Page 267 - Pipeline Pigging Technology

Pipeline Pigging Technology

This trade-off between suspension and rinsability required the use of
proven high-sealant pigs and, in most cases, a modified design of the gel train.
The gel which was used for cleaning the Fulmar line in the North Sea (290km,
20in) to a cleanliness level of 10 microns proved that:

1. RPG was fully rinsable and no residue was left at the pipe wall;
2. no effect on the drying period occurred;
3. subsequent pig runs found no debris in the line;
4. the gel did not trap debris against the pipe wall.

Hydrocarbon gels

Gelled hydrocarbons, such as diesel, kerosene or, in many cases, line
product, can be mixed as the base fluid, giving the high sealing efficiency
characteristic of gel pigs. They are used in operational oil or gas pipelines
where aqueous systems are unacceptable, either run alone if displaced by
liquids, or usually with a mechanical pig when displaced by gas.
In gas pipelines, continuous injection of corrosion inhibitor may need to
be supplemented with a periodic slug treatment. Sticky diesel gels can be
loaded with up to 20% of an inhibitor, and when injected ahead of a routine
mechanical pig run, give a satisfactory laydown on the whole pipe circumfer-
ence throughout its length, with internal flow within the pig allowing
continuous migration of fresh inhibitor to the pipe wall. When injected into
the line, the gel spreads along the pipe base, until launching of the mechanical
pig bulldozes it into a diameter-filling 'gelly pig'. Gas transmission continues
during gel injection, although the peak rate may have to be temporarily
reduced.
An important additional benefit, if not the joint objective, of a diesel gel run
is that it will flush out condensate, or water that has dropped out and
accumulated in the line. In a wet or rich-gas pipeline, especially if irregularly
contoured, even frequent conventional pigging can by-pass considerable
quantities of such liquids.
The gelling chemicals contain no organo-chlorines and will not poison
refinery catalysts, and are disposed of either by flaring or by dilution of the gel
by an acceptable hydrocarbon.
The sealant and cleaning gels are usually aqueous systems, prepared from
fresh water or seawater, and are both biodegradable and have no adverse
environmental effects when discharged at sea.
It should be stressed that all the gel systems are designed for a specific
application and that close liaison between the engineers responsible for the

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