Optical fiber system for monitoring strain and internal pressure of oil and gas pipeline

Abstract

The invention provides an oil and gas pipeline monitoring system based on a distributed optical fiber technology. The oil and gas pipeline monitoring system can monitor the strain and the internal pressure of a pipeline at the same time. The distributed monitoring of the axial strain of the pipeline and the quasi-distributed monitoring of the internal pressure are realized by laying the accompanying strain optical fiber beside the pipeline and winding and fixing the optical fiber in the circumferential direction of the pipeline at intervals. The system utilizes the BOTDA or BOTDR host to demodulate the data measured by the optical fiber and transmits the data to the local or cloud computer for processing, so that the pipeline axial strain and the area with large pressure change in the pipeline can be effectively monitored, and the system is used for providing corresponding early warning information. According to the method, the optical fiber originally used for strain monitoring can also be used for pressure monitoring, and the effect that one fiber has multiple purposes is achieved. The system is simple in design, easy to implement and suitable for real-time monitoring and maintenance management of large-scale oil and gas pipelines.

Description

Technical Field

[0001] The invention uses distributed optical fiber technology to monitor the strain and internal pressure of an oil and gas pipeline, and belongs to the technical field of oil and gas pipeline monitoring. Background Art

[0002] Distributed fiber optic technology is widely used in safety monitoring of oil and gas pipelines. Fiber optic technology based on BOTDA (Brillouin optical time domain analysis) or BOTDR (Brillouin optical time domain reflectometry) can be used to monitor the strain or temperature of oil and gas pipelines. Fiber optic technology based on DTS (distributed temperature sensing) can only monitor the temperature of oil and gas pipelines, and this technology is mostly used for leakage monitoring of oil and gas pipelines. Optical fiber technology based on DAS (distributed acoustic sensing) is used to monitor pipeline vibration, leakage, and third-party interference by monitoring sound waves or vibrations near the optical fiber.

[0003] When using BOTDA and BOTDR technologies to monitor the strain of oil and gas pipelines, a common solution is to lay a tightly sleeved accompanying strain optical fiber within a short distance next to the oil and gas pipeline. The direction of this optical fiber is consistent with the length direction of the pipeline. Since the distance between the optical fiber and the pipeline is relatively close, the strain sensed by the optical fiber can be approximated as the axial strain borne by the pipeline. When suffering from geological disasters such as mudslides, landslides, and earthquakes, the pipeline may be subjected to large strains, posing a safety hazard. Strain monitoring of the optical fiber can achieve strain warning for the pipeline. This monitoring technology has the advantage of being distributed and can perform nearly continuous strain monitoring of tens of kilometers of oil and gas pipelines. The monitoring interval depends on the sampling interval, which is usually between 0.25 and 4m.

[0004] According to the different oil and gas pipeline design specifications and actual operating conditions, the oil and gas pressure transported by the oil and gas pipeline is usually 10 0 to 10 1 MPa level. During the transportation of oil and gas, the internal pressure of the oil and gas pipeline will fluctuate within a certain range, which can be monitored by the pressure sensor installed on the pipeline. The typical installation interval of the pressure sensor is 10 to 50 km, which means that there is no data on the internal pressure of the oil and gas pipeline within the area of ​​tens of kilometers between two adjacent pressure sensors.

[0005] In the oil and gas pipeline engineering projects where strain-accompanying optical fibers are laid, there have been no reports on the additional use of optical fibers to monitor the internal pressure of the oil and gas pipelines. Summary of the invention

[0006] The technical problem to be solved by the present invention is that the existing distributed optical fiber technology is limited to monitoring the strain of the pipeline in the engineering project of oil and gas pipeline strain monitoring, and does not additionally use optical fiber to monitor the internal pressure of the pipeline.

[0007] Compared with the disclosed engineering applications, the characteristics and advantages of the present invention are that while using optical fiber technology to perform distributed strain monitoring of oil and gas pipelines, it can additionally realize quasi-distributed monitoring of the internal pressure of the pipeline, which helps to obtain pressure measurement data in areas where the pressure changes greatly inside the pipeline.

