Page 47 - Pipeline Pigging Technology

Pipeline Pigging Technology

Ultrasonics (U/S)

The principle of ultrasonic inspection is also very simple. A transducer
emits a pulse which travels at a known speed. On entering the pipe wall, there
is an echo, and another as the pulse reflects off the back wall. The time taken
for these echoes to return provides a virtually-direct reading of the wall
thickness.
Again, although the principle is very simple, it too has some drawbacks.
The first, and arguably the most important, is that the sound will only travel
through a homogeneous liquid. The word "homogeneous" is almost as
important as the word "liquid" in this context, as such things as gas bubbles
and wax floculation can affect the results.
Another important point for the HI tool designer to keep in mind is that
the transducers must be maintained square to the surface of the pipe wall to
within a very few degrees, or the echo will be missed. This poses particular
problems on bends.
Pipetronix has carried out a great deal of development work in order to
introduce its "UltraScan" tool (seepages 335-342). There is less information
available as to precisely what these developments are, but clearly they are
significant - because they work!
Although the internals may remain a mystery, the most prominent external
feature is the transducer array at the rear (Fig.8). It is also probably the most
important development to date. The distance from the transducer to the pipe
wall is called the 'stand-off. Most manufacturers, notably NKK, TDW and
AMS, use a stand-off of more than one inch (25mm), but Pipetronix has
embedded the transducers into a polyurethane cage which is towed behind
the pig. The cage flexes, maintaining the transducers in a close and constant
relationship with the pipe wall, even when passing through bends or
reductions in diameter. This also presumably makes it less susceptible to
changes in the homogeneity of the liquid in which it is immersed.
There is a constant search for new methods and materials to further
improve or expand the various ILI services, especially in the field of metal-loss
detection and measurement. A typical example is in extending the use of U/S
tools to gas lines. This has now been achieved very successfully on a number
of occasions by running two conventional pigs in the line at either end of a
slug of liquid (usually a gel) in which the U/S tool travels.

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