; σ a is the longitudinal stress caused by the external pressure of the pipeline; F a is the pipeline axial force; A p is the cross-sectional area of ​​the pipeline; M is the maximum bending moment of the pipeline; W is the section modulus of the pipeline

[0049] S5.2. By calculating the temperature gradient of the pipeline, the additional thermal stress on the pipeline is obtained:

[0050]

[0051] in, is the temperature gradient; ΔT grad is the temperature difference between the inside and outside of the pipe wall;

[0052] S5.3. Use Von Mises equivalent stress to express plastic stress σ plastic The influence on the pipeline and the plastic stress σ plastic Take into account the equivalent stress;

[0053] S5.4, based on principal stress and thermal stress σ thermal and plastic stress σ plastic , the equivalent stress σ of the pipeline is obtained by comprehensive calculation eq .

[0054] As a further improvement of the technical solution, the S5.3, using the Von Mises formula to express the equivalent stress σ of the pipeline eq , the specific method is as follows:

[0055]

[0056] Among them, σ eq is the equivalent stress.

[0057] As a further improvement of this technical solution, the equivalent stress and the dynamic allowable tensile strength are compared to preliminarily determine the risk of pipeline rupture, and the rupture failure risk of the pipeline is finally determined by establishing a progressive damage model of the pipeline material. The specific method steps are as follows:

[0058] S6.1. Comparison of equivalent stress σ eq and dynamic allowable tensile strength σ allow (t):

[0059] If σ eq ≤σ allow (t), the pipeline is in a safe state;

[0060] If σ eq >σ allow (t), there is a risk of pipeline rupture and proceed to S6.2;

[0061] S6.2. Establish a progressive damage model and ultimately determine the pipeline rupture failure risk based on the progressive damage model.

[0062] As a further improvement of the technical solution, in S6.2, a progressive damage model is established, and the rupture failure risk of the pipeline is finally determined based on the progressive damage model. The specific method steps are as follows:

[0063] S6.2.1. Define the damage factor D(t) in the progressive damage model as the cumulative quantitative index of internal damage of pipeline materials, where the range of the damage factor D(t) is:

[0064] 0≤D(t)≤1;

[0065] S6.2.2, based on damage factor D(t), equivalent stress σ eq and dynamic allowable tensile strength σ allow (t) Constructing a progressive injury model:

[0066]

[0067] Among them, σ eq (t) is the equivalent stress at time t; γ is the damage adjustment coefficient of the pipeline material; is the microcrack accumulation rate at time t′;

[0068] S6.2.3. The rupture failure risk of the pipeline is finally determined based on the damage factor D(t) in the progressive damage model:

[0069] When D(t)≥1, the pipeline is broken;

[0070] When D(t)<1, the pipeline is normal.

[0071] Compared with the prior art, the present invention has the following beneficial effects:

[0072] 1. In this method for integrity assessment of high-pressure natural gas pipelines under jet fire, based on the dynamic fatigue accumulation effect and the progressive damage model of materials under high temperature, the damage accumulation of pipelines under high temperature and cyclic stress environment can be tracked in real time, and the dynamic allowable tensile strength of the pipeline can be accurately calculated.

[0073] 2. In this method for evaluating the integrity of a high-pressure natural gas pipeline under jet fire, an accurate failure assessment of the pipeline under multi-directional stress is achieved by combining a comprehensive calculation model of temperature gradient stress, plastic deformation effect and equivalent stress. BRIEF DESCRIPTION OF THE DRAWINGS

[0074] Figure 1 The figure is a flow chart of the overall method of the present invention. DETAILED DESCRIPTION

[0075] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0076] Example:

[0077] See also Figure 1 As shown, this embodiment provides a method for evaluating the integrity of a high-pressure natural gas pipeline under jet fire, comprising the following steps:

[0078] S1. Collect total heat flux and ambient temperature data, and calculate the jet flame temperature by establishing a heat balance equation;

[0079] In this embodiment S1, the total heat flux and ambient temperature data are collected, and the jet flame temperature is calculated by establishing a heat balance equation. The specific method is as follows:

[0080] S1.1. Collect total heat flux q net And ambient temperature data T ∞ ;

[0081] S1.2. Establish the heat balance equation to calculate the jet flame temperature T f :

[0082]

[0083] Among them, q total is the total heat flux; h conv is the convective heat transfer coefficient; T f is the jet flame temperature; T ∞ is the ambient temperature; σ is the Stefan-Boltzmann constant.

