Analytical Method for Pipeline Stress Sensitivity Under Landslide Action

Abstract

A method for analyzing pipeline stress sensitivity under landslide action comprises the following steps: establishing a pipeline constitutive model by applying a Ramberg-Osgood model; assigning the pipeline constitutive model and pipeline performance parameters to a pipe unit of finite element software for mechanical stress and strain analysis to establish the pipeline model; determining soil physical and mechanical indicators according to soil type; defining three soil springs to simulate pipe-soil interaction in three directions of a buried pipeline, and calculating pipe-soil interaction parameters of the buried pipeline in combination with the soil physical and mechanical indicators, the outer diameter of the buried pipeline, and the burial depth; assigning the pipe-soil interaction parameters to a soil spring unit in the finite element software to establish a soil spring model; establishing a finite element model for stress analysis of the buried pipeline under landslide action by applying the pipe unit; simulating the stress distribution and maximum stress of the buried pipeline under landslide action; and varying the model parameters to obtain the influence of different factors on the stress distribution of the buried pipeline under landslide action and the maximum stress, thereby accurately analyzing the pipe-soil interaction under landslide action.

Description

Technical Field

[0001] The present invention relates to the technical field of pipeline stress analysis, and in particular to a method for analyzing pipeline stress sensitivity under landslide action. Background Art

[0002] Due to my country's complex terrain, oil and gas pipelines inevitably pass through geological disaster zones, presenting significant potential safety hazards. Pipeline landslides account for over 70% of pipeline geological disasters, making them the most significant geological disaster for oil and gas pipeline safety. Landslides pose a significant threat to the safe operation of oil and gas pipelines. Once the maximum additional stress acting on an oil and gas pipeline exceeds the allowable stress, it can cause excessive displacement or deformation, potentially leading to rupture and leakage, seriously compromising pipeline safety. Therefore, analyzing pipeline stress under landslide conditions to ensure safe pipeline operation and effectively prevent accidents has become a pressing research topic in this field.

[0003] To analyze pipeline stress under landslides, determine the safety status of oil and gas pipelines, provide timely warnings, and propose appropriate measures, it is necessary to deeply understand and comprehend the dynamic deformation characteristics of landslides and the interaction between pipes and soil (pipelines and soil). The actual stress conditions of pipelines in landslide sections are affected by many factors, including pipeline depth, internal pressure, landslide width, and landslide displacement. Currently used analytical methods struggle to clearly describe the interaction between pipes and soil, and the complex evaluation methods derived from theoretical derivations are difficult to apply in practice. Therefore, developing an analytical method for analyzing pipeline stress sensitivity under landslides to accurately analyze the interaction between pipes and soil, study the influence of different factors on pipeline stress conditions, and analyze the distribution of pipeline stress under landslides, thereby assisting in the development of scientific measures and strategies for preventing and controlling pipeline hazards under landslides, has become a pressing technical issue in this field and holds significant practical and guiding significance. Summary of the Invention

[0004] The purpose of this technical solution is to analyze the impact of different factors on the stress conditions of pipelines in order to propose an analytical method for the stress sensitivity of pipelines under landslides. This analytical method can accurately analyze the interaction between pipes and soil and the stress distribution pattern of pipelines under landslides, thereby providing strong data support for scientific prevention and control measures and strategies for pipeline hazards under landslides.

[0005] To achieve the above objectives, the present technical solution provides a method for analyzing pipeline stress sensitivity under landslide action, the steps of the analysis method comprising:

[0006] Establish a pipeline constitutive model: Among them, ε is the pipe strain, σ is the stress on the pipe, its unit is MPa, E is the elastic modulus of the pipe, its unit is MPa, σ s is the yield strength of the pipeline, the unit is MPa, α is the yield offset of the pipeline, and n is the hardening index of the pipeline;

[0007] Establish pipeline model: pipeline constitutive model And the elastic modulus E of the pipeline, the yield strength σ of the pipeline s , the yield offset α of the pipeline, the hardening index n of the pipeline, the Poisson's ratio of the pipeline, the stress σ of the pipeline and the corresponding pipeline strain ε are assigned to the pipe element in the finite element software used for mechanical stress and strain analysis to establish a pipeline model;

[0008] Determine the physical and mechanical indicators of the soil: Determine the physical and mechanical indicators of the soil according to the type of soil;

[0009] Calculate the pipe-soil interaction parameters for buried pipelines: Define three soil springs for the buried pipeline to simulate the horizontal soil pressure, axial soil friction, and vertical soil pressure. Combined with the physical and mechanical indicators of the soil, the outer diameter of the buried pipeline, and the burial depth, calculate the ultimate resistance and yield displacement in the horizontal direction, the ultimate resistance and yield displacement in the axial direction, and the ultimate resistance and yield displacement in the vertical direction.

