Case Study 4: The Evaluation of Cathodic Protection Effectiveness
Corrosion Engineering International, LLC was requested in 2014 to evaluate the effectiveness of a Cathodic Protection system protecting a 12” coated steel pipeline, which supplied high-pressure natural gas to various regulator stations feeding the surrounding metropolitan area. Corrosion Engineering International, LLC was tasked to identify the cause of the ailing cathodic protection system and provide design recommendations to meet criteria established in Code of Federal Regulations; Title 49 Part 192 Subpart I.
Upon review of client historical information, CEI personnel proceeded to conduct a baseline interrupted survey to determine the present cathodic protection system condition. This initial evaluation confirmed the established cathodic protection system boundaries, rectifier current and voltage output, and pipeline polarization at various test station locations. The field data indicated that 90% of the pipeline polarization was below required criteria with several test locations near native steel potentials. By comparing theoretical current calculations and rectifier current output, Corrosion Engineering International determined the rectifier current output would normally be sufficient to protect a 12” pipeline with less than 10% coating loss. However, the present polarization indicated the system had greater than anticipated coating loss or direct contact with a foreign structure. CEI personnel performed a Pipeline Current Mapping test to evaluate CP current distribution and Alternating Current Voltage Gradient to evaluate the coating condition. The combined test data indicated that two large coating holidays were present within 50 feet of the anode bed which was causing an ineffective current distribution across the entire pipeline.
Corrosion Engineering International recommended a direct examination of the coating deficiencies conducted and any coating repairs performed to improve the coating condition. The direct examination identified two uncoated 12” pipeline tees, which supplies natural gas to a regulator station, were the culprit. The pipeline tees were coated, which caused the cathodic protection current distribution to normalize, and all but a few locations now meet the required criteria. However, the platinum impressed current anode bed was also determined to be near depletion and required replacement.
Corrosion Engineering International was directed to provide a cathodic protection design for the replacement anode bed with design calculations, anode bed specification, installation drawings, and anode bed placement. While evaluating the present location, CEI determined soil resistivities would cause unnecessarily high installation costs due to the presence of rock and be ineffective in providing the required current output to protect 100% of the 12” natural gas pipeline. CEI personnel evaluated various locations along the pipeline and determined that three viable locations were suitable for the new impressed current mix metal oxide anode bed. Corrosion Engineering International provided the necessary design calculations, anode bed specifications, installation drawings, with a recommendation for two anode bed placement locations for improved current distribution.