Application of Computational Modeling to Predict the Effectiveness of CP on a PCCP Transmission Pipeline

Transcription

1 Application of Computational Modeling to Predict the Effectiveness of CP on a PCCP Transmission Pipeline Robert A. Adey, Andres Peratta, and John M W Baynham CM BEASY Ltd Ashurst Lodge Southampton, Hampshire, SO40 7AA UK ABSTRACT Prestressed Concrete Cylinder Pipe (PCCP) is a rigid pipe designed to take optimum advantage of the tensile strength of steel and of the compressive strength and corrosion inhibiting properties of concrete and is frequently used for water transmission. PCCP consists of a steel cylinder embedded in a concrete core, which is helically wrapped with high-strength, hard-drawn wire after curing. The wire is embedded in thick cement slurry and coated with a dense cement mortar. While the cement mortar and additional coatings usually protect the prestressing wires from corrosion, in certain circumstances chlorides can diffuse into the mortar and reach the wires. Therefore PCCP transmission pipelines can also be protected by CP systems to mitigate the risk of corrosion damage when chlorides are high in the soil. This paper describes a computer modeling study which was designed to determine the protection provided by a CP system, to evaluate different design options, and to optimize the design. Results will be presented showing the model predictions for the different cases considered. Keywords: cathodic protection, PCCP, computer modeling. INTRODUCTION Prestressed Concrete Cylinder Pipe (PCCP) is a rigid pipe designed to take optimum advantage of the tensile strength of steel and of the compressive strength and corrosion inhibiting properties of concrete and is frequently used for water transmission. PCCP consists of a steel cylinder embedded in a concrete core, which is helically wrapped with high-strength, hard-drawn wire after curing. The wire is embedded in thick cement slurry and coated with a dense cement mortar. While the cement mortar and additional coatings usually protect the prestressing wire from corrosion, in certain circumstances chlorides can diffuse into the mortar and reach the wire and can cause chlorideinduced corrosion of the prestressing wire. Therefore PCCP transmission pipelines can also be

2 protected by cathodic protection (CP) systems to mitigate the risk of corrosion damage to the wire in high chloride environments. The CP engineer has a number of options in the choice of the CP system to protect the pipeline ranging from coated or uncoated pipe, sacrificial anode ground beds, and Impressed Current Cathodic Protection (ICCP) systems. Within these overall choices there are also many design options for sacrificial systems including the type, size and distribution of anode ground beds. For ICCP systems the anodes can be located at discrete locations or ribbon type anodes buried near the pipe. In addition the power supplies and the cabling introduce more choices and impact the performance of the protection system. For PCCP pipelines extending over hundreds of km it is important to identify the most cost effective system which must also be robust and provide the integrity needed over the pipeline life. In this paper computer modeling is used to aid the CP engineer in identifying the optimum system and to test its robustness and the protection provided under the different environmental conditions experienced along the pipeline length. CP MODELING TOOL Until the development of reliable computer modeling techniques the prediction of the detailed protection levels provided by CP systems was extremely difficult and relied on the skill and experience of the corrosion engineer and extensive post commissioning surveys. The condition of coatings on the pipeline surface changes over its lifetime due to the action of third parties and deterioration of the coating itself etc. Although increased current demand from the CP system can indicate the presence of damage, the location and extent is unknown as well as the local level of protection provided. The development of modeling techniques has provided the necessary tools to predict the protection levels provided by a CP system as part of an up front design process as well as providing tools to interpret data from surveys and monitoring. In this work the BEASY Corrosion and CP software (1) is used to predict levels of CP protection on buried PCCP water transmission pipelines 8,9,10. The main objective of the modeling tool is to determine the distribution of electric potential, electric field, and current in the electrolyte, the anodes performance and the protection provided to the pipeline. This provides the CP engineer with information necessary to assess the level of protection of the pipeline life against corrosion, to provide a better interpretation of the data collected by the remote monitoring system from the field, and to predict possible vulnerabilities in the coating, and determine the existence of foreign structures which may interfere with the CP system protection. Conceptual Model and Modeling Approach The conceptual model consists of the pipeline network immersed in a layered heterogeneous partially or totally saturated soil, considered as an electrolyte. The conductivity may vary spatially or over time due to changes in the groundwater level etc. The CP system consists of a series of anodes, either sacrificial or with impressed currents, installed in the vicinity of the pipeline. The anodes are electrically interconnected by means of discrete electrical elements such as rectifiers, diodes and/or resistors between them and/or between a number of connection nodes in the pipeline. (1) C M BEASY LTD

