A buried pipeline monitoring sensor and its sensing method and inversion evaluation method

Abstract

The application discloses a buried pipeline monitoring sensor and a sensing method and inversion evaluation method thereof, and belongs to the technical field of buried pipeline monitoring. The buried pipeline monitoring sensor comprises a sensor base body, a special fixture, a bellows and an optical fiber. The special fixture is arranged at one end of the sensor base body. An arc-shaped groove is arranged below the special fixture. The bellows is sleeved outside the sensor base body. The optical fiber is attached to the upper and lower surfaces of the sensor base body. The optical fiber is connected with an OFDR demodulator device. The sensor base body is a spring steel sheet. The monitoring sensor is fixed to the outer surface of the buried pipeline. The monitoring sensor can be used for precise positioning of the buried pipeline and inversion strain evaluation of the operation state of the pipeline.

Description

Technical Field

[0001] This invention relates to the field of buried pipeline monitoring technology, and in particular to a buried pipeline monitoring sensor, its sensing method, and its inversion evaluation method. Background Technology

[0002] Buried pipelines are widely used in the oil and gas industry. However, China is a country prone to earthquakes, and buried pipelines inevitably traverse active fault zones. After an earthquake, fault displacements of several meters can occur in localized areas. For example, after the 2011 Northeast Pacific earthquake, the maximum displacement of the Yanheng Fault reached 2.1 meters, and after the 1999 Taiwan earthquake, the maximum surface fault displacement reached 12 meters. Under the influence of faults, the structural integrity of buried pipelines is severely compromised, causing localized buckling or tensile failure, significantly impacting pipeline lifespan and posing a serious threat to the local environment.

[0003] To ensure pipeline safety at fault locations, monitoring and assessment are essential. Some studies employ the finite element method (FEM) to analyze the impact of factors such as fault zone angle and displacement on pipelines, including soil deformation, pipeline deformation, pipeline axial strain, and bending strain. These studies have clarified the mechanism of fault zone interaction with pipelines. However, given the complexity of soil, FEM alone cannot fully and quantitatively assess the impact of fault zones on pipelines under different geological conditions. Furthermore, some researchers use simplified models and analytical methods to evaluate pipeline performance, such as the Winkler model and the three-beam model. These methods offer real-time performance, but simplified models may not capture the complexity of pipe-soil interactions. In addition, physical model testing methods such as centrifuge tests, shaking table tests, and large-scale indoor tests have proven effective in reducing uncertainties during pipeline condition assessment. In general, deploying strain or displacement monitoring sensors on the pipeline is the most direct assessment method.

[0004] Existing pipeline monitoring sensors include strain gauges, vibrating wire strain gauges, fiber optic gratings, distributed optical fibers, pipeline robots, and coaxial cables. However, pipelines are typically linear engineering projects, and the actual location of faults is highly random. The first three monitoring technologies are point-based methods and cannot accurately monitor the most dangerous sections. In engineering projects, the diameter of buried natural gas pipelines can reach over 1 meter, and the strain on the pipeline surface is proportional to the diameter. Under dynamic testing conditions, the monitoring range of strain sensors should be at least greater than 3%. The emerging distributed optical fiber monitoring range is limited, generally within 1%, which restricts its application in large strain monitoring. Existing research indicates that coaxial cables have a range greater than 3%; however, coaxial cables are still in the laboratory research stage, and their practical application is not yet mature. Pipeline robots cannot monitor during operation. In summary, there is currently limited research on methods for monitoring large deformations in pipelines with fracture zones, and a method capable of monitoring large deformations in pipelines is urgently needed. Summary of the Invention

[0005] The purpose of this invention is to provide a buried pipeline monitoring sensor, its sensing method, and its inversion evaluation method to solve the problems mentioned in the background art.

[0006] To achieve the above objectives, the present invention provides a buried pipeline monitoring sensor, comprising a sensor base, a special clamp, a corrugated pipe, and an optical fiber. The special clamp is disposed at one end of the sensor base, and an arc-shaped groove is provided below the special clamp. The corrugated pipe is sleeved on the outside of the sensor base. The optical fiber is attached to the upper and lower surfaces of the sensor base and is connected to an OFDR demodulator. The sensor base is a spring steel sheet with a thickness of 2 mm and a width of 10 mm.

[0007] This invention also provides a sensing method for a buried pipeline monitoring sensor, comprising the following steps:

[0008] Step 1: Divide the beam structure into several inverse beam elements and measure the surface strain components of each inverse beam element. The surface strain components of the inverse beam element include the measured axial strain and bending curvature of the inverse beam element.

[0009] Step 2: Construct a least squares function for each inverse beam element, substitute the theoretical values ​​of strain and curvature into the least squares function, introduce curvature boundary conditions, and obtain the final least squares function;

[0010] Step 3: Simplify the longitudinal deformation gradient to a constant to obtain a new node coordinate vector u. The node coordinate vector can be simplified to... Get u and The relationship between them;

[0011] Step 4: Iteratively solve u and u using Newton's method. The relationship between them is obtained by iteratively solving for the corresponding element in each unit. Then, the structure is assembled to obtain the overall displacement of the structure;

[0012] Step 5: Calculate the precise displacement of any point inside each unit using shape functions to obtain the continuous deformation result of the structure.

