A method for quantitative detection of magnetic signals

Abstract

The application provides a magnetic signal quantitative detection method, comprising the following steps: step 1, establishing a classical magnetic charge model, and defining a transmission compensation factor based on the classical magnetic charge model; step 2, constructing a radial transmission compensation factor; step 3, constructing an axial transmission compensation factor; step 4, describing the additional attenuation of the static magnetic shielding effect of the ferromagnetic material by the radial transmission compensation factor obtained in step 2 and the axial transmission compensation factor obtained in step 3, and introducing the classical magnetic charge model in step 1 for improvement to obtain an improved magnetic charge model, so as to solve the problem that the magnetic signal transmission process is affected when passing through the ferromagnetic wall thickness in the prior art, the amplitude of the magnetic flux leakage detection signal is reduced, the damage degree of the defect is underestimated, and the safety and service life of the material are incorrectly judged.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of quantitative research of magnetic flux leakage signals, and particularly relates to a magnetic signal quantitative detection method. BACKGROUND

[0002] In engineering practice, ferromagnetic material surface will produce metal volume defects, if the detection probe and the defect are respectively located at two surfaces of the material, the magnetic signal transmission process will be affected by the ferromagnetic wall thickness, which is mainly reflected in the amplitude reduction of the magnetic flux leakage detection signal, so that the defect damage degree is underestimated, and the safety and service life of the material are wrongly judged. SUMMARY

[0003] Therefore, the technical problem to be solved by the application is to provide a magnetic signal quantitative detection method, which solves the problem in the prior art that the magnetic signal transmission process is affected when passing through the ferromagnetic wall thickness, which is mainly reflected in the amplitude reduction of the magnetic flux leakage detection signal, so that the defect damage degree is underestimated, and the safety and service life of the material are wrongly judged.

[0004] In order to solve the above problems, the application provides a magnetic signal quantitative detection method, which comprises the following steps:

[0005] Step 1: establishing a classical magnetic charge model, defining a transmission compensation factor based on the classical magnetic charge model;

[0006] Step 2: constructing a radial transmission compensation factor;

[0007] Step 3: constructing an axial transmission compensation factor;

[0008] Step 4: describing the additional attenuation caused by the static magnetic shielding effect by using the radial transmission compensation factor obtained in step 2 and the axial transmission compensation factor obtained in step 3, and introducing the classical magnetic charge model in step 1 for improvement to obtain an improved magnetic charge model.

[0009] Optionally, step 1 specifically comprises: establishing a classical magnetic charge model: when the magnetic field passes through the defect of the ferromagnetic material, magnetic charges with different polarities are distributed on the two end faces of the defect, forming a magnetic charge accumulation, and then generating a magnetic field, a rectangular coordinate system is established at the center of the bottom of the defect, the X axis is along the axial direction of the pipeline, the Y axis is along the radial direction of the pipeline, and the Z axis is along the circumferential direction of the pipeline, the axial length of the outer defect is D x , the circumferential width of the outer defect is D z , the depth of the outer defect is D y , the coordinates of any magnetic charge source on the side wall are (x m , y m , z m ), and the radial signal H Y and the axial signal HX may be expressed as:

[0010]

[0011] where p is the magnetic charge density of the defect sidewall, μ0 is the vacuum permeability, r is the distance from the magnetic charge source to the detection point P, D y is the outer defect depth, D z is the outer defect circumferential width.

[0012] Optionally, step 1 further comprises: defining a transmission compensation factor based on the classical magnetic charge model, and the steps are as follows:

[0013] Step 1.1: Establish the shielding effectiveness, the formula is as follows:

[0014]

[0015] where SE is the shielding effectiveness, H out represents the magnetic field strength outside the shielding shell, H in represents the magnetic field strength inside the shielding shell, the determination of SE is related to the direction of the magnetic field, which is divided into two cases of radial magnetic field component and axial magnetic field component, wherein the radial magnetic field component H out_Xc is the axial magnetic field component H out_Zc ;

[0016] Step 1.2: Based on the shielding effectiveness, define the transmission compensation factor, that is, f shield , let: According to the direction of the magnetic field, it is also divided into two cases of radial transmission compensation factor f shield_xc and axial transmission compensation factor f shield_zc , let:

[0017] Optionally, step 2 specifically comprises: establishing a cylindrical shell shielding body and its cylindrical coordinate system (rc, θ, Zc), when the cylindrical shell type shielding body is subjected to the radial direction of the shielding shell outside the magnetic field, the free space inside and outside the cylindrical shielding body and the shielding shell inside satisfies the Laplace equation under the cylindrical coordinate system:

[0018]

[0019] wherein, Laplace operator, U out , U shield , U in respectively represent the scalar magnetic potential of the outer surface of the cylindrical cavity, the wall of the cylindrical cavity and the inner surface of the cylindrical cavity, R is the outer diameter of the cylindrical shell, r is the inner diameter of the cylindrical shell;

[0020] When the ferromagnetic material shield is affected by external constant magnetic field, the boundary condition of formula (3) satisfies the continuity theorem, and the solution of scalar magnetic potential of each space region can be obtained by solving formula (3), as shown in the following formula:

