Calculation method of stress intensity factor at crack tip of butt girth weld of pipeline with misalignment

Abstract

The invention discloses a method for calculating the stress intensity factor of a crack tip of a butt girth weld of a pipe with misaligned edges, comprising the following steps: step 1, geometric model establishment and material property setting step; step 2, crack and boundary load setting step; step 3, unit grid division and output value setting step; step 4, task submission for solution and subsequent finite element verification; the invention provides a simple method for calculating the stress intensity factor of a crack tip of a butt girth weld of a pipe with misaligned edges, provides a method for obtaining the stress intensity factor of a crack tip in real time in engineering, and provides a new idea for calculating the stress intensity factor value.

Description

Technical Field

[0001] The invention belongs to the technical field of structural engineering and relates to a method for calculating a stress intensity factor at a crack tip of a butt girth weld of a misaligned pipeline. Background Art

[0002] The transportation of oil and natural gas in various countries in the world mainly relies on oil and gas pipelines. There are a certain number of serious defects in pipelines. Accidents caused by corrosion, cracks and mechanical damage are very frequent every year, and leakage and explosion accidents occur from time to time. Therefore, improving the ability to predict accidents is the key to achieving safe and efficient oil and gas transportation. The stress intensity factor at the crack tip is one of the criteria for judging pipeline rupture. Therefore, it is necessary to calculate the stress intensity factor of defective pipelines. At present, there is no corresponding calculation formula for the stress intensity factor of the "crack + misalignment" composite defect in the girth weld. The present invention discusses the calculation of the stress intensity factor of the crack and misalignment composite defect in the girth weld. Summary of the invention

[0003] The purpose of the present invention is to provide a method for calculating the crack tip stress intensity factor of a butt girth weld of a pipe with misaligned edges, thereby solving the problem that the crack tip stress intensity factor of a butt pipe with a "misaligned edge + crack" composite defect cannot be calculated under existing conditions.

[0004] The technical solution adopted by the present invention is a method for calculating the stress intensity factor at the crack tip of a butt weld of a pipe with misaligned edges, which is specifically implemented in the following steps:

[0005] Step 1: Establish the geometric model of the butt weld of the pipe with misaligned edges and set the cross-sectional properties;

[0006] Step 2: pre-crack definition and analysis step setting;

[0007] Step 3: Load, boundary conditions and output settings;

[0008] Step 4: Task submission for solution and post-processing.

[0009] The present invention is also characterized in that:

[0010] Step 1 is specifically implemented as follows:

[0011] Step 1.1, using 3D modeling software or finite element software to build models of the crack body and the non-crack body respectively, and merge them into an overall geometric model;

[0012] Step 1.2: Use software to define the mechanical properties parameters of the round tube and weld materials, and assign the material parameters to the corresponding structural parts;

[0013] Step 2 is specifically implemented as follows:

[0014] Step 2.1, create an analysis step, select the static general mode and edit the increment of the analysis step, set the minimum and maximum increment steps;

[0015] Step 2.2, insert crack, select the specified crack surface to insert the crack, and then select the normal and propagation direction of the crack surface;

[0016] Step 2.3: Apply loads and constraints to the pipeline structure. First, apply an all-round fixed constraint on the left end face of the pipeline. Then, select the pressure load type when applying internal pressure and axial load to the pipeline.

