CN108680607B

CN108680607B - Pipeline crack corrosion monitoring method based on multidirectional alternating current potential drop

Info

Publication number: CN108680607B
Application number: CN201810998931.3A
Authority: CN
Inventors: 甘芳吉; 黄仕磊; 李文洋
Original assignee: Sichuan University
Current assignee: Sichuan University
Priority date: 2018-08-30
Filing date: 2018-08-30
Publication date: 2021-02-26
Anticipated expiration: 2038-08-30
Also published as: CN108680607A

Abstract

The invention provides a pipeline crack corrosion detection method based on multidirectional alternating current potential drop, which firstly provides a multidirectional current method, exciting currents are introduced from four different directions to obtain four groups of voltage ratios, the maximum voltage ratio is selected to be substituted into a depth solving formula according to the relation between the electrode crack direction and the voltage ratios, and the depth of cracks is solved. The multi-directional current method of the invention changes the included angle (complementary angle) range of the crack position and the current pole connecting line from 0-90 degrees to 67.5-90 degrees, and obviously improves the accuracy of crack depth solving.

Description

Pipeline crack corrosion monitoring method based on multidirectional alternating current potential drop

Technical Field

The invention belongs to the technical field of pipeline detection, and particularly relates to a pipeline crack corrosion monitoring method based on multidirectional alternating current potential drop.

Background

Pipeline corrosion is the most dangerous one of the many risks that oil and gas pipelines face, and 70% -90% of pipeline safety accidents are caused by pipeline corrosion.

At present, the commonly used pipeline corrosion detection technology, including a relatively advanced Field Signature Method (FSM), also has high detection precision only for local corrosion (localized corrosion) and homogeneous corrosion (general corrosion); the newly proposed Alternating Current Field Signature Method (ACFSM) estimates the corrosion depth by using the measurement data of a plurality of frequency points, and can improve the detection accuracy of crack corrosion, but the Method requires that the included angle between the crack direction and the Current electrode connecting line direction is greater than 45 degrees, cannot solve the crack with the included angle less than 45 degrees, and can limit the detection range of crack defects because the Current excitation electrode connecting line direction is parallel to the pipeline axial direction.

Therefore, in order to meet the actual detection requirements, it is necessary to deeply research the random crack corrosion detection technology.

Disclosure of Invention

The invention aims to solve the defects in the prior art and provides a pipeline crack corrosion detection method based on multidirectional alternating current potential drop, which can be used for more accurately detecting the corrosion depth of random cracks by applying exciting currents in different directions to a detected pipeline to change the range of an included angle (complementary angle) between the crack direction and the current pole connecting line direction from 0-90 degrees to 67.5-90 degrees.

The invention adopts the following technical scheme:

the pipeline crack corrosion detection method based on the multidirectional alternating current potential drop comprises the following steps:

step 1, welding and arranging a plurality of groups of test electrodes in the circumferential direction of the test pipeline.

And 2, generating an excitation signal by the signal generator, generating an excitation current by the power amplifier, sequentially introducing the excitation current to the input electrode from four different directions, wherein the frequency of the power amplifier is adjustable, and the amplitude of the excitation current is adjustable.

Step 3, measuring the voltage of the crack-free normal pipeline by using a high-precision phase-locked amplifier to obtain a normal pipeline test voltage value, inputting the normal pipeline test voltage value into a computer, measuring the voltage to be measured in the area to be measured of the test pipeline by using the high-precision phase-locked amplifier, and inputting the measurement result into the computer to obtain four groups of different voltage ratios;

and 4, selecting the maximum voltage ratio to substitute the following depth solving formula according to the relation between the crack direction and the voltage ratio, and solving by using a computer:

and 5, obtaining the depth of the pipeline crack according to the result displayed by the computer.

The further technical scheme is that in the

step

2, 4 groups of test electrodes arranged in the circumferential direction refer to the horizontal direction, the vertical direction and the diagonal direction, wherein the input electrodes and the output electrodes are arranged oppositely, and the connecting lines of the input electrodes and the output electrodes pass through the part to be tested.

The further technical scheme is that in the step 3, when the high-precision phase-locked amplifier is used for testing the voltage of the area to be tested, the testing direction of a connecting line of an input electrode for inputting the excitation current and an output electrode for outputting the excitation current is consistent with the testing direction of a connecting line of the measuring probe.

