Manzanillo paper 04 oct 2009

NUMERICAL MODELING OF CATHODIC PROTECTION SYSTEM LOOKING FOR PRESENT CONDITION EVALUATION AND IMPROVMENT OF PIPELINE NETWORK AT MANZANILLO, MEXICO. Arturo Godoy1, Roberto Ramírez1, Leonardo De Silva Muñoz1, Lorenzo M. Martinez-dela-Escalera1, Hernan Rivera1, Jorge Canto1, Corrosion y Proteccion Ingeneria, S.C. Rio Nazas 6. Cuernavaca, Morelos. Mexico.

62290.

Carlos Lopez Andrade,2 Cecil Knight,2 PEMEX Refinacion. Torre Ejecutiva, Marina Nacional 329, Col. Huasteca, México, Distrito Federal, C.P. 11311.

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. The capability to design, apply and even to evaluate the optimal operation of cathodic protection Systems depends of the consideration of the fundamental variables, and having alternative tools as data modeling, the optimization perspective becomes real as it has been demonstrated in multiple reports. Under consequence of the three different stages that are impacted bye the modeling: a) After work, which help in the understanding of the effects of each variable in an installation or structure; b) At the same time. Together to the implementation process information and calculation that allow the materials optimization, geometries and economical resources; c) Before the work, which is presented in groups with experience and it allows the design of installations and the own building In case of cathodic protection modeling, it is expected to determine the actual conditions and to discard those that does not affect to the CP operation. Numerical modeling of CP Systems has been used to study the current and pipe/soil potential behavior. Based on the boundary element method, with discrete approaches, the CatPro software is applied (developed by the Vrije Universiteit Brussel and the von Karman Institute for Fluid Dynamics. Building models from the identified geometries in field and the pipeline conditions, a effective modeling methodology is established, where considerations as the kind of soil, electric resistivity, pipeline geometry, type and position of the involved anodes, metal characteristics, electric interconnections, type and quality of coatings, isolator joints and any element that is involved in the operation conditions of the CP systems.

This work is focused on the study of a pipeline network at the city of Manzanillo, found in the Pacific Coast of Mexico, in the Colima State. We used modeling methods for the study of pipelines located inside the city, which transport hydrocarbon products from the maritime terminal to a thermoelectric central (south) and to a storage tank farm (north). This pipeline network has actually an ICCP system, which is not enough for covering the corrosion protection expectative; the use of several tools for helping to make decisions that reduce costs of system rehabilitation.

The validation of the initial model is made against pipe/soil potential data obtained from the CIS, which results in retro-alimentation data about the quality and size of defects in the used coating for the models, obtaining a model that represents the CP system actual conditions. After having a validated model, it is obtained a new model with modifications to the system that allow an optimal redesign, increasing the information and the parameters to make sustained decisions at the minimal cost. Once the rehabilitation of the CP system, based on the modeled results, the new pipe/soil potential values are compared with the predicted by the model, and the conjunction of experimental and theoretical data gives information of substantial elements for the pipeline network maintenance and generates a perspective of continual parameters evaluation in critical sites predictable from the modeling procedure.

1. INTRODUCTION Cathodic protection systems (CPS) are vital components for the maintenance of hydrocarbon pipelines. Improper designs of CPS can lead to high operating costs, poor corrosion protection or overdimensioned and costly system components. In general, CPS design relies mostly on experience, experimental data and heuristics; nevertheless the use of simulation software has become a valuable tool for CPS design that allows testing different configurations quickly and at a low cost before actual installation of the system. Variables like electrical current, rectifiers’ voltage, and anode bed size and localization can be modified in the simulation environment to determine the optimal system configuration in terms of cost and corrosion protection capacity. CPS numerical modeling is also useful during and after the installation of a cathodic protection system by helping engineers to make decisions in order to optimize the use of material and economical resources, and to understand the behavior of the system and the effects of the different variables that define it. Finally, in the case of defective or inefficient cathodic protection systems that need some refurbishing or a redesign, simulation software can help understand the limitations of the actual system, and thus assist to determine the best course of action.

This work presents the redesign of the cathodic protection system of a pipeline network owned by PEMEX in the city of Manzanillo, found in the Pacific Coast of Mexico, which is used to transport hydrocarbons from a maritime terminal to a thermoelectric power central, located at the south zone of the city, and to a storage and distribution terminal (SDT) in a tank farm configuration at the Northeast of the city. The site was characterized using sub-metric precision localization equipment and DCVG, CIS, and pipe/soil potential gradient measurements. Numerical modeling was used to study the current and pipe/soil potential behavior, and to redesign the cathodic protection system

of the pipeline network. Once the cathodic protection system was refurbished, new pipe/soil potentials were taken and compared with the model.

