A method for determining allowable crack size of girth weld of oil and gas pipeline based on stress design

Abstract

The application discloses a kind of based on stress design oil and gas pipeline girth weld allowable crack size determination method, target point is set in crack driving force curve coordinate system;The length or depth of crack is fixed, the length and depth of crack and elastic-plastic finite element model are used to obtain crack driving force curve;The relative position between crack driving force curve and the target point is judged, when crack driving force curve passes through target point, the length and depth of crack are the allowable crack size of girth weld to be solved;When crack driving force curve does not pass through target point, the quantity that is not fixed in the length and depth of crack is adjusted until it passes through target point;Using a plurality of different girth weld allowable crack size, draw critical crack size length and depth relationship curve;Using the curve to determine the allowable crack size of oil and gas pipeline girth weld.The application avoids the establishment of intermediate failure evaluation diagram, under the premise of guaranteeing evaluation precision, simplifies the calculation method of critical allowable crack size.

Description

Technical Field

[0001] This invention belongs to the field of engineering technology, specifically relating to a method for determining the allowable crack size of circumferential welds in oil and gas pipelines based on stress design. Background Technology

[0002] Pipelines transporting oil and natural gas are one of the world's main energy supply methods, and with the continuous growth of energy demand, pipeline mileage will continue to increase. The safe operation of pipelines is a prerequisite for their construction. Natural gas pipelines, in particular, are susceptible to explosions and fires if leaks occur, causing enormous economic losses and even personal injury. Statistical analysis of past pipeline failure factors shows that circumferential weld fracture is one of the most significant causes. As a crucial structural element connecting pipelines, circumferential welds inevitably introduce defects during the welding process. Planar cracks are the most dangerous defect in circumferential welds, and their direction is typically along the welding direction, perpendicular to the pipeline's axial load.

[0003] For a given circumferential weld, when the weld's mechanical properties (strength, fracture toughness) and external load are determined, the allowable crack size (length, depth) can be determined based on fracture mechanics principles. When this crack size exceeds the specified acceptance criteria (based on process acceptance standards or engineering critical evaluation), the circumferential weld needs to be cut and re-welded or repaired. Therefore, determining the allowable crack size of a circumferential weld under given operating conditions provides an important foundation for the safe design, operation, maintenance, assessment, and repair of oil and gas pipelines.

[0004] Currently, the allowable crack size of pipe circumferential welds is often determined using a failure assessment diagram (FAD). However, when constructing a failure assessment diagram using standards such as BS7910 Level 2, the influence of crack size on the failure assessment diagram is not considered, resulting in a conservative allowable crack size for the circumferential weld, and the process is cumbersome. Summary of the Invention

[0005] To address the problems existing in the prior art, the present invention aims to provide a method for determining the allowable crack size of circumferential welds in oil and gas pipelines based on stress design. The present invention avoids the establishment of intermediate failure assessment diagrams and simplifies the calculation method of critical allowable crack size while ensuring assessment accuracy.

[0006] The technical solution adopted in this invention is as follows:

[0007] A method for determining the allowable crack size of circumferential welds in oil and gas pipelines based on stress design includes the following steps:

[0008] Set target point: Set target point in the crack driving force curve coordinate system. The horizontal axis of the target point is the critical far end stress, and the vertical axis is the critical fracture toughness of the circumferential weld - crack tip opening displacement. The critical far end stress is the maximum allowable value of the far end stress, and the far end stress is the stress at the center position of the pipe at one end of the circumferential weld.

[0009] Obtain the crack driving force curve: With the crack length or depth fixed, the crack driving force curve is obtained using the crack length and depth and the established elastoplastic finite element model.

[0010] Determine and adjust the relative position: Determine the relative position between the obtained crack driving force curve and the target point. When the obtained crack driving force curve passes through the target point, the length and depth of the crack at this time are the allowable crack size of the circumferential weld. When the obtained crack driving force curve does not pass through the target point, adjust the unfixed amount in the length and depth of the crack until the crack driving force curve passes through the target point.

