JP2005257393A - Specifying method of corrosion spot of structure - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a specifying method of a corrosion spot of a structure capable of specifying highly accurately the corrosion spot of the structure, and specifying a loss factor of an optical fiber. <P>SOLUTION: In this method for specifying the corrosion spot of the structure 82 generating gaseous hydrogen h by corrosion, by storing one or a plurality of optical fibers 86, generation of the gaseous hydrogen h is detected by allowing light having the wavelength for detecting the gaseous hydrogen h generated by corrosion to enter the optical fibers 86, and when generation of the gaseous hydrogen h is detected, purge gas n is supplied into the structure 82 in a section where the gaseous hydrogen h is generated, to thereby purge the gaseous hydrogen h in the structure 82 to the outside of the structure 82, and then the change of a transmission loss generated in the optical fibers 86 is detected by allowing light having the wavelength for detecting the gaseous hydrogen hn to enter the optical fibers 86, to thereby specify a generation spot of the gaseous hydrogen hn. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for identifying a corrosion location of a structure that houses an optical fiber.

  As a structure that houses one or a plurality of optical fibers and is supplied with hydrogen gas by corrosion, for example, there is an optical fiber composite ground wire (OPGW) 81 as shown in FIG. The OPGW 81 has a function as an aerial ground wire that receives lightning strikes and a communication function using a built-in optical fiber. In this OPGW 81, an aluminum pipe unit 84 is configured by an aluminum tube (Al tube) 82 having an airtight structure and an aluminum grooved spacer 83 provided therein, and a plurality of optical fibers are provided in the groove 85 of the spacer 83. The optical fiber unit 87 for communication which twisted 86 is accommodated, and the steel tube aluminum wire (AC wire) 88 is twisted around the outer periphery of the aluminum pipe 82. A slight gap a is formed between the aluminum tube 82, the spacer 83, and the optical fiber unit 87.

  As shown in FIG. 10, the OPGW 81 is installed above a power transmission line (not shown) between a plurality of steel towers 1 provided at a steel tower span t (generally about 200 to 500 m), and below the power transmission line. The optical connection boxes for OPGW (JB) 2a, 2b... Provided in the steel tower 1 with a junction box interval b (generally about 2 to 3 km, or about 3 to 8 diameters of the steel tower 1) The young (upstream) side and the old (downstream) side of the fiber are connected to form a line.

  In OPGW 81, as shown in FIGS. 9A to 9D, there is a problem that a hole is opened in the Al pipe 82 due to corrosion. Specifically, water w accumulates in the gap between the Al tube 82 and the AC wire 88, and impurities such as Cl- are concentrated by evaporation of the accumulated water w (FIG. 9A). Cl − causes pitting corrosion in the Al tube 82 and the AC wire 88 (FIG. 9B). Pitting corrosion progresses and the iron of AC line 88 is exposed (FIG. 9C). Corrosion is accelerated by the galvanic action of aluminum and iron, and a through hole 91 is generated in the Al pipe 82.

  Water w enters the inside of the Al pipe 82 from the through-hole 91, and (i) the infiltrated water w freezes and expands due to a decrease in temperature and compresses the optical fiber, or (ii) an aluminum corrosion product (aluminum hydroxide) Occurring and compressing the optical fiber may increase the transmission loss of the optical fiber and cause a communication line failure. In addition, hydrogen gas is generated when water reacts with aluminum at the corrosion location of the Al pipe 82.

  There are the following methods 1) to 3) as methods for identifying the corrosion site of OPGW.

1) Airtight measurement method As shown in FIG. 11, the aluminum tubes and the junction boxes 2a, 2b (see FIG. 12) have an airtight structure, so that nitrogen gas is injected from one junction box 2a by the nitrogen gas cylinder 111, and the other In the connection box 2b, the internal pressure is monitored by the pressure sensor 112, and when the pressure becomes the same as that on the injection side, the airtightness is good. That is, there is no hole (through hole) in the aluminum tube of the OPGW 81 between the connection boxes 2a and 2b. judge. If the same pressure as the injection side is not reached, it is determined that the airtightness is poor, that is, the aluminum tube has a through hole.

  As shown in FIG. 12, the junction box 2a has an airtight structure by closing a lid on a case with packing. Since the OPGW introduction port 122 and the extension cable hole 123 are provided in the side wall 121 of the connection box 2a, nitrogen gas is injected from the extension cable hole 123.

2) Method for measuring the hydrogen concentration in the junction box Since the hydrogen gas generated inside the OPGW aluminum tube diffuses in the longitudinal direction of the OPGW and stays in the junction box, the hydrogen gas concentration in the junction box is measured using a detector tube or the like. . If hydrogen gas is detected in the measured junction box, it is determined that corrosion of the aluminum pipe has occurred in the OPGW on one side or the opposite side connected to the junction box, or the OPGW on both sides.

