A finite element simulation method for pipeline crush sensitivity analysis under bending moment and water pressure

Abstract

The present invention relates to the field of finite element simulation technology, and in particular to a finite element simulation method for analyzing the sensitivity of pipeline crushing under bending moment and water pressure, comprising the following steps: establishing a three-dimensional pipeline model and a rigid body pressure head model; assigning shell unit section properties and material properties to the three-dimensional pipeline model; establishing different forms of bending moment models; applying an angle or section displacement to the bending moment model to cause the pipeline to bend and deform, applying external water pressure to crush the pipeline and recording the data; calculating the error rate between the recorded crushing water pressure value and the crushing water pressure value required by the specification, and if the error rate meets the requirements, performing a sensitivity factor analysis. The method provided by the present invention accurately analyzes the results and then expands the influence of bending curvature, diameter-to-thickness ratio and ellipticity sensitivity factors on pipeline buckling and crushing based on different bending moment types, which has practical engineering guidance significance.

Description

Technical Field

[0001] The present invention relates to the technical field of finite element simulation, and in particular to a finite element simulation method for analyzing pipeline crush sensitivity under bending moment and water pressure. Background Art

[0002] Submarine pipelines are a key component of offshore oil and gas field development and production systems, serving as the fastest, safest, and most economical means of transporting offshore oil and gas. The development trend of submarine pipelines requires continuous improvement in their strength. During installation and operation, submarine pipelines are subject to a combination of loads, including bending loads caused by soil movement during installation and operation. Failure manifests as bending failure primarily due to bending loads. Failure during installation or operation can cause localized buckling of the pipeline, which can propagate and lead to collapse, resulting in serious safety consequences and economic losses.

[0003] Among the many external loads, bending moment and water pressure, as the main external loads during laying and long-term operation, have a significant impact on the geometric profile and compressive resistance of the pipeline. There are three methods for submarine pipeline strength analysis: theoretical analysis, numerical simulation, and experimental research. Researchers will use a combination of multiple methods when studying submarine pipeline analysis to improve the accuracy and applicability of the results. Analysis of pipeline buckling and crush sensitivity factors usually establishes a theoretical model, uses finite element numerical analysis and corresponding experimental research, and generally simplifies the bending moment load to an ideal bending moment acting on both ends of the pipeline. However, the actual load conditions during laying and operation are generally more complex, and there will be three-point bending, four-point bending, uniformly distributed load and other forms of action, as well as differences in the pipeline's own sensitivity factors, which affect the deformation position and compressive properties of the pipeline. Therefore, in combination with the actual needs of the project, it is necessary to classify and study different bending rectangles and analyze their mechanical influence on pipeline buckling and crushing. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a finite element simulation method for analyzing the sensitivity of pipeline crushing under bending moment and water pressure. By modeling with ABAQUS, specific bending moment types such as end bends and three-point bends can be simulated to analyze the influence of different bending moment types on pipeline buckling and crushing. The analysis results are relatively accurate. Based on the different bending moment types, the influence of bending curvature, diameter-to-thickness ratio and ovality sensitivity factors on pipeline buckling and crushing are further expanded, providing a reference for actual engineering.

[0005] The present invention is achieved through the following technical solutions:

[0006] A finite element simulation method for analyzing pipeline crush sensitivity under bending moment and water pressure comprises the following steps:

[0007] S1: Use ABAQUS shell elements to build three-dimensional models of submarine pipelines with different diameter-to-thickness ratios and ovality, as well as a rigid body pressure head model for point bending simulation, and transfer the corresponding modeling data to the visualization module;

[0008] S2: Assign shell unit cross-sectional properties, material behavior elasticity and plasticity properties to the three-dimensional model of the submarine pipeline, and transmit the shell unit cross-sectional properties, material behavior elasticity and plasticity property data to the visualization module;

[0009] S3: Assemble the submarine pipeline three-dimensional model according to different types of bending moments to establish multiple different forms of submarine pipeline bending moment models;

[0010] S4: After setting boundary conditions and loads in the load module according to the requirements of different submarine pipeline bending moment models, mesh the submarine pipeline bending moment model and verify its convergence. Then, apply rotation angle or cross-sectional displacement to different submarine pipeline bending moment models to cause the pipeline to bend and deform, and apply external water pressure to crush the pipeline.

[0011] S5: Record the angle or cross-sectional displacement, bending curvature, and external water pressure data when the pipe is crushed, and transmit the data to the visualization module;

[0012] S6: The visualization module records the collapse water pressure values ​​under different bend rectangular types, pipe diameter-thickness ratios, bend curvatures, and initial ovality, and compares them with the corresponding specifications. If the specifications are met, the error rate between the recorded collapse water pressure value and the collapse water pressure value required by the specifications is calculated. If the error rate meets the requirements, a sensitive factor analysis is performed.

