Dynamic calibration method for aging strain measurement optical fiber

Abstract

The invention discloses a dynamic calibration method for an aged strain measurement optical fiber, and relates to an oil and gas pipeline structure health monitoring technology. The service cycle of an oil and gas pipeline is long, a BOTDA system can cause the strain-frequency shift coefficient drift of an optical fiber in long-term operation, and the measurement precision is reduced. According to the invention, a vibrating wire strain gauge is used as a reference, and the dynamic calibration of the optical fiber is realized on site. The system is composed of vibrating wire strain gauges, a data acquisition instrument, a tight tube optical fiber, a loose tube optical fiber, a BOTDA host and a computer, and three vibrating wire strain gauges and three sections of tight tube optical fibers are simultaneously arranged on the section of a pipeline to obtain two types of independent strain data. And the computer compares the Brillouin frequency shift measured by the BOTDA system by taking a strain value measured by the vibrating wire strain gauge as a reference, calculates a strain-frequency shift coefficient of the aged optical fiber and periodically updates the strain-frequency shift coefficient, so that a strain measurement result is dynamically corrected. The method has the advantages of being free of disassembly, low in cost and easy to implement, and the long-term monitoring precision of the BOTDA system can be remarkably improved.

Description

Technical Field

[0001] This invention relates to the field of structural health monitoring technology for oil and gas pipelines, specifically a method for dynamically calibrating an aging distributed fiber optic strain monitoring system using a vibrating wire strain gauge. Background Technology

[0002] Oil and gas pipelines are crucial infrastructure for energy transportation, characterized by their long distances, complex environmental conditions, and long service life. Establishing a reliable pipeline stress and strain monitoring system is of great significance for ensuring the safety of oil and gas transportation and extending the service life of pipelines.

[0003] Current pipeline stress-strain monitoring technologies are mainly divided into two categories: point-based and distributed. Point-based monitoring, represented by vibrating wire strain gauges, can achieve high-precision strain measurement at the installation location, exhibiting good long-term stability and low drift rate. Furthermore, most products include built-in temperature measurement capabilities, allowing for temperature compensation during strain acquisition. However, point sensors can only acquire strain information at limited locations, failing to achieve full-line coverage and thus limiting their application in overall safety assessments of long-distance pipelines.

[0004] Distributed fiber optic sensing technology relies on the transmission characteristics of optical fibers themselves. For example, BOTDA technology, based on the Brillouin scattering principle, obtains strain and temperature information by detecting the frequency shift of scattered light inside the fiber. BOTDA technology can utilize looped optical fibers to achieve continuous distributed strain monitoring of oil and gas pipelines, covering a range of tens of kilometers. Tight-buffered fibers based on BOTDA technology can sense both strain and temperature simultaneously, while loose-buffered fibers can only sense temperature. Temperature compensation is generally required when using BOTDA technology for strain monitoring. A maximum stress monitoring system for oil and gas pipelines using BOTDA technology can employ the following scheme: a tight-buffered fiber is applied back and forth three times on the oil and gas pipeline to measure strain at different locations on the cross-section for calculating the maximum stress; simultaneously, an idle loose-buffered fiber in the communication cable is used to form a loop, which not only completes the closed path of the BOTDA system but also eliminates the influence of temperature on strain measurement. BOTDA technology typically relies on the linear relationship between strain and frequency shift calibrated at the factory to calculate strain values.

[0005] However, during long-term service, optical fibers are affected by various factors such as environmental humidity and heat, ultraviolet radiation, chemical corrosion, and mechanical fatigue, which cause changes in the physical properties of the fiber and thus cause a drift in the strain-frequency shift coefficient. This drift directly affects the measurement accuracy of BOTDA technology.

[0006] The common solution is to remove the fiber optic cable from the field and send it back to the laboratory for recalibration of the strain-frequency shift coefficient. This invention takes a different approach, using vibrating wire strain gauges to dynamically calibrate the strain fiber. Vibrating wire strain gauges exhibit high accuracy and low drift characteristics during long-term use, and their built-in temperature sensors enable temperature compensation of the strain data. By simultaneously deploying three vibrating wire strain gauges and three sections of distributed tight-buffered strain fiber at a certain cross-section of the pipeline, two independent sets of strain data can be obtained simultaneously at the same cross-section. The data from the vibrating wire strain gauges serves as a precise reference, while the data from the distributed fiber provides a continuous strain distribution across the entire pipeline. By comparing the two types of data at the same cross-section, the strain-frequency shift coefficient of the BOTDA system under field service conditions can be deduced, thus achieving dynamic calibration of the aging strain fiber. This significantly improves the strain measurement accuracy of the BOTDA system, providing more reliable data assurance for the safe operation of oil and gas pipelines. Summary of the Invention

