The reliability and assessment of using TFI magnetic intelligent pigs for the detection of longitudinal defects in gas pipelines

摘要

Longitudinal Defects are the most dangerous ones for pipeline integrity, being affected by hoop stresses. Longitudinal cracks cause most of problems for pipeline operators: as soon as they are detected, pipeline operation has to be immediately stopped. The problem of reliable detection of longitudinal cracks in oil pipelines was solved long ago, using ultrasonic ILI tools, though it was very difficult to apply such tools in gas pipelines. So the only choice was to develop and expand opportunities of magnetic in-line inspection technology, especially the transverse flux inspection (TFI) one. Five years ago it was impossible to detect longitudinal defects in gas pipelines, using magnetic ILI tools. However, nowadays, Spetsneftegaz has gained extensive experience of using high resolution TFI tools, having created the data bank of longitudinal defects recorded by more than 30 thousand km runs of TFI tools, being able not only to guarantee reliability of longitudinal defects detection, but also to assess actual errors of TFI inspections on the basis of the data bank. Such assessment is important due to the following main reasons: 1. At present, most of pipeline operators still prefer to use only MFL tools to inspect their pipelines, thus reducing 50% reliability of defects detection. 2. Absence of data on availability of longitudinal defects makes it impossible to assess actual state and operation ability of an inspected gas pipeline as well as to perform reliable analysis of defects growth in future. 3. Additional valuable information on sizes of defects can be obtained, when superimposing MFL & TFI plots. 4. Comparative technical & economic analysis of efficiency of magnetic and UT inspections can be made.