3 The model is considered as a multi-domain problem within the context of the Boundary Element Method (BEM) 1. It is basically composed of at least two regions: exterior and interior. The exterior region consists of the electrolyte (heterogeneous layered soil), while the interior consists of the highly conductive metallic pipelines involved in the calculation. The coupling between the two problems is done by imposing the corresponding continuity boundary conditions in the common interface. These matching conditions are provided by the polarization curve, which describes the voltage drop (over-voltage) between metal and electrolyte in terms of the normal component of the current density flowing throughout the common interface. Solution Approach The Boundary Element Method (BEM) has been widely used to simulate cathodic protection systems for underground and offshore structures. 2,3,4 The governing equations solved describe the IR drop through the electrolyte and the electrode kinetics on the metallic surfaces. The most significant advantages of the method are first that the formulation is based on the fundamental solution of the leading partial differential operator in the governing equation, and second that it requires only mesh discretization on the boundaries of the problem in the form of triangles or quadrilaterals. The former aspect confers high accuracy, while the latter substantially simplifies the pre-processing stage of the model, since volume discretization is not needed. PCCP PIPELINE As shown in Figure 1, Prestressed Concrete Cylinder Pipe (PCCP) consists of a steel cylinder embedded in a concrete core, which is helically wrapped with high-strength, hard-drawn wire after curing. The wire is embedded in a thick cement slurry and coated with a dense cement mortar. Figure 1: PCCP construction details.

4 In order to model the performance of the protection system, the following main elements of the system must be modelled: The geometry of the pipe and its internal structure, the surrounding electrolyte/soil and the anodes and other electrical connections The cement mortar, the concrete and any coating applied to the pipe The polarization properties of the steel wire embedded in the mortar and of the steel pipe liner The surrounding soil resistivity The characteristics of the anodes such as the current for ICCP anodes and polarization data for sacrificial anodes The electrical resistance of the pipe and all the electrical connections between the anodes and the pipe The importance of each of the above will depend to a certain extent upon the choice of the type of CP system but all were included in the model to enable all design options to be considered. STRATEGY FOR CP MODELING OF A PCCP PIPELINE While it is possible to estimate the resistance of the layers of mortar and coating the most accurate approach is to build up an equivalent polarization model for the pipe which represents the internal IR drops and the steel polarization. Therefore a local model was created including the different elements of the pipe structure and its reinforcement. With this model the current flow through the mortar and concrete can be determined as well as the potentials on the surfaces of the reinforcing wires and on the surface of the steel pipe liner for any set of conditions. This data can then be used in large-scale models of the PCCP and the CP system and its environment. This type of model can also be subsequently used to identify conditions on the surfaces of the reinforcing wires and on the surface of the steel pipe liner for the conditions identified in the large scale model. The modeling strategy is as follows: Step 1 Create a local model of the PCCP structure, including details of: The metal liner and surrounding concrete (on outside of the liner) The reinforcement wires The mortar Any coating on the surface of the mortar Use the local model to identify the IR drop across the various coatings and mortar from the wire or steel liner and calculate the potential and current density on the outer surface of the PCCP. The polarization properties of all the metallic elements inside the pipe are considered in the model.