[0013] Optimized, the modeling data input in step S1 include pipe length, pipe diameter, and pipe thickness, and the default initial ellipticity is zero.

[0014] Optimized, when assigning the shell element section properties in step S2, 5 Simpson integration points are assigned in the thickness direction, and the assigned material behavior elastic and plastic properties include the elastic modulus, buckling limit and Poisson's ratio of the material.

[0015] Furthermore, the submarine pipeline bending moment model established in step S3 includes an end bending model, a three-point bending moment model and a pressure head bending model.

[0016] Furthermore, in step S3, the submarine pipeline bending moment model is meshed according to the global size ratio.

[0017] When establishing the optimized end bending model, first set the constraints at both ends, then perform hinge constraints on the middle section to fix the pipe model, then apply a rotation radius at both end constraints to simulate the end bending, and then apply a pressure load to simulate the external water pressure until the pipe collapses, and then perform the buckling and crush sensitivity factor analysis of the end bending model according to the S5-S6 method.

[0018] Furthermore, when establishing the three-point bending moment model, the two ends are first hinged and a coordinate system displacement constraint is applied to the middle section. Then, a pressure load is applied to simulate the external water pressure until the pipe collapses. Then, the buckling and crushing sensitivity factors of the three-point bending moment model are analyzed according to the methods of S5-S6. Furthermore, in the visualization module of S6, when comparing the crushing water pressure values ​​recorded under different bending rectangles, pipe diameter-to-thickness ratios, bending curvatures, and ovality with the corresponding specification requirements, the following method is used:

[0019] D1: First, consider the external hydrostatic pressure and calculate the crushing pressure P of different submarine pipeline bending moment models by formula (1) c (t);

[0020]

[0021] Where D is the nominal outer diameter, t is the nominal wall thickness, f o is the ellipticity, P el (t) and P p (t) is the intermediate pressure, E is the elastic modulus, ν is the Poisson's ratio, f y is the characteristic yield strength, α fab is the manufacturing coefficient;

[0022] D2: Calculate the error rate according to (2) based on the collapse water pressure value recorded by S6 and the calculated collapse pressure, and compare the calculated error rate with the set threshold. If the calculated error rate is less than or equal to the set threshold, the accuracy of the established submarine pipeline bending moment model meets the requirements. If the calculated error rate is greater than the set threshold, re-establish the submarine pipeline bending moment model by adjusting the grid density, shell unit cross-sectional properties, and material behavior elastic and plastic properties. Execute step D3 until the accuracy of the submarine pipeline bending moment model meets the requirements.

[0023]

[0024] D3: Considering the combined effect of bending moment and external pressure on the cross section of the submarine pipeline bending moment model, the collapse water pressure P is calculated using formula (3): c, then calculate the error rate according to (2) based on the collapse water pressure value recorded by S6 and the calculated collapse pressure, and compare the calculated error rate with the set threshold. If the calculated error rate is less than or equal to the set threshold, the accuracy of the submarine pipeline bending moment model established meets the requirements. If the calculated error rate is greater than the set threshold, the submarine pipeline bending moment model is re-established by adjusting the grid density, shell unit section properties, and material behavior elastic and plastic properties until the accuracy of the submarine pipeline bending moment model meets the requirements and then execute step D4:

[0025]

[0026] in: α c is the velocity stress parameter; M Sd is the design bending moment load, M p (t) represents the plastic bending moment capacity, M p (t) = f y ·(D―t) 2 t;P min is the minimum sustained internal pressure; γ m is the material grade coefficient, γ sc is the safety level resistance coefficient;

[0027] Furthermore, sensitivity analysis was conducted on each influencing factor:

[0028] First, other influencing factors were kept constant. Different bending curvatures were set for different submarine pipeline bending moment models. The collapse water pressure values ​​were recorded and the collapse water pressure-bending curvature curve was plotted. A sensitivity analysis of the collapse water pressure-bending curvature curve showed that the bending curvature and collapse water pressure showed an approximately negative linear relationship for both the end bending model and the three-point bending moment model. In addition, the collapse water pressure of the three-point bending moment model was greater than that of the end bending model under the same bending curvature.

