Magnetic Flux Leakage Pigs or Ultrasonic Pigs? the Case for Combined Intelligent Pig Inspections

This paper describes a case study where a pipeline operator was faced with features in the pipe wall that proved difficult to conclusively identify. An ultrasonic (UT) pig inspection indicated that the pipeline contained numerous deep external defects. However, subsequent excavations indicated that the reported external defects were in fact mainly laminations and/or inclusions. To verify that this was the case for the whole pipeline the operator ran both a magnetic flux leakage (MFL) and an ultrasonic inspection tool in the same inspection programme, using the magnetic flux leakage tool to confirm the absence of part-wall defects and hence demonstrating the presence of laminations. Introduction Pipeline operators now have a wide choice of inline inspection tools (pigs) that have the capability to deliver a consistently high-level of reporting of pipeline features and defects. The two main internal inspection technologies are magnetic flux leakage (MFL) and ultrasonic (UT). Each has its benefits, however, each has its inherent limitations and neither can identify all possible pipeline defects. A case study is described in this paper where a pipeline operator was faced with features in the pipe wall that proved difficult to conclusively identify. An internal ultrasonic pig inspection had indicated that the pipeline contained many deep external defects. However, subsequent excavation and external inspection of the pipeline for verification indicated that the reported features were not as serious as initially thought: the defects identified by the external examinations were not all external defects but were, in fact, mainly laminations and/or inclusions in the pipe wall. The pipeline's operating conditions meant that laminations were not considered an integrity issue for this pipeline 1 , whereas the reported external defects reported were. Consequently, the pipeline operator required a method of inspection that could conclusively identify the features in the pipeline, without any ambiguity regarding external features or laminations. After consultation with Penspen Integrity, the pipeline operator opted to run both a magnetic flux leakage pig and an ultrasonic pig in the same inspection. The magnetic flux leakage tool used can detect metal-loss defects but not laminations. The ultrasonic tool used can accurately identify internal metal-loss, but can have difficulties when differentiating between external metal-loss and laminations. Consequently, the combined inspection can use the ultrasonic tool to identify the location of possible external metal-loss (or laminations) and the magnetic flux leakage tool can be used to confirm metal-loss, hence conclusively demonstrating where laminations are present …