Patent Publication Number: US-7711217-B2

Title: Active sensor, multipoint active sensor, method for diagnosing deterioration of pipe, and apparatus for diagnosing deterioration of pipe, and apparatus for diagnosis deterioration of pipe

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2007-106025 filed on Apr. 13, 2007, the entire contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an active sensor, a multi-point active sensor, a method of diagnosing deterioration of a pipe, and an apparatus for diagnosing deterioration of a pipe, capable of judging existence of a malfunction such as a pipe wall-thickness reduction caused by a high-temperature steam in an atomic power plant and a heat power plant, and a pipe corrosion in a chemical factory and an incineration plant, and capable of identifying the part having a trouble. 
   2. Description of Related Art 
   A pipe wall-thickness reduction and a pipe corrosion are conventionally inspected on periodic inspections by using an ultrasonic flaw detecting method and an X-ray transmission method. In the ultrasonic flaw detecting method, a probe that transmits and receives ultrasonic waves is brought into contact with a surface of a pipe, for example, and ultrasonic waves of various frequencies are propagated to an inside (pipe wall part) of the pipe. Then, by receiving the ultrasonic waves that have been reflected on a flaw in the pipe wall part of the pipe or a rear surface of the pipe and returned therefrom, a state of the pipe wall part of the pipe can be grasped. 
   A position of the flaw can be obtained by measuring a time period between the transmittance of the ultrasonic waves and the reception thereof. A size of the flaw can be obtained by measuring a height of the received echo (intensity of the ultrasonic waves that have been reflected and returned) and a range where the echo appears. 
   Such an ultrasonic flaw detecting method is mainly used in an atomic power plant, for detecting a plate thickness and a lamination (side cutting appearing in a cut surface of the plate) of a material, and detecting a fusion deficiency of a fused part and a base material by welding, and a crack generated in a thermally affected part. In addition, with respect to a build up welding for reinforcing a nozzle opening, a branch, and a pipe joint, which are disposed around a pressure vessel of an atomic reactor, the ultrasonic flaw detecting method is applied to a base material directly below a build-up welded part, a fused part, and a build-up deposited part (see, Atomic Energy and Design Technique, Okawa Shuppan, (1980), pp. 226 to 250 (Giitiro Uchigasaki, et al.)). 
   On the other hand, the X-ray transmission method can detect a pipe wall-thickness reduction, without detaching a heat insulation material from the pipe. In the X-ray transmission method, data, which haven been provided by a serial radiographic apparatus such as an X-ray CT scanner, are subjected to a high-speed image processing by using a powerful computer, so as to make an image of the overall object with a fault image showing different X-ray transmittances. 
   Recently, there is known a method capable of simultaneously taking a picture of substances of different X-ray transmittances, by a simple system including only a sheet-like color scintillator (fluorescent screen) and a CCD camera. The color scintillator emits three primary colors of light, i.e., red (R), green (G), and blue (B), with a luminescent ratio changing in accordance with a transmission amount. This method is used for observing a pipe wall-thickness reduction and for inspecting foreign matters in a thermal/atomic power plant and an oil/chemical complex. 
   However, in the above ultrasonic flaw detecting method, it is necessary to measure the thickness of a pipe at not less than 1000 positions, and thus it is difficult to conduct the method during a periodic inspection. Further, when the thickness of the pipe is measured, it is necessary to stop the plant in consideration of a temperature constraint, which results in decrease in availability factor. 
   On the other hand, in the X-ray method capable of detecting a malfunction through the heat insulation member of the pipe, although the method can measure a distribution of the thickness of the pipe, the method is not widely used because an apparatus therefor is expensive. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in view of the above circumstances. The object of the present invention is to provide an active sensor, a multi-point active sensor, a method of diagnosing deterioration of a pipe, and an apparatus for diagnosing deterioration of a pipe, capable of inspecting, while a plant is running, a pipe over a wide area thereof for a short period of time, and of reducing the time and the number of steps required for the inspection, at a low manufacturing cost. 
   The present invention is an active sensor positioned on an outside of a pipe so as to detect a thickness of the pipe, the active sensor comprising: an oscillator capable of inputting oscillatory waves into the pipe and scanning a frequency of the oscillatory waves within a desired range; and an optical fiber sensor mounted on the pipe, the optical fiber sensor detecting the oscillatory waves generated in the pipe. 
   Due to this structure, there can be obtained, at a low manufacturing cost, a thin active sensor capable of simply inspecting a pipe while a plant is running, and of significantly reducing the time and the number of steps required for the inspection. 
   The present invention is an active sensor positioned on an outside or an inside of a pipe so as to detect a thickness of the pipe, the active sensor comprising: an oscillator capable of inputting oscillatory waves into the pipe and scanning a frequency of the oscillatory waves over a desired range; and an optical fiber sensor mounted on the pipe, the optical fiber sensor detecting the oscillatory waves generated in the pipe. 