[0021]

[0022] Wherein, U out , U shield and U in represent the scalar magnetic potential of the outer surface of the cylindrical cavity, the wall of the cylindrical cavity and the inner surface of the cylindrical cavity respectively, H0 represents the leakage magnetic field generated by the external defect without shielding by the cylindrical shielding shell, R is the outer diameter of the cylindrical shell, r is the inner diameter of the cylindrical shell, μ r is the relative magnetic permeability;

[0023] The obtained U in , U out in formula (4) are substituted into respectively, so that the expression of the magnetic field intensity H in of the inner wall of the shielding shell and the magnetic field intensity H out of the outer wall of the shielding shell are obtained, and the obtained H in , H out are substituted into formula (2) to obtain the shielding effectiveness SE Xc of the cylindrical shielding shell to the radial magnetic field;

[0024] Based on the obtained SE Xc , SE Xc is substituted into formula , so that the radial transmission compensation factor f shield_Xc is obtained as follows:

[0025]

[0026] Wherein, H in , H out are the magnetic field intensity of the inner and outer walls of the shielding shell, R a is the average radius of the cylindrical shell, Δt is the residual wall thickness, μ r is the relative magnetic permeability, R is the outer diameter of the cylindrical shell, r is the inner diameter of the cylindrical shell, D y is the depth of the external defect, and t is the pipe wall thickness.

[0027] Optionally, step 3 further comprises: when the cylindrical shell type shielding body is affected by an external magnetic field parallel to the axis of the cylinder, the diffusion equation in the magnetic quasi-static field condition is satisfied inside the shielding shell, as shown in formula (6):

[0028]

[0029] Wherein, H shield_Zcis the axial component of the magnetic leakage field inside the shield shell, γ is the propagation constant, R is the outer diameter of the cylindrical shell, and r is the inner diameter of the cylindrical shell;

[0030] In addition, since the boundary condition of formula (6) satisfies the continuity theorem, the solution of H shield_Zc can be obtained by solving formula (6), and the shielding effectiveness SE shield_Zc of the cylindrical shield shell to the axial magnetic field is obtained based on the solution of H Zc , which is shown in formula (7):

[0031]

[0032] where I0 is the modified Bessel function of the first kind of order 0, K0 is the modified Bessel function of the second kind of order 0, K1 is the modified Bessel function of the second kind of order 1, I2 is the modified Bessel function of the first kind of order 2, γ is the propagation constant, R is the outer diameter of the cylindrical shell, r is the inner diameter of the cylindrical shell, and μ r is the relative magnetic permeability;

[0033] In the in-pipe detection model, the magnetic leakage signal is usually analyzed under the condition of a static magnetic field, and under the limit condition of a static magnetic field, SE Zc can be written as:

[0034]

[0035] where SE Zc is the shielding effectiveness of the cylindrical shield shell to the axial magnetic field, R a is the average radius of the cylindrical shell, Δt is the residual wall thickness, μ r is the relative magnetic permeability, γ is the propagation constant, R is the outer diameter of the cylindrical shell, D y is the outer defect depth, t is the pipe wall thickness, f is the frequency;

[0036] Substituting SE Zc into formula , the axial transmission compensation factor f shield_Zc can be obtained, and the formula is:

[0037]

[0038] where f shield_Zc is the axial transmission compensation factor, SE Zc is the shielding effectiveness of the cylindrical shield shell to the axial magnetic field, R a is the average radius of the cylindrical shell, Δt is the residual wall thickness, μ r is the relative magnetic permeability, γ is the propagation constant, R is the outer diameter of the cylindrical shell, D y is the outer defect depth, t is the pipe wall thickness, f is the frequency.

[0039] Optionally, step 4 specifically comprises:

[0040] The radial component H of the magnetic leakage signal that can be effectively detected under the action of the pipe wall, the axial component H of the magnetic leakage field that can be effectively detected eff_Y , the axial component H of the magnetic leakage field that can be effectively detected eff_X is expressed as:

[0041]

[0042] Wherein, the definition: H out_Y = H Y , H out_X = H X , and f shield_Xc in formula (5) and f shield_Zc in formula (9) are respectively substituted into formula (1) in the form of formula (10), to obtain an outer defect analytical model under the influence of pipe wall thickness:

[0043]

[0044] Wherein, R is the outer diameter of the cylindrical shell, t is the pipe wall thickness, D y is the outer defect depth, μ r is the relative magnetic permeability, ρ is the magnetic charge density of the defect side wall, μ0 is the vacuum magnetic permeability, D z is the outer defect circumferential width, x m , y m , z m are the three-axis coordinates of any magnetic charge source on the side wall, x, y, z are the three-axis coordinates of any point P in space, γ is the propagation constant, Δt is the remaining wall thickness, f is the frequency, and the magnetic leakage detection signal of the outer wall defect in the long oil and gas pipeline is quantified through formula (11).

