WO1997036150A1 - Arrangement for monitoring equipment - Google Patents

Arrangement for monitoring equipment Download PDF

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Publication number
WO1997036150A1
WO1997036150A1 PCT/GB1997/000850 GB9700850W WO9736150A1 WO 1997036150 A1 WO1997036150 A1 WO 1997036150A1 GB 9700850 W GB9700850 W GB 9700850W WO 9736150 A1 WO9736150 A1 WO 9736150A1
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WO
WIPO (PCT)
Prior art keywords
optical fibre
temperature
arrangement
fibres
primary optical
Prior art date
Application number
PCT/GB1997/000850
Other languages
French (fr)
Inventor
Michael George Harrop
Original Assignee
Bicc Public Limited Company
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Bicc Public Limited Company filed Critical Bicc Public Limited Company
Priority to AU21693/97A priority Critical patent/AU2169397A/en
Publication of WO1997036150A1 publication Critical patent/WO1997036150A1/en

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
    • G01M3/00—Investigating fluid-tightness of structures
    • G01M3/002—Investigating fluid-tightness of structures by using thermal means
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
    • G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
    • G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
    • G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
    • G01D5/353—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre
    • G01D5/35383—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using multiple sensor devices using multiplexing techniques
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01K—MEASURING TEMPERATURE; MEASURING QUANTITY OF HEAT; THERMALLY-SENSITIVE ELEMENTS NOT OTHERWISE PROVIDED FOR
    • G01K11/00—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00
    • G01K11/32—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00 using changes in transmittance, scattering or luminescence in optical fibres

