ABSTRACT
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.
INTRODUCTION
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. Both pipes are generally made of the same grade of carbon steel. Each cased crossing consists of carrier pipe, casing pipe, separators, and end seals. The casing pipe has centralizers designed to maintain an equal radial distance between the carrier pipe and the casing pipe throughout the casing length using the separators. The casing pipe ends may be sealed, filled with wax, or left open to the atmosphere. Cased pipeline segments can range from ~20- to >300-ft long. 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 end seals degrade and the annular space between the casing and the carrier pipe is filled with a mixture of soil and water. This results in electrolytic coupling between the two pipes. The electrolytic coupling can potentially divert the cathodic protection (CP) current from the rest of the pipeline under a certain set of conditions. A simplified model is developed to simulate electrolytic coupling between the carrier and casing pipe, and to estimate diversion of the CP current from rest of the pipeline for a given set of conditions.