[0007] The technical problem to be solved by this invention is that during the long-term service of oil and gas pipelines, the strain-frequency shift coefficient of the BOTDA system drifts due to fiber optic aging, thereby reducing the measurement accuracy. A method is needed to achieve dynamic calibration on site without disassembling the fiber.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: a dynamic calibration method for aging strain measuring optical fiber, wherein the system using the method includes a vibrating wire strain gauge installed on an oil and gas pipeline, a data acquisition instrument, a tight-buffered optical fiber for strain monitoring and a loose-buffered optical fiber for temperature compensation, a BOTDA host, and a computer.

[0009] The vibrating wire strain gauge is equipped with a temperature sensor for temperature compensation and is welded to the oil and gas pipeline. It is used to obtain high-precision, temperature-compensated strain data at the installation location, and three vibrating wire strain gauges are installed on one cross-section of the oil and gas pipeline.

[0010] The tight-buffered optical fiber used for strain monitoring is laid along the length of the oil and gas pipeline and is pasted back and forth three times on the oil and gas pipeline to form three segments, which are used to measure the strain at different points on the cross section. These three points are very close to the three points where three vibrating wire strain gauges are installed.

[0011] The loose-tube and tight-tube optical fibers used for temperature compensation are connected to form a loop, which is connected to the optical transmitter and optical receiver ports of the BOTDA host.

[0012] The data acquisition device wirelessly transmits the temperature-compensated strain data acquired by the vibrating wire strain gauge to the computer.

[0013] The computer is connected to the BOTDA host via a network cable, obtains the frequency shift data measured by the tight-buffered fiber, and uses the frequency shift data in the loose-buffered fiber at the corresponding position for temperature compensation.

[0014] The computer uses strain data measured at different times by a vibrating wire strain gauge as a reference to calculate the strain-frequency shift coefficient of the fiber optic cable at the location of the vibrating wire strain gauge within the corresponding time interval. This yields the dynamic strain-frequency shift coefficient of the aging fiber, which is the coefficient after dynamic calibration of the aging fiber. Specific steps include:

[0015] S1. Use vibrating wire strain gauges to obtain temperature-compensated baseline strain data at specific locations in oil and gas pipelines;

[0016] S2. The BOTDA host measures the Brillouin frequency shift data v1 of the tight-buffered fiber at the corresponding location;

[0017] S3. The BOTDA host measures the Brillouin frequency shift data v2 of the loose-tube fiber at the corresponding location;

[0018] S4. Calculate the difference between v1 and v2, Δv = v1 – v2, which is the Brillouin frequency shift data after considering temperature compensation;

[0019] S5. Using the reference strain data obtained in S1 and Δv obtained in S4, the strain-frequency shift coefficient of the aged optical fiber is calculated by the proportional relationship between the change in the reference strain data and Δv.

[0020] S6. The strain-frequency shift coefficient obtained from S5 is used to calibrate the strain measurement results of the optical fiber over a length of distance at that specific location.

[0021] Preferably, the computer performs dynamic updates of the fiber strain-frequency shift coefficient at intervals of 6 months to 1 year.

[0022] Preferably, the laser wavelength emitted by the BOTDA host is around 1550nm. Taking standard single-mode fiber G.652.D as an example, the corresponding strain-frequency shift coefficient is between 18με / MHz and 23με / MHz. This value will change with fiber aging.

[0023] The beneficial effects of this invention are: by dynamically calibrating the strain-frequency shift coefficient of the aging optical fiber, the problem of decreased measurement strain accuracy caused by long-term operation of the optical fiber is effectively solved. It has advantages such as low cost and easy implementation, and can significantly improve the long-term reliability of the monitoring system. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the system of the present invention;

[0025] Figure 2 for Figure 1A cross-sectional view at section AA;

[0026] Figure 3 for Figure 1 A cross-sectional view at section BB.

[0027] The labels in the attached diagram have the following meanings: 1. Oil and gas pipeline; 2A. Twelve-point tight-buffered optical fiber; 2B. Three-point tight-buffered optical fiber; 2C. Nine-point tight-buffered optical fiber; 3. Connecting optical fiber; 4. Loose-buffered optical fiber; 5. BOTDA host; 6A. Optical transmitter port; 6B. Optical receiver port; 7. Network cable; 8. Computer; 9A. Twelve-point vibrating wire strain gauge; 9B. Three-point vibrating wire strain gauge; 9C. Nine-point vibrating wire strain gauge; 10. RS232 line; 11. Data acquisition instrument. Detailed Implementation Plan

[0028] The present invention will now be described in further detail with reference to the accompanying drawings.

