Buried pipeline monitoring sensor and sensing method and inversion evaluation method thereof

Abstract

The invention discloses a buried pipeline monitoring sensor and a sensing method and an 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 specially-made clamp, a corrugated pipe and an optical fiber, the specially-made clamp is arranged at one end of the sensor base body, an arc-shaped groove is formed in the lower portion of the specially-made clamp, and the corrugated pipe is arranged in the arc-shaped groove; the corrugated pipe is sleeved outside the sensor base body, the optical fibers are pasted on the upper surface and the lower surface of the sensor base body, the optical fibers are connected with OFDR demodulator equipment, the sensor base body is a spring steel sheet, and the monitoring sensor is fixed on the outer surface of a buried pipeline. By adopting the monitoring sensor, accurate positioning of the buried pipeline can be realized, and the running state of the pipeline can be evaluated through inversion strain.

Description

Technical Field

[0001] The present invention relates to the technical field of buried pipeline monitoring, and in particular, to a buried pipeline monitoring sensor, a sensing method thereof, and an inversion evaluation method. Background Art

[0002] Buried pipelines are widely used in the oil and gas fields. China is a country with frequent earthquakes, and buried pipelines inevitably cross active fault zones. After an earthquake, a fault offset of several meters can occur at local positions. For example, the maximum displacement of the Yanheng Fault after the 2011 Tohoku earthquake along the Pacific coast reached 2.1 m, and after the 1999 Taiwan earthquake, the maximum surface fault displacement was up to 12 m. Under the action of the fault, the structural integrity of the buried pipeline will be severely damaged, resulting in local buckling or tensile failure, seriously affecting the pipeline life and posing a serious threat to the local environment.

[0003] To ensure the safety of pipelines at fault positions, it is very necessary to monitor and evaluate the pipelines. Some studies use the finite element method to analyze the influence of factors such as fault zone angle and displacement on pipelines from indicators such as soil deformation, pipeline deformation, pipeline axial strain, and bending strain. These studies make the action mechanism of the fault zone on the pipeline clearer. However, due to the complexity of the soil, it is impossible to completely and quantitatively evaluate the influence of fault zones under different geological conditions only by finite element analysis. In addition, some scholars evaluate the performance of pipelines by using analytical methods through simplified models, such as the Winkler model, the three-beam model, etc. These methods are instant, but the simplified models may not be able to capture the complexity of the pipe-soil interaction. In addition, physical model test methods such as centrifuge tests, shaking table tests, and large-scale indoor tests have been proven to be able to effectively reduce the uncertainty in the process of pipeline state evaluation. Generally speaking, arranging strain or displacement monitoring sensors on the pipeline is the most direct evaluation method.

[0004] Existing pipeline monitoring sensors include: strain gauges, vibrating wire strain gauges, fiber Bragg gratings, distributed optical fibers, pipeline robots, coaxial cables, etc. However, pipelines are a typical linear project, and the specific position where an actual fault occurs has strong randomness. The first three monitoring technologies are all point-type monitoring methods and cannot accurately monitor the most dangerous cross-section; in engineering, the diameter of buried natural gas pipelines can reach more than 1 m, and the strain on the pipeline surface is proportional to the diameter. In the case of dynamic testing, the monitoring range of the strain sensor should be at least greater than 3%. The monitoring range of the emerging distributed optical fiber is limited, generally within 1%, and it is limited in large-strain monitoring. Existing research shows that the monitoring range of coaxial cables is greater than 3%, but coaxial cables are still in the laboratory research stage and the actual application is still immature; pipeline robots cannot monitor during the operation period. To sum up, there is little research on the method for monitoring large deformations of pipelines in fault zones at present, and there is an urgent need for a method that can realize large deformation monitoring of pipelines. Summary of the Invention

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

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

[0007] The present invention also provides a sensing method for a buried pipeline monitoring sensor, including 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, and introduce the curvature boundary condition to 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, and the node coordinate vector can be simplified to Obtain the relationship between u and ;

[0011] Step 4: Use the Newton method to iteratively solve the relationship between u and . After obtaining the corresponding for each unit through iterative solution, assemble them to obtain the overall displacement of the structure;

[0012] Step 5: Calculate the exact displacement of any point inside each unit through the shape function 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:

[0014]

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

[0016] Preferably, the node coordinates of the inverse beam element are expressed as:

[0017]

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

[0019]