Definitions

  • This invention relates to the monitoring of equipment, and especially to the monitoring of the strain or the temperature of equipment by optical methods.
  • optical fibres to monitor the temperature of equip ⁇ ment by time domain reflectometry, which involves sending a pulse of light into one end of the fibre and observing the backscattered light at that end of the fibre as a func ⁇ tion of time.
  • Certain properties of the backscattered light are highly dependent on the temperature of the part of the fibre at which the light was backscattered, such as the intensity of the Raman anti-Stokes lines, so that, for example as described in UK patent application No. 2,140,554, by measuring the intensity of those lines, or by measuring the ratio of the intensity of the Raman anti-Stokes lines to the intensity of the Raman Stokes lines, as a function of time from the launch of the light pulse, a temperature profile along the optical fibre can be obtained.
  • ⁇ 2nd where ⁇ is the wavelength of the light, n is the refractive index of the fibre core, and d is the spacing of the lines in the grating.
  • a strain gauge for an engineering structure comprises:
  • the fibre is preferably looped around the supports a plurality of times so that the change in length of the optical fibre as a result of strain of the structure is greater than the change in separation of the supports.
  • the fibre(s) preferably contain Bragg gratings, either in that part of the fibre that will be stretched by the strain in the structure, or at both ends of that part so that the separation between the Bragg gratings will change with the strain in the structure. If desired fibres may be arranged in two orthogonal directions over the surface in order to obtain information about the strain in those directions, and/or in order to provid e temperature compensation.
  • one or more fibres such as a pair r adjacent fibres located in orthogonal direc- tions, may be subject to the strain of the surface, while one or more other fibres, e.g. another pair of orthogonal fibres, may be subject only to temperature variations and will provide temperature compensation.
  • Such a system may be provided as a package located in a housing that is welded to the surface.
  • an arrangement for monitoring the strain or temperature at different positions of a system that is at ele- vated temperature which comprises a primary optical fibre that extends along those parts of the system whose temperature or strain is to be monitored, means for launch ⁇ ing light into the primary optical fibre and means for detecting backscattered light exit ⁇ ing from the primary optical fibre, thermal insulation located between the optical fibre and the system so that the temperature of the primary optical fibre is lower than the temperature of the locations of the system, and a plurality of secondary optical fibres each of which is connected to the primary optical fibre at a different location on the primary optical fibre and extends through the insulation to one of the positions of the system whose strain or temperature is to be monitored, each of the secondary optical fibres having a reflector whose reflection is strain or temperature dependent, and each secondary optical fibre being protected against the temperature of the system.
  • the arrangement according to the present invention has the advantage that the bulk of the optical fibre that is employed, i.e. the primary optical fibre, need not be protected against the effects of the temperature of the system that is to be monitored, and so can be of relatively low cost, while only those parts of the arrangement that see the temperatures need employ relatively high temperature fibres.
  • a standard optical fibre cable can be laid throughout a building that houses the equip ⁇ ment to be monitored, and specific high temperature optical fibre sensors can be tapped into the optical fibre cable at the desired positions.
  • One application for which the ar ⁇ rangement according to the invention is particularly suitable is in the strain monitoring of hot pipes, for example steam pipes.
  • the primary optical fibre can be laid along the steam pipe outside the insulation or even spaced from the pipe by a significant distance in order to ensure that it does not experience any temperature that is so high that it would be damaged, and secondary optical fibre spurs extend through the insulation at a num- ber of locations spaced apart from one another along the length of the steam pipe into contact with the pipe.
  • optical fibre that is used for the spur will depend on the tem ⁇ perature that it is expected to experience during operation. If it is expected to experi ⁇ ence temperatures in the range of from 100°C to 800°C, it is preferably formed with a temperature resistant coating, for example a metal coating (e.g. gold or copper), a ce ⁇ ramic doped metal coating, or a carbon or polyamide coating, that is deposited directly on the fibre cladding, and does not include any plastics jacket. Such coatings may be formed by evaporation, sputtering or any other appropriate method.
  • a metal coating e.g. gold or copper
  • ce ⁇ ramic doped metal coating e.g. gold or copper
  • carbon or polyamide coating e.g. carbon or polyamide coating
  • the secondary optical fibres may be tapped into the primary optical fibre in the simplest arrangement simply by means of a number of couplers.
  • 3dB couplers it may be possible to employ 3dB couplers, although it is preferred for a lower intensity of light to be coupled into the secondary optical fibres since the use of 3dB couplers will significantly reduce the intensity of light in the principal optical fibre at each coupling point.
  • the primary optical fibre may be formed with a number of stubs that are spliced into the fibre along its length and to which the secondary optical fibres can be connected or spliced at whichever positions are desired.
  • the primary optical fibre can be provided at various locations along its length with surface flats so that it has a substantially "D"-shaped cross-section at those locations, and the ends of the secondary fibres are also provided with corresponding flats.
  • This enables the secondary fibres to be coupled to the primary optical fibre by means of so-called D- fibre couplers or evanescent couplers, for example as described in UK patent applica ⁇ tions Nos. 2,238,398 A and 2,242,754 A.
  • D- fibre couplers or evanescent couplers for example as described in UK patent applica ⁇ tions Nos. 2,238,398 A and 2,242,754 A.
  • the secondary optical fibres form part of a strain gauge comprising a plurality of supports for the optical fibre that are located on a surface of the system and are spaced apart from one another over a part of the surface, and the optical fibre is looped around the supports so that it extends between the supports, the optical fibre being fixed to the supports so that the length of the part of the fibre extending between the supports will vary in accordance with strain of the surface of the structure.
  • a strain gauge is described in our copending UK patent application No. 9521957.2.
  • This strain gauge has the advantage that it inco ⁇ orates a length of optical fibre that is significantly greater than the dimension of the area of the structure that is being monitored.
  • the optical fibre or fibres will normally contain one or more reflectors so that light will be caused to pass in both directions along that part of the optical fibre extend- ing between the supports.
