Mechanical Characteristics of Suspended Buried Pipelines in Coal Mining Areas Affected by Groundwater Loss

Abstract

Research on the deformation characteristics and failure modes of buried pipelines under local suspension conditions caused by groundwater loss in coal mining subsidence areas is conducive to grasping the failure evolution law of pipelines and providing technical support for the precise maintenance of gathering and transportation projects and the coordinated mining of gas and coal resources. First, a test system for monitoring the deformation of pipelines under loading was designed, which mainly includes pipeline load application devices, end fixing and stress monitoring devices, pipeline end brackets, and stress–strain monitoring devices. Then, a typical geological hazard faced by oil and gas pipelines in the gas–coal overlap area—local suspension—was used as the engineering background to simulate the field conditions of a 48 mm diameter gas pipeline with a localized uniform load. At the same time, deformation, top–bottom strain, end forces, and damage patterns of the pipeline were monitored and analyzed. The results show that the strain at the top and bottom of the pipeline increased as the load increased. In this case, the top was under pressure, and the bottom was under tension, and the conditions at the top and bottom were opposite.. For the same load, the strain tended to increase gradually from the end to the middle of the pipeline, and at the top, it increased significantly more than at the bottom. The tensile force carried by the end of the pipeline increased as the applied load increased, and the two were positively correlated by a quadratic function. The overall deformation of the pipeline evolved from a flat-bottom shape to a funnel and then to a triangular shape as the uniform load increased. In addition, plastic damage occurred when the pipeline deformed into a triangular shape. The results of the investigation clarify for the first time the mathematical relationship between local loads and ultimate forces on pipelines and analyze the evolution of pipeline failure, providing a reference for pipeline field maintenance. Based on this, the maximum deformation of and the most vulnerable position in natural gas pipelines passing through a mining subsidence area can be preliminarily judged, and then the corresponding remedial and protection measures can be taken, which has a certain guiding role for the protection of natural gas pipelines.

1. Introduction

The development of coal and associated resources forms the foundation of the current energy development model. The strata of the Ordos Basin are rich in high-quality mineral resources such as coal, oil, natural gas, uranium, and coalbed methane and are located in different layers of the same basin. These resources have formed a unique pattern of space superposition in the process of sedimentary metamorphism for hundreds of millions of years. Among these superimposed resources, the overlap of coal, oil, and natural gas resources in the basin forms a typical area of cross exploitation of coal-bearing resources.

When studying the phenomenon of surface movement and deformation in mining areas, it is found that the maximum surface subsidence in some mining areas often exceeds the actual mining thickness of coal seams [1,2]. After the mining of coal resources, ‘three zones‘ are produced, namely, a caving zone, a water-conducting fracture zone, and a bending subsidence zone [3,4,5,6]. Affected by the development of the water-conducting fracture zone, the coal strata will have a hydraulic connection with the aquifer in the overlying rock and soil mass, causing the water level of the aquifer in the overlying rock and soil mass of the coal seam to drop [7], ultimately affecting the surface loose layer. The range of surface subsidence is expanding, and the amount of surface subsidence is excessive., which can easily lead to the local suspension of natural gas pipelines in the subsidence area. At this time, The stress on the pipeline’s top, bearing the overburden rock and with a suspended bottom, can easily result in deformation and failure of the pipeline, which in turn leads to accidents such as leakage, poisoning, or explosion [8,9,10,11]. Therefore, research on local suspended pipelines in the subsidence area is of great significance for mastering the stress and deformation characteristics of buried pipelines and their accurate and reasonable maintenance.

In recent years, experts in related fields at home and abroad have conducted extensive research on buried pipeline systems by using analytical methods, dynamic models, similar simulation systems, and other specific methods. In terms of theoretical analysis, based on the theory of the elastic foundation beam, Wang Xiaolong et al. established a mechanical model of pipe–soil interaction when buried steel pipelines are partially suspended. Using the deformation coordination between the suspended section and the non-suspended section of the pipeline, the deflection and internal force of the partially suspended pipeline were analyzed [12]. Based on the elastic layered theory, Wang et al. used the transfer matrix method and the finite difference method to establish a calculation method of underground pipeline deformation under the condition of soil heterogeneity [13]. Wang Xiaolin et al. used the probability integral method to predict the three-dimensional deformation of the surface in the subsidence area. Combined with factors such as the axial action between the pipeline and the soil and the nonlinearity of the pipeline material, a simplified evaluation formula for the maximum stress and strain of the buried pipeline in the subsidence area was proposed [14]. Ren Jiandong et al. analyzed the deformation equation of pipelines under different coupling states of pipeline–soil by the elastic foundation beam model [15]. Based on the theory of continuous pipelines, Zhou Xiancheng et al. analyzed the influence of tunnel excavation on the buried pipeline considering factors such as stratum displacement settlement, pipeline section stiffness, and joint stiffness [16].