The preferred technical scheme is that the frequency of the power amplifier is selected to be 59-500Hz, and the amplitude of the exciting current is selected to be 2A.

Preferably, in step 2, the frequency of the power amplifier is selected to be 100 Hz.

The invention has the beneficial effects that:

1. the invention provides a multidirectional current method for the first time, exciting currents are introduced from four different directions to obtain four groups of voltage ratios, and the maximum voltage ratio is selected to be substituted into a depth solving formula according to the relation between the electrode crack direction and the voltage ratios; the multi-directional current method of the invention changes the included angle (complementary angle) range of the crack position and the current pole connecting line from 0-90 degrees to 67.5-90 degrees, thereby obviously improving the crack depth and the solving precision.

2. The invention adopts the principle of the AC potential drop technology, so the invention has the advantages of small exciting current, high measuring sensitivity, strong anti-interference capability and the like.

3. Compared with the traditional potential drop technology ACFSM which adopts single excitation current parallel to the axis of the pipeline, the method disclosed by the invention quantitatively solves the depth of the random crack defect for the first time, and shows that when the current flows through the crack defect, the direction of the crack influences the distribution of a current field according to simulation and experimental data results, so that the voltage of the measuring electrode is changed.

Drawings

FIG. 1 is a schematic diagram of skin current distribution;

FIG. 2 is a graph showing voltage profiles at different thicknesses for a test;

FIG. 3(a), FIG. 3(b), FIG. 3(c), FIG. 3(d), FIG. 3(e) are schematic views of the connection lines between the five crack directions and the electrodes;

FIG. 4 is a depth map of the ratio of cracks at different angles;

FIG. 5 is a schematic view of the multi-directional current measurement of the present invention;

FIG. 6 is a graph of maximum ratio depth for two crack defects from simulation testing.

FIG. 7 is a diagram of an experimental setup for the method of the invention;

FIG. 8 is a schematic diagram of a metal plate probe layout;

FIG. 9 is a schematic back view of the experimental plate.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the technical solutions of the present invention are described below clearly and completely, and it is obvious that the described embodiments are some, not all embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

Interpretation of terms:

1.1 skin Effect

Alternating excitation currents (AC) of different frequencies are passed through the conduit and the current distribution follows the skin effect. Formula for calculating skin depth δ:

mu in the above formula_rIs the relative permeability of the material; mu.s₀Is a vacuum magnetic conductivity; σ is the conductivity of the material; f is the frequency of the excitation current. The current distribution is schematically shown in fig. 1.

From the equation (1), when a high-frequency excitation current is applied, the current is concentrated on the outer wall of the pipeline, the current gradually permeates into the inner wall along with the reduction of the frequency of the excitation current, and the current density J (r) in the pipeline wall meets the equation:

wherein,

wherein I is the current amplitude with angular frequency omega; r is the outer radius of the pipe; r is the radial distance; j. the design is a square₀(kr) is a zeroth order Bessel function of the first kind, J₁(kR) is a first order Bessel function of the first kind.

According to the relationship between the current density and the voltage, the expression of the voltage value U (r) is as follows:

where l is the spacing of the measurement electrodes.

By replacing the Bessel function in equation (3) with an exponential function approximation, the voltage value amplitude can be expressed as:

measuring the contact depth d between the electrode and the outer wall of the pipeline₀The voltage amplitude U measured by the measuring electrode after the exciting current is applied is represented by formula (5):

the wall thickness of the pipeline is T, and the original voltage value measured before the pipeline is not corroded is U_(d＝0)After the pipeline is put into production, the actually measured voltage value is U_(d). When the corrosion defect of the inner wall of the pipeline is extremely shallow (d ≈ 0), the following conditions are always satisfied along with the reduction of the frequency:

U_(d)/U_(d＝0)≈1；

if there is a uniform corrosion defect with depth d at the bottom, measuring the voltage value U after delta is T-d_(d)Remains unchanged, U_(d)/U_(d＝0)≈m/U_(d＝0)(m is a constant); when the defect is a crack defect, the current around the defect "layer" also penetrates downward as the frequency decreases after δ ═ T-d, and U also penetrates downward_(d)<m，U_(d)/U_(d＝0)<m/U_(d＝0)。

U of crack defect_(d)/U_(d＝0)The value can be determined from U in two extreme cases (no corrosion defect, uniform corrosion defect)_(d)/U_(d＝0)Linear superposition approximation:

in the formula a₁,a₂,a₃Is a constant related to the measured material property.

