EAGE 69th Conference & Exhibition — London, UK, 11 - 14 June 2007

E005Electromagnetic Technology for DeterminingTechnical Conditions of Oil and Gas PipelinesA. Mousatov* (Mexican Petroleum Institute), E. Nakamura (MexicanPetroleum Institute), V. Shevnin (Mexican Petroleum Institute), O. Delgado(Mexican Petroleum Institute) & E. Pervago (Mexican Petroleum Institute)

SUMMARYIn this paper we present an electromagnetic technology for non destructive surface inspection of metallicpipelines that includes the determination of pipeline position and depth, assessment of technical conditionof an insulation coating, and estimation of operating state of a cathodic protection system. This technologyis based on the approximation of a metallic pipeline by a heterogeneous transmission line. In the model,the pipeline with damage of the insulated coating (variations of leakage resistance) and deterioration ofthe tube wall thickness (changes of tube impedance) is represented by uniform transmission line segments(piecewise model). This approach reduces the problem dimensions to 1D and allows us to obtain theanalytical solution for calculating the voltage, current, and magnetic field distributions for single andgroup interconnected pipelines. Using the transmission line model we have simulated the voltage andcurrent distributions for two interconnected pipelines with variable insulated coating in the low frequencyrange. The field operations consist in measurements of (1) the magnetic field created by the currentflowing in a pipeline, (2) voltages on the control posts of cathodic protection, and (3) soil resistivity. Wepresent the example of this technology application for the practical inspection of group interconnected oilpipelines.

EAGE 69th Conference & Exhibition — London, UK, 11 - 14 June 2007

INTRODUCTION The assessment of pipeline integrity is an important problem for the safe

transportation of oil and gas. During pipeline exploitation several factors such as: the quality of transported products, microtectonic movements, design problems with the cathodic protection system, etc. affect pipelines and change their technical condition. These changes consist in the destruction of pipe insulation coating, development of corrosion process, and reduction of pipeline wall thickness. These negative processes cause significant economic losses and environment contamination. The estimation of pipeline integrity is performed by internal and external inspection techniques. Internal methods are mainly used to evaluate the wall corrosion and cracking. To estimate the insulation coating damage and condition of the cathodic protection system, nondestructive external electromagnetic and electrical methods on the earth surface are applied.

Electrical methods such as Close Interval Pipe-to-Soil Potential and Direct Current Voltage Gradient use measurements of DC electrical field to assess the effectiveness of cathodic protection and to locate defects of the insulation coating (Masilela, 1998; Pawson, 1998). The disadvantage of these methods consists in their dependence on the environmental resistivity and requirements of pipeline position knowledge. To determine the pipeline trajectory and electrical-insulation quality, the Pipeline Current Mapper technique, based on the measurements of the magnetic field above a pipeline, is applied. However, all these methods provide only qualitative information about the insulation coating damage.

In this paper we present an electromagnetic surface technology for the external inspection of metallic pipelines that allows determining the pipe position and depth, resistance of the insulation coating (leakage resistance), the presence of interconnections, and current distributions in a group of pipelines. This technology is based on the approximation of metallic pipelines by heterogeneous transmission lines (Kaufman, 1989; Morgan, 1993; Mousatov and Nakamura, 2001; Mousatov et al., 2004). Using the transmission line approach we have simulated the distribution of the voltage, current and magnetic field as functions of the leakage resistance and tube impedance along a single and interconnected pipelines.

The field operations consist in measurements of (1) the magnetic field created by the current, flowing in the pipeline from an external low frequency generator, (2) voltages only on the control posts of cathodic protection system, and (3) soil resistivity (Mousatov et al., 2003). The experimental data processing includes the inversion of magnetic profiles to determine the pipeline geometric trajectory and distribution of current flowing in the pipes. The leakage resistance is determined as the ratio of the voltage to the current difference per length unit (leakage current). From the leakage resistance we can calculate the insulated coating resistance by applying a correction for the resistivity of surrounding pipeline formations.

We have successfully applied this technology for the practical inspection of several oil and gas pipelines in Mexico.

THEORETICAL BACKGROUND

For describing the electromagnetic field into and above a metallic pipeline we have approximated the pipeline by a heterogeneous transmission line (HTL). Transmission line theory was applied by Kaufman (1989) for measuring the formation resistivity through a cased borehole. Morgan (1993) used the homogeneous transmission line (TL) model to calculate current for cathodic protection source.

In our model, the pipeline with damage of the insulation coating (variations of the leakage resistance) and deterioration of the tube wall thickness (changes of tube impedance) is represented by uniform transmission line segments (piecewise model). This approach reduces the problem dimensions to 1D and allows us to obtain the analytical solution for calculating the voltage and current distribution along the pipeline and the electromagnetic field above the pipeline.

