FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

DISCUSSION
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The results of this project illustrate that focused beams can be used
successfully in conjunction with a simple mapping technique for defect
sizing. Other, more sophisticated, techniques have shown comparable or
better accuracy (3,8). The added complexity, however, does tend to isolate
them to the laboratory with several years of development before being
ready for field use, and require a higher qualification of inspectors than
is presently employed for pipeline inspection.
The project also restated standard problems encountered during contact
inspection due to weld bead geometry. Although the leading edges of the
wedge assemblies were ground off, the outer weld bead was sometimes still
wide enough to prevent full coverage of a known defect. Depending on the
nature of the defect and the working range limits of the lens, this could
result in loss of sizing accuracy. However, by lens design for longer
steel paths (i.e., in the region of 1 to 2 skips of the reflected beam,
instead of 0 to 1 skip as used here) this can be readily overcome.
Weld bead geometry can also confuse interpretation by introducing
extraneous signals, or significantly altering the sound path length. When
the ID weld bead is offset from the centre of the weld, sound may reflect
off the bottom of the weld bead, instead of off the pipe ID, before
reaching the target defect. In such instances an ID defect may appear to
be alternately located at midwall or at the weld root, depending on the
relative position of the transducer. Are there two defects located here,
or one? Focused beam inspection is as vulnerable to operator
interpretation in this area as any other technique which relies on
time-of-flight information to perform its function.
Transducer selection is also an area of vulnerability for this technique.
The use of transducers with inappropriate beam characteristics would
significantly degrade the outcome of any sizing attempt. Similarly,
variations due to instrumentation effects (instrument/probe interaction)
must be considered for optimum accuracy(6). At the extreme, the unfocused
beam characteristics must be determined using the actual instrument/
transducer combination intended for the inspection for input to the lens
design process.
Although the basic ultrasonic theory is well developed, it does not readily
lend itself to overcoming the specific limitations outlined above. In
response to such needs, computer modelling is presently being used as an
aid to understanding the physical processes involved. One such example is
based on solution of the first order equations of elasticity using finite
difference methods to model sound wave propagation through solids(9). The
results of such modelling (e.g. see Figure 6) will be used to develop
guidelines for optimization of transducer selection and inspection
procedure specification.
CONCLUSIONS
- Focused ultrasonic beams can be used in conjunction with simple mapping
techniques used in standard practice to give accurate sizing of
pipeline girth weld defects.