[0013] Preferably, the measured axial strain and bending curvature of the inverse beam element are expressed as follows:

[0014]

[0015] Where h is the thickness of the inverse beam element. Let be the measured strain on the upper surface of the i-th inverse beam element. Let be the measured strain on the lower surface of the i-th inverse beam element.

[0016] Preferably, the nodal coordinates of the inverse beam element are represented as follows:

[0017]

[0018] Where r represents the nodal coordinates of the inverse beam element, S represents the shape function matrix, and u is the nodal coordinate vector of the inverse beam element, expressed as:

[0019]

[0020] In the formula, r1 and r2 represent the two coordinate axes of the local coordinate system within the inverse beam element, x is the length coordinate of the node on the inverse beam element in its undeformed state, and l is the original length of the inverse beam element. Since it is a known quantity, it can be obtained from the boundary conditions. The quantity to be solved;

[0021] The shape function matrix S is written as:

[0022]

[0023] Among them, S i Let S represent a shape function, i = 1, 2, 3, 4. i Represented as:

[0024] S1=1-3ξ 2 +2ξ 3 ;

[0025] S2=ξ-2ξ 2 +ξ 3 ;

[0026] S3=3ξ 2 -2ξ 3 ;

[0027] S4=-ξ 2 +ξ 3 ;

[0028] Where ξ = x / l, and the range of ξ is 0-1, representing the position of the interpolation point in the inverse beam element;

[0029] The theoretical values ​​of strain and curvature are obtained using continuum mechanics methods:

[0030]

[0031] Where r′ represents the first derivative of matrix r with respect to x, T represents the transpose of the matrix, S′ represents the first derivative of matrix S with respect to x, r″ represents the first derivative of matrix r with respect to x, S′1 represents the first derivative of the first row of matrix S with respect to x, S″2 represents the second derivative of the second row of matrix S with respect to x, S′2 represents the second derivative of the second row of matrix S with respect to x, and S″1 represents the second derivative of the first row of matrix S with respect to x. f represents the longitudinal deformation gradient, which is simplified to a constant 1.

[0032] The preferred least squares function is expressed as:

[0033]

[0034] Where ε and k represent the measured axial strain and measured bending curvature, respectively, w t and w b These represent the weighting coefficients for the axial strain term and the bending curvature term, respectively, both taken as 1 and l. e Indicates the length of the reverse beam element;

[0035] Substituting the theoretical values ​​of strain and curvature into the least squares function yields:

[0036]

[0037] In the formula, S t and S b This is an abbreviation used to facilitate subsequent calculations.

[0038] Preferably, by introducing curvature boundary conditions, we obtain:

[0039] w k ∥k0(u)-k0∥ 2 =w k (u T S k u-k0) 2 ;

[0040]

[0041] In the formula, w k The weighting coefficient representing the boundary condition term is set to 1, S k For ease of subsequent calculations, k0 represents the curvature of the end node of the previous inverse beam element, which can be obtained from the previous known element.

[0042] The formula for obtaining the final weighted least squares function is:

[0043]

[0044] Among them, R t R b R k All of these are simplifications made to facilitate subsequent calculations.

[0045] Preferably, the new node coordinate vector is:

[0046]

[0047] Where θ represents the rotation angle of the node position;

[0048] Simplified Represented as:

[0049]

[0050] in, and These represent the simplified coordinate vectors of the two node positions of the inverse beam element;

[0051] u and The relationship between them can be written as:

[0052]

[0053] Preferably, Newton's method is used to iteratively solve for u and The relationship between them requires the use of gradient vectors and Hessian matrices;

[0054] The gradient vector is represented as:

[0055]

[0056] in, and Abbreviations used for ease of subsequent calculations;

[0057] The Hessian matrix is ​​represented as:

[0058]

[0059]

[0060] Among them, H t H b H k These are all abbreviations used for the convenience of subsequent calculations;

[0061] Boolean matrix B i The expression is as follows:

[0062]

[0063] Among them, b j Represents a Boolean variable;

[0064] The iterative formula based on Newton's method is:

[0065]

[0066] in, and Let represent the coordinate vector iteration results of the second node of the inverse beam element in the k-th and k+1-th iterations, respectively. This represents the nodal coordinate vector of the inverse beam element obtained through iterative calculation.

[0067] Preferably, the assembly formula is as follows:

[0068]

[0069] In the formula, r i g This represents the coordinate vector of the i-th node in the global coordinate system, where the superscript g indicates that the coordinates are in the global coordinate system. This represents the rotation angle of the i-th node in the global coordinate system.

[0070] This invention also provides an inversion evaluation method, comprising the following steps:

[0071] S1. Fix the monitoring sensor to the surface of the pipe to ensure that the monitoring sensor and the pipe can deform together. Based on the strain of the sensor substrate surface obtained by the monitoring sensor, the sensor deformation trend is reconstructed in real time using a sensing method.

