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IBP1196 11 DEVELOPMENT OF A METHODOLOGY FOR PREDICTING THE BEHAVIOR OF ELECTRIC CURRENT IN METALLIC STRUCTURES UNDER THE INFLUENCE OF CATHODIC PROTECTION SYSTEMS 2

September 10-22

Hemán Rivera!, Lorenzo M. Martínez 2, Jorge J. Cantó , Lorenzo Martínez-Gomez3

Copyright 2011, Brazilian Petroleum, Gas and Biofuels Institute - lBP This Technical Paper was prepared fo r presentation at the Rio Pipeline Conference & Exposition 2011 , held between September, 20­ 22, 201 L in Rio de Janeiro. This Technical Paper was selected for presentation by the Technical Committee of the event. The material as il is presented. does not necessarily represent Brazilian Petroleum. Gas and Biofuels lnstilule' opinion or that of its Members or Representatives. Authors consent to the publication of thi s Technical Paper in the Rio Pipeline Conference & Exposition 2011.

Abstract The behavior of electric cUITent from cathodic protection systems depends on many factors , like geometric and electrical properties of materials and media involved. [n most of the systems this behavior can be predicted during the design phase but in certain conditions like the presence of foreign structures or complex geometries, a detailed analysis is needed to ensure that the structures will be adequately protected and that the system will not cause any interference, which could represent a threat in the integrity of nearby structures. Certain tools allow the analysis of each system and to achieve a good prediction of its behavior, either existing or not yet installed, to be able to determine whether there are risks associated with their operation, or even predict ifthey have caused damage to sorne structure and the magnitude of it. The tools used in the development of the methodology included fieJd tests, analytical methods, computer programs for modeling, boundary element analysis and integrity assessment tools. It is noteworthy that unlike other methods of integrity assessment such as ECDA the proposed methodology is not focused on the evaluation of a structure, but in the operation of a cathodic protection system. The application of this methodology will enhance the safe operation of a cathodic protection system and even preven! operation accidents associated with poor performance or electrical interference generated by it.

Keywords : Cathod ic protection, interference

2

Engineer - Universidad Autónoma del Estado de Morelos Ph.D Materials Science - Corrosión y Protección Ingeniería S.e.

3

Ph.D Materials Science - 1 Jniversidad Nacional Alltónoma ele Méxiro

1 Mechanical

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1. Introduction In the simplest cases, the cathodic protection systems are intended to provide electrical current in order to decrease the corros ion velocity of metallic structures in contact with soil or water. The Impressed Current Cathodic Protection systems (ICCP) use a power source that creates a potential difference between the anodes and the metallic structure rising voltage gradients in the soil, not only in the area between the anodes and the structure but in the vicinity of the groundbed.

The affected surface and the magnitude of the gradients depend mainly in the potential difference (i.e. the output voltage of the power supply) and the electrical properties of the soi l. When other metallic structures are present, they can be affected by the voltage gradients as unintended current paths which may result in corrosion damages to the interfered structure and reductions in the cathodic protection system performance.

The interferences due to cathodic protection systems in pipelines are relatively easy to identify, measure and mitigate, since they usually occur in areas where the only metallic structures in the soil are the pipelines. But when the systems are installed close to bigger facilities , where other structures such as pipelines, buried tanks, electrical grounding systems and construction structures are present, the behavior of the electrical current may be difficult to predict and control. In these cases a process of analysis during the design and measurements in the construction and commissioning phase will be required to prevent possible stray currents across the structures under the effect of voltage gradients.

The experience in several study cases has become into a methodology to develop the analysis and prediction of the current behavior combining analytical equations with numerical modeling and field surveys such as described in the External Corros ion Direct Assessment (ECDA) methodologies.

Many factors are involved in the currents paths. For the analysis of the current behavior, the electrical properties of all elements involved and the geometry and distribution of those elements will be the base for the prediction of current distribution.

2. Case Studies The proposed methodology is based on the analysis of several cases of study where the current behavior has been a concern in terms of having the possibiJity of electrical interference and the effect ofthis on the corros ion rates in both, protected and unprotected structures involved. A brief description of each case is presented to define the conditions and the applied tooJs for each case.

2.1. Interfered pipelines in MazatIan (Sin, Mex)

In 2008 a CP survey over the pipelines in Mazatlan used for fuel transport from the maritime terminal to the storage terminal , showed the presence of an electrical interference due to a cathodic protection system intended to protect an isolated pipellne which feeds fuel to ยกhe local power plant.