[0029] Figure 1 This is a schematic diagram of the system of the present invention. Three tight-buffered optical fibers are attached to the main body of the oil and gas pipeline 1, distributed along the length of the pipeline 1 at three points on the cross-section. These are referred to as twelve-point tight-buffered fiber 2A, three-point tight-buffered fiber 2B, and nine-point tight-buffered fiber 2C. These three tight-buffered fibers, together with the connecting fiber 3 and an idle loose-buffered fiber 4 in the communication optical cable, form a loop, which is respectively connected to the optical transmitter port 6A and the optical receiver port 6B of the BOTDA host 5. The laser beam is emitted from the optical transmitter port 6A of the BOTDA host 5, passes sequentially through the connecting fiber 3, twelve-point tight-buffered fiber 2A, connecting fiber 3, three-point tight-buffered fiber 2B, connecting fiber 3, nine-point tight-buffered fiber 2C, connecting fiber 3, loose-buffered fiber 4, and connecting fiber 3, before returning to the BOTDA host 5 from the optical receiver port 6B. BOTDA host 5 acquires Brillouin frequency data from 12-point tight-buffered fiber optic cable 2A, 3-point tight-buffered fiber optic cable 2B, and 9-point tight-buffered fiber optic cable 2C, corresponding to strain without temperature compensation; it also acquires Brillouin frequency data from loose-buffered fiber optic cable 4 for temperature compensation of strain. The Brillouin frequency data measured by BOTDA host 5 is transmitted to computer 8 via network cable 7. Three vibrating wire strain gauges, designated as 12-point vibrating wire strain gauge 9A, 3-point vibrating wire strain gauge 9B, and 9-point vibrating wire strain gauge 9C on section AA of the oil and gas pipeline 1, are used to measure the strain at 12, 3, and 9 points of section AA, respectively. These three vibrating wire strain gauges contain temperature sensors, and the temperature-compensated strain data obtained is transmitted to data acquisition unit 11 via RS232 line 10, and then wirelessly to computer 8.

[0030] Figure 2 for Figure 1A cross-sectional view at section AA. Next to the twelve-point tight-buffered fiber 2A, the three-point tight-buffered fiber 2B, and the nine-point tight-buffered fiber 2C at section AA, twelve-point vibrating wire strain gauges 9A, 3-point vibrating wire strain gauges 9B, and nine-point vibrating wire strain gauges 9C are installed respectively. The three vibrating wire strain gauges at section AA are used to calculate the aging dynamic strain-frequency shift coefficient of the optical fiber at their respective locations.

[0031] Figure 3 for Figure 1 The cross-sectional view at section BB. There are no vibrating wire strain gauges at section BB, only twelve-point tight-buffered fiber 2A, three-point tight-buffered fiber 2B, and nine-point tight-buffered fiber 2C.

[0032] During the construction of oil and gas pipeline 1, a portion of tight-buffered optical fibers 2A, 2B, or 2C was individually cut and calibrated in a laboratory to determine the initial fiber strain-frequency shift coefficient. For example, in a temperature-controlled laboratory, a tensile tester precisely applied a 100 με tension to the cut tight-buffered fiber. If the BOTDA host measured a Brillouin frequency shift of 5 MHz for this tight-buffered fiber, then the initial fiber strain-frequency shift coefficient was C0 = 100 / 5 = 20.0 με / MHz. For a period of time after the completion of oil and gas pipeline 1, such as within 5 years, computer 8 could process the Brillouin frequency shift measured in tight-buffered fibers 2A, 2B, and 2C using C0 to obtain the strain change without any problems. However, over time, such as after 5 years, tight-buffered fibers 2A, 2B, and 2C, and loose-buffered fiber 4, showed some aging. At this point, computer 8 needs to recalculate the strain-frequency shift coefficients based on the strain data measured by vibrating wire strain gauges 9A, 9B, and 9C, and the Brillouin frequency shifts of the tightly-buffered optical fibers 2A, 2B, and 2C and the loosely-buffered optical fiber 4 at the locations of the vibrating wire strain gauges. For example, at a certain moment in the 6th year of operation of oil and gas pipeline 1, the temperature-compensated strain measured by the twelve-point vibrating wire strain gauge 9A is 72 με, the Brillouin frequency of the corresponding twelve-point tightly-buffered optical fiber 2A is 10.9081 GHz, and the Brillouin frequency of the loosely-buffered optical fiber 4 is 10.9023 GHz; after a certain interval, such as 6 hours later, the temperature-compensated strain measured by the twelve-point vibrating wire strain gauge 9A is 223 με, the Brillouin frequency of the corresponding twelve-point tightly-buffered optical fiber 2A is 10.9172 GHz, and the Brillouin frequency of the loosely-buffered optical fiber 4 is 10.9043 GHz. The Brillouin frequency shift caused by temperature is (10.9043-10.9023)*1000=2MHz. The aging dynamic strain-frequency shift coefficient of the twelve-point tight-buffered fiber 2A, after eliminating the temperature effect, is: C1=(223-72) / [(10.9172-10.9081-2*0.001)*1000]=21.3με / MHz. This C1 serves as the corrected strain-frequency shift coefficient for the entire twelve-point tight-buffered fiber 2A, used for calculating the shift from the Brillouin frequency to the corresponding strain change. Similarly, the three-point vibrating wire strain gauge 9B can be used to correct the three-point tight-buffered fiber 2B to obtain the aging dynamic strain-frequency shift coefficient C2; the nine-point vibrating wire strain gauge 9C can be used to correct the nine-point tight-buffered fiber 2C to obtain the aging dynamic strain-frequency shift coefficient C3. The above steps can be repeated in the 7th and 8th years of operation of the oil and gas pipeline 1 to perform dynamic calibration of C1, C2, and C3.

