Page 225 - Pipeline Pigging Technology

Pipeline Pigging Technology

OIL LINES

Oil pipelines represent a simple problem when compared to gas lines.
Firstly, the volume of product required to depressurize the line is very small,
meaning we can work with a totally-depressurized system without wasting
product. Secondly, if the line is decommissioned and flooded with water,
there are very few problems associated with re-commissioning, as the water
can usually be handled in the production facilities.
The options for oil lines are therefore relatively straightforward, and
depend usually on the volume of product involved.
For lines of small volume, the simplest solution is to displace the product
with water, allowing the work to take place under safe conditions. Even when
all the product has been displaced, it is prudent to utilize a low-pressure
isolation device in the form of a sphere or stopper to ensure that any
vaporisation of hydrocarbon from wax, etc., does not come into contact with
the worksite, particularly if welding is going to take place.
For larger-volume systems, the pipeline can usually be isolated locally to
prevent having to displace all the product from the line. This can be done by
displacing one or more pigs down the riser and onto the seabed with water.
It is important in this scenario to evaluate the differences in elevation of the
two ends of the line, taking into account the differing static heads caused by
having one end of the line full of oil and one full of water. Again a secondary
isolation is usually installed after cold cutting at the new valve location and
prior to welding.
Under both of these scenarios, testing of the completed works is easily
undertaken by hydrotesting. In the second case, this can be carried out with
the isolation pig still in position so that product is still kept well away from the
new works being tested.
On completion of the work, the pig can be displaced back to the worksite
by displacing with oil from the far end or, by launching another pig, the train
can be pushed out to the far end.

GAS LINES

On gas lines, the problems associated with the valve installation are much
greater. Firstly, we have to vent off large quantities of gas to reduce the
pressure in the line. Secondly, if we introduce water into the line, we have in
most instances to dry the line in order to recommission it, in order to prevent

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