  • the increase in length may be measured by a reflectometry method in which light is sent along the fibre and reflected back to a detector and changes in the length of the fibre alter the time taken before the light is detected at the detector.
  • a detector may be formed by a mirror, a Bragg grating formed in the fibre, or even, in the broadest aspect of the invention, simply a cleaved end of the fibre.
  • the reflector, and any additional elements that may be present can be located at a position remote from the supports, so that if the structure to be monitored is subjected to very high temperatures or is otherwise located in a hostile environment, only that part of the or each optical fibre that is looped around the supports need be located in that environment.
  • the optical fibre may contain a strain-sensitive reflector such as a Bragg grating in that part of the fibre that extends between the supports.
  • a Bragg grating the spacing of the grating will vary in accordance with strain of the surface.
  • light of a broad wavelength spectrum could be launched into the optical fibre and the wavelength of the reflected light would vary in accordance with the strain of the surface.
  • a Bragg grating whose grating spacing varies along its length and to launch monochromatic light into the optical fibre.
  • the position along the optical fibre at which the grating spacing matches the light wavelength will vary with the strain on the surface and the path length of the light will change accordingly.
  • Bragg gratings may be formed by exposing the optical fibre to beams of ultra ⁇ violet radiation that vary in intensity or which interfere with one another so as to gen ⁇ erate a periodic variation of refractive index of the fibre core along its length.
  • the gratings may be formed by a number of methods, for example by a light induced method as described in US patent No. 4,474,427, a two-beam interferometry method as described in international patent application No. WO 86/01303 or a phase mask method as described in US patent No. 5,367,588, the disclosures of which are inco ⁇ o ⁇ rated herein by reference.
  • the optical fibre is coated with a temperature resistant layer in order to protect it from the temperature it will experience, it will be necessary to form the Bragg grating in the fibre before deposition of the coating.
  • any reflectometry method may be employed to obtain an indication of the strain or temperature of the system.
  • the arrangement may employ a time- domain reflectometry (OTDR) method in which pulses of light are launched into the primary optical fibre, and the backscattered light is analysed as a function of time.
  • OTDR time- domain reflectometry
  • OFDR frequency-domain approach
  • a correlation OTDR method can be em ⁇ ployed in which a pseudo random sequence of pulses is launched into the optical fibre.
  • the reflected signal can be logically ANDed with a time delayed version of the input sequence to generate an autocorrelation function of the signal for each reflector.
  • the reflectors can be polled in rotation by setting the time delays of the input sequence to match the zero phase difference peak corresponding to each reflector.
  • the primary optical fibre is interrupted at the said different locations thereon, and each secondary optical fibre is connected to each free end of the primary optical fibre to form a loop.
  • the secondary optical fibres will be spliced into the primary optical fibre, but other methods of connection such as an evanescent coupler could still be employed.
  • Figure 1 is a schematic representation of part of a high temperature steam pipe that is monitored by such an arrangement
  • Figure 2 is a schematic representation of part of the system of figure 1 on a larger scale; and Figure 3 is a schematic representation of part of an alternative system shown on the same scale as figure 2.
  • a steam pipe 1 is employed in an electrical power station to carry superheated steam at about 600°C from the boiler to the turbine, and typically comprises a seam welded steel pipe of diameter in the range of from 0.1 to lm surrounded by insulation 2 of thickness in the range of from 0.1 to lm. Strain on the pipe 1 is sensed at a number of points 3, 4, 5 and 6 by means of an optical backscattering arrangement comprising an optical fibre cable 10 that extends along the length of interest of the pipe 1 and has a number of spur fibres 12, 14, 16 and 18, each such fibre extending from the fibre 10 to the pipe 1 at one of the points of in ⁇ terest, through the insulation surrounding the pipe 1 to the pipe itself.
  • the optical ca ⁇ ble 10 comprises one or more optical fibres that are coated with conventional acrylate coatings that are capable of being subjected to temperatures in the region of 80 to 100°C, while spur fibres 12, 14, 16 and 18 are formed with gold coatings that are ca- pable of being exposed to temperatures in excess of 600°C.
  • the spur fibres 12, 14, 16 and 18 may be connected to the fibre cable 10 by standard 3dB couplers, D-fibre taps, splices or any other means.
  • the spur fibres 12, 14, 16 and 18 may be located within protective tubings if desired, for example formed from steel, in order to protect the fibres from mechanical abuse between the optical cable and the pipe, and may be bonded directly to the pipe, or may be attached to the pipe by means of an arrangement as described in our co ⁇ pending British application No. 9521957.2 mentioned above. If such an arrangement is employed, it is possible for one of the protuberances about which the fibre is looped to extend radially outwardly from the pipe 1 through the insulation and to the optical cable 10.
  • the optical fibre 10 is connected to a pulsed laser 20 which sends a number of interrogation pulses 22 along the fibre 10 and thence to the spur fi- bres 12, 14, 16 and 18 where they are reflected by the Bragg gratings and returned to a detector 24.
  • the detected signal is then passed to a standard microcomputer in whose memory is stored a reference reflection signal of the system against which the reflected signal can be compared. If strain is being monitored, the reference reflection signal may comprise a map of the intensity of the signal at one or more defined wavelengths corresponding to the reflected wavelengths of the Bragg gratings.
  • one of the spur fibres 18 is passed from the optical cable 10 to the pipe 1 through the insulation 2 within a stainless steel tube 30 to the surface of the pipe 1 where it extends into a housing 32 located on the surface of the pipe 1 inside which the fibre 18 is looped a number of times around a pair of protuberances whose separation will change in accordance with the strain experienced by the pipe 1.
  • Figure 3 shows an altemative arrangement for monitoring strain in a steam pipe 1 by means of a primary optical fibre 10, and a number of secondary optical fi- bres, one of which, fibre 30 is shown.
  • the primary optical fibre is interrupted at each of the different locations at which the pressure is to be measured, and a length of the secondary fibre is spliced into the pri ⁇ mary optical fibre to form a loop, so that the initial and reflected signals must pass through the looped secondary fibres.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Length Measuring Devices By Optical Means (AREA)
  • Optical Transform (AREA)