[0011] Repeat the process of obtaining the crack driving force curve and judging and adjusting the relative position to obtain multiple sets of different allowable crack sizes for circumferential welds;

[0012] Using multiple sets of different allowable crack sizes for circumferential welds, a curve showing the relationship between the length and depth of the critical crack size was plotted.

[0013] The allowable crack size of the circumferential weld of oil and gas pipelines is determined by using the relationship curve between the critical crack size length and depth.

[0014] Preferably, the process of establishing the elastoplastic finite element model includes:

[0015] Establish a geometric model of an oil and gas pipeline containing cracks;

[0016] The geometric model of the oil and gas pipeline is meshed to obtain the finite element model of the oil and gas pipeline.

[0017] By setting the boundary conditions and material constitutive model of the oil and gas pipeline finite element model, the elastic-plastic finite element model is obtained.

[0018] Preferably, the geometric model of the cracked oil and gas pipeline is established by evaluating the macroscopic metallographic structure and hardness cloud map of the circumferential weld using on-site welding process assessment.

[0019] When meshing the geometric model of the oil and gas pipeline, a spider web-type mesh is used for the mesh at the crack tip;

[0020] The boundary condition for the finite element model of an oil and gas pipeline is the critical far-end stress.

[0021] Preferably, the material constitutive model of the oil and gas pipeline finite element model includes the material constitutive models corresponding to the pipe material, weld metal, and heat-affected zone material; the material constitutive model is a stress-strain relationship;

[0022] The constitutive model of the material in the heat-affected zone is obtained using the following equation:

[0023]

[0024] σ HAZ (ε p )=(1-λ)σ BM (ε p )

[0025] Among them, H HAZ H represents the average hardness of the heat-affected zone obtained from the hardness contour plot. BM σ represents the average hardness of the pipe body obtained from the hardness contour plot, λ represents the softening degree of the heat-affected zone, and σ represents the average hardness of the pipe body obtained from the hardness contour plot. BM (ε p ) represents the flow stress equation for the pipe body, σ HAZ (ε p ) represents the flow stress equation of the heat-affected zone.

[0026] Preferably, when judging and adjusting relative positions, the following relationship is defined:

[0027] Δ=δ m -F(σ c )

[0028] Where Δ represents the critical fracture toughness of the circumferential weld - crack tip opening displacement δ m With crack driving force curve F(σ) c The critical distal stress σ c The algebraic difference of the corresponding crack tip opening displacement;

[0029] When 0≤Δ≤ε, it indicates that the crack driving force curve passes through the target point;

[0030] When Δ > ε or Δ < 0, it indicates that the crack driving force curve has not passed through the target point.

[0031] Preferred method: When Δ > ε, the process of adjusting the unfixed amounts in the crack length and depth until the crack driving force curve passes through the target point includes:

[0032] Set increment: Set the increment of the unfixed amount in the crack length and depth;

[0033] Obtain a new quantity: Add the increment to the unfixed quantity to obtain a new unfixed quantity;

[0034] Determine if the target point has been reached: Re-acquire the crack driving force curve with a fixed amount and a new unfixed amount, and determine if the crack driving force curve has passed the target point. If it has, stop adjusting the unfixed amount in the crack length and depth.

[0035] Otherwise, repeat the process of setting the increment, obtaining the new value, and determining whether the target point has been reached, until the crack driving force curve passes through the target point.

[0036] Preferred method: When Δ < 0, the process of adjusting the unfixed amounts in the crack length and depth until the crack driving force curve passes through the target point includes:

[0037] Set increment: Set the increment of the unfixed amount in the crack length and depth;

[0038] Obtain a new quantity: Subtract the increment from the unfixed quantity to obtain a new unfixed quantity;

[0039] Determine if the target point has been reached: Re-acquire the crack driving force curve with a fixed amount and a new unfixed amount, and determine if the crack driving force curve has passed the target point. If it has, stop adjusting the unfixed amount in the crack length and depth.

[0040] Otherwise, repeat the process of setting the increment, obtaining the new value, and determining whether the target point has been reached, until the crack driving force curve passes through the target point.

[0041] Preferred values: ε is 0-1mm, and the crack driving force curve is not lower than the target point.

