Page 31 - Pipeline Pigging Technology

Pipeline Pigging Technology

SPECIALIST APPLICATIONS

The field of pigging is expanding towards ever more sophisticated devices
and specialist applications. In particular, the requirement to survey pipelines
to detect not only dents and buckles, but also corrosion pitting and cracks has
lead to the development of intelligent pigs. Pigging systems have also evolved
to satisfy other demands such as the ability to paint the internal bore, or to
install a retrievable subsea safety valve similar to a down-hole safety valve, or
to plug the pipeline so that maintenance can be carried out without a shut
down, and so on. The following paragraphs look at these applications, which
are also summarized in Fig.4.

Magnetic-flux leakage intelligent pigs

A brief mention was made above of the regular use of calliper pig surveys
to detect pipeline geometry defects and compare with a baseline run during
commissioning. More sophisticated techniques allow die determination of
wall thickness over the entire pipe surface as well as picking up dents, buckles
and pipe ovality. One such technique is magnetic-flux leakage detection.
The principle of magnetic-flux leakage detection is used to determine the
volume of metal loss, and hence the size of defect. The pigs will function in
both gas and liquid lines. Since the shape of the magnetic output trace has to
be interpreted, the characterization is often improved by running a series of
surveys over a number of years to establish trends.
The alternative to using an intelligent pig to survey the wall thickness of
the line is to take ultrasonic measurements at key points along the pipeline
such as bends, crossings, tees, etc. Such measurements could easily miss a
problem and lead to a false sense of security; they are no match for the
comprehensive information obtained via intelligent pigs, but are obviously
much cheaper.

Ultrasonic intelligent pigs

Using the internal fluid as a couplant, ultrasonic pigs measure the wall
thickness of the entire pipeline surface. Since it is a direct measurement of
wall thickness, the interpretation is more straightforward than for a magnetic-
flux pig. They are better suited to liquid lines and cannot be used in gas lines
without a liquid couplant. Otherwise, the advantages over external ultrasonic
scanning are the same as for the magnetic-flux pigs.

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