NUMERICAL MODELING FOR CP DIAGNOSIS AND SOLUTIONS TO INTERFERENCE PROBLEMS IN SHORE FUEL OIL AND GAS PIPELINE NETWORKS IN THE PACIFIC COAST OF MEXICO.
Arturo Godoy,1 Jorge Canto,1 Roberto Ramírez,1 Hernan Rivera,1 Lorenzo M. Martinez-dela-Escalera,1 Corrosion y Proteccion Ingeneria, S.C. Rio Nazas 6. Cuernavaca, Morelos. Mexico. 62290. Jorge A. Ascencio3, and Lorenzo Martínez2,3 Instituto de Ciencias Físicas, Universidad Nacional Autonoma de Mexico, Ave Universidad 1001, Col. Chamilpa, Cuernavaca, Morelos. CP 62210. *
Also at Corrosion y Proteccion Ingenieria SC
ABSTRACT. Numerical modeling is a valuable tool for the diagnosis and the development of solution scenarios for the CP systems of complex pipeline networks. It is also very valuable to perform comparative and validation analysis once the engineering of the retrofitting based on the numerical modeling has been completed. We report CP modeling employing Finite Boundary Element numerical techniques that were applied during the retrofitting of poorly performing CP systems in locations where the electrolyte resistivity varies significantly as is the case of shore pipelines involving soil and seawater environments in the coastal regions of Mazatlan and Rosarito in the Pacific Coasts of Mexico. The case of the gas and fuel oil pipeline network at Rosarito, due to the very special social environment around the pipeline ROW, allowed very few options for the location of new CP facilities. Therefore modeling and anticipating the results of the CP performance were crucial. We report the trial studies developed to reach a solution that, when the retrofitting was completed, substantially improved the CP performance and ensured a very good pipe to soil potential distribution coverage along the pipeline network considering the segments immersed both in seawater and in soil. The case of the pipeline network of Mazatlan, also for fuel oil and gas was successfully analyzed and solved employing the Finite Boundary Element methods. The calculations involved the solution to a CP interference problem in a segment of the pipeline, as well as the support for the retrofitting engineering that allowed passing from a very limited CP coverage to a pipe to soil potential distribution in compliance with the standard codes.
1. INTRODUCTION.During the last years, the use of numerical modeling by Computers has increased its power and consequently the impact to different fields has been increasing in a equivalent proportion to the new computers capabilities. Particularly the us of numerical modeling in corrosion and pipelines industries has been vastly applied because the opportunity that it gives to understand the right of way (ROW) behavior and also to understand the cathodic protection behavior with its multiple variables. A specific opportunity segment is the use of this kind of virtual laboratory in the study and prediction of the behavior of ROWs that show a complicated access, or when there are not simple mechanisms to monitor enough elements. Lets remember that the tremendous capabilities of the numerical modeling can be observed in the three different stages: • •
•
To evaluate the behavior of structures that already exist and just a few punctual data can be obtained; this allows understanding the possible complication, specific risk points or critical operation condition. During the building and setup of structures, which allows improving the capabilities of the system, besides to reduce the resources because the perspective that the modeling can offer to evaluate alternatives in case of simple or grave complications during the construction or the setup of the installations. Before the building to evaluate the different variables for getting the optimal design, testing the different conditions that can be achieved for different configurations, conditions of the media and even the understanding of future possible complications to prevent the mechanisms to solve possible events.
In this way and considering the conditions of the PEMEX pipelines facilities at Mazatlan, Sinaloa and Rosarito, Baja California, in the Pacific Coasts of Mexico, we show in this work the mechanisms to identify solutions for apparently complicated problems; as the case of a mixed pipeline, which is submerged in seawater and part of it is buried in the near coast, with the complexity of evaluating the potentials in the submerged pipe segment as it is in Rosarito installations. Or the presence of interferences in structures that are sharing ROW, as it happened in Mazatlan. In this work we are focused to show the way that we used the numerical modeling to determine the solutions to the problems, increasing details in the case of Rosarito because the multiple variables that this required and exemplifying the way that we determined the problem and the proposed solution for Mazatlan. We take advantage of the big capacity of the boundary methods to consider the different variables that affect the cathodic protection systems as the current quantity, path and diameter of the pipelines, electrolyte resistivity, coating quality, electrical isolation elements, electric train lines, among others. The two different sites that are studied in this paper correspond to equivalent environmental conditions, even that the problems to attack are plenty different. Both are localized in the North region of the pacific coast of Mexico, both places are marked in figure 1a with details about their installations for Rosarito and Mazatlan PEMEX installations in figures 1b and 1c respectively.
