Survey of a 10" buried line

This work was carried out on a network of water injection lines at an oil processing facility. These were partially above ground, but the majority were buried. The concerns were external corrosion around the soil to air interface and where the coating had been damaged in the underground sections. Tests were carried out either from above ground sections or from 'bell hole' excavations. An example of the Teletest ® equipment set up on an above ground section is shown below. The result shown is from a buried section, which was totally inaccessible at the time of inspection. The scale has been enlarged to show the feature of interest, which is about 26m from the transducer. There are welds at 11.5 and 25.5m. The first is shown off-scale; the steepness of the DAC curves indicates the higher levels of attenuation, which are generally observed on buried lines with protective coatings. It should also be noted that the scatter on the baseline (similar to ultrasonic 'grass') is greater for such lines.

Teletest inspection Teletest inspection chart

Immediately beyond the second weld is an additional signal at 26m from the transducer, with associated mode converted components, which are plotted in colour (marked '+' on the plot). This region was reported as a moderate to severe defect and the area was excavated. The pipe was found to be heavily corroded at that point and a repair was put in place immediately. The section removed is shown below. The weld can be clearly seen with the heavy corrosion adjacent to it. It is unlikely that this defect would have been detected by any other means before it had caused a failure of the line.

Teletest inspection of defect Copyright © 2004 TWI Ltd

Information and advice from TWI and its partners are provided in good faith and based, where appropriate, on the best engineering knowledge available at the time and incorporated into TWI's website in accordance with TWI's ISO 9001:2000 accredited status. No warranty expressed or implied is given regarding the results or effects of applying information or advice obtained from the website, nor is any responsibility accepted for any consequential loss or damage.