[0020] In the formula, r 1 and r 2 respectively 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 the undeformed state, l is the original length of the inverse beam element, is a known quantity and can be obtained from the boundary conditions, is the quantity to be solved;

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

[0022]

[0023] where S i represents the shape function, i = 1, 2, 3, 4, and S i is expressed as:

[0024] S 1 = 1 - 3ξ 2 + 2ξ 3 ;

[0025] S 2 = ξ - 2ξ 2 + ξ 3 ;

[0026] S 3 = 3ξ 2 - 2ξ 3 ;

[0027] S 4 = -ξ 2 + ξ 3 ;

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

[0029] The theoretical values of strain and curvature are obtained by the method of continuous medium mechanics:

[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 second derivative of matrix r with respect to x, and S'1 The first row of matrix S differentiated once with respect to x, S″ 2 The second row of matrix S differentiated twice with respect to x, S′ 2 The second row of matrix S differentiated once with respect to x, S″ 1 The first row of matrix S differentiated twice with respect to x, f represents the longitudinal deformation gradient, which is simplified to the constant 1.

[0032] Preferably, the least - squares function is expressed as:

[0033]

[0034] where ε and k represent the measured axial strain and the measured bending curvature respectively, w t and w b represent the weight coefficients of the axial strain term and the bending curvature term respectively, both taken as 1, l e represents the length of the inverse beam element;

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

[0036]

[0037] In the formula, S t and S b are abbreviations for convenience in subsequent calculations.

[0038] Preferably, introducing the curvature boundary condition, we get:

[0039] w k ∥k 0 (u)-k 0 ∥ 2 =w k (u T S k u - k 0 ) 2 ;

[0040]

[0041] In the formula, w k represents the weight coefficient of the boundary condition term, taken as 1, S k is an abbreviation for convenience in subsequent calculations, k 0 represents the bending curvature at the end - node position of the previous inverse beam element, k 0 can be obtained from the previous known element;

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

[0043]

[0044] Among them, R t , R b , R k are all simplifications for facilitating subsequent calculations.

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

[0046]

[0047] Among them, θ represents the rotation angle of the node position;

[0048] Simplified is expressed as:

[0049]

[0050] Among them, and respectively represent the coordinate vectors of the two node positions of the inverse beam element after simplification;

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

[0052]

[0053] Preferably, the Newton method is used to iteratively solve the relationship between u and and the gradient vector and Hessian matrix are required;

[0054] The gradient vector is expressed as:

[0055]

[0056] Among them, and are abbreviations for facilitating subsequent calculations;

[0057] The Hessian matrix is expressed as:

[0058]

[0059]

[0060] Among them, H t , H b , H k are all abbreviations for facilitating subsequent calculations;

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

[0062]

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

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

[0065]

[0066] Among them, and respectively represent the iterative results of the coordinate vectors of the second node of the inverse beam element at the k-th and k+1-th times, represents the node coordinate vector of the inverse beam element obtained by iterative calculation.

[0067] Preferably, the assembly formula is:

[0068]

[0069] In the formula, r i g represents the coordinate vector of the i-th node in the global coordinate system, and the superscript g represents in the global coordinate system, represents the rotation angle of the i-th node in the global coordinate system.

[0070] The present invention also provides an inversion evaluation method, including the following steps:

[0071] S1. Fix the monitoring sensor on the surface of the pipeline to ensure that the monitoring sensor and the pipeline can deform synergistically. Based on the strain on the surface of the sensor matrix obtained by the monitoring sensor, use the sensing method to reconstruct the deformation trend of the sensor in real time;

[0072] S2. Perform fitting processing on the displacement monitoring results of the monitoring sensor, and the fitting method is a seventh-degree polynomial;

[0073] S3. Adopt the finite element analysis method to construct an equi-proportion pipeline model, adopt the three-segment line model of the pipeline material, load the deformation result obtained by fitting in step S2 into the pipeline model as the displacement load, calculate the state of the pipeline after deformation, and extract the strain on the surface of the pipeline as the inversion strain result;

[0074] S4. Select the area where the inversion strain result exceeds the elastic strain of the pipeline material as the pipeline hidden danger area;

[0075] S5. Adopt the same fitting method and finite element analysis method as in steps S2 and S3 to analyze the pipeline hidden danger area, and obtain the extreme values of the pipeline strain at each loading stage;

[0076] S6. Accurately locate the pipeline deformation through the displacement data of the monitoring sensor, and at the same time assist in judging the pipeline strain state through the extreme values of the pipeline inversion strain, so as to determine the state of the pipeline.

