Defects in external corrosion control for 3PE coating pipelines

In this paper, a section of 3PE coating pipeline was taken as an example for the comparative analysis between the statistics of external metal loss distribution on ILI data and the close interval potential survey(CIPS) data of ECDA. It is shown that the external metal loss is relatively concentrated on the first 20 km pipe section where the off potential of cathodic protection(CP) does not satisfy the CP criterion. Based on the comparison between two ILI data with an interval of 9 years, the external metal loss at the girth weld seams presents an increasing trend. It is indicated from the excavation inspection results that CP system can protect the 3PE coating holidays effectively, instead of the coating holidays at the field joints. Obviously, the defects in external corrosion control for 3PE coating pipelines are located at the field joints. Therefore, the key points for routine operation and management of 3PE coating pipelines are pointed out and the suggestions on the type selection of field joint coatings at the stage of pipeline design are put forward.