JP3007390B2 - Measuring method and measuring device for coating coverage area of underground pipe - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method and an apparatus for measuring the area of a coating defect of a metal pipe buried underground.

(Prior art) Many of the main pipelines that transport gas and oil are buried underground, so they suffer from soil corrosion and electrolytic corrosion (corrosion caused by DC current leaking from the railway tracks into the ground). easy. Therefore, in order to prevent such soil corrosion and electrolytic corrosion, conventional pipes or bituminous insulation coatings are used to cover the pipes, and at the same time, the coatings may be damaged or damaged. In order to prevent electrolytic corrosion when the deterioration of the coating has progressed, electrolytic protection is performed to keep a state in which a direct current is constantly flowing into the defective coating portion.

In order to evaluate whether the anticorrosion effect on the corroded portion of the buried pipe subjected to cathodic protection (called a coating defect) is sufficient, it is necessary to operate the coating defect of the buried pipe from the ground. However, conventionally, a needle electrode method and a Pearson method are known as a method of searching for a coating / covering defect.

With the needle electrode method, a DC current (repeated intermittently to make it easy to separate from stray currents) is applied to the buried pipe from the grounding electrode, and the potential gradient (voltage between two points at a constant interval) on the ground surface is applied. This is an exploration method that moves on a pipeline while measuring, and determines that a location where the potential gradient is maximum is a coating-covering defect.

On the other hand, the Pearson method has the same measurement principle as the needle electrode method,
Instead of a direct current, an alternating current that is easy to distinguish from a stray current flows into the buried pipe from the ground electrode, and a potential gradient on the ground surface is encouraged to promote an AC voltage response. "Corrosion prevention technology" Vol.22, No.9, p.383).

In any case, it is sufficient to find out the coating defect.

(Problems to be Solved by the Invention) By the way, the anticorrosion effect of the coating and covering defect is evaluated as good if the anticorrosion current flowing through the unit area of the defective portion is equal to or more than a certain value. In this case, a large error often occurs because the area of the coating is qualitatively and relatively estimated from the height of the potential gradient peak. In other words, the height of the peak of the potential gradient obtained by the needle electrode method or the Pearson method differs significantly depending on the burial depth of the tube from the ground surface and, for large-diameter tubes, depending on whether the tube is on the surface side or the opposite side of the tube. The measurement area of the mounting defect is affected,
There is a problem that it is difficult to obtain an accurate area of the coating and covering defect, and thus the evaluation of the anticorrosion effect cannot be correctly performed.

The present invention has been made in view of the above points, and has as its object to accurately measure the area of a coating defect of a metal pipe buried underground coated with a coating for corrosion protection.

(Means for Solving the Problems) In order to achieve the above object, according to the present invention, an alternating current is applied to a metal pipe which is coated and buried in the ground, and an AC potential distribution generated around a coating defect is provided. Is detected by at least three reference electrodes inserted into the surrounding ground, and the AC potential distribution is analyzed to calculate the defect area of the coating. Further, in the present invention, in order to carry out this method, an AC power supply for applying an alternating current to a metal tube which has been coated and buried underground, and an AC power supply around a coating defect of the metal tube. At least three reference electrodes inserted into the ground at a distance, an AC voltmeter switchably connected to the reference electrode, and an AC potential distribution around the coating defect output from the AC voltmeter. An apparatus for measuring the area of the coating-covering defect by analyzing and calculating means for calculating the area of the coating-covering defect was constructed.

(Operation) The potential distribution generated around the coating defect of the buried pipe is detected by the reference electrode, and the area of the coating defect is calculated by analyzing the potential distribution.

(Example) Hereinafter, the present invention will be described with reference to the drawings.

FIG. 1 is a schematic diagram of an apparatus for measuring a defect area of a coating device according to the present invention.

In the figure, reference numeral 1 denotes a metal underground pipe provided with a coating 1a, 2 denotes a coating / covering defect of the buried pipe 1, and 3 denotes an AC current to the coating / covering defect 2 of the buried pipe 1 from underground. Ground electrode for pouring (a galvanic anode for cathodic protection can be substituted), 4
Is an AC power supply for supplying a constant level of AC current between the buried pipe 1 and the ground electrode 3, and 5 a, 5 b, and 5 c are inserted from the ground into the ground near the coating defect 2 of the buried pipe 1. This is a reference electrode for a book, and the distance between the electrodes is, for example, 50 cm. 6 is an AC voltmeter for measuring an AC voltage response between each of the reference electrodes 5a, 5b, 5c and the buried tube 1 (a lock-in amplifier having high selectivity for a specific frequency is optimal); 7 is the reference electrode 5
This is a scanner for switching the connection between a, 5b, and 5c.