3) Measurement by OTDR using hydrogen absorption wavelength light Hydrogen gas diffuses to the inside of the optical fiber, and the transmission loss of light propagating in the optical fiber increases with light of a specific wavelength. The specific wavelength is a wavelength at which loss occurs due to absorption by hydrogen molecules of hydrogen gas diffused inside the optical fiber (hereinafter referred to as “hydrogen absorption wavelength”), which is a near-red wavelength used in optical fiber communication. In the outer region, as indicated by the characteristic line 131 in FIG. 13, the wavelengths in the 1.24 μm, 1.6 to 1.7 μm band, and the like correspond to this (for other wavelengths, refer to Non-Patent Document 1). However, the characteristic line 131 shows a value when the hydrogen gas partial pressure is 1 atm (hydrogen gas 100% atmosphere).

  As shown in FIG. 14, in the optical pulse tester (Optical Time Domain Reflectometer) (hereinafter referred to as OTDR) 141, by using hydrogen absorption wavelength light, transmission loss due to hydrogen gas (hereinafter referred to as “ It is possible to measure the distribution in the longitudinal direction of the portion where the hydrogen absorption loss occurs) and to grasp the section where the hydrogen absorption loss occurs (junction box interval b).

  Specifically, since the optical distribution board 143 of the communication station 142 and the connection box 2s of the young terminal of the OPGW 81 are already connected, the communication station 142 is provided with an OTDR 141, and this OTDR 141 and the optical distribution board 143 And connect. The OTDR 141 transmits hydrogen absorption wavelength light (for example, light having a wavelength of 1.24 μm) as measurement light to the optical fiber of the OPGW 81, and measures the light reception level of the backscattered light.

  In the example of the measurement result shown in FIG. 15, the transmission loss distribution (6.0 dB / km) of the hydrogen absorption wavelength light (here, light having a wavelength of 1.24 μm) indicated by a solid line has a large slope, that is, the loss is high. Since there is a large part (from a distance of about 0 km to about 2.5 km), this part is judged to be a place where hydrogen absorption loss has occurred, and it is considered that corrosion of the OPGW aluminum pipe has occurred at this position. Yes.

  The prior art document information related to the invention of this application includes the following.

JP-A-9-219114 JP 2002-218614 A JP 2002-218615A JP 60-196702 A JP-A-63-18309 Kuwasui, 2 others, “Effect of hydrogen on optical submarine cable transmission loss”, research on international communications, no. 129, pp. 345-354, June 1986

  By the way, the corroded OPGW section needs to be replaced, but in order to shorten the replacement section as much as possible (to reduce the cost), it is necessary to accurately grasp the corrosion position. Since the minimum unit of re-laying is usually the steel tower span t in FIG. 10, the specification of the corrosion position requires an accuracy of the steel tower span t (generally about 200 to 500 m) or less. However, the methods 1) to 3) described above have the following problems.

  1) The advantage of the airtightness measurement method is that it can directly grasp the airtightness failure of the aluminum tube. However, the connection boxes 2a, 2b... Are usually installed at intervals of about 3 to 8 diameters of the steel tower 1. Therefore, in this method, even if it is known that there is a hole in the aluminum pipe of OPGW 81 between the junction boxes 2a and 2b, it is not possible to specify at which position (location, between the tower diameters) the hole is present.

  Further, it is necessary to prevent nitrogen gas from flowing through the OPGW 81 that is not the object of investigation. Therefore, it is necessary to divide the OPGW 81 between the connection boxes 2a and 2b, and it is necessary to stop all communication lines and cut optical fibers, resulting in a high measurement cost. Since the measurement work requires about 3 days / location, it is not realistic.

  2) The method of measuring the hydrogen concentration in the junction box is advantageous in that it can be easily measured and the generation of hydrogen gas can be directly grasped. However, since hydrogen gas can flow from the OPGW on both sides into the junction box, it is impossible to determine which side of the OPGW has flowed hydrogen gas.

  That is, since the measurement result of the hydrogen concentration is an OPGW determination from the measured junction box to the junction boxes adjacent to both sides thereof, the corrosion position (between the tower diameters) cannot be specified as in 1).

  3) The measurement method by OTDR using the hydrogen absorption wavelength light is that the transmission loss distribution of the optical fiber built in all the OPGW 81 is measured only from one end of the optical fiber built in the OPGW 81, for example, the communication station 142. It is a great advantage over 1) and 2) that it can be grasped and the location of hydrogen absorption loss can be identified. However, the hydrogen absorption loss location that can be grasped by the OTDR 141 is the location where hydrogen gas exists, not the actual location where corrosion occurs. Since hydrogen gas diffuses in the longitudinal direction of the OPGW 81, hydrogen absorption loss is observed even between steel tower diameters where corrosion has not occurred. In fact, as described with reference to FIG. 15, in the actual line measurement data, hydrogen absorption loss is observed in the OPGW over the entire junction box interval.

  Also, even at a specific wavelength where hydrogen absorption loss occurs, the loss increase may be very similar to hydrogen absorption loss due to other factors (such as microbend loss). is there. In particular, in the case of light having a wavelength of 1.625 μm, which is one of the hydrogen absorption wavelength lights, the influence of other factors becomes significant.

