Page 341 - Pipeline Pigging Technology

Pipeline Pigging Technology

Deriving acceptable defect curves

The equations necessary for deriving acceptable corrosion defect curves
are given above (or the acceptance levels in the ANSI/ASME Code[12] can be
adopted). The selection of safety factors for use in Eqn(l) will be the
responsibility of the pipeline operator, but the hydrotest safety factor has the
advantage of being directly related to code and pre-service requirements. In
some codes (particularly for oil pipelines) the hydrotest level is relatively low
(e.g. IP6[12]), and it may be better to use a higher hydrotest level in deriving
a safety margin, e.g. 100% SMYS as used in the ANSI/ASME B31A Code [5], [ 12],
to ensure a reasonable safety factor.

Deriving repeat inspection intervals

The acceptable defect curves can be used during repeat inspections.
These can be combined with corrosion rate data to predict increases in
corrosion depth with time, Fig. I4(a). The curves, with safety factors included,
can also be used to both predict when any downrating of operating pressure
is needed or when it would be necessary to re-inspect the line to avoid
downrating, Fig.l4(b).

CONCLUDING REMARKS

A defect assessment methodology for corroded pipelines, based on the
above sections, can be proposed. Fig. 15 summarizes the methodology, and it
is recommended that this type of methodology is applied to future assess-
ments of corroded pipelines. It can be applied to pipelines containing limited
corrosion or extensive corrosion. However, there are some limitations, and
these are listed in Ref.3. For example, the interaction of neighbouring
corrosion pits is not well understood. However, the methodology will be
applicable to most corrosion types, despite these limitations.
It should be emphasized that a defect assessment is only as good as the
defect inspection report. If the report is inaccurate, the defect assessment will
be inaccurate. Therefore, a reliable, accurate inspection tool is required if the
above methodology is to be applied. These tools can be expensive, but they
allow defect assessments which avoid expensive repairs to the pipeline.

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