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00673 A NOVEL APPROACH TO CATHODIC PROTECTION DESIGN FOR MARINE PIPELINES: PART I - AN INCLUSIVE ATTENUATION EQUATION

Picture for 00673 A NOVEL APPROACH TO CATHODIC PROTECTION
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Product Number: 51300-00673-SG
ISBN: 00673 2000 CP
Author: Patrick Pierson, Keith P. Bethune, William H. Hartt, and Palaniswam Anathakrishnan
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A first principles based equation for potential attenuation along a marine pipeline or riser that is cathodically polarized by multiple, equally spaced, identical galvanic anodes and which incorporates the electrolyte (anode), coating, and metallic path resistances, as well as the pipe polarization resistance, is derived. Finite Difference Method (FDM) solutions for this equation show that the potential profile consists of a relatively abrupt polarization decay within the first several meters of an anode and an essentially constant potential beyond this for cases where anode spacing is less than about one km (this distance varies with pipe and anode dimensions and properties and with exposure conditions). For anode spacings greater than this, metallic path resistance becomes important such that a potential gradient results along the entire pipe length. Comparison of the FDM solutions for the case of a pipeline of typical dimensions and marine exposure conditions with results from Boundary Element Modeling (BEM) indicates excellent agreement between the two for situations where the metallic resistance is negligible. For cases where this term is not negligible (relatively large anode spacings), the FDM solutions are the more accurate since BEM does not incorporate metallic path resistance. The potential attenuation projected by the classical equation of Uhlig is shown to be non-conservative compared to the FDM and BEM solutions because of its failure to consider the electrolyte component of circuit resistance. Anode current output determinations based upon the derived equation and upon BEM are shown to be in excellent mutual agreement. It is concluded that the derived equation has utility for design of pipeline cathodic protection systems and for analysis of data therefrom, particularly in cases where anode spacing is sufficiently large that metallic path resistance is non- negligible. Keywords: Pipelines, cathodic protection, seawater, galvanic anode, potential attenuation, anode current output, computer modeling.
A first principles based equation for potential attenuation along a marine pipeline or riser that is cathodically polarized by multiple, equally spaced, identical galvanic anodes and which incorporates the electrolyte (anode), coating, and metallic path resistances, as well as the pipe polarization resistance, is derived. Finite Difference Method (FDM) solutions for this equation show that the potential profile consists of a relatively abrupt polarization decay within the first several meters of an anode and an essentially constant potential beyond this for cases where anode spacing is less than about one km (this distance varies with pipe and anode dimensions and properties and with exposure conditions). For anode spacings greater than this, metallic path resistance becomes important such that a potential gradient results along the entire pipe length. Comparison of the FDM solutions for the case of a pipeline of typical dimensions and marine exposure conditions with results from Boundary Element Modeling (BEM) indicates excellent agreement between the two for situations where the metallic resistance is negligible. For cases where this term is not negligible (relatively large anode spacings), the FDM solutions are the more accurate since BEM does not incorporate metallic path resistance. The potential attenuation projected by the classical equation of Uhlig is shown to be non-conservative compared to the FDM and BEM solutions because of its failure to consider the electrolyte component of circuit resistance. Anode current output determinations based upon the derived equation and upon BEM are shown to be in excellent mutual agreement. It is concluded that the derived equation has utility for design of pipeline cathodic protection systems and for analysis of data therefrom, particularly in cases where anode spacing is sufficiently large that metallic path resistance is non- negligible. Keywords: Pipelines, cathodic protection, seawater, galvanic anode, potential attenuation, anode current output, computer modeling.
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