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

[0078] (1) The sensor has the advantages of distributed monitoring, small size, high precision, etc., 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 applicable to large deformation scenarios such as fracture zones;

[0080] (3) The improved iANCFs method and the unique sensor structure design make the sensor not affected by temperature;

[0081] (4) The proposed strain inversion evaluation method can realize large strain monitoring of the pipeline and accurately evaluate the state of the pipeline.

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

[0083] Figure 1 Schematic diagram of the buried pipeline monitoring sensor and its application method according to the embodiment of the present invention;

[0084] Figure 2 Schematic diagram of the structure of the buried pipeline monitoring sensor according to the embodiment of the present invention;

[0085] Figure 3 Verification experiment diagram of the buried pipeline monitoring sensor according to the embodiment of the present invention;

[0086] Figure 4 Comparison diagram of the calculated displacement of the buried pipeline monitoring sensor and the actually measured displacement by the rangefinder according to the embodiment of the present invention;

[0087] Figure 5 Schematic diagram of the structure of the beam structure according to the embodiment of the present invention;

[0088] Figure 6 Calculation flow chart of the sensing method of the buried pipeline monitoring sensor according to the embodiment of the present invention;

[0089] Figure 7 Comparison diagram of the reconstructed displacements of the iANCFs, iFEM and CB methods according to the embodiment of the present invention;

[0090] Figure 8 Variation diagram of the relative error of the iANCFs method with the number of elements according to the embodiment of the present invention;

[0091] Figure 9 Schematic diagram of the installation position of the monitoring sensor according to the embodiment of the present invention;

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

[0093] Figure 11 It is a strain distribution diagram of the upper surface of the monitoring sensor at different moments during the test for the embodiment of the present invention;

[0094] Figure 12 It is a measured strain distribution diagram of the pipeline surface at different moments during the test for the embodiment of the present invention;

[0095] Figure 13 It is a comparison diagram of the measured pipeline deformation by the displacement sensor and the measured pipeline deformation by the lead screw at different moments during the test for the embodiment of the present invention;

[0096] Figure 14 It is a flow chart of the inversion evaluation method for the embodiment of the present invention;

[0097] Figure 15 It is a comparison diagram of the inversion strain and the measured pipeline strain by the distributed optical fiber for the inversion evaluation method for the embodiment of the present invention;

[0098] Figure 16 It is an effect diagram of the seventh-degree polynomial fitting of the pipeline deformation at typical moments for the inversion evaluation method for the embodiment of the present invention;

[0099] Figure 17 It is a development trend diagram of the extreme values of the pipeline strain at different loading stages for the inversion evaluation method for the embodiment of the present invention;

[0100] Figure 18 It is a pipeline state diagram at different loading stages obtained by inversion for the inversion evaluation method for the embodiment of the present invention;

[0101] Reference numerals

[0102] 1. Sensor base; 2. Special fixture; 3. Bellows; 4. Optical fiber; 5. Arc-shaped groove. Detailed implementation manners

[0103] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. The components of the embodiments of the present invention described and illustrated herein generally can be arranged and designed in a variety of different configurations. Therefore, the detailed description of the embodiments of the present invention provided herein is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0104] It should be noted that like reference numerals and letters refer to like items in the following figures. 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] As Figure 1-2 shown, the present invention provides a buried pipeline monitoring sensor, including a sensor base body 1, a special fixture 2, a corrugated pipe 3 and an optical fiber 4. The special fixture 2 is arranged at one end of the sensor base body. An arc-shaped groove 5 is arranged below the special fixture 2 so as to fit onto the pipeline surface. The corrugated pipe 3 is sleeved outside the sensor base body 1. The optical fiber 4 is pasted on the upper and lower surfaces of the sensor base body 1. The optical fiber 4 is connected to an OFDR demodulator device. The monitoring sensor is fixed on the outer surface of the buried pipeline. The sensor base body 1 is a spring steel sheet with a thickness of 2 mm and a width of 10 mm. The corrugated pipe 3 is a stainless steel corrugated pipe with an inner diameter of 10 mm.

[0107] Installation process: Paste the optical fiber on the upper and lower surfaces of the sensor base body 1, then pass the sensor base body 1 through the corrugated pipe 3, and finally connect the optical fiber 4 to the OFDR demodulator to monitor the strain on the surface of the base body in real time. Taking the obtained strain as the input quantity of the algorithm, the displacement of the base body can be obtained.

