Stg35 cis and interference nov19 2007f

CIS STUDY AND INTERFERENCE ROOT CAUSE ANALYSIS OF AN EARLY EXTERNAL CORROSION LEAKAGE OF A PRODUCTION GAS PIPELINE IN THE GAS FIELDS IN NORTHEAST MEXICO

J. Canto* and H. Rivera* Corrosion y Proteccion Ingeneria, S.C. Rio Nazas 6. Cuernavaca, Morelos. Mexico. 62290.

N. Pesce Omnitronic S.A. San Martín Sur 36 Dpto 5 y 6. Mendoza.

Godoy Cruz, Mendoza. Argentina. 62290.

H. C. Albaya Sistemas de Protección Catódica S.A. Tronador 1126, piso 5° A. Buenos Aires. Capital Federal. Argentina. C1427CRX. J. L. Luna Baez and R. Perez-Polanco Petroleos Mexicanos.

Marina Nacional Torre PEMEX Piso 23.

México, Distrito Federal. Mexico. 01000.

A. Alanis PEMEX Exploracion y Produccion.

Blvd. Lázaro Cárdenas No. 605, Col. Anzalduas Reynosa, Tamaulipas. MexicoJ. A. Ascencio and L. Martinez-Gomez** Instituto de Ciencias Físicas, Universidad Nacional Autonoma de Mexico.

Avenida Universidad s/n, Colonia Chamilpa Cuernavaca, Morelos. 62210, Mexico. *Also at Facultad de Ciencias Quimicas e Ingenieria, Universidad Autonoma del Estado de Morelos.

Avenida Universidad 1001, Colonia Chamilpa Cuernavaca, Morelos. 62210, Mexico. ** Also at Corrosion y Proteccion Ingeniería, S. C.

ABSTRACT The gas production in Northern Mexico has experienced significant growth in the last years. The fields are now exploited by a number of gas producers increasing the complexity of the Cathodic Protection(CP) systems. We report a case of study of a one year old 10� pipeline segment which was damaged by interference current discharge causing a significant number of gas leaks, and over 14 deep corrosion pits. The pipeline was fully removed and reconstructed along the discharge zone of over 2 miles. A CIS study was performed on the pipeline to find a zone of current pick up nearby an anode bed in service to other pipelines. This anode bed caused the pick up of current for over a year, and the CIS study also allowed to reveal the discharge zone.

A thorough documentation of this case study was possible due to the development of the CIS, as well as the access to the construction work during the repairs of the pipeline to the actual pits, and the assessment of the state of the Fusion Bonded Epoxy (FBE) and its defects. The calculations regarding the amount of current pick up and discharge, the metal losses associated to the interference and the correlation to the actual pits was made possible. This work lead to the establishment of recommendations for the CP design, construction and commissioning in the area, the coatings inspection, the CIS methodology and the needs of consideration of the many CP systems operating in this heavily congested zone of Northern Mexico.

Keywords: Cathodic protection, interference, pipelines, gas, anode bed, rectifier, corrosion, pits, fusion bonded expoxy, coating, root cause.

INTRODUCTION The natural gas production in Mexico has increased very rapidly. In the last five years the increment was 45%, 16% of which happened in the last year. Recently the Mexican gas production has reached 6.5 billion cubic feet per day. The gas fields of Northeast Mexico are remarkably expanding and have been signaled as one of the most productive and expanding in the country, where now about 1.5 billion of cubic feet are produced per day. The gas production fields have accelerated expansion in the last years due to growing gas prices and demand, and the incorporation of private participation in the production and transport of gas in the zone. Hundreds of new production wells are constructed every year, as well as many hundreds kilometers on new gathering and transmission pipelines.

Cathodic Protection (CP) interference has been an increasing cause of concern in this region due to the multiplicity of pipelines as well as CP systems, the impaired remoteness of the anode beds due to the congested environment, and the communication problems between the pipeline owners. CP design and construction in heavily congested production fields are to be performed following a through field inspection of the pipelines to be protected, and a very careful identification of other existing pipelines, anode beds and facilities. There must also be a well documented background of information of adjacent pipeline networks and installations, as well as the regular soils and current demand studies.1-4 Anodic interference is a very undesirable condition in the context of corrosion control of pipelines.

