A temperature-compensated strain monitoring system for oil and gas pipelines

Abstract

This invention discloses a temperature-compensated strain monitoring system for oil and gas pipelines, belonging to the field of structural health monitoring. The system includes a BOTDA host, a MEMS optical switch, tight-buffered optical fiber, loose-buffered optical fiber, and a computer. The MEMS optical switch switches the optical path in stages, measuring the spurious strain of the tight-buffered fiber and the temperature data of the loose-buffered fiber separately during the monitoring cycle. The computer then performs interpolation, alignment, and temperature compensation processing on the data, effectively eliminating the influence of temperature on strain measurement. This invention innovatively employs an optical path switching method, solving the problem in traditional solutions where signal attenuation due to excessively long temperature compensation fibers prevents long-distance monitoring. This allows the system to extend the effective monitoring distance to 20-40 km while maintaining temperature compensation accuracy. This system is particularly suitable for long-distance oil and gas pipeline strain monitoring projects using BOTDA technology, significantly improving the accuracy of strain measurement and its engineering applicability.

Description

Technical Field

[0001] This invention proposes a temperature compensation method for projects using BOTDA technology to monitor strain in long-distance oil and gas pipelines. This method is used to more accurately obtain the strain after temperature compensation and belongs to the field of structural health monitoring. Background Technology

[0002] In recent years, distributed fiber optic sensing technology has been widely used in the field of structural health monitoring due to its advantages such as high sensitivity, strong resistance to electromagnetic interference, and wide coverage. Among them, BOTDA technology based on the Brillouin scattering principle can monitor the strain distribution of long-distance pipelines by measuring the Brillouin frequency shift, and has become one of the important means of strain monitoring for oil and gas pipelines.

[0003] Because BOTDA technology is inherently sensitive to changes in ambient temperature, the monitored frequency shift contains both strain and temperature information, making it difficult to accurately distinguish the effects of both in practical applications. Without effective temperature compensation, significant errors in strain monitoring data can easily occur, affecting the accuracy of judgments. Therefore, introducing an efficient temperature compensation mechanism into BOTDA strain monitoring is one of the key challenges in its current engineering applications.

[0004] In most existing projects using BOTDA technology for strain monitoring of oil and gas pipelines, a tight-buffered strain gauge fiber is laid alongside the pipeline, running parallel to its length. The strain measured by the strain gauge fiber is approximated as the strain borne by the pipeline. An alarm is triggered when the strain exceeds a threshold. Since BOTDA technology requires the optical fibers to form a loop, the tight-buffered fiber is used to measure strain, while one core (loose-buffered fiber) of the communication optical cable laid alongside the pipeline forms the loop. The frequency shift in the tight-buffered fiber measured by BOTDA technology is a pseudo-strain, resulting from both strain and temperature changes. The loose-buffered fiber can only sense frequency shifts caused by temperature changes, so it can be used to compensate for the pseudo-strain measured by the tight-buffered fiber. This scheme, using one loose-buffered fiber core from the communication optical cable to form a loop while simultaneously performing temperature compensation, is practical and effective for short- to medium-distance monitoring. Considering the attenuation of the optical signal in the fiber, the effective measurement distance of common BOTDA technologies does not exceed 40 km. For long distances, such as monitoring distances exceeding 20 km, the above temperature compensation scheme is no longer applicable. Using the aforementioned temperature compensation scheme, a total of 40 km of tight-buffered optical fiber (20 km long) and loose-buffered optical fiber (20 km long for loop formation) can monitor oil and gas pipelines up to 20 km long. Without temperature compensation, a total of 80 km of tight-buffered and loose-buffered optical fiber (40 km long for loop formation) can monitor oil and gas pipelines up to 40 km long. However, the 40 km long loose-buffered optical fiber, due to signal attenuation, can only be used for loop formation and not for temperature compensation. How to monitor oil and gas pipelines within a length range of 20 km to 40 km while considering temperature compensation is an engineering problem that this invention aims to solve. Summary of the Invention

[0005] The technical problem to be solved by this invention is: in engineering projects that use BOTDA technology to monitor the strain of oil and gas pipelines, how to increase the monitoring distance of oil and gas pipelines as much as possible while taking temperature compensation into account.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: a strain monitoring system for oil and gas pipelines with temperature compensation, comprising a BOTDA host, a MEMS optical switch, a tight-buffered optical fiber, a loose-buffered optical fiber, and a computer.

