The presence of defects in pipelines is a concern especially in petrochemical applications where the service integrity of pipes is a fundamental requirement to avoid
process interruptions and to fulfil safety standards. Guided wave inspection is now
routinely used in industry for screening long lengths of pipe for corrosion, any
suspect areas then being followed up with conventional ultrasonic thickness gauging.
However, this is difficult in cases where the suspect area is inaccessible (e.g. buried
pipelines or pipes passing though walls), so it would be very useful to apply guided
wave techniques for sizing as well as the detection and location of defects. This
target is challenging due to the complexity of the profiles encountered in practice.
The present work aims to improve the understanding of the scattering of the fundamental torsional mode T(0, 1) from complex shaped discontinuities and to determine
the controlling parameters of this phenomenon. The overall analysis starts with a
study of the reflection from axi-symmetric tapered steps and notches in pipes. After
that the scattering from three dimensional (3D) defects with different shapes has
been studied. Firstly,
flat-bottomed defects with different surface profiles have been
analyzed, and then the study of the reflection behavior from 3D defects with varying
depth profile has been carried out. All of the work presented here uses the T(0,1)
mode for inspection.
It is revealed that the reflection coefficient maxima from axi-symmetric tapered
defects decrease with increasing frequency as the slope of the taper becomes more
gradual, this effect being more pronounced when the ratio of the average defect
length to the wavelength increases. Tapered defects are therefore expected to be
more difficult to detect at higher inspection frequencies; this effect is more evident
for shallower tapers. It is also found that at a given maximum depth of a finite
discontinuity, the peak of the reflection coefficient from a defect is linearly dependent
on the circumferential extent of the defect, and is independent of its shape. The
results from these analyses have been used to propose a practical approach to determine the maximum depth of a complex discontinuity from the reflection coefficient behavior, provided that the external circumferential extent of the defect is known.
This method has been applied to real corrosion patches and the results validated
with experiments. Its main limitation is on defects with a gradual corrosion section
profile, but with a sudden change of the depth over a small circumferential region. It
is shown then that a possible way to diagnose sharp circumferential profile changes
is to measure the reflection coefficient spectrum at frequency higher than usually
used in long range guided wave inspection.