External coatings are used routinely to protect transmission pipelines from conditions that promote corrosion. However, over time coatings may disbond, allowing corrosion to occur. To detect metal loss due to corrosion defects, transmission pipeline operators often use magnetic flux leakage (MFL) in-line inspection tools. These tools do not detect the cause of an actual problem, i.e., failure of a coating and the presence of a corrosive environment; rather, they detect only the result, i.e., a defect that may permanently alter the pressure capacity of the pipeline. Metal loss is the most common...
External coatings are used routinely to protect transmission pipelines from conditions that promote corrosion. However, over time coatings may disbond, allowing corrosion to occur. To detect metal loss due to corrosion defects, transmission pipeline operators often use magnetic flux leakage (MFL) in-line inspection tools. These tools do not detect the cause of an actual problem, i.e., failure of a coating and the presence of a corrosive environment; rather, they detect only the result, i.e., a defect that may permanently alter the pressure capacity of the pipeline. Metal loss is the most common defect that occurs at a disbond; however, it is not the only defect that can occur. Stress corrosion cracking (SCC) almost always occurs at disbonds. Information about disbonded and missing coatings, soil models, cathodic protection assessment, and related data could be used to assess a pipeline for the potential of cracking. Direct inspection of the coating could indicate potential problems that could lead to pipeline degradation that affects serviceability. SCC can be detected using in-line inspection technology, but such tools have a limited availability and limited success in natural gas pipelines, and the cost of inspection is high compared to MFL inspection. A novel method for the in-line evaluation of a protective coating on a pipeline was shown to be feasible. The method involves the merging of two technologies, magnetic flux leakage (MFL) and electromagnetic acoustic transducers (EMATs). The results of the testing performed at the Pipeline Simulation Facility showed three technical accomplishments. First, EMATs could be designed to work with the magnetic fields produced by an MFL tool, although fields 2 to 3 times greater are usually optimal for EMAT sensors. Second, the EMAT transmitter and receiver could be mounted on an MFL inspection tool without interfering with the corrosion detection sensors. Finally, the EMAT sensors as implemented on the MFL pig could detect missing coating and disbonds on wrapped tar coating. While technical issues remain that would broaden the applicability, the immediate implementation of this technology for specific coating problems is possible.