Page 26 - Pipeline Pigging Technology

Why pig a pipeline?

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

Cleaning after construction

After construction, the pipeline bore typically contains dirt, rust, and
millscale; for several reasons it is normal to clean these off. The most obvious
of these is to prevent contamination of the product. Gas feeding into the
domestic grid, for example, must not be contaminated with participate
matter, since it could block the jets in the burners downstream. A similar
argument applies to most product lines, in that the fluid is devalued by
contamination.
A second reason for cleaning the pipeline after construction is to allow
effective use of corrosion inhibitors during commissioning and operation. If
the product fluid contains corrosive components such as hydrogen sulphide
or carbon dioxide, or the pipeline has to be left full of water for some time
before it can be commissioned, one way of protecting against corrosive attack
is by introducing inhibitors into the pipeline. These are, however, less
effective where the steel surface is already corroded or covered with millscale,
since the inhibitors do not come into intimate contact with the surface they
are intended to protect.
Thirdly, the flow efficiency is improved by having a clean line and keeping
it clean. This applies particularly to longer pipelines where the effect is more
noticeable.
It will be seen from the above that most pipelines will require to be clean
for commissioning. Increasingly, operators are specifying that the pipe
should be sand blasted, coated with inhibitor and the ends capped after
construction in order to minimize the post-construction cleaning operation.
A typical cleaning operation would consist of sending through a train of pigs
driven by water. The pigs would have wire brushes and would permit some
by-pass flow of the water so that the rust and millscale dislodged by the