JP3451349B2 - Method for detecting paint film damage on buried coated steel pipe - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a coating film damage detection method for a buried coating steel pipe, which detects contactlessly a coating film damage portion of a coating steel pipe buried in the ground.

[0002]

2. Description of the Related Art As a method for detecting a coating film damage (defect) of a coated steel pipe buried in the ground in a non-contact manner from the ground surface, for example, an alternating current is applied between the coated steel pipe and a grounding electrode in the ground. There is a method called the potential method that measures the damage position by applying a voltage, passing a current through the steel pipe, and measuring the potential distribution generated on the ground surface by the current flowing into the ground from the coating damage. is there. In coating film damage detection by the actual potential method,
By using the cathodic protection terminal and the Mg anode (ground electrode) provided in the buried steel pipe, a low-frequency sine wave signal is applied between the buried steel pipe and the ground electrode in the ground to generate a low-frequency AC current. Shed. Then, along the coated steel pipe, 2 at regular intervals on the ground surface.
Distribute the potential difference by arranging two electrodes, scanning the two electrodes along the buried coated steel pipe, detecting the potential difference signals at two points by the two electrodes, and detecting the signals. The intensity and phase of the. Then, the damaged position is specified from the pattern of the potential difference distribution on the ground surface.

[0003]

In the conventional coating film damage detecting method for a buried coating steel pipe, as described above, the two electrodes are scanned along the buried coating steel pipe and the two electrodes are scanned. The damage position is identified from the pattern of the potential difference distribution on the ground surface detected by the electrodes of. However, since scanning is performed from the ground surface, there is a problem that it is not possible to determine at which position in the circumferential direction of the pipe body the damage of the coated steel pipe is. Furthermore, it is possible to estimate the outflow current from the damaged area and the damaged area based on the strength of the potential difference detected on the ground surface, but it is difficult to accurately estimate the damaged area because the depth of damage cannot be determined. There is a problem. In addition, when the damage of the buried coated steel pipe is at the lower surface of the pipe body, the potential difference signal detected on the ground surface becomes small due to the influence of the pipe body existing above the damage body, so that the minute difference of the bottom surface of the pipe body There is also a problem that it is difficult to detect damage.

The present invention has been made in order to solve the above-mentioned problems, and enables the detection of minute damages on the lower surface of the pipe body of the buried coating steel pipe and specifies the position in the circumferential direction. The present invention provides a method for detecting coating film damage of a buried coating steel pipe, which enables the above.

[0005]

(1) A coating film damage detection method for a buried coating steel pipe according to one aspect of the present invention is a method of detecting a low frequency between a buried coating steel pipe and a grounding electrode in the ground. An AC voltage is applied, and a potential difference between two points is detected by two electrodes arranged in the depth direction in a vertical hole provided near the buried coating steel pipe, and the vertical direction of the two electrodes is detected. By changing the depth in the hole, the potential difference distribution in the ground along the vertical hole is obtained, and the damaged position of the buried coated steel pipe is detected based on the potential difference distribution.

In the coating film damage detection method according to one aspect of the present invention, for example, a low-frequency AC voltage is applied between one point of the buried coating steel pipe and the grounding electrode in the ground to generate an AC current in the steel pipe. Inflow. When the coating of the steel pipe is damaged and the steel pipe itself and the ground are in contact with each other, a part of the current flowing through the steel pipe flows into the ground, and the current flowing out from the damaged portion causes a potential distribution.

In the conventional damage detection test, two electrodes with a constant interval are arranged on the ground surface, the potential difference between the two electrodes is measured, and the potential difference due to the potential distribution generated by the outflow current from the damage is measured. To detect. As shown in Fig. 8, the potential distribution on the ground surface is maximized at the position directly above the damage (the position where the distance from the damage is the minimum), and decreases locally as the distance from the damage increases. Potential distribution. The potential difference detected by the two electrodes installed on the ground surface is 0 when the position of each electrode is equidistant around the damage, and at other positions, the potential difference from the damaged position of each of the two electrodes is Since a + or-potential difference is measured by the difference in distance, the damaged site is determined from the change in these potential differences depending on the position (potential difference distribution).