[0010] Establish a soil spring model: The outer diameter and burial depth of the buried pipeline, as well as the ultimate resistance and yield displacement in the horizontal direction, the ultimate resistance and yield displacement in the axial direction, and the ultimate resistance and yield displacement in the vertical direction of the buried pipeline are assigned to the soil spring unit in the finite element software used for mechanical stress and strain analysis to establish the soil springs in the horizontal, axial, and vertical directions of the buried pipeline;

[0011] Establishing a finite element model for stress analysis of buried pipelines under landslides: Based on the establishment of a pipeline model using pipe units using finite element software for mechanical stress and strain analysis, the length of the pipe unit, the width of the non-landslide section and the width of the landslide section of the buried pipeline are set in the pipe unit, and soil springs in the horizontal, axial, and vertical directions of the buried pipeline established by connecting the soil spring units are set at the nodes of each pipe unit;

[0012] Simulating the stress of a buried pipeline under the action of a landslide: applying the pipe unit to constrain the six degrees of freedom of the end of the buried pipeline, setting the operating internal pressure of the buried pipeline, and applying displacement loads in at least one of the horizontal, axial, and vertical directions to the soil springs established by the soil spring unit connected to the landslide section of the buried pipeline, so as to obtain the stress distribution and maximum stress of the buried pipeline under the action of the landslide;

[0013] Stress sensitivity analysis of buried pipelines under landslides: Based on the finite element model for stress analysis of buried pipelines under landslides, the buried depth, operating internal pressure, landslide section width, and at least one of the horizontal, axial, or vertical displacement loads on the landslide section are changed to determine the effects of different factors on the stress distribution of buried pipelines under landslides and the maximum stress.

[0014] The above-mentioned analysis method of the present technical solution can establish a finite element model for stress analysis of buried pipelines under landslides for oil and natural gas transmission pipelines. By changing one or more parameter values ​​of the buried pipeline's burial depth, operating internal pressure, landslide section width, and landslide displacement, the influence of these parameter values ​​on the stress of the buried pipeline can be analyzed, thereby accurately analyzing the interaction between the pipe and the soil, controlling the sensitivity of the buried pipeline stress under landslides, determining the stress state of the buried pipeline, judging the safety level of the buried pipeline, and providing strong data support for scientific prevention and control measures and strategies for pipeline hazards.

[0015] As another implementation of this technical solution, the elastic modulus E of the pipeline, the Poisson's ratio of the pipeline, the yield strength σ of the pipeline s The values ​​of the yield offset α of the pipeline and the hardening index n of the pipeline are obtained by sampling the steel of the buried pipeline and performing axial tensile tests on the sampled steel.

[0016] As another implementation of this technical solution, when the test conditions are insufficient, the elastic modulus E of the pipeline, the Poisson's ratio of the pipeline, the yield strength σ of the pipeline s The values ​​of the pipeline's yield offset α and the pipeline's hardening index n can also be obtained through the parameter values ​​of the Ramberg-Osgood model corresponding to different steel grades in the "GB / T 50470 Technical Specification for Seismic Resistance of Oil and Gas Pipeline Engineering".

[0017] As another implementation of the present technical solution, the physical and mechanical indicators of the soil are obtained through soil mechanics testing methods, wherein the physical and mechanical indicators of the soil include soil cohesion, total bulk density, effective bulk density and pipe-soil friction angle.

[0018] As another implementation of this technical solution, the ultimate axial resistance of the buried pipeline is: Where D is the outer diameter of the buried pipeline, its unit is m, c is the cohesion of the soil, its unit is Pa, H is the burial depth of the buried pipeline, its unit is m, is the effective bulk density of the soil, its unit is N / m 3 , β is the viscosity coefficient, Its unit is kPa / 100, δ is the pipe-soil friction angle, δ=fφ, φ is the soil internal friction angle, f is the friction coefficient and f=0.6, K0 is the soil pressure coefficient, K0=1-sin(φπ / 180);

[0019] The ultimate resistance of buried pipelines in the horizontal direction is: Among them, N ch is the horizontal bearing capacity coefficient of hard clay or loose clay and is 0 when c=0. N qh is the horizontal bearing capacity coefficient of dense sand or loose sand and is 0 when φ=0°, N qh =a+b(x)+c(x 2 )+d(x 3 )+e(x 4 ), the above a, b, c, d, e coefficients are obtained by interpolation, and the interpolation table is as follows:

