Page 273 - Pipeline Pigging Technology

Pipeline Pigging Technology

hydrocarbon fluid which had the physical property of expansion -when
subjected to low temperatures within the pipeline system was a potential
isolation technique.

PIPE FREEZING

Nowsco was contracted to develop a remote pipe-freezing system capable
of undertaking one or more pipeline freezes simultaneously, each freeze
being remote from the freeze cooling equipment. The technique was origi-
nally designed for subsea freezing and line isolation, but also has many
applications in production and transmission systems. In this technique the
fluid to be frozen would be displaced through the pipeline and arrested
conventionally, and a freeze jacket installed around the outside of the line
would allow cooling to take place at a localized position, in a controlled
manner.
Pipe-freezing techniques have been available for a number of years, and
usually involve either liquid nitrogen or carbon dioxide as the cooling
medium, which are externally applied to the area of pipeline to be frozen. The
fluids inside the line, usually water, are reduced in temperature until they
form solid plugs. Experience has shown that these plugs are capable of
withstanding very high differential pressures, and pipe freezing has become
a relatively-common technique.
In the applications envisaged by Nowsco, it was considered that control-
lability of the freeze was desirable, and therefore the design criteria called for
the freeze temperature on the outside of the pipe to be controlled to ±1°C.
It has been shown that even though low temperatures do not permanently
impair the pipeline steel, it becomes very brittle during the operation, and
therefore some potential clients would be happier not to go below -40° C for
many of the operations considered. At the same time, it was envisaged that a
number of freezes would be applied rather than a single freeze, and the
temperature of all the freezes would be controlled remotely from a single
point, minimizing the number of operators required to undertake the opera-
tion. Alarms where also required to be built into the system to monitor
deviations in circulating fluid temperatures. There have been examples in
pipe-freezing operations in the North Sea where liquid nitrogen had been
withdrawn from a vessel at a low rate on a continuous basis and passed
through small-diameter cryogenic hoses to a conventional freezing jacket.
Ambient heat had vaporised the nitrogen to a gas, and cool gas had been
circulating around the jacket rather than the intended liquid.

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