Page 249 - Pipeline Pigging Technology

Pipeline Pigging Technology

Fig.3. Up- and down-hill multi-phase flow; effects of inclination.
the pipeline system. Oil pipelines are best inhibited using oil-soluble water-
dispersible filming amine-type corrosion inhibitors which can disperse suffi-
ciently into stratified water layers to prevent corrosion beneath the water.

Inhibitor application with pigging

When inhibiting either gas or oil lines, pigs should first be used to sweep
out water and remove any sediment from the pipe wall. If liquids alone are
being displaced, a sealing pig would be sufficient. Cleaning pigs equipped
with wire brushes or scraping discs should be used if deposits such as wax or
scale are evident in the line. A film of inhibitor should then be applied using
periodic batch treatment with sealing pigs. Batching keeps the chemical in a
solid column ahead of the pig, as shown in Fig.3, allowing exposure to the
entire pipe surface. If pigs are not used, the slug of chemical will lose its
column form, leaving portions of the pipe unprotected. Batching, followed by
a continuous low-concentration injection programme, is recommended over
an injection programme alone, as there is no way to ensure that all of the pipe
wall has been treated.
A Canadian sour gas-gathering system in which corrosion failure occurred
is discussed in Refs 2 and 3. This system had been treated with a liquid-soluble
corrosion inhibitor in a continuous injection programme. Stratification ex-
isted in sections of the line, especially down-sloping portions. The liquid-
soluble inhibitor used provided excellent protection to the bottom of the line,
but the top sections of the line were left unprotected. These lines burst after

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