[0017] Step 2.2 specifically includes inserting a crack in the spceial in the toolbar, inserting a crack by setting assignseam to select a specified crack surface, establishing the area near the crack and the rest of the pipeline in two different parts, and then assembling the two parts independently through Assembly to form a complete pipeline; then using Tie binding to connect the part of the crack part with the part of the rest of the pipeline;

[0018] Step 3 is specifically implemented as follows:

[0019] Step 3.1, use singularity to set the crack tip;

[0020] Step 3.2, when setting the field output and history output, select several paths near the tip for integral solution; the integral path is selected according to the nodes near the crack, the number of contour integrals is set to 10, the output parameter is selected as Stress intensity factor, and the crack criterion is selected as the maximum energy release rate;

[0021] Specifically, step 3.1 is to modify the value of the Midside node parameter using singularity for the crack tip, that is, to change the position of the midside node on the edge of the unit by 1;

[0022] Step 4 is specifically implemented as follows:

[0023] Step 4.1. Submit and solve the stress intensity factor value, based on the contour integral method, using the finite element software to characterize the average parameter of the strain concentration characteristics at the crack tip. The two-dimensional expression of the contour integral is:

[0024] (1)

[0025] Where Γ is the integration path; W is the strain energy at any point on the integration loop / J; S ijis the partial tensor corresponding to the point; n is the unit vector along the external normal direction on the Γ loop;

[0026] In the case of linear elasticity, the J integral is expressed as the energy released when the crack expands per unit area, that is, the energy release rate G, and is also related to the stress intensity factor K I There are the following relationships:

[0027] (2)

[0028] Where, E is the elastic modulus of the material / GPa; ν is the Poisson's ratio of the material;

[0029] Step 4.2: When reading the finite element calculation results, after the model calculation results converge, read the calculation results of the 2nd to 10th integral paths and take their average value;

[0030] Step 4.3: According to the Stress Intensity Factor Manual, the uniform tensile stress σ on the pipeline and the stress intensity factor at the crack front K I The size is:

[0031] (3)

[0032] (4)

[0033] In the formula, F I is the crack shape correction factor; F is the axial tension / N; R 0 、R i is the outer radius and inner radius of the pipe / mm; a is the crack depth / mm;

[0034] Step 4.4: Calculate the stress intensity factor value at the crack tip of the model and compare it with the theoretical value in the stress intensity factor manual to verify the reliability of the finite element modeling method for defective pipelines used in this study.

[0035] The beneficial effects of the present invention are:

[0036] The method for calculating the stress intensity factor of the crack tip of the butt ring weld of a pipe with misaligned edges of the present invention obtains a method for calculating the stress intensity factor of a composite defect of a weld, providing a new idea for calculating the stress intensity factor; it can more clearly characterize the stress and strain distribution in the area near the crack tip, predict the failure position of the weld during use, and guide actual use. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1It is a schematic diagram of a symmetrical model of a girth weld pipeline in an example of a method for calculating a crack tip stress intensity factor of a girth weld butt welded pipeline with misaligned edges of the present invention;

[0038] Figure 2 The calculation method of the stress intensity factor of the crack tip of the butt weld of the pipeline with misaligned edges in the present invention is

[0039] Schematic diagram of crack locations in the method example (left is inner surface crack, right is buried crack);

[0040] Figure 3 It is a schematic diagram of crack area setting in an example of a method for calculating a crack tip stress intensity factor of a butt girth weld of a pipe with misaligned edges according to the present invention;

[0041] Figure 4 It is a schematic diagram of load and constraint application in an example of a method for calculating the stress intensity factor of a crack tip of a butt girth weld of a pipe with misaligned edges according to the present invention;

[0042] Figure 5 It is a schematic diagram of structural finite element mesh division in an example of a method for calculating a crack tip stress intensity factor of a butt girth weld of a pipe with misaligned edges according to the present invention;

[0043] Figure 6 It is a schematic diagram of the integral path setting in an example of the method for calculating the crack tip stress intensity factor of the butt girth weld of a pipe with misaligned edges according to the present invention. DETAILED DESCRIPTION

[0044] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0045] The present invention provides a method for calculating the stress intensity factor of the crack tip of the butt weld of a pipe with misaligned edges, which is specifically implemented according to the following steps:

[0046] Step 1: Establish the geometric model of the butt weld of the pipe with misaligned edges and set the section properties:

