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Proceedings of IPC 2004 International Pipeline Conference October 4 - 8, 2004 Calgary, Alberta, Canada

IPC04-0223
ELECTROM AGNETIC SURFACE METHOD FOR THE PIPELINE PERIODICAL INSPECTION BASED ON THE HETEROGENEOUS TRANSMISSION LINE MODEL
Aleksandr Mousatov/Mexican Petroleum Institute amousat@imp.mx Edgar K. Nakamura/Mexican Petroleum Institute enakamu@imp.mx Vladimir Shevnin/Mexican Petroleum Institute vshevnin@imp.mx

ABSTRACT The superficial electromagnetic methods are widely applied for non-destructive control of pipeline technical condition. For this purpose we have developed the theory and field technology for the pipeline inspection that allows: 1) determining the pipeline trajectory (plane position and depth), 2) delimitation of pipeline intervals with an insulation coating damage, and 3) estimating the condition of a cathodic protection system. The proposed method is based on the approximation of a metallic pipeline by a heterogeneous transmission line with variable distributed parameters: resistance and inductance along a tube (tube wall thickness) and leakage conductance. The field operations consist in measurements of magnetic field above a pipeline and voltage only on the control posts. The interpretation process includes the iterative inversion of the magnetic component in order to determine the pipeline depth and current distribution along pipeline. The current difference corresponds to leakage current and allow estimate the leakage resistance. We illustrate the practical application of this method using the obtained results for pipeline inspections in Mexico. Based on the voltage and current simulations for the infralowfrequency band (0-1kHz) we have evaluated the feasibility of the time lapse measurements of the magnetic field in some fixed points for the early prediction of an insulation damaging process. KEYWORDS Pipeline inspection, electromagnetic thickness, insulation damage, magnetic field.

electromagnetic methods consists in the non-contact measurement of magnetic field created by the current flowing in the pipeline [1-3]. This current is produced by the cathodic protection station or can be excited using an external generator. The external AC generator is connected directly to the pipeline through the control unit of cathodic protection system and an electrode grounded in a remote point ("infinity"). The operating frequencies are selected in low frequency range to apply an inductive magnetic antenna that allows measuring magnetic fields components. The electrical stations of cathodic protection system use rectifiers of industrial frequency current and produce both the DC and AC components. Frequently, the magnetic-field observation carries out at frequency of 120 Hz, which corresponds to the second harmonic of the industrial frequency. In this work we consider the pipeline inspection method based on the superficial measurements of electromagnetic field components. We treat a metallic pipeline as heterogeneous transmission line (TL). This approximation allows representing a pipeline with insulation damages and variable tube-wall thickness. Based on the transmission line approach we simulated the distributions of voltage, current and magnetic field along a pipeline as functions of the leakage resistance and tube impedance. We present the results of this technique application to the practical inspection of pipeline condition. This example developed the field technology and the interpretation of the magnetic field measurements to determine the depth and position of the pipeline axis as well to delimitate the intervals with coating failure of different stages. Taking into account that the voltage behavior is mainly defined by the integral pipeline parameters, we can reconstruct the voltage distribution along pipeline using observations only on several control posts. The leakage resistance is determined as the ratio of the voltage to the current difference per length unit (leakage current).

method,

wall

INTRODUCTION The pipeline inspection includes the determination of their position, depth, condition of insulated coating, and quality of cathodic protection. The effective inspection by superficial

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Additionally, we have considered the feasibility of use the time lapse measurements of the magnetic-field in several fixed points for monitoring the dropping of the pipeline insulation resistance. NOMENCLATURE C - leakage capacitance per unit interval of 1 m [Fm L - pipe inductance per unit interval of 1 m [Hm R - pipe resistance per unit interval of 1 m [m
-1

HETEROGENEOUS TRANSMISSION LINE FOR METALLIC PIPELINE APPROXIMATION The equivalent electric scheme of the pipeline as a transmission line with distributed parameters is given in Fig. 1.

]

.
-1

V(x)

I l (x)

V(x)+dV(x) I(x)+d I(x)

G - leakage conductance per unit interval of 1 m [ -1 m
-1

]

A
.
z x

I(x)

-1

] ]. ]

.
y

dx Pipe metal

Y - leakage admittance per unit interval of 1 m [ -1 m Z - pipe impedance per unit interval of 1 m [m dI(x) ­ change of current along the pipeline [A]. dV(x) ­ change of voltage along the pipeline [V]. dx -length element of the transmission line [m]. Hy - magnetic-field horizontal component [Am Iil(x) - leakage current on an interval i [A/m] Ti(x) - leakage resistance on an interval i [m T T
env i ins i
-1 -1 -1

-1

]

.

