Page 94 - Pipeline Pigging Technology

Pigging for flexible pipes

Corrosion monitoring can also be a problem, because most NDT tools have
been primarily developed to aid in the determination of global corrosion
processes rather than local ones. Because of the rough bore of flexible pipe
and due to the irregular geometry of the inner steel carcass or liner, turbulent
flow conditions can exist which can aggravate the predominant corrosion
mechanism, local crevice attack. Due to the generally-high chloride contents
in well fluids and in consideration of increasing reservoir temperatures (up to
130°Q, particular attention needs to be paid to steel selection and monitoring
carcass surface condition.

PIGGING CONSIDERATIONS

Pigging experience with flexible pipes has been largely confined to
applications outside Brazil and generally where hydrate or wax build-up in the
pipeline can be expected. This requirement will probably be introduced as
Petrobras moves into deep-water developments where low fluid tempera-
tures can be expected. Pigs can help maintain the reliability of a pipeline
system generally by: reducing pressure drop, improving flow capacity, and
controlling the build-up of sand, liquid, wax, and hydrates. Some pigging
operations, such as scraping and inhibition, can also play a central role in
boosting the corrosion protection of the pipeline system. Pigging frequencies
and selection of pigs will depend on the operator's philosophy, the degree
and rate of deposition on the pipe wall, and governing critical constraints.
Probably the greatest use of pigs in flexible pipe occurs during factory
release testing (for pipes on storage reels) or during system hydrotesting. Pigs
are used (principally for non-bonded pipes) for filling and dewatering pur-
poses as well as to determine pipe obstructions. In non-bonded pipe, the
inner liner (polymer) or carcass (steel) is not formed around a fixed mandrel
as with some bonded pipes, and therefore some i.d. variations can exist. Also,
when pressurizing/depressurizing a pipe, air can pass through the gaps in the
carcass structure, making it not always possible to remove entrapped air.
Pigging is therefore used to improve air-removal operations and following
pressure test completion, to dewater long-length flowlines.
When considering pig selection, it is important to note certain factors
concerning the construction of flexible pipes. Firstly, there will be variations
in i.d. along the bore of the steel pipe/flexible pipe route. The manufactured
diameter of flexible pipe generally comes in even numbers (e.g. 2in, 4in, 6in)
and tolerances on i.d. are much tighter than for steel pipe, typically 2-3% or
less. This fact means that at end connector areas, restrictions to pigging could

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