[0045] Beneficial effects

[0046] The magnetic signal quantitative detection method provided in the embodiment of the application is based on the static magnetic shielding theory, proposes a transmission compensation factor, combines a classical magnetic charge model, and establishes an improved magnetic charge model. The model can solve the problem that in the prior art, the magnetic signal transmission process is affected by the ferromagnetic wall thickness, which is concentrated in the amplitude reduction of the magnetic leakage detection signal, so that the defect hazard degree is underestimated, and the safety and service life of the material are incorrectly judged. The specific application is, for example, the identification of the outer defect of the long oil and gas pipeline. BRIEF DESCRIPTION OF DRAWINGS

[0047] Figure 1 The figure is a change rule diagram of the magnetic leakage signal under different pipe wall thicknesses of the embodiment of the application;

[0048] Figure 2A leakage magnetic signal variation law graph under different defect depths of the embodiment of the present application is shown in the figure.

[0049] Figure 3 A leakage magnetic signal variation law graph under different defect lengths of the embodiment of the present application is shown in the figure.

[0050] Figure 4 A leakage magnetic signal characteristic value comparison graph of the same size inner and outer defects of the embodiment of the present application is shown in the figure.

[0051] Figure 5 A comparison between the classical model and the improved model when the wall thickness changes of the embodiment of the present application is shown in the figure.

[0052] Figure 6 A comparison between the classical model and the improved model when the defect depth changes of the embodiment of the present application is shown in the figure.

[0053] Figure 7 A comparison between the classical model and the improved model when the defect axial length changes of the embodiment of the present application is shown in the figure.

[0054] Figure 8 A cylindrical shell shielding body establishment graph of the embodiment of the present application is shown in the figure. DETAILED DESCRIPTION

[0055] For reference Figures 1 to 8 As shown in the figure, according to the embodiment of the present application, a magnetic signal quantitative detection method takes the leakage magnetic internal detection of outer defects of a long oil and gas pipeline as an example, and includes the following steps:

[0056] Step 1: Establish a classical magnetic charge model, and define a transmission compensation factor based on the classical magnetic charge model;

[0057] Further, the magnetic charge model has been widely applied to the analysis of leakage magnetic signals. When a magnetic field passes through a defect of a ferromagnetic material, magnetic charges of different polarities are distributed on the two end faces of the defect, forming a magnetic charge accumulation, and then generating a magnetic field. A rectangular coordinate system is established at the center bottom of the defect, the X-axis is along the axial direction of the pipeline, the Y-axis is along the radial direction of the pipeline, and the Z-axis is along the tangential direction of the pipeline. The axial length of the outer defect is D x , the circumferential width of the outer defect is D z , the depth of the outer defect is D y , the coordinates of any magnetic charge source on the side wall are (x m , y m , z m ), and the radial signal H y and the axial signal H x of the magnetic field formed at any point P(x, y, z) in space can be respectively represented as:

[0058]

[0059] where p is the magnetic charge density of the defect sidewall, μ0 is the vacuum permeability, r is the distance from the magnetic charge source to the detection point P, D y is the outer defect depth, D z is the outer defect circumferential width.

[0060] Taking the quantification of the outer wall defect in the long-distance pipeline magnetic flux leakage internal detection as an example, in the classical magnetic charge model, it is assumed that the material in the path of the leakage magnetic signal propagating to the probe is the same, but for the outer wall defect internal detection, this ignores the change of part of the material in the propagation path, that is, the actual situation is that part of the filler is a ferromagnetic material pipe wall, and the ferromagnetic metal component can absorb more magnetic lines of force because of its high permeability, so that the magnetic lines of force scattered in the air are reduced, that is, under the action of the ferromagnetic material pipe wall, the outer wall surface leakage magnetic field will be attenuated and then transmitted to the inner wall surface. The shielding coefficient (shielding ratio) is generally used to describe the attenuation ability of the ferromagnetic material to the magnetic field:

[0061]

[0062] where SE is the shielding effectiveness, H out represents the outer wall magnetic field strength of the shielding shell, H in represents the inner wall magnetic field strength of the shielding shell, and the determination of SE is related to the direction of the magnetic field, which is divided into two cases of radial magnetic field component and axial magnetic field component, wherein the radial magnetic field component H out_Xc is the axial magnetic field component H out_Zc .

[0063] In actual calculation, the loss of the leakage magnetic signal of the outer wall defect in the transmission process of the pipe wall needs to be compensated in order to accurately evaluate the size of the outer defect and better perform integrity analysis. In order to study the transmission characteristics of the leakage magnetic signal of the outer wall defect in internal detection, based on the shielding effectiveness, a transmission compensation factor f shield is defined, that is: According to the direction of the magnetic field, it is also divided into axial transmission compensation factor f shield_zc and radial transmission compensation factor f shield_xc , that is:

[0064] Further, the transmission compensation factor is a coefficient for describing the attenuation of the pipe wall to the leakage magnetic signal, and its determination is related to the direction of the magnetic field, which is divided into two cases of radial magnetic field and axial magnetic field.