[0007] The optical transmitter and receiver ports of the BOTDA host are connected to a MEMS optical switch, and then to tight-buffered and loose-buffered optical fibers.

[0008] The computer-controlled BOTDA host and MEMS optical switch cyclically monitor the oil and gas pipeline. Each monitoring cycle is divided into two steps: the first step monitors the frequency shift caused by pseudo-strain in the tight-buffered optical fiber, and the second step monitors the frequency shift caused by temperature in the loose-buffered optical fiber.

[0009] In the first monitoring step, the MEMS optical switch causes the laser to be emitted from the optical transmitting port of the BOTDA host, first through the tight-buffered fiber, then through the loose-buffered fiber, and back to the optical receiving port of the BOTDA host.

[0010] In the second monitoring step, the MEMS optical switch causes the laser to be emitted from the optical transmitting port of the BOTDA host, first through the loose-tube fiber, then through the tight-tube fiber, and back to the optical receiving port of the BOTDA host.

[0011] The computer performs interpolation processing on the data measured in the first and second steps, so that the interpolated data are aligned in the same position;

[0012] The computer uses the temperature data in the loosely fitted fiber obtained in the second step to perform temperature compensation on the pseudo strain data in the tightly fitted fiber obtained in the first step at the same location, eliminating the influence of temperature and obtaining the true strain data.

[0013] The loose-tube optical fiber is one core of the communication optical cable that is laid synchronously with the oil and gas pipeline, and is laid together with the tight-tube optical fiber next to the oil and gas pipeline.

[0014] Preferably, the sampling interval of the BOTDA host is 1 to 8 m.

[0015] The beneficial effects of this invention are: by using MEMS optical switches and BOTDA main units in combination, the optical path is switched in stages to achieve pseudo-strain and temperature separation measurement of the same section of oil and gas pipeline. This avoids the problem in the original solution where the signal attenuation caused by the excessive length of the temperature compensation optical fiber makes it unsuitable for long-distance monitoring. This allows the system to cover a longer monitoring range (e.g., 20 to 40 km) while ensuring temperature compensation, thus improving the system's application capabilities. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the system of the present invention;

[0017] Figure 2 This is a schematic diagram of interpolation between pseudo-strain data points and temperature data points.

[0018] The labels in the attached diagram have the following meanings: 1. Tight-buffered optical fiber, 2. Loose-buffered optical fiber, 3. Oil and gas pipeline, 4. Computer, 5. Network cable, 6. BOTDA host, 7. RS232 line, 8. MEMS optical switch, 9. Optical transmitting port, 10. Connecting optical fiber, 11. Port A, 12. Port B, 13. Optical receiving port, 14. Pseudo-strain data, 15. Temperature data, 16. Interpolated pseudo-strain data, 17. Interpolated temperature data. Detailed Implementation Plan

[0019] The present invention will now be described in further detail with reference to the accompanying drawings.

[0020] Figure 1 This is a schematic diagram of the monitoring system. Tight-buffered fiber 1 and one loose-buffered fiber 2 from the communication optical cable are laid beside the oil and gas pipeline 3. Computer 4 controls the BOTDA host 6 via network cable 5 and the MEMS optical switch 8 via RS232 cable 7. Computer 4 configures the BOTDA host 6 to perform cyclic monitoring, with each cycle consisting of two steps. During the first monitoring step, a laser beam is emitted from the optical transmitter port 9 of the BOTDA host 6, passes through the connecting fiber 10 into the MEMS optical switch 8, then through port A 11 of the MEMS optical switch 8, through the connecting fiber 10, into the tight-buffered fiber 1, through the connecting fiber 10, then back to the loose-buffered fiber 2, through the connecting fiber 10, through port B 12 of the MEMS optical switch 8, through the optical receiver port 13 of the MEMS optical switch 8, and finally to the BOTDA host 6. When the second step of monitoring is performed, the computer 4 controls the MEMS optical switch 8 to switch so that the laser emitted from the optical transmitting port 9 no longer enters the tight-buffered fiber through port A 11, but instead enters the loose-buffered fiber 2 through port B 12. Correspondingly, when the laser returns from the tight-buffered fiber 1 after passing through the loose-buffered fiber 2, it enters the MEMS optical switch 8 through port A 11, and then enters the BOTDA host 6 through the optical receiving port 13.