Since the magnitude of the potential difference detected by the two electrodes is determined by the current flowing out from the damage of the buried coated steel pipe and the distance from the damage to the electrode, the potential difference detected by the electrodes and the damage The size of the damage can be estimated if the depth in the ground is known, but the measurement on the ground surface determines where the damage is on the circumference of the pipe body of the coated steel pipe, especially on the upper surface or the lower surface. Therefore, the estimated damage size includes a depth error corresponding to the diameter of the coated steel pipe.

In the present invention, the above-mentioned potential method is used for special characteristics.
A vertical hole is formed at an arbitrary distance from the coated steel pipe on the ground surface of the specified coated steel pipe, and two electrodes are inserted at regular intervals in the depth direction in the vertical hole. Then, the potential difference in the depth direction along the vertical hole is detected. As shown in Fig. 1, the potential distribution in the depth direction along the vertical hole is the same as the potential distribution on the ground surface, where the distance from the damage is the minimum (
Depth) is the maximum, and there is a local potential distribution that decreases as the distance from the damage increases, so when the positions of the two electrodes in the vertical hole are changed, the The potential difference becomes 0 at the position where the distances are equal, and the inverted potential difference is detected at the upper and lower positions. Since the position of damage (depth) is specified from this potential difference distribution pattern, by combining it with the measurement on the ground surface, the damage position on the circumference of the pipe body of the coated steel pipe of damage is specified and the magnitude of damage is determined. (Dimension) can be accurately estimated.

In addition, in the measurement by the electrodes installed on the ground surface, since the ground surface is generally paved, the detection signal may decrease depending on the installation state of the paved surface and the electrodes. In addition, since the distance between the buried coated steel pipe and the ground surface (the depth of the steel pipe) is constant, the limit of the measurable signal strength limits the size of the damage that can be detected, making it difficult to detect minute damage. is there. Furthermore, if the damage is on the underside of the body of the buried coated steel pipe, the signal size is further suppressed due to the influence of the body of the pipe above the damage, which further limits the detectable size.

However, according to the present invention, since the measurement is performed by the electrode inserted in the vertical hole and the electrode and the ground are directly contacted with each other, the potential difference can be stably and efficiently detected. Further, in the present invention, since the strength of the detected potential difference is determined by the distance from the buried coating steel pipe to the vertical hole, by installing the vertical hole near the buried coating steel pipe, the detectable potential difference strength is increased. Then, the potential difference corresponding to the potential distribution generated by the smaller damage can be detected. Further, even when the damage is on the lower surface of the buried coating steel pipe, the potential difference can be detected without being affected by the pipe body, and therefore damage on the lower face of the pipe body, particularly minute damage can be detected. It will be possible.

(2) A method for detecting coating film damage in a buried coated steel pipe according to another aspect of the present invention is the same as in the above (1), wherein the vertical hole is symmetrical with respect to the pipe axis of the buried coated steel pipe. A plurality of holes are provided at different positions, and the flaw detection position in the circumferential direction of the pipe body of the buried coated steel pipe is obtained by obtaining the potential difference distribution in the depth direction for each vertical hole.

According to the present invention, the potential difference distribution measured in the plurality of vertical holes is determined by the damage position of the buried coated steel pipe and the distance from the damage to the electrode in the vertical hole. Since it is symmetrical with respect to and is equidistant from the buried coating steel pipe, if the damage of the buried coating steel pipe is at the center position (top or bottom) of the pipe body, measure at the vertical hole at the symmetrical position. The pattern of the potential difference distribution in the depth direction is the same, but when the damage is on either the left or right side of the tubular body, the potential difference change pattern measured in each vertical hole is the same in the vertical hole on the damaged side. Large and small on the side where no damage is present. Since the position of the damage in the depth direction and the position on the left or right of the damaged tubular body can be known from the pattern of the potential difference distribution measured in each vertical hole, the accurate position on the damaged tubular body can be specified.