5 Step 2 Use the results of the local model to construct a curve relating the potential on the outer surface of the PCCP to the current density through the outer surface of the PCCP. In the following sections, this is referred to as an equivalent polarization curve Step 3 Perform a large scale simulation of the polarization behavior of the outer surface of the PCCP under CP applying the equivalent polarization curve to the outer surface. This model provides the potentials on the outer surfaces of the PCCP. Step 4 Using relationships derived from the detailed model the potentials obtained from the large scale model can be used to determine the potentials on the surfaces of the reinforcement wires and on the surface of the steel liner. Step 1 and 2 - The Local Model The local model aims to capture all the features of the internal structure of the pipe as shown in Figure 2. As the pipe is essentially axisymmetric a two dimensional model can be used to simulate the behavior. Figure 3 shows the two dimensional BEM model used to simulate the internal structure of the pipe. The key features are the electrical resistivity of the mortar, and the polarization properties of the metallic wires and liner. Data is available from a number of sources 5,6 but in this study the data in reference 5 was chosen as the most appropriate as this provided data for steel in new concrete and for steel in chloride contaminated concrete as shown in Figure 4. Reinforcing Spacing ~ wires 2 to 5 mm Backfill around pipe Coating Thickness ~ 1 mm 20 mm 180 mm Mortar Concrete Steel liner Axis of the pipe (not to scale) Concrete Figure 2: Cross section of PCCP showing the different features included in the model. The model was used to create an equivalent polarization curve for the PCCP by predicting the response of the pipe to a series of values of potential specified on the external surface of the pipe. In this way a series of pairs can be determined for the: Potential at outer surface of the PCCP Current density through the outer surface of the PCCP

6 Such data can be displayed graphically, in the form of an equivalent polarization curve. COATING MORTAR WIRES SOIL Geometry of the 2D model, taking advantage of symmetry SURFACE OF STEEL LINER Figure 3: The two dimensional model used to simulate the internal behavior of the pipe Voltage (mv SCE) Chloride contaminated Uncontaminated current density (ma/m**2) Figure 4: Polarization data for steel in reinforced concrete from L. Bertolini et al. 5 This data provides the relationship between the current density on the steel surface and the potential. The results from this model provide a relationship between the potential outside the pipe to that on the steel wire and on the steel liner. In the model the liner and wire are assumed to be electrically contiguous A series of parameter studies were performed, using different scenarios, as follows:

7 Various values of concrete and mortar resistivity (500 Ohm m and 100 Ohm m) Various polarization curves for the steel liner and steel wires. Without coating, and with coating of resistivity 5000 Ohm m The scenarios considered are shown in Table 1. Table 1 Summary of the Scenarios Considered Using the Local Model to Determine the Equivalent Polarization Properties of the PCCP Scenario Concrete Resistivity External Coating Steel Polarization 1 Low No Passive 2 Low No Corroding 3 Low 5000 Ohm m Passive 4 Low 5000 Ohm m Corroding 5 High No Passive 6 High No Corroding 7 High 5000 Ohm m Passive 8 High 5000 Ohm m Corroding Figure 5 and Figure 6 show the equivalent polarization curve for the PCCP for the various scenarios considered. This data provides all the information needed to simulate the performance of various corrosion control options using the large scale model. No Coating coated LOW RESISTIVITY No Coating Coated HIGH RESISTIVITY Figure 5: Equivalent polarization curve, showing the relationship between potential and current density on the outer surface of the PCCP, CHLORIDE CONTAMINATED.

8 No Coating Coated No Coating LOW RESISTIVITY Coated HIGH RESISTIVITY Figure 6: Equivalent polarization curves, showing the relationship between potential and current density on the outer surface of the PCCP, UNCONTAMINATED. Step 3 - The Global Model Having established the behavior of PCCP under various levels of CP protection, PCCP can be considered as a simple pipe in the global model with the equivalent polarization properties. One CP option considered was to protect the pipe using vertical Zinc anode ground-beds positioned 5.5 meters to either side of the axis of the pipe and distributed at 6m intervals along the length of the pipeline 7. The anodes on one side of the pipe are connected to a header cable as shown in Figure 7. Because of the length of the cables and the internal resistances of the various components it is expected that the current flowing to the pipe will vary significantly depending upon the distance the anode is from the connection to the pipe. Therefore, the maximum current is expected near the connection and minimum flow remote from the connection. One of the objectives of the model is therefore to identify the variation in the current flow and hence the protection potential on the pipe caused by the IR drop which occurs principally in the connection cables. The distribution of the anodes surrounding the pipe is shown in Figure 8 and Figure 9. In a similar way to step 1, the model was constructed to allow the sensitivity of the various design parameters to be assessed, such as the resistivity of the soil surrounding the pipe, the anode characteristics and the electrical connections. Therefore the model was used to simulate various scenarios to allow the robustness of the design to be assessed. Note the polarization properties of the zinc anodes were included in the model. A key aspect of the design was the electrical connections and their impact on the distribution of current on the anodes and the pipe.

