Page 71 - Pipeline Pigging Technology

Pipeline Pigging Technology

limited or zero by-pass;
vendor's detailed drawings should be submitted with quotations;
valves should be designed to be suitable for vacuum drying or resistant
to glycol drying if necessary.

Pipe internal diameter

The pipe internal diameter should be kept constant. The wall thickness of
the pipeline determines the internal diameter of all pipeline components
(valves, bends, tees, flanges, etc.).
The wall thickness changes for road and river crossings as well as for
platform risers should be studied to assess the feasibility of adding extra
thickness to the outside wall to accommodate the greater strength require-
ments at these locations. Maximum deviation of internal diameter from the
nominal should be kept to below the figures given in the following table:

Nominal diameter (in) Maximum deviation (mm)

4 4
6 6
8-12 10
14-20 14
20-36 16
36 and over 20

Any internal diameter changes should be made with a transition piece of
1:5 minimum slope. Special care should be taken with the pipeline design
where diameter changes occur towards the ends of gas pipelines.

Pig-trap stations

Pig-trap stations can be subdivided into groups:

permanent stations for onshore pipelines;
portable stations for onshore pipelines;
permanent topside stations for offshore pipelines; and
permanent subsea stations for offshore pipelines.

Permanent pig-trap stations for onshore pipelines differ mainly in layout
from those for topsides' installation offshore due to space limitations. Simi-

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