[0029] By controlling other influencing factors, different diameter-to-thickness ratios were set for different submarine pipeline bending moment models, and the collapse water pressure values ​​were recorded. The collapse water pressure-diameter-to-thickness ratio curve was drawn. By analyzing the collapse water pressure-diameter-to-thickness ratio curve, it was found that under the combined action of bending moment and water pressure, as the diameter-to-thickness ratio increases, the pipeline's ability to resist bending moment decreases. The collapse water pressure of the three-point bending moment model at equivalent bending curvatures at different diameter-to-thickness ratios is greater than that of the end bending model, but as the diameter-to-thickness ratio increases, the collapse water pressures of the two tend to approach each other.

[0030] Then, by controlling other influencing factors unchanged, setting different initial ovality, recording the collapse water pressure value, and drawing the collapse water pressure-initial ovality curve and analyzing it, it is concluded that when the initial ovality of the end bending model and the three-point bending moment model is a small negative value, the collapse water pressure increases with the increase of the initial ovality. When it reaches a certain negative initial ovality, the collapse water pressure decreases with the increase of the initial ovality. Moreover, the collapse water pressure of the three-point bending moment model is not always greater than that of the end bending model under the same sensitivity factors of curvature and diameter-to-thickness ratio.

[0031] Furthermore, the visualization module in S6 compares the collapse water pressure values ​​recorded under different bend shapes, pipe diameter-to-thickness ratios, bend curvatures, and ovality with the corresponding specification requirements in the following manner:

[0032] E1: Calculate the crushing pressure P of different submarine pipeline bending moment models under the combined action of bending moment and external pressure using formula (4) c (t);

[0033]

[0034] Where: P0 is the collapse pressure of the pipe under pure water pressure, k b is the curvature of the pipe under pure bending moment, k max is the maximum curvature actually experienced by the pipe, g(Δ) is a function of the initial ellipticity, g(Δ)=(1+20Δ) ―1 , △ is the initial ellipticity;

[0035] E2: Calculate the error rate according to (2) based on the collapse water pressure value recorded by S6 and the calculated collapse pressure, and compare the calculated error rate with the set threshold. If the calculated error rate is less than or equal to the set threshold, the accuracy of the established submarine pipeline bending moment model meets the requirements. If the calculated error rate is greater than the set threshold, re-establish the submarine pipeline bending moment model by adjusting the grid density, shell unit section properties, and material behavior elastic and plastic properties until the accuracy of the submarine pipeline bending moment model meets the requirements and then jump to step D4.

[0036] Advantageous Effects of the Invention

[0037] The present invention provides a finite element simulation method for analyzing pipeline crush sensitivity under bending moment and water pressure, which has the following advantages:

[0038] (1) Modeling is performed by ABAQUS to simulate specific bending moment types such as end bending (i.e., ideal pure bending) and three-point bending. The influence of different bending moment types on pipeline buckling and crushing can be analyzed, and the analysis results are relatively accurate.

[0039] (2) Based on different bending moment types, the influence of bending curvature, diameter-to-thickness ratio and ellipticity sensitivity on pipeline buckling and crushing can be further analyzed, which has good engineering guidance significance. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 It is the overall flow chart of the present invention;

[0041] Figure 2 is a stress-strain diagram of the material of the present invention;

[0042] Figure 3 This is a schematic diagram of establishing reference points for the end bend model of the present invention;

[0043] Figure 4 This is a schematic diagram of the corner applied to the end bend model of the present invention;

[0044] Figure 5 Schematic diagram of the cross-sectional displacement applied to the three-point bending model of the present invention;

[0045] Figure 6 It is the crushing water pressure-bending curvature curve of the present invention;

[0046] Figure 7 This is the collapse water pressure-diameter-to-thickness ratio curve of the present invention;

[0047] Figure 8 It is the collapse water pressure-initial ellipticity curve of the present invention;

[0048] Figure 9-12 It is a schematic diagram of the rigid body indenter model of the present invention. DETAILED DESCRIPTION

[0049] A finite element simulation method for analyzing pipeline collapse sensitivity under bending moment and water pressure. The specific flow chart is as shown in the attached figure. Figure 1 As shown, it needs to be redrawn, which includes the following steps:

[0050] S1: Use ABAQUS shell elements to establish three-dimensional models of submarine pipelines with different pipe diameter-to-thickness ratios and ellipticities, as well as rigid body pressure head models for point bending simulation, and transfer the corresponding modeling data to the visualization module; different forms of bend rectangles can be established by assembling different rigid body pressure head models. Shell elements are used for pipeline simulation. Shell elements can simulate structures where the scale in one direction (the thickness of the submarine pipeline) is much smaller than the scale in other directions, and the stress and sum along the thickness direction are negligible. Specifically, the S4R element, that is, the reduced integration 4-node shell element, can be used. It is a general shell element type with the advantages of stable performance and a wide range of applications. It is conducive to reducing operation time and focusing on the overall compression changes of the pipeline. It meets the conditions for establishing shell elements. The specific modeling data are shown in Table 1:

