Page 270 - Pipeline Pigging Technology

Pig-tnto-place plugs and slugs

PIG-INTO-PLACE PLUGS AND SLUGS

INTRODUCTION

Following the Piper Alpha tragedy in the North Sea, and other accidents
around the world in the last few years, a large number of operators and
legislative bodies are beginning to require that emergency isolation systems
are available on the appropriate pipelines, enabling the systems to be safely
shut down in an emergency. There are many pipeline systems throughout the
world which cannot be fully isolated should there be a problem at a particular
point within the transmission system.
The purpose of this paper is to describe a number of techniques which are
being successfully used, as well as ones presently under development, to
enable the pipeline to be isolated without requiring the complete system to
be decommissioned. Obviously, there is a significant cost advantage in
working on a line while it is still full of product, as long as this can be
undertaken safely and quickly. The alternative option would be to drain the
line of product and either flood the system or free the line of gas prior to
starting work. Either option can have not only economic effects in the local
region, but also affect the complete distribution network.
In 1988 it was recognized that there may well be an application for a subsea
intervention system which would enable additional pipelines to be tied into
a main trunk line without decommissioning the complete pipeline. In a
typical North Sea scenario, we may have a 200+km pipeline which has been
dried at the time of commissioning down to the dewpoint of -20°C, and
operated in a controlled manner since then. The time required to recommission
the pipeline back to the acceptable standards for product delivery is such that
after the installation of a spool piece into the pipeline and subsequent testing,
a further 10-15 days may be required to dry the pipeline. It was for this initial
intervention requirement that a number of isolation designs were selected for
further evaluation. The systems evaluated and developed to operational
status have the following in common:

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