In terms of numerical simulations, Gregory C. Sarvanis et al. proposed an analytical methodology for the strain analysis of pipelines subjected to permanent ground-induced actions in geohazard areas, which resulted in closed-form expressions for pipeline deflection and strains [17]. Wang Kunyuan et al. defined the safety factor of pipelines through the FISH language built-in FLAC3D and obtained the sinking law and safety factor for pipelines at different mining stages through numerical calculation and analysis [18,19]. Zheng Dahai et al. established a finite-element model based on the soil spring model and proposed that the axial and vertical displacements of a pipeline increase with the increase in the mining thickness and the deformation reaches extreme values in the middle and inner edges of the subsidence area [20,21]. Ren Jiandong et al. analyzed the deformation curve of a pipeline and the stress state of the top and bottom under the influence of coal mining through ABAQUS [22]. Regarding similarity simulation, Xu Ping et al. designed a test system for the study of pipe–soil interactions in subsidence soil and measured the sinking deformation of pipelines, the earth pressure around pipelines, and the axial strain on pipelines in the presence of different levels of soil subsidence [23,24].

Through similar simulations, Wang et al. indicated that the bending deformation in the center of the subsidence area and the oblique tensile deformation at the inflection point were the main causes of pipeline failure [25]. Zhou Min et al. analyzed the deformation characteristics of buried pipelines under the action of surface soil subsidence with similar simulation results and considered that the deformation curve of pipelines under the influence of coal mining conforms to the Gaussian function and that there is a phenomenon of pipe–soil separation [26,27,28]. Chen Fu-Quan et al. proposed a new mechanical model to compute the earth pressure above a pipeline considering pipe–soil interactions and the three-dimensional soil arching effects of cohesive fills, based on the Terzaghi vertical sliding surface theory [29]. Ming Yang et al. researched the pipe–FC interaction mechanism and compared it with the pipe–soil interaction mechanism under normal fault displacements caused by earthquakes [30]. To clarify the impact of excavation unloading on buried pipelines, Yukun Li et al. established a finite-element three-dimensional pipe–soil model, investigated the mechanical response of pipelines under layered excavation, and evaluated various parameters impacting the response [31]. Yingnan Xu et al. established a theoretical model for the safety prediction of gas pipelines in mining subsidence areas based on the elastic free energy theory, constructed a 3D model of a pipe–sand soil system by using ABAQUS (version 6.14), analyzed the characteristics of ground surface and pipeline settlement combined with on-site measured data, and revealed the temporal and spatial evolution law of pipeline load and deformation under the condition of diagonal intersections of the pipeline and high-strength mining working face [32].

At present, research on buried pipelines mainly focuses on the stress and deformation of pipelines under the influence of coal mining and on the determination of failure location and mainly adopts theoretical and numerical analysis methods. At the same time, some scholars are also studying the internal force and deformation of partially suspended pipelines through theoretical analysis, but they have not carried out practical experimental research on the stress state of partially suspended gas pipelines [33,34,35,36]. In view of this, an experimental device for monitoring the local stress of buried pipelines was designed using a 1:1 replica pipeline, considering the material parameters and the parameters of natural gas pipelines laid in mine fields with a diameter of 48 mm. In this paper, a similar simulation test platform was established to study the deformation characteristics of pipelines and the stress characteristics at their top and bottom, and the functional relationship between the end force and the load was analyzed, which provides a reference for the deformation and later maintenance of buried oil and natural gas pipelines under gas–coal cross mining.

