Ultrasonic Crack Detection
Ultrasonic tools for crack detection contain transducers oriented at a different angle to the pipe wall. This ensures that ultrasonic shear waves propagate along a 45° path through the wall. Both internal and external cracks reflect the energy. Calculation of crack location is done with the time of flight; the crack depth is determined depending on the signal amplitude in relation to saturation amplitude of a reference reflector.
Cracks are most commonly situated along the long seam weld and are axial oriented of the pipe, due to the hoop stress. However, in girth weld areas circumferential oriented cracks can appear. Another occasion for these types of cracks may be high bending stresses which might be caused by geohazzards and pipeline movement.
Crack tools are available from 6" to 48", both orientations of cracks can be addressed. Combined inspection tools for wall thickness and crack inspection in one tool are also available, including multi diameter tools, e.g. 36"/48" corrosion/crack combo. Typical specifications for UT crack tools are: detection of cracks with a minimum length of 20 mm (0.79 in) and depth of 1 mm (0.04 in) in base material and at welds, 2 mm (0.08 in) depth in welds. Most of these tools are sensitive enough to detect flaws with only 0.5 mm in depth. In general the specification in length and depth is the same for cracks with different orientations. Cracks at and in welds are typically below 0.01 mm in width; the majority of SCC cracks is grouped around 0.01mm.
Conventional UT crack tools are able to detect cracks below 0.001 mm opening with high probability.
Crack Inspection Tools
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%.