Page 358 - Pipeline Pigging Technology

Corrosion surueys with the 'UltraScan' pig

tape, and the unit has 10.5-in reels which can handle approx. 4 GByte of data
on 28 tracks. If the untreated data on the ultrasonic module was to be
recorded, the distance between two ultrasonic pulses being only 2.5mm
apart, then the magnetic tape capacity of a 40-in pig would only be sufficient
for 10km of pipeline
Owing to the nature of the data, it can be compressed without loss of
information. The pig is equipped with an on-board multi-microprocessor
system to carry out this function; the data flow is constantly monitored and
compressed in such a manner that only 1/10 of the otherwise necessary
storage space is actually occupied on the magnetic tape. Hence, it is possible
to store 100km of pipeline on one magnetic tape without loss of any data.
Since the 40-in pig has two magnetic tape recorders, it is therefore possible
to store 200km on tape; this is equivalent to 80 GByte of ultrasonic data.
The magnetic tape's storage capacity is only used efficiently if the data is
stored at the maximum baud rate at the selected tape speed. In order to
achieve this, the data supplied by the data-compression microprocessor is
stored first in the cache register. Once this is full, the data will be transmitted
to the magnetic tape storage in a serial mode at a constant baud rate. A second
cache register acts in the meantime as an intermediate storage for the
continuous flow of data. Rechargeable silver-zinc batteries provide the
necessary power in pigs with large diameters. These batteries represent the
most up-to-date technology, and have twice the energy density of nickel-
cadmium batteries. Pigs for smaller diameters use primary lithium cells, since
rechargeable batteries do not provide sufficient energy, given the limited
space available.

Data analysis

The entire software for data evaluation was written for IBM AT-compatible
computers, allowing analysis of the recorded data directly at site. A very
important feature is the representation of defects through quasi-three-dimen-
sional colour charts. Fig.4 shows such a chart: the wall thickness is shown in
the bottom section and the stand-off in the top section. The chart is composed
of many parallel lines, each one representing the data from one sensor in
colour. The y-axis represents, therefore, the unfolded wall, and the x-axis the
distance travelled. A section of pipeline without defects, with the sensor at
normal stand-off, is shown as white. As soon as a defect appears, a colour spot
will become visible. The size of the spot indicates the area extension of the
defect.
The data evaluation is carried out in steps. Since the magnetic tape unit is
not efficient for handling the data because it does not have random access, all

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