Detection and Sizing of Axial Cracks

Axial orientated cracks are often invisible to the eye, and can weaken a pipeline causing devastating failure. Active crack growth in operational pipelines can be caused by:

  • Stress-corrosion cracking (SCC)
  • Hydrogen induced cracking (HIC)
  • Thermal and/or mechanical fatigue

SCC: Often occurs as crack colonies that are usually aligned axially on external side of the pipe. A number of factors can contribute to their initiation and growth, including high-stress, steel susceptibility and corrosive environment in connection with damaged coating. Under certain stress conditions, however, SCC can also develop in the transverse direction (transverse SCC).

HIC: Usually start as embedded cracks that run parallel to the pipe wall. Cracks may join up stepwise, at different levels through the pipe wall, to create through-wall cracks. HIC is caused by hydrogen resulting from suitable chemical components in the pipeline medium (e.g. H2S). Atomic hydrogen diffuses into the pipe wall and recombining of hydrogen atoms into hydrogen molecules takes place at inclusions or lamination's causing blistering. The resulting high-pressure within these blisters promotes crack growth.

Thermal and/or mechanical fatigue: Cracks are caused by cyclic loading due to mechanical and/or thermal stresses. They are often generated at sharp geometry changes like for example at the edges of the long seam. Crack growth can take place at even small cyclic stresses generated e.g. by pumping. Usually, these cracks are oriented axially as the hoop stress it the principal stress component.

Our Ultrasonic Tools

Evo Series UC

Ultrasonic crack inspection enables early detection and sizing of crack-like irregularities. This allows the pipeline operator to take appropriate measures to avoid pipeline failures caused by cracks.

The principle of the ultrasonic crack inspection tool is based on the 45° angle beam technique using shear wave. Due to the so-called corner reflection, even minor cracks from approximately 1 mm (0.04 in) on wards give quite strong reflections.

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Ultrasonic Crack Detection

Evo Series 1.0 UC fleet improves speed capabilities and resolution for crack inspections. A maximum inspection speed of 4 m/s and an axial resolution of 1.5 mm (0.06 in) has become standard. Tools can detect and size features with minimum dimension of 20 mm (0.78 in) by 1 mm (0.04 in) in base material at a POD greater than 90%.