Page 95 - Pipeline Pigging Technology

Pipeline Pigging Technology

exist. Also, as the nominal bore of the corresponding steel pipe will be less (by
5-10%) than that of the flexible bore, there is every chance that standard pig
sealing arrangements will be inadequate. To prevent fluid by-pass, a double-
cup arrangement is therefore recommended.
The steel materials used for the inner carcass are generally made from
stainless to 316L, austenitic steel (6% Mo, 21% Cr), or duplex. When wire
brushes or steel gauging plates are used, their material compatibility must be
ensured to prevent damage or contamination to the stainless steel (or
sometimes to the brushes themselves).
When selecting cups, blades or gauging plates for use on pigs, it is also
important to note that carcass wall thicknesses are generally only of the order
of several millimetres. Their profile is a convex wave shape and spaces will
exist between adjacent waves. This means that inappropriate pig selection
could cause extended blades to jam or even become obstructed in the pipe.
Flexible pipes are by definition and application flexible in catenary, i.e.
they are not rigid in bend areas and are likely to have changing radii of
curvature. Particularly for dynamic catenary riser applications, pigging should
not be considered for radii generally less than 5D, bearing in mind pipe
minimum bend radii are generally 8-10 times i.d. Should small radii be
required, a steel arch or bend restrictor may be required to safely control
curvature.
When using sensing pigs to determine ovality or assess pipe internal
condition, further care must be taken, as flexible pipe is a naturally slightly
oval structure and will be even more so after elongation and at areas of
greatest bending. When considering using intelligent pigs, it should be noted
that these devices have been specifically developed for large-bore steel pipe.
They largely operate on the principles of magnetic flux (whereby distur-
bances in an induced magnetic field are related to metal loss); or they use
ultrasound inspection (whereby contact probes issue short ultrasonic pulses
through the pipe wall and sound transit time is converted to wall thickness
measurement). Difficulties exist with these devices due to: flexible pipe's
relatively-small bore; the thinness of the steel carcass (0.5-4.Omm); and
because of the problems of ultrasonic wave scatter in individual pipe layers.
In summary, pig selection should be carefully made with regard to the
special aspects of flexible pipe construction and in view of the need for the
pig to pass through without becoming obstructed or causing damage.

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