[0108] It should be noted that the optical fiber will be affected by temperature when measuring strain, but this device can offset the temperature influence during the process of the optical fiber measuring strain, which is achieved jointly based on the structural characteristics of the sensor and the iANCFs method. First, in terms of the structural design of the monitoring sensor, the corrugated pipe is used as the outer shell to protect the sensor. While ensuring the coordinated deformation of the monitoring sensor and the pipeline, the sensor base body can axially deform, ensuring that the sensor base body is not affected by longitudinal forces. Secondly, the iANCFs method simplifies the measured axial strain to a constant, which is not affected by temperature. When calculating the bending strain, the strains on the upper and lower surfaces are subtracted, offsetting the temperature influence.

[0109] Combining the optical fiber sensing technology with the shape sensing method to make a monitoring sensor can monitor the displacement of the structure in real time. The distance between the optical fiber and the centroid of the sensor base body is much smaller than the distance between the optical fiber directly pasted on the pipeline surface and the centroid of the pipeline. Therefore, there is no need to worry about exceeding the optical fiber range during the monitoring process.

[0110] Next, analyze the accuracy and temperature sensitivity of the monitoring sensor:

[0111] 1. In this embodiment, it is designed as Figure 3In the verification test shown, the length of the sensor is 5.8 m. One end of the fabricated sensor is fixed using a special fixture 2. Seven marking points are set on the surface of the corrugated pipe 3 at intervals of 0.8 m. The positions of the seven marking points under different deformations at five stages are recorded using a tape measure and a laser rangefinder respectively. At the same time, the strain of the sensor is collected using an optical fiber demodulator, and the displacement of the sensor is calculated through the sensing method. Refer to Figure 4 , the two displacement trends are highly consistent, and the error percentage of the maximum displacement is less than 1%. During the test, the maximum strain of the optical fiber 4 is approximately 350 με, which is much smaller than the strain monitoring range of the optical fiber. This sensor can also achieve deformation monitoring with a larger range.

[0112] 2. Since the temperature difference in the indoor test is small, on the basis of the measured strain of the sensor, the influence of temperature on the strain measurement result is increased, and the temperature sensitivity of the sensor is analyzed. The calibration coefficients of the spectral drift of the optical fiber 4 with respect to strain and temperature are -6.67 and -0.638 respectively, that is, when the temperature increases by 1 °C, the strain measurement value will increase by 10.45. Analyze the influence of temperature increments of 5 °C, 10 °C, and 15 °C on the displacement measurement result of the sensor, and define the temperature error δ T as follows:

[0113]

[0114] where r i 0 represents the coordinate of the i-th node when the temperature increment is 0 °C, and r i represents the coordinate of the i-th node when the temperature increment is not 0 °C, and n is the total number of nodes.

[0115] Calculate the temperature errors of the iANCFs considering the correction effect and the iANCF without considering the temperature effect under different temperature increments respectively. Since the iANCFs consider the influence of temperature, the temperature will not affect the displacement calculation result. The temperature error corresponding to the iANCF increases significantly with the increase of the temperature increment.

[0116] Example 2

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

[0118] This 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 inverse beam element). First, divide the beam structure into several inverse beam elements, and measure the strain components on the surface of each inverse beam element.

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

[0121]

[0122] where h is the thickness of the inverse beam element, is the measured strain on the upper surface of the i-th inverse beam element, is 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, r 1 and r 2 respectively represent the two coordinate axes of the local coordinate system within the inverse beam element, x is the length coordinate of the upper node of the inverse beam element in the undeformed state, l is the original length of the inverse beam element, is a known quantity and can be obtained from the boundary conditions, is the quantity to be solved.

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

[0129]

[0130] where S i represents the shape function, and i = 1, 2, 3, 4.

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

[0132] S 1 = 1 - 3ξ 2 + 2ξ 3 ;

[0133] S 2 = ξ - 2ξ 2 + ξ 3 ;

[0134] S 3 = 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 by the method of continuum mechanics:

[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 second 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 first derivative of the second row of matrix S with respect to x, 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 affected by longitudinal forces, for the convenience of solving the calculation, it can be simplified to a constant, that is, f ≈ 1.

[0140] Construct the least - square function for each inverse beam element, as shown in the following formula:

[0141]

[0142] where ε and k represent the measured axial strain and the measured bending curvature respectively, w t and w b represent the weight coefficients of the axial strain term and the bending curvature term respectively, both taking 1, and l e represents the length of the inverse beam element.