Pipe Coating

.

]

.
Fig 1. Transmission line model o f a metallic pipeline. (A) Vo ltage and current variat ions in the lo ngitudinal pipeline section. (B) Equivalent electrical scheme o f transmissio n line.
-1

Ii(x) ­ current along the pipeline on an interval i [A].

­ insulation resistance on an interval i [m

­ environmental resistance on an interval i [m
-1

]

.

]

]

.

.

In the frequency domain the variations of current I(x) and voltage V(x) in the pipeline are expressed by following equations:
dV( x ) = -ZI( x ) dx dI(x ) = -YV (x ) dx

Vi(x) ­ voltage on an interval i [V]. h - depth of the pipeline axis [m]. t ­ time [s]. x - point of observation [m]. - skin-layer thickness [m]. µ - magnetic permeability of pipeline [Hm
-1

(1) ( 2)

]

.

where: Z=R+jL Y=G+jC The pipe impedance Z and leakage admittance Y are defined by the next parameters, distributed per unit interval of 1 m: pipe resistance R, pipe inductance L, leakage conductance G and leakage capacitance C. The decrease in current along a pipeline takes place due to the leakage of current through pipe insulation into an environment. Therefore this leakage current Il(x) in a point x can be defined as derivative of current I(x) flowing along a pipeline for each i-th interval:
Ii ( x ) = -
l

µo - permeability of free space and has a value of 4x10-7 [Hm -1 ] i - attenuation constant for an interval i [m i - phase constant for an interval i [m i ­ wavelength on interval i [m].
-1 -1

]

]

.

.
-1

i - propagation constant for an interval i [m

]

.
-1

i - pipeline conductivity for an interval i [ -1 m m - environmental resistivity [m] ins ­ is the insulation resistivity [m]

]

.

(x,t) - relative variation of the magnetic field in the measuring point (x,t) relative variation of magnetic field on the pipeline interval x. r - pipeline wall thickness [m].

dI( x ) dx

( 3)

Taking into account that the real pipeline has the variable wall thickness and insulation resistance we represented such pipeline as a heterogeneous transmission line composed by piecewise-uniform intervals [4-5].

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For each i-th interval where the TL parameters are constant, from expressions (1) and (2) we can obtain
d 2 Vi ( x ) 2 - i Vi ( x ) = 0 (4) dx 2 d 2 I i (x) 2 - i I i (x) = 0 (5) 2 dx where: i = Z i Yi - propagation constant for an interval i.

For the frequencies >0, the resistance and inductance relate to a skin-layer thickness . When 10, the equations (14) and (15) have to be substituted by following:
R
iac

=

1 2r0
1 2r0

µ , 2 i
µ . 2

(16)

L iac =

(17)

The propagation constant i is a complex quantity that can be presented through an attenuation constant i and phase constant i, related with the wavelength i as i=2/i: i=i+ji. The solution of equations (4) and (5) can be written for the i-th interval as following:
Vi ( x ) = A i e
- i x

For operating frequencies in the low range (0-10 kHz) we can neglect the influence of leakage capacitance because the leakage conductance G>>C. The leakage resistance T (T=1/G) can be presented as a sum of the insulation Tins and environmental Tm resistances
T=T
ins

+ Bi e
-i x

ix

(6)
i x

I i ( x ) = i (A i e Zi

+T

m

( 18)

- Bi e

)

(7)

The terms with the positive and negative exponents correspond to the outgoing and reflected components of voltage and current. The unknown coefficients Ai and Bi of equations obtained by using between homogeneous intervals and the points of homogeneous interval current functions are continuous
Vi (d i ) = Vi+1 (d i+1 ) Ii (d i ) = I i+1 (d i+1 )

The resistance of the insulation coating is calculated taking into account that the leakage current is radially directed from a tube surface [1]:
T
ins