  Further, when hydrogen absorption loss occurs over several kilometers in the OPGW 81, the total amount of transmission loss increases, and the length of the OPGW that can be measured at one time is greatly limited. That is, in the measurement with light of a specific wavelength (for example, 1.24 μm), the attenuation of the measurement light of OTDR 141 is large, and it is impossible to measure the transmission loss distribution of the entire OPGW 81 only by measurement from the communication station 142. Therefore, it is impossible to evaluate the location where the hydrogen absorption loss occurs.

  In other words, in this method, even if the transmission loss increase interval (generally, about the junction box interval) in which hydrogen gas is diffused can be determined, the cause of the transmission loss is identified, and further the factor that increases the transmission loss occurs. It is not possible to identify the location where the corrosion occurs, that is, the location where the corrosion has occurred (or between the tower diameters where the corrosion has occurred).

  Accordingly, an object of the present invention is to solve the above-mentioned problems, and to provide a method for identifying a corroded portion of a structure that enables highly accurate identification of a corroded portion of a structure and identification of a factor causing transmission loss of an optical fiber. It is to provide.

  The present invention was devised to achieve the above object, and the invention of claim 1 specifies one or a plurality of optical fibers and specifies a corrosion location of a structure to which hydrogen gas is supplied by corrosion. In this method, light having a wavelength for detecting hydrogen gas generated by corrosion is incident on the optical fiber to detect whether hydrogen gas is generated. When hydrogen gas generation is detected, hydrogen gas is generated. A purge gas is supplied into the structure in a section where the hydrogen gas in the structure is purged outside the structure, and then light having a wavelength for detecting the hydrogen gas is incident on the optical fiber and generated in the optical fiber. This is a method for identifying the corrosion location of a structure that identifies the location where hydrogen gas is generated by detecting a change in transmission loss.

  According to a second aspect of the present invention, in the detection of the presence or absence of the generation of hydrogen gas, light having a wavelength with a large transmission loss due to hydrogen gas and a transmission loss due to hydrogen gas than light having a wavelength having a large transmission loss due to hydrogen gas 2. The method for identifying a corroded portion of a structure according to claim 1, wherein light having at least two wavelengths and light having a smaller wavelength is used.

  A third aspect of the present invention is the method for identifying a corroded portion of a structure according to the first or second aspect, wherein light of a hydrogen absorption wavelength is used to detect a change in transmission loss generated in the optical fiber.

  The invention according to claim 4 is the method for identifying a corrosion site of a structure according to claim 1 or 2, wherein only light having a wavelength with a large transmission loss due to hydrogen gas is used as light having a hydrogen absorption wavelength.

  The invention according to claim 5 is the method for identifying a corrosion location of the structure according to any one of claims 1 to 4, wherein the structure is an aluminum pipe.

  A sixth aspect of the invention is a method for identifying a corrosion location of a structure according to any one of the first to fifth aspects, wherein the purge gas is air or nitrogen.

  According to a seventh aspect of the invention, the measurement of the transmission loss is performed by using any one of optical fibers housed in the structure. Is the method.

  Invention of Claim 8 is the identification method of the corrosion location of the structure in any one of Claims 1-7 which makes the structure which accommodates the said 1 or several optical fiber into an optical fiber composite aerial ground wire. .

  According to the present invention, the following excellent effects are exhibited.

  (1) The corrosion location of the structure can be specified with high accuracy.

  (2) The section to be repaired (replaced) can be shortened, so that the repair cost can be reduced and the construction period can be shortened.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described with reference to the accompanying drawings.

  The present invention is a method for identifying a corroded portion of a structure containing an optical fiber. In the following description, the present invention is applied to a method for identifying a corroded portion of an OPGW aluminum pipe described in FIG. I will explain it.

  The method for identifying the corrosion location of the structure according to the present embodiment is based on the above-described 3) measurement method by OTDR using hydrogen absorption wavelength light, but the identification of the corrosion location (position), that is, corrosion. In order to distinguish between the location where the hydrogen gas is diffused without being corroded and the location where only the OTDR measurement is performed and the purge gas injection from the junction box is used.

  FIG. 1 is a schematic diagram showing one step of a method for identifying a corroded portion of an aluminum pipe according to a preferred embodiment of the present invention.

  As shown in FIG. 1, OPGW 81 is installed above a transmission line (not shown) (such as the top of the tower) between a plurality of towers 1 provided with a tower span t (generally about 200 to 500 m). An optical connection box for OPGW (JB) 2s, 2a provided on the tower 1 below the transmission line with a connection box interval b (generally about 2-3 km or about 3-8 diameters of the tower 1). , 2b... Are connected to the young (upstream) side and the old (downstream) side of the optical fiber. Since the optical wiring board 4 of the communication station 3 and the connection box 2s of the young terminal of the OPGW 81 are already connected, the OTDR 5 is provided in the communication station 3 and the OTDR 5 and the optical wiring board 4 are connected.

  First, whether or not hydrogen gas is retained in the aluminum pipe of OPGW81 is detected by measurement using this OTDR5.