(3) A coating film damage detection method for a buried coating steel pipe according to another aspect of the present invention, in which a low-frequency AC voltage is applied between the buried coating steel pipe and a grounding electrode in the ground, The first electrode is inserted into one vertical hole and the second electrode is inserted into the other vertical hole of the plurality of vertical holes provided at a position symmetrical with respect to the pipe axis of the buried coating steel pipe. , The potential difference inside the ground when the depths of the first and second electrodes from the ground surface are made the same, and the potential difference obtained when the depths of the first and second electrodes are changed Detect the damaged part of the buried coated steel pipe based on the distribution.

In the coating film damage detection method according to another aspect of the present invention, the first electrode is inserted into one of the plurality of vertical holes provided in the vicinity of the buried coating steel pipe, and The second electrode is inserted into the vertical hole of, and installed at an arbitrary depth from the ground surface of the first and second electrodes, and the potential difference between the electrodes is measured. Since the potential at each electrode position (depth) is determined by the damage position and the distance between the damage and the electrode, the change pattern of the potential difference distribution obtained when the first and second electrode depths are changed is used to calculate the buried coated steel pipe. The damaged area on the tube can be determined.

(4) A method for detecting coating film damage of a buried coating steel pipe according to another aspect of the present invention is the same as the above-mentioned detection methods (1) to (3), wherein the buried coating steel pipe and the grounding electrode in the ground. As an alternating voltage applied between and, an alternating voltage related to a random signal or a pseudo-random signal is used, and a reference signal extracted from the alternating voltage and a correlation signal between the detection signal of the potential difference between the electrodes are processed. Then, the potential difference distribution is obtained by using the peak value as a representative value of the potential difference between the two points. (5) A coating film damage detection method for a buried coating steel pipe according to another aspect of the present invention is the method for detecting a coating film damage of a buried coating steel pipe between a buried coating steel pipe and a grounding electrode in the ground according to the above-mentioned detection methods. As the AC voltage applied to the, using the AC voltage according to the pseudo-random signal, and a reference signal of the same pattern as the AC voltage,
Correlation processing is performed with the detection signal of the potential difference between the electrodes, and the peak value is used as the representative value of the potential difference between the two points to obtain the potential difference distribution.

According to another aspect of the present invention, the above (4) and (5)
In the coating film damage detection method of the embedded coating steel pipe, by performing a correlation process using a random signal or a pseudo-random signal as an AC signal applied to the embedded coating pipe,
Since the noise component is removed and the S / N is improved, it is possible to detect the damaged position of the buried coating steel pipe with high sensitivity, high accuracy and certainty.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiment 1. 1 (A) and 1 (B) are a diagram showing a configuration of an apparatus to which a coating film damage detection method for a buried coating steel pipe according to a first embodiment of the present invention is applied and a sectional explanatory view thereof. In FIGS. 1 (A) and 1 (B), 1 and 2 are electrodes, 3 is a buried coated steel pipe, 4 is damage, 5 is a detection device, and 6
Is a cable, 7 is a power feeding device, 8 is a terminal, 9 is a ground electrode, and 10 is a vertical hole.

In this embodiment, a low-frequency (440 Hz) AC voltage is applied between the terminal 8 and the ground electrode 9 of the buried coated steel pipe 3 from the power feeding device 7 to make the buried coated steel pipe 3
AC current is flown into. The current flowing into the buried coating steel pipe 3 is determined by the amplitude of the voltage applied between the terminal 8 and the ground electrode 9 and the resistance between the terminal 8 and the ground electrode 9 (ground resistance of the steel pipe to the ground). It differs depending on the burying environment of the coated steel pipe 9. Since a part of the current flowing through the buried coating steel pipe 3 flows out into the ground through the damage 4, the value of the current flowing through the steel pipe decreases with the distance from the feeding point. In the detection test, power is supplied from the terminal 8 so that the current value is 1 to 2 A so that the current value at a level where damage can be detected can be maintained over a range of several km.

