23/11/2016
Instead of classical wall thickness measurements, Long
Range Ultrasonic Testing (LRUT), a guided wave
ultrasonic NDT technology developed for detecting
metal loss and defects in pipe
-work can be used. From
one location up to 25 meters to the right and 25 meters
to the left of the pipeline can be scanned. Compared to
classical wall thickness, for LRUT there is no need for
removing all insulation and it gives information about
the entire pipe tested, not just in one point.
The LRUT system employs low frequency guided
waves that operate just above audible frequencies and
propagate from a transducer array fixed around the
pipe. At these frequencies a liquid coupling between
the transducers and the surface is not necessary, as
satisfactory ultrasonic coupling is achieved by
pneumatic pressure applied to the back of the
transducers to maintain an intimate contact with the
pipe surface. The uniform spacing of the ultrasonic
transducers around the pipe circumference allows
guided waves to be generated which propagate
symmetrically about the pipe axis. These may be
visualized as a circular wave that sweeps along the
pipe. The whole of the pipe wall thickness is excited
by the wave motion, the pipe acting as a wave guide.
When the sound wave encounters a change in pipe wall
thickness, whether an increase or a decrease, then a
proportion of the sound energy is reflected back to the
transducer array, thereby providing a mechanism for
the detection of discontinuities. The presence of these
signals is a strong indicator of discontinuities such as
corrosion. The test data are collected by the laptop
computer. The software displays the A
-scans in both
the forward and backward directions. Distance
Amplitude Correction (DAC) curves are plotted on the
display. Based on the dotted, green and red Curve (dB
values), anomalies were categorized in Cat.1, Cat.2
and Cat.3. Focusing on anomalies through additional
measurements gives us a final result of the follow up
priority.
