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

摘要

Cathodic Protection (CP) design guidelines and associated equations for buried pipelines were developed for conventional trench installations, and may be poorly suited for Horizontal Directional Drilling (HDD) installations. Pipeline segments installed by the HDD method are likely to experience coating defects and damage that are not typical in 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 properly describe the coating defects and damages observed in conventional trench installations. HDD installations can result in long axially-oriented coating damage with aspect ratios of 100 or more when a rock with a sharp edge scrapes against the pipeline segment being pulled through the borehole. Coating damages with a large aspect ratio are likely to experience substantially different corrosion patterns than damages with a small aspect ratio because (ⅰ) corrosion rates at the damaged locations are strongly and nonlinearly dependent on electrode potential at the exposed metal surface and (ⅱ) the electrode potential is governed by the distance to the anode and the corrosion rate. In this paper, various options to estimate electrical current for protecting coating defect locations on the HDD-installed locations are discussed.