[0021] The purpose of switching the MEMS optical switch 8 is to ensure that when performing pseudo-strain monitoring, the laser first enters the tight-fitting fiber 1 and then the loose-fitting fiber 2, thereby ensuring the accuracy and effectiveness of pseudo-strain measurement; when performing temperature monitoring, the laser first enters the loose-fitting fiber 2 and then the tight-fitting fiber 1, thereby ensuring the accuracy and effectiveness of temperature measurement.

[0022] The data obtained by computer 4 during the first and second monitoring steps are generally not aligned along the length of the oil and gas pipeline 3. Therefore, interpolation processing is required for the pseudo-strain data 14 and temperature data 15, such as... Figure 2 As shown, four pseudo-strain data points 14 and four temperature data points 15 are used as examples. After interpolation, the interpolated pseudo-strain data points 16 and interpolated temperature data points 17 are aligned in pairs along the length of the oil and gas pipeline 3. A feasible interpolation scheme is linear interpolation.

[0023] When the laser wavelength is 1550nm and the optical fiber is SMF-28, a 1MHz frequency shift corresponds to a strain change of approximately 20με or a temperature change of approximately 1ºC. The specific coefficients should be determined experimentally. Assume that at a certain location measured by computer 4, the initial frequency of the interpolated pseudo-strain data 16 is 10.801GHz, and the measured frequency is 10.901GHz; at the same location, the initial frequency of the interpolated temperature data 17 is 10.823GHz, and the measured frequency is 10.843GHz. The pseudo-strain change is (10.901-10.801)×1000×20=2000με, and the temperature change is (10.843-10.823)×1000×1=20ºC. The true strain change, after removing the temperature effect, is [(10.901-10.801)-(10.843-10.823)]×1000×20=1600με. The true strain change is 20% lower than the pseudo-strain change.

[0024] Specific embodiments of the present invention have been described above, but the present invention is not limited to the specific implementations described above. Those skilled in the art can make various changes in form and detail within the scope of the claims, without affecting the essence of the present invention.

Claims

1. An oil and gas pipeline strain monitoring system with temperature compensation, characterized in that, The system comprises: a BOTDA host, a MEMS optical switch, a tight-fitting optical fiber, a loose-fitting optical fiber, and a computer; the light emitting port and the light receiving port of the BOTDA host are connected with the tight-fitting optical fiber and the loose-fitting optical fiber in sequence through the MEMS optical switch; the computer is used to control the BOTDA host and the MEMS optical switch to perform cyclic monitoring, each monitoring cycle comprising two steps: first, measuring the pseudo-strain frequency shift caused by strain and temperature in the tight-fitting optical fiber; second, measuring the frequency shift caused by temperature in the loose-fitting optical fiber; the MEMS optical switch switches the light path direction in the two monitoring steps respectively, so as to realize that the optical signal preferentially passes through the tight-fitting optical fiber or the loose-fitting optical fiber respectively; the computer performs interpolation processing on the data obtained in the two steps and aligns them at the same position, uses the temperature change frequency measured by the loose-fitting optical fiber to perform temperature compensation on the pseudo-strain frequency measured by the tight-fitting optical fiber, eliminates the temperature influence, and obtains the real strain data; the tight-fitting optical fiber and the loose-fitting optical fiber are laid along the oil and gas pipeline synchronously, and the loose-fitting optical fiber is a core optical fiber in a communication optical cable.

2. The monitoring system of claim 1, wherein, The MEMS optical switch has at least two switchable ports connected with the tight-fitting optical fiber and the loose-fitting optical fiber respectively, so as to realize switching of the laser path between the two optical fibers.

3. The monitoring system of claim 1, wherein, The computer processes the collected pseudo-strain data and temperature data by using an interpolation method, so that the interpolated data are aligned in the length direction of the oil and gas pipeline.

4. The monitoring system of claim 1, wherein, The sampling interval of the BOTDA host is 1 to 8 meters.

5. The monitoring system of claim 1, wherein, The calculation formula of the real strain value is: Real strain change = frequency coefficient × [(pseudo-strain frequency change) - (temperature frequency change)], wherein the frequency coefficient is obtained by experiment calibration.

6. The monitoring system of claim 1, wherein, The system is suitable for monitoring the oil and gas pipeline with a length of 20 km to 40 km, and can realize high-precision strain measurement after temperature compensation within the range.

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