[0051] Table 1 Model parameters

[0052]

[0053] S2: Assign shell unit cross-sectional properties, material behavior elasticity and plasticity to the three-dimensional model of the submarine pipeline, and transfer the shell unit cross-sectional properties, material behavior elasticity and plasticity data to the visualization module; when assigning shell unit cross-sectional properties, five Simpson integration points can be assigned in the thickness direction, which is equivalent to dividing the unit into six layers in the thickness direction. Gaussian integration points have higher calculation accuracy than Simpson integration points. The more integration points there are, the more accurate the calculation results are. Create materials and assign them specific elastic and plasticity properties, elastic modulus, Poisson's ratio, etc., focusing on analyzing the crushing strain. The specific implementation material can be X65 steel, and refer to the corresponding stress-strain diagram of the steel as shown in the attached figure. Figure 2 As shown, the material is given elastic and plastic strain.

[0054] S3: According to different types of bending moments, assemble the submarine pipeline three-dimensional model to establish a variety of submarine pipeline bending moment models; through the assembly of different rigid body pressure head models, the specific rigid body pressure head model can be as shown in the attached Figure 9-12 As shown, a variety of different forms of submarine pipeline bending moment models can be established, which have a wider range of adaptability.

[0055] When assembling the end bend model: first hinge and fix the coupling reference point of the middle section of the submarine pipeline 3D model, and set the coupling reference points at both ends, as shown in the attached figure. Figure 3 As shown in the figure, the angle α is applied to the coupling reference points at both ends to control the bending curvature k of the submarine pipeline three-dimensional model. Figure 4 When assembling the three-point bending model: first, the reference points of the cross-section coupling at both ends are hinged and fixed, and then the curvature radius is controlled to achieve the corresponding equivalent bending curvature k by applying the displacement of the middle section, as shown in the attached figure. Figure 5 Specifically, the equivalent curvature k can be achieved by applying displacement to different pressure points according to the number of different pressure heads.

[0056] S4: According to the requirements of different submarine pipeline bending moment models, after setting the boundary conditions and loads in the load module, the submarine pipeline bending moment model is meshed and the convergence is verified. Then, the different submarine pipeline bending moment models are applied with angle or cross-sectional displacement to cause the pipeline to bend and deform, and external water pressure is applied to cause the pipeline to collapse. Setting different boundary conditions according to different types of bending moments can simulate the corresponding curvature and bending moment.

[0057] S5: Record the angle or cross-sectional displacement, bending curvature, and external water pressure data when the pipe is crushed, and transmit the data to the visualization module;

[0058] S6: The visualization module records the collapse water pressure values ​​under different bend rectangular types, pipe diameter-thickness ratios, bend curvatures, and initial ovality, and compares them with the corresponding specifications. If the specifications are met, the error rate between the recorded collapse water pressure value and the collapse water pressure value required by the specifications is calculated. If the error rate meets the requirements, a sensitive factor analysis is performed.

[0059] Optimized, the modeling data input in step S1 include pipe length, pipe diameter, and pipe thickness, and the default initial ellipticity is zero.

[0060] Optimized, when assigning the shell element section properties in step S2, 5 Simpson integration points are assigned in the thickness direction, and the assigned material behavior elastic and plastic properties include the elastic modulus, buckling limit and Poisson's ratio of the material.

[0061] Furthermore, the submarine pipeline bending moment model established in step S3 includes an end bending model, a three-point bending moment model and a pressure head bending model.

[0062] When establishing an end bending model, first set constraints at both ends: constrain U1 and UR2 with UR3 (U1 = 0, UR2 = UR3 = 0), then apply a hinge constraint to the midsection (U1 = U2 = U3 = 0) to fix the pipeline model. Apply a UR1 rotation angle at both ends to simulate end bending, then apply a pressure load to simulate external water pressure to study the pipeline's crush pressure. This allows analysis of the sensitivity factors to buckling and crushing of submarine pipelines under end bending moments.

[0063] When establishing a three-point bending moment model, the initial step is to set the hinges acting on the corresponding reference points at both ends, that is, the x, y, and z directions are fixed (U1 = U2 = U3 = O). Coordinate system displacement (U1, U2, U3) or rotation (UR1, UR2, UR3) constraints are applied to the mid-section reference points. U1 is the movement of the reference point in the X-axis direction, U2 is the movement of the reference point in the Y-axis direction, U3 is the movement of the reference point in the Z-axis direction, UR1 is the rotation of the reference point in the X-axis direction, UR2 is the rotation of the reference point in the Y-axis direction, and UR3 is the rotation of the reference point in the Z-axis direction. By adding the above-mentioned boundary conditions with different displacements to the reference points, the three-point bending moment type is simulated. Then, a pressure load is applied to simulate external water pressure and study the pipeline crush pressure. This allows for an analysis of the sensitivity factors of submarine pipeline buckling and crushing under three-point bending moment conditions.

