5,000 KM ROW CP SURVEY ANALYSIS AND THE POTENTIAL ADVANTAGES OF THE 100 MV POLARIZATION CRITERION FOR THE CP OF AGED COATING OIL AND GAS PIPELINES IN GULF AND NORTH OF MEXICO
J. Canto*, L. M. Martinez de la Escalera*, H. Rivera* and A. Godoy* Corrosion y Proteccion Ingeneria, S.C. Rio Nazas 6. Cuernavaca, Morelos. Mexico. 62290. C. G. Lopez Andrade PEMEX Gas y Petroquímica Básica Marina Nacional Torre B Piso 8. México, Distrito Federal. Mexico. 01000 E. Rodríguez Betancourt DCO, Petroleos Mexicanos. Marina Nacional Torre PEMEX Piso 23. México, Distrito Federal. Mexico. 01000. H. C. Albaya Sistemas de Protección Catódica S.A. Tronador 1126, piso 5° A. Buenos Aires. Capital Federal. Argentina. C1427CRX. Norberto Pesce Omnitronic S.A. San Martín Sur 36 Dpto 5 y 6. Mendoza. Godoy Cruz, Mendoza. Argentina. 62290. J. 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.
ABSTRACT A survey of over 5,000 Km of pipelines was developed in 2006 to assess the CP performance of the pipelines located in the main ROWs of the Mexican States of Veracruz, Tamaulipas, Nuevo Leon, Coahuila, Durango, y Chihuahua. Historically the CP surveying in the region has been done measuring the ON potentials and applying the –850 mV criterion to these measurements. By employing a set of several satellite synchronized by GPS instrumentation, the instant OFF potentials presently reported resulted considerably more electropositive, and the compliance to the -850 mV criterion was found under 50%, in a set of over 5,000 measurements in this important region of Mexico. Besides the needs of new CP infrastructure, we report the main causes that impair the compliance of the CP to the -850 mV criterion of the NACE RP 0169 – 2002. Among these are the problems of isolation of the pipelines to main processing units such as refineries, pumping or metering stations. Findings of irregular, non instrumented or even controlled electrical interconnections between the pipelines in production or abandoned, were also pointed out as major causes for the insufficient CP. Old and damaged coatings of pipelines with many years in service contribute tu limit the CP compliance to codes. Out of service and poorly coated pipelines were found sharing the CP systems in the ROWs, with very deleterious effect on the overall CP performance due to the elevated current demand of abandoned pipelines lacking coating maintenance. In the present scenario a remarkable effort is to be made to pursue the electrical isolation of the pipelines, the needed coating reparations, as well as the use of the 100 mV potential criterion was concluded to be a sound approach to maximize the pipeline operational compliance to the integrity management programs in the Gulf and North of Mexico. Keywords: Cathodic protection, isolating joints, coatings, remoteness, CIS DCVG surveys; pipeline; Gulf of Mexico, 100mV criterion.
