Page 337 - Pipeline Pigging Technology

Pipeline Pigging Technology

A METHODOLOGY

The above sections can be combined to develop a methodology for
assessing the significance of corrosion in pipelines. The methodology can be
divided into three parts:

1. processing corrosion data;
2. modelling corrosion;
3. deriving acceptable defect curves with safety factors.

Processing corrosion data

Figs 3-10 give methods of obtaining corrosion rates and highlighting
suspect spools from on-line inspection data.
For a single on-line inspection, a 'weak link' approach is recommended.
This means determining the most severe defect in a pipeline and the
significance of this defect governs the pipeline integrity. In practice, a
number of defects, of different sizes and shapes, will be reported that are
above agreed defect reporting levels. As the failure stress of corrosion is
related to both corrosion length and depth, it is necessary to determine the
significance of all these defects (e.g. Fig.4).
Repeat inspections may allow an estimate of corrosion rate; Figs 3-6 give
methods of determining this rate.

Modelling corrosion

A high-resolution magnetic-based on-line inspection can give a reliable
estimate of corrosion size. For a single inspection, the maximum size of the
corrosion should be used in setting defect acceptance levels; this means that
all defects are conservatively modelled as 'flat-bottomed' (see Eqns(l) and
(2)). Additionally, it may be necessary to take account of inspection tool sizing
tolerances in the depth and length inputs into Eqns(l) and (2).
For repeat inspections, it maybe necessary to model the corrosion rate as
well as the defect size. A variety of models are possible; Fig. 13 gives three
examples of modelling corrosion and corrosion rate. In practice, it may be
necessary to evaluate all such models and take lower bound values. Modelling
of pitting corrosion and rates is given in Fig.10.

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