=

ins d ln 2 d

2 1

( 19)

are found from the system the boundary conditions conditions in the source. In connection the voltage and ( 8) (9)

where: ins ­ is the insulation resistivity, d1 and d2 - are internal and external diameters of an isolation layer. The environmental resistance for the small pipeline depth (about a few meters) can be estimated using the expression for the current line on the surface of a half-space with the environmental resistivity m
Tm = m d ln d
3 2

The source conditions for the voltage or current generator are
V1(0) = V0 I 1 ( 0) = I
0

( 20)

(10) ( 11)

In the last interval, when the distance from the source point tends to infinity the voltage and current go to zero.
V1( x ) 0 x I 1( x ) 0 x

The quantity d3 is the effective radius in an environment, on which the voltage value can be assumed equal to zero (according to accuracy needed for the practical application d3103 d2). Magnetic field The magnetic field created by the current flowing in the pipeline can be calculated on the basis of Biot - Savart law. The magnetic-field component that is perpendicular to the pipeline axis is expressed trough the convolution integral [1]:
H y (, x, y, h ) = 1 4

(12) (13)

Transmission line parameters The pipe resist defined by the the inner radius 1 Ridc = i 2r0 r ance Ri according to Chipman [6], for =0 is pipe conductivity i, the wall thickness r and r 0: . (14)

-

[h



2

+ y + (x - x
2

I(, x p )hdx

p p

)]

2 3/ 2

(21)

where h is the depth of pipeline axis and x is the point of magnetic field measurement. When the current change in the finite integration interval (-a, a) is close to be linear and a > 10 h 2 + y 2 , the horizontal component Hy along the profile perpendicular to the pipeline axis can be calculated with accuracy better than 0.5 % from expression

The inductance of pipe, for =0 and r0>>r [6] is given as
µ Lidc = 8 4 3 r 2 r - 15 0 r 1 - r 10 0
2 3 r r 0

(15)

where: µ­ is the magnetic permeability of pipe material.

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H y (, x ,0, h ) =

I(, x )h 2(h 2 + y

2

)

(22)

This equation shows that the horizontal magnetic field corresponds to the current value in the measuring point and its distribution has the similar behavior as current flowing along pipeline. DISTRIBUTIOS OF THE ELECTROMAGNETIC-FIELD CHARACTERISTICS ALONG PIPELINE To study the influence of the insulation damages and changes of the tube wall thickness we have calculated the distribution of the electromagnetic-field characteristic along a pipeline with variable parameters. We treated such pipelines as a heterogeneous transmission line and performed simulation solving system of equations (6-13). The transmission line parameters were evaluated from equations (16-17) for the operating frequencies of 625 Hz. We used the following pipe electromagnetic properties: electrical conductivity =106 and magnetic permeability µ=50µo. We considered two cases of the electromagnetic-field excitations by a voltage and current generators. The generator was connected to the pipeline in the point x=0. The magnetic field was calculated at the altitude 1 m above the pipeline axis. Insulation damage. The model of the pipeline with insulation coating variations consists of three zones with different leakage conductance (fig. 2). In the first zone the presence of undamaged insulation with a leakage conductance G1=10-4 is supposed. The second zone introduces a part of the pipeline with strong damage of insulation and it is characterized by a high leakage conductance G2=10-2. Center of this zone is in a point 250 m. Its width is variable and equal to 10, 30 and 100 m. In the third zone the leakage conductance G3=10-3 corresponds to medium grade of the insulation change.

Fig.3. Distribut ion of vo ltage (A), and current (B) on the interval 150350m(C). Distribut ion of magnet ic field on the interval 150-350m for the model fro m fig. 2. (Current generator).

Fig. 2. Model of a leakage resistance distribut ion for different length of a zone of strong insulat ion damage: Model 1 - 100 m. ; Model 2 -30 m.; Model 3 - 10 m. Center of a zone is in the point X=250 m.

For current generator the voltage graphs for three models have a similar behavior and low sensitivity to the zone of the high leakage conductance (Fig. 3A). The presence of the zone with strong insulation damage affects only the absolute value of voltage that vary on 10% when the length of the second zone is changed in one order. The voltage is characterized by integral propagation factor of the whole pipeline.

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The current distribution reflects clearly the variations of the leakage conductance (Fig. 3B). The breakpoints in the current curves correspond to the contacts of three zones with a different leakage resistance. The current distribution in the non-uniform pipeline is defined by the local characteristic of the transmission line in the measurement point. The behavior of the horizontal component of the magnetic field along pipeline axis is similar to the current distribution (Fig. 3C).