Rosarito
Mazatlan
Figure 1. Satellite image that allows identifying the localization of Rosarito, Baja California (Marked with the red frame) and Mazatlan, Sinaloa (yellow frame) on the pacific coas of Mexico.
It is important to notice the presence of the urban zone in the considerations of risk, so the evaluation and particularly the necessity of reducing possible problems of corrosion and eventual accidents are crucial; in this way, PEMEX searched for the most advanced technologies to be have this region protected to reduce risk during the pipelines operation, Let study and show each case separately to understand its implications and variables for the numerical modeling application.
2. ROSARITO, BAJA CALIFORNIA Site characterization and diagnostic The facilities of this terminal are property of PEMEX refinacion and operated by the residence of Rosarito. Through Coast Marine facilities outside, this terminal caters PEMEX Magna, PEMEX Premium, PEMEX diesel, turbosine and heavy fuel oil to the State of Baja California; this received tank-ships from the Salina Cruz Maritime Terminal with products of Antonio Dovalí Jaime de Salina Cruz or importations from United States. In addition to products by PEMEX Refinery, the terminal download national and imported liquefied gas and imported propane, which is delivered to the refrigerate terminal of Rosarito property by PEMEX Gas and Basic Petrochemical. As it is illustrated in figure 2a, the maritime facilities outside coast have three moorings; the first one is a conventional bollard with 5 buoys and allows the operation of ships up to 45,000 tons of death weight. Its structure is of 2 ¾’’ diameter chains and an anchor of 8 tons en each halter. The type of superstructure is with cylindrical buoys of horizontal type and it works with 12’’ diameter waterspouts. The flux direction is from ship to land. The second bollard is conventional of 5 buoys and allows the operation of a shiptank up to 30,000 tons of death weight, with a similar superstructure type, and water spouts of 10’’ diameter, while the flux direction is in both directions from ship to land and land to ship. The third ones is a single buoy type “CALM” and it allows the operation of ships up to 60,000 tons of death weight. Its superstructure is four halters of 3 ½’’ diameter grade “U”, three with two anchors of 15 tons each. The type of superstructure is a single-buoy with diameter of 11 meters. It works with two
waterspouts of 16’’ and reductions to 12’’ of diameter. The flux direction is only the ship to land.
a
Monoboya
b
SDT Conventional bollard
Single buoy
Figure 2. a) Scheme of the PEMEX pipelines and facilities at Rosarito and b) a pie plot of the measured potentials that achieve the -850mV criterion.
The Quay 1 is connected to the storage terminal by means of two pipelines, the first is an universal pipeline of 16 " diameter and 150 meters of length and the second is a fuel oil pipeline of 20-inch diameter and 150 meters of length. The single buoy connects to the terminal for two universal pipelines of 24-inch diameter and 3.5 kilometers in length.The transport of distilled products in the port of Rosarito, Baja California, is done with tank-ship pumping to the Residence of Port Operations in Rosarito and from there to the storage and distribution terminal (SDT) of Rosarito, with this is provided the fuel supply to the zone. Preliminary revision of the cathodic protection systems revealed that exist the necessity of reinforce them to achieve adequate potentials in the whole ROW. When the project initiate the 75% of the potentials were found more electropositive that -850mV, as it can be observed in the figure 2b. For the understanding of the origin of the low potentials, a deep analysis of the cathodic protection systems was realized. A couple of rectifiers were operating and one is out of operation because problems with the power supply of one phase. In the three cases of the anodic beds, they showed values of circuit resistance from 17 to 158 ohms, which were associated to cabling failures in the rectifier output, besides the thieving or vandalism. In a similar way, two or the three anodic beds had exceeded its useful life and they required to be substituted. Modeling. The strategy to make a model to understand the behavior involved to include in it the next parameters; geometry, bed configuration and electric bridges (obtained by trace method); specifications of diameter, steel type, anode bed type, anodes type, coatings, isolation against alien structures or facilities; measurement of electrolyte resistivity and coating quality. The model is also improved by using iterative procedure up to get pipe/soil potentials similar to the evaluated in the site.
