Page 113 - Pipeline Pigging Technology

Pipeline Pigging Technology

requirements from the UK Department of Energy and Norwegian Petroleum
Directorate.
Three methods of inspection are used on the submarine sections of the
pipelines:

Acoustic survey by side-scan sonar: This method allows an overall general
inspection of the pipelines. It provides information on the trench and burial
condition of the lines, detects significant changes on free spans (sections
where the pipeline is not supported by the sea bed) and identifies areas where
the sea bed has been disturbed (anchor scars, etc.).
Because of the relatively-low cost per km and the speed of the method, the
whole length of each pipeline is surveyed acoustically once a year.

Inspection by remote operated vehicle (ROV): This method allows a close
detailed inspection on specific areas of the pipelines. Its main objectives are:
to inspect the external condition of the pipeline, including its coatings and
features (anodes, supports, etc.); to monitor the level of cathodic protection;
to provide further and more accurate information on free spans and burial
condition; and finally to detect the presence of debris (anchors, fish nets,
etc.).
Due to the high cost per km and the slowness of this method, only specific
areas of the lines are inspected each year. The inspection scope is defined so
that all non-buried areas are surveyed at least once in a five-year cycle. Any
significant free spans detected by the latest acoustic inspection are included
in the next ROV inspection.

Internal inspection by intelligent pigging: This method allows a full
assessment of the pipe wall condition along the whole length of the line
(including risers). It detects anomalies in the pipe geometry (ID restrictions)
and the pipe wall (corrosion, etc.). The Frigg pipelines are inspected by
intelligent pig once every four years.
Acoustic and ROV surveys are used in conjunction, as the results provided
by the acoustic inspection, normally carried out during spring, are used to
define the scope of the ROV campaign which takes place during summer. Any
remedial action required will be decided during or after the ROV survey and
will normally be carried out in autumn.
As a consequence, two critical periods for result analysis can be identified:

after the acoustic campaign, when the scope of the ROV inspection has
to be finalized;
after the ROV campaign, to plan the remedial action required.

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