Abstract

An arrangement for monitoring the strain or temperature at different positions of a system that is at elevated temperature, comprises a primary optical fibre (10) that extends along those parts of the system whose temperature or strain is to be monitored, means (20) for launching light into the primary optical fibre and means (24) for detecting backscattered light exiting from the primary optical fibre. Thermal insulation (2) is located between the optical fibre and the system so that the temperature of the primary optical fibre is lower than the temperature of the locations of the system, and plurality of secondary or spur optical fibres (12, 14, 16, 18) each of which is connected to the primary optical fibre at a different location on the primary optical fibre extends though the insulation to one of the positions of the system whose strain or temperature is to be monitored. Each of the secondary optical fibres has a reflector whose reflection is strain or temperature dependent, and is protected against the temperature of thesystem, for example by being located within a protective steel tubing.

Description

ARRANGEMENT FOR MONITORING EQUIPMENT
This invention relates to the monitoring of equipment, and especially to the monitoring of the strain or the temperature of equipment by optical methods.
It has been proposed to use optical fibres to monitor the temperature of equip¬ ment by time domain reflectometry, which involves sending a pulse of light into one end of the fibre and observing the backscattered light at that end of the fibre as a func¬ tion of time. Certain properties of the backscattered light are highly dependent on the temperature of the part of the fibre at which the light was backscattered, such as the intensity of the Raman anti-Stokes lines, so that, for example as described in UK patent application No. 2,140,554, by measuring the intensity of those lines, or by measuring the ratio of the intensity of the Raman anti-Stokes lines to the intensity of the Raman Stokes lines, as a function of time from the launch of the light pulse, a temperature profile along the optical fibre can be obtained.
It has also been proposed in UK patent application No.2,281,618 A to employ Bragg gratings in the optical fibre to measure the temperature or strain at specific points along it. The presence of a Bragg grating in the optical fibre will cause interfer¬ ence of light impinging on the grating if the wavelength of the light is related to the spacing of the grating by the Bragg formula:
λ = 2nd where λ is the wavelength of the light, n is the refractive index of the fibre core, and d is the spacing of the lines in the grating.
Thus, a portion of the light will be backscattered from each Bragg grating provided that the grating has the appropriate spacing. If the spacing of any grating is changed by thermal expansion of the optical fibre, the intensity of the backscattered light will be affected. The Bragg gratings need not be used solely for monitoring temperature, but instead can be employed to monitor strain, since any strain in the equipment that is coupled to the fibre will cause the spacing of the gratings to change. An example of the use of Bragg gratings as strain gauges is given in our copending UK patent appli- cation No. 9521957.2, the disclosure of which is incoφorated herein by reference. In that application, a strain gauge for an engineering structure comprises:
a) a plurality of supports for an optical fibre that are, or can be, located on the sur¬ face of the structure and are spaced apart from one another over a part of the surface, and
b) at least one optical fibre that is looped around the supports so that it extends between the supports, the optical fibre being fixed to the supports so that the part of the fibre extending between the supports will vary in accordance with the strain of the surface of the structure.
The fibre is preferably looped around the supports a plurality of times so that the change in length of the optical fibre as a result of strain of the structure is greater than the change in separation of the supports. Also, the fibre(s) preferably contain Bragg gratings, either in that part of the fibre that will be stretched by the strain in the structure, or at both ends of that part so that the separation between the Bragg gratings will change with the strain in the structure. If desired fibres may be arranged in two orthogonal directions over the surface in order to obtain information about the strain in those directions, and/or in order to provide temperature compensation. Thus, for ex¬ ample, one or more fibres, such as a pair r adjacent fibres located in orthogonal direc- tions, may be subject to the strain of the surface, while one or more other fibres, e.g. another pair of orthogonal fibres, may be subject only to temperature variations and will provide temperature compensation. Such a system may be provided as a package located in a housing that is welded to the surface.
Although the use of Bragg gratings in optical fibres for temperature measure¬ ment and strain measurement will work in principle, such systems can only be used in those environments that will not damage the optical fibre.
According to the present invention, there is provided an arrangement for monitoring the strain or temperature at different positions of a system that is at ele- vated temperature, which comprises a primary optical fibre that extends along those parts of the system whose temperature or strain is to be monitored, means for launch¬ ing light into the primary optical fibre and means for detecting backscattered light exit¬ ing from the primary optical fibre, thermal insulation located between the optical fibre and the system so that the temperature of the primary optical fibre is lower than the temperature of the locations of the system, and a plurality of secondary optical fibres each of which is connected to the primary optical fibre at a different location on the primary optical fibre and extends through the insulation to one of the positions of the system whose strain or temperature is to be monitored, each of the secondary optical fibres having a reflector whose reflection is strain or temperature dependent, and each secondary optical fibre being protected against the temperature of the system.