To have a reliable model, demand of current tests were implemented with help of temporal systems. Once polarized the pipeline system, it was carried out a survey of potentials to all along the ROW, including the marine area, where we account with help of specialized divers and equipment for marine potentials monitoring. With the information obtained in field, the model was calibrated about specific conductance of each one of the pipes in the different segments up to match experimental and calculated values. It was then concluded that we had a model that behaves with good approximation to reality as it is shown in figure 3a, where red sections involve high corrosion risks because they have not enough polarization potential.
a
b
c
d
km
Figure 3. Numerical modeling analysis; a) Original model for the submerged pipeline section with problems in a long section, b) potential gradient for the whole pipelines network showing clearly the sites where the anodes are, c) the evaluation of the effect over the submerged segment with the deep anode bed setup and d) the plot for the optimal operation conditions of the improved cathodic protection system.
Different designs were tested up to find an optimal configuration. The results derived into the installation of two anodic superficial beds in both arrival sites to the SDT and a deep anode bed in the beach foot, which allow reaching more electronegative polarization potentials in the totality of the pipelines length fulfilling the -850 mV even in the region between the SDT and the single-buoy as it is shown in figure 3b and 3c. In fact, the fast attenuation that the potential displays especially in the marine zone implied that the necessity of the installation of a deep anode bed was imperative; this is because this kind of anode bed allows a better remote influence and the security of keeping protected the pipeline in the seawater as can be observed from the plot of figure 3d. The use of numerical modeling also allows the understanding of the effect of the impressed current and consequently the cathodic protection system is performed with the enough support to be implemented. In figure 4, which illustrate the mechanisms to
select the optimal characteristics and operation conditions for the deep anode bed and the rectifier of the SDT. It is clear from figure 4a that when the current of the deep anode bed is increased, the pipeline that goes to the single-buoy is impacted, while the manipulation each of them (anode bed or rectifier) of together derive into the configuration of potentials in the intermediate region (figure 4a and 4b).
a
b
Figure 4. Analysis of the influence to the polarization potential along the pipelines by the impressed current amount of a) the deep anode bed and b) the rectifier of the SDT.
The potential improvement and the consequent protection to the pipeline network was confirmed by evaluating potentials in the submerged region of the pipeline and in general along the pipelines, getting the values that imply a full cover of the cathodic protection system as it can be reviewed from the table 1.
m
Before ON
OFF
0 380 600 800 1000 1800 2000 2400
-0.686 -0.742 -0.617 -0.204 -0.63 -0.526 -0.518 -0.898
-0.686 -0.742 -0.617 -0.204 -0.63 -0.526 -0.517 -0.88
M
After ON
OFF
0 579 603 766 887 1702 1881 2704
-1.215 -1.08 -1.01 -1.017 -1.087 -1.084 -1.326 -1.247
-1.108 -0.916 -0.876 -0.897 -0.985 -0.982 -1.15 -1.148
CRITERIO -0.85 -0.85 -0.85 -0.85 -0.85 -0.85 -0.85 -0.85 -0.85
Table 1. Polarization potentials for evaluations before and after the cathodic protection enhancement.
The specific impact of the numerical modeling to the cathodic protection improvement is clear in this case, because the calculation of the different parameters allowed determining the kind of anodes to use and the requirement of impressed current to keep protected the whole metallic structure.