The current value flowing out from the damaged portion is determined by the ground voltage value of the steel pipe at the damaged portion and the contact resistance between the damage and the ground, and is generally proportional to the damaged area. A potential distribution is generated on the ground surface above the damage by the current flowing out from the damage. By detecting this potential distribution,
Identify the damaged part of the coated steel pipe on the surface. The potential distribution is determined by the current value flowing out from the damage, the depth of damage, and the electrical characteristics of the ground, and is proportional to the damage area like the current. In addition, since the alternating current is flowing in the coated steel pipe 3, the generated potential distribution also fluctuates at the same frequency as the current frequency.

In this embodiment, for example, the depth of the buried coated steel pipe 3 is set to 2 m, and the damaged portion of the specified coated steel pipe is covered.
A vertical hole 10 having a diameter of 60 mm is drilled to a depth of 3.5 m at a position 1 m away from the buried coating steel pipe 3 on the ground surface, and electrodes 1 and 2 are inserted into the vertical hole. The electrodes 1 and 2 are fixed so that the distance between them is 1 m, and each electrode is a cable 6
Is connected to the detection device 5 via an input terminal and an AC signal corresponding to the potential difference between the electrodes 1 and 2 is input to the detection device 5. Here, the metal electrodes 1 and 2 are fixed to the non-conductive rod, inserted into the vertical holes, and water is injected into the vertical holes to maintain the contact between the electrodes 1 and 2 and the ground. .

The detection device 5 detects the AC signal detected by the electrodes 1 and 2 and measures its intensity and phase. In this embodiment, when the electrode position in the vertical hole is changed in the depth direction and changes in the intensity and phase of the detected potential difference signal are observed, the detected potential difference changes depending on the position of the electrode, and each electrode is equidistant from the damage. In this case, the result was 0, and the result that the potential difference signal was inverted at the upper and lower positions was obtained, and it was confirmed that there was damage, and the position (depth) of damage was specified from the position where the detected potential difference was 0.

Further, in the present embodiment, the vertical hole position is set to 1 m from the steel pipe, but it is possible to increase the detected potential difference signal intensity by making the vertical hole closer to the steel pipe.
It is possible to detect minute damage that could not be detected by ordinary exploration from the ground surface or damage existing on the lower surface of the pipe. Further, in the present embodiment, the electrode interval is set to 1 m, but the potential difference signal can be more efficiently measured by changing the electrode interval in accordance with the potential distribution pattern in the ground.

Embodiment 2. FIG. 2 is a diagram showing a configuration of an apparatus to which a coating film damage detection method for a buried coating steel pipe according to a second embodiment of the present invention is applied. In the present embodiment, two vertical holes 10 are provided at positions equidistant from each other about the pipe axis of the buried coating steel pipe 3, and two electrodes 1 and 2 are inserted into each vertical hole to form a deep hole. The potential difference is measured while moving in the vertical direction. For example, the position of each vertical hole 10 is 1 m from the buried coating steel pipe 3, and the interval between the electrodes to be inserted is also 1 m. Electrode 1,
In the depth direction pattern of the potential difference signal measured in the vertical hole by 2, the intensity of the potential difference signal at the electrode on the side surface of the pipe body having the damage of the buried coating steel pipe 3 becomes larger, and the damage causes the potential difference. It is determined that the signal is on the side of a vertical hole with a large signal, and the position in the depth direction is specified from the pattern of the potential difference distribution, so if there is damage, the position of damage on the circumference of the buried coated steel pipe is determined. You can ask.

Embodiment 3. FIG. 3 is a diagram showing a configuration of an apparatus to which a coating film damage detection method for a buried coating steel pipe according to a third embodiment of the present invention is applied. In this embodiment, the vertical holes 10 are provided at equidistant positions symmetrical about the pipe axis of the buried coating steel pipe 3.
Two electrodes were provided, and one electrode 1 and 2 was inserted into each vertical hole, and the potential difference was measured. The position of each vertical hole 10 is 1 m from the buried coating steel pipe. Since the potential at each electrode position (depth) is determined by the distance between the damaged part and the electrode, the potential damage distribution pattern obtained when the depths of electrodes 1 and 2 are changed at the same time indicates the damage on the circumference of the buried coated steel pipe. The position can be calculated.