  Detection of the presence or absence of hydrogen gas generation is performed by measuring the distribution of transmission loss of light in the longitudinal direction of an optical fiber, as represented by OTDR, using light of a plurality of wavelengths of two or more wavelengths (the explanation is simple). Therefore, in the following, it is assumed that the measurement of the distribution of light transmission loss in the longitudinal direction of the optical fiber is performed using OTDR, and the measurement is referred to as OTDR measurement.)

  The measurement light used for this OTDR measurement is less affected by hydrogen gas generated by corrosion of the aluminum tube (having a smaller hydrogen absorption loss) and more affected by hydrogen gas than light of wavelength λa. It is assumed that the light has a wavelength of λb that is easy (having a large hydrogen absorption loss) and at least two wavelengths.

  In the present embodiment, light having a wavelength of 1.31 μm that is difficult to be absorbed by hydrogen gas is used as measurement light having a wavelength λa. (This wavelength is usually used in communication lines.) In addition, as the measurement light having the wavelength λb, light having a wavelength that causes absorption loss due to hydrogen gas of 2 dB / km or more is used. One light having a wavelength of 1.24 μm or a light having a wavelength of 1.625 μm was used. Although it is sufficient that the wavelength has different hydrogen absorption loss, it is more preferable that the wavelength λa is not a hydrogen absorption wavelength and the wavelength λb is a hydrogen absorption wavelength as in this example. In addition, when the hydrogen absorption wavelength is selected as the wavelength λb, the measurement light may be a wavelength in the 1.6 to 1.7 μm band such as 1.59 and 1.7 μm as shown in FIG. May be used (see FIG. 13 and Non-Patent Document 1).

  The communication station 3 transmits the light with the wavelength λa and the light with the wavelength λb into one of the optical fibers of the OPGW 81 by the OTDR 5, and receives the backscattered light of each light. Measure each level.

  Here, it is utilized that the transmission loss of light propagating through the optical fiber increases at a constant rate according to the hydrogen gas partial pressure at the hydrogen absorption wavelength. The above-described characteristic line 131 in FIG. 13 shows a value when the hydrogen gas partial pressure is 1 atm (hydrogen gas 100% atmosphere), but the shape of the characteristic line other than the hydrogen gas partial pressure is 1 atm. Similar to the shape (see Non-Patent Document 1).

  In other words, since the ratio of the transmission loss increase value of each wavelength due to hydrogen gas is unique and constant, the ratio of the transmission loss increase value of each wavelength due to hydrogen gas between each connection box is By taking into account the initial loss of each optical fiber, the presence or absence of hydrogen gas generation is detected, and only the section (between adjacent junction boxes) where the transmission loss factor is the hydrogen absorption loss is specified. However, the initial loss of the optical fiber means sound loss such as connection loss and transmission loss of the optical fiber itself, and bending loss.

  For example, when light having a wavelength of 1.31 μm and light having a wavelength of 1.24 μm are used for the measurement, the transmission loss (dB / km) of light having a wavelength of 1.31 μm is plotted on the horizontal axis, and light having a wavelength of 1.24 μm is transmitted. When the loss (dB / km) is taken on the vertical axis, the measurement result of the actual line as shown in FIG. 2 is obtained. However, each transmission loss is an average value of transmission loss in each junction box section.

  As shown in FIG. 2, when each transmission loss is about 1 to 7 dB / km in the light with a wavelength of 1.24 μm and about 0.38 to 0.58 dB / km in the light with a wavelength of 1.31 μm, As described above, when the vicinity of the characteristic line 21 having a large inclination is reached, since each transmission loss is greatly different, it is considered that a hydrogen absorption loss (× in the figure) occurs, and hydrogen gas stays in the section. Is determined.

  On the other hand, when the transmission loss of both the light with a wavelength of 1.24 μm and the light with a wavelength of 1.31 μm are almost the same and are concentrated in the vicinity of about 0.36 to 0.4 dB / km (◯ in the figure). Indicates that the transmission loss is almost the same regardless of the location, so it is regarded as the transmission loss (sound loss) of the optical fiber itself, and hydrogen gas stagnation and other factors occur in the section. Judge that it is not.

  Further, when the transmission loss of light having a wavelength of 1.24 μm and light having a wavelength of 1.31 μm is in the vicinity of the characteristic line 22 having a small inclination as in the case of about 0.37 to 0.52 dB / km (in the drawing) ), Transmission loss differs depending on the location, but since there is a difference in loss that does not reach the ratio of hydrogen absorption loss, it is considered that it is not hydrogen absorption loss, for example, bending loss. Although there is no stagnation of hydrogen gas in the section, it is determined that some loss has occurred.

  The same applies when light having a wavelength of 1.31 μm and light having a wavelength of 1.625 μm are used. That is, as shown in FIG. 3, each transmission loss is about 0.6 to 2.7 dB / km for light with a wavelength of 1.625 μm and about 0.38 to 0.62 dB for light with a wavelength of 1.31 μm. When it is near the characteristic line 31 with a large inclination as in the case of / km, it is determined that hydrogen absorption loss has occurred, and it is determined that hydrogen gas is retained in the section.