[0064] Furthermore, in step S3, the submarine pipeline bending moment model is meshed according to the global size ratio, and the mesh can be refined in the key area. Before the pipeline simulation, the pipeline model is meshed and the independence convergence verification is performed. The more mesh units, the more accurate the result, but the longer the calculation time. By establishing pipeline models with different mesh densities, the test crushing pressure P is tested.c The results were compared to ensure the reliability and practicality of the model.

[0065] When establishing the optimized end bending model, first set the constraints at both ends, then perform hinge constraints on the middle section to fix the pipe model, then apply a rotation radius at both end constraints to simulate the end bending, and then apply a pressure load to simulate the external water pressure until the pipe collapses, and then perform the buckling and crush sensitivity factor analysis of the end bending model according to the S5-S6 method.

[0066] Furthermore, when establishing the three-point bending moment model, the two ends are first hinged and a coordinate system displacement constraint is applied to the middle section. Then, a pressure load is applied to simulate the external water pressure until the pipe collapses. Then, the buckling and crushing sensitivity factors of the three-point bending moment model are analyzed according to the methods of S5-S6. Furthermore, in the visualization module of S6, when comparing the crushing water pressure values ​​recorded under different bending rectangles, pipe diameter-to-thickness ratios, bending curvatures, and ovality with the corresponding specification requirements, the following method is used:

[0067] D1: First, consider the external hydrostatic pressure and calculate the crushing pressure P of different submarine pipeline bending moment models by formula (1) c (t);

[0068]

[0069] Where D is the nominal outer diameter, t is the nominal wall thickness, f o is the ellipticity, P el (t) and P p (t) is the intermediate pressure, E is the elastic modulus, ν is the Poisson's ratio, f y is the characteristic yield strength, α fab is the manufacturing coefficient;

[0070] D2: Calculate the error rate according to (2) based on the collapse water pressure value recorded by S6 and the calculated collapse pressure, and compare the calculated error rate with the set threshold. If the calculated error rate is less than or equal to the set threshold, the accuracy of the established submarine pipeline bending moment model meets the requirements. If the calculated error rate is greater than the set threshold, re-establish the submarine pipeline bending moment model by adjusting the grid density, shell unit cross-sectional properties, and material behavior elastic and plastic properties. Execute step D3 until the accuracy of the submarine pipeline bending moment model meets the requirements.

[0071]

[0072] D3: Considering the combined effect of bending moment and external pressure on the cross section of the submarine pipeline bending moment model, the collapse water pressure P is calculated using formula (3): c, then calculate the error rate according to (2) based on the collapse water pressure value recorded by S6 and the calculated collapse pressure, and compare the calculated error rate with the set threshold. If the calculated error rate is less than or equal to the set threshold, the accuracy of the submarine pipeline bending moment model established meets the requirements. If the calculated error rate is greater than the set threshold, the submarine pipeline bending moment model is re-established by adjusting the grid density, shell unit section properties, and material behavior elastic and plastic properties until the accuracy of the submarine pipeline bending moment model meets the requirements and then execute step D4:

[0073]

[0074] in: α c is the velocity stress parameter; M Sd is the design bending moment load, M p (t) represents the plastic bending moment capacity, M p (t) = f y ·(D―t) 2 t;P min is the minimum sustained internal pressure; γ m is the material grade coefficient, γ sc is the safety level resistance coefficient;

[0075] First, the calculated error rate is compared with a set threshold when subjected to external hydrostatic pressure. If the calculated error rate is less than or equal to the set threshold, the next step is to compare and verify the submarine pipeline bending moment model cross section under the combined action of bending moment and external pressure. If the calculated error rate is greater than the set threshold, the submarine pipeline bending moment model is rebuilt by adjusting the mesh density, shell element cross-sectional properties, and material behavior elastic and plastic properties. This progressive comparison, verification, and correction method can further ensure the accuracy of the submarine pipeline bending moment model and the precision of the verification results.