3. MAZATLAN, SINALOA Site characterization and diagnostic In Mazatlan, the ROW has several pipelines that transport distilled products, fuel oil and COPE; a pipeline for fuel oil was built from the seawater to the CFE (federal commission of electricity –that is the responsible of the power production and distribution in Mexico -) power plant (CFE); and a set of the other three pipelines, which transport the hydrocarbons from the maritime terminal (MT) to a storage and distribution terminal (SDT) in the North of Mazatlan as is shown in the scheme of the figure 5a. This terminal is quite important for the region because it deliver the hydrocarbons to the region of Sinaloa State and part of the Nayarit state too. In the region of the selected area illustrated in figure 5b, the presence of multiple corrosion evidences after the bifurcation of the ROW, particularly in the pipelines segments (marked with red on the figure). This singularity was determined by using the CIS/DCVG evaluation over the pipelines, one of the plots are include in the figure 5c, where it can be found a couple of important zones with more electropositive values (around 4850 and 5045 meters) that match with the red marked segments that are localized near to the anode bed, so the study was focused to understand the behavior that is inducing it, with a first identified proposal associated to a interference mechanisms on the zone.
a
SDT
b Interference zone
Anode bed DF
CFE
Figure 5. a) Scheme of the PEMEX pipelines and facilities at Mazatlan and b) a selected area of the region where the corrosion problems were identified as induced by an interference behavior.
The evaluation of the ohmic drop allowed to identify the contribution of the rectifiers to the pipelines that showed problems of corrosion risks, and in figure 6 the evaluation for all the involved rectifiers is displayed. In the region of the 6.5 kilometers and withc a length of almost one kilometer is clearly identified the change of electronegativity, inducing that the pipeline acts as anode, not as cathode as it was expected.
OHMIC DROP (v)
Dock Rectifier
P. Suarez Rectifier
Bonfil Rectifier
Pozo 4 Rectifier
Electric Interference on Magna, Premium and Diesel Pipelines
LONGITUDE (m) Figure 6. Rectifiers influence to the beach pipelines denoting the electric interference effect.
The understanding of this process was sustained in the field work and also in the numerical modeling approaches. Then, the numerical modeling methods were focused to identify the conditions and parameters that contribute to the interference behavior.
The use of this methodology allowed to establish the requirement of a couple of electrical joints by making multiple configurations and to select the optimal for a better distribution of the current and the polarization potentials to achieve the -850 mV criterion. In figure 7a, the change on the electronegativity was understood as a problem of electrical connections, even more, the potentials gradient distribution allowed establishing that the interference behavior is induced by the anode bed that was considered to be protecting the pipeline that is connected to the CFE thermoelectric power plant.
a
B
Figure 7. Numerical modeling analysis of the pipelines network, where a) the polarization potential of the pipelines and b) the potential gradients in the electrolyte are displayed.
The use of these studies allowed identifying the zone where the electrical joints must be implemented and the results of this procedure were directly obtained with the plenty cathodic protection of the pipelines network.
CONCLUSIONS. The use of numerical modeling opened the perspective to improve the design of the cathodic protection systems in two different pipelines configurations and with varied problems. In Rosarito the impact was in the determination of the type of deep anode bed that allows protecting the submerged pipelines even in the remoteness zones. The numerical analysis also gave information about the amount of current that the system needs to have the optimal operation. The use of this methodology was also focused to solve the interference problem identified in Mazatlan, which involved the contribution of current impressed in two different pipelines, inducing the behavior of segments of the metallic structure as anodes instead cathodes, solving the problem with help of the electrical circuit correction. Both cases illustrated the plausible contribution of the numerical modeling methodologies in the understanding of problems in cathodic protection systems, the
identification of parameters or elements that allow the improvement of an existing system or the design of a new one, based on pre-evaluated data.
REFERENCES • •
W.S. Hall, G. Oliveto, Boundary Element Methods for Soil-Structure Interaction, Kluwer Academic Publishers, 2003, The Netherlands. Gernot Beer, Ian Smith, Christian Duenser, The boundary element method with programming, SpringerWienNewYork, 2008.
•
Leslie Bortels, “The use of Dedicated Simulation Software for the Design and Understanding of the Cathodic Protection of Underground Pipeline Networks under various Interference Conditions” Corrosion 2003, paper no 03202.
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ELSYCA SoftWare Documentation (User manual).