[0042] This invention also provides a system for determining the allowable crack size of circumferential welds in oil and gas pipelines based on stress design, comprising:

[0043] Target point setting module: used to set a target point in the crack driving force curve coordinate system. The horizontal axis of the target point is the critical distal stress, and the vertical axis is the critical fracture toughness of the circumferential weld - crack tip opening displacement. The critical distal stress is the maximum allowable value of the distal stress, and the distal stress is the stress at the center of the pipe at one end of the circumferential weld.

[0044] Crack driving force curve acquisition module: Used to fix the length or depth of the crack, and obtain the crack driving force curve using the crack length and depth and the established elastoplastic finite element model;

[0045] The relative position judgment and adjustment module is used to judge the relative position between the obtained crack driving force curve and the target point. When the obtained crack driving force curve passes through the target point, the length and depth of the crack at this time are the allowable crack size of the circumferential weld. When the obtained crack driving force curve does not pass through the target point, the unfixed amount in the length and depth of the crack is adjusted until the crack driving force curve passes through the target point.

[0046] Curve plotting module: Used to plot the relationship curve between the length and depth of the critical crack size using multiple sets of different allowable crack sizes of circumferential welds; wherein, multiple sets of different allowable crack sizes of circumferential welds are obtained by processing the different sizes of multiple sets of cracks through the crack driving force curve acquisition module and the judgment and adjustment relative position module.

[0047] Allowable crack size acquisition module: used to determine the allowable crack size of the circumferential weld of oil and gas pipeline using the relationship curve between the critical crack size length and depth.

[0048] This invention also provides a method for safety evaluation of pipelines with cracked circumferential welds, comprising the following steps:

[0049] Measure the size of cracks in pipes with cracked circumferential welds;

[0050] The safety of a pipe with a crack in its circumferential weld is determined based on the positional relationship between the point corresponding to the crack size and the curve relating the length and depth of the critical crack size. The point corresponding to the crack size and the curve relating the length and depth of the critical crack size are in the same coordinate system.

[0051] When the point corresponding to the crack size is located above the curve relating the length and depth of the critical crack size, it indicates that there is a safety hazard at the pipe circumferential weld and it needs to be addressed; otherwise, it indicates that the pipe circumferential weld is safe.

[0052] The present invention has the following beneficial effects:

[0053] This invention presents a method for determining the allowable crack size of circumferential welds in oil and gas pipelines based on stress design. This method considers the true geometric morphology and mechanical properties of the circumferential welds, overcoming the shortcomings of conventional failure assessment diagrams, which are relatively coarse, conservative, and unsuitable for studying patterns. Furthermore, by utilizing the driving force curve method, the material's toughness is not included in the crack driving force curve, facilitating the input of different ultimate loads and fracture toughnesses. This promotes the method's widespread adoption and understanding, ultimately providing technical support for ensuring the safe operation of oil and gas pipelines. This invention also utilizes an elastoplastic finite element model to obtain the allowable crack size of circumferential welds, avoiding the need for intermediate failure assessment diagrams and simplifying the calculation method for critical allowable crack size while maintaining assessment accuracy. Attached Figure Description

[0054] Figure 1(a) is a geometric model of a pipe with surface cracks in an embodiment of the present invention; Figure 1(b) is a lateral contour diagram of the cracks in an embodiment of the present invention; Figure 1(c) is a spider web-like mesh of a local area of ​​surface cracks in an embodiment of the present invention.

[0055] Figure 2 The constitutive (stress-strain relationship) curves of the welded metal of the circumferential weld, the pipe body, and the heat-affected zone in the embodiments of the present invention are shown.

[0056] Figure 3(a) is a macroscopic metallographic image of the circumferential weld of the welded joint in this invention; Figure 3(b) is a hardness cloud diagram of the circumferential weld of the welded joint in this invention; Figure 3(c) is a model diagram of the circumferential weld of the welded joint in this invention.

[0057] Figure 4 A diagram illustrating the defect tolerance determination method based on the driving force method in this invention embodiment (fixed defect length c, critical defect depth a);

[0058] Figure 5 This is a curve showing the relationship between the critical crack size length and depth obtained in an embodiment of the present invention. Detailed Implementation

[0059] The following specific embodiments further illustrate the technical solution for determining the allowable crack size of circumferential welds in oil and gas pipelines.