[0072] S2. Fit the displacement monitoring results of the monitoring sensor using a seventh-order polynomial.

[0073] S3. Using the finite element analysis method, construct a proportional pipe model. Using the pipe material three-segment model, apply the deformation result obtained in step S2 as a displacement load to the pipe model, calculate the state of the pipe after deformation, and extract the strain on the pipe surface as the inversion strain result.

[0074] S4. Select the region in the inversion strain results that exceeds the elastic strain of the pipeline material as the pipeline hidden danger area;

[0075] S5. Using the same fitting method and finite element analysis method as steps S2 and S3, the pipeline hidden danger area is analyzed to obtain the pipeline strain extreme values ​​at each loading stage.

[0076] S6. Accurately locate pipeline deformation by monitoring sensor displacement data, and simultaneously determine the pipeline strain state by inverting the extreme values ​​of strain in the pipeline.

[0077] Therefore, the present invention employs the above-mentioned buried pipeline monitoring sensor, sensing method, and inversion evaluation method, which has the following beneficial effects:

[0078] (1) The sensor has the advantages of distributed monitoring, small size and high precision, and will not affect the operation of the pipeline. The high-precision distributed displacement data provides reliable data for subsequent strain inversion evaluation.

[0079] (2) The sensor can be used for large deformation monitoring and is suitable for large deformation scenarios such as fracture zones;

[0080] (3) The improved iANCFs method and unique sensor structure design make the sensor unaffected by temperature;

[0081] (4) The proposed strain inversion assessment method can realize large strain monitoring of pipelines and accurately assess the condition of pipelines.

[0082] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0083] Figure 1 This is a schematic diagram of a buried pipeline monitoring sensor and its application method according to an embodiment of the present invention;

[0084] Figure 2 This is a schematic diagram of the structure of the buried pipeline monitoring sensor according to an embodiment of the present invention;

[0085] Figure 3 This is a verification experiment diagram of the buried pipeline monitoring sensor according to an embodiment of the present invention;

[0086] Figure 4 This is a comparison chart of the displacement calculated by the buried pipeline monitoring sensor and the displacement measured by the distance measuring instrument in an embodiment of the present invention.

[0087] Figure 5 This is a schematic diagram of the beam structure according to an embodiment of the present invention;

[0088] Figure 6 This is a flowchart illustrating the calculation process of the sensing method for the buried pipeline monitoring sensor according to an embodiment of the present invention.

[0089] Figure 7 This is a comparison diagram of the reconstructed displacements of the three methods iANCFs, iFEM and CB in this embodiment of the invention;

[0090] Figure 8 This is a graph showing the relative error of the iANCFs method in this embodiment of the invention as a function of the number of elements.

[0091] Figure 9 This is a schematic diagram showing the installation location of the monitoring sensor according to an embodiment of the present invention;

[0092] Figure 10 This is a comparison diagram of the pipeline before and after the test in an embodiment of the present invention;

[0093] Figure 11 The strain distribution diagrams of the upper surface of the sensor are shown at different times during the test of this embodiment of the invention.

[0094] Figure 12These are measured strain distribution diagrams of the pipe surface at different times during the test process of this embodiment of the invention;

[0095] Figure 13 This is a comparison diagram of the pipe deformation measured by the displacement sensor and the pipe deformation measured by the lead screw at different times during the test of this embodiment of the invention;

[0096] Figure 14 This is a flowchart of the inversion evaluation method according to an embodiment of the present invention;

[0097] Figure 15 This is a comparison diagram of the inversion strain of the inversion evaluation method according to an embodiment of the present invention and the measured strain of a distributed optical fiber pipeline;

[0098] Figure 16 This is a typical moment of the pipeline deformation seventh-order polynomial fitting effect diagram of the inversion evaluation method of this invention embodiment;

[0099] Figure 17 This is a graph showing the development trend of extreme values ​​of pipeline strain at different loading stages in the inversion evaluation method of this invention.

[0100] Figure 18 The inversion evaluation method of this invention is used to obtain pipeline state diagrams at different loading stages.

[0101] Figure Labels

[0102] 1. Sensor substrate; 2. Special fixture; 3. Corrugated tube; 4. Optical fiber; 5. Arc-shaped groove. Detailed Implementation

[0103] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0104] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0105] Example 1

[0106] like Figure 1-2 As shown, this invention provides a buried pipeline monitoring sensor, including a sensor base 1, a special clamp 2, a corrugated pipe 3, and an optical fiber 4. The special clamp 2 is disposed at one end of the sensor base, and an arc-shaped groove 5 is provided below the special clamp 2 to fit against the pipeline surface. The corrugated pipe 3 is sleeved on the outside of the sensor base 1, and the optical fiber 4 is attached to the upper and lower surfaces of the sensor base 1. The optical fiber 4 is connected to an OFDR demodulator. The monitoring sensor is fixed to the outer surface of the buried pipeline. The sensor base 1 is a spring steel sheet with a thickness of 2mm and a width of 10mm, and the corrugated pipe 3 is a stainless steel corrugated pipe with an inner diameter of 10mm.

