NoPig: A New Inspection Technique For Non-Piggable Pipelines
Proceedings Publication Date
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 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 unpiggable pipelines. The performance of the NoPig technique was significantly higher than existing corrosion detection and could also detect metal loss. The NoPig technique has been extensively tested with natural as well as artificial metal loss defects under field and laboratory conditions. The NoPig technique has been 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 the AC current passed through the pipeline from a special current source. For this purpose two contact points maximum 1 km apart must be available on the pipeline. The range between the contact points will be inspected. The inspection tool contains a sensor unit and a data collector unit. In the sensor unit many magnetic field sensors are combined to cover a step length of 1 m. During inspection the tool will be moved step by step above ground over the pipeline. On this matter the whole distance between two connection points will be inspected. 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 a metal loss is present. Using this principle metal loss defects will 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” (TÜV certified) which can be extended towards large diameters to 36” (experimentally tested). Both external and internal metal loss defects can be found.