Embodiment 4. FIG. 4 is a configuration diagram of an apparatus to which the coating film damage detection method for a buried coating steel pipe according to Embodiment 4 of the present invention is applied. In the present embodiment, the AC voltage applied from the power feeding device 7 between the terminal 8 and the ground electrode 9 is M
This is an example using a series signal, and the arrangement and scanning of the electrodes 1 and 2 and the like described in the first to third embodiments are applied, and here, the processing of the M series signal will be mainly described.

The power supply device 7 is composed of an M-sequence signal generator 21 and a power amplifier 22. The detection device 5 is composed of an M-sequence reference signal generator 23 and a correlation processing unit 24. The M-sequence signal can be generated by a shift register having a feedback loop as shown in FIG. The code length obtained by the 7-stage shift register 25 shown in FIG. 5 is 2 ⁷ −1 = 12.
7

FIGS. 6A and 6B are diagrams showing examples of the signal waveform of the M-sequence signal and its autocorrelation signal waveform. In FIG. 6, the horizontal axis represents time, the vertical axis represents the signal magnitude, and τa is the cycle of the clock given to the shift register.

The M-sequence signal is a pseudo-random signal having a periodicity, and has a periodicity of a code length of 63 here. Therefore, when the autocorrelation is taken, peaks as shown in FIG. To have. From this, it can be seen that when the cross-correlation with other signals is taken, only the signal whose pattern matches the M-sequence signal has the cross-correlation value having a high peak value. In the present embodiment, this property is utilized to reduce the noise signal.

In FIG. 4, the M series signal from the M series signal generator 21 is amplified by the power amplifier 22 and applied as a voltage between the buried coating steel pipe 3 and the ground electrode 9.
In this state, the positions of the electrodes 1 and 2 are changed as shown in the first to third embodiments, and the electrodes 1 and 2 detect the potential of the ground with which they are in contact. The detection device 5 takes in these potential differences. M-series reference signal generator 2 of detection device 5
Although 3 is electrically independent of the M-sequence signal generator 21, it is configured to generate an M-sequence signal having the same pulse pattern.

The correlation processing section 24 of the detection device 5 performs a cross-correlation operation between the reference signal from the M-series reference signal generator 23 and the potential difference signal (detection signal) between the electrodes 1 and 2. That is, when the detection signal is f (t) and the reference signal is g (t), the cross-correlation calculation result is shown as follows.

[0032]

[Equation 1] Here, T is a repetition period.

Since the detection signal f (t) contains an M-system signal component and the reference signal g (t) is an M-sequence signal having the same pattern as this, the cross-correlation calculation result has the same cycle as the reference signal. It has a peak value. FIG. 7 shows an example of the cross-correlation calculation result. In FIG. 7, the horizontal axis represents τ in equation (1), and the vertical axis represents Φfg (τ) in equation (1). By detecting this peak value and using it as the representative value of the detection signal, it is possible to detect the potential difference with high accuracy while suppressing the noise signal. And
By performing processing using this representative value, the damaged portion of the coating film can be reliably detected with high accuracy.

Since this processing method suppresses the noise component in the potential difference detection, it is possible to perform detection with high accuracy and high sensitivity. A conventionally known circuit may be used to perform this processing method. For example, a circuit that detects the point where the potential difference crosses zero, a method that sets a threshold value for the potential difference, and a circuit that detects the point that crosses zero after the absolute value of the potential difference exceeds this threshold, and the positive potential difference indicates the maximum value. There is a method using a circuit that detects the midpoint between the point and the point where the negative potential difference shows the maximum value.

In the above embodiment, an example using the M-sequence signal has been described, but a random signal can be used as the AC voltage to be applied. In that case, a random signal is extracted from the applied AC voltage and used as a reference signal.