Note that at least three reference electrodes 5a, 5b, and 5c are required, and grounding is performed so that the distance from the paint-covering defect 2 to each of the reference electrodes 5a, 5b, and 5c is different. At this time, the reference electrode is inserted into the ground to eliminate errors caused by disturbance of the AC electric field due to the ground surface and uneven resistance of concrete or asphalt on the pavement, so that the reference electrode is sufficiently brought into contact with the soil.
Reference numeral 8 denotes an arithmetic circuit having a microcomputer configuration for performing an arithmetic process described later using an output from the AC voltmeter 6. 9 denotes an area of the paint-covering defect which is a final result based on the result calculated by the arithmetic circuit 8. This is a display for displaying at a depth position.

Next, the measuring method of the present invention using the above-described measuring device will be described.

First, it is necessary to find the coating covering defect 2 of the buried pipe 1 before the measurement. There are several known methods besides the needle electrode method and the Pearson method described above. Particularly, in the case of a long buried pipe, it is convenient to use a movable coating and covering defect detector.

The search principle of this probe is to apply a current to a buried pipe and detect a potential change generated on the ground surface by a current leaking from a damaged defect by using a special receiver. An example is shown in FIG.

As shown in FIG. 2, a power supply 10 and a transmitter 11 are prepared on the surface of the ground, and a signal of a specific frequency is supplied from the speaker 11 to the pipe 1 buried under the ground. And recorder
When a probe 15 using conductive tires 14 is run along the pipe 1 on the ground, the probe data as shown in FIG. The positions of the covering defects A and B can be immediately grasped.

When the position of the coating covering defect is known in this way, mark it on the ground, and make a 50 cm deep pore there at intervals of about 50 cm,
The three reference electrodes 5a, 5b, 5c are inserted therein, and the scanner 7 and the AC voltmeter 6 are set (see FIG. 1).

Now, an alternating current of, for example, a frequency of about 255 Hz flows between the buried tube 1 and the ground electrode 3 by the alternating current power supply 4. The AC voltage response appearing between each of the reference electrodes 5a, 5b, 5c and the buried tube 1 is measured using the AC voltmeter 6 while switching the reference electrodes by the scanner 7. At this time, only the AC voltage response between one reference electrode (for example, 5a) serving as a reference and the buried pipe 1 is measured, and the rest are the reference reference electrode 5a and the reference electrode 5a.
The measurement and analysis accuracy is higher when the AC voltage response between b and 5c is measured and the measured value is corrected to calculate the AC potential distribution.

The AC potential distribution thus obtained is analyzed by the following method.

A conductive sphere of radius r buried in the soil where the soil is uniform (soil resistivity ρ is constant) and there are no obstacles around,
When an alternating current I _AC is applied from an infinite ground electrode buried at infinity, an AC potential E _AC measured at a point L away from the center of the conductive sphere is represented by the following equation (1). .

Here, in order to apply this formula to a coating defect that actually exists, when the coating defect is approximated by a semiconductive spherical surface having the same area, when the coating defect area is S, S and r The relationship is expressed as follows (see Kohei Hoshino's technical journal, “Anti-corrosion Technology”, Vol. 16, No. 22).

Therefore, when the soil is uniform and there is no influence of obstacles, and the ground electrode is sufficiently away from the coating defect, the AC potential E _AC (5a) at the reference electrode 5a is expressed by the following equation (3).

As shown in FIG. 4, the horizontal distance between the coating defect 2 of the underground pipe 1 and the reference electrode 5a inserted into the ground in the present invention is defined as X (5a), and D _REF
(5a), assuming that the depth of the coating-covering defect 2 from the ground surface is D,
The distance L (5a) between the coating covering defect 2 and the tip of the reference electrode 5a is:
It is represented by the following equation (4).

Therefore, equation (1) can be modified as follows.