[0033] To improve calibration accuracy, taking the twelve-point cross-section as an example, computer 8 selects multiple time points (e.g., intervals of 6 hours, 12 hours, 24 hours, etc.) to collect strain data and corresponding Brillouin frequency shift data of the twelve-point vibrating wire strain gauge 9A. By analyzing this data, computer 8 selects time periods with significant strain changes (e.g., strain changes exceeding 100 με) for calculating new strain-frequency shift coefficients. Specifically, for each valid time period, computer 8 calculates a temporary strain-frequency shift coefficient, and then performs a weighted average of these temporary coefficients (giving higher weights to time periods with large strain changes), ultimately obtaining a more accurate dynamic calibration coefficient C1. This multi-time-point sampling and weighted averaging method can effectively reduce random errors in single measurements and improve the reliability of calibration results. In subsequent years (e.g., the 7th and 8th years), the same multi-time-point sampling and weighted averaging method is used to ensure that each calibration accurately reflects the current aging state of the optical fiber. The calculation methods for the three-point and nine-point cross-sections are the same as those for the twelve-point cross-section. In addition, the computer 8 will automatically record the strain-frequency shift coefficient of each calibration to form an aging curve, providing data support for predicting the performance degradation trend of optical fibers.

[0034] Taking a twelve-point cross-section as an example, since the twelve-point vibrating wire strain gauge 9A is installed on a specific cross-section AA of the oil and gas pipeline 1, while the twelve-point tight-buffered optical fiber 2A and loose-buffered optical fiber 4 are continuously laid along the pipeline, their spatial positions may not be completely corresponding. In order to accurately calibrate the strain-frequency shift coefficient of the optical fiber, the computer 8 needs to align the data on the twelve-point tight-buffered optical fiber 2A and loose-buffered optical fiber 4 onto the twelve-point vibrating wire strain gauge 9A. The specific steps are as follows:

[0035] (1) Computer 8 aligns the measurement points of the twelve-point tight-fitting fiber 2A and loose-fitting fiber 4 along the length of the oil and gas pipeline 1 by linear interpolation.

[0036] (2) Computer 8 stores the installation location (such as GPS coordinates or pipeline mileage station number) of the twelve-point vibrating wire strain gauge 9A and matches the interpolated twelve-point tight-buffered fiber 2A and loose-buffered fiber 4 near the location.

[0037] (3) If the installation position of the twelve-point vibrating wire strain gauge 9A does not completely coincide with the above interpolation point, the Brillouin frequency shift of the optical fiber at that position is calculated by linear interpolation.

[0038] (4) Assume that the twelve - point vibrating wire strain gauge 9A is located at the pipeline mileage L0, and the nearest optical fiber data points of the interpolated twelve - point tight - jacket optical fiber 2A and loose - jacket optical fiber 4 are L1 and L2 (L1 < L0 < L2). The Brillouin frequencies of the twelve - point tight - jacket optical fiber 2A are f1 and f2 respectively, and the Brillouin frequencies of the corresponding loose - jacket optical fiber 4 are f3 and f4 respectively. Then the Brillouin frequency f of the tight - jacket optical fiber at the location of the twelve - point vibrating wire strain gauge 9A 01 can be calculated by linear interpolation:

[0039] The Brillouin frequency f of the loose - jacket optical fiber at the location of the twelve - point vibrating wire strain gauge 9A 02 is:

[0040] This method can ensure the accurate matching of the strain data of the twelve - point vibrating wire strain gauge 9A with the Brillouin frequencies of the optical fiber measurement points, improving the calibration accuracy. The spatial position matching methods of the three - point vibrating wire strain gauge 9B and the nine - point vibrating wire strain gauge 9C are the same as those of the twelve - point vibrating wire strain gauge 9A.