2. Engineering Background

The Ordos Basin is rich in mineral resources such as oil, coal, and natural gas, which leads to serious natural gas–coal cross-mining problems. With the exploitation of the coal resources, the development of a fracture zone caused by the collapse of rock strata in the goaf has directly caused the loss of water resources in the high phreatic aquifer. The loss of water resources further exacerbates surface subsidence, making the surface settlement value significantly greater than under normal conditions

According to statistics, five of the nine gas field blocks in the Ordos Basin overlap with coal mine fields to varying degrees. The following only analyzes the data results of natural gas pipeline length and overlap area of the Daniudi gas field and surrounding coal mine (

Table 1. Pipeline layout in overlapping area.

Overlapping Area Colliery	Overlapping Area/km2	Length of the Pipeline/km	Number of Pipelines/pcs
Taigemiao Coal Mine	252.94	40.21	—
Hulusu coal mine	53.30	22	—
Shilawusu coal mine	71.16	—	—
Wulan coal mine	52.41	—	2
Menkeqing Coal Mine	17.74	—	3

).

With the continuous mining of coal resources in the overlapping area, the surface overlying strata corresponding to the working face gradually migrate and deform, forming a dynamic moving subsidence basin on the surface. When a pipeline is located at the end of the suspended rock layer above the goaf of the coal mining face, the pipeline will be partially suspended under the tensile stress caused by the bending of the overlying rock layer. At this time, the pipeline is in a stress state at the top, and its bottom is suspended, which can easily lead to problems such as pipeline deformation and fracture failure. Especially in the case of high-intensity mining in the overlapping area, the development of a water-conducting fracture zone to the high-level aquifer leads to a decline in the aquifer water level, and the range and depth of surface subsidence will increase significantly, as shown in Figure 1.

It is important to consider the seriousness of the overlapping problem of gas fields and coal mines in the basin, the density of pipeline layout, the difficulty of human supervision, the lack of gas–coal coordinated mining, and other unsafe factors. Therefore, it is of great significance to carry out research on the stress and deformation of locally suspended buried pipelines in the subsidence area for mastering the stress and deformation of buried oil and gas pipelines in coal mining subsidence areas and their precise maintenance. At the same time, it is also helpful to reduce the safety risk caused by the deformation and failure of oil and gas pipelines in the overlapping area.

3. Pipeline Deformation in the Subsidence Area

In the process of redistribution of stress after coal seam mining, the overlying strata of the working face move to the direction of the goaf, so that the whole rock stratum from the direct roof to the surface moves and breaks, and the stratum settlement gradually moves to the surface, causing surface deformation and sinking. When the advancing distance of the working face is about 1/4~1/2 of the mining depth, the strata movement spreads to the surface, and the surface begins to sink. When the advancing distance is 1.2~1.4 times the mining depth, full mining is achieved, and ground subsidence reaches the maximum value. As the working face continues to advance, the scope of the inverted trapezoidal subsidence basin continues to expand, but the maximum subsidence value does not change [37]. After the working face has advanced, the overburden rock will collapse, bend, and sink to a certain extent in the direction of the goaf, and separation cracks will form between the strata. For a natural gas pipeline buried in the aeolian sand layer of the Ordos Basin, the migration of sand caused by stratum activity will lead to the gradual loss of support at the bottom of the pipeline, and the gravity of the upper overburden layer will act on the upper part of the pipeline and cause movement and deformation at the center of the goaf. At this time, the pipeline at the edge of the subsidence area can be regarded as a fixed end due to a lack of deformation, while the overall force of the pipeline is suspended in the middle of the subsidence area, and its deformation is funnel-shaped and directed towards the center of the goaf.

4. Experiment on the Mechanical Characteristics of the Local Suspension of a Buried Pipeline

4.1. Test System Layout

Referring to the geological conditions of 22106 working face of a mine in the gas–coal overlap area, a test bench for the ‘deformation evolution characteristics of natural gas pipelines under load‘ was designed. Considering the boundary effect, a guide chain was installed to control the displacement change of the end of the pipeline. By applying a uniform load to the middle area of the pipeline, the stress state of the pipeline was analyzed when the pipeline was loaded at the top while the bottom was suspended. The test bench was composed of a holder, a guide chain, an electronic hanging scale, a natural gas pipeline, a pulley, a steel strand, a rope buckle, a beam, a fastener, a strain gauge, a computer, and so on. The material of the test bench holder was 32a channel steel with a thickness of 6 mm, and the section size was 120 mm × 60 mm. In the middle of the holder, a cross steel tube beam was used to improve the strength of the holder, and the connection of the holder was fixed by welding. The pulley was 120 mm high and 50 mm in radius. The electric scale and the guide chain were connected by the steel strand and the rope buckle. The two ends of the pipeline were drilled separately, and the steel strand passed through the borehole to connect the electric scale with the pipeline. The lead chain was 2 t at both ends and 3 t in the middle, and the electric weighing range was 3 t, as shown in Figure 2. The beam was fixed in the middle area of the test pipeline by fasteners. By continuously tightening the length of the guide chain, a uniform load was applied to the middle of the pipeline. The load could be displayed in real time on the electric scale. After the expected load was reached, the pulling of the guide chain was stopped; so, the load could be stopped.