   Due to this structure, there can be obtained, at a low manufacturing cost, a thin active sensor capable of simply inspecting a pipe while a plant is running, and of significantly reducing the time and the number of steps required for the inspection. 
   The present invention is a multi-point active sensor comprising the plurality of aforementioned active sensors wherein the active sensors are linearly arranged or arranged in matrix. 
   Due to this structure, the thickness of the pipe can be measured and mapped over a wider area, whereby the malfunction of the pipe can be accurately detected. 
   The present invention is a method of diagnosing deterioration of a pipe using the aforementioned multi-point active sensor, the method comprising the steps of: inputting oscillatory waves into a pipe by the oscillator of at least one active sensor; detecting the oscillatory waves generated in the pipe by the optical fiber sensor of at least one active sensor; and calculating a thickness of the pipe by deriving a relationship between a frequency and a vibration strength, based on a frequency of the oscillatory waves inputted by the oscillator into the pipe and an amplitude of the oscillatory waves at this frequency detected by the optical fiber sensor. 
   Due to this structure, the thickness of the pipe can be measured and mapped over a wider area, whereby the malfunction of the pipe can be accurately detected. 
   The present invention is an apparatus for diagnosing deterioration of a pipe, comprising: the aforementioned multi-point active sensor; a waveform analysis unit connected to the respective active sensors, the waveform analysis unit calculating a thickness of a pipe by deriving a relationship between a frequency and a vibration strength, based on a frequency of oscillatory waves inputted by the oscillator of this active sensor into the pipe and an amplitude of the oscillatory waves at this frequency detected by the optical fiber sensor of this active sensor; a diagnostic database storing judgment threshold values relating to the deterioration of the pipe; and a diagnostic unit connected to the waveform analysis unit and the diagnostic database, the diagnostic unit comparing the thickness of the pipe calculated by the waveform analysis unit with the judgment threshold values stored in the diagnostic database, so as to diagnose the deterioration and the malfunction of the pipe. 
   Due to this structure, the deterioration and the malfunction of the pipe can be diagnosed in accordance with a size and a thickness thereof which may differ with the industry and the kind. In addition, it is possible, not only to calculate the thickness of the pipe so as to diagnose the deterioration and the malfunction of the pipe, but also to judge a lifetime of the pipe. 
   The present invention is a method for diagnosing deterioration of a pipe using the aforementioned multi-point active sensor, the method comprising the steps of: passively detecting oscillatory waves generated in a pipe by the optical fiber sensor of at least one active sensor; and analyzing the oscillatory waves generated in the pipe and detected by the optical fiber sensor, so as to detect deterioration and malfunction of the pipe. 
   Due to this structure, the deterioration and the malfunction of the pipe can be detected, without inputting oscillatory waves into the pipe by the oscillator of the active sensor. 
   According to the present invention, by using an active sensor including an oscillator capable of inputting oscillatory waves into a pipe and scanning a frequency of the oscillatory waves within a desired range, and an optical fiber sensor mounted on the pipe, the optical fiber sensor detecting the oscillatory waves generated in the pipe, the pipe can be inspected over a wide area thereof for a short period of time at a low manufacturing cost, while a plant is running. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view showing an active sensor in a first embodiment of the present invention. 
       FIG. 2  is a sectional view showing the active sensor in the first embodiment of the present invention. 
       FIG. 3  is a structural view showing an optical fiber sensor in the first embodiment of the present invention. 
       FIGS. 4(   a ) and  4 ( b ) are a plan view and a side view showing a multi-point active sensor in the first embodiment of the present invention. 
       FIG. 5  is a structural view showing an apparatus for diagnosing deterioration of a pipe in the first embodiment of the present invention. 
       FIG. 6  is a graph showing incident waves, reflected waves, and resonant waves, which are observed by the apparatus for diagnosing deterioration of a pipe in the first embodiment of the present invention. 
       FIG. 7  is a graph showing relationship between a frequency and a vibration strength, which is obtained by the apparatus for diagnosing deterioration of a pipe in the first embodiment of the present invention. 
       FIG. 8  is a graph showing a relationship between an inverse number of a plate thickness and a resonant frequency, which is obtained by the apparatus for diagnosing deterioration of a pipe in the first embodiment. 
       FIG. 9  is a structural view showing a multi-point active sensor and an active sensor in an alternative example 1 in the first embodiment of the present invention. 
       FIG. 10  is a perspective view showing an active sensor in an alternative example 2 in the first embodiment of the present invention. 
       FIG. 11  is a sectional view showing an active sensor in the alternative example 2 in the first embodiment of the present invention. 
       FIG. 12  is a structural view showing a multi-point active sensor in a second embodiment of the present invention. 
