Page 245 - Pipeline Pigging Technology

Pipeline Pigging Technology

1. paraffin deposition rates are greatest when chemicals are not used;
2. the cost for complete chemical inhibition of paraffins can be very
high;
3. allowing any pipeline or its instrumentation and metering systems to
become fouled with significant wax deposits is both unnecessary
and can lead to erroneous metering, possible loss of control of the
line, and greatly-increased pumping requirements.

Pigs should be run periodically to scrape off accumulated paraffin deposits
on the walls of the pipe which the chemical programme has not been able to
prevent. This will also lead to reduced chemical consumption, as the goal is
no longer complete prevention of deposits. Optimized programmes for
paraffin control in pipelines combine chemical treatments with pigging to:

1. maintain the line in a clean condition and enable it to be re-started in
a cold condition;
2. minimize the chances of sticking a pig, especially in offshore lines;
3. prevent flow capacity reductions or pressure drop increases through
the line;
4. keep instrumentation and sampling equipment clean and in working
order;
5. keep operating costs to a minimum.

When a pipeline has accumulated an excessive amount of paraffin build-
up, either through improper or no maintenance at all, caution should be used
in the design of the rehabilitation programme. When thick deposits are
present, it may not be feasible or cost effective to use chemicals for dispersal
of the wax, as very large volumes of the chemicals would be needed.
It can also be difficult and hazardous to try to move huge volumes of wax
with pigs through long pipelines, as it is very easy to create a blockage and may
require extraordinary pressures. Care must be taken to conservatively re-
move the wax in controllable amounts through use of progressive pigging
techniques. Once pigs have removed all of the wax physically possible,
chemicals should be used to treat the remaining paraffin.
As an example, a pigging programme to clean paraffin deposits was
reported for a North Sea oil pipeline [1]. An estimated 7500brls of paraffin
deposits had accumulated in the line over several years under low flow
conditions due to cooling of the oil as it passed beneath the sea. A flow
improver had been added to the oil to enable the line to be cold re-started in
the event of a shut-down and cooling of the line. Whereas the chemical had
undoubtedly reduced the rate of deposit formation, it had obviously not

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