  On the other hand, when the loss values of both the light with a wavelength of 1.625 μm and the light with a wavelength of 1.31 μm are substantially the same and concentrate in the vicinity of about 0.36 to 0.4 dB / km, It is determined that there is no loss due to stagnation of hydrogen gas or other factors in the section.

  In addition, the characteristic line has a small inclination as in the case of about 0.3 to 0.7 dB / km at a wavelength of 1.625 μm and about 0.37 to 0.52 dB / km in light having a wavelength of 1.31 μm. When it is close to 32, it is regarded as a loss that is not a hydrogen absorption loss (for example, a bending loss, etc.), and it is determined that some loss has occurred, although there is no stagnation of hydrogen gas in that section.

  From the above, it is detected whether hydrogen gas is retained in the aluminum pipe of OPGW81, and at the same time, the section where the hydrogen gas is retained (the section where the generated hydrogen gas has diffused) is specified with the accuracy of the junction box interval. Is done.

  Next, a method for specifying the section where hydrogen gas is retained in the aluminum pipe by the above-described method and then identifying where the hydrogen gas is generated in the section will be described.

  In the OPGW 81 in the section where hydrogen gas is generated, as shown in FIG. 5A, a corrosive spot (hydrogen gas supply source) c is formed in the gap a formed by the aluminum tube 82, the spacer 83, and the optical fiber unit. The hydrogen gas h generated from is diffused and filled. The hydrogen gas h is filled in the gap a because the water that has entered the OPGW 81 accumulates in the gap between the bottom of the outer peripheral surface of the aluminum tube 82 and the AC wire 88 due to gravity, and the accumulated water corrodes the aluminum tube 82. This is presumably because the through hole 91 shown in FIG. 9 is formed at the bottom of the aluminum tube 82.

  Here, as an example, the description will be made assuming that the section in which the hydrogen gas is staying is specified between the junction boxes 2a and 2b in FIG.

  As shown in FIG. 4, a purge gas cylinder 41 is connected to one of the connection boxes 2a, and purge gas is supplied (injected) from the connection box 2a into the gap a in the aluminum pipe of the OPGW 81 (see FIG. 8). The purge gas may be anything as long as it does not affect the transmission loss of the measuring light, but is preferably an inert gas, air, etc., preferably nitrogen gas. The pressure to be supplied needs to be equal to or higher than the external pressure, but is preferably about 1 atm. The purge gas is injected from the extension cable hole 123 provided in the side wall 121 of the junction box 2a as described with reference to FIG.

  When purge gas (in this example, the purge gas is nitrogen gas n) is injected, as shown in FIG. 5B, the hydrogen gas h in the gap a is caused to flow into the junction box 2a, After purging out of the aluminum tube 82 between 2b and reducing the hydrogen gas partial pressure in the gap a, the injection of nitrogen gas n is stopped.

  The amount of nitrogen gas n injected is such that hydrogen gas h present in the gap a in the aluminum tube 82 between the junction boxes 2a and 2b can be replaced with nitrogen gas n. Specifically, since the cross-sectional area of the gap a in the aluminum pipe 82 is known, the injection amount is obtained by calculation, and the time for opening the valve of the purge gas cylinder 41 in FIG. 4 is determined based on this injection amount. In addition, when a hydrogen gas concentration sensor is connected to the other connection box 2b and the hydrogen gas concentration in the connection box 2b is monitored with this hydrogen gas concentration sensor, the hydrogen gas is below a predetermined amount or no longer detected. The valve of the purge gas cylinder 41 may be closed.

  Although the amount of nitrogen gas n need not be strictly the amount described above, it is desirable that the amount of nitrogen gas n not be too large or that nitrogen gas n be continuously injected. This is to observe the process of hydrogen gas slowly escaping from the inside of the optical fiber by natural diffusion. That is, to facilitate observation of a change in transmission loss of the optical fiber due to hydrogen gas, which will be described later.

  After stopping the injection of the nitrogen gas n, the measurement light having the wavelength λb (for example, the wavelength of 1.24 μm, or the like) is applied to any one of the optical fiber units of the OPGW 81 by the OTDR 5 from the communication station 3 in FIG. Light with a hydrogen absorption wavelength such as a wavelength of 1.625 μm is transmitted and incident, and the light reception level (transmission loss distribution) of the backscattered light of this light is continuously measured.

  FIG. 6 shows an outline of the characteristics of transmission loss observed in the OTDR measurement in each state of FIG. However, in FIGS. 6A to 6C, the horizontal axis is the position in the longitudinal direction, and the vertical axis is the OTDR light reception level.

  As shown in FIG. 6 (a), the waveform 61a (FIG. 5 (a) and FIG. 5 (b) of the OTDR light reception level obtained by the OTDR measurement immediately after the hydrogen gas h in the gap a is removed (immediately after the measurement). (Corresponding to the measurement result in the state), the loss level is almost the same as before the hydrogen gas h is extracted, and the loss is observed as a whole. This is because the hydrogen gas inside the optical fiber is not immediately purged, and hydrogen gas exists in the optical fiber.