[0020] coefficient φ x a b c d e <![CDATA[N ch ]]> 0° H / D 6.752 0.065 -11.063 7.119 -- <![CDATA[N qh ]]> 20° H / D 2.399 0.439 -0.03 <![CDATA[1.059(10) -3 ]]> <![CDATA[-1.754(10) -5 ]]> <![CDATA[N qh ]]> 25° H / D 3.332 0.839 -0.09 <![CDATA[5.606(10) -3 ]]> <![CDATA[-1.319(10) -4 ]]> <![CDATA[N qh ]]> 30° H / D 4.565 1.234 -0.089 <![CDATA[4.275(10) -3 ]]> <![CDATA[-9.159(10) -5 ]]> <![CDATA[N qh ]]> 35° H / D 6.816 2.019 -0.146 <![CDATA[7.651(10) -3 ]]> <![CDATA[-1.683(10) -4 ]]> <![CDATA[N qh ]]> 40° H / D 10.959 1.783 0.045 <![CDATA[-5.425(10) -3 ]]> <![CDATA[-1.153(10) -4 ]]> <![CDATA[N qh ]]> 45° H / D 17.658 3.309 0.048 <![CDATA[-6.443(10) -3 ]]> <![CDATA[-1.299(10) -4 ]]> ;

[0021] The ultimate resistance of the buried pipeline in the vertical upward direction is: Among them, N cv is the vertical upward coefficient of hard clay or loose clay and when c=0, it is 0, N cv =2*(H / D)≤10, N qv It is the vertical upward coefficient of dense sand or loose sand and when φ=0°, it is 0, N qv =(φ*H / 44 / D)≤N q ;

[0022] The ultimate resistance of the buried pipeline in the vertical downward direction is: Among them, N c , N q , N γ is the bearing capacity coefficient, γ is the total bulk density of the soil,

[0023]

[0024]

[0025] N γ =e (0.18φ-2.5) .

[0026] As another implementation of the present technical solution, the axial yield displacement of the buried pipeline is: 3 mm for dense sand, 5 mm for loose sand, 8 mm for hard clay, and 10 mm for loose clay; the horizontal yield displacement of the buried pipeline is: Δp = 0.04*(H+D / 2) ≤ 0.15D; the vertical upward yield displacement of the buried pipeline is: 0.015H < 0.1D for both dense sand and loose sand, and 0.15H < 0.2D for both hard clay and loose clay; the vertical downward yield displacement of the buried pipeline is: 0.1D for both dense sand and loose sand, and 0.2D for both hard clay and loose clay.

[0027] As another implementation of the present technical solution, the finite element software used for mechanical stress and strain analysis can be ANSYS software or ABAQUS software, the pipe unit in the finite element software can be the PIPE20 pipe unit in ANSYS software or the PIPE31 pipe unit in ABAQUS software, and the soil spring unit in the finite element software can be the COMBIN39 unit in ANSYS software or the PSI unit in ABAQUS software. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a flow chart of the method for analyzing pipeline stress sensitivity under landslide action according to the present invention;

[0029] Figure 2 The graph of the Ramberg-Osgood model for X80 steel grade pipe;

[0030] Figure 3 Schematic diagram for defining soil springs in three directions for buried pipelines;

[0031] Figure 4a This is a schematic diagram of the change in the horizontal ultimate resistance and yield displacement performance of the soil spring;

[0032] Figure 4b This is a schematic diagram of the change in the ultimate resistance and yield displacement performance of the soil spring in the vertical direction;

[0033] Figure 4c This is a schematic diagram of the change in the axial ultimate resistance and yield displacement performance of the soil spring;

[0034] Figure 5 Schematic diagram of displacement load application under landslide action;

[0035] Figure 6 The stress distribution curve of the buried pipeline in hard clay soil;

[0036] Figure 7 The stress distribution curve of buried pipelines at different burial depths;

[0037] Figure 8The stress distribution curve of buried pipeline under different operating pressures;

[0038] Figure 9 The stress distribution curve of buried pipeline under different landslide section widths;

[0039] Figure 10 Figure 2 is a stress distribution curve of buried pipelines under different horizontal displacement loads.

[0040] Description of symbols in the accompanying drawings:

[0041] Steps S1 to S8; 1 buried pipeline; 2 soil spring; 3 landslide section; 4 node; KP horizontal soil pressure; KT axial soil friction; KQ vertical soil pressure. DETAILED DESCRIPTION

[0042] The detailed description and technical contents of the present invention are described below with reference to the accompanying drawings. However, the accompanying drawings are only provided for reference and illustration and are not intended to limit the present invention.

[0043] like Figure 1 As shown, the present invention provides a method for analyzing pipeline stress sensitivity under landslide action, the steps of the analysis method include:

[0044] Establish pipeline constitutive model S1: Among them, ε is the pipe strain, σ is the stress on the pipe, its unit is MPa, E is the elastic modulus of the pipe, its unit is MPa, σ s is the yield strength of the pipeline, with the unit of MPa, α is the yield offset of the pipeline, and n is the hardening index of the pipeline. In the present invention, considering that the pipeline is subjected to a large displacement load due to landslide, thus entering a plastic state, in order to accurately describe the elastic-plastic constitutive relationship of the pipeline, the present invention adopts the Ramberg-Osgood model (RO model) to better characterize the true constitutive relationship of the pipeline, thereby accurately calculating the actual stress distribution law of the pipeline. Figure 2 As shown in FIG, a constitutive model of a pipeline with steel grade X80 is established using the Ramberg-Osgood model to describe the corresponding curve relationship between stress and strain.