[0107] Installation process: Fiber optic cables are attached to the upper and lower surfaces of the sensor substrate 1. Then, the sensor substrate 1 is passed through the corrugated pipe 3. Finally, the fiber optic cable 4 is connected to the OFDR demodulator to monitor the strain on the substrate surface in real time. The obtained strain is used as the input of the algorithm to obtain the displacement of the substrate.

[0108] It is important to note that optical fibers are affected by temperature when measuring strain. However, this device can counteract the temperature effect on the optical fiber during strain measurement. This is achieved through a combination of sensor structural characteristics and the iANCFs method. Firstly, in terms of sensor structural design, a corrugated pipe is used as the outer shell to protect the sensor. This ensures that the sensor and the pipe deform in tandem while allowing the sensor substrate to deform axially, thus preventing longitudinal forces from acting on the sensor substrate. Secondly, the iANCFs method simplifies the measured axial strain to a constant, unaffected by temperature. Furthermore, when calculating bending strain, the strains on the upper and lower surfaces are subtracted, thus offsetting the temperature effect.

[0109] By combining fiber optic sensing technology with shape sensing methods, a monitoring sensor can be created to monitor structural displacement in real time. The distance between the fiber optic cable and the centroid of the sensor substrate is much smaller than the distance between the fiber optic cable directly attached to the pipe surface and the centroid of the pipe; therefore, there is no need to worry about exceeding the fiber optic cable's range during monitoring.

[0110] The accuracy and temperature sensitivity of the monitoring sensor will be analyzed next:

[0111] 1. This embodiment is designed as follows: Figure 3 The verification test shown used a 5.8m long sensor. One end of the fabricated sensor was fixed using a special clamp 2. Seven marker points were set at 0.8m intervals on the surface of the bellows 3. The positions of the seven marker points under different deformations in five stages were recorded using a measuring tape and a laser rangefinder. Simultaneously, the sensor strain was acquired using a fiber optic demodulator, and the sensor displacement was calculated using a sensing method. (Refer to...) Figure 4The two displacement trends are highly consistent, with the error percentage of the maximum displacement being less than 1%. During the experiment, the maximum strain of fiber 4 was approximately 350 με, which is much smaller than the strain monitoring range of the fiber. This sensor can also achieve deformation monitoring over a larger range.

[0112] 2. Due to the small temperature difference in the indoor test, the influence of temperature on the strain measurement results was added to the actual sensor strain measurement to analyze the sensor's temperature sensitivity. The calibration coefficients for the fiber optic 4 spectral drift and strain and temperature are -6.67 and -0.638, respectively, meaning that a 1℃ increase in temperature will lead to an increase of 10.45 in the strain measurement value. The effects of temperature increments of 5℃, 10℃, and 15℃ on the sensor displacement measurement results were analyzed, and the temperature error δ was defined. T as follows:

[0113]

[0114] Where, r i 0 r represents the coordinates of the i-th node when the temperature increment is 0℃. i The coordinates of the i-th node represent the temperature increments when they are not zero, and n is the total number of nodes.

[0115] We calculated the temperature errors of two methods under different temperature increments: iANCFs considering the correction effect and iANCFs without considering the temperature effect. iANCFs takes temperature into account, therefore temperature does not affect the displacement calculation results. However, the temperature error of iANCFs increases significantly with increasing temperature increment.

[0116] Example 2

[0117] This invention also provides a sensing method for a buried pipeline monitoring sensor, which obtains discrete strain on the surface of the sensor substrate through distributed optical fiber, reconstructs the sensor displacement based on the improved iANCFs shape sensing method, and realizes deformation monitoring of the structure.

[0118] The sensing method includes the following steps:

[0119] Given the length and thickness of the beam structure, refer to Figure 5 (The left side is the beam structure, and the right side is the reverse beam element). First, the beam structure is divided into several reverse beam elements, and the surface strain components of each reverse beam element are measured.

[0120] The surface strain components of the inverse beam element, including the measured axial strain and bending curvature, can be expressed as:

[0121]

[0122] Where h is the thickness of the inverse beam element. Let be the measured strain on the upper surface of the i-th inverse beam element. Let be the measured strain on the lower surface of the i-th inverse beam element.

[0123] The nodal coordinates of the inverse beam element can be expressed as:

[0124]

[0125] Where r represents the nodal coordinates of the inverse beam element, S represents the shape function matrix, and u is the nodal coordinate vector of the inverse beam element, which can be expressed as:

[0126]

[0127] In the formula, r1 and r2 represent the two coordinate axes of the local coordinate system within the inverse beam element, x is the length coordinate of the node on the inverse beam element in its undeformed state, and l is the original length of the inverse beam element. Since it is a known quantity, it can be obtained from the boundary conditions. Let be the quantity to be solved.

[0128] The shape function matrix S can be written as:

[0129]

[0130] Among them, S i Let i represent a shape function, where i = 1, 2, 3, 4.