INTRODUCTION The use of the 100 mV polarization criterion was concluded to be a sound approach to maximize the pipeline operational compliance to the integrity management programs in the Gulf and North of Mexico. By integrating the 100mV cathodic protection (CP) criterion and its procedures into the algorithms of integrity management and the CP regular surveys, the present needs of investments in refurbishing the CP infrastructure may be reduced.1-8 Particularly, in Southeast Mexico leaks related to pipeline failures resulted in over 500 incidents in 2005. Personal loses, injuries, deleterious effects to property and products and severe damages to environment were caused. Other than third party damage, external corrosion was the main cause of these failures. As a region Southeast Mexico is a world major producer of oil and gas. Of the total Mexican oil and gas production about 70% comes from Southeast Mexico. Pipelines transport crude oil and gas from wells to refineries and into final consumption. All together over 20,000 kilometers of active pipelines from 4 to 48 inches diameter are installed in over 60 rights of way (ROW) just in this area. FIELD PROCEDURES This work was dedicated to assess the current status of cathodic protection (CP) performance at the North and Gulf of Mexico employing six mobile CP laboratory units, all equipped with state of the art CP assessment devices including power plants, rectifiers, GPS (global position system) driven current interrupters, soil resistivity set, isolation joint tester, clamp ammeter, several types of half cells,
close interval survey and direct current voltage gradient, (CIS DCVG) equipment, pipe locators with PCM and ACVG technology, ultrasonic and coating gages and well as a pipe sized clamp ammeter up to 48 inch. Each team driving in two mobile laboratories where leaded by a NACE Certified CP specialist, working together with a field NACE Certified CP 2 engineer and two corrosion technicians. The procedure was focused on visual inspection and direct field measurement techniques, searching for the conditions of isolation systems, possible interference with other CP systems as well as dynamic sources, evaluating the coatings of the pipelines; these parameters were evaluated with help of satellite synchronized current interrupters, up to 5 along the ROW. From literature and the own professional experience from the group, assessment was focused to the next parameters: Rectifier performance, pipe to soil potential, isolation, anode bed performance (surface anodes and deep anodes), anode bed remoteness, and coating performance. The specialized instrumentation that equipped the mobile laboratories allowed diagnosing the control systems of exterior corrosion with the highest possible precision and evaluated directly in-situ of over 5,00 km of ROW, something never done in MĂŠxico by a single work group.
Figure 1. a) Map of North and Gulf of Mexico, where studied ROW´s are found and every test point is marked with red points. Besides a couple of selected areas to illustrate the no satisfactory values of pipeline/soil polarization potential (red squares) and the ones that show potential in normative compliance (green squares), also to a few rectifiers identified (RPC) from b) south of Coahuila state and near to Gulf of Mexico in the Tamaulipas state.
These ROW´s are located in the States of Veracruz, Tamaulipas, Nuevo Leon, Coahuila, Durango and Chihuahua up to the US border, as it is illustrated in figure 1a, which corresponds to the sites from where the studies were performed. The evaluated pipelines diameters go between 8 and 48 inches, while the transported products are crude oil for the north refineries, besides natural and liquefied petroleum (LP) gas, gasoline, diesel and Jet fuel both for national and international consumption.
RESULTS This study considered 5000km of ROW distributed in the Gulf and north of Mexico with very different landscape and weather conditions, as it is illustrated in figure 1; the sites evaluation allowed to recognize that most of the lines have no sufficient polarized potential according to the -850mV criterion. An interesting case of study is illustrated in figure 1b, In green “R� marked squares are the operating rectifiers and in red squares are shown the more electropositive than -850 mV pipe to soil potentials that where the widespread value. This ROW is shared by two 24 inch lines that supply oil to the mayor Refinery of Cadereyta in Nuevo Leon, and a 12 inch refined products pipeline. Rectifiers average an individual output over 50 A. Two main factors were identified. First, the ground bead showed no remoteness and where as close as 10 meters to the ROW. Second, both the 12 inch and one of 24 inch have aged cold tar coating with conductance equivalent to bare steel. Potential attenuation cause values to go more electropositive than -850 mV in less than one kilometer. From our field study, the corresponding values for the 5,205 pipeline/soil potential of the evaluated sites show an almost Gaussian distribution as it is clear in figure 2a; where it is plotted the pipeline/soil polarization potential versus the frequency. This figure allows to establish that most (54%) of the measured values (light gray columns) correspond to potentials with insufficient pipeline/soil polarization (based on the actual Mexican standard for CP); while 39% of the values (from -850 to -1100 mV) match with the range established as effective for CP (dark grey columns) and finally 6.4% of the values are evaluated as exceeded potentials (black columns), which are not recommendable values as they could damage the coating and even the steel; most of these cases are not polarized potentials due to the effect of IR drop associated to unidentified sacrificial anodes.
Figure 2. Statistical analysis of a) pipe soil polarization potential distribution and b) particular problems evaluated during the 5000 km survey of the main ROW´s from North and Gulf of Mexico.