[0084] S2. Calculate the net heat absorption intensity q of the pipeline based on the jet flame temperature net :

[0085]

[0086] Among them, q net is the net heat absorption intensity of the pipeline; a s is the pipeline absorption rate; ∈ f is the flame radiation rate under jet fire; ∈ s is the pipeline emissivity; A is the surface area of ​​the pipeline; T out is the outer wall temperature of the pipeline;

[0087] S3. Iteratively calculate the pipeline temperature after the pipeline absorbs heat according to the net heat absorption intensity;

[0088] In this embodiment S3, according to the net heat absorption intensity, the pipeline temperature after the pipeline absorbs heat is iteratively calculated, and the specific method steps are as follows:

[0089] S3.1. Use the linear approximation formula to express the change of pipeline specific heat capacity with temperature, as follows:

[0090] C p (T) = a + bT + cT 2 ;

[0091] Among them, C p (T) is the pipeline specific heat capacity; T is the temperature;

[0092] S3.2, determine the calculation time step Δt, and according to the net heat absorption intensity q net and pipeline specific heat capacity C p (T), calculate the pipeline temperature change ΔT after time step Δt at time t:

[0093]

[0094] Where m is the pipeline mass; ΔT is the change in pipeline temperature after a time step of Δt at time t; Δt is the time step; C p (T) is the specific heat capacity of the pipeline;

[0095] S3.3. Update the pipeline temperature T of the n+1th iteration n+1 :

[0096] T n+1 =T n +ΔT;

[0097] Among them, T n is the pipeline temperature of the nth iteration; T n+1 The pipeline temperature at the n+1th iteration;

[0098] S3.4. Repeat the iterative calculation of pipeline temperature until the pipeline rupture temperature is reached and then stop the iterative calculation.

[0099] S4. Calculate the allowable tensile strength based on the ultimate tensile strength of the pipeline material at different temperatures and the safety factor, and calculate the dynamic allowable tensile strength by considering the fatigue accumulation effect of the pipeline material at high temperature;

[0100] In this embodiment S4, the allowable tensile strength is calculated based on the ultimate tensile strength of the pipeline material at different temperatures in combination with the safety factor, and the dynamic allowable tensile strength is calculated taking into account the fatigue accumulation effect of the pipeline material at high temperature. The specific method steps are as follows:

[0101] S4.1. Find the ultimate tensile strength σ of the pipeline material limit (T);

[0102] S4.2, introduce the safety factor SF, at the ultimate tensile strength σ limit (T), calculate the allowable tensile strength σ of the pipeline allow :

[0103]

[0104] Among them, σ allow Allowable tensile strength for pipeline;

[0105] S4.3. Under high temperature environment, considering the influence of fatigue accumulation effect on pipeline material, calculate the dynamic fatigue stress σ of pipeline material fatigue (t);

[0106] S4.4. Calculate the dynamic allowable tensile strength σ over time based on the dynamic fatigue stress of the pipeline material allow (t).

[0107] In this embodiment S4.3, considering the influence of fatigue accumulation effect on pipeline material, the dynamic fatigue stress σ of pipeline material is calculated. fatigue (t), the specific method is as follows:

[0108]

[0109] Among them, σ fatigue (t) is the dynamic fatigue stress; N f is the fatigue life of pipeline material; cyclic (t′) is the cyclic stress, which indicates the cyclic stress borne by the material at time t′;

[0110] S4.4, combined with the dynamic fatigue stress of pipeline materials, calculate the dynamic allowable tensile strength σ over time allow (t), the specific method is as follows:

[0111] σ allow (t) = σ allow -σ fatigue (t);

[0112] Among them, σ allow (t) is the dynamic allowable tensile strength over time; σ allow Allowable tensile strength for the pipeline.

