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ELECTROMAGNETIC GROUND CONDUCTIVITY SURVEY TECHNIQUE IN PIPELINE INTEGRITY

Pablo Federico Cosentino 1

, Sergio Néstor Rio 1

1. Transportadora de Gas del Sur S.A.

Keywords: 1. Soil Resistivity; 2. External Corrosion Direct Assessment ECDA; 3. Stress Corrosion

Cracking SCC. 1 Introduction

Transportadora de Gas del Sur (TGS) S. A. is the leading gas transportation company in Argentina. TGS operates the one of the longest and oldest pipeline systems in Latin America (8500 Km of pipeline, 579.090 HP compression power, 74 MMm3/d of contracted capacity). It performs a variety of tasks within its integrity plan in order to operate its pipeline system at the highest level of reliability, optimizing human and material resources and reducing environmental, personal and business impacts.

In order to mitigate the deterioration process of buried pipeline infrastructure, the pipeline industry has widely adopted the use of protective coating systems and the application of cathodic protection (CP). As the pipeline infrastructure continues to age, it becomes a challenge to maintain both CP effectiveness and coating integrity.

External Corrosion and Stress Corrosion Cracking (SCC) are two forms of pipeline deterioration that depend on CP performance, coating integrity and the environment in which the pipeline is buried. If CP or the coating system fails, external corrosion or SCC will probably occur at certain locations along the pipeline. Soil characteristics could be used to predict areas prone to external corrosion or SCC.

The External Corrosion Direct Assessment (ECDA) is a structured process that is intended to improve safety by assessing and reducing the impact of external corrosion on pipeline integrity (3). Pipeline operators have traditionally managed several tools to control external corrosion, the ECDA integrates all the external corrosion control technology under one coordinated program.

TGS has adopted the ECDA process to asses the integrity of un-piggable pipelines. Design and construction data, operational history, corrosion control history and soil characteristics are used in the pre-assessment step to define the viability of applying ECDA and if is so, to define specific ECDA regions.

TGS highlights:

• 8500 km of natural gas pipelines.

• 579.090 HP compression power.

• 74 MMm3 of contracted capacity.

• 8 pipeline maintenance facilities.

• 24”, 30” and 36” diameter trunk lines.

• Average age of the pipeline system 30 years.

• Coating type:

o Asphalt: 5,640 km

o Tape: 1,230 km

o Three Layer Coating: 1,630 km

Figure 1 – TGS transportation system and highlights

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2 Development

Soil characteristics could be used to predict areas prone to external corrosion and SCC. Detailed soil

characterization is the most accurate way to identify those areas. Soil profile, chemical composition, geological classification surveys are conducted to gather the data, but it is an intensive task that requires time and resources.

For example TGS - High-pH SCC model study consisted of the following tasks:

• Provision and digital processing of high-resolution satellite images. High resolution IKONOS images (1 meter definition) in stereoscopic format are acquired and processed in order to analyse them in 3D. The processing included the following steps:

o Ground control point measuring using GPS technology.

o Creation of a digital elevation model.

o Image orthorectification.

o Creation of a digital mosaic.

• Detailed scale analysis of researched areas. This analysis is based on IKONOS images, the digital land model and the slope map, combined with a detailed review of previous research (including soil research and geological studies, etc.).

• Field work for the classification and detailed description of soils in areas under research. This research is carried out through test pits, well studies and natural cracks in the ground.

• A study of physical land characteristics is carried out, which includes:

• Relief and micro-relief

• Natural drainage

• Superficial drainage

• Flooding

• Surface flora

• Surface lithology

• Human influence

• The edaphic profile is also studied, including:

• Horizon description

• Texture

• Structure

• Permeability

• Effective depth

• Porosity

• Stoniness

• Rock outcrop proportion

• Organic material content

• Color

• Soil-pH reaction

• Calcium carbonate content

• Internal drainage

• Consistency

• Soil instability and flood risk

• Physical-chemical “in situ” parameters are also studied, in relation to the following:

o Hydrogen potential (pH)

o Specific conductivity

• Analytical lab description. Included mineral composition and crystalline substance description of researched soils/sediments, as well as chemical-analytical description in the aqueous phase.

In order to optimise this work, and extend the knowledge of the soil environment along the pipeline system right of way (ROW) in the first phase of the data gathering process, several variables were identified and studied with the aim to assess the variable significance to be used later as a filter to pin-point smaller areas for further studies.

TGS identified the soil parameters that can be related to deterioration mechanisms, studied the influence in the degradation process and collected information about the measuring tools available to gather the data in the field.

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Soil resistivity is the capacity to conduct electric current, a parameter that is widely used for soil corrosivity characterization, it is easy to measure with conventional methods and it can be used to filter segments of pipeline right of way (ROW) for further data collection.