[0036]

As described above, according to the present invention, the depth of damage is measured by measuring the potential difference distribution corresponding to the potential distribution generated by the damage existing in the buried coated steel pipe by the electrode inserted in the vertical hole. And / or it becomes possible to accurately measure the position of the buried coated steel pipe on the circumference of the pipe body and to accurately estimate the size of the damage. In addition, if the vertical hole position is made closer to the steel pipe, the strength of the detected potential difference signal is increased, and it is possible to detect minute damage or damage that is difficult to detect by ordinary measurement on the lower surface of the pipe body. It will be possible. Furthermore, by performing correlation processing using a voltage related to a random signal or a pseudo-random signal as an AC voltage applied to the embedded coating film pipe, noise components are removed and S / N is improved.
It is possible to detect a damaged portion of the coating film of the buried coating steel pipe with high sensitivity, accuracy and reliability.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a device to which a coating film damage position detecting method for a buried coating steel pipe according to a first embodiment of the present invention is applied and a cross-sectional explanatory view thereof.

FIG. 2 is a configuration diagram of an apparatus to which a coating film damage position detecting method for a buried coating steel pipe according to a second embodiment of the present invention is applied.

FIG. 3 is a configuration diagram of an apparatus to which a coating film damage position detection method for a buried coating steel pipe according to a third embodiment of the present invention is applied.

FIG. 4 is a configuration diagram of an apparatus to which a coating film damage position detecting method for a buried coating steel pipe according to a fourth embodiment of the present invention is applied.

FIG. 5 is a diagram showing an example of an M-sequence signal generation shift register.

FIG. 6 is a diagram showing an example of an M-sequence signal waveform and its autocorrelation signal waveform.

7 is a diagram showing an example of a correlation waveform between a reference signal and a detection signal according to the embodiment of FIG.

FIG. 8 is an explanatory diagram for explaining the operation of the conventional method.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shuji Chiba 88 Ono-cho, Tsurumi-ku, Yokohama-shi, Kanagawa Nihon Steel Pipe Works Co., Ltd. (56) References JP-A-10-239267 (JP, A) JP-A 63-100365 (JP, A) JP-A 63-20555 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) G01N 27/00-27/24

Claims (5)

(57) [Claims] 1. A low-frequency AC voltage is applied between the buried coating steel pipe and a grounding electrode in the ground, and is arranged in the depth direction in a vertical hole provided near the buried coating steel pipe. By detecting the potential difference between two points with two electrodes, and changing the depth in the vertical hole of the two electrodes,
A method for detecting coating film damage in an embedded steel pipe covered with coating, wherein the potential difference distribution in the ground along a vertical hole is obtained, and the position of damage to the embedded steel pipe coated is detected based on the potential difference distribution. 2. A pipe body of a buried coated steel pipe, wherein a plurality of said vertical holes are provided at positions symmetrical with respect to the pipe axis of the buried coated steel pipe, and the potential difference distribution in the depth direction is obtained for each vertical hole. The method for detecting coating film damage of a buried coating steel pipe according to claim 1, wherein a damage position in the circumferential direction is obtained. 3. A plurality of a plurality of pipes are provided at positions symmetrical with respect to the pipe axis of the buried coating steel pipe by applying a low-frequency alternating voltage between the buried coating steel pipe and a grounding electrode in the ground. 1 of the vertical holes
The potential difference inside the ground when the first electrode is inserted into one vertical hole, the second electrode is inserted into the other vertical hole, and the depths from the ground surface of the first and second electrodes are the same. Of the embedded coating-coated steel pipe based on the potential difference distribution obtained when the first and second electrode depths are changed, and the coating film of the embedded coating-coated steel pipe is characterized by Damage detection method. 4. A reference signal extracted from the AC voltage, wherein an AC voltage according to a random signal or a pseudo-random signal is used as the AC voltage applied between the buried coating steel pipe and the ground electrode in the ground. And a detection signal of a potential difference between the electrodes, correlation processing is performed, and a potential difference distribution is obtained using the peak value as a representative value of the potential difference between the two points. A method for detecting coating film damage of a buried coating steel pipe according to. 5. An alternating voltage according to a pseudo-random signal is used as an alternating voltage applied between a buried coated steel pipe and a grounding electrode in the ground, and a reference signal having the same pattern as the alternating voltage. 4. The potential difference distribution is obtained by performing correlation processing with a detection signal of the potential difference between the electrodes, and using the peak value as a representative value of the potential difference between the two points. A method for detecting coating film damage of a buried coating steel pipe as described.