The arrangement according to the present invention has the advantage that the bulk of the optical fibre that is employed, i.e. the primary optical fibre, need not be protected against the effects of the temperature of the system that is to be monitored, and so can be of relatively low cost, while only those parts of the arrangement that see the temperatures need employ relatively high temperature fibres. Thus, for example, a standard optical fibre cable can be laid throughout a building that houses the equip¬ ment to be monitored, and specific high temperature optical fibre sensors can be tapped into the optical fibre cable at the desired positions. One application for which the ar¬ rangement according to the invention is particularly suitable is in the strain monitoring of hot pipes, for example steam pipes. In the field of power generation, superheated steam is transferred from the boiler to the turbine at a temperature of about 600°C along pipes whose strain needs to be monitored in order to ensure that no failure oc¬ curs. According to the invention, the primary optical fibre can be laid along the steam pipe outside the insulation or even spaced from the pipe by a significant distance in order to ensure that it does not experience any temperature that is so high that it would be damaged, and secondary optical fibre spurs extend through the insulation at a num- ber of locations spaced apart from one another along the length of the steam pipe into contact with the pipe.
The nature of the optical fibre that is used for the spur will depend on the tem¬ perature that it is expected to experience during operation. If it is expected to experi¬ ence temperatures in the range of from 100°C to 800°C, it is preferably formed with a temperature resistant coating, for example a metal coating (e.g. gold or copper), a ce¬ ramic doped metal coating, or a carbon or polyamide coating, that is deposited directly on the fibre cladding, and does not include any plastics jacket. Such coatings may be formed by evaporation, sputtering or any other appropriate method.
The secondary optical fibres may be tapped into the primary optical fibre in the simplest arrangement simply by means of a number of couplers. In some instances it may be possible to employ 3dB couplers, although it is preferred for a lower intensity of light to be coupled into the secondary optical fibres since the use of 3dB couplers will significantly reduce the intensity of light in the principal optical fibre at each coupling point. Thus it is preferred for from 2 to 20%, and especially from 5 to 10% of the light to be coupled into the secondary optical fibre at each coupling point. Alter¬ natively, the primary optical fibre may be formed with a number of stubs that are spliced into the fibre along its length and to which the secondary optical fibres can be connected or spliced at whichever positions are desired. In another arrangement, the primary optical fibre can be provided at various locations along its length with surface flats so that it has a substantially "D"-shaped cross-section at those locations, and the ends of the secondary fibres are also provided with corresponding flats. This enables the secondary fibres to be coupled to the primary optical fibre by means of so-called D- fibre couplers or evanescent couplers, for example as described in UK patent applica¬ tions Nos. 2,238,398 A and 2,242,754 A. Using a 5% coupler/tap and 100% reflec¬ tivity gratings, with an input power of 1 mW, and assuming the detector has a dynamic range of -60dBm, the system would allow for 40 sensing points. If more detectors are needed, a number of individual fibres and/or cables can be multiplexed, or alterna- tively, a correlation OTDR method as discussed below may be used.
Preferably the secondary optical fibres form part of a strain gauge comprising a plurality of supports for the optical fibre that are located on a surface of the system and are spaced apart from one another over a part of the surface, and the optical fibre is looped around the supports so that it extends between the supports, the optical fibre being fixed to the supports so that the length of the part of the fibre extending between the supports will vary in accordance with strain of the surface of the structure. Such a form of strain gauge is described in our copending UK patent application No. 9521957.2. This strain gauge has the advantage that it incoφorates a length of optical fibre that is significantly greater than the dimension of the area of the structure that is being monitored. In addition, it is possible by the use of more than one secondary op¬ tical fibre, or by use of more than one reflector, only one of which is subject to strains of the system, to compensate the arrangement for temperature effects.
The optical fibre or fibres will normally contain one or more reflectors so that light will be caused to pass in both directions along that part of the optical fibre extend- ing between the supports. Thus, for example, the increase in length may be measured by a reflectometry method in which light is sent along the fibre and reflected back to a detector and changes in the length of the fibre alter the time taken before the light is detected at the detector. Such a detector may be formed by a mirror, a Bragg grating formed in the fibre, or even, in the broadest aspect of the invention, simply a cleaved end of the fibre. Such arrangements have the advantage that the reflector, and any additional elements that may be present, can be located at a position remote from the supports, so that if the structure to be monitored is subjected to very high temperatures or is otherwise located in a hostile environment, only that part of the or each optical fibre that is looped around the supports need be located in that environment. Alterna¬ tively the optical fibre may contain a strain-sensitive reflector such as a Bragg grating in that part of the fibre that extends between the supports. For example, in the case of a Bragg grating, the spacing of the grating will vary in accordance with strain of the surface. Thus, light of a broad wavelength spectrum could be launched into the optical fibre and the wavelength of the reflected light would vary in accordance with the strain of the surface. Instead, it may be appropriate to employ a Bragg grating whose grating spacing varies along its length and to launch monochromatic light into the optical fibre. In this case the position along the optical fibre at which the grating spacing matches the light wavelength will vary with the strain on the surface and the path length of the light will change accordingly.