[0131] Function S in the shape function matrix i (i = 1, 2, 3, 4) can be represented as:

[0132] S1=1-3ξ 2 +2ξ 3 ;

[0133] S2=ξ-2ξ 2 +ξ 3 ;

[0134] S3=3ξ 2 -2ξ 3 ;

[0135] S4=-ξ 2 +ξ 3 ;

[0136] Where ξ = x / l, and the range of ξ is 0-1, representing the position of the interpolation point in the inverse beam element.

[0137] The theoretical values ​​of strain and curvature can be obtained using continuum mechanics methods:

[0138]

[0139] Where r′ represents the first derivative of matrix r with respect to x, T represents the transpose of the matrix, S′ represents the first derivative of matrix S with respect to x, r″ represents the first derivative of matrix r with respect to x, S′1 represents the first derivative of the first row of matrix S with respect to x, S″2 represents the second derivative of the second row of matrix S with respect to x, S′2 represents the second derivative of the second row of matrix S with respect to x, and S″1 represents the second derivative of the first row of matrix S with respect to x. f represents the longitudinal deformation gradient. Since the monitoring sensor structure is not subjected to longitudinal force, it can be simplified to a constant, i.e., f≈1, for ease of calculation.

[0140] For each inverse beam element, a least squares function is constructed as shown in the following formula:

[0141]

[0142] Where ε and k represent the measured axial strain and measured bending curvature, respectively, w t and w b These represent the weighting coefficients for the axial strain term and the bending curvature term, respectively, both taken as 1 and l. e This indicates the length of the reverse beam element.

[0143] Substituting the theoretical values ​​of strain and curvature into the above equation yields:

[0144]

[0145] In the formula, S t and S b This is an abbreviation used to facilitate subsequent calculations.

[0146] Introducing curvature boundary conditions, it can be written as:

[0147] w k ∥k0(u)-k0∥ 2 =w k (u T S k u-k0) 2 ;

[0148]

[0149] In the formula, w k The weighting coefficient representing the boundary condition term is set to 1, S k For ease of subsequent calculations, k0 represents the curvature at the end node of the previous inverse beam element, which can be obtained from the previous known element. Therefore, the final weighted least squares formula is:

[0150]

[0151] Among them, R t Rb R k All of these are simplifications made to facilitate subsequent calculations.

[0152] The above equation constitutes the weighted least squares formula for solving the deformation of a single inverse element. For the entire structure, it can be assembled into a global formula; however, this would greatly increase the complexity of the solution. Therefore, this paper adopts an element-by-element solution method.

[0153] From the formula for the nodal coordinate vector of the inverse beam element above, it can be seen that the number of unknowns is still greater than the number of constraints. Since f can be simplified to a constant 1, the nodal coordinate vector u can be written as:

[0154]

[0155] Where θ represents the rotation angle of the node position.

[0156] Furthermore, the node coordinate vector can be simplified to

[0157]

[0158] in, and These represent the simplified coordinate vectors of the two node positions of the inverse beam element.

[0159] u and The relationship between them can be written as:

[0160]

[0161] Solving the above nonlinear equations using Newton's method requires the use of gradient vectors and Hessian matrices.

[0162] The gradient vector can be written as:

[0163]

[0164]

[0165] in, and This is an abbreviation used to facilitate subsequent calculations.

[0166] The Hessian matrix can be written as:

[0167]

[0168] Among them, H t H b H k These are all abbreviations used for the convenience of subsequent calculations.

[0169] Boolean matrix B i The expression is as follows:

[0170]

[0171] Among them, b j Represents a Boolean variable.

[0172] The iterative formula based on Newton's method is:

[0173]

[0174] in, and Let represent the coordinate vector iteration results of the second node of the inverse beam element in the k-th and k+1-th iterations, respectively. This represents the nodal coordinate vector of the inverse beam element obtained through iterative calculation.

[0175] The corresponding solution for each unit is obtained through iterative solution. After that, it needs to be assembled to obtain the overall displacement of the structure.

[0176]

[0177] In the formula, r i g This represents the coordinate vector of the i-th node in the global coordinate system, where the superscript g indicates that the coordinates are in the global coordinate system. This represents the rotation angle of the i-th node in the global coordinate system.

[0178] The displacements of each node on the structure can be obtained through the above calculations, referring to... Figure 6 Furthermore, the precise displacement of any point within each unit can be calculated using shape functions, thus obtaining the continuous deformation results of the structure.

[0179] Compared to the iANCF method, this sensing method is adaptively simplified in three aspects based on the sensor structure. First, the measured axial strain of each element is simplified to a constant of 0, offsetting the temperature-dependent strain measurement by the fiber optic sensor. Second, the longitudinal deformation gradient is simplified to a constant of 1. Third, the Newton iteration solution process is optimized, eliminating complex terms with minimal impact on the calculation results, thus improving computational efficiency while maintaining accuracy requirements. The first and second simplifications are predicated on the sensor structure designed in this study not being subjected to longitudinal forces. Finite element analysis verifies that the improved iANCF method has comparable accuracy to iANCF and can achieve real-time displacement reconstruction.

[0180] Next, we will analyze the performance of the iANCFs method from three aspects: accuracy comparison analysis, influence of element number division, and adaptability to long-distance complex deformation.