Wall thickness variations The changes of the tube wall thickness can be a consequence of erosion produced by products carried into the pipeline. To simulate this situation we considered the pipeline characterized by two zones with the same leakage conductivity (G=10-4) and different wall thickness. In terms of transmission lines parameters that changes represents variations on its impedance (Resistance and inductance). By the first zone ( 0m x < 250m ) the wall thickness is equal 8mm, 6mm, and 4mm while in the second zones ( 250 m x 500 m ) this parameter maintains constant (10mm). It means that impedance on first zone is bigger than second zone (Z1>Z2). The external tube radius is 10 cm. The results of numerical simulations using a voltage generator are showed on figs 5-6 and are represented by continues lines. The inverse situation on wall thickness is presented in the same figures by doted lines: 10mm of wall thickness on zone ( 0m x < 250m ) and 8mm, 6mm and 4mm in the second zone ( 250m x 500m ). The voltage distribution is sensitive and reflects the pipeline wall thickness variations (Fig 5) and does not depend on the generator type (voltage or current). The contact of two zones with different wall thickness reflects a breakpoint on voltage curves. In this case the voltage distribution is defined by the local characteristics of the transmission line on the measurement. point.

Fig. 4. The graphs of a leakage current Il (x) for model from fig. 2. (Current generator). The leakage current (Fig. 4) is the piecewise continuous function that practically repeats the variations of a leakage conductance. We consider this parameter as the most adequate characteristic to separate the zones of insulation damages and estimate the relative variation of the leakage resistance. The magnitude of leakage current Iil(x) for a homogeneous pipeline, according to equation (7), is:
Ii (x) =
l

i2 (A i e Zi

-i x

+ Bi e i x ) ,

(23)

where

i2 1 =. Z i Ti

For the non uniform of voltage (6) to a Eq. (3) corresponds current difference on
Ti ( x ) = Vi ( x ) l Ii (x )

pipeline with variable parameters the ratio leakage current (23) on the basis of the to magnitude of a leakage resistance to a an interval i. (24)

Fig. 5. Voltage distribution along the pipeline with wall thickness variation using a voltage generator. However, measurements of voltage only are possible between control posts because it directly corresponds to the voltage along the pipeline. Thus, an electric field measurement on the surface will not be correct.

From here follows, that it is possible to receive high resolution in estimation of a leakage resistance, which in fact is restricted by the accuracy of finding a current difference on a magnetic field. It is necessary to mark, that the measured magnetic field smoothes variations of a current depending on depth.

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1. Analysis of graphs and maps of the horizontal magnetic-field component. Correlation of anomalies to trace the pipeline axis position (red dotted line in Fig. 7A). The magnetic-field profile above the axis is shown in Fig. 7B. 2. Inversion of the magnetic field data obtained along the profiles perpendicular to pipeline axis using the equation (21) to determine the pipeline axis depth (Fig. 7C) and the value of current that flow along the pipeline (Fig. 7D). 3. Calculation of the leakage current per length unit (1 m) as the difference of current along of pipeline axis (Fig. 7E). 4. Reconstruction of the voltage distribution using values measured on the proximate control points of cathodic protection and the apparent propagation constant. The apparent propagation constant is defined as propagation constant on homogeneous transmission line. In this case the voltage values were V(-500) = 5.4 mV, V(0) =8.5 mV, V(+500) =3.3 mV and the propagation constants corresponded to =1.110-3 (for negative points) and =1.710-3 (for positive points). 5. Evaluation of the leakage resistance along of the pipeline (Fig.7F) based on equation (24) The values of a leakage resistance exceeding 0.8-1 kOhm.m correspond to the satisfactory state of isolation coating. The pipeline intervals with reduced leakage resistance from 800 up to 100 Ohm.m characterize the isolation with different grades of damages. Values of resistances less than 100 Ohm.m indicate a high grade of the isolation destruction. In such zones the corrosive processes can be progressed with high probability. The pipeline zones with the damaged isolation are potentially the most dangerous areas from the viewpoint of petroleum leakage.