[0143] Substituting the theoretical values of strain and curvature into the above formula, we can get:

[0144]

[0145] In the formula, S t and S b are abbreviations for the convenience of subsequent calculations.

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

[0147] w k ∥k 0 (u)-k 0 ∥ 2 = w k (u T S ku - k 0 ) 2 ;

[0148]

[0149] In the formula, w k represents the weight coefficient of the boundary condition term, taken as 1, and S k is a shorthand notation for facilitating subsequent calculations. k 0 represents the bending curvature at the end node position of the previous inverse beam element, and k 0 can be obtained from the previous known element. Therefore, the final weighted least - squares formula is:

[0150]

[0151] where R t , R b , R k are all simplifications for facilitating subsequent calculations.

[0152] The above formula 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 will greatly increase the complexity of the solution. Therefore, in this paper, the method of solving element by element is adopted.

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

[0154]

[0155] where θ represents the rotation angle at the node position.

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

[0157]

[0158] where and respectively represent the simplified coordinate vectors at the two node positions of the inverse beam element.

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

[0160]

[0161] To solve the above - mentioned non - linear equations using Newton's method, the gradient vector and the Hessian matrix are required.

[0162] The gradient vector can be written as:

[0163]

[0164]

[0165] Among them, and are abbreviations for facilitating subsequent calculations.

[0166] The Hessian matrix can be written as:

[0167]

[0168] Among them, H t 、H b 、H k are all abbreviations for facilitating subsequent calculations.

[0169] The Boolean matrix B i has the following expression:

[0170]

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

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

[0173]

[0174] Among them, and respectively represent the iterative results of the coordinate vectors of the second node of the inverse beam element at the k-th and k + 1-th times, represents the node coordinate vector of the inverse beam element obtained by iterative calculation.

[0175] After obtaining the corresponding to each element through iterative solution, it is necessary to assemble them to obtain the overall displacement of the structure.

[0176]

[0177] In the formula, r i g represents the coordinate vector of the i-th node in the global coordinate system, and the superscript g represents in the global coordinate system, represents the rotation angle of the i-th node in the global coordinate system.

[0178] Through the above calculations, the displacements of each node on the structure can be obtained. Referring to Figure 6 , in addition, the exact displacements of any point inside each element can also be calculated through the shape function, that is, the continuous deformation result of the structure can be obtained.

[0179] Compared with the iANCF method, this sensing method has been adaptively simplified from three aspects based on the sensor structure. First, the measured axial strain of each unit is simplified to a constant 0, offsetting the part affected by temperature when the fiber optic sensor measures strain; second, the longitudinal deformation gradient is simplified to a constant 1; third, the Newton iteration solution process is optimized, omitting the complex terms with little impact on the calculation results, and improving the calculation efficiency on the premise of ensuring the accuracy requirements. The prerequisite for the first and second simplifications is that the sensor structure designed in this study is not affected by longitudinal forces. Through finite element analysis, it is verified that the accuracy of the improved iANCFs method is equivalent to that of iANCF, and real-time displacement reconstruction can be achieved.

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

[0181] 1. Accuracy comparison of different methods

[0182] A cantilever beam model is established in the finite element simulation software, and a displacement load is applied at the end of the beam to simulate the deformation of the beam. The length of the beam is 1000 mm and the thickness is 2 mm. A fixed constraint is applied at one end of the beam, and a displacement load is applied at the other end, with the load magnitudes being 50 mm, 100 mm, 200 mm, 400 mm, and 600 mm respectively.

[0183] To compare the accuracies of the improved iANCFs, iFEM, and CB methods, V iANCFs 、V iFEM 、V CB are calculated respectively using the three methods. Among them, V iANCFs 、V iFEM 、V CB are the displacements obtained by strain reconstruction using the inverse absolute nodal coordinate method, inverse finite element method, and conjugate beam method respectively. All three methods divide the entire beam into 5 inverse beam elements, and the displacement values of the nodes of each element are calculated.

[0184] Referring to Figure 7 , since the inverse finite element method and the conjugate beam method do not consider the axial displacement change of the beam, the x-direction nodal coordinates of each element will not change. When the displacement at the end of the cantilever beam reaches 100 mm, although the vertical displacements calculated by the inverse finite element method and the conjugate beam method are not much different from the true values, their axial displacements have already had obvious errors. When the displacement at the end of the cantilever beam is greater than 100 mm, the error of the calculation results of the inverse finite element method and the conjugate beam method increases sharply.

