Page 266 - Pipeline Pigging Technology

Gels for commissioning and production

It should be noted at this time that there are two types of gel pig system
used. The first type is used always in conjunction with a mechanical pig to
prevent by-pass of displacing fluid; these have a lower viscosity than the
second type of polymer gels which are premoulded and have a very high
viscosity and can actually be used as a mechanical pig.
Train design: The amount of cleaning gel required is primarily dependent
on the maximum amount of debris expected. In new pipelines, this is usually
estimated at 0.05 kg/m 2 of pipe wall (assuming the line has been gauged
before). Using 4li of gel per kilogram of debris there is a more than adequate
margin for such contingencies as gel dilution, or more debris than expected.
A typical gel pig will tolerate 100% dilution and still carry the total expected
debris. Undiluted, it will carry several times this amount of debris with only
a limited increase in flow resistance.

Rtnsabtttty of gels: Following investigations into the success of the early
gel treatments it became apparent that gels were capable of supporting large
amounts of debris. It was, however, assumed that all of the removable debris
had been carried from the line by the gel. It was only at a later date, when
subsequent flushing and pigging removed further debris from the line, that
the efficiency of the chosen gel system was questioned.
Nowsco began an extensive research programme into the gel systems that
had been used on the operations. It was found that a thin layer of gel remained
trapped on the pipewall and that the subsequent pig did not remove all of the
gel. The gel layer left behind was found to vary from 1mm to O.lmm in
thickness. This layer effect was more noticeable when the gels were not
displaced by a pig and much larger volumes of gel were left behind.
Subsequent flushing of the line did not remove the gel, and it was found that:

1. remaining gel would become loose and entrain itself into the product
if not fully removed prior to the introduction of the product;
2. debris with a conventional gel train design may be trapped below this
film and remain in the line;
3. any remaining gel would have an adverse effect on the efficiency of
the drying process.

Nowsco has developed RPG (rinsable pipeline gel) as an alternative to the
existing gels in certain applications. This gel is fully rinsable but does not
break down on contact with water. It is, though, slowly diluted and its
suspension ability decreases with dilution. RPG is designed to be able to hold
its full debris load after 100% dilution by water has occurred.

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