[0041] When the BOTDA host 5 scans one cycle, the Brillouin frequencies of the twelve - point tight - jacket optical fiber 2A, three - point tight - jacket optical fiber 2B, nine - point tight - jacket optical fiber 2C, and loose - jacket optical fiber 4 at the same moment can be obtained. The data acquisition frequencies of the vibrating wire strain gauges 9A, 9B, and 9C are the same, but they may not be completely synchronized with the measurement moment of the BOTDA host 5. The computer 8 needs to perform time interpolation to ensure the temporal matching of the two sets of data. The specific method is as follows:

[0042] (1) The vibrating wire strain gauges 9A, 9B, and 9C usually collect strain data at fixed intervals (such as every minute), while the BOTDA host 5 may scan to obtain the optical fiber Brillouin frequencies at different frequencies (such as every 20 minutes).

[0043] (2) If the measurement moments of the two do not completely coincide, directly using the original data may lead to calibration errors. Therefore, time interpolation is required.

[0044] (3) Assume that the twelve - point vibrating wire strain gauge 9A measures strains ε1 and ε2 at times t and t2, and the BOTDA host 5 measures the Brillouin frequency f at time t0 (t < t0 < t2) 01 , then the strain ε0 of the twelve - point vibrating wire strain gauge 9A at time t0 is:

[0045] The time matching methods of the three - point vibrating wire strain gauge 9B and the nine - point vibrating wire strain gauge 9C are the same as those of the twelve - point vibrating wire strain gauge 9A.

[0046] 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. A dynamic calibration method for aging strain measuring optical fiber, wherein the system using the method includes three vibrating wire strain gauges installed on an oil and gas pipeline, a data acquisition instrument, a tight-buffered optical fiber for strain monitoring and a loose-buffered optical fiber for temperature compensation, a BOTDA host, and a computer. The three vibrating wire strain gauges are welded to the same cross section of the oil and gas pipeline; The tight-fitting optical fiber is pasted back and forth three times on the oil and gas pipeline to form three sections; The computer uses strain data measured at different times by a vibrating wire strain gauge as a reference to calculate the strain-frequency shift coefficient of the fiber optic cable at the location of the vibrating wire strain gauge within the corresponding time interval. This yields the dynamic strain-frequency shift coefficient of the aging fiber, which is the coefficient after dynamic calibration of the aging fiber. Specific steps include: S1. Use vibrating wire strain gauges to obtain temperature-compensated baseline strain data at specific locations in oil and gas pipelines; S2. The BOTDA host measures the Brillouin frequency shift data v1 of the tight-buffered fiber at the corresponding location; S3. The BOTDA host measures the Brillouin frequency shift data v2 of the loose-tube fiber at the corresponding location; S4. Calculate the difference between v1 and v2, Δv = v1 – v2, which is the Brillouin frequency shift data after considering temperature compensation; S5. Using the reference strain data obtained in S1 and Δv obtained in S4, the strain-frequency shift coefficient of the aged optical fiber is calculated by the proportional relationship between the change in the reference strain data and Δv. S6. The strain-frequency shift coefficient obtained from S5 is used to calibrate the strain measurement results of the optical fiber over a length of distance at that specific location.

2. The method according to claim 1, characterized in that, The computer periodically updates the strain-frequency shift coefficient dynamically, with an update cycle of 6 months to 1 year.

3. The method according to claim 1, characterized in that, The laser wavelength of the BOTDA host is about 1550nm, and the corresponding strain-frequency shift coefficient ranges from 18με / MHz to 23με / MHz.

4. The method according to claim 1, characterized in that, Three vibrating wire strain gauges and three tightly fitted optical fibers are arranged on the cross-section of the oil and gas pipeline at the twelve o'clock, three o'clock, and nine o'clock positions.

5. The method according to claim 1, characterized in that, The computer uses a linear interpolation method to align the fiber optic data measured by the BOTDA host with the strain measured by the vibrating wire strain gauge.

6. The method according to claim 1, characterized in that, The computer uses a linear interpolation method to align the fiber optic data measured by the BOTDA host with the strain measured by the vibrating wire strain gauge.

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