Page 306 - Pipeline Pigging Technology

Cable-operated and self-contained ultrasonic pigs

Last but not least, the risk of an intelligent pig getting stuck in an off-loading
line was considered too great. These lines are often old, sometimes with mitre
bends, dents or other unknown obstructions or features. To imagine an
obstacle without a rescue line in what is often a "life line" for a plant or refinery
was alone reason enough for operators not to apply intelligent pigs to off-
loading lines.
It is for all the above-mentioned reasons that RTD worked on a solution,
and decided to construct cable-operated ultrasonic pigs. In our solution, as
Fig.l shows, we use a motor-driven crawler. This self-propelled unit makes
the operation independent of pumping facilities.
The umbilical for transmission of signals to and from the inspection
crawler is reinforced for rescue purposes. An array of ultrasonic probes is
mounted at the front end of the inspection tool.
To deploy the tool, the pipeline has to be opened for several metres to
attach a simple open launch tray; apart from power supply and hoisting
equipment, no other facilities are needed. On-line presentation of results and
full control over speed and direction makes the pipeline inspection tool (PIT)
very attractive to pipeline owners.
To date, eight successful world-wide applications have proved the viability
of this concept.

THE ULTRASONIC STAND-OFF METHOD

The most suitable method of quantifying internal and external corrosion
is the stand-off technique as illustrated in Fig. 2. A circular array of transducers
is located at some distance from the inner pipe wall, and the liquid in the pipe,
usually oil or water, acts as the essential acoustic couplant. In this way both
the distance from the transducer to the pipe wall as well as the pipe wall
thickness can be measured. These readings can be undertaken simultane-
ously, and with an accuracy of far better than 1mm.
To obtain a fine grid of data, a small axial sampling interval of a few
millimetres is usually applied, while for circumferential coverage, a large
number of transducers are used; the size of the corrosion pits that can be
detected and quantified will depend on the type and number of transducers
employed.
Not only is the stand-off technique as shown in Fig.2 well-suited for the
measurement of internal corrosion (i.e. profile), but the array of transducers
is several centimetres away from the pipe, making the tool less vulnerable to
damage. This allows'a relatively-simple form of transducer suspension.
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