Page 85 - Reliability and Maintainability of In service Pipelines

74 Reliability and Maintainability of In-Service Pipelines

2.4.3 CLEANING

Regular cleaning is an industry standard for oil pipeline operators. These clean-
ings ensure that the pipeline is operating at peak efficiency and that corrosive
buildup is removed. Pipelines can be cleaned mechanically, with a tool known as
a pig, or chemically. Mechanical cleanings are the most commonly accepted prac-
tice to remove deposits within the pipeline. The pig is passed through the pipe-
line, scraping away at deposits until it is retrieved in a relatively clean condition.
Because operators usually cannot see the inside of the pipeline while cleaning,
the cleanliness of the pig was historically used as a proxy for the cleanliness of
the pipe. However, assuming the pipeline is clean when the pig is clean can be
inaccurate because the mechanical pig is not capable of perfectly removing depos-
its. As it passes through the pipe, it smears a small amount of debris along the
surface, creating a thin coating. This thin layer is compacted over multiple clean-
ings. It is possible for water and other corrosive elements to be trapped under-
neath this layer, which increases the risk of internal corrosion. The interior coat-
ing can also interfere with in-line inspection tools. The historical failures of
mechanical pigging have led to the quick adoption of chemical cleaning. Used in
conjunction with a mechanical pig, liquid chemicals can remove more debris in
fewer runs. While mechanical cleaning can be performed on both active and inac-
tive pipelines, chemical cleaning requires the pipeline to be temporarily deacti-
vated. The section to be cleaned must be closed off, after which it is be filled
with liquid chemicals. The flow rate and the pH of the chemicals are monitored
as the pipeline is cleaned—both values increase. Once the flow rate and pH level
off, the pipeline is flushed and reactivated. The chemical blends used are proprie-
tary but all follow a few key parameters. Ideal cleaning products have the follow-
ing properties: wetting, to reduce surface tension of deposits; emulsification, to
prevent hydrocarbons from redepositing down the line; detergency, to mobilize
hydrocarbon deposits; and dispersion, to keep the deposits in suspension by pre-
venting aggregation of particles.

2.4.4 CORRECTIVE ACTION

Simply, corrective action defines allowing a pipeline to run its full course until it
then fails; then once failure has occurred, continuing with a strategy to fix said
failure. Pipelines are monitored to ensure that knowledge of when failures will
occur don’t come as a complete surprise and can be attended to immediately
reduce damage costs and timeframes.