[0045] Establish pipeline model S2: pipeline constitutive model And the elastic modulus E of the pipeline, the yield strength σ of the pipeline s, the yield offset α of the pipeline, the hardening index n of the pipeline, the Poisson's ratio of the pipeline, and the stress σ on the pipeline and its corresponding pipeline strain ε are assigned to the pipe unit in the finite element software used for mechanical stress and strain analysis to establish a pipeline model. In actual implementation, since the range of landslides is generally large, and considering that this analysis method mainly analyzes the stress and strain distribution of the pipeline, combined with the fact that the pipe unit has tensile, compressive and bending properties and can describe plasticity and expansion characteristics, it is very suitable for stress sensitivity analysis of pipelines under landslides. Therefore, using the pipe unit to simulate the pipeline can well analyze the distribution of pipeline stress and strain, and can save computing resources. In the present invention, the finite element software used for mechanical stress and strain analysis can be selected from ANSYS software or ABAQUS software. Of course, other finite element software of the same type can also be selected, and the pipe unit in the finite element software can be selected from the PIPE20 pipe unit in ANSYS software or the PIPE31 pipe unit in ABAQUS software. In the present invention, ANSYS software is mainly used as an example.

[0046] Determine the physical and mechanical indicators of the soil S3: Determine the physical and mechanical indicators of the soil based on the type of soil.

[0047] Calculate the pipe-soil interaction parameter S4 of the buried pipeline: Figure 3 As shown, three soil springs 2 are defined for the buried pipeline 1 to simulate the horizontal soil pressure KP, axial soil friction KT and vertical soil pressure KQ of the buried pipeline 1, and the horizontal ultimate resistance P of the buried pipeline is calculated by combining the physical and mechanical indicators of the soil and the outer diameter and burial depth of the buried pipeline 1. u Combined with the yield displacement Δp ( Figure 4a As shown), the ultimate axial resistance T u Combined with the yield displacement Δt ( Figure 4c As shown) and the ultimate resistance Q in the vertical direction u (vertically upward) and Q d (vertically downward) and yield displacement Δq u (vertically upward) and Δq d (vertically downward)(combined Figure 4b Considering that the pipeline will deform due to the large displacement load imposed by the soil when it is subjected to landslide, and the pipeline will also have a reaction on the soil, it is very necessary to consider the interaction between the pipe and the soil when conducting stress sensitivity analysis of the pipeline under landslide.

[0048] Establish soil spring model S5: the outer diameter and burial depth of the buried pipeline and the ultimate resistance P in the horizontal direction of the buried pipeline are u and yield displacement Δp, axial ultimate resistance T u and yield displacement Δt and ultimate resistance Q in the vertical directionu , Q d and yield displacement Δq u , Δq d The COMBIN39 unit in ANSYS software is assigned to establish the soil springs in the horizontal, axial and vertical directions of the buried pipeline. Of course, the PSI unit in ABAQUS software can also be assigned to establish the soil springs in the horizontal, axial and vertical directions of the buried pipeline.

[0049] Establish the finite element model S6 for stress analysis of buried pipelines under landslide action: Based on the application of PIPE20 pipe unit in ANSYS software to establish the pipeline model, the length of the pipeline unit, the width of the non-landslide section of the buried pipeline 1 and the width of the landslide section 3 are set in the PIPE20 pipe unit, and the soil spring 2 in the horizontal, axial and vertical directions of the buried pipeline established by the COMBIN39 unit is set at the node 4 of each pipeline unit, as shown in the figure. Figure 5 shown.

[0050] Simulate the stress S7 of the buried pipeline under the action of landslide: Use the PIPE20 pipe element to constrain the six degrees of freedom of the end of the buried pipeline 1, set the operating internal pressure of the buried pipeline 1, and apply displacement loads in at least one of the horizontal, axial and vertical directions to the soil springs 2 connected to the landslide section 3 of the buried pipeline 1 by the COMBIN39 element (that is, the soil springs 2 in three directions at the nodes 4 of each pipeline element in the landslide section) to obtain the stress distribution and maximum stress of the buried pipeline 1 under the action of landslide.

[0051] Stress sensitivity analysis of buried pipelines under landslides S8: Based on the finite element model for stress analysis of buried pipelines under landslides, at least one parameter value of the buried depth of the buried pipeline 1, the operating internal pressure, the width of the landslide section 3, and the displacement load on the landslide section 3 in the horizontal, axial, or vertical directions is changed respectively to obtain the influence of different factors on the stress distribution of the buried pipeline 1 under the action of landslides and the maximum stress.