       FIG. 13  is a sectional view showing a state in which an optical fiber sensor is connected to a pipe. 
       FIG. 14  is a graph showing a relationship between an outer diameter of an optical fiber sensor and a vibration strength. 
   

   BEST MODE FOR CARRYING OUT THE INVENTION 
   First Embodiment 
   A first embodiment of an active sensor according to the present invention is described below, with reference to the drawings.  FIGS. 1 to 8 ,  FIGS. 13(   a ) and  13 ( b ), and  FIG. 14  are views showing the first embodiment of the present invention. 
   As shown in  FIGS. 1 and 2 , an active sensor  10  is positioned on an outside of a pipe  60 , and is used for detecting a thickness of the pipe  60 . The active sensor  10  has: an oscillator  15  capable of inputting oscillatory waves (ultrasonic waves) into the pipe  60  and scanning frequencies of the oscillatory waves within a desired range; and an optical fiber sensor  11  mounted on the oscillator  15  on a side of the pipe  60 , the optical fiber sensor detecting the oscillatory waves generated in the pipe  60 . 
   As shown in  FIG. 2 , the optical fiber sensor  11  is embedded in a high-temperature adhesive  12  filling a space between a pair of polyimide sheets  19   u  and  19   l . The polyimide sheet  191 , which is located on a lower part of  FIG. 2 , is attached to the pipe  60  with the high-temperature adhesive  12 . Between the upper polyimide sheet  19   u  and the oscillator  15 , there is disposed a holding member  13  that prevents a connection between an oscillation caused by the oscillator  15  and an oscillation propagating in the pipe  60  to be tested. In place of attaching the polyimide sheet  191  to the pipe  60  with the high-temperature adhesive  12 , the polyimide sheet  191  may be disposed on the pipe  60  by spraying. 
   It is preferable to optimize a size of the optical fiber sensor  11  in accordance with a plate thickness value of the pipe  60  to be measured and an oscillatory wavelength, considering an attenuation of ultrasonic waves propagating in the pipe  60 . Specifically, an inner diameter of the optical fiber sensor  11  is preferably not less than 5 mm which is a minimum size capable of avoiding a breaking of the optical fiber sensor  11  by bending. Meanwhile, it is desirable to optimize an outer diameter of the optical fiber sensor  11  based on a wavelength of the oscillatory waves propagating inside the pipe  60 . The outer diameter is preferably not more than a value that is obtained by adding, to the inner diameter, one half of the wavelength of the oscillatory waves propagating inside the pipe  60 . The standard number of winding turns of the optical fiber sensor  11  is 50. 
     FIGS. 13(   a ) and  13 ( b ) respectively show a state in which the optical fiber sensor  11  is connected to the pipe  60 . In  FIGS. 13(   a ) and  13 ( b ), illustration of the oscillator  16  is omitted and is not shown. The wave lines in  FIGS. 13(   a ) and  13 ( b ) show shapes of oscillatory waves propagating inside the pipe  60 . 
   As shown in  FIG. 13(   a ), in a case where the outer diameter of the optical fiber sensor  11  is not more than a value that is obtained by adding, to the inner diameter, one half of the wavelength of the oscillatory waves propagating inside the pipe  60 , since amplitudes of the oscillatory waves propagating in aligned sensing part  11   a  (described below) of the circular or elliptic optical fiber sensor  11  are oriented in the same direction, a large vibration strength can be provided. On the other hand, as shown in  FIG. 13(   b ), in a case where the outer diameter of the optical fiber sensor  11  is larger than a value that is obtained by adding, to the inner diameter, one half of the wavelength of the oscillatory waves propagating inside the pipe  60 , the amplitudes of the oscillatory waves propagating in the sensing part  11   a  undergo vibrations of opposite direction. Namely, since the vibration directions are balanced out by the opposite amplitudes, the vibration strength is lowered. In  FIGS. 13(   a ) and  13 ( b ), the reference number  11   r  represents a region in which the optical fiber sensor  11  is positioned, i.e., a region between the inner diameter and the outer diameter of the optical fiber sensor  11 . 
     FIG. 14  is a graph showing a relationship between the outer diameter of the optical fiber sensor  11  (referred to as “optical fiber sensor outer diameter” in  FIG. 14 ) and the vibration strength. Herein, a thickness of a test piece is 5 mm, a wavelength of oscillatory waves propagating in the test piece is 10.7 mm (sonic velocity: 5800 m/sec), and an inner diameter of the optical fiber sensor  11  is 10 mm. In a case where the outer diameter of the optical fiber sensor  11  is 15 mm, which is a value obtained by adding, to the inner diameter (10 mm) of the optical fiber sensor  11 , 5 mm which is about one half of the wavelength of the oscillatory waves, it can be seen that the sufficient vibration strength is obtained. On the other hand, in a case where the outer diameter of the optical fiber sensor  11  is a value obtained by adding, to the inner diameter of the optical fiber sensor  11 , a value larger than one half of the oscillatory wavelength (in a case where the outer diameter of the optical fiber sensor  11  is larger than 15 mm), it can be seen that the vibration strength is reduced. 