2. SITE CHARACTERIZATION AND DIAGNOSTIC. Manzanillo is an industrial and commercial port city, which is localized in the pacific coast of Mexico as it can be observed in the satellite image of figure 1a. The port regularly receives 26 shipping lines from 74 destinations in the World. It serves 15 states within the Mexican Republic whose economic activities represent 64% of the Mexico’s GDP. The port has a terminal that receives hydrocarbons for the coastal region of the Colima state and South of the Jalisco state (with around 1.2 million of habitants) and for the thermoelectric power of Manzanillo. The power plant, constituted by two turbo gas units, has an installed capacity of 1900 MW, which represents about 4% of the country’s total power production capability.

The pipeline network that connects the maritime terminal to the thermoelectric plant and the tank farm has some segments that are exposed to the atmosphere, which is an aggressive medium due to the marine environment. About 60% of the network lies inside the urban zone of Manzanillo, which imposes a risk of damage to urban infrastructure, population health hazards, and life loss if the pipelines are not properly maintained. The area is also sensitive to ecological damage due to the proximity to the coast and to the fact that a 2 km segment goes through the Tapeixtles lagoon.

The precise horizontal and vertical localization of the pipelines was determined using state of the art magnetometric detection equipment and a sub-metric precision Differential Global Positioning System (DGPS) that allowed identifying the scheme of figure 1b and 1c. Direct Current Voltage Gradient (DCVG) and Close Interval Survey (CIS) measurements were carried out on “on” and “off” modes of the cathodic protection system. Coating defects found with the DCVG equipment were thoroughly characterized by measuring pipe/remote soil potentials and voltage gradients around the points of interest. Rectifier inspections, soil electrical properties measurements, and electrical isolation tests between pipelines and surface installations were also performed.

The network has two main pipelines. The first is a 24” universal fuel pipeline that feeds gasoline (magna and premium) and diesel to the tank farm, which is an important distribution node of fuel for the region. The second is a 24” fuel oil pipeline that goes from the maritime terminal to an interconnection station known as “El Tajo” where it is divided into a 20” pipeline that is used to feed the thermoelectric central, and a 16” pipeline that connects “El Tajo” to the tank farm. The 16” pipeline is used to transport fuel oil coming from the maritime terminal to the tank farm, and to transport the stored fuel oil from the tank farm to the power plant. The network also has three pipeline segments that were used to transport fuel oil but are no longer in operation (figure 1c).

Figure 1. Generalities of Manzanillo. a) Satellite image to identify the localization of the city, b) aerial view of the pipeline network of Manzanillo, c) scheme of the pipelines present at the site.

The pipe/soil potentials obtained on “off� mode from the CIS showed that there were regions with potentials below the -850 mV criteria for the soil/pipe polarization potential with a Cu/CuSO4 reference cell, which indicated a high corrosion risk in such regions. In contrast, it was found that other parts of the pipeline network were overprotected, generating unnecessary operation costs. The survey also showed that there were electrical connections between adjacent pipes, which are not recommended for the proper function of the CPS. The inspection of the two rectifiers did not show any problems, nevertheless, the anode beds, consisting in 25 graphite anodes installed parallel to the pipelines and near the surface, were performing poorly. Few anodes were operating effectively and most of them were worn out; which resulted in an inadequate current distribution along the pipelines. In addition, it was found that the anode beds were too close to the pipelines, limiting their effective protection range.

The DCVG measurements detected 53 coating defects on the fuel oil pipeline going from the maritime terminal to the power plant, 25 defects on the universal fuel pipeline going from the maritime terminal to the tank farm, and 49 defects on the 16” fuel oil pipeline going from “El Tajo” to the tank farm. Of the detected coating defects, just 9 were critical and needed immediate attention by either modifying the CPS or by repairing the coating defects, 29 and 11 defects needed repair at the medium and long term respectively.

Figure 2 Operational conditions of the CP system before the it’s redesign, Pipe/Soil Off polarization potentials for a) the Maritime Terminal to the Thermoelectric plant pipelines, b) the Maritime Terminal to the Tank farm pipelines, and c) a summary of general observations derived from the field survey.

3. NUMERICAL MODELING In order to find an optimal configuration for the CPS, CatPro simulation software (developed by the Vrije Universiteit Brussel and the von Karman Institute for Fluid Dynamics) was used. Based on the boundary element method, the software is specifically designed for cathodic protection system modeling.

Before testing different configurations for the CPS, it is necessary to introduce a series of parameters that describe the system, and to validate the model by comparing its results with the field data. The parameters introduced into the model needed to obtain the best numerical approach are listed in table 2.