[0052] In the above analysis method, the elastic modulus E of the pipeline, the Poisson's ratio of the pipeline, and the yield strength σ of the pipeline s The values ​​of the yield offset α and the hardening index n of the pipeline are obtained by sampling the steel of the buried pipeline and performing axial tensile tests on the sampled steel. However, under insufficient test conditions, the elastic modulus E, Poisson's ratio and yield strength σ of the pipeline are not sThe values ​​of the pipeline's yield offset α and the pipeline's hardening index n can also be obtained through the parameter values ​​of the Ramberg-Osgood model corresponding to different steel grades in the "GB / T50470 Technical Specification for Seismic Resistance of Oil and Gas Transmission Pipeline Engineering", as shown in Table 1 below. Among them, for pipes made of metal, their elastic modulus E and Poisson's ratio are mutually corresponding and relatively fixed values, and therefore are not included in Table 1.

[0053] Steel Grade <![CDATA[σ s ]]> α n X42 350 2.004 17.72 X52 389 1.699 14.14 X56 417 1.519 17.79 X60 437 1.403 15.85 X65 459 1.288 19.90 X70 470 1.234 14.58 X80 530 0.981 20.12

[0054] Table 1.

[0055] In the above-mentioned analysis method, the physical and mechanical properties of the soil can be obtained through soil mechanics testing methods. These physical and mechanical properties include soil cohesion, total bulk density, effective bulk density, and pipe-soil friction angle. Obtaining the values ​​of soil cohesion, total bulk density, effective bulk density, and pipe-soil friction angle through soil mechanics testing methods is common knowledge, and therefore, this soil mechanics testing method will not be described in detail in this disclosure. Furthermore, the above-mentioned physical and mechanical properties of different soil types can also be obtained by consulting relevant literature.

[0056] In the above analysis method, the ultimate axial resistance of the buried pipeline is T u for: Where D is the outer diameter of the buried pipeline, its unit is m, c is the cohesion of the soil, its unit is Pa, H is the burial depth of the buried pipeline, its unit is m, is the effective bulk density of the soil, its unit is N / m 3 , β is the viscosity coefficient, Its unit is kPa / 100, δ is the pipe-soil friction angle, δ=fφ, φ is the soil internal friction angle, f is the friction coefficient and f=0.6, K0 is the soil pressure coefficient, K0=1-sin(φπ / 180);

[0057] Ultimate resistance P of buried pipeline in horizontal direction u for: Among them, N ch is the horizontal bearing capacity coefficient of hard clay or loose clay and is 0 when c=0. N qh is the horizontal bearing capacity coefficient of dense sand or loose sand and is 0 when φ=0°, N qh =a+b(x)+c(x 2 )+d(x 3 )+e(x 4 ), the above a, b, c, d, e coefficients are obtained by interpolation, and the interpolation table is as follows:

[0058] coefficient φ x a b c d e <![CDATA[N ch ]]> 0° H / D 6.752 0.065 -11.063 7.119 -- <![CDATA[N qh ]]> 20° H / D 2.399 0.439 -0.03 <![CDATA[1.059(10) -3 ]]> <![CDATA[-1.754(10) -5 ]]> <![CDATA[N qh ]]> 25° H / D 3.332 0.839 -0.09 <![CDATA[5.606(10) -3 ]]> <![CDATA[-1.319(10) -4 ]]> <![CDATA[N qh ]]> 30° H / D 4.565 1.234 -0.089 <![CDATA[4.275(10) -3 ]]> <![CDATA[-9.159(10) -5 ]]> <![CDATA[N qh ]]> 35° H / D 6.816 2.019 -0.146 <![CDATA[7.651(10) -3 ]]> <![CDATA[-1.683(10) -4 ]]> <![CDATA[N qh ]]> 40° H / D 10.959 1.783 0.045 <![CDATA[-5.425(10) -3 ]]> <![CDATA[-1.153(10) -4 ]]> <![CDATA[N qh ]]> 45° H / D 17.658 3.309 0.048 <![CDATA[-6.443(10) -3 ]]> <![CDATA[-1.299(10) -4 ]]> ;

[0059] Ultimate resistance Q of buried pipeline in vertical upward direction u for: Among them, N cv is the vertical upward coefficient of hard clay or loose clay and when c=0, it is 0, N cv =2*(H / D)≤10, N qv It is the vertical upward coefficient of dense sand or loose sand and when φ=0°, it is 0, N qv =(φ*H / 44 / D)≤N q ;

[0060] Ultimate resistance Q of buried pipeline in vertical downward direction d for: Among them, N c , N q , N γ is the bearing capacity coefficient, γ is the total bulk density of the soil,

[0061]

[0062]

[0063] N γ =e (0.18φ-2.5) .