Figure l. Geometrical distribution offuel pipelines in Mazatlan Figure 1 shows the geometrical distribution of the different structures involved in this interference. The groundbed (red dots) is intended to protect the pipeline in the segment :2 from the valve to the power plant (green line),

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the blue arrow represents the intended path of the current. The first anode of the groundbed is only 30 meters (98 ft) from the pipelines in segment I (yellow lines), creating an alternate current path from the anodes to the segment 1 pipelines, and from these pipelines to the protected pipeline in segment 2 (the interference current is represented by orange arrows).

The presence of interference represents potential corros ion damage in the segment 1 pipelines in the area where the interference current is discharged from the metal surface to the ground. Since the pipelines are under a paved street, direct inspections are not a convenient altemative. In order to estimate the magnitude of the damage it is necessary to develop and follow an assessment process, which consist in the next activities: •

Estimate the magnitude of the interference current and the time it has been acting over the pipelines.

• Determinate the zone of the pipelines where it has been a current discharge from interfered pipelines to the ground.

•

Evaluate the coating cond itions of the interfered pipelines in the discharge zone in the metal surface.

to

estimate the current density

• Calculate the metal loss and penetration rate over the pipelines.

The available tools used in this analysis are: •

Field CP measurements, including potential surveys, coating inspection (DCVG) and soil electrical properties.

These measurements allow to characterize the electrical system and to identify the charge and discharge zones.

• Analytical equations like Ohm 's Lay and Sundae 's equation are used to solve the system and calculate the magnitude of the interference current.

• Numerical Modeling and Finite Element Analysis are used to estimate the current behavior and determinate the current charge and discharge zones.

The diagram in Figure 2 shows the proposed methodology to generate a reliable damage evaluation and to design a solution for mitigation and integrity evaluation .

Figure 2 The proposed methodology for the interference analysis and mitigation includes all available tools, For this case study the combination of field surveys and numerical modeling showed the most probable areas for the current discharge. Using the algebraic analysis , the estimation of interference current allowed designing the mitigation method and in combination with the inspection procedures that showed a damaged coating, the damage prediction resulted in a very low penetration rate, For this particular case, it is possible to perform field tests, apply the proposed solution and evaluate its performance, because the pipelines and cathodic protection systems are already installed and they are accessible for an inspection, and its operation can be modified.

Two solutions were proposed to mitigate this interference; in short term an adjustable resistance was installed to connect the interfered pipelines to the cathodic protection system, giving an electronic path for the current to retum to its source instead of an electrorytic path wich results in corrosion damage for the interfered pipeline. The adjustable resistances helps to regulate the current supplied to the interfered pipelines without affecting the system performance over the protect structure.

In long term, a better solution is to replace the interfering groundbed with a deep anode system, finding a proper location to ensure that no current can be picked up by foreign structures,

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2.2 Madero Maritime Terminal (Tamps, Mex) The dry dock of the Madero Maritime Terminal is the largest dry dock in Latin America. It is a strategic facility for the operation of the Maritime Terminal and for the Distribution Department of PEMEX. It is constructed on the Panuco River on the state of Tamaulipas under environmental conditions that favor the corros ion of steel structures and the reinforcing steel of concrete structures. The Dry Dock is constructed mainly by sheet piling and a steel Lock Gate.

There are four main different metallic structures under the effects of the environment, the sheet pilling which supports the Dry Dock, Bay Dock and Ship Repair Dock, the Lock Gate , which is considered as a floating ship, the Dolphin's supporting piles and the Dry dock 's Gate Columns made ofreinforced concrete.

Different inspections were conducted in order to obtain an integral corrosion evaluation of the steel structures.

Particular conditions found on each structure demanded complex solutions to improve the actual corrosion control systems. For the atmospheric exposed areas the solution is based on special coatings to be applied in irregular surfaces with many cavities which not allow regular paints.

For the submerged and buried structures the best solution is the application of cathodic protection, but the geometry makes it difficult to predict the current behavior, and the magnitude of the non-visible metallic structures makes necessary field tests and numerical simulation to estimate the amount ofthe total current requirements.

The Madero Maritime Terminal has a steel sheet pilling divided in three zones: the Bay Dock, the Dry Dock and the Ship Repair Dock. In total, the sheet piling has an are a of 30,085 m", which mean s 60,170 ml of metallic structure that has to be protected against corrosion. Nevertheless, only 8 % (4606 ml ) of the sheet piling surface is protected by cathodic protection systems. Figure 3 shows the three areas where the sheet pilling is located.

Figure 3. Satellite image ofthe Ship Repair Dock, the Dry Dock and the Bay Dock. The yellow line shows the location of the sheet piling.