  When the hydrogen gas h in the gap a is expelled and the hydrogen partial pressure in the gap a decreases, the hydrogen gas diffused in the optical fiber gradually diffuses to the outside of the optical fiber, and the light transmission loss decreases (OTDR light reception). Level will increase). This process is referred to herein as a loss reduction process because hydrogen gas escapes and transmission loss decreases.

  At this time, as shown in FIG. 5C, the hydrogen gas h was extracted by the newly generated hydrogen gas hn in the section where the corrosion occurred and became the hydrogen gas supply source (corrosion point c). Since the hydrogen gas partial pressure is higher than that in the healthy section (the white area in the figure), the speed at which the hydrogen gas in the optical fiber escapes to the outside of the optical fiber is slow.

  At this time, as shown in FIG. 6 (b), in the waveform 61b of the OTDR light reception level obtained by OTDR measurement (corresponding to the measurement result in the state of FIG. 5 (c)), the speed at which the loss is reduced is low. Since section B in which the slope of the loss (OTDR light reception level) distribution remains large is observed, this section B is a section having a factor of increasing the hydrogen gas partial pressure, such as newly generating hydrogen gas hn. I understand. On the other hand, in the range other than the section B of the waveform 61b of the OTDR light reception level obtained by the OTDR measurement (section having a factor that increases the hydrogen gas partial pressure), the waveform of the OTDR light reception level obtained by the OTDR measurement of FIG. Loss is reduced overall compared to 61a.

  Therefore, by continuously monitoring the transmission loss (OTDR received light level) distribution with OTDR5 and examining section B where the loss is slow to decrease, the OPGW81 aluminum is narrower in the narrower range between the junction boxes 2a and 2b in FIG. It is possible to identify the corrosion site of the pipe.

  Thereafter, when a predetermined period elapses, as shown in FIG. 5D, the hydrogen gas hn newly generated from the corrosion location c diffuses in the longitudinal direction, and the OPGW 81 returns to the state of FIG. The waveform of the OTDR light reception level obtained by the OTDR measurement corresponding to FIG. 5D is the waveform 61a of the OTDR light reception level obtained by the OTDR measurement as shown in FIG.

  Here, the present inventors roughly estimated the distance of the corrosion section that can be specified by this method using light having a wavelength of 1.24 μm.

  When the hydrogen gas partial pressure is 1 atm, the hydrogen absorption loss of light having a wavelength of 1.24 μm is 8 dB / km (see FIG. 14 and Non-Patent Document 1). However, since the observation is performed in the process of loss of hydrogen gas (loss reduction process), assuming that the loss at a hydrogen gas partial pressure of about 1/4 atm, that is, about 2 dB / km should be observed, Since the measurement accuracy of the transmission loss of the OTDR used in general is about 0.1 dB, at present, it cannot be detected unless a loss of 0.2 dB or more occurs in the entire corrosion section. Therefore, since (0.2 dB) ÷ (2 dB / km) = 100 m, the identifiable corrosion section length is 100 m or more.

  As described above, in the method according to the present embodiment, first, the connection boxes 2a and 2b between which the hydrogen gas h is considered to be retained due to corrosion of the aluminum pipe 82 of the OPGW 81 are specified by OTDR5. Next, the hydrogen gas h in the aluminum pipe 82 between the junction boxes 2a and 2b is purged with a purge gas, and then the change in the optical fiber transmission loss due to the hydrogen gas is continuously observed between the junction boxes 2a and 2b by OTDR5. Then, a section in which the hydrogen gas by the newly generated hydrogen gas hn is regarded as staying in the aluminum pipe 82 is detected.

  Thereby, according to the method according to the present embodiment, it is possible to specify the corrosion location of OPGW, which has conventionally been specified only with an accuracy between junction boxes, with an accuracy (about 100 m) equal to or less than the steel tower span t. Therefore, for example, it can be specified with high accuracy that the aluminum pipe 82 of the OPGW 81 is corroded between the steel towers 1a and 1b between the junction boxes 2a and 2b of FIG.

  Further, since the purge gas simply drives out the hydrogen gas h in the aluminum tube between the junction boxes, the OPGW is divided as in the airtight measurement method described in the background art. Since there is no need to make it non-flowing, there is no need to stop the communication line or cut the optical fiber, the measurement cost is low, and the work time is short.

  When the connection box 2a, 2b is specified by OTDR5, light having a wavelength of 1.24 μm that causes absorption loss due to hydrogen gas, or light having a wavelength in the 1.6 to 1.7 μm band, and wavelength 1 that is not easily absorbed by hydrogen gas Because light of at least two wavelengths with light of .31 μm is used, in the case of light of a specific wavelength (for example, light of wavelength 1.24 μm) in which initial corrosion, which has been difficult in the past, or a large amount of hydrogen absorption loss occurs. Even if the loss amount is large, the measurement light of OTDR5 is attenuated without reaching far, and it is impossible to evaluate the total loss between each junction box, the corrosion location of OPGW81 can be identified efficiently by selecting the wavelength. it can.