Bragg gratings may be formed by exposing the optical fibre to beams of ultra¬ violet radiation that vary in intensity or which interfere with one another so as to gen¬ erate a periodic variation of refractive index of the fibre core along its length. The gratings may be formed by a number of methods, for example by a light induced method as described in US patent No. 4,474,427, a two-beam interferometry method as described in international patent application No. WO 86/01303 or a phase mask method as described in US patent No. 5,367,588, the disclosures of which are incoφo¬ rated herein by reference. Where the optical fibre is coated with a temperature resistant layer in order to protect it from the temperature it will experience, it will be necessary to form the Bragg grating in the fibre before deposition of the coating.
Any reflectometry method may be employed to obtain an indication of the strain or temperature of the system. For example, the arrangement may employ a time- domain reflectometry (OTDR) method in which pulses of light are launched into the primary optical fibre, and the backscattered light is analysed as a function of time. Alternatively, a frequency-domain approach (OFDR) may be used in which the light is modulated with a frequency that increases (i.e. is launched into the fibre in the form of a number of "chiφs") and the reflected signal is mixed with the initial signal to pro¬ duce a signal that has components corresponding to the sum and to the difference of the modulating frequencies of the initial and reflected signal, the signal is filtered to remove those components corresponding to the summation of the frequencies, a signal containing information of the reflecting system can be obtained. If it is desired to im- prove the dynamic range of the arrangement, a correlation OTDR method can be em¬ ployed in which a pseudo random sequence of pulses is launched into the optical fibre. The reflected signal can be logically ANDed with a time delayed version of the input sequence to generate an autocorrelation function of the signal for each reflector. The reflectors can be polled in rotation by setting the time delays of the input sequence to match the zero phase difference peak corresponding to each reflector.
In an alternative arrangement according to the invention, the primary optical fibre is interrupted at the said different locations thereon, and each secondary optical fibre is connected to each free end of the primary optical fibre to form a loop. Nor¬ mally, the secondary optical fibres will be spliced into the primary optical fibre, but other methods of connection such as an evanescent coupler could still be employed.
One arrangement in accordance with the present invention will now be de¬ scribed by way of example with reference to the accompanying drawings, in which:
Figure 1 is a schematic representation of part ofa high temperature steam pipe that is monitored by such an arrangement;
Figure 2 is a schematic representation of part of the system of figure 1 on a larger scale; and Figure 3 is a schematic representation of part of an alternative system shown on the same scale as figure 2.
Referring to the accompanying drawings, a steam pipe 1 is employed in an electrical power station to carry superheated steam at about 600°C from the boiler to the turbine, and typically comprises a seam welded steel pipe of diameter in the range of from 0.1 to lm surrounded by insulation 2 of thickness in the range of from 0.1 to lm. Strain on the pipe 1 is sensed at a number of points 3, 4, 5 and 6 by means of an optical backscattering arrangement comprising an optical fibre cable 10 that extends along the length of interest of the pipe 1 and has a number of spur fibres 12, 14, 16 and 18, each such fibre extending from the fibre 10 to the pipe 1 at one of the points of in¬ terest, through the insulation surrounding the pipe 1 to the pipe itself. The optical ca¬ ble 10 comprises one or more optical fibres that are coated with conventional acrylate coatings that are capable of being subjected to temperatures in the region of 80 to 100°C, while spur fibres 12, 14, 16 and 18 are formed with gold coatings that are ca- pable of being exposed to temperatures in excess of 600°C. The spur fibres 12, 14, 16 and 18 may be connected to the fibre cable 10 by standard 3dB couplers, D-fibre taps, splices or any other means.
The spur fibres 12, 14, 16 and 18 may be located within protective tubings if desired, for example formed from steel, in order to protect the fibres from mechanical abuse between the optical cable and the pipe, and may be bonded directly to the pipe, or may be attached to the pipe by means of an arrangement as described in our co¬ pending British application No. 9521957.2 mentioned above. If such an arrangement is employed, it is possible for one of the protuberances about which the fibre is looped to extend radially outwardly from the pipe 1 through the insulation and to the optical cable 10.
At one end thereof, the optical fibre 10 is connected to a pulsed laser 20 which sends a number of interrogation pulses 22 along the fibre 10 and thence to the spur fi- bres 12, 14, 16 and 18 where they are reflected by the Bragg gratings and returned to a detector 24. The detected signal is then passed to a standard microcomputer in whose memory is stored a reference reflection signal of the system against which the reflected signal can be compared. If strain is being monitored, the reference reflection signal may comprise a map of the intensity of the signal at one or more defined wavelengths corresponding to the reflected wavelengths of the Bragg gratings.
In the arrangement shown in figure 2, one of the spur fibres 18 is passed from the optical cable 10 to the pipe 1 through the insulation 2 within a stainless steel tube 30 to the surface of the pipe 1 where it extends into a housing 32 located on the surface of the pipe 1 inside which the fibre 18 is looped a number of times around a pair of protuberances whose separation will change in accordance with the strain experienced by the pipe 1.
Figure 3 shows an altemative arrangement for monitoring strain in a steam pipe 1 by means of a primary optical fibre 10, and a number of secondary optical fi- bres, one of which, fibre 30 is shown. In this arrangement, instead of the secondary optical fibres 30 being connected to the primary optical fibre 10 in the form of spurs, the primary optical fibre is interrupted at each of the different locations at which the pressure is to be measured, and a length of the secondary fibre is spliced into the pri¬ mary optical fibre to form a loop, so that the initial and reflected signals must pass through the looped secondary fibres.

