Method for early warning of section stress of oil and gas pipeline by using coaxial cable technology

Abstract

The invention relates to an oil and gas pipeline section stress early warning method based on a coaxial cable technology. According to the quasi-distributed coaxial cable technology, a plurality of sensors are arranged on a coaxial cable at intervals, the position change data of the sensors are obtained by using a coaxial cable demodulator, and the original strain between the adjacent sensors is calculated. In order to solve the problem that the maximum stress of the cross section cannot be accurately predicted at limited measurement points, a Spline interpolation method is introduced, high-density interpolation strain data is generated in a sensor coverage interval, the scheme of arranging three coaxial cables in combination with an oil and gas pipeline is adopted, the maximum strain of the cross section is calculated according to the national standard, and then the maximum stress of the cross section is obtained through Young modulus conversion. And when the stress exceeds a threshold value, early warning is triggered. According to the method, the data fluctuation trend is comprehensively considered through Spline interpolation, the extreme stress value is effectively predicted, the method is more reliable compared with non-interpolation or other interpolation schemes, the method is suitable for pipeline monitoring of short-distance high-risk areas (such as earthquake fault zones), and the method has the advantages of cost effectiveness and worse condition early warning.

Description

Technical Field

[0001] The present invention aims at a solution for monitoring the cross-sectional stress of an oil and gas pipeline by means of coaxial cable technology, and provides a data processing method for obtaining the maximum cross-sectional stress from the position change, which is used for early warning of the maximum cross-sectional stress and belongs to the field of structural monitoring. Background Art

[0002] Due to the long-distance characteristics of oil and gas pipelines, distributed optical fiber technology has been widely used in stress and strain monitoring projects of oil and gas pipelines. When using optical fiber technology, there is a strain acquisition point every 0.25 m to 4 m on the optical fiber, which is used for stress and strain monitoring of oil and gas pipelines with a length of dozens of kilometers. Having such dense acquisition points within such a long range makes the optical fiber technology have the characteristics of distribution, that is, it can be approximately considered that the measurement points are continuous within a distance range of dozens of kilometers. However, the optical fiber demodulation host required by distributed optical fiber technology has a relatively high price, which is more economical for a monitoring range of dozens of kilometers, but too costly for engineering projects with a monitoring distance of dozens of meters. Some engineering projects only need to monitor a distance of dozens of meters, such as oil and gas pipelines crossing seismic fault zones and easily landslide areas. In these projects, it is more appropriate to use quasi-distributed coaxial cable technology because the price of the coaxial cable demodulation host is relatively low.

[0003] In coaxial cable technology, a coaxial cable sensor is obtained by creating an impedance discontinuity point on the coaxial cable, and a coaxial cable demodulation host is used to transmit and receive microwaves on the coaxial cable with the sensor fabricated, so as to obtain the information of the sensor position. The microwave frequency transmitted and received by a typical coaxial cable demodulation host is 300 MHz. When the coaxial cable deforms, the position of the sensor changes, and thus the strain data can be indirectly calculated. Common methods for creating impedance discontinuity points include the extrusion method and the cavity method. Considering the propagation attenuation of microwaves in the coaxial cable, the length L that can be effectively recognized by the coaxial cable demodulation host for 300 MHz microwaves is within 50 m. In common coaxial cable projects, the interval ΔL between coaxial cable sensors is between 2 m and 12 m. Taking L = 32 m and ΔL = 4 m as an example, the number n of coaxial cable sensors is 8. Compared with the distributed optical fiber system with nearly continuous measurement points, the coaxial cable technology is quasi-distributed, between point type and distributed type, that is, there are n measurement points. How to reasonably utilize the position information obtained from these n measurements to calculate the strain, and further obtain the maximum cross-sectional stress for cross-sectional stress early warning is a problem worthy of research.

