Page 331 - Pipeline Pigging Technology

Pipeline Pigging Technology

Fig.9. Metal-loss readings along a channel.

metal loss readings (see Fig.7) can be identified. These spools can then be
closely scrutinised during a second run, and the reported corrosion can also
be assessed using the methods detailed later.

Repeat inspections

Following a repeat inspection, inspection data will be available for the
entire pipeline (e.g. Fig.3(b)) and individual spools, Fig.8; the most severely-
corroding spools can be determined using the type of procedure used in Fig.8.
Care should be taken in assessing the metal-loss readings from spools; this is
because corrosion can be preferential, so that corrosion rates determined
from mean metal-loss readings around the circumference of the pipe (Fig.3(b))
and along the axis of the pipe (Fig.9) can be misleading. Similarly, when
determining corrosion rates at specific areas of corrosion (i.e. pits), care
should be taken in allowing for general wall thickness corrosion as well as pit
corrosion, Fig. 10.

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