Page 274 - Pipeline Pigging Technology

Pig-into-place plugs and slugs

As failure of the plug could have severe consequences, freeze monitoring
is considered essential, and the system developed by Nowsco has been
designed to overcome these potential problems. The system comprises a
uniquely-designed jacket which is placed around the pipeline to be frozen. A
pair of special circulation hoses are connected from the jacket via a circulat-
ing pump to a heat exchanger; the coolant is circulated continuously around
the system and, as it passes through the heat exchanger, liquid nitrogen on
one side of the exchanger reduces the temperature of the coolant fluid,
enabling the surface temperature of the pipe to be reduced. Computer
simulation of cooldowns has enabled the inner core temperature of the plug
to be predicted, in different operating environments with various internal and
external temperatures.
A significant amount of work was undertaken to determine the best fluids
to be used for the freezing operation, both in the laboratory and in field trials.
Obviously water can be successfully frozen, and has been in the past; a
significant amount of research was therefore centred on developing a
hydrocarbon fluid which when cooled expanded rather than contracted, and
an acceptable fluid has now been identified.
To ensure that no voids are present in the pipe once the freeze fluid has
been displaced to its correct location, the use of gels to increase the viscosity
of the freeze fluid was evaluated. It was found that by gelling the fluid, void
spaces which were potentially present at the top of the liquid were mini-
mized. Operationally, the fluids were pigged into place in a pipeline train
rather than just relying on a single pig to provide the barrier between the
freezing fluids and the displacement fluid. Trials have been undertaken in 20-
in pipe loops where hydrocarbon-based gels have been frozen to -40°C and
withstood a 500-psi differential; in aqueous-based trials, 1,000psi differential
pressures have been withstood.
The minimum pipe-freeze length which has been employed traditionally
in pipe freezing has been three times the pipeline diameter, but in the field
tests undertaken this had been reduced to no more than 1.5 times pipe
diameter; however, wherever possible 3D plugs should be used. Obviously,
where very high differential pressures are to be withstood, the strength of the
plug is directly related to the diameter of the freeze and the length of the plug,
as well as to the structural composition of the frozen fluid.

GELS AND HIGH-SEALANT PIGS

The Nowsco group of companies has recently developed and deployed a
high-sealing high-pressure bi-directional pig train utilizing modified pipeline

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