[0113] S5. Calculate the principal stress and the equivalent stress of the pipeline based on the temperature gradient and the plastic deformation of the material;

[0114] In this embodiment S5, the principal stress is calculated, and the equivalent stress of the pipeline is calculated based on the temperature gradient and the plastic deformation of the material. The specific method steps are as follows:

[0115] S5.1. Calculate the principal stresses, which include the hoop stress σ hoop , axial stress σ axial and radial stress σ radial :

[0116]

[0117] Among them, σ hoop is the hoop stress; axial is the axial stress; σ radial is the radial stress; P is the pressure inside the pipe; D is the outer diameter of the pipe; d is the inner diameter of the pipe; r is the inner radius of the pipe; σ long is the longitudinal stress caused by the internal pressure of the pipeline; σ a is the longitudinal stress caused by the external pressure of the pipeline; F a is the pipeline axial force; A p is the cross-sectional area of ​​the pipeline; M is the maximum bending moment of the pipeline; W is the section modulus of the pipeline

[0118] S5.2. By calculating the temperature gradient of the pipeline, the additional thermal stress on the pipeline is obtained:

[0119]

[0120] in, is the temperature gradient; ΔT grad is the temperature difference between the inside and outside of the pipe wall;

[0121] S5.3. Use Von Mises equivalent stress to express plastic stress σ plastic The influence on the pipeline and the plastic stress σ plastic Take into account the equivalent stress;

[0122] S5.4, based on principal stress and thermal stress σ thermal and plastic stress σ plastic , the equivalent stress σ of the pipeline is obtained by comprehensive calculation eq .

[0123] In this embodiment S5.3, the Von Mises formula is used to express the equivalent stress σ of the pipeline. eq , the specific method is as follows:

[0124]

[0125] Among them, σ eq is the equivalent stress.

[0126] S6. Compare the equivalent stress and the dynamic allowable tensile strength, and determine the rupture failure risk of the pipeline by establishing a progressive damage model for pipeline materials;

[0127] In this embodiment S6, the equivalent stress and the dynamic allowable tensile strength are compared to preliminarily determine the risk of pipeline rupture, and the risk of pipeline rupture failure is finally determined by establishing a progressive damage model of the pipeline material. The specific method steps are as follows:

[0128] S6.1. Comparison of equivalent stress σ eq and dynamic allowable tensile strength σ allow (t):

[0129] If σ eq ≤σ allow (t), the pipeline is in a safe state;

[0130] If σ eq >σ allow (t), there is a risk of pipeline rupture and proceed to S6.2;

[0131] S6.2. Establish a progressive damage model and ultimately determine the pipeline rupture failure risk based on the progressive damage model.

[0132] In this embodiment S6.2, a progressive damage model is established, and the rupture failure risk of the pipeline is finally determined based on the progressive damage model. The specific method steps are as follows:

[0133] S6.2.1. Define the damage factor D(t) in the progressive damage model as the cumulative quantitative index of internal damage of pipeline materials, where the range of the damage factor D(t) is:

[0134] 0≤D(t)≤1;

[0135] S6.2.2, based on damage factor D(t), equivalent stress σ eq and dynamic allowable tensile strength σ allow (t) Constructing a progressive injury model:

[0136]

[0137] Among them, σ eq (t) is the equivalent stress at time t; γ is the damage adjustment coefficient of the pipeline material; is the microcrack accumulation rate at time t′;

[0138] S6.2.3. The rupture failure risk of the pipeline is finally determined based on the damage factor D(t) in the progressive damage model:

[0139] When D(t)≥1, the pipeline is broken;

[0140] When D(t)<1, the pipeline is normal.

[0141] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments, and the above embodiments and descriptions are only preferred examples of the present invention, and are not intended to limit the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, and these changes and improvements all fall within the scope of the present invention to be protected.