4.2. Test Equipment

As shown in Figure 2, the main equipment used in the test included electronic hanging scales, chain guides, strain gauges, pulleys, rope buckles, expansion screws, fasteners, anchor screws, etc. The electronic hanging scale model was OCS-NF-3T, the maximum bearing capacity was 3 t, the minimum bearing capacity was 20 kg, and the safety load was 125% of the maximum value; the chain guide model was HSZ-SERIES0.5-20/TON, the maximum range was 3 t; the strain gauge model was BZ2205C, with resolution of 0.1 με and accuracy of 0.1% ± 1 με, and the test point was 16 bits. The diameter of the pulley was 100 mm, and the pulley was arranged at the end of the bracket for connecting the electronic hanging scale and the guide chain. The rope buckle model was M15 and was used to fasten the steel strand; the diameter of the fastener was 80 mm, and the bolt model was M16. The bolt passed through the fastener on the pipeline and was fixed together with the beam to apply a uniform load to the pipeline. The expansion screw model M16 × 200 was used to fix the bracket to a horizontal surface, as shown in Figure 3.

One end of the experimental device was fixed on the cement ground, and the other end was fixed on the surface. Considering that the holder was fixed on the a surface, the bracket could easily move. The size of the surface holder position was 1000 mm × 1000 mm × 1000 mm (length × width × height) underground in the foundation pit, four anchor screws (model M16, length 1 m) were welded to fix the bearing in the middle of the foundation pit, and cement was poured. After the cement condensed and stabilized, the surface side holder was fixed on the anchor screw of the cement pouring body, as shown in Figure 3.

4.3. Procedure of the Test

(1).

Installation of the instruments and devices. The pipeline, holder, chain guides, electric scales, and the other instruments were assembled and connected according to the design.

(2).

Instrument calibration and inspection. After the installation of the test device was completed, the electronic lifting scale was started for zero adjustment; the strain gauge and the computer were started to check whether the wiring was complete and make the corresponding adjustments.

(3).

Experiment and record data. A load of 500 N was applied for the first time on 3.6 m area in the middle of the pipeline, and then increased by 1000 N each time. The strain data were recorded at the top and bottom of the pipeline, together with the force values at the ends and the settlement data.

4.4. Data Acquisition System Design and Layout

In order to analyze the deformation evolution of buried gas pipelines under the action of surface soil collapse, the reference parameters of natural gas pipelines laid in a gas field were simulated by using a 1:1 replica pipeline of 20 # steel, with an outer diameter of 48 mm, a wall thickness of 3 mm, and a length of 8.8 m. The strain gauge model used in this test was BX120-100AA (100 × 3), the resistance value was 120 ± 0.2 ohm, the sensitivity coefficient was 2.08 ± 0.1%, and the accuracy level was A. In order to record the strain evolution process of the pipeline in the process of soil collapse, we analyzed the strain distribution law and stress state characteristics of the pipeline. The strain gauges were arranged at the top and bottom of the pipeline and symmetrically on both sides every 1200 mm, with the middle of the pipeline as the center. Seven strain gauges were arranged at the top and bottom of the pipeline. In order to apply a uniform load to the pipeline, a 3.6 m area in the middle of the pipeline was connected with fasteners and beams. A downward tension was applied to the beams to achieve a uniform load in the 3.6 m area in the middle of the test pipeline, and the deformation evolution characteristics of the pipeline under uniform loading were simulated. Before the test, a ruler and an oil pen were used to mark the position of the strain gauge at the top of the pipeline. The strain gauge was pasted after the pipeline was placed horizontally and kept neat during the arrangement process, as shown in Figure 4.