[0064] In the above analysis method, the axial yield displacement Δt of the buried pipeline is: 3mm for dense sand, 5mm for loose sand, 8mm for hard clay, and 10mm for loose clay; the horizontal yield displacement Δp of the buried pipeline is: Δp=0.04*(H+D / 2)≤0.15D; the vertical upward yield displacement Δq u The yield displacement Δq of the buried pipeline in the vertical downward direction is: 0.015H<0.1D for both dense sand and loose sand, and 0.15H<0.2D for both hard clay and loose clay. d The density of dense sand and loose sand is 0.1D, and the density of hard clay and loose clay is 0.2D.

[0065] The following example describes the specific implementation of this analysis method:

[0066] This example uses a natural gas pipeline in my country as an example. The pipeline is made of X80 steel pipe, has an outer diameter of 1016 mm, a wall thickness of 14.6 mm, a buried depth of 2 m, an operating internal pressure of 10 MPa, and the soil surrounding the pipeline is hard clay. The pipeline length is set to 1000 m. The sections from 0 to 450 m and from 550 to 1000 m are non-landslide sections, while the section from 450 to 550 m is a landslide section. The width of the landslide section is 100 m, the horizontal displacement load is 2 m, and the length of the pipeline unit is 1 m.

[0067] The X80 steel pipe was sampled and subjected to axial tensile tests to obtain the pipeline constitutive model parameter values ​​of X80 steel, as shown in Table 2:

[0068]

[0069] Table 2.

[0070] Pipeline constitutive model The PIPE20 element in ANSYS software is assigned its parameter values ​​to build the pipeline model. The PIPE20 element is a uniaxial element with tensile, compressive, bending, and torsion properties. Each node of the element has six degrees of freedom: displacement along the nodal coordinates x, y, and z directions and rotation about the nodal coordinates x, y, and z axes. The element has plasticity, creep, and expansion properties.

[0071] The physical and mechanical indicators of hard clay were determined by soil mechanics testing methods, as shown in Table 3 below:

[0072]

[0073] Table 3.

[0074] Define three soil springs to simulate the pipe-soil interaction in three directions of the pipeline, combined with Figure 3 、 Figures 4a to 4c As shown in Table 4 below, the pipe-soil interaction parameters of the pipeline are calculated by combining the above physical and mechanical indicators with the outer diameter and burial depth of the pipeline, as shown in Table 4 below:

[0075]

[0076] Table 4.

[0077] The above-mentioned pipe-soil interaction parameters are assigned to the COMBIN39 element in ANSYS software to establish a soil spring model. COMBIN39 is a one-way element with nonlinear capabilities. A generalized force-deformation curve can be input to this element, effectively simulating the effects of soil on buried pipelines. This element can be used in any analysis. In one-, two-, and three-dimensional applications, this element has axial or torsional capabilities. The axial option (longitudinal) represents an axial tension-compression element, with three degrees of freedom per node: translation along the nodal coordinate system X, Y, and Z, ignoring bending and torsion. The torsional option (torsional) represents a pure torsion element, with three degrees of freedom per node: rotation about the nodal coordinate system X, Y, and Z, ignoring bending and axial loads. This element can only have large displacement capabilities when each node has two or three degrees of freedom.

[0078] Use PIPE20 pipe unit to create a pipeline with a length of 1000m, combined with Figure 5As shown, the length of the pipeline unit is set to 1m and is evenly distributed. The sections from 0 to 450m and from 550 to 1000m are non-landslide sections, and 450 to 550m is the landslide section 3. The width of the landslide section 3 is 100m. At the node 4 of each pipeline unit, there are three-directional soil springs 2 established by the COMBIN39 unit. The six degrees of freedom of the pipeline are constrained at the pipeline ends at 0m and 1000m. An operating internal pressure of 10MPa is applied to the entire pipeline, and a displacement load of 2m in the X direction (but displacement loads in the Y direction and / or Z direction can also be applied simultaneously or separately) is applied to the soil springs 2 connected to the pipeline in the landslide section 3 (i.e., the three-directional soil springs 2 connected to the node 4 of each pipeline unit in the section from 450 to 550m) (i.e., simulating a horizontal landslide) to perform finite element calculation and pipeline stress analysis under the action of landslide, and obtain the stress distribution of the pipeline and the stress distribution curve along the entire pipeline, as shown in FIG. Figure 6 As shown in the figure, the analysis shows that the maximum axial stress of the pipeline is 141.9 MPa.

[0079] Keeping other model parameters unchanged, calculate the stress distribution of the pipeline when the pipeline is buried at a depth of 1.5m, 2m, and 2.5m. Since the pipe-soil interaction parameters are related to the pipeline burial depth, when the pipeline burial depth changes, the pipe-soil interaction parameters need to be recalculated and the re-obtained pipe-soil interaction parameters are assigned to the COMBIN39 unit. The pipe-soil interaction parameters for different burial depths are shown in Table 5 below. The stress distribution of the pipeline at different burial depths is calculated. Figure 7 As the buried depth of the pipeline increases, the maximum axial stress of the pipeline also increases. The maximum axial stress of the pipeline at different buried depths is shown in Table 6 below:

[0080]

[0081] Table 5

[0082]

[0083] Table 6.