[0004] According to the definition of strain, the strain between two adjacent coaxial cable sensors is the change in their distance divided by the initial distance. However, the strain at any position is generally unknown, although it can be obtained by linear interpolation between two strain points before and after this position. However, linear interpolation only uses the data of the two adjacent strain points and does not consider the influence of all measurement points on this position. Especially when there are drastic changes among all strain points, the strain at this position obtained by linear interpolation is always between the strains of the two adjacent strain points, which is too conservative for cross-section stress warning. Since cross-section stress warning needs to consider the maximum stress of the cross-section within the length range of the coaxial cable monitoring the oil and gas pipeline, this value may not be exactly reflected by those several measurement values. It is necessary to predict the possible maximum cross-section stress in a worse case using a suitable strain interpolation method. The present invention recommends using Spline interpolation. This interpolation method problem does not exist in the optical fiber monitoring scheme because the points measured by the optical fiber are dense enough and the data is nearly continuous, so no interpolation processing is required or simple linear interpolation can be carried out.

[0005] The method for calculating the maximum strain of the cross-section from the strains of three points on the cross-section is given in the national standard "GB / T 40702-2021 Technical Specification for Geological Disaster Protection of Oil and Gas Pipelines". These three points present a "pin shape", a "120-degree type" or a "45-degree type". The maximum stress of the cross-section is obtained by multiplying the absolute value of the maximum strain of the cross-section by the Young's modulus of the oil and gas pipeline. Summary of the Invention

[0006] The technical problem to be solved by the present invention is: in the scenario where coaxial cable technology is used for stress and strain monitoring of oil and gas pipelines, how to predict the maximum stress of the pipeline cross-section in a worse case based on a limited number of discrete measurement points through a suitable strain interpolation method to improve the reliability of cross-section maximum stress warning.

[0007] To achieve the above object, the present invention adopts the following technical solution: A coaxial cable sensor is nominally set at intervals of ΔL on a coaxial cable, and a total of n are provided. At the initial moment when the coaxial cable sensor is installed on the oil and gas pipeline, the initial coordinates of these n sensors measured by the coaxial cable demodulation host are x i (i = 1...n). At a certain measurement moment, due to the stress and strain of the oil and gas pipeline, the measured coordinates of these n sensors are y i (i = 1...n). It is assumed that the position of the coaxial cable demodulator is the coordinate 0 point and remains unchanged all the time. The first original strain data e1 at the position of x1 / 2 is:

[0008] e1 = (y1 - 0) - (x1 - 0) / (x1 - 0) = (y1 - x1) / x1 Equation (1)

[0009] Let j = 2…n, at the position of (xj +x j-1 ) / 2 of the j-th original strain data e j is as follows:

[0010] e j = [(y j - y j-1 ) - (x j - x j-1 )] / (x j - x j-1 ) Equation (2)

[0011] For the measured original strain data e i (i = 1...n), as shown in Equation (1) and Equation (2), Spline interpolation is performed in the interval where the position is [ΔL, (n - 1 / 2)ΔL], the interpolation interval is equal, and the total number of interpolations is m, where m >> n. The strain data after Spline interpolation is denoted as E k (k = 1...m). The strain data E k after the above Spline interpolation is data of a coaxial cable. Three coaxial cables are arranged on the oil and gas pipeline, presenting a "pin shape", a "120-degree type" or a "45-degree type", and the strain calculation and interpolation methods of the other two coaxial cables are the same as those of the above coaxial cable. The coaxial cable sensors on the three coaxial cables are fixed on the oil and gas pipeline. Based on the "Technical Specification for Geological Disaster Protection of Oil and Gas Pipelines" (GB / T 40702-2021), the strain of the three coaxial cables after Spline interpolation is used to calculate the maximum stress of the cross-section, and an alarm is given when the maximum stress of the cross-section exceeds the warning threshold.

[0012] The above coaxial cable sensors are connected in series on the coaxial cable.

[0013] The above three coaxial cables are connected to three ports of the coaxial cable demodulator.

[0014] The coordinates x i and y i measured by the above coaxial cable demodulator are transmitted to a local or cloud computer via wire or wireless for the calculation of the original strain, Spline interpolation, the calculation of the maximum stress of the cross-section, and alarm.

[0015] Preferably, ΔL is between 2m and 12m.

[0016] Preferably, n is between 4 and 20.

[0017] Preferably, m is between 5n and 1000n.

[0018] Preferably, the microwave frequency emitted and received by the coaxial cable demodulator is between 200 MHz and 500 MHz.