  Thereafter, when detecting a section in which the hydrogen gas generated by the hydrogen gas hn newly generated in the aluminum pipe 82 is regarded as staying, light having a wavelength of 1.24 μm causing absorption loss due to the hydrogen gas, or 1.6 to Since light having a wavelength in the 1.7 μm band is used, the corrosion location of OPGW81 can be specified with high accuracy.

  Further, in general, in the OPGW, the outer diameter of the spacer 83 is slightly smaller than the inner diameter of the aluminum tube 82 as in the OPGW 81 in FIG. 8, and any one of the optical fiber units 87 is provided due to the gap a. If the optical fiber 86 is used for measurement, the hydrogen gas h and the newly generated hydrogen gas hn can be detected without measuring all the optical fibers 86.

  Since the OTDR measurement focuses on the ratio of the transmission loss value for each wavelength, the evaluation result is not easily influenced by the magnitude of the transmission loss. Therefore, it is possible to cope with the initial stage of corrosion with a relatively small loss increase, and it can be used as a constant monitoring method for OPGW.

  In addition, when specifying the connection box by OTDR, the initial loss of the optical fiber for each connection box is added to the ratio of the transmission loss increase value of each wavelength by the hydrogen gas for each connection box, so that The presence or absence of gas generation can be detected with higher accuracy, and only the section (between adjacent junction boxes) where the transmission loss factor is the hydrogen absorption loss can be specified.

  Furthermore, according to the present invention, the corrosion location (corrosion zone, corrosion range) of OPGW can be specified with high accuracy, so that the zone where OPGW is refurbished (replaced) can be shortened, and the repair cost can be reduced and the construction period can be shortened. It becomes.

  The purge gas is not limited to the nitrogen gas n because it only needs to achieve the purpose of lowering the partial pressure of the hydrogen gas in the aluminum pipe between the specified junction boxes. Any type of gas may be used as long as it does not increase loss.

  The above-described method is described as a method of observing the loss reduction process in which hydrogen gas escapes and transmission loss decreases, but a large amount of purge gas is injected into the aluminum tube between the specified junction boxes, and the aluminum tube (optical fiber) The hydrogen gas h (including the inside) is sufficiently expelled, the purge gas injection is stopped, and the generation of hydrogen gas hn newly generated again (a process in which transmission loss increases: hereinafter referred to as “loss increasing process”). ) May be used. However, the method of observing the loss reduction process is preferable for shortening the measurement time.

  This is because in the method of observing the loss reduction process, the rate of change in transmission loss (hereinafter referred to as “phenomenon rate”) is determined by the rate at which hydrogen gas diffuses from the inside of the optical fiber (diffusion constant). This is because the corrosion position can be specified by observation for about one week regardless of the degree of corrosion. On the other hand, in the loss increasing process, it is necessary to wait for an increase in hydrogen gas as the corrosion progresses, and this phenomenon speed is considerably slower than in the loss decreasing process. Moreover, since the phenomenon speed depends on the degree of corrosion, the necessary observation period cannot be predicted in advance.

  An OPGW (OP unit) 81 having a length of 100 m was used as a sample, and an aluminum tube 82 having a length of 26 m at the center was corroded by injecting dilute hydrochloric acid. By the method according to the present embodiment described above, a predetermined amount of nitrogen gas n is injected into the sample aluminum tube 82 as a purge gas, and the change in transmission loss after the injection is stopped is measured using light having a wavelength of 1.24 μm. Observed continuously. The experimental results are shown in FIG. However, in FIG. 7, the horizontal axis is the distance, and the vertical axis is the OTDR light reception level. In addition, the mesh part of FIG. 7 is a section of the corrosion range in which the aluminum tube 82 is disassembled after the measurement is finished and the corrosion is visually confirmed.

  As shown in FIG. 7, the waveform 71 (thin line in the figure) obtained by the OTDR measurement before nitrogen gas injection, and the average line 71a (the one-dot chain line in the figure) are generally observed to have a loss ( FIG. 5 (a) corresponds to FIG. 6 (a). Waveform 72 (thick dotted line in the figure) obtained by OTDR measurement after 1.8 days after the purge gas injection is stopped is just after hydrogen gas in the aluminum pipe has been purged (displaced), so transmission before and after purging hydrogen gas The level of loss hardly changes (state of FIG. 5B).

  However, in the waveform 73 (thick line in the figure) obtained by OTDR measurement after 8.8 days, which is about one week after the stop of the purge gas injection, and the average line 73a (two-dot chain line in the figure), the waveform 71 and Compared with the average line 71a, the transmission loss is reduced (the state of FIG. 5C, corresponding to FIG. 6B), and the transmission loss is slow to decrease (the inclination of the average line of the waveform remains large). A certain part, that is, a part where transmission loss remains large can be observed. Therefore, the portion where the transmission loss remains large is identified as the corrosion range (corrosion section). This result coincided with the section of the corrosion range in which the aluminum tube 82 was disassembled after the measurement was finished and visually confirmed that it was corroded, and the effectiveness of the present invention was verified.

