Page 246 - Pipeline Pigging Technology

Pigging and chemical treatment

prevented deposit formation. In addition, the pump pressure required to
move fluids through the line was nearly five times greater than that required
for a clean line.
Pigging was used to remove the paraffin deposits to prepare the line for a
corrosion survey by an intelligent pig. A premium was placed on ensuring
minimum risk to the line due to sticking a pig during removal of the paraffin
deposits, as this would have shut down the field. A progressive pigging
programme was developed to gradually remove deposits in a controlled
manner. Foam pigs were selected, as they can easily deform to accommodate
diameter restrictions. Further, with application of sufficient differential
pressure, foam pigs will compress and by-pass major obstructions. Soft
undersized foam pigs were used to start with, building up to harder and
tougher pigs as the line was progressively cleaned. Once a series of foam pigs
had been run, a pressure by-pass pig and several other mandrel pigs were used
in the final cleaning process.
Once the line was cleaned, it was found that a paraffin-treating chemical
was still required to prevent paraffins from clogging instrumentation and
sampling ports. A final programme was developed in which periodic pigging
was used in combination with chemical injection to maintain the line in good
condition.

CORROSION CONTROL IN PIPELINES

Corrosion is the most serious problem associated with pipeline mainte-
nance. There are enormous sums of money spent each year on prevention,
monitoring, inspection and repair of corrosion-related damage. Most corro-
sion programmes are treated chemically with inhibitors, which are used to
form a protective layer on the walls of the pipe by adhering to the metal or
corrosion product layer such as iron carbonate or iron sulphide. Corrosion
inhibitors come in several basic types, such as oil-soluble water-dispersible,
water-soluble, limited-solubility (gunkers), and volatile, and each performs
uniquely in different pipeline conditions. Inhibition can be applied in a batch
procedure where the persistent nature of a heavy protective film may last for
weeks or months. Or, inhibitors can be continuously injected into the
pipeline in low concentrations through a continuous injection programme,
where a thin film is gradually laid down and maintained over time. The
chemicals work very well, provided that an effective film can be established
through proper application.

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