[0019] The beneficial effects of the present invention are as follows: Aiming at the application of quasi-distributed coaxial cable technology in the stress and strain monitoring of oil and gas pipelines, an improved strain processing scheme is proposed, that is, Spline interpolation is performed on the measured strain, and the maximum stress of the cross-section in a worse case is predicted by using finite measurement points (quasi-distributed characteristics). BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a schematic diagram of the original strain data points on a coaxial cable of the present invention, taking n = 8 as an example;

[0021] Figure 2 It is a schematic diagram of Spline interpolation on three coaxial cables of the present invention, taking n = 8 and m = 5n as an example;

[0022] Figure 3 It is a schematic diagram of the cross-section distribution of three coaxial cables on an oil and gas pipeline, taking the "triangle" as an example;

[0023] Figure 4 It is an example of the present invention.

[0024] The meanings of the marks in the drawings are as follows: 1, coaxial cable; 2, coaxial cable sensor; 3, original strain data point; 4, coaxial cable demodulator; 5, port; 6, oil and gas pipeline; 7, interpolated strain data point; 8, cross-section; 9, computer. SPECIFIC IMPLEMENTATION MODE

[0025] The present invention will be further described in detail below with reference to the drawings of the specification.

[0026] Figure 1 It illustrates the calculation method of the original strain on the coaxial cable of the present invention. Eight coaxial cable sensors 2 are connected in series on the coaxial cable 1. The displacement at the left end of the coaxial cable 1 is always 0. The initial coordinates of these eight coaxial cable sensors 2 are x i (i = 1...8), and the coordinates at a certain moment after being subjected to stress and strain become y i (i = 1...8). At this time, the original strain data point 3 has coordinates of x1 / 2 and (x j +x j-1 ) / 2 (j = 2…8), and the corresponding original strain data is calculated from Equations (1) and (2).

[0027] Figure 2 It illustrates the Spline interpolation data points and cross-sections of three coaxial cables of the present invention. Three ports 5 of the coaxial cable demodulator 4 are respectively connected to three coaxial cables 1. The distribution of these three coaxial cables 1 on the oil and gas pipeline 6 is as Figure 3 shown, taking the "triangle" as an example. As Figure 2As shown, the original strain data point 3 is interpolated by Spline to obtain the interpolated strain data point 7. The original strain data points 3 on the three coaxial cables 1 do not necessarily align on one cross-section, but the interpolated strain data points 7 are grouped in threes and aligned on one cross-section 8 for the calculation of the maximum stress on the cross-section. The coaxial cable demodulator 4 is connected to the computer 9 by wired or wireless means, and the calculation of strain, Spline interpolation, calculation of the maximum stress on the cross-section, and early warning are carried out on the computer 9.

[0028] The method for calculating the maximum strain E of the cross-section from the strains of three points on the cross-section max is given in the "Technical Specification for Geological Disaster Protection of Oil and Gas Pipelines" (GB / T 40702-2021). The formula for the three points on the cross-section in a "pin" shape is as follows:

[0029]

[0030] where

[0031]

[0032]

[0033] where E L , E U , E R are the strains in the 90°, 0°, and 270° directions on the cross-section after Spline interpolation, with the direction directly above the cross-section being 0°. The maximum stress on the cross-section is:

[0034] σ max = E Y |E max | Equation (7)

[0035] where E Y is the Young's modulus of the oil and gas pipeline, and |E max | is the absolute value of E max .

[0036] Table 1 and Table 2 are the initial coordinates of 8 coaxial cable sensors and the measured coordinates at a certain moment after being subjected to stress and strain, respectively. Among them, the L, U, and R sensors correspond to the data of the three coaxial cables in the 90°, 0°, and 270° directions on the pipeline cross-section. At the initial moment, the nominal interval ΔL of the sensors is 4m, and the actual interval is measured by the coaxial cable demodulator and fluctuates around 4m, as shown in Table 1.

