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51315-6112-Electrical Current Requirement to Protect Coating Defect Locations on Pipeline Segments Installed Using Horizontal Directional Drilling

Picture for Electrical Current Requirement to Protect Coating Defect Locations on Pipeline Segments Installed Us
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Product Number: 51315-6112-SG
ISBN: 6112 2015 CP
Author: Pavan Shukla
Publication Date: 2015
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Oil and gas transmission pipelines are subject to external corrosion which can lead to dangerous explosive pipeline failures. Pipeline operators use two complementary practices to protect pipelines from corrosion: (i) pipe coatings and (ii) CP. Intact pipe coatings are effective at isolating metal surfaces from the surrounding environment thereby reducing corrosion but coatings may develop defects that permit locally elevated corrosion rates. Pipeline operators mitigate corrosion at coating defects by CP systems installed at intervals along the pipeline. These CP systems require proper design to assure protection but have associated operations and maintenance costs. CP systems are designed using guidelines developed for the coating defects expected in conventional trench installation practices. However trenches are undesirable in some locations spurring use of Horizontal Directional Drilling (HDD). Typical HDD installation locations include wetlands congested areas and road crossings.The CP design guidelines and associated equations for the buried pipelines were developed for conventional trench installations and may be poorly suited for HDD installations because pipeline segments installed by the HDD method experience coating defects that are not typical of conventional trench installations. Newman in 1966 developed CP design equations still in use today to represent small coating defects with an aspect ratio of less than 10 which are typical of defects observed in conventional trench installations. HDD installations can result in long axially oriented coating defects with aspect ratios of 100 or more when a rock with a sharp edge scrapes against the pipeline segment being pulled through the borehole. Defects with a large aspect ratio are likely to experience substantially different corrosion patterns than defects with a small aspect ratio because (i) corrosion rates in coating defects are strongly and nonlinearly dependent on electrode potential at the exposed metal surface and (ii) the electrode potential is governed in part by the distance to the anode and in part by the corrosion rate. In this paper we will present a methodology to estimate electrical current for protecting coating defect locations on the HDD-installed locations.
Oil and gas transmission pipelines are subject to external corrosion which can lead to dangerous explosive pipeline failures. Pipeline operators use two complementary practices to protect pipelines from corrosion: (i) pipe coatings and (ii) CP. Intact pipe coatings are effective at isolating metal surfaces from the surrounding environment thereby reducing corrosion but coatings may develop defects that permit locally elevated corrosion rates. Pipeline operators mitigate corrosion at coating defects by CP systems installed at intervals along the pipeline. These CP systems require proper design to assure protection but have associated operations and maintenance costs. CP systems are designed using guidelines developed for the coating defects expected in conventional trench installation practices. However trenches are undesirable in some locations spurring use of Horizontal Directional Drilling (HDD). Typical HDD installation locations include wetlands congested areas and road crossings.The CP design guidelines and associated equations for the buried pipelines were developed for conventional trench installations and may be poorly suited for HDD installations because pipeline segments installed by the HDD method experience coating defects that are not typical of conventional trench installations. Newman in 1966 developed CP design equations still in use today to represent small coating defects with an aspect ratio of less than 10 which are typical of defects observed in conventional trench installations. HDD installations can result in long axially oriented coating defects with aspect ratios of 100 or more when a rock with a sharp edge scrapes against the pipeline segment being pulled through the borehole. Defects with a large aspect ratio are likely to experience substantially different corrosion patterns than defects with a small aspect ratio because (i) corrosion rates in coating defects are strongly and nonlinearly dependent on electrode potential at the exposed metal surface and (ii) the electrode potential is governed in part by the distance to the anode and in part by the corrosion rate. In this paper we will present a methodology to estimate electrical current for protecting coating defect locations on the HDD-installed locations.
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