[0076] Furthermore, sensitivity analysis was conducted on each influencing factor:

[0077] First, other influencing factors are controlled to be constant, and different bending curvatures are set for different types of submarine pipeline bending moment models. In a specific embodiment, the bending curvature k1 can be set to 0.05m. ―1 , k2=0.1m ―1 , k3=0.15m ―1 , k4=0.2m ―1 , record the collapse water pressure value and draw the collapse water pressure-bending curvature curve, as shown in the attached Figure 6Through the sensitivity analysis of the collapse water pressure-bending curvature curve, it is found that the bending curvature and collapse water pressure show an approximately negative linear relationship for both the end bending model and the three-point bending moment model, and the collapse water pressure of the three-point bending moment model is greater than that of the end bending model under the same bending curvature.

[0078] Then, other influencing factors are controlled unchanged, and different diameter-to-thickness ratios are set for different types of submarine pipeline bending moment models. For example, under the same curvature, the diameter-to-thickness ratios are set to 15, 20, 25, 30, 40, and 50, respectively. The collapse water pressure value is recorded, and the collapse water pressure-diameter-to-thickness ratio curve is drawn. The details are shown in the attached figure. Figure 7 As shown in the figure, analysis of the collapse water pressure-diameter-to-thickness ratio curve reveals that under the combined effects of bending moment and water pressure, as the diameter-to-thickness ratio increases, the pipe's ability to resist bending moment decreases. The collapse water pressure of the three-point bending moment model at equivalent bending curvatures at different diameter-to-thickness ratios is greater than that of the end-bend model, but the two collapse water pressures approach each other as the diameter-to-thickness ratio increases. The diameter-to-thickness ratio of a pipe significantly affects its bending and compressive resistance. A larger diameter-to-thickness ratio correlates with poorer bending and compressive resistance and a lower collapse water pressure. Therefore, the diameter-to-thickness ratio of pipes should be smaller in deep and ultra-deepwater environments.

[0079] Then, other influencing factors are controlled to remain unchanged, that is, the parameters of the pipeline and the boundary conditions of the model establishment remain unchanged, and the bending curvature k is controlled to be 0.2m. ―1 The initial ellipticity Δ0 is set to be constant. The definition of the initial ellipticity can be calculated by formula (4):

[0080]

[0081] Where: D x is the diameter length of the pipe section in the x-axis direction, D y is the length of the path in the y-axis direction. x 、D y The initial ellipticity is calculated by formula (4) to be -2%, -0.2%, 0, 0.2%, and 2%, respectively. The collapse water pressure value is recorded, and the collapse water pressure-initial ellipticity curve is drawn. Figure 8As shown, analysis of the collapse water pressure-initial ovality curve reveals that for both the end-bend model and the three-point bending moment model, the collapse water pressure increases with increasing initial ovality when the initial ovality is a small negative value. However, when the initial ovality reaches a certain negative value, the collapse water pressure decreases as the initial ovality increases. Furthermore, the collapse water pressure of the three-point bending moment model is not always greater than that of the end-bend model under the same sensitivity factors of curvature and diameter-to-thickness ratio. By observing the pipe cross-section after bending deformation, it is found that bending moment loads increase the ovality of the pipe cross-section, thereby reducing the pipe's collapse water pressure. Therefore, setting a reasonable negative ovality value for the pipe can increase the collapse water pressure under the combined action of bending moment and water pressure.

[0082] Furthermore, the visualization module in S6 compares the collapse water pressure values ​​recorded under different bend shapes, pipe diameter-to-thickness ratios, bend curvatures, and ovality with the corresponding specification requirements in the following manner:

[0083] E1: Calculate the crushing pressure P of different submarine pipeline bending moment models under the combined action of bending moment and external pressure using formula (4) c (t);

[0084]

[0085] Where: P0 is the collapse pressure of the pipe under pure water pressure, k b is the curvature of the pipe under pure bending moment, k max is the maximum curvature actually experienced by the pipe, g(Δ) is a function of the initial ellipticity, g(Δ)=(1+20Δ) ―1 , △ is the initial ellipticity;

[0086] E2: Calculate the error rate according to (2) based on the collapse water pressure value recorded by S6 and the calculated collapse pressure, and compare the calculated error rate with the set threshold. If the calculated error rate is less than or equal to the set threshold, the accuracy of the established submarine pipeline bending moment model meets the requirements. If the calculated error rate is greater than the set threshold, re-establish the submarine pipeline bending moment model by adjusting the grid density, shell unit section properties, and material behavior elastic and plastic properties until the accuracy of the submarine pipeline bending moment model meets the requirements and then jump to step D4.

[0087] By calculating the error rate in two ways, it can meet the verification requirements of DNV-ST-F101-2017 and API specifications at the same time, improving the accuracy of the simulation results and the applicability of the specifications.