9 Vertical zinc anode ground-beds at 6 metre intervals Cable connecting anodes on the other side of the PCCP PCCP Cable connecting anodes on one side of the PCCP Cable connects to the PCCP at 504 metre intervals Figure 7: Schematic of the Cathodic Protection Scheme to protect PCCP, showing PCCP, separate connection cables on either side, and vertical anode ground-beds at 6 m intervals along the pipeline. Vertical zinc anode ground-beds at 6 metre intervals PCCP Cable connects to the PCCP at 504 metre intervals Figure 8: Plan view of the PCCP and anode ground-beds.

10 Ground level Anode ground-bed PCCP Anode ground-bed Figure 9: Cross-section of the PCCP showing anode ground-beds to either side. Figure 10 shows the electrical connection between the anodes and the pipe. The diagram shows only one half of the circuit between the point where the header cable is connected to the pipe and the point mid distance between the connection points. Anode ground-beds Midpoint between the cable connections to the pipe Connection to pipe Rh Rh Rh Rh Rh Figure 10: Layout of the electrical connections between the anodes and the pipe. A number of cases were considered in the study and a few of the results are presented in Figures 11 to 14. Because of the attenuation, the performance of the individual anodes varies significantly as can be seen in Figure 11 where the variation of the potential along the pipe is shown between the electrical connections to the pipe.

11 Potential (mv SCE) Distance (metres) Figure 11: Predicted potential on the surface of PCCP along the length of the pipe from the cable connection point to the mid point showing the attenuation in the potential as the distance from the connection point increases. Soil resistivity is 1 ohm m. The impact of different values of soil resistivity can be clearly seen by comparing the pipeline potential distributions in Figure 11 and Figure 13. The latter figure shows that for soil resistivity 100 Ohm.m, potential on PCCP is: more positive shows significant variation between positions of the anode ground-beds Smallest current density on anodes and pipe at mid-way between connection points Largest current density on anodes and pipe close to the connection point Figure 12: Predicted current density on anodes and PCCP surface for the case of soil resistivity 1 ohm m.

12 Potential (mv SCE) Distance (metres) Figure 13: Predicted potential on the surface of PCCP from the cable connection point to the mid point showing the attenuation in the potential as the distance from the connection point increases. (100 ohm.m Soil Resistivity) The ripple in the potential is caused by the higher soil resistivity of 100 ohm.m which causes a larger potential drop between the anodes compared with the results for 1 ohm.m. Smallest current density on anodes and pipe at mid-way between connection points Largest current density on anodes and pipe close to the connection point Figure 14: Predicted current density on anodes and PCCP surface for the soil resistivity of 100 ohm m. Step 4 - Predicting the Condition of the Wire & Liner Having predicted the potentials on the external surface of the pipe using the large scale/global model for the various CP options considered, the conditions on the wire can be computed using the data developed earlier using the local model. Figure 15 shows typical data which can be developed from the model. The horizontal axis shows the potential on the outer surface of the pipe and vertical axis shows