[0181] 1. Comparison of accuracy of different methods

[0182] A cantilever beam model is created in finite element simulation software, and displacement loads are applied to the ends of the beam to simulate its deformation. The beam is 1000 mm long and 2 mm thick. Fixed constraints are applied to one end of the beam, and displacement loads are applied to the other end with magnitudes of 50 mm, 100 mm, 200 mm, 400 mm, and 600 mm.

[0183] To compare the accuracy of the improved iANCFs, iFEM, and CB methods, V was calculated using each of the three methods. iANCFs V iFEM V CB Among them, V iANCFs V iFEM V CB The displacements were obtained through strain reconstruction using three methods: the inverse absolute nodal coordinate method, the inverse finite element method, and the conjugate beam method. All three methods divided the entire beam into five inverse beam elements and calculated the displacement values ​​at each element's nodes.

[0184] Reference Figure 7 Since the inverse finite element method and the conjugate beam method do not consider the axial displacement of the beam, the nodal coordinates in the x-direction of each element will not change. When the displacement at the end of the cantilever beam reaches 100 mm, although the vertical displacement calculated by the inverse finite element method and the conjugate beam method is not significantly different from the actual value, the axial displacement has already produced a relatively significant error. When the displacement at the end of the cantilever beam is greater than 100 mm, the error in the calculation results of the inverse finite element method and the conjugate beam method increases sharply.

[0185] 2. Unit Quantity Division

[0186] The aforementioned 1m long cantilever beam was divided into different numbers of elements, and the impact of different element numbers on the method accuracy was analyzed. The relative error was defined as follows:

[0187]

[0188] Where, r i Let be the coordinate vector of the i-th node reconstructed by the iANCFs method. Let be the coordinate vector of the i-th node obtained from the finite element analysis; Let be the coordinate vector of the i-th node in the initial state of the cantilever beam, and n be the number of nodes.

[0189] Reference Figure 8The calculation results under different deformation conditions show relatively consistent convergence. When the cantilever beam is divided into 4 inverse elements, the relative error is less than 0.005. The above results prove that the iANCFs method can reconstruct the displacement of the structure with high accuracy with a relatively small number of inverse elements.

[0190] The monitoring sensor was applied to a large deformation model test of a buried pipeline under the action of a fracture zone to verify its reliability. The pipeline material used in the test was X42, with an outer diameter of 89 mm and a wall thickness of 3.5 mm. The fabricated pipeline monitoring sensor was externally protected by a corrugated pipe and fixed to one side of the pipeline in the horizontal direction using wire binding. Both ends of the monitoring sensor were fixed with special clamps to ensure that the normal of the monitoring sensor substrate always pointed to the centroid of the pipeline. Distributed optical fibers and 11 resistance strain gauges were attached to the other side of the pipeline in the horizontal direction to measure the axial strain of the pipeline. The pipeline monitoring sensor and the distributed optical fiber were connected in series to an optical fiber demodulation device, and the resistance strain gauges were connected to the acquisition equipment. Figure 9 .

[0191] The test chamber measures 6m x 1.1m x 1m. The left side is the movable section, which can move horizontally along a slide rail at the bottom of the chamber with the help of two jacks. The right side is the fixed section. The movement of the test chamber simulates a 90° angled fault displacement. Before the test, a geomembrane is laid on the inner wall of the test chamber to reduce friction between the soil and the chamber. Then, clay is filled to a predetermined height inside the chamber, with the soil compacted in layers during filling. A pipe equipped with monitoring sensors is placed inside the test chamber. The ends of the monitoring sensors are fixed to the side wall of the fixed section of the test chamber using magnetic bases. 1m long threaded rods are connected to the pipe at six different locations using clamps, with the ends of the threaded rods extending to the outside of the test chamber through openings in the side wall.

[0192] During the experiment, two jacks were used to control the displacement of the moving parts of the test chamber, with each movement in 25mm increments and a maximum movement distance of approximately 400mm. The displacement of the pipe was actually measured by extending the test chamber length through a measuring screw. After the experiment began, strain readings were continuously acquired from the optical fiber and strain gauges. The optical fiber demodulation device was an ODISI6000. Since the monitoring sensor itself can eliminate the influence of temperature, the effect of temperature changes on the strain measured by the optical fiber is not considered. Figure 10 .

[0193] Reference Figure 11 The extreme value of the surface strain of the monitoring sensor is about 400 με, which is much smaller than the monitoring range of the optical fiber, proving the large range advantage of the monitoring sensor.

[0194] Reference Figure 12 The measured results of strain gauges and distributed optical fibers are in excellent agreement, proving the accuracy of strain monitoring.

[0195] Starting from the 8th loading stage, the data from distributed optical fiber and strain gauge monitoring showed abnormal peak values ​​at the extreme strain locations in the pipeline. Both monitoring methods were effective and easy to install within the small elastic deformation range of the pipeline. However, they could not effectively monitor the pipeline strain when the strain was large under bending action.