Particularly the causes for the CP insufficiency are indicated in the obtained results during the inspection of the CP system components for each ROW. A resume of the main play factors in the 5370 studied installations is shown in figure 3a. Dark grey fragment of each bar corresponds to the components working properly at satisfactory pipeline/soil polarization potential, while light gray show the components associated to those components that presented fails or insufficiency. As it was previously mentioned, the evaluated components are rectifiers, pipe to soil potentials, isolation joints, anode beds (including both: surface and deep anodes), anode bed remoteness, and pipe coatings. It is evident here that there is a direct correlation between the operation conditions of the CP system components and the adequate potentials values. Consequently the CP insufficiency exists when: rectifiers are found not operating; there are no isolation joints or they fail; there are electrical bounds between pipelines and foreign structures; the coating systems are defective or the anode beds have insufficient remoteness. From figure 3b, we can clearly see that association based on statistical analysis, from those we can identify that the most important problems associated to insufficient potential are coating failures, omission of isolation joints and not desired electrical interconnectivities. In figure 3, three cases are illustrated. From one of the studied ROW´s, we exemplify how a pipeline is interconnected to the superficial installations of a compression station, which is shown 3a, the absence of isolation for the protected pipe induces a significant current loss from the cathodic protection system. Also ROW located in populated congested areas as well as social population problems cause electrical remoteness for ground beds to be difficult to achieve horizontally and deep anode technology need to be approached. However it was found extensive failure of this technology due to constructive and design faults. In figure 3c is possible to observe a deep anode in Monterrey with the ventilation pipe blocked as it was apparently used to pump the backfill down during the construction process. Current output and consequently pipe to soil potentials where affected as the resistance of the ground bed exceeds 10 ohms. Buried and unidentified electrical bonds have proved to be a common engineering practice all over the 5,000 km inspected. Illustrated case 3c shows the results of detection of a significant case of bonding in which, an old 36” oil pipeline coated with cold tar is bonded to a newer 12” refined product pipeline with Fusion Bonded Epoxy coating. In a desperate attempt to avoid damaging the 12” coating by excessive CP, the 100 amp insight rectifier was forced to work with a 670 mA output leaving the old 36” pipeline in almost natural potentials.
a
c
b
ELECTRICAL BOND
Figure 3. Common problems with the isolation of pipelines that derivate into important local reductions of the soil/pipeline polarization potential.
CONCLUSIONS This paper reports the unique and innovating experience of performing a CP survey to a large number of pipelines in the North and Gulf regions of Mexico. The variety of problems encountered in the actual field conditions has proved to be an enormous information provider allowing the work team to identify the main areas of opportunity for the improvement of the CP coverage. All together the problems converge in the fact of having more than 50% of the pipe to soil polarized potentials not complying with the standard -850mV criterion. This team has identified important areas of opportunity for the improvement of the CP performance both in operational practices and the improvement of the infrastructure, described as follows: 1. Lack of electrical definition of the pipelines in the ROW´s is a major issue limiting the effectiveness of CP. Isolating pipelines from huge metallic structures such as refinery, gas processing units, and power plants is a high priority activity in order to accomplish protective pipe to soil potentials in the ROW. There is the need to isolate the pipelines from metallic bridges and other grounded structures required to be build by the topographic accidents or customer pipeline derivations in all the thousands of kilometers of the ROW´s in Mexico.
