Page 79 - Pipeline Pigging Technology

Pipeline Pigging Technology

Comments

(a) It is more difficult to assess whether deposits are present in longer lines
(over 60km).
(b) The lines may be dirty either as a result of construction debris or debris
which has slowly accumulated over many years.
(c) Lines that are rapidly accumulating a layer of deposit require special
arrangements, i.e. a corrosion-inspection pig should be run immediately after
the cleaning programme.
(d) The normal cleaning runs maintain the flow requirements adequately.
The corrosion pig, however, introduces a magnetic field into the pipe wall via
very strong permanent magnets and brushes. These may scrape off more
deposits, which may interfere with the sensors' reading of magnetic signals.
It is clear that special arrangements have to be made to prevent failure of the
survey; it is suggested that a number of cleaning pigs are run at frequent
intervals, with the results from each run being carefully recorded and studied.
(e) The formation of so-called 'black dust' (iron sulphate) in gas pipelines
is caused by a reaction between the material of the pipe wall and the gas
content. The dust is usually very abrasive, wearing down discs/cups at a
tremendous rate. Again, it is very difficult to remove it from longer lines
(100km and over) due to the wear. Also, the dust may ignite when exposed
to the air, and so stringent safety precautions are recommended.
Since the debris is usually concentrated in the most interesting portions of
the pipeline (the bottom of the pipe cross-section, low spots, etc.), lack of
recorded data may reduce the efficiency of the survey by up to 80%.
Debris accumulation can result in:

mechanical failure of the inspection pig, jamming the odometer wheel
system (loss of location reporting);
lift-off of the magnetic brushes, and consequent loss of magnetic field
(reducing the level of detection);
lift-off of the sensors, and consequent failure to detect magnetic-flux
leakage (reducing the level of detection);
accumulation of ferrous debris disturbing the sensor readings (confus-
ing the detected data);
total or partial destruction of the corrosion-inspection pig itself.

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