Required parameters for a numerical model • • • • • • • • •

Pipeline geometry (length and diameter) Pipeline depth Localization of Electrical bonds Localization of Isolation joints Type and conditions of mechanical coating Electrical connections with other structures Type and number of anodes Localization and orientation of the anode beds Environmental conditions (specially soil resistivity)

Table 1. Required parameters for a numerical model The use of the numerical modeling software allowed determining a theoretical behavior of the system, shown in figure 2a, which has a similar tendency than the experimental measured profile. This way we confirmed that the model reproduced the conditions before the rehabilitation, meaning that any modification to the model could be also reproduced in the new CPS design. Once the model was validated, different modifications to the CPS system were tested until an optimal configuration was found.

The information obtained from the model gave the CPS design team enough elements to redesign the system optimizing CPS rehabilitation and operation costs.

The proposed CPS modifications were basically three: eliminate electrical connections between pipelines; improve the shunt box using individual connections to the anode bed, which increased the current alimentation capabilities of the system; and install deep anode beds to enhance the protection of the most remote pipeline sections. Once the CPS was rehabilitated, new pipe/soil potentials were measured and compared with those predicted by the model. The results are presented in figure 3, where a good correlation between the model and the real system behavior can be observed. Both experimental and theoretical pipe/soil potentials were between -0.850 and -1.2 V vs. Cu/CuSO4, showing that the new CPS was working properly without neither unprotected or overprotected pipeline sections.

Besides the benefits of allowing fast and low cost CPS design, a model of the system coupled with experimental data, gives valuable information for the pipeline network maintenance, like determining critical coating defects, and allows to predict the behavior of the system under different hypothesized situations like changes in the soil resistivity due to flooding or drought.

a

Redesigned CP system Numerical modeling of redesigned CP system

b

Redesigned CP system Numerical modeling of redesigned CP system

Figure 3. Calculated “off� polarization potentials for pipelines from marine terminal a) to the thermoelectric power plant and b) to the storage and distribution terminal.

The calculated potential distribution gradient plots also showed a better protection for remote pipeline sections (figure 5). Particularly the effect that was achieved with the modification of the anodes type, was the remoteness as it can be observed by a simple comparison between both configurations the corresponding to the conditions before and after the CP rehabilitation (Figs. 5a and 5b respectively).

a

b

Figure 5. Potential distribution before and after the cathodic protection system rehabilitation.

4. CATHODIC PROTECTION SYSTEM REHABILITATION The reengineering of the cathodic protection system included several tasks among, which stand out the following: geostratigraphic studies that delivered resistivity profiles of the soil on the proposed sites for deep anode beds installation; perforation of wells for the anode beds installation; installation of the anode beds with each anode connected individually to a shunt box; and installation of a reference cell near the pipelines for each rectifier. The procedures for the implementation of the corrective actions are shown in figure 6, in order to illustrate the specific tasks.

During the operation of the new CPS, the electrical circuit resistances were found to be lower than those measured on the previous system, which is a good indicator of the better performance of the new system. The electrical resistance of the “Alameda” rectifier was reduced from 0.7 to 0.4 Ohms while the resistance of the “Tapeixtles” rectifier was reduced form 1.3 to 1.1 Ohms. Another positive characteristic of the new system was that the total current drained was 50% lower than the original value.

Figure 6. Images of the correction actions of the cathodic protection system; a) deep well perforation, b) deep well bed, c) Shunt box with individual connections to the MMO anode, d) Rectifiers adjustment

CONCLUSIONS

Befor

A cathodic protection system of a hydrocarbon pipeline network in the port of Manzanillo, México was characterized, modeled and rehabilitated.

The CPS before rehabilitation did not protect the network properly. Some pipeline sections had a pipe/soil potential below recommended limits while others were overprotected generating unnecessary operation costs. The anode beds were deteriorated and performed poorly. Several coating defects were detected.

Numerical modeling of the cathodic protection system was used to determine the optimal configuration of the system that represented the lowest installation and operation costs, and an adequate protection capacity. Modeling allowed testing several configurations in fast and at a low cost before the implementation of any modification to the system.

The rehabilitation of the CPS consisted on the elimination of electrical connections between pipelines, improvement of the shunt box using individual connections to the anode bed, and installation of deep anode beds.

The rehabilitated CPS performed as predicted by the simulation software, adequately protecting the pipeline network without the need to perform pipeline coating repair works.

REFERENCES • •

Port Authority of Manzanillo, http://www.puertomanzanillo.com.mx/php/eng/ W.S. Hall, G. Oliveto, Boundary Element Methods for Soil-Structure Interaction, Kluwer Academic Publishers, 2003, The Netherlands.

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Gernot Beer, Ian Smith, Christian Duenser, The boundary element method with programming, SpringerWienNewYork, 2008.

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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).