NoPig: An Above-Ground Inspection Technique for Non-Piggable Pipelines

Non-piggable pipelines account for thousands of kilometres around Europe alone. The integrity management of these pipelines requires some non-destructive evaluation techniques, which can be applied without inserting inspection tools into pipelines. Some indirect techniques like DC voltage gradient or current mapping are used for this purpose. These methods can detect coating damages like holidays and thus can only be damages like holidays and thus can only be used for external corrosion detection; they give no information regarding metal loss. The NoPig technique was developed as an alternative (or supplementary) technique for inspecting non-piggable pipelines and can detect metal loss. The NoPig technique has been extensively tested with natural as well as artificial metal loss defects under field and laboratory conditions. This technique is used successfully across Europe and overseas for inspecting gas, oil and fuel pipelines. The principle of the NoPig technique is based on measurement and analysis of the surrounding AC magnetic field around the inspected pipeline. The magnetic field is induced by AC current passed through the pipeline from a special current source, which is connected via two contacts to the pipeline. The range between the contact points is inspected. The inspection tool contains a sensor unit and a data collector unit. During inspection the tool is moved step by step above ground over the pipeline allowing inspection over the whole distance between the two connection points. The magnetic field data is obtained and analysed at different frequencies. Due to the skin effect and stray magnetic flux (in case of metal loss) a frequency-dependent difference in magnetic field data takes place if metal loss is present. Using this principle metal loss defects can be detected. Information concerning metal loss, the depth of coverage and current plots are supplied after a NoPig inspection. The current plots are used for detection of current leakages through damaged coating. The NoPig inspection tool can be used for seamless as well as for long seam pipelines within the diameter range of 3" to 16" (TUV certified) and can be extended towards large diameters up to 36" (experimentally tested). Both external and internal metal loss defects independent of their clock position can be found.