Claims

CLAIMS:
1. An arrangement for monitoring the strain or temperature at different positions of a system that is at elevated temperature, which comprises a primary optical fibre that extends along those parts of the system whose temperature or strain is to be monitored, means for launching light into the primary optical fibre and means for detecting backscattered light exiting from the primary optical fibre, thermal insulation located between the optical fibre and the system so that the temperature of the primary optical fibre is lower than the temperature of the locations of the system, and a plurality of secondary optical fibres each of which is connected to the primary optical fibre at a different location on the primary optical fibre and extends through the insulation to one of the positions of the system whose strain or temperature is to be monitored, each of the secondary optical fibres having a reflector whose reflection is strain or temperature dependent, and each secondary optical fibre being protected against the temperature of the system.
2. An arrangement as claimed in claim 1, wherein the secondary fibres include one or more Bragg gratings as its reflector.
3. An arrangement as claimed in claim 1 or claim 2, wherein the secondary fibres are provided with a temperature resistant coating.
4. An arrangement as claimed in claim 3, wherein the temperature resistant coat- ing comprises a metal coating, a ceramic doped metal coating, a carbon coating or a polyamide coating.
5. An arrangement as claimed in any one of claims 1 to 4, wherein the secondary fibres are connected to the primary fibre by means of D-fibre couplers.
6. An aπangement as claimed in any one of claims 1 to 5, wherein from 2 to 20% of the light in the primary optical fibre to be coupled into the secondary optical fibre at each coupling point.
7. An arrangement as claimed in any one of claims 1 to 6, which contains upto 40 secondary fibres.
8. An arrangement as claimed in any one of claims 1 to 6, which contains more than 40 secondary fibres, and contains a plurality of primary optical fibres that are multiplexed, or employs a correlation reflectometry method.
9. An arrangement as claimed in any one of claims 1 to 8, wherein the secondary optical fibres extend from the primary optical fibre in the form of a plurality of spurs.
10. An arrangement as claimed in any one of claims 1 to 8, wherein the primary optical fibre is interrupted at the said different locations thereon, and each secondary optical fibre is connected to each free end of the primary optical fibre to form a loop.
PCT/GB1997/000850 1996-03-28 1997-03-26 Arrangement for monitoring equipment WO1997036150A1 (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU21693/97A AU2169397A (en) 1996-03-28 1997-03-26 Arrangement for monitoring equipment