In order to stop the general ized corrosion of the sheet pi lIing, an impressed current cathodic protection system using distributed semi-deep anodes was proposed. Current demand measurements were performed showing that approximately 0.48 amperes per linear meter of sheet piling was needed; meaning a total of 614 A. In addition a three dimensional model of the sheet piling and its surrounding media was performed using the boundary elements method.

Simulations allowed determining the optimal distribution of the anodes. Figure 4 shows in a color scale, the potential distribution calculated with the numerical modeling.

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Figure 4. Results of the tmee dimensional model ing of the a) Ship Repair Dock and b) Bay Dock.

The area known as "cofferdam" is a corridor with two parallel sheet piling sections attached to each other by steel beams. By modeling such structure, it was proposed to install anodes between the sheet piling in order to protect both strtictures and the anchor beams.

The calculated amount of current required to protect the sheet pilling represents a risk of electrical interference due to the voltage gradients generated by the groundbeds. The Maritime Terminal is a big facility and severa! metallic structures are in the vicinity of the dry dock, including pipelines. storage tanks and reinforced concrete strti ctu res . For this reason, it is important to evaluate the behavior of the electric current and considerate it in the cathodic protection system design , any additional device that may contribute to interference mitigation.

Since the cathodic protection has not been installed yet, numerica! modeling and algebraic analysis are the available tools for this evaluation. The numerical modeling allows establishing the behavior of current and identifying possible interfered structures, while the algebraic analysis gives a good approximation of magnitudes.

For this case, the risk is detected in the design stage, so there is no certainty of the interference occurrence, but it is necessary 10 assure there won 't be any condition that may threaten the integrity of any structure. An iterative process of solutions, designs and results evaluation is now included in the methodology.

Figure 5. Evaluation methodology for the Madero Maritime Terminal new CP designo 2.3 AC interference in 8" gas pipeline in Guadalajara (Jal, Mex) During the planning stage for a new gas pipeline in an urban zone ofGuadalajara city, a threat was identified in a segment where the pipeline's right of way is shared with a two-circuit transmission power line. The construction project includes the installation of a pipeline of 8" for natural gas distribution. The section in which the pipeline will share the right ofway with power lines has a length of 1.6 km (1 mi). In Figure 6 is represented the shared ROW track.

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Figure 6. 8" Pipeline on the section shared ROW with power ยกines.

The pipeline will be built at an approximate depth of 1.2 meters below ground level, most of it will be under a paved street and a seetion of 350 meters in natural terrain. The pylons support a couple of three-phase circuits with a vertical distribution for each circuit, and each of them has a guard in the up position.

The objective of the interference evaluation is to calculate the induced voltage that may affeet the pipeline in order to estimate ifthere may be an electric hazard for the operative personal or the public . Additional calculations are required to determinate the minimum distance between the pipeline and each pylon foot to avoid a fault condition that may transfer an elevated amount of current to the pipeline. .

By induction tests on site it is possible to estimate the longitudinal electric field acting over an exposed pipeline. This test is performed by placing an insulated conductor on the route ofthe pipeline. At one end ofthe cable is grounded by a buried rod , and at the opposite end a voltmeter is used to measure AC voltage between the cable and another buried rod. The voltage obtained. divided by the length of the conductor results in an approximate value of the longitudinal electric field, induced by power lines.

The obtained value is only an approximation since it does not represent all the conditions of the pipeline, and they ean only determine a value for the operating eonditions of the lines in the time when the measurement was made, however transmitted power can vary according to the schedule and even the season ofyear. This value may observe the magnitude scale in which rhey are expecred ro behave. and wi 11 serve to collate and val idate the values obtained analytically using rhe equations.

The results of field tests showed rhe resulrs in equation 1: (1)

Where V is the maximum expected voltage in both ends of the shared ROW segment. The behavior of rhe AC voltage along rhe pipeline is shown in Figure 7.

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25 20 15 10

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-25 Distance (m)

Figure 7 Expeeted behavior of ground-to-pipeline AC voltages along the segment Furthermore phasor algebra equations were applied to pred iet the effeets of the transmission Iines on the pipeline based on the geometrie and operating parameters reported by the operator of the power lines. The results are presented below in equation 2: .

