Page 25 - Pipeline Pigging Technology

Pipeline Pigging Technology

the operation of the pipeline by blocking downstream filters, damaging pump
impellers, jamming valves open, and so on. In some instances the pipeline
operator may reason that small amounts of debris can be tolerated, but in most
cases the construction team will have to show that any debris has been
removed. The only way of doing so efficiently and convincingly is to run a pig
through the line.
Typically, once a section of pipeline has been completed, an air-driven pig
is sent through the line to sweep out the debris. The sections are kept short
so that the size of compressor and volume of compressed air are minimized.

Debris removal offshore

Offshore pipelines need to be constructed free of debris for the same
reasons as onshore pipelines. Strict control of the working practices on board
the lay barge minimizes the amount of debris entering the pipe in the first
place. The firing-line arrangement lends itself to having a pig a short distance
down inside the pipeline being pulled along by a wire attached to the barge.
As the lay barge moves forward, the pig is drawn through the pipeline driving
any debris before it.

Gauging

Often the landline debris-removal operation is combined with gauging to
detect dents and buckles. The operation proves that the pipeline has a
circular hole from one end to the other. Typically an aluminium disc with a
diameter of 95% of the nominal bore is attached to the front of the pig and is
inspected for marks at the end of the run. The pig would also carry a pinger
emitting an audible signal, so that if a dent or buckle halted the pig the
construction crew could locate it and repair the line.
Offshore, the most likely place for a buckle to develop during pipe laying
is in the sag bend just before the touchdown on the seabed. To detect this, a
gauging pig is pulled along behind the touchdown point. If the vessel moves
forward and the pig encounters a buckle, the towing line goes taut indicating
that it is necessary to retrieve and replace the affected section of line pipe.

Calliper pigging

Calliper pigs are used to measure pipe internal geometry. Typically they
have an array of levers mounted in one of the cups as shown in Fig. 1; the levers