[0060] This invention proposes a method for determining the allowable crack size of circumferential welds in oil and gas pipelines under axial tensile loads based on stress design. This method establishes an elastoplastic finite element model considering the actual geometry and mechanical properties of the circumferential weld. Using fracture mechanics theory, it constructs a crack driving force curve in the circumferential weld under external loads and combines this with the fracture toughness requirements of circumferential welds in pipelines under construction to obtain the allowable critical crack size. This method can provide a reference for the formulation of non-destructive testing and acceptance standards for newly built pipelines and also provide technical guidance for identifying potential quality hazards in circumferential welds of in-service pipelines, ensuring the safe operation of pipelines.

[0061] This invention relates to a method for determining the allowable crack size of circumferential welds in oil and gas pipelines based on stress design, comprising the following steps.

[0062] Step 1: Finite element model establishment. For in-service pipelines, a geometric model of the cracked oil and gas pipeline is established using the macroscopic metallographic data and hardness contour plot of the circumferential weld, as determined by the on-site welding process evaluation. A mesh is then created, referencing... Figure 1(a)-Figure 1(c) For crack tips, a spider web-like mesh is required to obtain better convergence results;

[0063] Step 2: Set the boundary conditions for the finite element model and determine the critical maximum stress. Set the internal pressure of the pipe and apply an axial load to the pipe end. The load is controlled by the distal stress σ, which is defined as the stress at the center of the pipe at the circumferential weld end to avoid the influence of the circumferential weld and the pipe end. The critical distal stress σ is determined based on the actual load or standard requirements. c ;

[0064] Step 3: Define the material constitutive models for the finite element model. Considering that the pipeline comprises three different materials—pipe material, weld metal, and heat-affected zone material—three different elastoplastic finite element constitutive models (stress-strain relationships) will be input into the finite element model. For in-service pipelines, the pipe material and weld metal constitutive models are obtained from the circumferential welds in the welding process qualification. The heat-affected zone constitutive model is obtained using the following equation:

[0065]

[0066] σ HAZ (ε p )=(1-λ)σ BM (ε p )

[0067] Among them, H HAZ H represents the average hardness of the heat-affected zone obtained from the hardness contour plot. BM σ represents the average hardness of the pipe body obtained from the hardness contour plot, λ represents the softening degree of the heat-affected zone, and σ represents the average hardness of the pipe body obtained from the hardness contour plot. BM (ε p ) represents the flow stress equation for the pipe body, σ HAZ (ε p () represents the flow stress equation for the heat-affected zone;

[0068] Step 4: Select the critical fracture toughness of the circumferential weld - crack tip opening displacement δ m And combined with the critical distal stress σ c Establish the critical state point (σ) of the circumferential weld in the crack driving force curve coordinate system. c δ m );

[0069] Step 5: Determine the critical crack size. The critical crack depth is determined under a fixed crack length using the following steps:

[0070] 1) Fixed crack length c x Set the initial crack depth a0;

[0071] 2) The relationship curve between the crack tip opening displacement δ and the distal stress σ is obtained using a finite element model, i.e., the crack driving force curve F(σ). Definition:

[0072] Δ=δ m -F(σ c (1)

[0073] Where, F(σ) c ) represents the critical distal stress σ c The corresponding crack tip opening displacement δ. Δ represents δ m σ in the crack driving force curve cThe algebraic difference of the crack tip opening displacement is: Δ > 0 indicates that the crack depth is within the critical crack depth, Δ < 0 indicates that the crack depth exceeds the critical crack depth, and Δ = 0 indicates that the crack depth is in the critical state.

[0074] 3) When 0 ≤ Δ ≤ ε (ε can be 0-1 mm), let the crack depth be a. y This step is now complete, and the length and depth of the crack at this point are the desired allowable crack size for the circumferential weld.