   The above oscillator  15  is formed from an electromagnet oscillator. To be specific, as shown in  FIGS. 1 and 2 , the oscillator  15  has a permanent magnet  16  positioned so as to generate a magnetic flux in a normal line direction of a pipe surface  60   f  (in the A direction shown by the arrow in  FIG. 1 ), and an electric coil  17  disposed on the permanent magnet  16  on a side of the optical fiber sensor  11 . In place of disposing the electric coil  17  on the permanent magnet  16  on the side of the optical fiber sensor  11 , the electric coil  17  may be wound around the permanent magnet  16 . In addition, instead of electric coil  17 , there may be used a conductive layer of an optical fiber sensor which is coated with a conductive material such as a metal. 
   The optical fiber sensor  11  is formed from a fiber-optic Doppler (FOD) sensor (see,  FIGS. 3(   a ) to  3 ( d ) that detects a kinetic strain of the pipe  60 , which is generated by the oscillatory waves inputted from the oscillator  15  into the pipe  60 . With the use of such an optical fiber sensor  11 , strains and vibrations can be detected as the Doppler effect of light based on the FOD principle. 
   As shown in  FIGS. 1 and 3 , the optical fiber sensor  11  has the circularly winding sensing part  11   a . As shown in  FIGS. 1 and 2 , the oscillator  15  is located at a center of the sensing part  11   a.    
   The sensing part  11   a  of the optical fiber sensor  11  are subjected to a heat-resistant process, such as a heat-resistant coating using gold, nickel, silica, and polyimide, and/or a narrow tube. Thus, the active sensor  10  can be mounted on even a position where a temperature thereof is raised to a high temperature (between about 350° C. and 750° C.). 
   As shown in  FIG. 3(   d ), the optical fiber sensor  11  has a core  41  formed from a quartz line or the like, and a clad  42  made of quartz and covering the core  41 . As shown in  FIG. 3(   a ), connected to one end of the optical fiber sensor  11  is a light source  5  that supplies a light beam of a predetermined wavelength, such as a laser beam, into the optical fiber sensor  11 . Connected to the other end of the optical fiber sensor  11  is a photodetector  6  that detects a deviation of the wavelength which is caused by the kinetic strain in the pipe by the Doppler effect when the light beam has passed through the optical fiber sensor  11 . 
   As described above, since the optical fiber sensor  11  is formed from a fiber-optic Doppler (FOD) sensor, the optical fiber sensor  11  is strained in accordance with strain rates (εx; strain rate in an x direction, εy; strain rate in a y direction) generated in the pipe  60 , so that a light beam P incident on the optical fiber sensor  11  from the light source  5  at a frequency f 0  repeatedly reflects in the core  41  of the sensing part  11   a  of the optical fiber sensor  11  so as to produce the Doppler effect (see,  FIG. 3(   d )), and emerges at a frequency f 0 ±fd to the photodetector  6  (see,  FIG. 3(   a )). 
     FIG. 3(   b ) is a partial enlarged view of the sensing part  11   a  of the optical fiber sensor  11 .  FIG. 3(   c ) is a further enlarged view of  FIG. 3(   b ).  FIG. 3(   d ) is a view showing a state in which the light beam P repeatedly reflects in the core  41  of the sensing part  11   a  of the optical fiber sensor  11 . 
   The deviation of the frequency fd is concretely represented as the following (Expression 1).
 
 f   d   =n   eq   NπR   av ( + )/λ 0   (Expression 1)
 
   in which: 
   n eq ; transmission refractive index in fiber 
   N; winding number 
   R av ; average winding diameter 
   λ 0 ; wavelength of incident light beam 
   As shown in  FIGS. 4(   a ) and  4 ( b ), by linearly (serially) arranging the plurality of active sensors  10 , a multi-point active sensor  20  can be obtained. The respective active sensors  10  are connected to each other by connection members  22  having a plasticity and a flexibility.  FIG. 4(   a ) is a plan view showing the multi-point active sensor  20  from above, and  FIG. 4(   b ) is a side view showing the multi-point active sensor  20  from the lateral side. 
   To be specific, as shown in  FIG. 4(   a ), each of the active sensors  10  is located in the connection member  22  having a projection  23  and a recess  24 . The projection  22  of each connection member  22  is fitted in the recess  24  of the adjacent connection member  22 , whereby each of the connection members  22  is connected to the connection members  22  adjacent thereto. 