The two ends of the test frame were arranged with electric scales; the pipeline end was connected with the electric crane through the steel strand, and the electric scale was connected with the pulley and the guide chain through the steel strand. After applying a uniform load to the middle of the pipeline, the force at both ends could be displayed in real time by the electric scale. Researchers record the bearing load in the middle of the pipeline and the tension value at both ends after each load is applied. When the pipeline is fixed at both ends and a uniformly distributed load is applied in the middle, a certain bending deformation will occur. The two ends of the pipeline are connected by thin lines. After applying the expected load, the distance between each position of the pipeline and the thin line was measured to evaluate the settlement of the pipeline.

5. Experimental Results and Analysis

5.1. Strain Distribution at the Top of the Pipeline

The strain gauges at the top of the pipeline were arranged from left to right, and a uniform load was applied to the middle of the pipeline. The strain (ε/−10⁻⁶) evolution from A1 to A7 is shown in Figure 4. The positive strain represents tension, and the negative strain represents compression. Under the action of its own gravity and the applied load, the internal bending moment caused bending deformation and produced additional bending axial stress. The strain in A1 increased gradually with the increase in the load in the range of 480~2500 N, decreased gradually with the increase in the load in the range of 2500~6500 N, and reached the maximum at 2500 N (Figure 5a). The strain in A2 increased gradually with the increase in the load in the range of 480~3550 N, decreased gradually with the increase in the load in the range of 3550~6500 N, and reached the maximum when the applied load reached 3550 N (Figure 5b). A1 and A2 were initially without load, so it was considered that the strain distribution at the top of the pipeline at the edge of the load increased first and then decreased. The strain at the A3 position at the end of the uniform bearing section of the pipeline gradually increased with an increase of the load in the range of 480~4470 N, and gradually decreased with the increase in the load in the range of 4470~6500 N and reached the maximum at the applied load of 4470 N (Figure 5c); the strain in A4 located in the center of the uniform bearing section increased gradually with the increase in the load in the range of 480~6500 N (Figure 5d). As the load increased, the position of the maximum strain gradually shifted to the center of the pipeline from position A1 to the center position A4.

It can be seen from Figure 5e that the strain at the top of the entire pipeline was less than zero, indicating that the top of the pipeline was subjected to pressure under load. Through comparative analysis, it was found that only the strain in A4 in the center of the pipeline increased with the increase in the load under uniform loading, and the strain in the other positions increased first and then decreased with the increase in the load. The overall analysis showed that regardless of the uniform load, the strain at the top of the pipeline was symmetrical in the center in A4 after the load was applied, and the maximum strain was in the center of the pipeline.

5.2. Strain Distribution at the Bottom of the Pipeline

The bottom strain gauge was arranged from the left to the right of the pipeline, and the applied load was located in the center of the pipeline. Therefore, when determining the strain at the bottom of the pipeline and performing data analysis and processing, only the strain generated at the B1–B4 positions could be analyzed. The strain distribution and overall evolution at the B1–B4 positions are shown in Figure 5. Under the double influence of its own gravity and the applied load, the internal bending moment caused the bending deformation of the pipeline and produced additional bending axial stress. The strain at the B1 position of the pipeline increased gradually with the increase in the load in the range of 480~2500 N, decreased gradually with the increase in the load in the range of 2500~6500 N, and reached the maximum at the applied load of 2500 N (Figure 6a); the strain at the B2 position of the pipeline increased gradually with the increase in the load in the range of 480~3550 N, decreased gradually with the increase in the load in the range of 3550~6500 N, and reached the maximum at the applied load of 3550 N (Figure 6b); however, the B1 and B2 positions of the pipeline were in the stage of unapplied load. Therefore, it was considered that the strain distribution at the bottom of the pipeline at the edge of the applied load increased first and then decreased. The strain at the B3 position at the end of the uniform bearing section of the pipeline gradually increased with the increase in the load in the range of 480~4470 N, gradually decreased with the increase in the load in the range of 4470~6500 N, and reached the maximum at the applied load of 4470 N (Figure 6c); the strain at B4 located in the center of the uniform bearing section increased gradually with the increase in the load in the range of 480~6500 N (Figure 6d). It was found that with the increase in the load, the maximum strain position gradually shifted to the center of the pipeline from the end position B1 to the center position B4 of the pipeline.