[0185] 2. Element number division

[0186] The above cantilever beam with a length of 1 m is divided into different numbers of elements respectively, and the influence of different numbers of elements on the method accuracy is analyzed. The relative error is defined as follows:

[0187]

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

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

[0190] The monitoring sensor is applied to the large deformation model test of buried pipelines under the action of fracture zones to verify the reliability of the monitoring sensor. The pipeline material used in the test is X42, with an outer diameter of 89 mm and a wall thickness of 3.5 mm. The external part of the fabricated pipeline monitoring sensor is protected by a corrugated pipe and fixed on one side of the pipeline in the horizontal direction by wire binding. Both ends of the monitoring sensor are fixed by special fixtures to ensure that the normal direction of the monitoring sensor matrix always points to the centroid of the pipeline. Distributed optical fibers and 11 resistance strain gauges are pasted on 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 are connected in series and then connected to the optical fiber demodulation device, and the resistance strain gauges are respectively connected to the acquisition device, referring to Figure 9 .

[0191] The size of the test box is 6 m * 1.1 m * 1 m. The left side is the movable part, which can move horizontally along the slide rail at the bottom of the test box under the action of two jacks, and the right side is the fixed part. The 90° dip fault displacement is simulated by the dislocation of the test box. Before the test starts, a layer of geomembrane is laid on the inner wall of the test box to reduce the frictional force between the soil and the test box. Then, the clay is filled to a predetermined height inside the test box, and the soil is compacted layer by layer during the filling process; the pipeline equipped with the monitoring sensor is placed in the test box, and the end of the monitoring sensor is fixed on the side wall of the fixed part of the test box through a magnetic base. 1 m long lead screws are connected at 6 different positions of the pipeline by clamps, and the ends of the lead screws extend outside the test box through the openings on the side wall of the test box.

[0192] During the test, two jacks were used to control the displacement of the moving part of the test box. 25 mm was taken as one order, and the maximum moving distance was about 400 mm. The displacement of the pipeline was actually measured by measuring the length of the lead screw extending out of the test box. After the test started, the strains of the optical fiber and the strain gauge were continuously collected, and the optical fiber demodulation device was ODISI6000. Since the monitoring sensor itself can eliminate the influence of temperature, the influence of temperature change on the strain measured by the optical fiber was not considered. Refer to Figure 10 .

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

[0194] Refer to Figure 12 . The measured results of the strain gauge and the distributed optical fiber were in good agreement, proving the accuracy of the strain monitoring.

[0195] Starting from the 8th loading stage, abnormal peaks occurred in the monitoring data of the distributed optical fiber and the strain gauge at the position of the extreme value of the pipeline strain. The two monitoring methods had good effects and were easy to install within the range of small elastic deformation of the pipeline. When the strain was large under bending action, the pipeline strain could not be effectively monitored.

[0196] Extract the surface strain data of the monitoring sensor and reconstruct the pipeline displacement by using the iANCFs method. The results are as Figure 13 shown. The distributed displacement of the pipeline measured by the monitoring sensor was in good agreement with the trend of the actual displacement of the lead screw at 6 positions, proving that the iANCFs method could still accurately reconstruct the displacement trend under such complex strain trends as Figure 11 .

[0197] Example 3

[0198] Refer to Figure 14 . The present invention also provides an inversion evaluation method, including the following steps:

[0199] S1. Monitor the pipeline deformation: Fix the monitoring sensor on the surface of the pipeline to ensure that the monitoring sensor and the pipeline can deform synergistically. Based on the surface strain of the sensor matrix obtained by the monitoring sensor, use the sensing method to reconstruct the deformation trend of the sensor in real time, as Figure 13 shown.

[0200] S2. Fit the pipeline deformation trend: 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 the bellows, there are inevitably gaps. Therefore, the pipeline deformation trend is not smooth at local positions, and it is necessary to first perform fitting processing on the displacement monitoring results of the monitoring sensor. The fitting method is a seventh-degree polynomial.