[0065] Step 2: Construct the radial transmission compensation factor;

[0066] Further, a cylindrical shell shielding body and its cylindrical coordinate system (r c , θ, Z c ) are established, as shown in Figure 8 , the cylinder shell axis is along the Z cWhen the cylindrical shell type shielding body is subjected to the radial direction shielding shell outer wall magnetic field H out , the free space inside and outside the cylindrical shielding body and the shielding shell interior satisfy Laplace equation in cylindrical coordinate system:

[0067]

[0068] Wherein, Laplace operator, U out , U shield , U in respectively represent the scalar magnetic potential of the outer surface of the cylindrical cavity, the cylindrical cavity wall and the inner surface of the cylindrical cavity, R is the outer diameter of the cylindrical shell, and r is the inner diameter of the cylindrical shell.

[0069] When the ferromagnetic material shielding is subjected to the external constant magnetic field, the boundary condition of formula (3) satisfies the continuity theorem, and the solution of the scalar magnetic potential of each space region obtained by solving formula (3) is as follows:

[0070]

[0071] Wherein, U out , U shield , U in respectively represent the scalar magnetic potential of the outer surface of the cylindrical cavity, the cylindrical cavity wall and the inner surface of the cylindrical cavity, H0 represents the leakage magnetic field generated by the external defect without shielding by the cylindrical shielding shell, R is the outer diameter of the cylindrical shell, r is the inner diameter of the cylindrical shell, and μ r is the relative permeability.

[0072] Substitute U in , U out in formula (4) into , the expression of the inner wall magnetic field strength H in and the outer wall magnetic field strength H out of the shielding shell can be obtained. In the pipeline detection, the pipe wall acts as a shielding body, has the property of high relative permeability, and its radius is usually much larger than the pipe wall, and the obtained H in , H out values are substituted into formula (2) to obtain the shielding coefficient SE Xc of the cylindrical shielding shell to the radial magnetic field.

[0073] Based on the obtained SE Xc , SE Xc is substituted into formula , and the radial transmission compensation factor f shield_Xc is obtained as follows:

[0074]

[0075] Wherein, H in , H outis the magnetic field strength inside and outside the shielding shell, R a is the average radius, Δt is the residual wall thickness, μ r is the relative permeability, R is the outer diameter of the cylindrical shell, r is the inner diameter of the cylindrical shell, D y is the outer defect depth, t is the pipe wall thickness.

[0076] Step 3: Construct the axial transmission compensation factor;

[0077] Further, when the cylindrical shell type shielding body is subjected to an external magnetic field parallel to the axis of the cylinder, the diffusion equation inside the shielding shell satisfies the magnetic quasi-static field condition, as shown in equation (6).

[0078]

[0079] where H shield_Zc is the axial component of the magnetic leakage field inside the shielding shell, γ is the propagation constant, R is the outer diameter of the cylindrical shell, and r is the inner diameter of the cylindrical shell;

[0080] In addition, since the boundary conditions of equation (6) satisfy the continuity theorem, the solution of H shiel_Zc can be obtained by solving equation (6). Based on the solution of H shield_Zc , the shielding effectiveness SE Zc of the cylindrical shielding shell to the axial magnetic field can be obtained, as shown in equation (7):

[0081]

[0082] where I0 is the first kind 0 order modified Bessel function, K0 is the second kind 0 order modified Bessel function, K1 is the second kind 1 order modified Bessel function, I2 is the first kind 2 order modified Bessel function, γ is the propagation constant, R is the outer diameter of the cylindrical shell, r is the inner diameter of the cylindrical shell, and μ r is the relative permeability. In the in-pipe detection model, the magnetic leakage signal is analyzed under the static magnetic field condition, and under the static magnetic field limit condition, SE Zc can be written as:

[0083]

[0084] where SE Zc is the shielding effectiveness of the cylindrical shielding shell to the axial magnetic field, R a is the average radius of the cylindrical shell, Δt is the residual wall thickness, μ r is the relative permeability, γ is the propagation constant, R is the outer diameter of the cylindrical shell, D y is the outer defect depth, t is the pipe wall thickness, f is the frequency.

[0085] Substitute SE Zc into equation Then the axial transmission compensation factor f can be obtained. shield_Zc The formula is:

[0086]

[0087] Among them, f shield_Zc It is the axial transmission compensation factor, SE Zc It refers to the shielding effectiveness of the cylindrical shielding shell against the axial magnetic field, R. a The mean radius of the cylindrical shell is μ, Δt is the remaining wall thickness, and μ is the average radius of the cylindrical shell. r γ is the relative permeability, γ is the propagation constant, R is the outer diameter of the cylindrical shell, and D is the relative permeability. y Where t is the depth of the external defect and t is the pipe wall thickness. f It refers to frequency.

[0088] Step 4: The radial transfer compensation factor obtained in Step 2 and the axial transfer compensation factor obtained in Step 3 are used to describe the additional attenuation caused by the static magnetic shielding effect, and are introduced into the classical magnetic charge model in Step 1 for improvement, resulting in the improved magnetic charge model.

[0089] Furthermore, the static magnetic shielding effect of ferromagnetic materials will cause additional attenuation of the leakage magnetic signal when detecting external defects from within, which will cause deviations in actual measurements. Therefore, the radial and axial transmission compensation factors obtained in steps 2 and 3 are used to describe the additional attenuation caused by the static magnetic shielding effect, and are introduced into the classical magnetic charge model for improvement.