Claims

1. A method for evaluating the integrity of a high-pressure natural gas pipeline under jet fire, characterized in that: The following steps are involved: S1. Collect total heat flux and ambient temperature data, and calculate the jet flame temperature by establishing a heat balance equation; S2. Calculate the net heat absorption intensity of the pipeline under the jet fire according to the jet flame temperature; S3. Iteratively calculate the pipeline temperature after the pipeline absorbs heat according to the net heat absorption intensity; S4. Calculate the allowable tensile strength based on the ultimate tensile strength of the pipeline material at different temperatures and the safety factor, and calculate the dynamic allowable tensile strength by considering the fatigue accumulation effect of the pipeline material at high temperature; S5. Calculate the principal stress and the equivalent stress of the pipeline based on the temperature gradient and the plastic deformation of the material; S6. Compare the equivalent stress with the dynamic allowable tensile strength, and determine the rupture failure risk of the pipeline by establishing a progressive damage model of the pipeline material.

2. The method for evaluating the integrity of a high-pressure natural gas pipeline under jet fire according to claim 1, characterized in that: In S1, the total heat flux and ambient temperature data are collected, and the jet flame temperature is calculated by establishing a heat balance equation. The specific method is as follows: S1.

1. Collect total heat flux q net And ambient temperature data T ∞ ; S1.

2. Establish the heat balance equation to calculate the jet flame temperature T f : Among them, q total is the total heat flux; h conv is the convective heat transfer coefficient; T f is the jet flame temperature; T ∞ is the ambient temperature; σ is the Stefan-Boltzmann constant.

3. The method for evaluating the integrity of a high-pressure natural gas pipeline under jet fire according to claim 1, characterized in that: In S2, the net heat absorption intensity of the pipeline under the jet fire is calculated according to the jet flame temperature. The specific method steps are as follows: S2. Calculate the net heat absorption intensity q of the pipeline based on the jet flame temperature net : Among them, q net is the net heat absorption intensity of the pipeline; a s is the pipeline absorption rate; ∈ f is the flame radiation rate under jet fire; ∈ s is the pipeline emissivity; A is the surface area of ​​the pipeline; T out is the pipeline outer wall temperature.

4. The method for evaluating the integrity of a high-pressure natural gas pipeline under jet fire according to claim 1, characterized in that: In S3, according to the net heat absorption intensity, the pipeline temperature after the pipeline absorbs heat is iteratively calculated. The specific method steps are as follows: S3.

1. Use the linear approximation formula to express the change of pipeline specific heat capacity with temperature, as follows: C p (T)=a+bT+cT 2 ; Among them, C p (T) is the pipeline specific heat capacity; T is the temperature; S3.2, determine the calculation time step Δt, and according to the net heat absorption intensity q net and pipeline specific heat capacity C p (T), calculate the pipeline temperature change ΔT after time step Δt at time t: Where m is the pipeline mass; ΔT is the change in pipeline temperature after a time step of Δt at time t; Δt is the time step; C p (T) is the specific heat capacity of the pipeline; S3.

3. Update the pipeline temperature T of the n+1th iteration n+1 : T n+1 =T n +ΔT; Among them, T n is the pipeline temperature of the nth iteration; T n+1 The pipeline temperature at the n+1th iteration; S3.

4. Repeat the iterative calculation of pipeline temperature until the pipeline rupture temperature is reached and then stop the iterative calculation.

5. The method for evaluating the integrity of a high-pressure natural gas pipeline under jet fire according to claim 1, characterized in that: In S4, the allowable tensile strength is calculated based on the ultimate tensile strength of the pipeline material at different temperatures in combination with the safety factor, and the dynamic allowable tensile strength is calculated taking into account the fatigue accumulation effect of the pipeline material at high temperature. The specific method steps are as follows: S4.

1. Find the ultimate tensile strength σ of the pipeline material limit (T); S4.2, introduce the safety factor SF, at the ultimate tensile strength σ limit (T), calculate the allowable tensile strength σ of the pipeline allow : Among them, σ allow Allowable tensile strength for pipeline; S4.

3. Under high temperature environment, considering the influence of fatigue accumulation effect on pipeline material, calculate the dynamic fatigue stress σ of pipeline material fatigue (t); S4.

4. Calculate the dynamic allowable tensile strength σ over time based on the dynamic fatigue stress of the pipeline material allow (t).