[0075] 4) When Δ > ε, set a crack depth increment Δa, and the new crack depth a i+1 =a i +Δa, where i = 0, 1, ..., and the crack length remains fixed at c. x The crack depth is a i+1 Repeat steps 2)-3);

[0076] 5) When Δ < 0, set a crack depth increment Δa, and the new crack depth a i+1 =a i -Δa, where i = 0, 1, ..., and the crack length remains fixed at this point as c. x The crack depth is a i+1 Repeat steps 2)-3);

[0077] Finally, the crack length c was obtained. x Critical crack depth a under certain conditions y That is, the critical defect size of the circumferential weld at this time is (c x ,a y ).

[0078] Step 6: Repeat steps 2)-5) by changing different fixed crack lengths, and use the obtained critical crack size length c. x and depth a y By plotting the relationship curve and interpolating, the critical depth that the circumferential weld can be allowed under any given length condition can be obtained.

[0079] Step 7: Use the obtained critical crack size (length, depth) curve to evaluate the safety of the pipeline. When a crack exceeding the critical size is detected in the circumferential weld, the pipeline needs to be repaired.

[0080] For a fixed depth, setting an initial crack length and performing steps 2) to 7) is also acceptable.

[0081] Example

[0082] In this embodiment, the X80 pipe circumferential weld has a surface crack to represent a crack-type defect in the pipe circumferential weld, as shown in Figures 1(a) and 1(b). The steel pipe has a thickness of t = 22 mm and an outer diameter of D = 1219 mm.

[0083] To save computation time, a symmetrical model was adopted. A spiderweb-type refined mesh element with an element size less than 0.1 mm was used at the crack propagation front, as shown in Figure 1(c). The constitutive (stress-strain relationship) curves of different regions of the circumferential weld metal, the pipe body, and the heat-affected zone are shown in Figure 1(c). Figure 2 As shown.

[0084] Based on the actual macroscopic metallographic features (Figure 3(a)) and hardness contour plot (Figure 3(b)) of the welded joint, a simplified finite element model is abstracted. The relevant welded joint parameters are shown in Figure 3(c). Determining the critical defect tolerance requires calculation using multiple finite element models containing cracks of different sizes. The specific calculation process is as follows:

[0085] 1) Determine the toughness requirement of the weld (δ) m ) and critical load (σ c The current research determines δ based on the "Technical Specifications for Welding of Oil and Gas Pipeline Engineering - Part 1: Line Welding". m =0.254mm. For pipelines designed based on stress, according to the ASME B31.8 standard, the axial stress specified in the pipeline design is usually 90% SMYS (the minimum yield stress specified for steel pipes), which is 500MPa for X80 steel pipes, thus determining the defect tolerance assessment point A.

[0086] 2) Fix the length c1 and depth a0 of a certain crack, and obtain the crack driving curve F(σ);

[0087] 3) When F(σ) is below the evaluation point A, such as Figure 4 Increasing F1(σ) increases crack depth. When F(σ) is above evaluation point A, as shown... Figure 4 For F3(σ), subtract the crack depth until the driving force curve finally approaches the evaluation point A, such as Figure 4 As shown in F2(σ), the crack depth is a1 at this point;

[0088] 4) Determine the critical defect length c1 and depth a1 under this condition;

[0089] 5) Select a new crack length c2, and repeat steps 1)-5) to finally obtain the critical allowable crack curve of the circumferential weld, as shown below. Figure 5 As shown.

[0090] Step 6), when conducting a safety evaluation of pipes with cracked circumferential welds:

[0091] Measure the crack size in a pipe with a circumferential weld containing a crack, and mark the points corresponding to the crack size (length, depth) on the coordinate system containing the critical crack size length-depth relationship curve (e.g., ...). Figure 5 (as shown);

[0092] When the point corresponding to the crack size is located above the curve relating the length and depth of the critical crack size, it indicates that there is a safety hazard at the circumferential weld of the pipeline and it needs to be addressed; otherwise (i.e., located on or below the curve), it indicates that the circumferential weld of the pipeline is safe.