Interference is prone to occur when in construction new pipelines the CP design overlooks the existing facilities in the field. While current pick up from foreign CP systems does not affect those segments of the pipeline, the discharging segments may be severely affected by metal loss. It is common that both the pick up and discharge currents account to several amperes in magnitude, each ampere yielding metal losses in the discharge segments near to 10 Kg of steel per year. The study reported in this work

involved a visual inspection of excavated pipeline, CIS, and other CP assessment studies in the region with reported leaks, and soil properties.

CASE OF STUDY In this paper we present the case of a 5,600 m length 10� pipeline which failed nearly after a year of service. The gas pipeline construction was performed using API grade X65 steel and FBE coating. The failure consisted of a number of perforations from the exterior of the pipeline through the metal by means of localized pitting, as is shown in figure 1. A general scope of the aerial view of the extreme of the pipeline affected by the localized corrosion and the reported leaks (the scale bar corresponds to 500 m). The failures caused the release of natural gas, but were detected in time to avoid explosions. The type of problems identified in the metal are illustrated in the figure, and they are associated to the sites where they were found, establishing a preferential region for the pitting formation. Fresh mechanical damage of the coating by scratching during installation is evident in i and v; besides coating blistering is clear in ii and iii; and deep metal loss can be observed (viii), and full penetration is also shown in iv, vi and vii.5

ii

iii

iv

v

vi

i

vii

viii

500 m Figure 1. Aerial view of the region with reported leaks and corrosion problems. Examples of pitting evidence are shown from eight different sites.

In order to discard other causes that could be associated to corrosion pitting, we considered relevant ones, as the case of the possibility of microbiological induced corrosion (MIC), fundamentally

associated to the presence of microorganisms in the soils, which derive into accelerate metal degradation in punctual sites.7-8 Consequently the soil properties have to be considered and they were studied in our analysis.

We identify that the soil in this region is desert type, dry most of the year. At the time of the present study the rain season was about to end. The acidity of the soil was tested and the pH average was 6. The chloride contents in the oil are 35 to 40 ppm. Soil resistivity varies between 1,500 Ohm-cm to 7,500 Ohm – cm averaging 4,100 Ohm – cm. MIC of the soil was tested employing both APB and SRE commercial microbiology sets for testing in 5 points along the pipeline trench. The soil in the pipeline trench was wet employing distilled water and sterile syringes were used to extract samples and inoculate the vials. All tests resulted negative for MIC. FOREIGN ANODE BED CPR

CURRENT INTERFERENCE INCOMING CURRENT INTERFERENCEDISCHARGE

KM 1.900 KM 0.500

CPR KM 0.300

KM 0.000

NORTH

Figure 2. Aerial view of the region with the corrosion affected pipeline. A foreign anode bed is identified in the top of the image, while the zones with current interference incoming and discharge are illustrated.

Also the leaks evidence are shown by yellow marked regions.

A CP system consisting of 25 vertical graphite anodes and low resistivity coke backfill anode bed, and a 100V X 100A rectifier were installed at the position marked in green as CPR at the North extreme of this pipeline, as depicted in figure 2. However for several reasons this CP system did not operate most of the time. The field inspection over the ROW revealed that an anode bed belonging to other (foreign) pipelines was located at a distance of just 10 m from the pipeline of this study at the South extreme. The foreign anode bed was oriented in the parallel direction as shown in figure 2. In the figure, we can also identify the sites were leaks (yellow marks) were reported, which are localized at a distance of least around 4 km from the position of the foreign anode bed (Km 1.900, 0.500, 0.300 and the reference 0.000 at the North of the registered area).