[0201] S3. Inverting pipeline strain: Using the finite element analysis method, an equi-proportion pipeline model is constructed. The three-segment line model of the pipeline material is adopted. The deformation results obtained by fitting in step 2 are used as displacement loads and applied to the pipeline model. The state of the deformed pipeline is calculated, and the strain on the pipeline surface is extracted as the inversion strain result. The inversion strain is compared with the strain of the buried pipeline measured by the distributed optical fiber. As Figure 15 shown. In the first 8 loading stages, the inversion results have quite high accuracy. The prerequisite for accurately inverting the pipeline strain is that the monitoring sensor can accurately monitor the horizontal displacement and axial distance change of the pipeline, which cannot be achieved by traditional sensors.

[0202] S4. Determining the pipeline hidden danger area: Select the area where the inversion strain result exceeds the elastic strain of the pipeline material as the pipeline hidden danger area. From Figure 15 it can be determined the area where the pipeline enters the elastoplastic deformation. Select the position of 1.5m - 2.5m at the fixed end part in the fracture zone model test as the pipeline hidden danger area.

[0203] S5. Separately fitting and inverting the pipeline hidden danger area: Referring to Figure 16 , as the fault displacement increases, at the position where the pipeline displacement gradient changes greatly, the fitting result cannot fully reflect the true deformation of the pipeline. Therefore, the finite element analysis method same as that in step S3 is used to analyze the pipeline hidden danger area, and the extreme values of the pipeline strain at each loading stage can be obtained. As Figure 17 shown. In the first 10 loading stages, the extreme values of the pipeline strain obtained by inversion are in agreement with the measured extreme values of the pipeline strain. After the 10th loading stage, no effective measured extreme value data of the strain are obtained by the distributed optical fiber and the strain gauge.

[0204] S6. Pipeline state assessment: The displacement data of the monitoring sensor can achieve accurate positioning of the pipeline deformation, and the extreme values of the pipeline inversion strain can assist in judging the pipeline strain state. Figure 18 The specific positions of the pipeline states at different loading stages in the stress-strain diagram can clearly determine the pipeline state. The conclusion of the pipeline state assessment is that the pipeline enters the elastoplastic stage starting from the 8th loading stage, but the pipeline does not fail until the end of the test.

[0205] Therefore, the present invention adopts the above-mentioned buried pipeline monitoring sensor, its sensing method and inversion and assessment method, which can achieve distributed deformation monitoring, and the monitoring sensor is not affected by temperature, can be used for large deformation monitoring, and the proposed strain inversion and assessment method can accurately assess the pipeline state.

[0206] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions of the present invention or make equivalent replacements, and these modifications or equivalent replacements cannot make the modified technical solutions 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: The following steps are involved: Step 1, dividing the beam structure into a number of inverse beam units, and measuring the surface strain component of each inverse beam unit, where the surface strain component of the inverse beam unit includes the measured axial strain and bending curvature of the inverse beam unit; Step 2: construct a least square function for each inverse beam element, bring the theoretical values ​​of strain and curvature into the least square function, introduce the curvature boundary condition, and obtain the final least square function; Step 3: Simplify the longitudinal deformation gradient to a constant and obtain a new node coordinate vector u. The node coordinate vector can be simplified to Get u with the relationship between; Step 4: Use Newton's method to iteratively solve u and The relationship between the two is solved iteratively to obtain the corresponding After that, assemble them to get the overall displacement of the structure; Step 5: Calculate the precise displacement of any point inside each unit through shape function 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 inverted beam element are expressed as: Where h is the thickness of the inverse beam element, is the measured strain on the upper surface of the i-th inverse beam element, is the measured strain on the lower surface of the i-th inverse beam element.

3. The sensing method of a buried pipeline monitoring sensor according to claim 2, characterized in that: The node coordinates of the inverse beam element are expressed as: Among them, r represents the node coordinates of the inverse beam element, S represents the shape function matrix, and u is the node coordinate vector of the inverse beam element, which is expressed as: Where r1 and r2 represent the two coordinate axes of the local coordinate system in the inverse beam element, x is the length coordinate of the node on the inverse beam element in the undeformed state, l is the original length of the inverse beam element, is a known quantity, which can be obtained from the boundary conditions. is the quantity to be solved; The shape function matrix S is expressed as: Among them, S i represents shape function, i=1,2,3,4, S i It is expressed as: S1=1-3ξ 2 +2x 3 ; S2=ξ-2ξ 2 +ξ 3 ; S3=3ξ 2 -2ξ 3 ; S4=-ξ 2 +ξ 3 ; Where ξ=x / l, the range of ξ is 0-1, representing the position of the interpolation point in the inverse beam element; The theoretical values ​​of strain and curvature are obtained by the continuum mechanics method: Among them, 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 row of matrix S with respect to x, S″2 represents the second row of matrix S with respect to x twice, S′2 represents the second row of matrix S with respect to x once, S″1 represents the first row of matrix S with respect to x twice, f represents the longitudinal deformation gradient, which is simplified to a constant of 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: Where ε and k represent the measured axial strain and the measured bending curvature, respectively, and w t and w b They represent the weight coefficients of the axial strain term and the bending curvature term, both of which are 1 and l e represents the length of the inverse beam element; Substituting the theoretical values ​​of strain and curvature into the least squares function, we obtain: In the formula, S t and S b This is an abbreviation for the convenience of subsequent calculations.

