Dents are among the anomalies which may be a threat for the safe operations on high-pressure pipelines. They are mainly due to civil works and other third-party activities, in the event of mechanical impact and local deformation on the pipeline. Dents are characterized by a plastic deformation which causes changes in the magnetic properties of the ferromagnetic pipewall. Large Stand-off Magnetometry (LSM) is a contactless technology which can contribute to check the integrity of pipelines from the surface. It measures magnetic changes in order to detect and locate anomalies such as dents and metal loss (corrosion).

Skipper NDT, a company at the forefront of LSM technology, conducted trials in 2019 in partnership with TOTAL and CETIM (Centre des Etudes Techniques des Industries Mécaniques), to apprehend changes in the magnetic properties around dents. The objective is to improve the results of the LSM inspections entrusted by pipeline operators.

The trials were conducted in a laboratory under controlled conditions. A proprietary bench was designed for this purpose. Dents with depth of 5%, 10% and 15% were machined on X42 pipes of 6 inches diameter, 7 mm of wall thickness and 12 m length. Multiple gauges were disposed around the area of the dents to measure the strain during the machining. The mechanical behavior was simulated by Ansys finite elements software to have an accurate knowledge of the stress all around the dents and confirm the measurements from the gauges. Magnetic properties were measured before and after the machining of dents by high accuracy Fluxgate magnetometers at several heights.

This paper describes the trials and the results observed in term of magneto-mechanical behavior. Its conclusion mentions the improvements which can be expected for the detection and the identification of dents on pipelines by the LSM technology.