It can be seen from Figure 6e that the strain at the bottom of the entire pipeline was greater than zero, indicating that the bottom of the natural gas pipeline was subjected to tension under load. Through a careful comparative analysis, it was found that only the strain in B4 in the center of the pipeline increased with the increase in the load under uniform loading, and the strains at the other positions increased first and then decreased with the increase in the load. The overall analysis shows that regardless of the uniform load, the strain at the bottom of the pipeline was roughly symmetrical about the center position B4 of the pipeline after each load was applied, and the maximum strain was in the center of the pipeline.

5.3. Stress Characteristics of Pipeline Top and Bottom

The pipeline was subjected to an external load to produce a bending moment, which manifested as bending of the pipeline itself, thus indirectly reflecting the stress state of the pipeline at different positions. Through the analysis of Figure 7, it can be concluded that the pipeline with length L was bent under uniform loading; the top length was obviously smaller than the original length of the pipeline, and the bottom length was greater than the original length. It can be considered that the top of the pipeline was compressed by pressure, and the bottom was stretched by tension. The strain at the top and bottom of the pipeline was represented in the same diagram to analyze the stress state of the top and bottom of the pipeline. The stress state at the top and bottom of the pipeline was opposite. The top was compressed, and the bottom was pulled, which is consistent with the theoretical analysis of the stress characteristics at the top and bottom of the pipeline, indicating that the test results were reasonable and reliable. Comparing the strain increment at the top and bottom of the pipeline under uniform loading, it was found that the strain increment at the top of the pipeline was obviously larger than that at the bottom, and under the same load, the strain at the top of the pipeline was also significantly larger than that at the bottom, which indicates that under uniform loading, the first deformation and failure position of the pipeline was located at the top of the center of the loaded pipeline.

5.4. Stress Characteristics of the Pipeline End

According to the experimental design, the stress scatter diagram at both ends of the pipeline after applying a uniform load is shown in Figure 8. With the increase in the applied load, the force at both ends of the pipeline gradually increased, and the two were positively correlated. Through correlation curve fitting of the force at the end of the pipeline, it was found that the load applied in the middle of the pipeline and the load at the end of the pipeline showed a quadratic correlation, and the fitting degree was 99.7%. The fitting equation is Formula (1):

$$y=-480+0.94x+7.97x^2$$

(1)

In the formula, x is the load applied to the middle of the pipeline in N; y is the force load at the end of the pipeline in N.

5.5. Deflection Deformation Characteristics of the Pipeline

Through statistical analysis of the settlement data of the pipeline after the uniform load was applied, curves of the pipeline settlement with the loading were drawn, as shown in Figure 9. Under the action of a uniform load, the middle part of the pipeline gradually changed shape from the initial flat-bottom shape to the funnel shape with the increase in the load.

When the load was gradually increased from 0 to 1500 N, the middle part of the pipeline remained roughly flat, indicating that the pipeline was in the elastic stage without plastic failure. Between 1500 and 4470 N, the middle of the pipeline gradually changed shape from a flat-bottom shape to the funnel shape. Between 3550 and 4470 N, the sinking of the pipeline at A3 and A5 did not change significantly, but the A4 position in the middle of the pipeline obvious sunk, indicating that the middle of the pipeline reached its own strength limit when the load was 4470 N. Elastic–plastic failure occurred. As the load continued to increase from 4470 to 6500 N, the middle of the pipeline transformed from a smooth funnel shape to a straight triangular shape, and the deformation of the pipeline was particularly obvious when the load was 6500 N, indicating that plastic deformation at the A4 damage position in the middle of the pipeline continued to increase as the plastic failure of the pipeline continued with the application of the load. Based on this, it was concluded that the deformation of the pipeline evolved from a flat-bottom shape to the funnel shape and to a triangular shape under a uniform load.

The strain law for the top and bottom of the middle of the pipeline under a uniform load revealed the stress characteristics and deformation behavior of the pipeline under a uniform load. As the applied load gradually increased, the overall deformation of the pipeline progressed, and the shape changed from a flat-bottom to a funnel shape and finally evolved into a triangular shape, and plastic failure occurred when the pipeline became triangular. Through an in-depth understanding of these laws, the performance and safety of pipelines can be better evaluated, which provides a scientific basis for the design, construction, and maintenance of pipelines.