[0196] Extracting surface strain data from monitoring sensors and reconstructing pipe displacement using the iANCFs method, the results are as follows: Figure 13 As shown in the figure, the distributed displacement of the pipeline measured by the monitoring sensors matches the trend of the measured displacement of the lead screw at six locations, proving that the iANCFs method is effective in such cases. Figure 11 Even under such complex strain trends, it can still accurately reconstruct the displacement trend.

[0197] Example 3

[0198] Reference Figure 14 The present invention also provides an inversion evaluation method, comprising the following steps:

[0199] S1. Monitoring Pipeline Deformation: A monitoring sensor is fixed to the pipeline surface, ensuring that the sensor and pipeline deform in tandem. Based on the strain acquired by the sensor substrate surface, a sensing method is used to reconstruct the sensor's deformation trend in real time, such as... Figure 13 As shown.

[0200] S2. Pipeline Deformation Trend Fitting: Since the stiffness of the monitoring sensor is much smaller than that of the pipeline, and the monitoring sensor and the pipeline are indirectly in contact through a corrugated pipe, there is an unavoidable gap. Therefore, the pipeline deformation trend is not smooth in some local locations. It is necessary to first fit the displacement monitoring results of the monitoring sensor. The fitting method is a 7th-order polynomial.

[0201] S3. Inverting Pipe Strain: Using the finite element method, a scaled-down pipe model is constructed. A three-segmented model of the pipe material is used. The deformation result obtained in step 2 is applied as a displacement load to the pipe model. The deformed state of the pipe is calculated, and the strain on the pipe surface is extracted as the inverted strain result. The inverted strain is compared with the measured pipe strain using distributed optical fiber. Figure 15 As shown. In the first 8 loading stages, the inversion results have a fairly high accuracy. The premise for achieving accurate inversion of pipeline strain is that the monitoring sensors can accurately monitor the changes in the horizontal displacement and axial distance of the pipeline, which is something that traditional sensors cannot do.

[0202] S4. Identify Pipeline Hazardous Areas: Select areas in the inversion strain results that exceed the elastic strain of the pipeline material as pipeline hazardous areas. Figure 15 The process can determine the region where the pipeline enters the elastoplastic deformation zone. The 1.5m-2.5m position of the fixed end in the fracture zone model test is selected as the pipeline hazard area.

[0203] S5. Individual fitting and inversion of pipeline hazard areas: Refer to Figure 16 As the fault displacement increases, the fitting results cannot fully reflect the true deformation of the pipeline at locations where the pipeline displacement gradient changes significantly. Therefore, the same finite element analysis method as in step S3 is used to analyze the pipeline hazard area, which can obtain the extreme values ​​of pipeline strain at each loading stage, such as... Figure 17 As shown. In the first 10 loading stages, the inverted extreme values ​​of pipe strain matched the measured extreme values ​​of pipe strain. After the 10th loading stage, the distributed optical fiber and strain gauges did not obtain valid measured extreme value data of strain.

[0204] S6. Pipeline condition assessment: Monitoring sensor displacement data can enable precise positioning of pipeline deformation, and pipeline strain extreme values ​​can help determine the pipeline strain state. Figure 18 By specifying the exact location of the pipeline state at different loading stages in the stress-strain diagram, the pipeline state can be clearly determined. The pipeline state assessment conclusion is that the pipeline enters the elastoplastic stage starting from the 8th loading stage, but the pipeline was not damaged until the end of the test.

[0205] Therefore, the present invention employs the above-mentioned buried pipeline monitoring sensor, sensing method, and inversion evaluation method to achieve distributed deformation monitoring. Furthermore, the monitoring sensor is unaffected by temperature and can be used for large deformation monitoring. The proposed strain inversion evaluation method can accurately assess the state of the pipeline.

[0206] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A sensing method for a buried pipeline monitoring sensor, characterized in that, Includes the following steps: Step 1: Divide the beam structure into several inverse beam elements and measure the surface strain components of each inverse beam element. The surface strain components of the inverse beam element include the measured axial strain and bending curvature of the inverse beam element. Step 2: Construct a least squares function for each inverse beam element, substitute the theoretical values ​​of strain and curvature into the least squares function, introduce curvature boundary conditions, and obtain the final least squares function; Step 3: Simplify the longitudinal deformation gradient to a constant to obtain the new node coordinate vector. u The node coordinate vector can be simplified to ,get u and The relationship between them; Step 4: Solve iteratively using Newton's method. u and The relationship between them is obtained by iteratively solving for the corresponding element in each unit. Then, the structure is assembled to obtain the overall displacement of the structure; The new node coordinate vector is: ； in, The angle representing the node's position; Simplified Represented as: ； in, and These represent the simplified coordinate vectors of the two node positions of the inverse beam element; u and The relationship between them can be written as: ； Solving iteratively using Newton's method u and The relationship between them requires the use of gradient vectors and Hessian matrices; The gradient vector is represented as: ； ； ； in, , , , and These are all abbreviations used for the convenience of subsequent calculations; The Hessian matrix is ​​represented as: ； ； ； ； in, , , These are all abbreviations used for the convenience of subsequent calculations; Boolean matrix B i The expression is as follows: ； in, Represents a Boolean variable; The iterative formula based on Newton's method is: ； ； in, and They represent the first k and k The coordinate vector iteration result of the second node of the +1 inverse beam element This represents the nodal coordinate vector of the inverse beam element obtained through iterative calculation; Step 5: Calculate the precise displacement of any point inside each unit using shape functions to obtain the continuous deformation result of the structure.