Soil resistivity is a variable that can be related to High pH SCC and the external corrosion mechanism and plays a very important role:

• CP current distribution depends on soil resistivity variation along the ROW and coating faults location.

• The TGS SCC predictive model seeks for low resistivity areas, no more than 3 km apart from the Impressed Current Cathodic Protection Units (ICCPU) with high current output during long periods of time:

o Higher CP current density at coating faults leads to cathodic disbondment.

o High pH microenvironment formation and shielding of CP current at coating faults.

• Due to the high conductivity values of low resistivity soils, the electrolytic current in the soil produces higher corrosion rates in the exposed metal surface at coating faults.

• Rapid changes in soil resistivity can be related to changes in the soil composition that produce corrosion cells due to differential aeration in the surrounding soil along the pipeline ROW (clay – sand interface).

• ECDA process seeks multiple indications gathered with indirect inspection methods, to locate coating faults with an insufficient cathodic protection level in a corrosive environment.

The mathematical definition of resistivity can be interpreted as the electrical resistance between opposite faces of a unit cube of material; the reciprocal of conductivity. Resistivity is used in preference to conductivity as an expression of the electrical character of soils (and waters) since it is expressed in whole numbers. When a metallic structure is immersed in a conductive medium, the ability of the medium to carry current will influence the magnitude of galvanic currents and cathodic protection currents.

Resistivity ]Ohm x cmL

AR [ ×=ρ

Electrical resistance ]OhmI

VR [ =

Conductivity

=

cm

S

ρσ

1

Figure 2. Mathematical relationship between resistivity and conductivity

Resistivity of the soil is a function of soil matrix and the interstitial fluid (moisture) that can conduct electric current. Since interstitial fluids have much higher conductivity than the mineral phase by itself, their presence and the degree of mineralization can significantly reduce resistivity of a soil. Thus, typical soil resistivity measurements represent a composite measure of the moisture content of a soil and dissolved electrolytes in the soil water. Clay minerals, due to the particularities of their crystal structure, have a relatively high conductivity; so their presence will reduce the resistivity of a sample.

Table 1 describes the relationship between soil resistivity and degree of corrosivity defined on a relative scale.

Resistivity (ohm-cm) Degree of Corrosivity

< 500 Very Corrosive

500 - 1,000 Corrosive

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Resistivity (ohm-cm) Degree of Corrosivity

1,000 - 2,000 Moderately Corrosive

2,000 - 10,000 Mildly Corrosive

> 10,000 Progressively Less Corrosive

Table 1. Resistivity Ranges for Levels of Soil Corrosivity (4).

3 Tool selection

There are many methods to gather resistivity data in the field that are used successfully in pipeline integrity for many years, a brief explanation of each method is listed below:

• Soil Box

This method is used for field or laboratory measurement of soil resistivity, both in situ at excavations and for samples removed from the ground and processed later.

The equipment required for field resistivity measurements consists of a current source, a suitable voltmeter, ammeter, or galvanometer, four metal electrodes connected to the soil box, and the necessary wiring to make the connections as is shown in figure 3.

The soil box itself is made of non conductive materials and the electrodes in the outer faces have a known surface A, through the sample a current will be established and the voltage drop will be measured in the inner electrodes that are separated a distance a . The soil should be well-compacted in layers in the soil box, with air spaces eliminated as far as practicable. Fill the box flush to the top. The current is generated with a AC source to avoid the error due to the electrode polarization with direct current DC.

The relationship between the soil resistivity of the sample and the electrical parameters and dimensions in the box is shown in the equation (1).

I

VR

a

AR

=

×=ρ

(1)

Figure 3. Soil Box connections.

Where:

R : calculated resistance [Ω].

A: cross sectional area of the container perpendicular to the current flow [cm2].

a : inner electrode spacing [cm].

ρ: soil resistivity [Ω cm].

• 4 pin or Wenner Method

The most extended method in practice of measuring the average resistivity of large volumes of undisturbed earth is the four-point method. Typically the soil resistivity is measured at discreet points along the pipeline.

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Small electrodes are buried in four small holes in the earth, all at depth b and spaced (in a straight line) at intervals a. A test current I is passed between the two outer electrodes and the potential V between the two inner electrodes is measured with a potentiometer or high-impedance voltmeter. The current is generated with a AC source to avoid the error due to the electrode polarization with direct current DC.

The relationship between the soil resistivity of the sample, the electrical parameters and dimensions of the pin-arrangement is shown in the equation (2).

I

VR

ba

a

ba

a

Ra

=

+

−

+

+

=

2222

2

4

21

4πρ

(2)

Figure 4. 4 pin-method connections.

