Defect Sizing Accuracy
A major problem with inspection systems is that each potential user runs their own validation program using their own equipment and scope of work. This leads to differences of opinion regarding the inspection systems capabilities regarding defect sizing. For example height size accuracies vary from as little as +/-0.5mm to +/-2mm. These discrepancies can be explained by:
- Inspection procedure
- Design of calibration blocks
- Differences in validation methodology
- Type and shape of defects inspected
- Material used for validation program (e.g., grain size variations, attenuation variations)
- Position in the weld
- Pipe wall thickness
- Properties of the material in which the defect lies (yield strength mismatch)
- Service conditions of the pipeline
Some general rules of thumb are:
Surface breaking defects are roughly twice as severe as the same size embedded defects. Weld reinforcement makes embedded defects far less critical for the structural integrity than surface breaking ones, however the beneficial effect of the weld reinforcement on the weld performance vanishes for surface breaking defects.
Some of the advantages of using AUT in combination with an engineering critical assessment are:
- Reduces the amount of unnecessary weld repairs
- Reduction of weld reject rates which results in a faster installation time
- Ability to size the through-thickness dimension of flaws