   In  FIG. 5  (see, also  FIG. 1 ), connected to each electric coil  17  of the oscillator  15  of the active sensor  10  of the multi-active sensor  20  is an oscillation controller  32  that supplies an alternating current to the electric coil  17 . The oscillation controller  32  is provided with a function generator (not shown) capable of scanning a frequency of the alternating current supplied by the oscillation controller  32 . In addition, the oscillation controller  32  is capable of adjusting an intensity of the current to be supplied. 
   As shown in  FIG. 5 , an apparatus for diagnosing deterioration of a pipe is composed of: the above multi-point active sensor  20 ; a waveform analysis unit  31  connected to the oscillation controller  32  and the photodetector  6 , the waveform analysis unit  31  calculating a thickness of the pipe  60 ; a diagnostic database  33  storing judgment threshold values relating to the deterioration of the pipe  60 ; and a diagnostic unit  35  connected to the waveform analysis unit  31  and the diagnostic database  33 , the diagnostic unit  35  comparing the thickness of the pipe  60  calculated by the waveform analysis unit  31  with the judgment threshold values stored in the diagnostic database  33  so as to diagnose the deterioration of the pipe  60 . 
   Herein, the waveform analysis unit  31  calculates the thickness of the pipe  60  by deriving a relationship between a frequency and a vibration strength, based on a frequency of the oscillatory waves inputted into the pipe  60  by the oscillator  15  of the active sensor  10  in the multi-point active sensor  20  and an effective value of an amplitude of the oscillatory waves or a frequency spectral intensity obtained by Fourier converting the oscillatory waves at the frequency detected by the optical fiber sensor  11  of the active sensor  10 . 
   Connected to each of the oscillators  15  is a switching mechanism (not shown) which can be selectively switched on and off from a remote position. Thus, it is possible to select the active sensor(s)  10  to be activated in the multi-point active sensor  20 . Accordingly, which point(s) of the pipe  60  to be measured can be freely selected. 
   Next, an effect of this embodiment as structured above is described. 
   At first, a relationship between a frequency of oscillatory waves inputted into the pipe  60  by the oscillator  15  of the active sensor  10  and the thickness of the pipe  60  is described. 
   As shown in  FIG. 6 , when a relationship of “λ=2L” is satisfied between the thickness L of the pipe  60  and the wavelength λ of the oscillatory waves inputted into the pipe  60 , the oscillatory waves (incident waves) inputted into the pipe  60  and the oscillatory waves (reflected waves) detected by the optical fiber sensor  11  sympathetically vibrate so that resonant waves are observed. The resonant waves herein mean reflected waves which are observed after the incident waves are stopped (after a time point T 1  in  FIG. 6 ). The reference character T 0  shows a time point at which the incident waves are started to be inputted, and T 1  shows a time point at which the incident operation is stopped. 
   Thus, the thickness L of the pipe  60  can be measured by a reverse operation from the wavelength λ. Namely, when the following condition is satisfied, the ultrasonic wave resonates. 
   2d=λ Expression (2) in which a thickness of a metal plate is d and a wavelength of an ultrasonic wave is λ. 
   This can be rewritten with a frequency f of the ultrasonic wave to obtain the following Expression (3). Thus, when a resonant frequency and a sonic velocity can be grasped, the plate thickness can be reversely operated. 
   
     
       
         
           
             
               
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   For example, in a case where the pipe  60  formed from a steel plate having a thickness of 15 mm is measured, when ultrasonic waves at a frequency of 200 kHz are inputted, the resonance occur. 
   After the optical fiber sensor  11  whose winding number is 50 is attached with an instant adhesive to surfaces of SUS 304 (stainless steel) plates having the same diameter of 200 mm, and thicknesses of 5 mm, 7 mm, 10 mm, 15 mm, 20 mm, 25 mm, and 30 mm, sine waves amplified to 150 Vp-p by an amplifier at every 1 kHz in increment in a range from 50 kHz to 500 kHz are generated. Then, resonant waves corresponding to each frequency can be detected ( FIG. 6 ). 
   Then, when a value obtained by integrating an intensity (voltage value) of the resonant waves in a preset time period relative to the time is defined as “vibration strength”, vibration strengths at the respective frequencies can be derived (see,  FIG. 6 ). From the vibration strengths at the respective frequency as obtained above, a relationship between the frequency and the vibration strength can be derived, which is shown in  FIG. 7 . From the frequency when the vibration strength is highest (frequency corresponding to the region surrounded by the elliptic circle), the resonant frequency can be derived.  FIG. 8  shows a relationship between a resonant frequency and an inverse number of the plate thickness of SUS 304. It can be understood from  FIG. 8  that the relationship defined by the Expression (3) is satisfied between the resonant frequency and the inverse number of the plate thickness of SUS304. 
   Next, a method of diagnosing deterioration and malfunction of the pipe  60  is described. 