Where:

R : calculated resistance [Ω].

a: electrode separation [cm].

b: electrode depth [cm].

ρ: soil resistivity [Ω cm].

If b < 0.1 a, we can assume b = 0

The equation (2) becomes Raπρ 2=

• Novaprobe

This instrument uses a probe that is provided with a measuring sensor in the tip capable of measuring the resistivity and pH.

This kind of instruments is calibrated using solutions of known resistivity and the instrument directly displays the recorded value of soil resistivity in the surroundings of the probe tip.

Figure 5. Soil probe measuring the resistivity over the pipeline ROW.

• Electromagnetic ground conductivity

The electromagnetic ground conductivity (EMGC) survey technique has been used for decades in geological applications. The application of this method to measure the resistivity along the ROW of pipelines has grown in the last decade. The method uses a transmitter coil Tx to produce a magnetic field, the

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magnetic field induces an eddy current in the soil, this electrical current generates a magnetic field that can be measured by a receiver coil Rx (Figure 6).

Figure 6. EM ground conductivity principle

The quadrature (out of-phase) component of the secondary magnetic field from these loops is linearly related to subsurface conductivity in units of millisiemens/meter, this is valid under certain constrains that are incorporated into the design of the EM instruments.

According to equation 3 the relationship between the intensity of the magnetic fields is lineal and the ratio depends on the soil conductivity when the inter-coil spacing and the frequency has been set for the exploration at a depth of interest.

4

2

0si

H

H

p

s ××××≅

σωµ (3)

Where:

Hs : secondary magnetic field at the receiver coil.

Hp: primary magnetic field at the receiver coil.

σ: soil conductivity.

ω: 2πf

f: frequency.

s: intercoil spacing.

µ0: permeability of free space.

The conductivity is the mathematical inverse of electrical resistivity as the equation 4 shows:

ρσ

1= (4)

σ: soil conductivity.

ρ: soil resistivity.

The linearity is valid in the range of 100 Ohm x cm to 1.000.000 Ohm x cm, at lower values of resistivity the relationship is not longer valid as it is shown in figure 7.

The in-phase component can be used to detect both ferrous and nonferrous metal. These two measurements provide a means of assessing changes in geology and the possible presence of buried pipelines or other utilities that may influence resistivity measurements.

These surveys are non-invasive, above ground, electronic surveys, which measure the ability of soil to conduct electrical currents. These surveys have an advantage over the Wenner 4 Pin method in so far as data is electronically collected and recorded continuously, rather than at a series of discrete points.

The instruments typically survey to depths of 3.0 meters and approximately 1800 data points can be collected per hour. The data logger records the conductivity readings in units of millisiemens/meter (mSm).

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Figure 7. Plot of indicated conductivity versus true conductivity.

• Comparative of Measuring Methods

In the following table we compare the advantages and limitations of each method:

Soil resistivity measurement methodology

Advantages and limitations.

Soil Box 4-pin (Wenner) Novaprobe Electromagnetic (EM)

Electrical contact problems.

None. Very sensitive to pin-contact issues in highly resistive soil

top layer.

Depend on moisture content.

None.

AC Interference. None. Depends on Instrument AC

rejection.

Depends on Instrument AC

rejection.

Sensible in low-end performance instruments.

Exploration at different depths.

Soil sample of each layer at different

depths.

Excellent. Good Fixed depth in instruments with

fixed frequency and intercoil spacing.

Daily average survey production.

Depends on sampling distance.

Depends on sampling distance.

Depends on sampling distance.

10 km with 2 meter sampling distance.

Soil resistivity profile resolution.

High resolution cost prohibitive due to exploration wells.

High resolution increases the cost

dramatically.

High resolution increases the cost

dramatically.

Excellent.

Survey in rocky soils.

Soil sample must be prepared, in situ

measurements can be difficult to obtain.

Depend on the rocky content of the

top layer.

Not possible. Contact-free measurement not affected by rocky

soils.

Personnel training and qualification

Technician with general knowledge.

Technician with general knowledge

Technician with general knowledge

Technician with general knowledge

GPS coupling and Data integration with GIS

Not common in standard

instruments.

Not common in standard

instruments.

Measured value not coupled with GPS

coordinates.

Measured value and GPS coordinates

data logging function.

Instrument cost 1,000 U$ 1,000 U$ 2,000 U$ 20,000 U$ or more depends on instrument complexity

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The methods used to gather resistivity data were compared, the advantages and disadvantages of each method were pointed out. The results of this comparative chart show that EM technology could be the most suitable methodology to survey long segments of ROW, economically, gathering one resistivity value at regular intervals at walking pace.

This tool was selected and tested to ensure that the resistivity data gathered with this method was accurate. The preliminary comparisons with the available data of four pin method resistivity values was successful -see figure 8-. Therefore, field survey was planned and executed.

