A Simplified Model to Simulate Electrolytic Coupling in Cased Pipeline Crossings

摘要

To protect an oil or gas pipeline from external damage or stress that can occur when a segment of the pipeline crosses or goes under a highway, railroad, or river, a casing pipe is typically installed that surrounds the carrier pipeline. The U.S. Department of Transportation's Pipeline and Hazardous Materials Safety Administration estimates there are thousands of cased crossings nationwide and hundreds if not thousands located in high consequence areas. Over time, the annular space between the casing and the carrier pipe may be filled with soil and water, resulting in electrolytic coupling between the two pipes. A simplified model is developed to simulate electrolytic coupling between the carrier and casing pipe, and to estimate diversion of the cathodic protection current from the rest of the pipeline for a given set of conditions. This model calculates current densities at the casing-soil and casing-annulus interfaces for different coatings at the carrier pipe and other relevant parameters. Model results show that electrolytic coupling has the potential to degrade the cathodic protection level at the carrier pipe, but the effects are dependent on the quality of the casing coating. This model may be useful for evaluating the potential for pipe degradation at cased pipeline crossings.