Page 271 - Pipeline Pigging Technology

Pipeline Pigging Technology

1. they are capable of withstanding a significant differential pressure;
2. the system has to be reliable and repeatable, with fail-safe systems to
prevent failure;
3. the barrier system has to be easily introduce into the pipeline;
4. the barrier system should not cause any damage either to the pipe
wall or to the integrity of the pipeline system;
5. the system has to be easily removable following completion of the
work.

GEL ISOLATION

Through its downhole applications, Nowsco has developed a cross-linked
aqueous-based gelling system which has been used for temporary abandon-
ment of well bores. The properties of this particular gel are well known, and
in practise lengths of gel 150-200ft long placed inside 7-in internal diameter
pipe have been able to withstand in excess of 250psi differential pressure.
The major field problem with this particular system is that gellation takes
place rapidly and the plug has to be displaced into location within a very short
time for it to be able to form a coherent barrier.
Gel technology has also been used extensively by Nowsco in the pipeline
commissioning field, where both aqueous and hydrocarbon systems were
used to clean, pig and lay down chemicals on pipelines, at the time of
commissioning, and also subsequently during their operational life.
Using this experience as a database, it was decided to develop a gel system
which could be pumped into place, where the gel would have a controlled
gellation time and an controlled viscosity, enabling it temporarily to isolate a
pipeline system.
The design criteria also called for the life of the gel plug to be accurately
determined; this was carried out by chemically controlling the degradation of
the gel after a predetermined time. To enable the testing to take place,
pipeline test loops were built and extensive research undertaken in the
laboratory in the UK. The test loop design was slightly unusual in so much that
air-actuated valves allowed the gel train to go round the loop continuously,
simulating the passage of gel down a line of whatever length was required. For
practical purposes, we utilized a gel system inside an 8-in test loop, and for the
tests it was determined that the train would be displaced 20km, prior to
slowing the gel train, and allowing it to hydrate.

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