   At first, oscillatory waves are inputted into the pipe  60  by the oscillators  15  of the active sensors  10  of the multi-point active sensor  20 . Specifically, by supplying an alternating current to the electric coil  17  of the oscillator  15  by the oscillation controller  32 , the Lorentz force is applied to the permanent magnet  16 , to thereby input transversal waves to the pipe  60  in a thickness direction thereof (see,  FIGS. 1 and 5 ). 
   A frequency of the alternating current is changed by using the function generator of the oscillation controller  32 , and the alternating current is scanned with a desired frequency bandwidth. By connecting an amplifier to the function generator, it is possible to optionally change both a frequency and an intensity of the input waves. 
   On the occasion of construction or periodic inspection of the pipe  60 , such a multi-point active sensor  20  is preferably mounted on an elbow portion and an orifice downstream portion of the pipe  60 , which are susceptible to erosion and corrosion. 
   Then, the oscillatory waves generated in the pipe  60  are detected, and are sent to the photodetector  6  by the optical fiber sensor  11  of the active sensor  10  (see,  FIG. 5 ). Since the oscillator  15  is positioned at the center of the sensing part  11   a , the oscillatory waves generated in the pipe  60  can be detected with an improved sensitivity (see,  FIGS. 1 and 2 ). In addition, since the optical fiber sensor  11  is formed from a fiber-optic Doppler sensor, the waves can be detected with an excellent sensitivity over a wide frequency bandwidth ranging from 0 Hz (excluding zero) and several MHz. 
   Then, based on a frequency of the oscillatory waves inputted into the pipe  60  by the oscillator  15  of the active sensor  10 , and an amplitude of the oscillatory waves at this frequency detected by the optical fiber sensor  11  of this active sensor  10 , a relationship between the frequency and the vibration strength is derived by the waveform analysis unit  31  connected to the oscillation controller  32  and the photodetector  6  (see,  FIG. 7 ). Thereafter, the waveform analysis unit  31  derives a resonant frequency based on the relationship between the frequency and the vibration strength, and calculates the thickness of the pipe  60  based on the Expression (3) or the graph shown in  FIG. 8 . 
   Then, the thickness of the pipe  60  calculated by the waveform analysis unit  31  and the judgment threshold values stored in the diagnostic database  33  are compared to each other, and the deterioration of the pipe  60  or the malfunction of the pipe  60  is diagnosed. 
   In this manner, since the deterioration of the pipe  60  is diagnosed with the use of the judgment threshold values stored in the diagnostic database  33 , the pipe  60  can be diagnosed in accordance with its size and thickness, which may differ with the industry and the kind. In addition, it is possible, not only to calculate the thickness of the pipe  60  so as to diagnose the deterioration and the malfunction of the pipe  60 , but also to judge a lifetime of the pipe  60 . 
   As has been described above, by mounting the multi-point active sensor  20  on the outside of the pipe  60 , it is possible to, while a plant is running, calculate the thickness of the pipe  60  so as to diagnose the deterioration and the malfunction of the pipe  60  over a wide area, for a short period of time, without detaching an heat-insulation material from the pipe  60 . Thus, the time and the number of steps required for the inspection can be significantly reduced. Accordingly, the time for the periodic inspection can be reduced, and the corrective maintenance service can be improved. 
   As shown in  FIGS. 4(   a ) and  4 ( b ), the respective active sensors  10  included in the multi-point active sensor  20  are connected to each other by the connection members  22  having a plasticity and a flexibility. Thus, the multi-point active sensor  20  can be mounted on a curved portion and an elbow portion of the pipe  60 , whereby portions of the pipe  60  where a thickness thereof is prone to be reduced can be inspected. 
   Since the active sensor  10  in this embodiment can be manufactured from the electric coil  17 , the permanent magnet  16 , and the optical fiber sensor  11 , a manufacturing cost for the active sensor  10  is considerably inexpensive. 
   The reliability of the multi-point active sensor  20  can be prolonged, by reducing the size of the oscillator  15  of the active sensor  10  so as to restrain a sensing area, by optimizing a distance between the active sensors  10 , by enhancing a connection between the active sensors  10 , by enhancing a decomposability when measuring a thickness, by improving a heat-resistant property of the adhesive  12 , and by improving an absorbance of the active sensor  10  at an elbow portion of the pipe  60 . 
   Further, the use of the smaller oscillator  15 , which can be driven at a low voltage, and the optical fiber sensor  11 , which can detect a wave with a short FOD gauge length, can enhance practical usefulness. 
   The precision of measuring the thickness of the pipe  60  is determined by parameters such as a power of the oscillatory waves from the oscillator  15  (capacity of the amplifier connected to the function generator), a magnetic force of the permanent magnet  16 , the turning number of the electric coil  17 , a sensitivity of the optical fiber sensor  11  itself (the turning number of the optical fiber sensor  11 ), and a heat resistance. 