[0047] Step 1.1, use abaqus module components to build a three-dimensional solid model of the circular tube and weld, and merge them into an overall geometric model;

[0048] Step 1.2, open the software parameter setting section, select mechanical properties, select elastic force, set Young's modulus and Poisson's ratio, and assign the set material properties to the corresponding structural parts;

[0049] Step 2: Pre-crack definition and analysis step settings:

[0050] Step 2.1, set the analysis step, select Static, General type of analysis step, set reasonable minimum and maximum increment steps to ensure sufficient calculation step length and improve the calculation speed as much as possible;

[0051] Step 2.2, insert crack through spceial in the toolbar, and set assign seam

[0052] Select the specified crack surface to insert the crack. After specifying the crack front, select the normal and propagation direction of the crack surface. At this time, the crack related settings are completed;

[0053] Step 2.3, apply loads and constraints to the pipeline structure. First, apply all-round fixed constraints on the left end face of the pipeline. Then, when applying internal pressure and axial load to the pipeline, select the pressure load type. The internal pressure varies according to the thickness of the pipeline wall, and different pressure loads are applied to the inner surface of the pipeline.

[0054] Step 3: Load, boundary conditions and output settings:

[0055] Step 3.1. The area near the weld crack is the key analysis area, and the mesh accuracy is high. The crack tip has singularity, so a singular unit derived from the degeneration of the quarter point is required. The singular unit is a secondary unit, but there is no such secondary unit that can be used directly in ABAQUS, so it needs to be set using singularity. The rest of the pipeline has a simple structure and does not require high mesh accuracy.

[0056] Step 3.2: When setting the field output and history output, select several paths near the tip for integration.

[0057] Solve. The integral path is selected based on the nodes near the crack. In order to make the result more accurate, the number of contour integrals is set to 10, the output parameter is the stress intensity factor, and the crack criterion is the maximum energy release rate.

[0058] Step 4: Task submission, solution and post-processing:

[0059] Step 4.1, submit and solve the value of stress intensity factor. When reading the finite element calculation results, read the calculation results of the 2nd to 10th integral paths after the model calculation results converge, and take the average value;

[0060] Step 4.2: According to the Stress Intensity Factor Manual, the uniform tensile stress σ on the pipeline and the stress intensity factor at the crack front are K I The size is:

[0061] (3)

[0062] (4)

[0063] in, FI is the crack shape correction factor; F is Axial tension / N; R 0 、R i为 Pipe outer radius, inner radius / mm; a is Crack depth / mm;

[0064] Step 4.3: Calculate the stress intensity factor value at the crack tip of the model and compare it with the theoretical value in the stress intensity factor manual to verify the reliability of the finite element modeling method for defective pipelines used in this study.

[0065] Example

[0066] The calculation method of the stress intensity factor at the crack tip of the girth weld of the butt joint of the pipeline with misaligned edges is as follows. Taking a natural gas pipeline as an example, the girth weld defect accounts for the largest proportion of its failure. The pipeline material used is X80 pipeline steel with an outer diameter of 1422 mm; the mechanical properties of X80 pipeline steel are shown in Table 1 below:

[0067] Table 1

[0068]

[0069] The pipe specifications have four wall thicknesses of 17.8 / 21.4 / 25.7 / 30.8 mm; the operating internal pressure of the pipe with a wall thickness of 17.8 mm is 10 Mpa, and the operating internal pressure of the pipes of other specifications is 12 Mpa; the cases of various crack sizes with different depth-to-width ratios a / c and a / t and the case of buried cracks considering 2a / t are shown in Table 2:

[0070] Table 2

[0071]

[0072] This example uses a pipe wall thickness of 21.4 mm, an outer diameter of 1422 mm, an axial tensile load of 166.5 MPa, and crack sizes a / t of 0.1 / 0.2 / 0.3 / 0.4 / 0.5 for calculation;

[0073] Step 1 is implemented as follows:

[0074] Step 1.1, use abaqus module components to build a 3D solid model of the round tube and weld.