1.2, multidirectional Current

A pipe with a length of 400mm, an inner diameter of 140mm and an outer diameter of 160mm was subjected to finite element analysis, and the finite element software was COMSOL Multiphysics 5.0. The measuring area is positioned in the middle of the pipeline, the distance between the probes is 20mm, the amplitude of the injection current is 2A, and the defect depth is 2mm, 2.5mm, 3mm, 3.5mm, 4mm, 4.5mm, 5mm, 5.5mm and 6 mm.

The material parameters are shown in table 1:

it can be known from the formula (3) that the voltage distribution at different 'thickness layers' is different when the excitation current frequency is different, and in order to ensure that the penetration current contacts the 'defect layer', a proper excitation current needs to be set, and the voltage distribution is obtained when the frequency is 2.5KHz and 500Hz based on the voltage (equal voltage distribution) at each radial distance during direct current, as shown in fig. 2.

As can be seen, the excitation current has a frequency that is too high, e.g., 2.5KH_ZIn the process, the current cannot completely penetrate through the pipeline wall, under the condition, the penetrating current cannot contact a 'defect layer' with shallow depth, and the measured signal cannot reflect defect information; the excitation current frequency is reduced, the skin depth is increased, and the penetration current can reach the 'defect layer' of each depth, such as the frequency of 500 Hz.

Considering that the wall thickness of the pipeline is 10mm, the maximum skin depth is 10mm, and the current frequency corresponding to the maximum skin depth can be calculated to be 59Hz according to the formula (1) and the parameters in the table 1.

Taking four position defects in FIG. 3(b) and FIG. 3(e) as examples (b)

The angle between the crack direction and the electrode connecting line direction).

Fig. 3(a) shows a defect-free pipe, where the measured voltage value is U (d is 0, f is 100Hz) as the original voltage:

the voltage values U of the defects at the four positions of the graph 3(b) to the graph 3(e) are measured at different depths_(d)And U_(d＝0)The results obtained are shown in FIG. 4.

As can be seen from FIG. 4, when the crack direction forms an angle with the electrode connecting line direction

When the angle is less than 45 degrees, U_(d)/U_(d＝0)The value of (D) and the crack depth do not conform to equation (6), where U_(d)/U_(d＝0)It cannot be used to solve for defect depth;

when in use

When the angle is larger than 45 degrees, the angle is changed,

the closer to 90 degrees, U_(d)/U_(d＝0)The more the relation between the value of (A) and the crack depth follows the exponential distribution form of the equation (6), the more U is expressed_(d)/U_(d＝0)The closer the crack depth calculated by substituting equation (6) to the actual depth,

the solving precision can reach 97.16% when the angle is 90 degrees; and simultaneously, simulation data also shows that:

the closer to 90 degrees, the U_(d)/U_(d＝0)The larger the value of (c).

Thus, for solving the problem for the depth of random cracks, use is made of

The closer to 90 degrees, the higher the measurement precision, and a multidirectional current method is innovatively provided. By adding three groups of exciting currents, so that

The range was changed from 0-90 deg. to 67.5-90 deg., and fig. 5 is a measurement diagram.

A set of characterizing signals can be obtained for the same defect at the same frequency:

get the largest U_(d)/U_(d＝0)And substituting the values into an equation (6) to solve the crack depth.

2. The pipeline crack corrosion detection method based on the multidirectional alternating current potential drop comprises the following steps:

The further technical scheme is that in the

step

3. Example 1 simulation test Using a Flat Panel model

The flat model is solved accurately.

The length of the flat plate is 220mm, the width is 220mm, the thickness is 10mm, the distance between measuring probes is 20mm, the amplitude of the injection current is 2A, and the frequency is 100HZ, which is shown in detail in FIG. 6.

The defect depth is 2mm, 2.5mm, 3mm, 4mm, 4.5mm, 5mm, 5.5mm, 6mm, and the material parameters are as follows in Table 2:

taking 0 degree crack defect and 45 degree crack defect as examples, the measurement results obtained under the above simulation conditions are:

taking the value of the 0-degree defect for depth fitting, wherein the formula is as follows:

the results of the 45 degree defect calculation using equation (7) are shown in table 2 above.