Claims

1. A method for determining the allowable crack size of circumferential welds in oil and gas pipelines based on stress design, characterized in that, The process includes the following: Set target point: Set a target point in the crack driving force curve coordinate system. The horizontal axis of the target point is the critical far end stress, and the vertical axis is the critical fracture toughness of the circumferential weld - crack tip opening displacement. The critical far end stress is the maximum allowable value of the far end stress, and the far end stress is the stress at the center position of the pipe at one end of the circumferential weld. Obtain the crack driving force curve: With the crack length or depth fixed, the crack driving force curve is obtained using the crack length and depth and the established elastoplastic finite element model. The process of establishing the elastic-plastic finite element model includes: establishing a geometric model of an oil and gas pipeline with cracks; meshing the geometric model of the oil and gas pipeline to obtain the finite element model of the oil and gas pipeline; setting the boundary conditions and material constitutive properties of the finite element model of the oil and gas pipeline to obtain the elastic-plastic finite element model. The material constitutive models include those of the pipe, weld metal, and heat-affected zone material; the material constitutive models are stress-strain relationships; wherein, the material constitutive model of the heat-affected zone material is obtained using the following equation: in, H HAZ This represents the average hardness of the heat-affected zone obtained from the hardness contour map. H BM This represents the average hardness of the pipe body obtained from the hardness contour plot, where λ represents the degree of softening in the heat-affected zone. The equation representing the flow stress in the pipe body. The flow stress equation representing the heat-affected zone; Determine and adjust the relative position: Determine the relative position between the obtained crack driving force curve and the target point. Specifically, define the following relationship: Δ=δ m -F(σ c ), where Δ represents the critical fracture toughness of the circumferential weld - crack tip opening displacement δ m With crack driving force curve F(σ) c The critical distal stress σ c The algebraic difference of the crack tip opening displacement; when 0≤Δ≤ε, it indicates that the crack driving force curve passes through the target point; when Δ>ε or Δ<0, it indicates that the crack driving force curve does not pass through the target point; when the obtained crack driving force curve passes through the target point, the length and depth of the crack at this time are the allowable crack size of the circumferential weld; when the obtained crack driving force curve does not pass through the target point, adjust the unfixed amount in the length and depth of the crack until the crack driving force curve passes through the target point; Repeat the process of obtaining the crack driving force curve and judging and adjusting the relative position to obtain multiple sets of different allowable crack sizes for circumferential welds; Using multiple sets of different allowable crack sizes for circumferential welds, a curve showing the relationship between the length and depth of the critical crack size was plotted. The allowable crack size of the circumferential weld of oil and gas pipelines is determined by using the relationship curve between the critical crack size length and depth.

2. The method for determining the allowable crack size of circumferential welds in oil and gas pipelines based on stress design, as described in claim 1, is characterized in that... The geometric model of the cracked oil and gas pipeline was established by evaluating the macroscopic metallographic structure and hardness cloud map of the circumferential weld using on-site welding process assessment. When meshing the geometric model of the oil and gas pipeline, a spider web-type mesh is used for the mesh at the crack tip; The boundary condition for the finite element model of an oil and gas pipeline is the critical far-end stress.

3. The method for determining the allowable crack size of circumferential welds in oil and gas pipelines based on stress design, as described in claim 1, is characterized in that: When Δ > ε, the process of adjusting the unfixed quantities in the crack length and depth until the crack driving force curve passes through the target point includes: Set increment: Set the increment of the unfixed amount in the crack length and depth; Obtain a new quantity: Add the increment to the unfixed quantity to obtain a new unfixed quantity; Determine if the target point has been reached: Re-acquire the crack driving force curve with a fixed amount and a new unfixed amount, and determine if the crack driving force curve has passed the target point. If it has, stop adjusting the unfixed amount in the crack length and depth. Otherwise, repeat the process of setting the increment, obtaining the new value, and determining whether the target point has been reached, until the crack driving force curve passes through the target point.