2. The sensing method of a buried pipeline monitoring sensor according to claim 1, characterized in that, The measured axial strain and bending curvature of the inverse beam element are expressed as follows: ； ； in, h For the thickness of the inverse beam element, For the first i Measured strain on the upper surface of a reverse beam element For the first i Measured strain on the lower surface of a reverse beam element.

3. The sensing method of a buried pipeline monitoring sensor according to claim 2, characterized in that, The nodal coordinates of the inverse beam element are represented as follows: ； in, r Represents the node coordinates of the inverse beam element. S Represents a shape function matrix, u Let be the nodal coordinate vector of the inverse beam element, expressed as: ； In the formula, and These represent the two coordinate axes of the local coordinate system within the inverse beam element. x These are the length coordinates of the nodes on the inverse beam element in its undeformed state. l This represents the original length of the inverse beam element. Since it is a known quantity, it can be obtained from the boundary conditions. The quantity to be solved; Shape function matrix S Represented as: ； in, S i Represents shape functions, i =1,2,3,4 S i Represented as: ； ； ； ； in, ξ=x / l , ξ The range is 0-1, representing the position of the interpolation point in the inverse beam element; The theoretical values ​​of strain and curvature are obtained using continuum mechanics methods: ； ； in, Representative matrix r right x One of the guides, This represents the transpose of the matrix. Representative matrix S right x Find the first derivative. Representative matrix r right x Find the first derivative. Representative matrix S The first line x Find the first derivative. Representative matrix S The second line x Find the second derivative. Representative matrix S The second line x Find the first derivative. Represents the first row of matrix S x Find the second derivative. f=| r / x | , f This represents the longitudinal deformation gradient, which is simplified to a constant 1.

4. The sensing method of a buried pipeline monitoring sensor according to claim 3, characterized in that, The least squares function is expressed as: ； in, and k These represent the measured axial strain and the measured bending curvature, respectively. and These represent the weighting coefficients for the axial strain term and the bending curvature term, respectively, both set to 1. Indicates the length of the reverse beam element; Substituting the theoretical values ​​of strain and curvature into the least squares function yields: ； ； ； In the formula, and This is an abbreviation used to facilitate subsequent calculations.

5. The sensing method of a buried pipeline monitoring sensor according to claim 4, characterized in that, Introducing curvature boundary conditions, we obtain: ； ； In the formula, The weighting coefficient representing the boundary condition term is set to 1. This abbreviation is for the convenience of subsequent calculations. This represents the curvature at the end node of the previous inverse beam element. This can be obtained from the previously known unit; The formula for obtaining the final weighted least squares function is: ； in, R t , R b , R k All of these are simplifications made to facilitate subsequent calculations.

6. The sensing method of a buried pipeline monitoring sensor according to claim 5, characterized in that, The assembly formula is: ； In the formula, Indicates the first i The coordinate vectors of each node in the global coordinate system, with superscript... g Indicates that in the global coordinate system, Indicates the first i The rotation angle of each node in the global coordinate system.

7. A buried pipeline monitoring sensor, applied to the sensing method of a buried pipeline monitoring sensor as described in any one of claims 1-6, characterized in that: The sensor includes a sensor substrate, a special clamp, a corrugated tube, and an optical fiber. The special clamp is located at one end of the sensor substrate, and an arc-shaped groove is provided below the special clamp. The corrugated tube is sleeved on the outside of the sensor substrate. The optical fiber is attached to the upper and lower surfaces of the sensor substrate and is connected to an OFDR demodulator. The sensor substrate is a spring steel sheet with a thickness of 2 mm and a width of 10 mm.

8. An inversion evaluation method, applied to the sensing method of a buried pipeline monitoring sensor as described in any one of claims 1-6, characterized in that, Includes the following steps: S1. Fix the monitoring sensor to the surface of the pipe to ensure that the monitoring sensor and the pipe can deform together. Based on the strain of the sensor substrate surface obtained by the monitoring sensor, the sensor deformation trend is reconstructed in real time using a sensing method. S2. Fit the displacement monitoring results of the monitoring sensor using a seventh-order polynomial. S3. Using the finite element analysis method, construct a proportional pipe model. Using the pipe material three-segment model, apply the deformation result obtained in step S2 as a displacement load to the pipe model, calculate the state of the pipe after deformation, and extract the strain on the pipe surface as the inversion strain result. S4. Select the region in the inversion strain results that exceeds the elastic strain of the pipeline material as the pipeline hidden danger area; S5. Using the same fitting method and finite element analysis method as steps S2 and S3, the pipeline hidden danger area is analyzed to obtain the pipeline strain extreme values ​​at each loading stage. S6. Accurately locate pipeline deformation by monitoring sensor displacement data, and simultaneously determine the pipeline strain state by inverting the extreme values ​​of strain in the pipeline.

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