[0084] Keeping other model parameters unchanged, calculate the stress distribution of the pipeline when the internal pressure of the pipeline is 6MPa, 8MPa, 10MPa and 12MPa, as shown in the following example: Figure 8 As shown in Table 7, as the internal pressure of the pipeline increases, the maximum axial stress of the pipeline also increases. The maximum axial stress of the pipeline under different internal pressures of the pipeline is shown in Table 7 below:

[0085]

[0086] Table 7.

[0087] Keeping other model parameters unchanged, calculate the stress distribution of the pipeline when the landslide width is 75m, 100m, 125m and 150m, as shown in the following example: Figure 9 As shown in Table 8, as the width of the landslide section increases, the maximum axial stress of the pipeline also increases. The maximum axial stress of the pipeline under different landslide section widths is shown in Table 8 below:

[0088]

[0089] Table 8.

[0090] Keeping other model parameters unchanged, calculate the stress distribution of the pipeline when the horizontal displacement load is 1.5m, 2m, 2.5m and 3m, as shown in the following example: Figure 10 As shown in Table 9, as the displacement load increases, the maximum axial stress of the pipeline also increases. The maximum axial stress of the pipeline under different displacement loads is shown in Table 9 below:

[0091]

[0092] Table 9.

[0093] Of course, this analysis method can also be used to keep other model parameters unchanged, while changing the pipeline burial depth, operating internal pressure, landslide section width and two or more model parameters in the horizontal, axial and vertical directions to calculate and analyze the stress distribution and maximum stress of the pipeline.

[0094] The above-mentioned analysis method of the present invention can establish a finite element model for stress analysis of buried pipelines under the action of landslides for oil and natural gas transmission pipelines. By changing one or more parameter values ​​of the buried pipeline's burial depth, operating internal pressure, landslide section width, and landslide displacement, the influence of these parameter values ​​on the stress of the buried pipeline can be analyzed, thereby accurately analyzing the interaction between the pipe and the soil, controlling the sensitivity of the buried pipeline stress under the action of landslides, determining the stress state of the buried pipeline, judging the safety level of the buried pipeline, and providing strong data support for scientific prevention and control measures and strategies for pipeline hazards.

[0095] The above are only preferred embodiments of the present invention and are not intended to limit the patent scope of the present invention. Other equivalent changes made using the patent concept of the present invention should fall within the patent protection scope of the present invention.

Claims

1. A method for analyzing pipeline stress sensitivity under landslide action, characterized by the following steps: include: Establish a pipeline constitutive model: Among them, ε is the pipe strain, σ is the stress on the pipe, its unit is MPa, E is the elastic modulus of the pipe, its unit is MPa, σ s is the yield strength of the pipeline, the unit is MPa, α is the yield offset of the pipeline, and n is the hardening index of the pipeline; Establish pipeline model: pipeline constitutive model And the elastic modulus E of the pipeline, the yield strength σ of the pipeline s , the yield offset α of the pipeline, the hardening index n of the pipeline, the Poisson's ratio of the pipeline, the stress σ of the pipeline and the corresponding pipeline strain ε are assigned to the pipe element in the finite element software used for mechanical stress and strain analysis to establish a pipeline model; Determine the physical and mechanical indicators of the soil: Determine the physical and mechanical indicators of the soil according to the type of soil; Calculate the pipe-soil interaction parameters for buried pipelines: Define three soil springs for the buried pipeline to simulate the horizontal soil pressure, axial soil friction, and vertical soil pressure. Combined with the physical and mechanical indicators of the soil, the outer diameter of the buried pipeline, and the burial depth, calculate the ultimate resistance and yield displacement in the horizontal direction, the ultimate resistance and yield displacement in the axial direction, and the ultimate resistance and yield displacement in the vertical direction. Establish a soil spring model: The outer diameter and burial depth of the buried pipeline, as well as the ultimate resistance and yield displacement in the horizontal direction, the ultimate resistance and yield displacement in the axial direction, and the ultimate resistance and yield displacement in the vertical direction of the buried pipeline are assigned to the soil spring unit in the finite element software used for mechanical stress and strain analysis to establish the soil springs in the horizontal, axial, and vertical directions of the buried pipeline; Establishing a finite element model for stress analysis of buried pipelines under landslides: Based on the establishment of a pipeline model using pipe units using finite element software for mechanical stress and strain analysis, the length of the pipe unit, the width of the non-landslide section and the width of the landslide section of the buried pipeline are set in the pipe unit, and soil springs in the horizontal, axial, and vertical directions of the buried pipeline established by connecting the soil spring units are set at the nodes of each pipe unit; Simulating the stress of a buried pipeline under the action of a landslide: applying the pipe unit to constrain the six degrees of freedom of the end of the buried pipeline, setting the operating internal pressure of the buried pipeline, and applying displacement loads in at least one of the horizontal, axial, and vertical directions to the soil springs established by the soil spring unit connected to the landslide section of the buried pipeline, so as to obtain the stress distribution and maximum stress of the buried pipeline under the action of the landslide; Stress sensitivity analysis of buried pipelines under landslides: Based on the finite element model for stress analysis of buried pipelines under landslides, the buried depth, operating internal pressure, landslide section width, and at least one of the horizontal, axial, or vertical displacement loads on the landslide section are changed to determine the effects of different factors on the stress distribution of buried pipelines under landslides and the maximum stress.