  As described above, in the specific description of the measurement, the OTDR is taken as an example of a measuring instrument for measuring the transmission loss distribution of the optical fiber, but it can be measured in the same manner as long as it is a method of measuring the transmission loss distribution by light. The measuring instrument is not limited to OTDR alone.

  In the above-described embodiment, the method for identifying the corrosion site of OPGW as a structure has been described. However, the present invention is not limited to OPGW, and any power transmission line or communication cable having a configuration in which an optical fiber is accommodated in an aluminum tube is used. It can also be applied to methods for identifying corroded areas in facilities that have optical fibers arranged inside an aluminum tube as a structure, such as various cables included, and conversely, aluminum tubes with optical fibers arranged for observation. Moreover, if the observation target is not only an aluminum pipe but also hydrogen gas is generated by corrosion, the corrosion location can be specified similarly.

  Furthermore, the optical fiber is stored in a structure (storage container) where hydrogen gas is present in the surrounding atmosphere and the partial pressure of hydrogen gas is usually low, and the storage container is abnormal (corrosion due to breakage, cracking, melting, etc.). Thus, an abnormal location can be identified in the same way for equipment under circumstances where hydrogen gas is supplied from the outside and the optical fiber is exposed to the hydrogen gas.

It is the schematic which shows 1 process of the identification method of the corrosion location of the structure which is preferable embodiment of this invention. It is a figure which shows an example of the actual track | line measurement result in the process of FIG. It is a figure which shows an example of the actual track | line measurement result in the process of FIG. It is the schematic which shows 1 process of the identification method of the corrosion location of the structure which concerns on this Embodiment. FIG. 5A to FIG. 5D are diagrams schematically showing a longitudinal section of the OPGW in the process of FIG. 6A is a waveform diagram obtained by the OTDR measurement in FIGS. 5A and 5B, FIG. 6B is a waveform diagram obtained by the OTDR measurement in FIG. 5C, and FIG. ) Is a waveform diagram obtained by the OTDR measurement in FIG. It is a wave form diagram obtained by the OTDR measurement in an Example. It is a cross-sectional view which shows an example of OPGW. FIGS. 9A to 9D are diagrams (enlarged view of portion A in FIG. 8) schematically showing the progress of corrosion of OPGW. It is the schematic which shows the track | line structure of OPGW. It is the schematic which shows the airtight measuring method of background art. It is the schematic of an OPGW connection box. It is a figure which shows the wavelength dependence of the optical fiber transmission loss increase by hydrogen gas. It is the schematic which shows the OTDR measuring method using the hydrogen absorption wavelength light of background art. It is a wave form diagram obtained by OTDR measurement which shows an example of the measurement result of FIG.

Explanation of symbols

1 Steel tower 2s, 2a, 2b ... Optical connection box for OPGW 3 Communication station 5 OTDR
81 OPGW (structure)
82 Aluminum pipe (structure)
86 Optical fiber b Connection box interval t Steel tower span n Nitrogen gas (purge gas)
h Hydrogen gas hn Newly generated hydrogen gas

Claims (8)

  A method of housing one or a plurality of optical fibers and specifying a corrosion location of a structure to which hydrogen gas is supplied by corrosion, and entering light having a wavelength for detecting hydrogen gas generated by corrosion into the optical fiber. When the generation of hydrogen gas is detected, purge gas is supplied to the structure in the section where hydrogen gas is generated, and the hydrogen gas in the structure is purged outside the structure. Then, the location where hydrogen gas is generated is identified by detecting the change in the transmission loss generated in the optical fiber when light having a wavelength for detecting hydrogen gas is incident on the optical fiber. How to identify corroded parts.   The detection of the presence or absence of generation of hydrogen gas includes light having a wavelength with a large transmission loss due to hydrogen gas and light having a wavelength with a transmission loss due to hydrogen gas smaller than light having a wavelength with large transmission loss due to hydrogen gas. The method for identifying a corrosion location of a structure according to claim 1, wherein light having at least two wavelengths is used.   The method for identifying a corroded portion of a structure according to claim 1 or 2, wherein light of a hydrogen absorption wavelength is used to detect a change in transmission loss generated in the optical fiber.   The method for identifying a corroded portion of a structure according to claim 1 or 2, wherein only light having a wavelength with a large transmission loss due to hydrogen gas is light having a hydrogen absorption wavelength.   The method for identifying a corrosion location of a structure according to any one of claims 1 to 4, wherein the structure is an aluminum pipe.   The method for identifying a corrosion location of a structure according to any one of claims 1 to 5, wherein the purge gas is air or nitrogen.   The method for identifying a corrosion site of a structure according to any one of claims 1 to 6, wherein the transmission loss is measured using any one of optical fibers housed in the structure. The method for identifying a corroded portion of a structure according to any one of claims 1 to 7, wherein the structure that houses the one or more optical fibers is an optical fiber composite ground wire.

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