13 the predicted potential on the wire. For example, for a pipe potential on the surface of -900mv (SCE), the potential around the circumference of the wire is shown in the figure as varying between points A and B. A B θ WIRE COATING MORTAR Outer surface of PCCP DIAGRAM NOT TO SCALE Potential around circumference of reinforcement wire (mv) LOW RESISTIVITY A B Potential at outer surface of PCCP (mv) Figure 15: Predicted potentials on the PCCP wire as a function of the external potential (vs.sce) for the case of Low resistivity concrete where SC 1, 2, 3 and 4 correspond to the Scenarios (SC) defined in Table 1. Similar data can be developed for the case of High Resistivity Concrete as shown in Figure 16. The results clearly show that there is a greater variation in the potential for the case of the higher resistivity concrete and that there is a drop of approximately 30mV compared with the low resistivity case. Obviously these results can only be used for compatible models. Therefore the local Low resistivity data can be only used to interpret results from the large scale model based on the Low resistivity PCCP data. Outer surface of PCCP HIGH RESISTIVITY A B θ WIRE COATING MORTAR DIAGRAM NOT TO SCALE Potential around circumference of reinforcement wire (mv) A B Potential at outer surface of PCCP (mv) Figure 16: Predicted potentials on the PCCP wire as a function of the external potential (vs.sce) for the case of High resistivity concrete.

14 Similar data can be developed for the liner as shown in Figure 17. θ WIRE Surface of the steel liner COATING MORTAR Outer surface of PCCP DIAGRAM NOT TO SCALE Potential along the surface of the steel liner (mv) WITH LOW RESISTIVITY Potential at outer surface of PCCP (A) (mv) Figure 17: Predicted potentials on the PCCP liner as a function of the external potential (vs.sce) for the case of Low resistivity concrete. CONCLUSIONS The work presented has demonstrated how a multiscale modeling study can support the design of a CP system to test the effectiveness and the robustness of the design under various conditions. The information can be used to optimize the final design. Model results have been validated on similar studies where correlation with subsequent field data has been good. Such a study provides the means to interpret pipeline potential data to determine information on the potential and current densities on the prestressing wire and steel liner within the PCCP. REFERENCES 1. C.A. Brebbia, J.C.F. Telles, L.C. Wrobel, Boundary Element Techniques, (Berlin Heidelberg, NY, Tokyo: Springer Verlag, 1984). 2. D. P. Riemer, M. E. Orazem, Modeling Coating Flaws with Non-Linear Polarization Curves for Long Pipelines, in Corrosion and Cathodic Protection Modeling and Simulation, Volume 12 of Advances in Boundary Elements, R. A. Adey, editor, WIT press, Southampton, 2005, pp D. P. Riemer, M. E. Orazem, Application of Boundary Element Models to Predict the Effectiveness of Coupons for Accessing Cathodic Protection of Buried Structures, Corrosion, 56 (2000): pp

15 4. R. A. Adey, J. Baynham, Design and Optimization of Cathodic Protection Systems Using Computer Simulation, CORROSION/2000, paper no. 723 (Houston, Texas: NACE Int., 2000). 5. L. Bertolini, F. Bolzoni, A.Cigada, T. Pastore and P. Pedeferri, Cathodic Protection of New and Old Reinforced Concrete Structures, Corrosion Science 35, 5-8 (1993): pp A. A. Sagues and S.C. Kranc, On The Determination of Polarization Diagrams of Reinforcing Steel in Concrete, Corrosion Science 48, 8 (1992): pp A.Muharemovic, N.Behlilovic, I.Turkovic, Functional Relationship Between Cathodic Protection Current/Potential and Duration of System Deployment in Desert Conditions, Advances In Engineering Software (2011) (Accepted for publication). 8. A. Peratta, J. Baynham, and R. Adey, Mathematical Modeling Applications in Reinforced Concrete Structures. A. Peratta, J. Baynham, and R. Adey, CORROSION/2010, (Houston, Texas. USA, NACE Int. 2010). 9. Andres B Peratta, John M W Baynham, and Robert A. Adey. A Computational Approach For Assessing Coating Performance In Cathodically Protected Transmission Pipelines. CORROSION/2009, (Houston, Texas, USA. NACE Int. 2009). 10. A. Peratta, J. Baynham, R. Adey, Gervasio F. Pimenta Intelligent Remote Monitoring System For Cathodic Protection Of Transmission Pipelines, CORROSION/2009, (Houston, Texas: USA. NACE Int. 2009).