Fig. 6. Current distribution along the pipeline with wall thickness variation using a voltage generator. The current distribution for all models has a similar behavior and reflects a low sensitive to the pipeline wall thickness variation (Fig 6). This variation affects the absolute value of current distribution and no breaks are presented, this is a reason for which voltage generator is optimal for wall thickness changes. The measurements of voltage between two control post letting us to detect the absolute variations of the pipeline thickness change. So, the joint analysis of electromagnetic distributions along the pipeline allows to estimate and to distinguish variations on insulation damage and wall thickness. Example of pipeline inspection We have applied the method presented above to study technical condition of various pipelines in different areas of Mexico including urban zones [4-5]. Here we consider one example to illustrate the field operations and processing steps. The field observations have consisted in the measurements of the horizontal component of the magnetic field. We have used the external generator with current of 100 mA at the frequency of 625 Hz. (The point of the generator connection to the pipeline is considered as the coordinate origin). The measuring system have consisted of the induction magnetic antenna with sensitivity 10 mV/(mA/m) and the amplifier with a high selective filter. The observations were carried out along profiles of 20m length perpendicular to the pipeline axis with the step 1 m. The selection of an interval between profiles depends on the investigation resolution required and time of measurements. In this case the interval was 25m. The measurements on one profile have taken about 3-4 minutes. Such a magnetic-field mapping can be performed at any time of the year and its efficiency is determined by accessibility of a survey area. The processing of magnetic-field data for estimating isolation damages on the basis of current values and leakage resistance [7-8] has included the following steps:

Fig. 7. A, B, C Estimation of leakage resistance on magnet ic field distribut ion above pipeline. (A)- map of the measured magnetic field, (B) - a magnet ic field graph along pipeline axis, (C) - depth of the pipeline axis.

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Fig. 7. D,E.F. Estimat ion o f leakage resistance on magnet ic field distribut ion above pipeline. (A)- map o f the measured magnet ic field, (B) - a magnetic field graph alo ng pipeline axis, (C) - depth of the pipeline axis, (D) ­ current, flowing in the pipeline, (E) - current leakage fro m the pipeline in a surrounding medium, (F) - leakage resistance.

FEASIBILITY OF PERIODICAL INSPECTION Based on the voltage and current calculations for the frequency band we have estimated the feasibility predicting the process of insulation damage. In this time lapse measurements of the magnetic field are perf several fixed points along pipeline.

Fig. 8. Horizontal co mponent of the magnetic field for the model wit h variable leakage conductance. (A) - DC current generator, (B) - DC vo ltage generator.

infralow of early case the ormed in

To simulate this situation we considered the pipeline model composed by three zones. The wall thickness (10 mm) and external radius (10cm) are assumed constant along the pipeline for all periodical measurements. The tube conductivity and magnetic permeability are =106and µ=50µo. We considered two cases of the electromagnetic-field excitations: voltage and current generators. The generator was connected to the pipeline in the point x=0. The magnetic field was calculated at the altitude 1 m above the pipeline axis. In the first zone ( 0 x < 225 m ) and third ( 275 < x ) zones the leakage conductivity G0=10-4 corresponds to a pipeline with the non-damaged insulation. The second zone ( 225 x 275 m ) represents the pipeline interval where the insulation coating vary with time. In this interval the leakage conductance increases consequently on 20% (G1=1.25x10-4), 100% (G2=2x10-4), and 200% (G3=4x10-4).

Taking into account that the horizontal magnetic-field component is directly proportional to the current value in the point of observation we can used this characteristic measured in fixed points along pipeline for monitoring the change of pipeline technical parameters during exploitation. The graphs of the Hy time­lapse measurements are sensitive to the variation of the leakage conductance in the central model zone (Fig. 8A-B). To evaluate the pipeline insulation changing we have calculated the relative variation of magnetic field on the pipeline interval x: H y (x ,0) - H y (x , t ) (x , t ) = x100% (25) H y (x,0) where: Hy(x,0)=Hy(x,0)-Hy(x+x,0) is the initial difference of the magnetic filed in the interval x, Hy(x,t)=Hy(x,t)-Hy(x+x,t) is the difference of the magnetic filed in the same interval at the moment t that correspond the different value of the leakage resistance.