[0037] Table 1. Initial Coordinates of Coaxial Cable Sensors

[0038] <![CDATA[x1(m)]]> <![CDATA[x2(m)]]> <![CDATA[x3(m)]]> <![CDATA[x4(m)]]> <![CDATA[x5(m)]]> <![CDATA[x6(m)]]> <![CDATA[x7(m)]]> <![CDATA[x8(m)]]> L sensor 4.0010 8.0020 12.0040 15.9960 20.0020 23.9940 27.9930 31.9930 U sensor 3.9980 7.9970 12.0030 16.0030 19.9960 24.0020 28.0040 31.9960 R sensor 3.9960 7.9930 11.9940 16.0020 19.9930 24.0070 28.0090 32.0030

[0039] Table 2. Measured Coordinates of Coaxial Cable Sensors at a Certain Moment after Being Subjected to Stress and Strain

[0040] <![CDATA[y1(m)]]> <![CDATA[y2(m)]]> <![CDATA[y3(m)]]> <![CDATA[y4(m)]]> <![CDATA[y5(m)]]> <![CDATA[y6(m)]]> <![CDATA[y7(m)]]> <![CDATA[y8(m)]]> L sensor 4.0016 8.0050 12.0052 16.0002 20.0032 23.9994 27.9942 31.9990 U sensor 3.9981 7.9974 12.0031 16.0035 19.9961 24.0026 28.0041 31.9967 R sensor 3.9954 7.9900 11.9928 15.9978 19.9918 24.0016 28.0078 31.9970

[0041] To demonstrate the superiority of Spline interpolation, the method of the present invention is used to process the data in Table 1 and Table 2 to obtain the maximum sectional stress, and compare it with the results of non-interpolation, linear interpolation, pchip interpolation, and cubic interpolation. The comparison results are shown in Figure 4 . In Figure 4 's calculation, the number of interpolation points m = 200, and the Young's modulus E of the oil and gas pipeline Y = 210 GPa. When performing cubic interpolation, since this algorithm requires the sampling interval to be consistent, the sampling coordinates do not use x1 / 2 and (x j + x j-1 ) / 2 (j = 2... 8), but approximately replace them with 2m, 6m, 10m... 30m. Assuming that the early warning value of the maximum sectional stress is 300 MPa, it can be seen from Figure 4 that the maximum sectional stress obtained by Spline interpolation exceeds 300 MPa. At this time, an early warning of the maximum sectional stress needs to be given, while the maximum sectional stresses obtained by non-interpolation, linear interpolation, pchip interpolation, and cubic interpolation are all less than 300 MPa. Therefore, Spline interpolation gives the maximum sectional stress under a worse possibility. The mechanism of this worse maximum sectional stress prediction by Spline interpolation lies in that it comprehensively considers the volatility of the original data points and the changing trend among them, and reasonably predicts the overshoot or undershoot generated at the missing positions of the original data points.

[0042] The specific embodiments of the present invention have been described above, but the present invention is not limited to the above-mentioned specific implementation schemes. Those skilled in the art can make various forms and details changes within the scope of the claims, and these do not affect the essence of the present invention.

Claims

1. An early warning system for the maximum stress of the cross-section of an oil and gas pipeline using coaxial cable technology, characterized in that, It includes a coaxial cable demodulator, a computer, and three coaxial cables arranged along the length direction of the oil and gas pipeline, where coaxial cable sensors are connected in series on the coaxial cables; The coaxial cable sensors are fixed on the oil and gas pipeline; The coaxial cable demodulator measures the information on the position change of the coaxial cable sensors; The computer calculates the original strain from the information on the position change, then performs Spline interpolation, and the interpolated strain data is used for the calculation of the maximum stress of the cross-section. When the threshold is exceeded, an early warning is given.

2. The warning system according to claim 1, wherein The three coaxial cables present a "pin shape", a "120-degree type", or a "45-degree type" on the oil and gas pipeline.

3. The warning system according to claim 1, wherein The coaxial cable demodulator has at least three channels, and each channel is connected to a coaxial cable respectively.

4. The warning system according to claim 1, characterized in that The spacing between the coaxial cable sensors connected in series on the coaxial cable is between 2m and 12m.

5. The warning system according to claim 1, wherein The number of coaxial cable sensors connected in series on the coaxial cable is 4 to 20.

6. The warning system according to claim 1, wherein The microwave frequency emitted and received by the coaxial cable demodulator is between 200MHz and 500MHz.

Images