   In the above embodiment, the oscillator  15  formed from an electromagnetic oscillator is described by way of example. However, not limited thereto, there may be used an oscillator  15  formed from a piezoelectric oscillator having a piezoelectric element. When such a piezoelectric oscillator is used, a strong oscillation can be provided at a lower electric power. 
   Further, in the above embodiment, the optical fiber sensor  11  having the circularly winding sensing part  11   a  is described by way of example. However, not limited thereto, there may be used an optical fiber sensor  11  having an elliptically winding sensing part. When such an optical fiber sensor  11  having the elliptically winding sensing part is used, the optical fiber sensor  11  can have an anisotropy. 
   Alternative Example 1 
   Next, an alternative example 1 of the first embodiment is described with reference to  FIGS. 9(   a ) to  9 ( c ). In the alternative example 1 of the first embodiment shown in  FIGS. 9(   a ) to  9 ( c ), in place of using the multi-point active sensor  20  in which the plurality of active sensors  10  are linearly arranged, there is used a multi-point active sensor  20  in which the plurality of active sensors  10  are arranged in matrix. Other structures of the alternative example 1 are substantially the same as those of the first embodiment shown in  FIGS. 1 to 8 . 
   In the alternative example 1 shown in  FIGS. 9(   a ) to  9 ( c ), the same parts as those in the first embodiment shown in  FIGS. 1 to 8  are shown by the same reference numbers, and a detailed description thereof is omitted. 
   As shown in  FIG. 9(   b ), the multi-point active sensor  20  in this embodiment has the plurality of active sensors  10  arranged in matrix. To be more specific, as shown in  FIG. 9(   b ), in the multi-point active sensor  20 , there are arranged, in a square area of 100 mm by 100 mm, the nine active sensors  10  of about 30 mmφ in 3×3 matrix. 
   As shown in  FIG. 9(   c ), each of the active sensors  10  included in the multi-point active sensor  20  of the alternative example 1 has: an oscillator  15  mounted on a pipe surface  60   f  of a pipe  60 , the oscillator  15  inputting oscillatory waves into the pipe  60 , and an optical fiber sensor  11  mounted on an outer surface of the pipe  60  so as to surround the oscillator  15 , the optical fiber sensor  11  detecting oscillatory waves generated in the pipe  60 . The oscillator  15  and the optical fiber sensor  11  are attached to the outer surface of the pipe  60  with a heat-resistant adhesive  12  (or tackifier). 
   As shown in  FIGS. 9(   b ) and  9 ( c ), the respective active sensors  10  are connected to each other by a connection member  22   a  made of a silicon sheet. On the connection member  22   a  and at an outer periphery of the optical fiber sensor  11 , there is disposed a case  29  made of metal or engineering plastic. A space between the oscillator  15  and the case  29  is filled with silicon  27 . 
   By using such a multi-point active sensor  20 , a thickness of the pipe  60  can be measured and mapped over a wider area, whereby the deterioration and the malfunction of the pipe  60  can be more precisely detected. 
   In the alternative example 1, there is described by way of example the active sensor  10  including the oscillator  15  mounted on the outer surface of the pipe  60  and the optical fiber sensor  11  mounted on the outer surface of the pipe  60  so as to surround the oscillator  15 . However, not limited thereto, there may be used an active sensor  10  including an oscillator  15 , and an optical fiber sensor  11  mounted on the oscillator  15  on a side of the pipe  60 , as shown in the first embodiment. 
   To the contrary, there may be used, as the active sensor in the first embodiment, an active sensor  10  as shown in the alternative example 1 including an oscillator  15  mounted on an outer surface of a pipe  60  and an optical fiber sensor  11  mounted on the outer surface of the pipe  60  so as to surround the oscillator  15 . 
   Alternative Example 2 
   Next, an alternative example 2 of the first embodiment is described with reference to  FIG. 10  and  FIGS. 11(   a ) and  11 ( b ). In the alternative example 2 shown in  FIG. 10  and  FIGS. 11(   a ) and  11 ( b ), in place of using the oscillator  15  including the permanent magnet  16  positioned so as to generate a magnetic flux in a normal line direction of the pipe surface  60   f  (in the A direction shown by the arrow in  FIG. 10)  and the electric coil  17  disposed on the permanent magnet  16  on a side of the optical fiber sensor  11 , there is used an oscillator  15  including a pair of permanent magnets  16  positioned so as to generate a magnetic flux in a direction perpendicular to a normal line direction of the pipe surface  60   f  (in the A direction shown by the arrow in  FIG. 10 ), and an electric coil  17  disposed between the pair of permanent magnets  16 . Other structures of the alternative example 2 are substantially the same as those of the first embodiment shown in  FIGS. 1 to 8 . 
   In the alternative example 2 shown in  FIG. 10  and  FIGS. 11(   a ) and  11 ( b ), the same parts as those in the first embodiment shown in  FIGS. 1 to 8  are shown by the same reference numbers, and a detailed description thereof is omitted. 