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The Hy difference is the linear function of the conductance G at the moment of measurements and so the (x,t) represents the relative leakage resistance variations. Thus, the estimation of the relative time variations of the pipeline parameters does not require the magnetic field inversion for determining the pipeline depth and current. To simplify the field operations it is convenient to carry out periodical measurements only in the points which are close to control unit of cathodic protection system. For this purpose we have estimated the sensitivity of the (x,t) function (Table 1) for the large interval x=400 m (x=50 m). Table 1
PERCENTA GE OF INSULA TION DA M A GE 20% 100% 200% CURRENT GENERA TOR Hy(x,t ) (%) FOR DC 3 12.2 36.3 Hy (x,t) (%) FOR 625HZ 2.8 11.2 33 VOLTAGE GENERA TOR Hy(x,t ) (%) FOR DC 3.12 12.5 37.5 Hy (x,t) (%) FOR 625HZ 3.1 12.4 37.2

Other characteristic that can be applied for the periodical inspection and delimitation of zone with anomalous time behavior is proportional to the relative current variations in the measuring point Hy ( x,0) - Hy( x , t ) x100 (26) ( x , t ) = Hy( x ,0) The (x,t) graphs for the model considered reflect the local change in the pipeline insulation coating (fig. 9).

CONCLUSIONS We have presented the superficial electromagnetic method for the pipeline inspection based on the approximation of pipelines by heterogeneous transmission line. The developed theory allows fast simulating as the distributions of current and voltage along a pipeline as well in the surrounding environment. This approximation provides the inversion of the magnetic-field data measured above a pipeline into technical characteristics as a pipeline position, depth and resistance of the insulation coating. The last parameter allows estimating the grade of insulation damage. The modeling results demonstrate that the technique proposed is sensitive to the wall thicknesses of pipelines and can be used to estimate the integral wall thinning during the exploitation process. We have demonstrated the feasibility of the time lapse measurements of the magnetic fields in several fixed points along the pipeline for the early prediction of an insulationdamage process. ACKNOWLEDGMENTS The authors consider as a pleasant debt to express gratitude to the Mexican Petroleum Institute where this study was fulfilled. REFERENCES [1] Mousatov A., Nakamura E., 2001, "Transmission line approximation of pipelines with cathodic protection", Proc., SAGEEP 2001, Denver, Colorado. ERP ­ 5, pp. 10. [2]Mousatov A., Nakamura E., Shevnin V., Modin I., 2001, "InspecciÑn y control periÑdico de ductos aplicando mediciones del campo electromagnÈtico producido por sistemas de protecciÑn catÑdica". Memorias TÈcnicas 6th Congreso Internacional de Ductos, MÈrida ,YucatÀn, MÈxico. DT-13. 7p.

The rise of the leakage conductance on 20% only in the central zone of the 50 m length results in the relative variation (x,t) about 3% in interval 400m. Such sensitivity of magnetic field difference to the leakage conductance increment is sufficient to detect the initial phase of the insulation coating destruction. To localize the anomalous pipeline parts with time variable properties the density of observation points can be incremented. For example, we can use the binary search technique in the anomalous zone to increase both the spatial resolution and value of the (x,t). This methodology allows optimizing field operations for pipeline monitoring.

Fig 9. Relat ive variat ion of the magnet ic field in the measuring point for the model wit h the variable leakage conductance (current generator).

[3] Shevnin V.A. and Modin I.N., 1999, "Geoecological inspection of oil industrial enterprises", Moscow Editors, 511 pp. (In Russian).

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[4]Mousatov A., Nakamura E., Shevnin V., Delgado O., 2002, "Application of electromagnetic methods for estimation of technical conditions of oil and gas pipelines in Mexico", Proc., SAGEEP 2002, Las Vegas, NV., IDA-3, pp. 9. [5] Mousatov A., Shevnin V., Delgado O., Nakamura E., Mejia A., 2003, "Application of superficial electromagnetic methods for localization and inspection of oil and gas pipeline's technical condition", Proc., SAGEEP 2003, San Antonio TX USA, pp. 10. [6]Chipman R. A., 1968, "Theory and problems of transmission lines", McGraw Hill book Company. [7] A. Mousatov, E. Nakamura, V. Shevnin, O. Delgado, A. Mejia., 2002, Copyright register. "TecnologÌa de estimaciÑn del estado de aislamiento de ductos utilizando mediciones superficiales del campo electromagnÈtico". Register number: 03-2002-071812212000-01 [8] Mousatov A., Nakamura E., Shevnin V., 2003, "MÈtodo para determinar el estado de aislamiento de ductos utilizando mediciones de fugas de corriente". Patent pending PA/a/2003/005187.

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