For frequency domain, the current I(ω,x) and voltage V(ω,x) variations in each uniform interval i of the equivalent TL satisfy Helmholtz’s equations:

EAGE 69th Conference & Exhibition — London, UK, 11 - 14 June 2007

0),x(Vdx

),x(Vdi

2i2

i2

=ωγ−ω (1) 0),x(I

dx),x(Id

i2i2

i2

=ωγ−ω (2)

where iii YZ=γ is the propagation factor.

The impedance Zi and admittance Yi of the equivalent transmission line depend on distributed resistance (Ri,), inductance (Li), conductance (Gi), and capacitance (Ci,): iii LiRZ ω+= ,

iii CiGY ω+= . The transmission line distributed parameters Ri, Li,Gi, and Ci are defined through the physical properties of the metallic pipe (electrical conductivity and magnetic permeability), their constructive characteristics (tube diameter, wall thickness), resistance of the insulation coating, and soil resistivity (Chipman, 1968, Mousatov at al,. 2004). In the low frequency range, the tube impedance Zi is a complex function and depends on a skin layer thickness while the leakage admittance Yi corresponds to the active leakage conductivity (Gi>>ωCi). The solution of equations (1) and (2) can be written for the i-th interval as follows:

xi

xii

ii eBeA)x(V γγ− += (3) )eBeA(Z)x(I xi

xi

1iii

ii γγ−− −γ= (4)

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 are found from the system of equations obtained by using the boundary conditions (voltage and current are continuous functions at the points of interval connections) and conditions at the source and end of line.

The magnetic field created by the current flowing in the pipeline can be calculated on the basis of the Biot - Savart law. The horizontal magnetic-field component perpendicular to the pipeline axis is expressed trough the convolution integral

( )[ ]∫∞

∞− −++

ωπ

=ω 2/32p

22

ppy

xxyh

hdx)x,(I41)h,y,x,(H (5)

where h is the depth of the pipeline axis and x is the point of magnetic field measurement. For the current variation on the integration interval given by equation (4) the horizontal magnetic field above a pipe axis corresponds to the current value at the measuring point and its behavior along a pipeline is the similar to the current distribution. To study the influence of the coating damage we have simulated the voltage and current along pipeline with variable leakage conductance. In Fig.1 the graphs of the voltage and current distributions are presented for different tube magnetic permeabilities (µ1=50µ0, µ2=10µ0, µ3=µ0) and resistance of insulated coating: T1=104 Ωm (0≤x≤200m, the interval of perfect insulation), T2=102 Ωm (200m≤x<300m, zone of coating damage), T3=103 Ωm (300m≤x, good state of insulation). For simulations we have used the following pipe parameters: conductivity σ=107 Ωm, diameter d=0.2 m, tube wall thickness ∆r=0.01 m. The DC and AC (625 Hz) current generator is connected to the initial point of pipeline (x=0).

EAGE 69th Conference & Exhibition — London, UK, 11 - 14 June 2007

Fig. 1. A)Voltage and B)Current distributions along the pipeline with the variable leakage resistance.

The modeling results demonstrate that the voltage distribution has low spatial resolution and it is characterized by integral propagation factor of the whole pipeline. Thus, it is possible to reconstruct the voltage distribution using only the voltage measurements obtained on several control points of cathodic protection. The current graphs depend strongly on the variations of the leakage conductance and allow the zone with insulation damage to be delimited. In any point of transmission line xm the leakage resistance (Ti=Gi

-1) corresponds to the ratio between the voltage and leakage current ∆Ii, which is defined as current derivative: Ti(xm)=V(xm)/∆Ii(xm). From this characteristic, taking into account the resistivity of surrounding pipeline formations, we can calculate the resistance of the insulation coating that is a direct quantitative parameter for describing the pipeline’s technical conditions.

Frequently, for group of pipelines a common system of cathodic protection is used and pipelines appear connected to each other. These connections significantly affect the current and voltage distributions. To model this situation we have applied the following additional boundary conditions at connection points: (1) the same voltage value for all pipelines and (2) the sum of input and output currents should be equal. An example of the voltage and current distributions for two interconnected pipelines is presented in Fig.2. The graphs 1 and 2 correspond to pipelines with impedance Z1=2.5x10-4, Z2=1x10-3and different leakage resistance T1=103 Ω/m (good insulation coating) T2=10 Ω/m (damaged insulation). In the initial point x=0 these pipelines are connected to the current generator. The graphs 3 and 4 represent the voltage and current distributions when they have additional interconnection in the point x=400 m.