Applications Claiming Priority (2)

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GBGB9606498.5A GB9606498D0 (en) 1996-03-28 1996-03-28 Arrangement for monitoring equipment
GB9606498.5 1996-03-28

Publications (1)

Publication Number Publication Date
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Cited By (5)

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WO2006102259A2 (en) 2005-03-23 2006-09-28 Shell Internationale Research Maatschappij B.V. Underwater structure monitoring systems and methods
WO2007110574A1 (en) * 2006-03-25 2007-10-04 Qinetiq Limited Fibre optic sensor arra
WO2009068095A1 (en) * 2007-11-29 2009-06-04 Prysmian Cables & Systems Limited A device for applying a fiber-optic monitoring system to a component to be monitored
WO2009068094A1 (en) * 2007-11-29 2009-06-04 Prysmian S.P.A. Method and system for fiber-optic monitoring of spatially distributed components
WO2009115422A1 (en) * 2008-03-18 2009-09-24 Siemens Aktiengesellschaft Device for monitoring tanks on a ship

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Publication number Priority date Publication date Assignee Title
EP0326199A2 (en) * 1988-01-29 1989-08-02 Plessey Overseas Limited An optical sensing apparatus
US4950886A (en) * 1989-06-30 1990-08-21 Claus Richard O Partially reflecting optical fiber splice for temperature and strain measurement
US5426297A (en) * 1993-09-27 1995-06-20 United Technologies Corporation Multiplexed Bragg grating sensors

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Publication number Priority date Publication date Assignee Title
EP0326199A2 (en) * 1988-01-29 1989-08-02 Plessey Overseas Limited An optical sensing apparatus
US4950886A (en) * 1989-06-30 1990-08-21 Claus Richard O Partially reflecting optical fiber splice for temperature and strain measurement
US5426297A (en) * 1993-09-27 1995-06-20 United Technologies Corporation Multiplexed Bragg grating sensors

Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2006102259A2 (en) 2005-03-23 2006-09-28 Shell Internationale Research Maatschappij B.V. Underwater structure monitoring systems and methods
WO2007110574A1 (en) * 2006-03-25 2007-10-04 Qinetiq Limited Fibre optic sensor arra
US9599489B2 (en) 2006-03-25 2017-03-21 Optasense Holdings Ltd. Fibre optic sensor array
NO343604B1 (en) * 2006-03-25 2019-04-08 Optasense Holdings Ltd Fiber optic sensor series
WO2009068095A1 (en) * 2007-11-29 2009-06-04 Prysmian Cables & Systems Limited A device for applying a fiber-optic monitoring system to a component to be monitored
WO2009068094A1 (en) * 2007-11-29 2009-06-04 Prysmian S.P.A. Method and system for fiber-optic monitoring of spatially distributed components
US8391653B2 (en) 2007-11-29 2013-03-05 Prysmian Cables & Systems Limited Device for applying a fiber-optic monitoring system to a component to be monitored
US8488114B2 (en) 2007-11-29 2013-07-16 Prysmian S.P.A. Method and system for fiber-optic monitoring of spatially distributed components
WO2009115422A1 (en) * 2008-03-18 2009-09-24 Siemens Aktiengesellschaft Device for monitoring tanks on a ship

Also Published As

Publication number Publication date
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AU2169397A (en) 1997-10-17

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