Page 172 - Pipeline Pigging Technology

Inspection of the Forties sea line

INSPECTION OPERATION RESULTS

Each time the inspection vehicle was run through the pipeline, an initial
assessment was carried out on the recorded data to ascertain both the quality
of the data and also distance of pipeline inspected.
Full data processing was carried out at the On-Line Inspection Centre,
involving transference of data from inspection tape to computer tape. All data
was then fully evaluated using the extensive computing facility at the Centre.
The data produced showed that corrosion was evident in the pipeline
characteristic of individual corrosion pitting, general corrosion containing
pitting, large areas of pipe-wall thinning and selective attack of girth welds.
The corrosion was detected from the start of the pipeline for approximately
29km, gradually reducing with distance from the launch.
It was noticed that within this area some pipe spools existed that had
resisted corrosion attack even when adjacent pipe spools had shown corro-
sion.
From the outset, it was necessary to produce the inspection results in
formats that allowed BP to:

determine the general condition of the pipeline;
using fracture mechanics specialists, to evaluate the effect of the
condition of the line on its operating integrity;
determine a derating curve for the pipeline validated by subsequent
inspections.

As a first step, a computer listing was produced (Fig.7) giving weld
numbers down the line, relative distance between each weld, and their
absolute distance from launch. Values of pipe wall thickness for each spool
were added to this list, but because of the very large number of readings
involved in the inspection process, the values were given as:

1) mean value - average value for each spool;
2) maximum value - the maximum value obtained in the spool, this
value also showing the presence of buckle arresters;
3) minimum value - the value of the thinnest area of pipe in the spool;
and
4) standard deviation - a figure which gave an indication of the
variability of the wall thickness over the entire spool and hence
overall condition of that spool.

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