[0090] The radial component H of the leakage magnetic field that can be effectively detected by the pipe wall when detecting external defects from within is the magnetic leakage signal. eff_Y The axial component H of the leakage magnetic field that can be effectively detected eff_X Represented as:

[0091]

[0092] Wherein, H is defined as: out_Y =H Y H out_X =H X f in formula (5) shield_Xc With f in formula (9) shield_Zc Substituting formula (10) into formula (1) respectively, we can obtain the analytical model of external defects under the influence of pipe wall thickness:

[0093]

[0094] Where R is the outer diameter of the cylindrical shell, t is the wall thickness, and D... y It is the depth of the external defect, μ r ρ is the relative permeability, ρ is the magnetic charge density of the defect sidewall, μ0 is the free permeability, and D is the relative permeability.z It is the circumferential width of the external defect, x m y m z m Let be the three-axis coordinates of any magnetic charge source on the sidewall, x, y, z be the three-axis coordinates of any point P in space, γ be the propagation constant, and Δt be the residual wall thickness. f It is the frequency, and the leakage magnetic flux detection signal for detecting defects on the outer wall of a long-distance oil and gas pipeline can be quantified by formula (11).

[0095] Formula (11) can be used to quantify the magnetic flux leakage signal of external wall defects in long-distance oil and gas pipelines. The magnetic flux leakage signal of external wall defects is related to the pipe wall thickness, defect size, and pipeline properties. The proposed external defect magnetic charge model can effectively describe the distribution characteristics of the magnetic flux leakage field of pipeline external wall defects, and has scientific guiding significance for defect identification and quantitative assessment.

[0096] Using the theory of static magnetic shielding, a transmission compensation coefficient is introduced to compensate for the propagation attenuation of leakage magnetic signals in ferromagnetic materials, and finally an improved model for the quantification of leakage magnetic signals is established.

[0097] Furthermore, the present invention also includes a theoretical analysis section for verifying the accuracy of the algorithm. The theoretical analysis section takes X70 steel in actual engineering applications as the research object and establishes a model. The specific steps are as follows:

[0098] By obtaining f from formula (5) shield_Xc Substituting formula (1) into formula (10) yields H. eff_Y f obtained from formula (9) shield_Zc Substituting formula (1) into formula (10) yields H. eff_X The transmission compensation factor characterizes the additional attenuation of the magnetic signal caused by the magnetic shielding effect of the ferromagnetic pipe wall. This coefficient is associated with the classical magnetic charge model to establish a quantitative calculation model for the leakage magnetic field detection of defects on the outer wall of long-distance pipelines.

[0099] Furthermore, the present invention also includes a part for calculating and analyzing the magnetic flux leakage signal of the detection of external defects under the action of pipe walls of different thicknesses, the magnetic flux leakage signal of the detection of external defects under the action of different defect depths, and the magnetic flux leakage signal of the detection of external defects under the action of different defect lengths.

[0100] (1) Calculation of magnetic flux leakage signal for detecting external defects under the action of pipe walls of different thicknesses:

[0101] Construct an external defect with an axial length of 2D x =20mm, the circumferential width of the external defect is 2D z =40mm, external defect depth is D y=2.4mm rectangular defect size, set lift value is 1mm, wall thickness is t = 7mm ~ 17mm, interval is 1mm, the detection range of leakage magnetic field is scanned along the X-axis pipe axial direction through the defect center -75mm ~ 75mm area, according to formula (11) to explore the influence law of ferromagnetic wall thickness on the leakage magnetic signal characteristics of external wall defects.

[0102] Figure 1 As shown, the axial component of the magnetic flux leakage signal exhibits a maximum value characteristic, with the peak value decreasing as the wall thickness increases. The radial component presents a sinusoidal waveform with peak-valley characteristics, and the peak and valley values ​​gradually decrease as the wall thickness increases. This indicates that the magnetic flux leakage signal intensity at the external defect decreases with increasing pipe wall thickness. When the wall thickness increases from 7 mm to 17 mm, the axial maximum value decreases by 2989.88 A / m, and the radial peak-valley value decreases by 4592.50 A / m.

[0103] (2) Calculation of magnetic flux leakage signal detected inside external defects under different defect depths:

[0104] Construct an external defect with an axial length of 2D x =6mm, the circumferential width of the external defect is 2D z =20mm, external defect depth is D y =0.6mm~7.2mm rectangular defect size, lift-off value 1mm, wall thickness 15mm, the detection range of leakage magnetic field along the X-axis -200mm~200mm, according to formula (11) the influence law of external defect depth on leakage magnetic signal characteristics.

[0105] Figure 2 As shown, under the same wall thickness, with the increase of the external defect depth, the axial component maximum value V max With radial component peak and valley values ​​V p-v All show a non-linear increasing trend. The increase is relatively small within the defect depth range of 0mm to 5mm, but increases rapidly in the range of 5mm to 7mm. When the external defect depth is 7.2mm, V max The amplitude reached 7687 A / m. The radial signal exhibited peaks and troughs with a spacing of 20 mm, corresponding to the axial length of the defect. V p-v The amplitude reached 5382 A / m.