5. The sensing method of a buried pipeline monitoring sensor according to claim 4, characterized in that: Introducing the curvature boundary condition, we get: w k ||k0(u)-k0|| 2 =w k (u T S k u-k0) 2 ; In the formula, w k Represents the weight coefficient of the boundary condition term, which is taken as 1, S k For the convenience of subsequent calculations, k0 represents the bending curvature of the end node of the previous inverse beam element. k0 can be obtained from the previous known element. The formula for the final weighted least squares function is: Among them, R t , R b , R k 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 new node coordinate vector is: Among them, θ represents the rotation angle of the node position; Simplified It is expressed as: in, and Respectively represent the simplified coordinate vectors of the two node positions of the inverse beam element; u and The relationship between can be written as:

7. The sensing method of a buried pipeline monitoring sensor according to claim 6, characterized in that: Newton's method is used to iteratively solve u and The relationship between requires the use of gradient vectors and Hessian matrices; The gradient vector is expressed as: in, and This is an abbreviation for the convenience of subsequent calculations; The Hessian matrix is ​​expressed as: Among them, H t , H b , H k These are all abbreviations for the convenience of subsequent calculations; Boolean Matrix B i The expression is as follows: B i =[b j ] 1×6 , Among them, b j Represents a Boolean variable; The iterative formula based on Newton's method is: in, and They represent the coordinate vector iteration results of the second node of the inverse beam element for the kth and k+1th times respectively. Represents the node coordinate vector of the inverse beam element obtained by iterative calculation.

8. The sensing method of a buried pipeline monitoring sensor according to claim 7, characterized in that: The assembly formula is: In the formula, Represents the coordinate vector of the ith node in the global coordinate system. The superscript g indicates that it is in the global coordinate system. Represents the rotation angle of the i-th node in the global coordinate system.

9. A buried pipeline monitoring sensor, applied to the sensing method of a buried pipeline monitoring sensor as claimed in any one of claims 1 to 8, characterized in that: It includes a sensor base, a special fixture, a bellows and an optical fiber. The special fixture is arranged at one end of the sensor base, an arc-shaped groove is arranged below the special fixture, the bellows is sleeved on the outside of the sensor base, the optical fiber is pasted on the upper and lower surfaces of the sensor base, and the optical fiber is connected to an OFDR demodulator device. The sensor base is a spring steel sheet with a thickness of 2 mm and a width of 10 mm.

10. An inversion evaluation method, applied to a sensing method of a buried pipeline monitoring sensor as claimed in any one of claims 1 to 8, characterized in that: The following steps are involved: S1. Fix the monitoring sensor on the pipeline surface to ensure that the monitoring sensor and the pipeline can deform in coordination. Based on the surface strain of the sensor substrate obtained by the monitoring sensor, a sensing method is used to reconstruct the deformation trend of the sensor in real time. S2. Fitting the displacement monitoring result of the monitoring sensor, using a seventh-order polynomial as the fitting method; S3, using the finite element analysis method to construct a proportional pipeline model, using the three-fold line model of the pipeline material, loading the deformation result obtained by fitting in step S2 as a displacement load into the pipeline model, calculating the state of the pipeline after deformation, and extracting the strain on the pipeline surface as the inversion strain result; S4, selecting the area exceeding the elastic strain of the pipeline material in the inversion strain result as the pipeline hidden danger area; S5, using the same fitting method and finite element analysis method as steps S2 and S3 to analyze the potential danger area of ​​the pipeline, and obtain the extreme value of pipeline strain at each loading stage; S6. Accurately locate pipeline deformation by monitoring sensor displacement data, and assist in judging pipeline strain state through pipeline inversion strain extreme value, so as to determine pipeline state.

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