[0008] In order to achieve the above-mentioned purpose, the present invention adopts the following technical scheme: in an oil and gas pipeline engineering project where strain optical fiber is laid, the strain optical fiber is wound and fixed in the circumferential direction of the oil and gas pipeline at intervals of a certain distance ΔL; the optical fiber laid along the length direction of the oil and gas pipeline measures the axial strain borne by the pipeline; the optical fiber wound and fixed in the circumferential direction of the oil and gas pipeline obtains the internal pressure of the oil and gas pipeline by measuring the circumferential strain of the pipeline; the data measured by the optical fiber is demodulated by a BOTDA host or a BOTDR host, and transmitted to a local or cloud computer for calculation and processing.

[0009] When the strain-accompanying optical fiber is laid along the length of the oil and gas pipeline, it is 0.25 to 1 m away from the oil and gas pipeline to ensure that the distance between the optical fiber and the oil and gas pipeline is close enough, and the strain borne by the optical fiber is approximately equivalent to the axial strain borne by the oil and gas pipeline.

[0010] When the strain-accompanying optical fiber is wound and fixed in the annular direction of the oil and gas pipeline, it is wound at least 3 times in order to obtain multiple annular strain data measurement points in the annular direction, so that the calculation of the internal pressure is more accurate.

[0011] When the data obtained by the strain-matched optical fiber measurement is demodulated by the BOTDA host, the strain-matched optical fiber and the unused cores in the multi-core communication optical cable beside the oil and gas pipeline form a loop and are connected to the optical transmitting port and the optical receiving port of the BOTDA host for double-end measurement.

[0012] When the data obtained by the strain-matched optical fiber measurement is demodulated by the BOTDR host, the strain-matched optical fiber is connected to the optical port of the BOTDR host to perform single-ended measurement.

[0013] Preferably, ΔL is between 5 and 2000 m.

[0014] Preferably, the above-mentioned winding and fixing method is fixed by applying glue. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is a schematic diagram of the present invention using a BOTDA host for monitoring; Figure 2 This is a schematic diagram of the present invention using a BOTDR host for monitoring; Figure 3 This is the finite element model of the oil and gas pipeline of the present invention.

[0016] The meanings of the marks in the attached drawings are as follows: 1. Oil and gas pipeline, 2. Strained optical fiber, 2A. Straight optical fiber, 2B. Wrapped optical fiber, 3. Fiber core in communication optical cable, 4. BOTDA host, 5. Optical transmitting port, 6. Connecting optical fiber, 7. Optical receiving port, 8. Local or cloud computer, 9. BOTDR host, 10. Optical port. DETAILED DESCRIPTION

[0017] Below Figure 1 Figure 2 The monitoring system shown is described in detail with reference to an example, but the protection scope of the present invention is not limited to the embodiment.

[0018] Figure 1 The accompanying strain optical fiber 2 and the fiber core 3 in the communication optical cable are laid beside the oil and gas pipeline 1 shown. The laser is emitted from the optical emission port 5 in the BOTDA host 4 and enters the accompanying strain optical fiber 2 through the connecting optical fiber 6. The accompanying strain optical fiber 2 is divided into a straight optical fiber 2A part and a winding optical fiber 2B part. The straight optical fiber 2A is laid along the length direction of the oil and gas pipeline, and the measured strain corresponds to the average axial strain of the pipeline at the location. The winding optical fiber 2B is wound and fixed in the circumferential direction of the oil and gas pipeline, and the measured strain is the circumferential strain of the pipeline. Figure 1 There are three places where the optical fiber 2B is wound: position I, position II, and position III, which are used to measure and calculate the internal pressure of the pipeline at those three places. After the laser leaves the strain-accompanying optical fiber 2, it passes through the connecting optical fiber 6 and enters the core 3 in the communication optical cable, and then returns to the optical receiving port 7 in the BOTDA host 4 through the connecting optical fiber 6. The strain data measured by the BOTDA host 4 is transmitted to the local or cloud computer 8 for calculation and processing.

[0019] Figure 2 The monitoring system shown is similar to Figure 1 Similar, except that Figure 1 The system uses a BOTDA host 4 and needs to use the fiber core 3 in the communication optical cable to form a loop. Figure 2 The system uses a BOTDR host 9 to perform single-ended measurement, and the laser is emitted from an optical port 10 without forming a loop, which is more practical when there is a lack of available communication optical cable cores.

