Page 42 - Pipeline Rules of Thumb Handbook

General Information 29

PIPELINE PIGGING

Sizing plates

Often the debris-removal operation, after completion of When constructing offshore pipelines, the most likely place
construction, is combined with gauging to detect dents and for a buckle to occur during the lay operation is in the sag
buckles. This operation will prove that the pipeline has a cir- bend just before the pipe touches the bottom. A gauging pig
cular hole from end to end. Typically an aluminum disc with can be placed inside the pipe and pulled along the pipe. If
a diameter of 95% of the nominal inside diameter of the pipe the lay barge moves forward and the pig encounters a buckle
is attached to the front of a pig and is inspected for marks at or dent, the pull line will become taut. This indicates that it
the end of the run. The pig could also be equipped with a will be necessary to pick up and replace the dented section
transmitter to facilitate tracking the location of the pig. If the of pipe.
pig hangs, this will facilitate easy location of the pig to locate
the dent/buckle.

Caliper pigging

Caliper pigs are used to measure pipe internal geometry. Caliper pigs can be used to gauge the pipeline. The ability
Typically they have an array of levers mounted in one of the to pass constrictions such as a dent or buckle means that the
cups. The levers are connected to a recording device in the pig can be used to prove that the line is clear with minimum
body. As the pig travels through the pipeline the deflections risk of jamming. This is particularly useful on subsea pipelines
of the levers are recorded. The results can show up details and long landlines where it would be difficult and expensive
such as girth-weld penetration, pipe ovality, and dents. The to locate a stuck pig. The results of a caliper pig run also form
body is normally compact, about 60% of the internal diame- a baseline record for comparison with future similar surveys,
ter, which combined with flexible cups allows the pig to pass as discussed further below.
constrictions up to 15% of bore.

Cleaning after construction

After construction, the pipeline bore typically contains dirt, Therefore, most pipelines will require to be clean com-
rust, and millscale; for several reasons it is normal to clean missioning. Increasingly, operators are specifying that the
these off. The most obvious of these reasons is to prevent pipe should be sand blasted, coated with inhibitor and the
contamination of the product. Gas feeding into the domestic ends capped after traction in order to minimize the post-
grid, for example, must not be contaminated with particulate construction cleaning operation. A typical cleaning operation
matter, since it could block the jets in the burners down- would consist of sending through a train of pigs driven by
stream. A similar argument applies to most product lines, in water. The pigs would have wire brushes and would permit
that the fluid is devalued by contamination. some bypass flow of the water so that the rust and millscale
A second reason for cleaning the pipeline after construction is dislodged by the brushing would be flushed out in front of
to allow effective use of corrosion inhibitors during commis- the pigs and kept in suspension by the turbulent flow. The
sioning and operation. If product fluid contains corrosive pipeline would then be flushed and swept out by batching pigs
components such as hydrogen sulfide carbon dioxide, or the until the particulate matter in the flow has reduced to accept-
pipeline has to be left full of water for some time before it can be able levels.
commissioned, one way of protecting against corrosive attack is Following brushing, the longer the pipeline the longer it
by introducing inhibitors into the pipeline. These are, however, will take to flush and sweep out the particles to an acceptable
less effective where the steel surface is already corroded or level. Gel slugs are used to pick up the debris into suspen-
covered with millscale, since the inhibitors do not come into inti- sion, cleaning the pipeline more efficiently. Gels are specially
mate contact with the surface they intended to protect. formulated viscous liquids that will wet the pipe surface, pick
Thirdly, the flow efficiency is improved by having a clean up and hold particles in suspension. A slug of gel would be
line and keeping it clean. This applies particularly to longer contained between two batching pigs and would be followed
pipelines where the effect is more noticeable. by a slug of solvent to remove any traces of gel left behind.