2. More CP current output from the currently installed rectifiers. The present Mexican CP regulations limit the ON or OFF pipe to soil potentials in the aim of protecting the coatings and minimizing other deleterious hydrogen effects. Either the ON potentials at drainage points are to be limited to -2500mV or the OFF potentials are limited to -1100 mV. The conservative application of both limits often causes the rectifiers being operated at very low current output. Due to considerable high IR drops, the -2500 mV limit complying may cause the OFF potential to be significant less electronegative than -1100 mV, and current outputs were far below the capacity of the CP systems. Most of the rectifiers found in the field, over 97% have a capacity of 100 A, and actually they were operated with an output averaging 14 A. This team sees an area of opportunity in raising the average from 14 A output to 50 A per rectifier - anode bed. 3. Many of the lines inspected did not have records of CIS, DCVG or ACVG surveys at all, there were others that showed records to be up to 5 to 10 years old and very few actually got surveyed for coating failures and repaired. Coating quality is a major component for external corrosion control, and a very good partner in having a successful CP practice. Coating surveys and repairs should be generalized to all active pipelines and enforced to be performed with a frequency of about 5 years. Inspection during pipelines coatings repairs was also found to be a need in the light of the field practices observed during this survey. Surface preparation, environmental conditions, coating application procedures and care of the coated surface before burying are the reasons for the critical need of having certified coating inspection enforced in the field. 4. The history of vandalism of the last decades has leaded to the depletion of the test stations infrastructure. Presently in many cases the ROW have only one test station per kilometer, and all pipelines sharing the ROW are interconnected in between and only to this test station. For the proper control of the CP current demanded by each of the pipelines, considering coating conditions, temperature and diameter, the test stations should be reinstalled and the interconnections removed. Recent test station technologies may be used where communication to the public needs to be made very explicit about the role of test stations in the safe operation of the pipelines, the hazards of manipulating the test stations and the penalties regarding vandalism. The unworthiness of the newest test station light weight polymeric materials for other uses may also contribute for the longer operational service of the test stations. 5. Remoteness was found to be a rare characteristic of the anode beds of the CP systems, causing a significant extra factor of potential attenuation and insufficient CP current in long segments of the pipelines not so far away from rectifiers. Remoteness allows a more uniform distribution of CP currents along the pipelines having impressed current systems. Deep anode solutions may help for the case of congested ROW located in conflictive zones were lateral extensions of land are unavailable. 6. The use of the 100mV criterion may also help in solving the problem of complying pipeline integrity management program requirements for external corrosion control. It is conceivable that the old and poorly coated pipelines may not ease the compliance to the -850mV criterion that has been the target for decades in Mexico, or for that the investments in new CP infrastructure may just be too much. A recommended practice manual was prepared to advice in the correct application method and technical limitations of the 100 mV criteria. Also a field practical example was performed for 5 km at a 16� pipeline where the -850 mV criteria hasn’t been able to be met for several years.
ACKNOWLEDGEMENTS We thank Osvaldo Flores and Maura Casales for the valuable technical support. We also thank Jaime Gutierrez at PEMEX Refinacion, Armando Alanis of PEMEX Exploration and Production, and Jorge Gomez of PEMEX Gas for the valuable support in the field. REFERENCES 1. Macdonald, K.A., and A. Cosham. “Best practice for the assessment of defects in pipelines - gouges and dents.” Eng. Fail. Anal. 12, 5 (2005):p. 720. 2. Jack, T.R., Wilmott, M.J., Sutherby R.L., and R.G. Worthingham. “External corrosion of line pipe - A summary of research activities”. Mat. Perf. 35, 3 (1996):p 18. 3. Holtsbaum, W. B. “Electrical safety and cathodic protection rectifiers”. Mat. Perf. 44, 6 (2005): p. 26. 4. Hall, S. C. “Cathodic protection criteria for prestressed concrete pipe - An update”. Mat. Perf. 37, 11 (1998): p. 14. 5. Funahasi, M., Bushman, J. B. “Technical Review of 100 mV Polarization Shift Criterion for Reinforcing Steel in Concrete”. Corrosion 47, 5, (1991): p. 376. 6. Benson, R. C. “A review of soil resistivity measurements for grounding, corrosion assessment, and cathodic protection”. Mat. Perf. 41, 1 (2002): p. 28. 7. Glass, G. K. “The 100-mV potential decay cathodic protection criterion”. Corrosion 55, 3 (1999): PBD. 8. Didas, J. “Cathodic protection criteria and its application to mature pipelines” Mat. Perf. 39, 4 (2000): p. 26.