[0020] When the oil and gas pipeline 1 is just built and has not yet transported oil and gas, it is assumed that at position I, the winding optical fiber 2B measures n hoop strain data, and the average value of these n strain data is recorded as e0. At a certain moment of transporting oil and gas, the average value of these n strain data becomes e1. Ignoring the fluctuation of soil pressure on the oil and gas pipeline 1, the internal pressure of the oil and gas pipeline at position I at this moment is: p=EY*(e1-e0)*t / (Rt) (Formula 1)

[0021] Where EY is the Young's modulus of the pipe, t is the pipe wall thickness, and R is the outer radius of the pipe. These calculations are performed in a local or cloud computer 8. The internal pressure calculation method at position II and position III is the same as that at position I.

[0022] In order to verify the accuracy of (Equation 1), the finite element model based on COMSOL software was used to simulate the oil and gas pipeline with internal pressure, such as Figure 3 As shown. The outer radius of the pipeline is R=355.5mm, the wall thickness is t=8.4mm, the length is L=7.11m, the material is steel, the Young's modulus is EY=210GPa, and the Poisson's ratio is 0.3. The pipeline is fixed at one end at z=0; at the other end at z=L, it can move in the z direction, and the x and y directions are fixed. The oil and gas pipeline is not subjected to force in the initial state, and the internal pressure p is zero, that is, e0 in (Equation 1) is zero. The external force Fx in the x direction is loaded at the middle of the pipeline at z=L / 2. According to (Equation 1), when it is known that e0 is zero and the internal pressure is p, the annular strain of the pipeline is: e1=p*(Rt) / (EY*t) (Formula 2)

[0023] The x-direction displacement and hoop strain at z=L / 2 in the middle of the pipe obtained by finite element simulation are recorded as Ux and E1, respectively, which are the average values ​​of the circumference at z=L / 2 on the outer surface of the pipe. Table 1 shows the Ux and E1 calculated by finite element under different Fx and p, as well as the hoop strain e predicted by (Equation 2) 1。 It can be seen from Table 1 that Ux is affected by Fx but not by p; E1 and e1 are affected by p but not by Fx, and E1 and e1 are well matched, with the absolute value of the relative error not exceeding 1.5%. The above analysis demonstrates the accuracy of (Equation 1).

[0024]

[0025] The specific embodiments of the present invention are described above, but the present invention is not limited to the specific embodiments described above. Those skilled in the art can make various changes in form and details within the scope of the claims, which do not affect the essential content of the present invention.

Claims

1. A system for simultaneously monitoring the axial strain and internal pressure of oil and gas pipelines using optical fiber technology, characterized in that In the oil and gas pipeline project, strain-accompanying optical fibers are laid along the length of the oil and gas pipeline; the optical fibers are wound and fixed in the circumferential direction of the oil and gas pipeline at intervals; the straight optical fibers laid along the length of the oil and gas pipeline measure the axial strain borne by the pipeline; the wound optical fibers wound and fixed in the circumferential direction of the oil and gas pipeline obtain the internal pressure of the oil and gas pipeline by measuring the circumferential strain of the pipeline; the data measured by the optical fibers are demodulated by the BOTDA host or the BOTDR host, and transmitted to the local or cloud computer for calculation and processing.

2. According to the scheme of claim 1, when a BOTDA host is used for demodulation, the cores of the accompanying strain optical fiber and the communication optical cable form a loop and are connected to the optical transmitting port and the optical receiving port of the BOTDA host for double-end measurement.

3. According to the solution of claim 1, when a BOTDR host is used for demodulation, the accompanying strain optical fiber is connected to the optical port of the BOTDR host for single-ended measurement.

4. The solution according to claim 1, characterized in that The interval between a location where the optical fiber is wound at one point in the oil and gas pipeline and a location where the optical fiber is wound at the next point is between 5 and 2000 m.

5. The solution according to claim 1, characterized in that: Each time the optical fiber is wound around, at least three turns are wrapped around the oil and gas pipeline.

6. The solution according to claim 1, characterized in that: The winding and fixing method adopts the method of applying glue to fix.

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