[0075] Saint-Venant theorem, in order to avoid the local effect of the boundary, the pipeline length is selected as 1600mm, and the established three-dimensional solid model is merged into the overall geometric model. Since the analysis is symmetrically distributed, 1 / 4 of the cracked pipeline is selected as the calculation model (such as Figure 1 shown);

[0076] Step 1.2, open the software parameter setting section, select mechanical properties, select elastic force, set the elastic modulus, Poisson's ratio and material density, and assign the set material properties to the corresponding structural parts;

[0077] Step 2 is implemented as follows:

[0078] Step 2.1, set the analysis step, select Static, General type of analysis step, set the total analysis step to 10000 steps, the initial step to 0.01, and the maximum step to 0.1;

[0079] Step 2.2, insert crack through spceial in the toolbar, and insert crack by setting assign seam to select the specified crack surface; after specifying the crack front, select the normal and extension direction of the crack surface. At this time, the crack related settings have been completed (such as Figure 2 , Figure 3 shown);

[0080] Step 2.3: To improve the computational efficiency and facilitate subsequent research, the area near the crack and the rest of the pipeline are established in two different parts, and then these two parts are assembled independently (Independent instance) through Assembly to form a complete pipeline; then the part of the crack part and the part of the rest of the pipeline are connected by Tie binding;

[0081] Step 2.4: Apply loads and constraints to the pipeline structure. First, apply all-round fixed constraints on the left end face of the pipeline to ensure that the pipeline will not move significantly during the calculation process. The internal pressure is different according to the thickness of the pipeline wall. Different pressure loads are applied to the inner surface of the pipeline. The axial tensile load is applied to the axial end face with -166.5Mpa or -277.5Mpa (such as Figure 4 shown);

[0082] Step 3 is implemented as follows:

[0083] Step 3.1: This study uses a three-dimensional eight-node linear hexahedral structural unit C3D8R, and weld crack

[0084] The crack tip is singular, so a singular unit derived from the degeneration of the quarter point is required. The singular unit is a secondary unit, but there is no such secondary unit that can be used directly in ABAQUS. It is necessary to use singularity to set the value of the Midside node parameter to 0.25, that is, to set the middle node at 1 / 4 of the unit edge. The rest of the pipeline has a simple structure and does not require high mesh accuracy (such as Figure 5 shown);

[0085] Step 3.2, when setting the field output and history output, select several paths near the tip for integral solution; the integral path is selected based on the nodes near the crack. In order to make the result more accurate, the number of contour integrals is set to 10, the output parameter is the stress intensity factor, and the crack criterion is the maximum energy release rate. When selecting the path, use the auxiliary lines set in advance to select the nodes to make the selected path more regular (such as Figure 6 shown);

[0086] Step 4 is implemented according to the following steps:

[0087] Step 4.1: Submit and solve the stress intensity factor value. When reading the finite element calculation results,

[0088] After the model calculation results converge, read the calculation results of the 2nd to 10th integral paths and take their average value;

[0089] Step 4.2: Substitute the parameters in the example into the following formula for calculation:

[0090] (3)

[0091] (4)

[0092] in, F I is the crack shape correction factor; F is the axial tension / N; R 0 、R i is the outer radius and inner radius of the pipe / mm; a for Crack depth / mm;

[0093] Step 4.3: Obtain the finite element calculation results K I Compare with the results obtained from theoretical calculations in the Stress Intensity Factor Handbook;

[0094] Finite element calculation results K I The results obtained from the theoretical calculation in the stress intensity factor manual are shown in Table 3 below:

[0095] Table 3

[0096]

[0097] It can be seen that the calculation results fit well, the maximum calculation error does not exceed 3%, and the calculation accuracy is high; it can be considered that the finite element model for calculating the stress intensity factor of cracked pipes established in this study is reliable.