4. The method for determining the allowable crack size of circumferential welds in oil and gas pipelines based on stress design, as described in claim 1, is characterized in that: When Δ < 0, the process of adjusting the unfixed quantities in the crack length and depth until the crack driving force curve passes through the target point includes: Set increment: Set the increment of the unfixed amount in the crack length and depth; Obtain a new quantity: Subtract the increment from the unfixed quantity to obtain a new unfixed quantity; Determine if the target point has been reached: Re-acquire the crack driving force curve with a fixed amount and a new unfixed amount, and determine if the crack driving force curve has passed the target point. If it has, stop adjusting the unfixed amount in the crack length and depth. Otherwise, repeat the process of setting the increment, obtaining the new value, and determining whether the target point has been reached, until the crack driving force curve passes through the target point.

5. The method for determining the allowable crack size of circumferential welds in oil and gas pipelines based on stress design, as described in claim 1, is characterized in that: The value of ε is 0-1mm, and the crack driving force curve is not lower than the target point.

6. A system for determining the allowable crack size of circumferential welds in oil and gas pipelines based on stress design, characterized in that, include: Target point setting module: used to set a target point in the crack driving force curve coordinate system. The horizontal axis of the target point is the critical far end stress, and the vertical axis is the critical fracture toughness of the circumferential weld - crack tip opening displacement. The critical far end stress is the maximum allowable value of the far end stress, and the far end stress is the stress at the center position of the pipe at one end of the circumferential weld. Crack driving force curve acquisition module: Used to fix the length or depth of the crack, and obtain the crack driving force curve using the crack length and depth and the established elastoplastic finite element model; The process of establishing the elastic-plastic finite element model includes: establishing a geometric model of an oil and gas pipeline with cracks; meshing the geometric model of the oil and gas pipeline to obtain the finite element model of the oil and gas pipeline; setting the boundary conditions and material constitutive properties of the finite element model of the oil and gas pipeline to obtain the elastic-plastic finite element model. The material constitutive models include those of the pipe, weld metal, and heat-affected zone material; the material constitutive models are stress-strain relationships; wherein, the material constitutive model of the heat-affected zone material is obtained using the following equation: in, H HAZ This represents the average hardness of the heat-affected zone obtained from the hardness contour map. H BM This represents the average hardness of the pipe body obtained from the hardness contour plot, where λ represents the degree of softening in the heat-affected zone. The equation representing the flow stress in the pipe body. The flow stress equation representing the heat-affected zone; The relative position determination and adjustment module is used to determine the relative position between the obtained crack driving force curve and the target point. Specifically, the following relationship is defined: Δ=δ m -F(σ c ), where Δ represents the critical fracture toughness of the circumferential weld - crack tip opening displacement δ m With crack driving force curve F(σ) c The critical distal stress σ c The algebraic difference of the crack tip opening displacement; when 0≤Δ≤ε, it indicates that the crack driving force curve passes through the target point; when Δ>ε or Δ<0, it indicates that the crack driving force curve does not pass through the target point; when the obtained crack driving force curve passes through the target point, the length and depth of the crack at this time are the allowable crack size of the circumferential weld; when the obtained crack driving force curve does not pass through the target point, adjust the unfixed amount in the length and depth of the crack until the crack driving force curve passes through the target point; Curve plotting module: Used to plot the relationship curve between the length and depth of the critical crack size using multiple sets of different allowable crack sizes of circumferential welds; wherein, multiple sets of different allowable crack sizes of circumferential welds are obtained by processing the different sizes of multiple sets of cracks through the crack driving force curve acquisition module and the judgment and adjustment relative position module. Allowable crack size acquisition module: used to determine the allowable crack size of the circumferential weld of oil and gas pipeline using the relationship curve between the critical crack size length and depth.

7. A method for evaluating the safety of pipelines with cracked circumferential welds, based on the method for determining the allowable crack size of circumferential welds in oil and gas pipelines according to claim 1, characterized in that... The process includes the following: Measure the size of cracks in pipes with cracked circumferential welds; The safety of a pipe with a crack in its circumferential weld is determined based on the positional relationship between the point corresponding to the crack size and the curve relating the length and depth of the critical crack size. The point corresponding to the crack size and the curve relating the length and depth of the critical crack size are in the same coordinate system. When the point corresponding to the crack size is located above the curve relating the length and depth of the critical crack size, it indicates that there is a safety hazard at the pipe circumferential weld and it needs to be addressed; otherwise, it indicates that the pipe circumferential weld is safe.

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