Page 72 - Pipeline Pigging Technology

Pipeline design for pigging

larly, subsea installations differ from the rest because of the necessity for
remote-control operation, as well as because of the generally-harsher environ-
mental aspects of subsea operations.
For toxic (H 2S-laden) products, pig-trap station piping should be extended
with flushing connections to allow the toxic product to be expelled from the
trap prior to opening. Otherwise, the layout of the piping will be similar for
both liquid and gas service.
Besides sampling points and filters, pig traps are the only piping compo-
nents that are opened during normal operations and, as such, require that
extreme care shall be taken with their design to protect operational staff.
Pig-trap stations should be laid out so that the functions of valves and by-
passes are clearly indicated. Standardization of layout is therefore recom-
mended, as is colour-coding of flushing piping and valves to highlight their
functions.

Portable pig traps

Portable pig traps should only be applied in the sizes of 12-in nominal
diameter and below. They should only be considered if the capital investment
involved outweighs the operational expenditure. This will only be the case if
a large number of the same sized pig traps are used in a pipeline grid, requiring
a low-frequency pigging operation (e.g. inspection pigging). There is not
much experience available in the use of portable traps to date.

Offshore traps

Pig traps on platforms may differ in layout from onshore installations due
to space limitations. The connections may be in the vertical plane to save
space.
Vertical receiving traps are not recommended; vertical launching traps
have proved to be of limited success, and should be limited to the absolute
minimum in the smaller sizes only. Multiple sphere-launching traps should
also be designed to handle inspection pigs; a cartridge design can be
considered for such an installation.

Editor's note:
Readers are referred to the paper given by Cees Bal at the series of
seminars "Pipelinepigging.... an art or a science?" organized by Pipeline
Equipment Benelux for further detailed information about pig-trap design.
The author's address is PO Box 186, 2700 AD Zoetermeer, Netherlands.

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