[0088] In summary, the present invention provides a finite element simulation method for analyzing the sensitivity of pipeline crushing under bending moment and water pressure. Modeling is carried out through ABAQUS, and specific bending moment types such as end bends and three-point bends are simulated. The influence of different bending moment types on pipeline buckling and crushing can be analyzed. The analysis results are relatively accurate, and the influence of bending curvature, diameter-to-thickness ratio and ellipticity sensitivity factors on pipeline buckling and crushing is further expanded on the basis of different bending moment types, which has practical engineering guidance significance. The above is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention can have various changes and variations. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A finite element simulation method for analyzing pipeline crush sensitivity under bending moment and water pressure, characterized in that: The steps include: S1: Use ABAQUS shell elements to build three-dimensional models of submarine pipelines with different diameter-to-thickness ratios and ovality, as well as a rigid body pressure head model for point bending simulation, and transfer the corresponding modeling data to the visualization module; S2: Assign shell unit cross-sectional properties, material behavior elasticity and plasticity properties to the three-dimensional model of the submarine pipeline, and transmit the shell unit cross-sectional properties, material behavior elasticity and plasticity property data to the visualization module; S3: Assemble the submarine pipeline three-dimensional model according to different types of bending moments to establish multiple different forms of submarine pipeline bending moment models; S4: After setting boundary conditions and loads in the load module according to the requirements of different submarine pipeline bending moment models, mesh the submarine pipeline bending moment model and verify its convergence. Then, apply rotation angle or cross-sectional displacement to different submarine pipeline bending moment models to cause the pipeline to bend and deform, and apply external water pressure to crush the pipeline. S5: Record the angle or cross-sectional displacement, bending curvature, and external water pressure data when the pipe is crushed, and transmit the data to the visualization module; S6: The visualization module records the collapse water pressure values ​​under different bend rectangular types, pipe diameter-thickness ratios, bend curvatures, and initial ovality, and compares them with the corresponding specifications. If the specifications are met, the error rate between the recorded collapse water pressure value and the collapse water pressure value required by the specifications is calculated. If the error rate meets the requirements, a sensitive factor analysis is performed.

2. The finite element simulation method for analyzing pipeline crush sensitivity under bending moment and water pressure according to claim 1 is characterized in that: The modeling data input in step S1 include the pipe length, pipe diameter, and pipe thickness, and the default initial ellipticity is zero.

3. The finite element simulation method for analyzing pipeline crush sensitivity under bending moment and water pressure according to claim 1 is characterized in that: When assigning the shell element cross-section properties in step S2, five Simpson integration points are assigned in the thickness direction, and the assigned material behavior elastic and plastic properties include the elastic modulus, buckling limit, and Poisson's ratio of the material.

4. The finite element simulation method for analyzing pipeline crush sensitivity under bending moment and water pressure according to claim 1 is characterized in that: The submarine pipeline bending moment model established in step S3 includes an end bending model, a three-point bending moment model, and a pressure head bending model.

5. The finite element simulation method for analyzing pipeline crush sensitivity under bending moment and water pressure according to claim 1 is characterized in that: In step S3, the submarine pipeline bending moment model is meshed according to the global size ratio.

6. The finite element simulation method for analyzing pipeline crush sensitivity under bending moment and water pressure according to claim 4 is characterized in that: When establishing the end bending model, first set the constraints at both ends, then apply hinge constraints to the middle section to fix the pipe model, then apply a rotation radius at the two end constraints to simulate the end bending, and then apply a pressure load to simulate the external water pressure until the pipe collapses. Then, use the S5-S6 method to analyze the buckling and crush sensitivity factors of the end bending model.

7. The finite element simulation method for analyzing pipeline crush sensitivity under bending moment and water pressure according to claim 4 is characterized in that: When establishing the three-point bending moment model, first set the two ends to be hinged, and apply the coordinate system displacement constraint to the middle section. Then, apply the pressure load to simulate the external water pressure until the pipe collapses. Then, the buckling and crushing sensitivity factors of the three-point bending moment model are analyzed according to the S5-S6 method.