6. The method for evaluating the integrity of a high-pressure natural gas pipeline under jet fire according to claim 5, characterized in that: In S4.3, the influence of fatigue accumulation effect on pipeline material is considered and the dynamic fatigue stress σ of pipeline material is calculated. fatigue (t), the specific method is as follows: Among them, σ fatigue (t) is the dynamic fatigue stress; N f is the fatigue life of pipeline material; cyclic (t′) is the cyclic stress, which indicates the cyclic stress borne by the material at time t′; S4.4, combined with the dynamic fatigue stress of pipeline materials, calculate the dynamic allowable tensile strength σ over time allow (t), the specific method is as follows: s allow (t)=σ allow -s fatigue (t); Among them, σ allow (t) is the dynamic allowable tensile strength over time; σ allow Allowable tensile strength for the pipeline.

7. The method for evaluating the integrity of a high-pressure natural gas pipeline under jet fire according to claim 1, characterized in that: In S5, the principal stress is calculated, and the equivalent stress of the pipeline is calculated based on the temperature gradient and the plastic deformation of the material. The specific method steps are as follows: S5.

1. Calculate the principal stresses, which include the hoop stress σ hoop , axial stress σ axial and radial stress σ radial : s axial =s long +s a ; Among them, σ hoop is the hoop stress; axial is the axial stress; σ radial is the radial stress; P is the pressure inside the pipe; D is the outer diameter of the pipe; d is the inner diameter of the pipe; r is the inner radius of the pipe; σ long is the longitudinal stress caused by the internal pressure of the pipeline; σ a is the longitudinal stress caused by the external pressure of the pipeline; F a is the pipeline axial force; A p is the cross-sectional area of ​​the pipeline; M is the maximum bending moment of the pipeline; W is the section modulus of the pipeline; S5.

2. By calculating the temperature gradient of the pipeline, the additional thermal stress on the pipeline is obtained: in, is the temperature gradient; ΔT grad is the temperature difference between the inside and outside of the pipe wall; S5.

3. Use Von Mises equivalent stress to express plastic stress σ plastic The influence on the pipeline and the plastic stress σ plastic Take into account the equivalent stress; S5.4, based on principal stress and thermal stress σ thermal and plastic stress σ plastic , the equivalent stress σ of the pipeline is obtained by comprehensive calculation eq .

8. The method for evaluating the integrity of a high-pressure natural gas pipeline under jet fire according to claim 7, characterized in that: S5.3, using the Von Mises formula to express the equivalent stress σ of the pipeline eq , the specific method is as follows: Among them, σ eq is the equivalent stress.

9. The method for evaluating the integrity of a high-pressure natural gas pipeline under jet fire according to claim 1, characterized in that: In S6, the equivalent stress and the dynamic allowable tensile strength are compared to preliminarily determine the risk of pipeline rupture, and the risk of pipeline rupture failure is finally determined by establishing a progressive damage model of the pipeline material. The specific method steps are as follows: S6.

1. Comparison of equivalent stress σ eq and dynamic allowable tensile strength σ allow (t): If σ eq ≤σ allow (t), the pipeline is in a safe state; If σ eq >σ allow (t), there is a risk of pipeline rupture and proceed to S6.2; S6.

2. Establish a progressive damage model and ultimately determine the pipeline rupture failure risk based on the progressive damage model.

10. The method for evaluating the integrity of a high-pressure natural gas pipeline under jet fire according to claim 9, characterized in that: In S6.2, a progressive damage model is established, and the rupture failure risk of the pipeline is finally determined based on the progressive damage model. The specific method steps are as follows: S6.2.

1. Define the damage factor D(t) in the progressive damage model as the cumulative quantitative index of internal damage of pipeline materials, where the range of the damage factor D(t) is: 0≤D(t)≤1; S6.2.2, based on damage factor D(t), equivalent stress σ eq and dynamic allowable tensile strength σ allow (t) Constructing a progressive injury model: Among them, σ eq (t) is the equivalent stress at time t; γ is the damage adjustment coefficient of the pipeline material; is the microcrack accumulation rate at time t′; S6.2.

3. The rupture failure risk of the pipeline is finally determined based on the damage factor D(t) in the progressive damage model: When D(t)≥1, the pipeline is broken; When D(t)<1, the pipeline is normal.

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