[0106] (3) Calculation of magnetic flux leakage signal detected inside external defects under different defect lengths:

[0107] Construct an external defect with an axial length of 2D x =40mm~120mm, the circumferential width of the external defect is 2D z =20mm, external defect depth is D y=7mm rectangular external defect size, lift-off value 1mm, wall thickness 15mm, leakage magnetic field detection range along X-axis -100mm~100mm, according to formula (11) to study the influence law of external defect axial length on leakage magnetic signal characteristics.

[0108] Figure 3 As shown, with the increase of defect length, the signal waveform of the axial component exhibits double maxima, and V max The axial length of the defect decreases non-linearly with increasing defect length. The axial length of the inner wall defect increases from 40 mm to 120 mm, and the axial V... max Attenuation of 3086 A / m, radial V p-v The axial V of the outer wall defect decreased by 11184 A / m. max Attenuation of 1341 A / m, radial V p-v The attenuation was 791 A / m; the axial distance between the two maxima of the axial signal gradually increased with a clear trend, and the axial distance between the peaks and valleys of the radial component increased significantly with the increase of the defect length. Comparing the analytical results of defects of the same size on the inner and outer walls, the axial V of the outer defect... max Radial V p-v The amplitude of the inner defect is significantly reduced. Axial V max The attenuation rate increased from 75.5% to 80.6%, with the radial peak-to-trough attenuation rate reaching as high as 91.8%.

[0109] As another preferred embodiment, the present invention also includes an experiment to verify the variation law of leakage magnetic signal detected in the external defect. The experiment uses three X70 pipes with wall thicknesses of 8 mm, 12 mm and 15 mm, and inner diameters of 602 mm, 598 mm and 595 mm, respectively. The 8 mm thick pipe is welded from four sections of pipe with a length of 1510 mm. The 12 mm and 15 mm thick pipes are each welded from five sections of pipe with a length of 1510 mm. Multiple sets of rectangular defects are prefabricated on the outer wall of the test pipes with different wall thicknesses using wire cut electrical discharge machining (WEDM).

[0110] The experimental setup mainly includes an internal magnetic flux leakage detector and a winch.

[0111] Use a winch to drag the magnetic flux leakage detector through the pipe being tested.

[0112] The internal magnetic flux leakage detector mainly consists of the following parts:

[0113] The drive system includes: a drive cup for capturing the pressure difference of the transport medium to propel the magnetic flux leakage detector within the pipeline; in simulation experiments, a winch provides the driving power for the detector; a support cup for maintaining posture and preventing friction, ensuring smooth operation of the detector; a universal joint for steering adjustment; and a mileage wheel for recording defect locations. The excitation and magnetic conduction system includes: a permanent magnet, steel brushes, and a yoke. The permanent magnet is made of NdFeB material, and its coercivity can provide an axial excitation magnetic field of 15000 A / m to supersaturate the pipeline; the steel brushes and yoke form a magnetic circuit with the pipe wall. The measurement and storage system includes: a ring probe array and a device for storing measurement data; the ring probe array uses a Hall sensor MLX90393 with a magnetic field resolution of up to 16 bits, capable of resolving magnetic flux leakage signals as small as 3.22 μT, with a detection range exceeding 1000 mT. The preamplifier circuit is made of non-magnetic steel, and the connection between the front end and the Hall element and the pipe wall is made of a highly permeable and wear-resistant material. The axial sampling distance of the probe is 2mm, and there is a 1mm lift-off value (air gap) between the probe and the tube wall to reduce resistance and protect the probe. Since magnetic flux leakage detection uses a very strong permanent magnet, and the commonly used storage devices are hard disks or tape drives, once affected by a strong magnetic field, the data on them will be destroyed, or even the storage device itself will be damaged. Therefore, a flash hard disk is used.

[0114] Depend on Figure 4 It can be seen that, under the same pipe and defect size conditions, the radial and axial signal amplitudes of the inner wall defect are significantly higher than those of the outer wall defect. This indicates that the pipe wall has an attenuating effect on the magnetic signal of the outer wall defect, fully demonstrating the rationality of the transmission compensation factor. Furthermore, the axial and radial components of the defect leakage magnetic signal are consistent with the analytical waveform. The axial component exhibits a maximum value, while the radial component presents a sinusoidal waveform with peaks and valleys, verifying the accuracy of the analytical model.

[0115] Depend on Figure 5 It can be seen that as the wall thickness increases, the axial and radial signal characteristic values ​​of the experimental data show a nonlinear decreasing trend, consistent with the theoretical analysis of the improved model, while the classical magnetic charge model shows no response to changes in wall thickness. For the improved model, when the wall thickness varies from 8mm to 12mm, the axial signal V... max The attenuation reached 60.16%, and the radial signal V p-v The attenuation rate reached 59.33%; the axial signal V in the 12mm to 15mm segment max The attenuation amplitude is 21.05%, and the radial signal attenuation V p-vThe amplitude was 20.67%, indicating that the thicker the wall, the smaller the attenuation rate, and the less sensitive the eigenvalue response becomes. To more fully compare the influence of the classical model and the model after wall thickness compensation on the magnetic signal characteristics, the absolute errors between the eigenvalues ​​of the two models and the experimental results were extracted and compared. The absolute errors between the classical model and the experimental results both increased with increasing wall thickness, reaching as high as 80.00% at a wall thickness of 15mm. The absolute error between the improved model and the experimental results was significantly reduced compared to the classical model, making it more consistent with reality. Especially for thinner pipes, the error between the experimental and improved models did not exceed 10%. Moreover, at a wall thickness of 15mm, the absolute error between the two models reached 75.06%, fully demonstrating that the improved model outperformed the classical model and was suitable for evaluating external defects.