2. The method for analyzing pipeline stress sensitivity under landslide according to claim 1, characterized in that: The elastic modulus E of the pipeline, the Poisson's ratio of the pipeline, and the yield strength σ of the pipeline s The values ​​of the yield offset α of the pipeline and the hardening index n of the pipeline are obtained by sampling the steel of the buried pipeline and performing axial tensile tests on the sampled steel.

3. The method for analyzing pipeline stress sensitivity under landslide according to claim 1, characterized in that: The elastic modulus E of the pipeline, the Poisson's ratio of the pipeline, and the yield strength σ of the pipeline s The values ​​of the yield offset α of the pipeline and the hardening index n of the pipeline are obtained through the parameter values ​​of the Ramberg-Osgood model corresponding to different steel grades in the "GB / T 50470 Technical Specification for Seismic Resistance of Oil and Gas Pipeline Line Engineering".

4. The method for analyzing pipeline stress sensitivity under landslide according to claim 1, characterized in that: The physical and mechanical indicators of the soil are obtained through soil mechanics testing methods, and the physical and mechanical indicators of the soil include soil cohesion, total bulk density, effective bulk density and pipe-soil friction angle.

5. The method for analyzing pipeline stress sensitivity under landslide according to claim 1, characterized in that: The ultimate axial resistance of buried pipelines: Where D is the outer diameter of the buried pipeline, its unit is m, c is the cohesion of the soil, its unit is Pa, H is the burial depth of the buried pipeline, its unit is m, is the effective bulk density of the soil, its unit is N / m 3 , β is the viscosity coefficient, Its unit is kPa / 100, δ is the pipe-soil friction angle, δ=fφ, φ is the soil internal friction angle, f is the friction coefficient and f=0.6, K0 is the soil pressure coefficient, K0=1-sin(φπ / 180); Ultimate resistance of buried pipelines in horizontal direction: Among them, N ch is the horizontal bearing capacity coefficient of hard clay or loose clay and is 0 when c=0. N qh is the horizontal bearing capacity coefficient of dense sand or loose sand and is 0 when φ=0°, N qh =a+b(x)+c(x 2 )+d(x 3 )+e(x 4 ), the above a, b, c, d, e coefficients are obtained by interpolation, and the interpolation table is as follows: ； Ultimate resistance of buried pipeline in vertical upward direction: Among them, N cv is the vertical upward coefficient of hard clay or loose clay and when c=0, it is 0, N cv =2*(H / D)≤10, N qv It is the vertical upward coefficient of dense sand or loose sand and when φ=0°, it is 0, N qv =(φ*H / 44 / D)≤N q ; Ultimate resistance of buried pipeline in vertical downward direction: Among them, N c , N q , N γ is the bearing capacity coefficient, γ is the total bulk density of the soil, N γ =e (0.18φ-2.5) 。 6. The method for analyzing pipeline stress sensitivity under landslide according to claim 5, characterized in that: Axial yield displacement of buried pipelines: 3mm for dense sand, 5mm for loose sand, 8mm for hard clay, and 10mm for loose clay; Horizontal yield displacement of buried pipeline: Δp=0.04*(H+D / 2)≤0.15D; The yield displacement of buried pipelines in the vertical upward direction is 0.015H<0.1D for both dense sand and loose sand, and 0.15H<0.2D for both hard clay and loose clay; The yield displacement of buried pipelines in the vertical downward direction is: 0.1D for dense sand and loose sand, and 0.2D for hard clay and loose clay.

7. The method for analyzing pipeline stress sensitivity under landslide according to claim 1, characterized in that: The finite element software used for mechanical stress and strain analysis is ANSYS software or ABAQUS software, the pipe unit is the PIPE20 pipe unit in ANSYS software or the PIPE31 pipe unit in ABAQUS software, and the soil spring unit is the COMBIN39 unit in ANSYS software or the PSI unit in ABAQUS software.

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