Example 2 tests were carried out using 5 aluminium panels

Experimental materials: 5 pieces of 220mm by 10mm aluminum plates, the entire experimental set-up is shown in FIGS. 7-8.

Before the unprocessed defect, sequentially passing through I_in1、I_in2、I_in3、I_in4Electrodes inject 2A, 100Hz AC into the plate, as shown in FIGS. 5 and 8, and high precision digital lock-in amplifiers SR850 are used to measure the probe pairs (P)₁-P₂)、(P₁-P₃)、(P₂-P₃)、(P₂-P₄) In betweenOriginal voltage U_1(d＝0)、U_2(d＝0)、U_3(d＝0)、U_4(d＝0)。

Secondly, respectively engraving 5 cracks at the bottoms of the 5 flat plates, wherein the No. 1-3 flat plates have 0-degree crack defects, the depths are respectively 2mm, 4mm and 6mm, and the widths are respectively 2 mm; no. 4-5 flat plates were 45 degree crack defects with depth distributions of 3mm, 5mm, as shown in FIG. 9.

Then, by I_in1、I_in2、I_in3、I_in4The electrodes, as shown in FIG. 8, can inject 2A current, 59-500Hz AC, preferably 100Hz AC, into the plate in turn, and measure U_1(d)、U_2(d)、U_3(d)、U_4(d)。

The measured voltage was compared to the original voltage and the results obtained are shown in table 3 below:

note: u shape₁₀＝U_1(d＝0),U₂₀＝U_2(d＝0),U₃₀＝U_3(d＝0),U₄₀＝U_4(d＝0)。

Maximum U corresponding to each depth_(d)/U_(d＝0)The values are substituted into equation (7), and the results are shown in table 4 below.

Therefore, the multi-directional current method can solve the random crack depth, compared to the case where the random position defect cannot be detected in the past.

The method for solving the depth by using the voltage ratio in the formula (7) can eliminate environmental interference and improve the anti-interference capability of the measurement system.

Conclusion analysis:

the traditional potential drop technology adopts single exciting current parallel to the axis of the pipeline, and the invention utilizes the included angle between the current and the crack

At 90 deg., the maximum ratio of the measured voltage to the original voltage and the best function of the ratio and the defect depth are supplied with AC exciting current from four directions, so that

From 0-90 degrees to 67.5-90 degrees, and the maximum voltage ratio is selected to calculate the defect depth.

Experimental data of simulation tests show that the new calculation method improves the detection precision of random cracks and can detect the corrosion condition more effectively.

Finally, it should be noted that: the above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.

Claims

1. The pipeline crack corrosion detection method based on the multidirectional alternating current potential drop is characterized by comprising the following steps of:

step 1, welding and arranging a plurality of groups of test electrodes in the circumferential direction of the test pipeline to enable the included angle between the crack direction and the electrode connecting line direction

Step 2, the signal generator generates an excitation signal and generates an excitation current through the power amplifier, the excitation current is sequentially introduced to the input electrode from four different directions, the frequency of the power amplifier can be adjusted, and the amplitude of the excitation current can be adjusted;

the original voltage value measured before non-corrosion is U_(d＝0)；

After the pipeline is put into production, the actually measured voltage value is U_(d)；

2. The method for detecting the pipeline crack corrosion based on the multi-directional alternating current potential drop is characterized in that in the step 2, the frequency of a power amplifier is selected to be 59-500Hz, and the amplitude of an excitation current is selected to be 2A.

3. The method for detecting pipeline crack corrosion based on multidirectional alternating current potential drop as claimed in claim 2, wherein the frequency of the power amplifier is selected to be 100 Hz.

4. The method for detecting pipeline crack corrosion based on multi-directional alternating current potential drop as claimed in claim 1, wherein in step 2, 4 groups of test electrodes arranged in the circumferential direction are in the horizontal direction, the vertical direction and the diagonal direction, wherein the input electrode and the output electrode are arranged oppositely, and the connecting lines of the input electrode and the output electrode pass through the part to be tested.

5. The method for detecting pipeline crack corrosion based on multi-directional alternating current potential drop as claimed in claim 1, wherein in step 3, when the high-precision phase-locked amplifier is used for testing the voltage of the area to be tested, the connection line of the input electrode for inputting the excitation current and the output electrode for outputting the excitation current is consistent with the connection line test direction of the measurement probe.