PIPE/SOIL POTENTIAL, V

The foreign CP system remained in service almost to the time of the present study providing a CP current of 34 A. A current interrupter was installed on the foreign CP system and a close interval study was performed over the pipeline. The pattern of the ON and instant OFF potentials revealed a clear interference condition where the pipeline captured current along the first 100m approximately, and discharged along the resting 5,500m of pipeline, as shown in figure 3. The instant OFF to ON potential difference was about 1V along the discharge zone. Pipeline conductance measurements performed in the field lead to an estimate of the pipe resistance along the segment affected by the interference allowing to an estimate of the current discharge of about 0.6A during the year of service of the pipeline.

-5 -4

ON POTENTIAL

CP Interference charge

OFF POTENTIAL

-3 -2

Reported leaks

-1 CP Interference discharge

1V

0 1 Foreign anode bed

5

4

3

2

1

0

DISTANCE, Km

Figure 3. Instant OFF to ON pipe/soil potential evaluated for different distances, denoting the effect of the CP interference discharge – charge effect.

The metal loss associated to the accumulative effect of this current discharge could have been over 5,000 grams and could have caused about 15 pits per kilometer, of the size shown in figure 4, also similar to the effects observed in figure 1. In figure 4a, the pit evidence correspond to a 0.8 cm diameter with corrosion products around the main pitting evidence, here the appearance could suggests that coating is just affected in the region of the own pit. However, observing the case shown in figure 4b, more metal is lost, and the pit diameter is of 1.2 cm, with a deteriorated coating zone of more than 5 cm, which is associated coating disbondment and metal loss exposed during the examination of the samples, by the removal of the corrosion products and residues from the affected zone. This team worked along about 1km length of open trench of the pipeline and found 14 pits of the type shown in figure 1 and 4.

a

1 cm

b

1 cm

Figure 4. Pitting evidenc of two different regions. a) With evidence of corrosion residues and b) when the material is detached around the pitting site.

DISCUSSION AND COMMENTS The interfering anode bed was promptly disconnected and a new one was installed in a remote position.

The evidence denote the interference effect, which is also associated to mechanical degradation of the coating system to generate rapid corrosion damage in the pipelines, and we can then mention the next consideration.

Two commentaries regarding the case of study can also be made. One refers to the design of the CP system installed, although almost never in service. It is well known that the preferred and almost single design for many of the CP systems installed in Mexico is a 100V X 100 A rectifier, along with an anode bed of 25 vertical anodes. Of a sample of 244 CP systems in the region, over 95% were 100V X 100A in capacity. This particular case of study where a new FBE coated relatively short pipeline was to be protected, even in a very conservative estimate could not demand a CP current of more than 500 mA. The CP design based on a 100A rectifier was very much in excess, and actually a few Mg anodes could have been a more suitable CP solution.

The second commentary is regarding the precautions needed for the coating integrity during construction as well as the convenience of a temporary CP system in order to not to expose the pipeline for long periods during construction and commissioning. The soil in this region, as shown in figure 5 contains many small and jagged rocks which when neglected caused damage to the coatings, and induced the localized pits already reported. Failure of the coating can induce much higher corrosion penetration due to localization in the absence of CP. The coating damage could have minimized by allocating the new pipeline over a bed of sand.

CONCLUSIONS The present case of study was used to stress the importance of introducing specialized and certified CP design in this important, productive and expanding gas producing region. The layout and construction of

new pipelines as well as the corresponding CP systems should anticipate all the potential CP interference situations caused by the increasing conglomeration of pipelines and well casings.

A direct root cause of the early failure of this pipeline, as well as its reconstruction in full was associated to a well defined CP interference situation, where quantitative calculations allowed estimating the metal losses, and correlated to the approximate numbers and sizes of the pits found in the field inspection. The recommended actions were directed to the disconnection and removal of the interfering anode bed, and its further relocation. The analysis of the CP design for this pipeline proved extensive over design, and a recommendation was issued to provide a solution based on sacrificial anodes. A note was given on the importance of preserving the integrity of the pipeline coating during construction and service, since many of the pits occurred at points where the coating was mechanically damaged.

ACKNOWLEDGMENTS We are grateful to PEMEX Exploration and Production for the help in performing the field work. We also thank the technical support of Anselmo Gonzalez, Maura Casales and Osvaldo Flores.

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