[0116] Depend on Figure 6 It can be seen that when the defect depth increases from 0.6 mm to 7.2 mm, the axial component V max It increased by 2090%, radial component V p-v The growth rate increased by 2430%. As the defect depth increased, the axial and radial signal characteristic values ​​in the experimental data showed an exponential growth trend, consistent with the theoretical analysis of the improved model. In contrast, the classical magnetic charge model showed a near-linear (or logarithmic) growth trend, with a low agreement with the experimental growth rate. The error histogram shows that the errors between the improved model and the experimental results are smaller than those of the classical model. Furthermore, for shallow external defects, the maximum error between the two models reached 9.78%, indicating that the improved model outperformed the classical model.

[0117] Depend on Figure 7 It can be seen that as the defect length increases, the axial and radial signal eigenvalues ​​of the experimental data both exhibit an exponential decay trend, consistent with the theoretical analysis of the improved model. Comparing the experimental results, as the defect length increases from 70mm to 120mm, the improved model differs from the experimental axial results by 42.26%, 34.84%, 25.17%, 28.11%, 23.82%, and 18.95%, respectively, showing a decreasing trend. The radial results differ by 14.48%, 4.36%, 5.85%, 0.12%, 9.77%, and 6.95%, respectively, indicating that the improved model is suitable for long external defects. The evaluation showed that the radial signal quantization accuracy was high. The axial results of the classical model differed from the experimental results by 48.62%, 55.65%, 66.24%, 60.90%, 65.23%, and 70.53%, respectively, showing an increasing trend. The radial results differed by 472.71%, 475.79%, 466.04%, 467.20%, 449.56%, and 448.93%, respectively, and all of these differences were much smaller, fully demonstrating the superiority of the improved model.

[0118] This invention presents a quantitative calculation method for leakage magnetic field defects in the outer wall of long-distance pipelines based on the theory of static magnetic shielding. This method provides scientific guidance for effectively describing the distribution characteristics of leakage magnetic fields in pipeline outer wall defects, and for defect identification and quantitative assessment.

[0119] It will be readily understood by those skilled in the art that the aforementioned advantageous methods can be freely combined and superimposed without conflict.