   As shown in  FIG. 10  and  FIGS. 11(   a ) and  11 ( b ), the oscillator  15  in this alternative example has the pair of permanent magnets  16  positioned so as to generate a magnetic flux in a direction perpendicular to a normal line direction of the pipe surface  60   f , and the electric coil  17  disposed between the pair of permanent magnets  16 . Thus, by supplying an alternating current to the electric coil  17  disposed between the pair of permanent magnets  16  from an oscillation controller  32 , the Lorentz force can be applied to the pair of permanent magnets  16  positioned so as to generate a magnetic flux in a direction perpendicular to a normal line direction of the pipe surface  60   f , to thereby input longitudinal waves into the pipe  60  in a thickness direction thereof (see,  FIGS. 5 and 10) . 
   As shown in  FIG. 11(   b ), a holding member  13  may be provided between the permanent magnets  16  and a polyimide sheet  19   u . Alternatively, as shown in  FIG. 11(   a ), the provision of the holding member  13  between the permanent magnets  16  and the polyimide sheet  19   u  may be omitted. 
   Second Embodiment 
   Next, a second embodiment of the present invention is described with reference to  FIG. 12 . In the second embodiment shown in  FIG. 12 , a waveform analysis unit  31  has the following three functions, i.e., (1) a frequency analysis function considering a burst behavior at a high frequency area (discrimination from a steady noise), (2) a behavior observation function of standing waves at a low frequency area (discrimination from a steady noise), and (3) a “steady”/“non-steady” observation function utilizing a neutral network and the like. In addition, a diagnostic database  33  stores information relating to deterioration and malfunction of a pipe  60 , the information being to be compared to oscillatory waves generated in the pipe  60  for some reason or other which are detected by an optical fiber sensor  11  of an active sensor  10 . Other structures of the second embodiment 2 are substantially the same as those of the first embodiment shown in  FIGS. 1 to 8 . 
   In the second embodiment shown in  FIG. 12 , the same parts as those in the first embodiment shown in  FIGS. 1 to 8  are shown by the same reference numbers, and a detailed description thereof is omitted. 
   At first, by the optical fiber sensor  11  of the active sensor  10 , oscillatory waves generated in the pipe  60  for some reason or other (for example, oscillatory waves caused by a bust impact, or oscillatory waves generated in an abnormal state which do not appear in a steady state) are passively detected (see,  FIG. 12 ). 
   Then, due to the (1) a frequency analysis function considering a burst behavior at a high frequency area (discrimination from a steady noise), (2) a behavior observation function of a standing wave at a low frequency area (discrimination from a steady noise), and (3) a “steady”/“non-steady” observation function utilizing a neutral network and the like, of the waveform analysis unit  31 , a waveform of the oscillatory waves generated in the pipe  60  or some reason or other is analyzed. 
   Then, a diagnostic unit  35  analyzes the oscillatory waves detected by the optical fiber sensor  11  of the active sensor  11 , referring to the information relating to deterioration and malfunction of the pipe  60 , which has been stored in the diagnostic database  33  beforehand, so that the deterioration and the malfunction of the pipe  60  is detected (see,  FIG. 5 ). 
   As shown in the first embodiment, the diagnostic unit  35  can also detect the thickness of the pipe  60  from a resonant frequency derived from an amplitude of the oscillatory waves detected by the optical fiber sensor  11 . 
   Thus, according to the apparatus for diagnosing deterioration of a pipe in this embodiment, it is not necessary to input oscillatory waves into the pipe  60  by the oscillator  15  of the active sensor  10 , which is necessary in the above first embodiment, but it is possible to passively detect oscillatory waves generated in the pipe  60  for some reason or other, to thereby detect the thickness of the pipe  60  and the deterioration and the malfunction of the pipe  60 . 
   Further, with the use of the information relating to the deterioration and the malfunction of the pipe  60 , which has been stored in the diagnostic database  33  beforehand, the pipe  60  can be diagnosed in accordance with its size and thickness, which may differ with the industry and the kind. Furthermore, it is possible, not only to calculate the thickness of the pipe  60  so as to diagnose deterioration and malfunction of the pipe  60 , but also to judge a lifetime of the pipe  60 . 
   According to the present invention, after oscillatory waves are inputted into the pipe  60  by the oscillator  15  of the active sensor  10 , the oscillatory waves generated in the pipe  60  can be actively detected by the optical fiber sensor  11  of the active sensor  10 , which is shown in as shown in the above first embodiment (including the alternative examples 1 and 2). Alternatively, oscillatory waves generated in the pipe  60  for some reason or other can be passively detected without inputting oscillatory waves into the pipe  60 , which is shown in the second embodiment. Therefore, the deterioration and the malfunction of the pipe  60  can be detected with a high probability.