5.6. Safety Analysis of the Pipeline

If the 3.6 m area of the buried pipeline with a diameter of 48 mm and a length of 8.8 m was in the mining subsidence area, the middle part of the pipeline would bear the overburden load F, which is expressed by Formula (2):

F = DLHρg

(2)

In the formula, D is the pipeline diameter, 48 mm; L is the length of the pipeline in the subsidence area, 3.6 m; H is the buried depth of the pipeline, 1.6 m; ρ is the bulk density of aeolian sand, corresponding to 1800 kg/m³; and g is the gravitational acceleration, 9.8 g/m².

The calculation Formula (2) shows that the bearing gravity of the 3.6 m area of the buried pipeline under the action of mining subsidence was 4877 N. Considering Figure 9, it was found that elastic–plastic failure occurred when the bearing load in the middle of the pipeline was 4470 N, and the sinking reached its own strength limit when at 33.8 cm. Therefore, it was considered that when the buried pipeline with a diameter of 48 mm was in the mining subsidence area with a length of 3.6 m, the surface subsidence reached 33.8 cm, which indicated pipeline failure.

6. Discussion

In this paper, a stress and deformation test system for buried natural gas pipelines under the action of simulated subsidence soil was designed and developed. It can be used with different loading methods (uniform loading or single-point loading) and pipelines with different diameters, in different subsidence ranges, and with different material parameters. Under the examined conditions, the stress and strain, deformation evolution distribution law, and stress characteristics at both ends of the pipeline were tested, providing a basis for evaluating the stress and deformation of buried pipelines under mining subsidence.

During the test, it was found that there were some shortcomings, as follows: (1) due to the influence of the deformation of the lower beam of the pipeline, uniform loading for long distances could not be guaranteed to fully achieve a uniform loading; (2) this test did not test pipelines in different materials. In further research, the pipeline uniform loading device can be improved, and a variety of natural gas pipeline in different materials can be tested to provide data guidance for the maintenance of natural gas pipelines in on-site production activities.

The test device is only applicable to the buried strata of natural gas pipelines, which are aeolian sand or geological strata with strong fluidity. In view of the high degree of adhesion and the difficult flow of the buried stratum of the pipeline, it is necessary to continue this research.

7. Conclusions

A large number of natural gas pipelines of different types and diameters are laid in the gas–coal overlapping area of the Ordos Basin. When a buried pipeline is within the influence range of the coal seam mining subsidence area, the movement and deformation of the surface overburden rock can easily lead to the deformation and failure of the buried pipeline, thus affecting the safe production of natural gas. According to this experimental study, the strain and maximum deformation position of a natural gas pipeline under loading are located at the middle of the pipeline. According to this, the maximum deformation and the most vulnerable position of a natural gas pipeline passing through the mining subsidence area can be preliminarily judged, and then the corresponding remedial and protection measures can be taken. Thus, the results of this study have a certain guiding role for the protection of natural gas pipelines. We drew the following conclusions:

• With the help of the test system, a local incremental load was applied to a natural gas pipeline with a diameter of 48 mm. It was found that the strain at the top and bottom of the pipeline increased with the increase in the load. The top of the pipeline was compressed, and the bottom was tensioned; the stress state at the top and bottom was opposite. Under the same load, the strain of the pipeline increased gradually from the end to the loaded position, and the increase in the strain at the top of the pipeline was significantly greater than that at the bottom.
• With the help of the test system to monitor the stress of the pipeline end under the load, it was found that the load applied in the middle of the pipeline had a quadratic nonlinear positive correlation with the stress load at the end, and with the increase in the applied load, the stress load at the end of the pipeline exceeded the applied load.
• With the help of the test system to monitor the deflection change of the pipeline under a load, it was found that with a gradual increase in the applied load, the overall deformation of the pipeline evolved from a flat-bottom shape to the funnel shape and finally to a triangular shape; plastic failure occurred when the pipeline became triangular.

According to the experimental analysis, the deformation and strain distribution law of the pipeline under loading and the functional relationship between the end of the pipeline and the stress on the area in the subsidence area were determined. These results can provide a reference for performance parameter design, field protection measures, and pipeline layout of buried natural gas pipelines in gas–coal overlapping areas.