Page 40 - Pipeline Pigging Technology

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an ILI tool, equipped with sensors, must carry data-gathering, processing and
storage equipment, as well as its own power source. It may travel hundreds
of miles in perhaps crude oil, at high pressures. It will often start and end its
journey via several 90° bends and a vertical riser - quite apart from the
somewhat less-than-delicate manner in which it will be handled by the
roustabouts...
It is not surprising, therefore, that a great many inspection techniques
which work in a laboratory will not work in a pipeline. And many millions of
dollars have been spent in proving this point.
We are therefore left with relatively-few techniques which are truly "tried
and tested" - and even these are subjected to almost constant further
development.

Geometry pigs

Electro-mechanical

The first ILI geometry tool was the TDW "Kaliper" pig (Fig.2); the early
versions utilized the electro-mechanical method, as a number of other
manufacturers still do today.
A series of fingers radiate from the centre of the pig. These are attached to
a rod which passes through a seal into a pressure-tight chamber. Inside the
chamber, a stylus mounted on the end of the rod rests on a paper chart
running between two rollers. One of the rollers is driven by a stepper motor,
actuated by a reed switch mounted in one (or both) of the arms, which in turn
is triggered by magnets buried in the odometer wheels.
Odometer wheels are a feature of almost all ILI tools, and are machined to
a diameter which gives a predetermined length of travel for each revolution
(typically 1ft).
As the pig passes a reduction in diameter, the fingers are deflected. This
moves the centre rod a certain distance (depending on the size of the
reduction), and so marks the chart accordingly. Thus, both the extent and the
location of the reduction are recorded, and can be seen on the chart when it
is removed at the end of the run. Skilled interpretation of the trace can
distinguish different types of reduction, such as a dent compared to ovality.

Electronic-mechanical

An obvious development of the electro-mechanical tool was to record the
movement of the stylus electronically, rather than on a paper chart. The

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