8. The finite element simulation method for analyzing pipeline crush sensitivity under bending moment and water pressure according to claim 1 is characterized in that: The visualization module in S6 compares the collapse water pressure values ​​recorded under different bend shapes, pipe diameter-to-thickness ratios, bend curvatures, and ovality with the corresponding specification requirements in the following manner: D1: First, consider the external hydrostatic pressure and calculate the crushing pressure P of different submarine pipeline bending moment models by formula (1) c (t); Where D is the nominal outer diameter, t is the nominal wall thickness, f o is the ellipticity, P el (t) and P p (t) is the intermediate pressure, E is the elastic modulus, ν is the Poisson's ratio, f y is the characteristic yield strength, α fab is the manufacturing coefficient; D2: Calculate the error rate according to (2) based on the collapse water pressure value recorded by S6 and the calculated collapse pressure, and compare the calculated error rate with the set threshold. If the calculated error rate is less than or equal to the set threshold, the accuracy of the established submarine pipeline bending moment model meets the requirements. If the calculated error rate is greater than the set threshold, re-establish the submarine pipeline bending moment model by adjusting the grid density, shell unit cross-sectional properties, and material behavior elastic and plastic properties. Execute step D3 until the accuracy of the submarine pipeline bending moment model meets the requirements. D3: Considering the combined effect of bending moment and external pressure on the cross section of the submarine pipeline bending moment model, the collapse water pressure P is calculated using formula (3): c , then calculate the error rate according to (2) based on the collapse water pressure value recorded by S6 and the calculated collapse pressure, and compare the calculated error rate with the set threshold. If the calculated error rate is less than or equal to the set threshold, the accuracy of the established submarine pipeline bending moment model meets the requirements. If the calculated error rate is greater than the set threshold, the submarine pipeline bending moment model is re-established by adjusting the grid density, shell unit section properties, and material behavior elastic and plastic properties until the accuracy of the submarine pipeline bending moment model meets the requirements and then execute step D4: in: α c is the velocity stress parameter; M Sd is the design bending moment load, M p (t) represents the plastic bending moment capacity, M p (t) = f y ·(D―t) 2 t;P min is the minimum sustained internal pressure; γ m is the material grade coefficient, γ sc is the safety level resistance coefficient; D4: Conduct sensitivity analysis on each influencing factor: First, other influencing factors were kept constant. Different bending curvatures were set for different submarine pipeline bending moment models. The collapse water pressure values ​​were recorded and the collapse water pressure-bending curvature curve was plotted. A sensitivity analysis of the collapse water pressure-bending curvature curve showed that the bending curvature and collapse water pressure showed an approximately negative linear relationship for both the end bending model and the three-point bending moment model. In addition, the collapse water pressure of the three-point bending moment model was greater than that of the end bending model under the same bending curvature. By controlling other influencing factors, different diameter-to-thickness ratios were set for different submarine pipeline bending moment models, and the collapse water pressure values ​​were recorded. The collapse water pressure-diameter-to-thickness ratio curve was drawn. By analyzing the collapse water pressure-diameter-to-thickness ratio curve, it was found that under the combined action of bending moment and water pressure, as the diameter-to-thickness ratio increases, the pipeline's ability to resist bending moment decreases. The collapse water pressure of the three-point bending moment model at equivalent bending curvatures at different diameter-to-thickness ratios is greater than that of the end bending model, but as the diameter-to-thickness ratio increases, the collapse water pressures of the two tend to approach each other. Then, by controlling other influencing factors unchanged, setting different initial ovality, recording the collapse water pressure value, and drawing the collapse water pressure-initial ovality curve and analyzing it, it is concluded that when the initial ovality of the end bending model and the three-point bending moment model is a small negative value, the collapse water pressure increases with the increase of the initial ovality. When it reaches a certain negative initial ovality, the collapse water pressure decreases with the increase of the initial ovality. Moreover, the collapse water pressure of the three-point bending moment model is not always greater than that of the end bending model under the same sensitivity factors of curvature and diameter-to-thickness ratio.

9. The finite element simulation method for analyzing pipeline crush sensitivity under bending moment and water pressure according to claim 1, characterized in that: The visualization module in S6 compares the collapse water pressure values ​​recorded under different bend shapes, pipe diameter-to-thickness ratios, bend curvatures, and ovality with the corresponding specification requirements in the following manner: E1: Calculate the crushing pressure P of different submarine pipeline bending moment models under the combined action of bending moment and external pressure using formula (4) c (t); Where: P0 is the collapse pressure of the pipe under pure water pressure, k b is the curvature of the pipe under pure bending moment, k max is the maximum curvature actually experienced by the pipe, g(Δ) is a function of the initial ellipticity, g(Δ)=(1+20Δ) ―1 , △ is the initial ellipticity; E2: Calculate the error rate according to (2) based on the collapse water pressure value recorded by S6 and the calculated collapse pressure, and compare the calculated error rate with the set threshold. If the calculated error rate is less than or equal to the set threshold, the accuracy of the established submarine pipeline bending moment model meets the requirements. If the calculated error rate is greater than the set threshold, re-establish the submarine pipeline bending moment model by adjusting the grid density, shell unit section properties, and material behavior elastic and plastic properties until the accuracy of the submarine pipeline bending moment model meets the requirements and then jump to step D4.

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