ZM = J . f

J(h - h' +2 J p i j 2rrfl'o )2 +d 2

. /10 . In -'--- -.jr7'(h=+=h=c')""z+=d""2' - - -

(2)

Where

f = Frquency in Hz

= Space Permitivity = 1.26x10- 6 H1m p = Soil Resistivity j = Complex operator = g /10

Since the lines are right over the pipeline, the value "d" is zero, so the equation simplifies being as follows: ' f I Z M = J' . /10' n

h-h' +2Jplj2rrfl'o h +h'

(3)

Assuming an extreme case in wh ich the three lines carry the maximum current of 1,250 A, the value of the LEF remain as the sum ofthe three mutual impedances multiplied by the maximum current, as shown in equation 4: E = ImaxCZยกAZ +18+ Zยก C + Z2A Z +28+ Z2 C)

E = 1,250[(-1.05511 X 10-5 + 3.6687 x lO-S}) - 0.021787

+ (-6.87823 X 10- 6 + 2,79192 x lO - s})] =

+ 0.080757 j [~]

(4)

The physical meani,ng ofthis value is a Linear Electric Field Induced with a magnitude: E = 0.083~ = 83~m

km

(5)

The obtained results by both methods show a significant difference, the main reason for this is that the field tests were conducted with actual operating conditions of high voltage lines (which are unknown) while the algebraic analysis considers the maximum operation condition reported by ยกhe ยกransmission line owner, In this case, the proposed methodology is aimed to predicting the effect of an external agent on a pipeline that has not yet been built. Its purpose is to detect a possible risk to the population and generate an efficient mitigation 7

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solution. Since there is no simulation tool for altemating cUlTent, available tools are the field tests in conjunction with the algebraic analysis.

The proposed methodology for this case study is shown in Figure 8.

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Figure 8. Proposed methodology for AC interference Analysis

3. Conclusions Electrical interferences are phenomena that can threaten the integrity of pipelines and also buried or submerged metallic structures for the security of the population. For detection and mitigation is necessary to keep in mind all the factors involved in the system and manage them properly. In some cases the solutions may be trivial and only requires a periodic inspection program for maintenance, but in more complex system or when the phenomenon occurs in high consequence areas (HCA) , the evaluation process and design of solutions for control and mitigation requires further analysis and verification processes that wiJI be more demanding.

For each case of study has been proposed a specific methodology that integrates and allows interaction of the different tools availabJe according to their own needs and objectives. From each case of study can be extracted common panems to establish a general methodology that alJows adaptation to any particular case to be a functional guide in solving real problems for the maintenance of pipelines. The general scheme proposed is shown in Figure 9:

Figure 9 General methodology for interference detection and solution designo •

Field SUfvey: During all the process, elements and parameters should be identified both, geometrical and electrical involved. lncludes general measurements and surveys. It is considered as an initial stage but in most cases the need for further studies at later stages can be detected.

•

Definition ofequivalent system: It is an abstraction ofthe important elements that make up the system and are directed to an efficient ·analysis. According to the specific needs of each case, the equivalent system can be approached in different ways as an equivalent circuit diagram, a three dimensional model or a flowchart.

•

Data analysis: At this stage, an analysis of data from field measurements according to the equivalent system approach is made. The analysis can be done by experimental methods, analytical tools or by computer resources and it is best to use all available tools, considering the limitations and cOlTections that may apply in each case. o

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Is the comparison of results to define a criterion based on the interpretatíon of them, ir is usually given greater validity to that resulls more conservative, but sound analysis can help the selection of the optimal solutíons. of the problem but also The chosen solution mus! consider not only effective ensure efficient operatíon of rhe system. The solution mus! al 50 be economically viable and inelude mechanisms and procedures for assessment and The proposed solutíon musl be evaluated borh from a functional standpoint and economlcally, and it should be íntegrated into the system to its effectiveness and If necessary the dala should be done to ensure that the proposed solution is functional and compatible with the system.

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4. References

SHWEHDI, M. U. M. Transmission Line EMF lnterference wilh Buried Pipeline: Essential & Cautions.

In: lntemational Conference on Non-Ionizing Radialion al UN EPRI. Power Line-Induced AC PotentĂ­al on Natural Gas v.

1983 VON BAECKMANN W., W W. Handbook of cathodic corros ion protection. Gulf Profesional Publishing. 1997 HOLTSBAUM, W Cathodic Protection Survey Procedures. NACE International. 2009 ALBAYA, H. MARTINEZ DE LA ESCA L. M.. CANTO, J., A.. CARRlLLO, ASCENCIO, J. A.) MART1NEZ-GOMEZ L.. Hybrid CP for an Airport Jet Fuel Pipeline. Materials Performance 48 40-45,2009.

US 1\aval Sea Command) Dock Inciden! (source: Web T.H. Deep Anode Systems. Loresco International. 1997 NACE Intemational. Cathodic Protection ist Course Manual. 2004 NACE lnternational. Cathodic Protection Interference. 2007

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