Since this equation holds for each of the reference electrodes, the same equation holds for the reference electrode (5b). As a result, the following equation is derived by solving the area S of the coating covering defect from these two equations.

Similarly, for the reference electrode 5b and the reference electrode 5c, the following formula is derived for the coating covering defect area S.

Equations (6) and (7) representing the two coating-covering defect areas S are equations composed of two unknowns, the area S of the coating-covering defect and the depth position D of the coating-covering defect. ,
By solving the simultaneous equations, the depth position D of the coating defect and the area S of the coating defect are obtained.

As a solution of the simultaneous equations, a plot is made as shown in FIG. 5 by plotting the depth position D of the coating defect on the horizontal axis and the area S of the coating defect on the vertical axis (log scale). And a method of solving the coordinates of the intersection A of these two curves is used.

In this embodiment, the equations (6) and (7) are written in advance in the arithmetic circuit 8, and the AC potential E _AC (5a) obtained from the AC voltmeter 6 for the reference electrodes 5a, 5b, 5c. ), E _AC
By substituting (5b) and E _AC (5c), the solution of the area S and the depth position D of the coating covering defect can be obtained. The solution of the simultaneous equations using such a microcomputer is already known and will not be described further.

In order to improve the analysis accuracy, a method of increasing the number of grounding reference electrodes, measuring the AC potential distribution in detail, and measuring the depth position and area of the coating covering defect from the distribution of the intersections of the plurality of curves. It is valid.

FIG. 6 shows a coating-coated metal pipe having a known coating-covering defect area buried in the ground, and coated with a reference electrode inserted at a depth of 70 cm into the ground by the method of the present invention. The result of estimating the area of the covering defect is shown. The horizontal axis in the figure is the actual defect area, and the vertical axis is the estimated defect area. From this figure, it can be seen that the area measuring method according to the present invention enables highly accurate estimation of the defect area. In the figure, the mark ○ indicates the case where the defect is located near the surface of the buried pipe, the mark △ or △ indicates the case where the defect is located on the side of the tube, and the mark □ indicates the case where the defect is located far from the surface of the tube. .

(Effects of the Invention) As described above, in the present invention, an alternating current is applied to a metal pipe buried in the ground covered with coating, and the AC potential distribution generated around the coating-covered defect is converted to the ground potential in the surrounding area. The area of the coating-covering defect is calculated by detecting with at least three reference electrodes inserted therein and analyzing the AC potential distribution thereof, so that the area of the defect reflecting the depth position of the defect is obtained. And accurate area measurements are obtained. Therefore, by using the defect area required by the present invention, the anticorrosion effect of the underground pipe can be correctly evaluated.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of an apparatus for measuring the area of a coating defect of an underground pipe according to the present invention, and FIG. 2 is a schematic diagram of a conventionally known system for detecting a coating defect of a movable underground pipe. Diagram, third
FIG. 4 shows an example of search data obtained from the paint-covered defect detector shown in FIG. 2, FIG. 4 is a view for explaining the principle of measuring the paint-covered defect area in the present invention, and FIG. FIG. 6 is a graph for explaining the analysis of the AC potential distribution in the measuring method according to the present invention, and FIG. 6 is a graph for demonstrating the effect of the measuring method according to the present invention. 1 ... Underground pipe, 1a ... Coating, 2 ... Coating defect
3: Ground electrode, 4: AC power supply, 5a, 5b, 5c: Reference electrode,
6 ... AC voltmeter, 7 ... Scanner, 8 ... Calculation circuit,
9 …… Display

Claims (2)

(57) [Claims] An AC current is applied to a metal tube buried in the ground by applying a coating, and an AC potential distribution generated around the coating defect of the metal tube is converted into a ground around the coating defect. A method for measuring the area of a coating defect of a buried underground pipe, wherein the area of the coating defect is calculated by detecting at least three inserted reference electrodes and analyzing an AC potential distribution thereof. Method. 2. An AC power supply for applying an AC current to a metal tube provided with a coating and buried in the ground, and at least three AC power sources inserted into the ground around the coating defect of the metal tube. The reference electrode of the present invention, an AC voltmeter switchably connected to the reference electrode, and an AC potential distribution around the coating defect output from the AC voltmeter is analyzed to determine the area of the coating defect. And a calculating means for calculating the coating area of the underground pipe.