EAGE 69th Conference & Exhibition — London, UK, 11 - 14 June 2007

Fig.2.A)Voltage and B)Current distributions for two interconnected pipelines with different insulation

resistances. PIPELINE INSPECTIONS USING ELECTROMAGNETIC TECHNOLOGY

We have applied the developed electromagnetic surface technology for the practical inspections of several oil and gas pipelines with different technical stages and environmental conditions. Here we present the example of the technology application to determine the insulation coating damage on a group of interconnected pipelines.

The field operations consisted of measurements of the magnetic-field components, voltage on control posts of cathodic protection, and soil resistivity. We have used the external AC (625 Hz) generator with a current of 100 mA, connected to the control posts. The measurement system included an induction antenna with sensitivity 10 mV/(mA/m) and amplification and filtering modules. The magnetic field measurements were carried out along profiles orthogonally oriented to the pipeline axis. The distance between profiles was 25-50m and corresponded to the points of soil resistivity estimations. In Fig. 3 the results of experimental study for one of five inspected pipelines are presented. By inversion of the magnetic field data we have obtained the depth of the pipeline axis and the value of current flowing in the pipeline (Fig. 3, A). The reconstruction of the voltage distribution along the pipeline was performed using observations on three control posts of cathodic protection (x=0m, x=520m and x=1100m) and assuming that the pipeline between two points can be approximated by a homogeneous transmission line (Fig. 3, B). The presence of minima on the current and voltage graphs is related to the interconnections between pipelines with different technical characteristics as was predicted by our modeling results (Fig.2). For each interval between two magnetic field profiles, the leakage current (current difference normalized to interval length) was calculated (Fig. 3, C). Based on the reconstructed voltage and obtained leakage current we have found the leakage resistance T along the pipeline (Fig. 3, D). The environmental resistance Tm (Fig. 3, F) was assumed as average resistance (blue line in

EAGE 69th Conference & Exhibition — London, UK, 11 - 14 June 2007

Fig.3,E) obtained from the measured resistivity of formations surrounding pipeline. The difference between leakage and environmental resistance corresponds to the insulated coating resistance Tins (Fig. 3, E) and characterizes quantitatively the insulation stage. The graphs of leakage and coating resistance indicate that the interval 320 - 960 m is characterized by damaged insulation and high probability of external corrosion.

Figure 3. Results for 8” oil pipeline: (A) current along pipeline, (B) voltage distributions, (C) leakage

current, (D) leakage resistance, (E) soil resistivity, and (F) insulated coating resistance. CONCLUSIONS

We have presented a surface electromagnetic technology for non destructive inspections of single and group interconnected pipelines. The technology proposed allows us to determine quantitatively such important pipeline technical characteristics as the pipeline position and depth, leakage resistance, and resistance of the insulation coating, all of which are required for correct estimation pipeline integrity and limitation of potentially hazardous intervals. This technology has been successfully applied for external inspections of several pipelines in Mexico with different technical and environmental conditions. To simulate the voltage, current, and magnetic field distributions along pipelines we have developed the heterogeneous transmission line model for the approximation of interconnected metallic pipelines with variable technical parameters. This approach provides an analytical solution for fast modeling of the magnetic field above pipeline and can be used for experimental data inversion. REFERENCES A.A. Kaufman, “Condutivity determination in formation having a cased well”. United States Patent No. 4,796,186, 1989. A. Mousatov, E. Nakamura, “Transmission-line approximation of pipelines with cathodic protection”. SAGEEP-2001, Denver, USA, March 2001, p.11. A. Mousatov, .E. Nakamura, V. Shevnin, ”Método para determinar el estado de aislamiento de ductos utilizando mediciones de fugas de corriente”. Patent pending PA/a/2003/005187. A. Mousatov, E. Nakamura, V. Shevnin, Electromagnetic surface method for the pipeline periodical inspection based on the heterogeneous transmission line model. V International Pipeline Conference, Calgary, 2004 . J. Morgan, “Cathodic Protection”. Second Edition, NACE publication, USA, 1993. R. A.Chipman, “Theory and problems of transmission lines”. McGraw Hill, USA, 1968. R.L. Pawson: "Close Interval Potential Surveys - Planning, Execution, Results”, Proceedings on Corrosion, NACE, paper 575, pp.16-21, 1998.

EAGE 69th Conference & Exhibition — London, UK, 11 - 14 June 2007

Z. Masilela and J. Pereira: "Using the DCVG technology as a quality control tool during construction of new pipelines". Engineering Failure Analysis, Vol. 5, No. 2, pp. 99-104, 1998.