Claims

1. The calculation method of the stress intensity factor at the crack tip of the butt weld of the pipeline with misaligned edges is characterized by: Please follow the steps below to implement: Step 1: Establish the geometric model of the butt weld of the pipe with misaligned edges and set the cross-sectional properties; Step 2: pre-crack definition and analysis step setting; Step 3: Load, boundary conditions and output settings; Step 4: Task submission, solution and post-processing; The step 3 is specifically implemented according to the following steps: Step 3.1, use singularity to set the crack tip; Step 3.2: When setting the field output and history output, select several paths near the tip for integral solution; The integral path is selected based on the nodes near the crack, the number of contour integrals is set to 10, the output parameter is the stress intensity factor, and the crack criterion is the maximum energy release rate; Specifically, the step 3.1 is that, for the crack tip part, the value of the Midside node parameter needs to be modified using singularity, that is, the position of the midside node on the edge of the unit is changed by 1; The step 4 is specifically implemented according to the following steps: Step 4.

1. Submit and solve the stress intensity factor value, based on the contour integral method, using the finite element software to characterize the average parameter of the strain concentration characteristics at the crack tip. The two-dimensional expression of the contour integral is: （1） Where Γ is the integration path; W is the strain energy at any point on the integration loop / J; S ij is the partial tensor corresponding to the point; n is the unit vector along the external normal direction on the Γ loop; In the case of linear elasticity, the J integral is expressed as the energy released when the crack expands per unit area, that is, the energy release rate G, and is also related to the stress intensity factor K I There are the following relationships: （2） Where, E is the elastic modulus of the material / GPa; ν is the Poisson's ratio of the material; Step 4.2: When reading the finite element calculation results, after the model calculation results converge, read the calculation results of the 2nd to 10th integral paths and take their average value; Step 4.3: According to the Stress Intensity Factor Manual, the uniform tensile stress σ on the pipeline and the stress intensity factor at the crack front K I The size is: （3） （4） In the formula, F I is the crack shape correction factor; F is the axial tension / N; R 0 、R i is the outer radius and inner radius of the pipe / mm; a is the crack depth / mm; Step 4.4: Calculate the stress intensity factor value at the crack tip of the model and compare it with the theoretical value in the stress intensity factor manual to verify the reliability of the finite element modeling method for defective pipelines used in this study.

2. The method for calculating the stress intensity factor at the crack tip of a butt weld of a pipe with misaligned edges according to claim 1 is characterized in that: The step 1 is specifically implemented according to the following steps: Step 1.1, using 3D modeling software or finite element software to build models of the crack body and the non-crack body respectively, and merge them into an overall geometric model; Step 1.2: Use software to define the mechanical properties parameters of the round tube and weld materials, and assign the material parameters to the corresponding structural parts.

3. The method for calculating the stress intensity factor at the crack tip of the butt weld of a pipe with misaligned edges according to claim 1 is characterized in that: The step 2 is specifically implemented according to the following steps: Step 2.1, create an analysis step, select the static general mode and edit the increment of the analysis step, set the minimum and maximum increment steps; Step 2.2, insert crack, select the specified crack surface to insert the crack, and then select the normal and propagation direction of the crack surface; Step 2.3: Apply loads and constraints to the pipeline structure. First, apply an all-round fixed constraint to the left end face of the pipeline. Then, select the pressure load type when applying internal pressure and axial load to the pipeline.

4. The method for calculating the stress intensity factor at the crack tip of a butt weld of a pipe with misaligned edges according to claim 3 is characterized in that: Specifically, the step 2.2 includes inserting a crack in the spceial in the toolbar, inserting the crack by setting assignseam to select the specified crack surface, establishing the area near the crack and the rest of the pipeline in two different parts, and then assembling the two parts independently through Assembly to form a complete pipeline; and then connecting the part of the crack part with the part of the rest of the pipeline by Tie binding.

Images