Page 357 - Pipeline Pigging Technology

Pipeline Pigging Technology

EQUIPMENT DESCRIPTION

The complete pig, seen in Fig. 2, consists of three modules with a sensor
carrier at the end of the tool. The individual pig modules are linked by flexible
universal joints, and have pressure-resistant bodies that carry the electronic
equipment for the survey. The first pig module rides on cups through the
pipeline. These cups guide the pig and simultaneously seal inside the pipe to
create the necessary differential pressure for propulsion. This module acts as
the towing unit for the whole pig train.
The other units are guided by rollers or cups with by-passes. The 24-in pig
seen in the illustration has a battery pack in the first module as power supply.
The second module holds the data storage and the multi-microprocessor
system for data processing.
The ultrasonic survey equipment is located in the third module, and a
multitude of ultrasonic sensors is mounted on the sensor carrier which is
towed behind.
In order to fulfil its duty, the pig must scan the entire surface of the pipe
during one run. To do so, the sensor carrier (Fig.3) is equipped with eight
sensor planes. The various sensors are mounted in such a fashion as to ensure
complete coverage of the pipe's surface. The sensor carrier must keep the
individual sensors perpendicular to the wall and ensure that the sensors are
kept at constant distance from the wall. A pig with 48-in diameter (1.2m) has,
for example, 448 sensors located around its circumference.
The individual ultrasonic sensors are connected via shielded cables to the
ultrasonic equipment inside the third module. These cables enter the third
module through a pressure-resistant bulkhead. The sensors have a special
design and are pressure-resistant up to 200bars to withstand the pressure
inside the pipeline.
64 sensors are combined to form a multiplex unit, each of which has a
central control board and a main amplifier which supplies the 64 modular
units. The individual sensors are excited by a 5-MHz ultrasonic pulse. The
maximum pulse repetition frequency is 400Hz per sensor. A 48-in pig has
seven multiplexed modular units for its 448 sensors. Two 8-bit words appear
at the output of each modular group: one for wall thickness, and the other for
stand-off. The ultrasonic module's data is passed on to the second pig module;
the data flow is approx. 400 kByte/sec.
For data storage, magnetic tape recorders are still used despite the recent
advances in semi-conductor storage technology, because magnetic tapes
have a higher data density per volume. The UltraScan pig, which is subject
to considerable acceleration inside the pipeline, has a magnetic tape unit that
was developed for airborne applications. It stores the data on a 1-in magnetic
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