Claims

1. A method for quantitative detection of magnetic signals, characterized in that, Includes the following steps: Step 1: Establish a classical magnetic charge model, and define a transfer compensation factor based on the classical magnetic charge model; Step 2: Construct the radial transfer compensation factor; Step 3: Construct the axial transmission compensation factor; Step 4: The radial transfer compensation factor obtained in Step 2 and the axial transfer compensation factor obtained in Step 3 are used to describe the additional attenuation caused by the static magnetic shielding effect of ferromagnetic materials, and are introduced into the classical magnetic charge model in Step 1 for improvement, to obtain the improved magnetic charge model. Step 1 also includes: defining a transfer compensation factor based on the classical magnetic charge model, as follows: Step 1.1: Establish shielding effectiveness, the formula is as follows: （2） Among them, SE represents shielding effectiveness. This represents the magnetic field strength of the outer wall of the shielding shell. This represents the magnetic field strength inside the shielding shell. The determination of the magnetic field depends on its direction and can be divided into two cases: radial magnetic field component and axial magnetic field component. The radial magnetic field component... The axial magnetic field component is ; Step 1.2: Based on shielding effectiveness, define the transmission compensation factor, i.e. ,make: Based on the direction of the magnetic field, it is also divided into radial transmission compensation factor. and axial transmission compensation factor Two scenarios, let: , ; Step 2 specifically includes: establishing the cylindrical shell shield and its cylindrical coordinate system. When a cylindrical shell-shaped shield is subjected to a radial external magnetic field, the free space inside and outside the cylindrical shield and the interior of the shield satisfy the Laplace equation in cylindrical coordinates: （3） in, Laplace operator, , , These represent the scalar magnetic potentials on the outer surface of the cylindrical cavity, within the cylindrical cavity wall, and on the inner surface of the cylindrical cavity, respectively. It is the outer diameter of the cylindrical shell. It is the inner diameter of the cylindrical shell; When a ferromagnetic material is shielded by an external constant magnetic field, the boundary conditions of formula (3) satisfy the continuity theorem. The scalar magnetic potential of each spatial region can be obtained by solving formula (3), as shown in the following equation: （4） in, , , These represent the scalar magnetic potentials on the outer surface of the cylindrical cavity, within the cylindrical cavity wall, and on the inner surface of the cylindrical cavity, respectively. This indicates the leakage magnetic field generated by external defects that is not shielded by the cylindrical shielding shell. It is the outer diameter of the cylindrical shell. It is the inner diameter of the cylindrical shell. It is the relative permeability; The result obtained from formula (4) , Substitute them separately The magnetic field strength H of the inner wall of the shielding shell can then be obtained. in The expression for H of the magnetic field strength on the outer wall of the shielding shell out and the obtained H in H out Substituting the values ​​into formula (2), the shielding effectiveness of the cylindrical shielding shell against the radial magnetic field can be obtained. ; Based on the obtained ,Will Substitute into the formula Then the radial transfer compensation factor is obtained. for: （5） Among them, H in H out It refers to the magnetic field strength inside and outside the shielding shell. It is the average radius of the cylindrical shell. It is the remaining wall thickness. It is the relative permeability. It is the outer diameter of the cylindrical shell. It is the inner diameter of the cylindrical shell. It is the depth of the external defect. It refers to the pipe wall thickness; Step 3 further includes: when the cylindrical shell shield is subjected to an external magnetic field parallel to the cylinder axis, the diffusion equation under the quasi-static field condition is satisfied inside the shield, as shown in formula (6): （6） in, It is the axial component of the leakage magnetic field inside the shielding shell. It is the propagation constant. It is the outer diameter of the cylindrical shell. It is the inner diameter of the cylindrical shell; Furthermore, since the boundary conditions of formula (6) satisfy the continuity theorem, H can be obtained by solving formula (6). shield_Zc The solution, based on H shield_Zc The solution yields the shielding effectiveness of the cylindrical shielding shell against the axial magnetic field. As shown in formula (7): （7） in, It is a first-order modified Bessel function of the 0th order. It is a modified Bessel function of the second kind, order 0. It is a first-order modified Bessel function of the second kind. It is a second-order modified Bessel function of the first kind. It is the propagation constant. It is the outer diameter of the cylindrical shell. It is the inner diameter of the cylindrical shell. It is the relative permeability; In pipeline inspection models, leakage magnetic signals are typically analyzed under static magnetic field conditions. Therefore, under the limiting conditions of the static magnetic field... It can be written as: （8） in, It refers to the shielding effectiveness of the cylindrical shielding shell against axial magnetic fields. It is the average radius of the cylindrical shell. It is the remaining wall thickness. It is the relative permeability. It is the propagation constant. It is the outer diameter of the cylindrical shell. It is the depth of the external defect. It is the pipe wall thickness. It is frequency; Will Substitute into the formula Then the axial transmission compensation factor can be obtained. The formula is: （9） in, It is the axial transmission compensation factor. It refers to the shielding effectiveness of the cylindrical shielding shell against axial magnetic fields. It is the average radius of the cylindrical shell. It is the remaining wall thickness. It is the relative permeability. It is the propagation constant. It is the outer diameter of the cylindrical shell. It is the depth of the external defect. It is the pipe wall thickness. It refers to frequency.

2. The method for quantitative detection of magnetic signals according to claim 1, characterized in that, Step 1 specifically includes: Establishing a classical magnetic charge model: When a magnetic field passes through a defect in a ferromagnetic material, magnetic charges of different polarities are distributed on the two end faces of the defect, forming a magnetic charge accumulation, which in turn generates a magnetic field. A rectangular coordinate system is established at the center of the bottom of the defect, with the X-axis along the pipe axis, the Y-axis along the pipe radial direction, and the Z-axis along the pipe circumference. The axial length D of the outer defect is set. x Circumferential width of external defects D z External defect depth D y The coordinates of any magnetic charge source on the sidewall are (x m y m , z m If the radial signal of the magnetic field formed at any point P(x, y, z) in space is... With axial signal They can be represented as: （1） in, The magnetic charge density of the defect sidewall, Where is the vacuum permeability, r is the distance from the magnetic charge source to the detection point P, and D is the magnetic permeability of vacuum. y It is the depth of the external defect, D z It is an external defect with a circumferential width.

3. The method for quantitative detection of magnetic signals according to claim 1, characterized in that, Step 4 specifically includes: The radial component H of the leakage magnetic field that can be effectively detected by the pipe wall when detecting external defects from within is the magnetic leakage signal. eff_Y The axial component H of the leakage magnetic field that can be effectively detected eff_X Represented as: （10） Wherein, the definition is: and in formula (5) With formula (9) Substituting formula (10) into formula (1) respectively, we can obtain the analytical model of external defects under the influence of pipe wall thickness: （11） in, It is the outer diameter of the cylindrical shell. It is the pipe wall thickness. It is the depth of the external defect. It is the relative permeability. The magnetic charge density of the defect sidewall, It is the vacuum permeability, D z It is the circumferential width of the external defect, x m、 y m、 z m Here, x, y, and z are the three-axis coordinates of any magnetic charge source on the sidewall, and x, y, and z are the three-axis coordinates of any point P in space. It is the propagation constant. It is the remaining wall thickness. It is the frequency, and the leakage magnetic flux detection signal for detecting defects on the outer wall of a long-distance oil and gas pipeline can be quantified by formula (11).

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