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Crack depth sizing without depth restriction

On Wednesday, 3rd May 2017, at the 12th annual Pipeline Technology Conference (PTC) in Berlin, Germany, NDT Global's Principal ILI Technology Consultant Herbert Willems presented a paper on "Sizing Crack Indications from Ultrasonic ILI: Challenges and Options".

The paper examines some aspects of ultrasonic inline inspection (ILI), regarding the task of crack inspection in particular. One common issue often experienced across the pipeline inspection industry is the uncertainty that comes with crack sizing.

The sizing of crack indications commonly comprises the evaluation of the length and the depth of cracks, based on the corresponding ultrasonic indications. This activity is part of the off-line data analysis process that comes after the performance of the physical ILI. Unfortunately, the technology applied has some limitations regarding depth sizing, as the reflection signal shows a saturation behavior for depths greater than approx. 4 mm (0.16 in). Recently announced by NDT Global, the addition of Enhanced Sizing to its ultrasonic (UT) crack inspection service allows for depth sizing without depth restriction by using a more advanced signal analysis. 

Identifying Pipeline Integrity Problems, Offering Solutions

Assessing crack-like defects is predominantly based on the length and depth of a crack, with depth of particular importance for the calculation of the safe operation pressure. A crack depth's determination relies on the recorded amplitudes from a reflection at the corner of the crack (corner reflection). This approach comes with inherent limitations, however, particularly pertaining to crack-like reflectors. Such limitations include, but are not limited to:

  • Influence of tilt angle
  • Influence of skew angle
  • Influence of pressure & temperature

The issue of uncertainty regarding crack depth arises due to the inaccessibility of some of the defect's characteristics, such as true crack shape, or tilt angle. This may result in under sizing of cracks in certain situations, especially where the reliability of depth sizing lies almost exclusively on the amplitude of the corner reflection with the additional drawback of limited depth range.

By introducing Enhanced Sizing to its UC inspection services, NDT Global will extend the depth range over the full wall thickness. This methodology represents a further step forward in constantly improving the reliability of in-line crack inspection. 

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