Page 265 - Pipeline Pigging Technology

Pipeline Pigging Technology

TYPES OF GEL

Three main types of gel pigs are commonly used for pipeline applications:

High-viscosity sealing gels

Sealant gels are based on the series of gels designed for downhole
fracturing techniques. These gels are visco-elastic and self-healing, with a
strong cohesive attraction, and are typically used in situations where contami-
nation of the product or pipe wall is not important.

Commissioning cleaning gel systems

Cleaning gel pigs are prepared from fresh water or seawater gelled with a
biodegradable polymer. The gel has visco-elastic and plastic flow properties
(pronounced yield-point and significant cohesive behaviour). The gels have
a high yield strength which ensures that the debris remains suspended even
if the gel is static for long periods.

Debris pick-up mechanism: Debris pick-up gels are usually and most
successfully run in conjunction with a following mechanical pig, displaced at
between 1 and 3 ft/sec to ensure that the gel is in plug flow during the pipeline
transit. In this flow regime, the core volume of gel moves as a semi-solid plug
at higher displacement velocity than gel on the wall; therefore there is little
exchange with the material, with the almost-stationary gel near the pipe wall.
During displacement the gel in this annular zone is removed from the pipe
wall by the mechanical pig, and flows forward into the core zone, forming a
'convection system'.
The gel is very adhesive to either previously loose or newly pig-loosened
debris. This debris is entrained and carried forward into the core by the action
of the following pig. In this system, debris cannot accumulate in front of the
pig causing it to stick, but is distributed evenly throughout the gel body.
As some of the debris pick-up gels are readily water-dispersible, and if pig
reliability is doubtful or a situation exists where mechanical pigs cannot be
used due to diameter changes, or launching/landing difficulties, and polymer
pigs are used, then the cleaning gel can be protected front and rear by a
sealant preventing dilution by entrained and by-passing water. Because of
their very different characteristics, gel and sealant gels do not readily inter-
mix.

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