Figure 8. EM resistivity values compared with the four pin method.

4 EMGC Survey Methodology

Field Work

The survey crew consisted in three technicians with a DGPS unit with submeter accuracy and a EM equipment capable of gathering soil conductivity data up to three meters depth. The crew surveyed up to 15 kilometers per day with a daily average of 10 km Figure 9.

The equipment experienced AC rejection problems in ROWs shared with overhead high voltage power lines, the measuring methodology used in this case was the 4 pin method.

The position of pipeline markers and pipeline features must be recorded on the data logs to make it possible to match results from the conductivity/resistivity surveys with data from other indirect inspection methodologies, using the axial position as the common denominator.

Data post processing

The collected data was checked for error daily and then sent to the GIS specialist for further processing.

Post processing included converting the conductivity values into resistivity values, data validation with resistivity values of prior field works, projecting the data in the correct datum, and control of GPS locations with the existing data layers of facilities, over the line surveys, etc.

Figure 9. EM conductivity survey crew in Patagonia desert.

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The ECDA implementation uses the EM data in the pre-assessment step to define regions, the environment is classified using resistivity values and soil sample composition results, the coating condition, pipeline characteristics and prior records of leaks and reparations. This information is taken into account to outline the limits of the regions.

Different GIS layers are used to define the regions, in this case a high resolution digital elevation model DEM, the resistivity layer, soil sample results from prior works and records of prior reparations are shown together (Figure 10) to define the limits of each region and to select the tools to be used to inspect the coating condition and assess the CP effectiveness.

Figure 10. GIS window with DEM and resistivity data.

The indirect inspection methods are used to assess the coating condition and CP effectiveness, the selection of Close Interval Survey (CIS) and Direct Current Voltage Gradient (DCVG) Surveys are used in conjunction with soil parameters to select the indications for direct examination.

Electronic alignment datasheet can be generated using the GIS platform. This kind of graphs are very useful to match several variables and establish correlations between CP and environmental variables, soil resistivity and records of previous records. Figure 11.

1000

2000

3000

4000

5000

6000

7000

6.9 7.0 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9

Progresiva (m)

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

Resistividad (Ohm x cm) Potencial ON Actual CIS ON CIS OFF -0.85 V

0%

10%

20%

30%

6.9 7.0 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9

Progresiva (m)

Profundidad de Defecto (%)

0.00

0.10

0.20

PM Ext Velocidad de corrosión (mm/A)

Figure 11. Data alignment sheet.

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5 Conclusions

TGS has analyzed the advantages and limitations of the continuous ground conductivity survey technique, compared this technique with other widely used techniques and implemented successfully the use of continuous ground conductivity data in the ECDA process and High pH SCC Model.

EMGC survey limitations:

• The resistivity exploration at different depths is more difficult with low range instruments than with the four pin method.

• AC induction can cause problems in the measuring electronics at certain frequencies.

• Linearity is valid with a lower limit of 100 ohm x cm.

EMGC survey advantages:

• Measuring technique without contact issues improves accuracy.

• Coupled with GPS hardware provides easy data integration with GIS.

• At walking pace surveyor could survey 10 km of pipeline ROW per day.

• Higher resolution than conventional methods.

High pH SCC Model and ECDA implementation:

• The EM survey data are used in the ECDA pre-assessment step to define regions.

• Indirect inspection methods are selected to assess the coating condition and CP effectiveness.

• The EM survey data are used in the preliminary phase of analysis to select regions for further data collection in the SCC Model.

• ECDA Indirect Inspection step identifies deficiencies in either coating or CP, and then EM measurements of resistivity will probably suffice to identify locations prone to corrosion.

• Severe DCVG Coating faults and CIS polarized CP levels more positive than -850 mVCSE in low resistivity soils are selected for direct examinations.

• Direct examination results are used to define corrosion rates for each region and inspections intervals. Soil sample results and root cause analysis are included in the pre-assessment step to refine the region definition criteria.

References [1] McNEILL, J. D. Electrical conductivity of soils and rocks. Geonics Limited Technical Note TN-5 . 1980. [2] McNEILL, J. D. Electromagnetic terrain conductivity measurements at low induction numbers. Geonics Limited Technical Note TN-6 . 1980. [3] NACE SP0502-2008. Pipeline External Corrosion Direct Assessment Methodology. [4] PEABODY, A.W. Peabody’s Control of Pipeline Corrosion. Bianchetti, R.L. editor, NACE, Houston, TX pp. 91. 2001. [5] ASTM G 57, “Standard Test Method for Field Measurement of Soil Resistivity Using the Wenner Four-Electrode Method” (West Conshohocken, PA: ASTM, 1995).