Pigging
Pig is an acronym
for Pipeline Inspection Gauge.
Pigging
is an in-line inspection (ILI) method
in which devices known as “pigs” are inserted into pipelines to clean and/or
inspect the pipeline. The main benefits of pigging a pipeline are to clear the
line of debris and also to gather information about the pipeline. The user then
uses this information to map out defects to aid repair crews in maintenance.
Types of pigs
a. Utility pigs: used to clean rubbish/debris from the pipeline or to
seal the pipeline
b. Inspection pigs: used to gather data/
information such as pipeline's diameter, curvature, bends, temperature,
anomalies on the pipe internal wall, metal loss, general corrosion, erosion
corrosion, pitting, weld anomalies, hydrogen induced cracking, the presence and
location of corrosion. Smart pig is an example. Two methods used for inspection are:
i. Magnetic
flux leakage (MFL) – sends magnetic flux into the pipe walls for leak,
corrosion and flaw detection
ii.
Ultrasonic
(UT) – measure ultrasonic sound wave echoes to determine pipe wall thickness
c. Specialty pigs: used to isolate /block off a
line so that maintenance can be performed eg. plug
d. Gel pigs: gelled liquids that can be used
alone or in conjunction with other types of pigs for various procedures
including debris removal, hydrostatic testing, product separation, dewatering,
condensate removal, removing a stuck pig and more
Pig trap
Pig trap is composed of a pig
launcher and a pig receiver.
Pig launchers and receivers are
normally classified as part of the pipeline and should be designed to
accommodate the full range of pressures to the same extent as the main
pipeline. They are normally classified as pressure vessels and designed and
manufactured either to BS 5500 or ASME VIII Div 1.
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| Figure 1: Pig launcher - receiver schematic [Source: enggcyclopedia.com] |
A pig launcher and receiver
comprise the main barrel in which the pig is loaded or removed from, the opening
device to allow loading or removal and the requisite valving to either force
gas through the launcher to propel the pig along the pipeline or to push it
into the receiver and flow past it. Launchers and receivers vary from each other
only in respect of potential slopes incorporated to assist launch or recovery
and the position of the off-takes. In practice a common design can serve both to
equal effect.
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| Figure 2: Pig receiving method [Source: PARS TECHNIC] |
The operation of pig launchers and
receivers needs to be carefully controlled and safety designed into the
equipment to prevent accidental opening of the main valve whilst the launcher
or receiver is open or opening of the launcher or receiver while pressure
remains in the launcher or receiver.
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| Figure 3: Typical pig launcher [source: Jamison products] |
HSE Precautions during Pigging Operation
To ensure, healthy, safe and secure
working environment during pigging, the following precautions should be adhered
to.
i.
The contractor shall seek permit for the pigging
operation.
ii.
Contractor shall ensure availability of PPE. All
workers in the supervised area must wear the appropriate PPE.
iii.
Clear the pig trap area of any debris or sand
before the operation start.
iv.
Minimize persons in the supervised area.
v.
When opening the pig trap, care must be taken to
minimize the amount of debris falling into the sump beneath it. Plastic may be
used to contain the debris if possible.
vi.
Erect barrier and notices around the pigging
area
vii.
Control procedures
o Fully depressurize and vent
the receiver before opening the door.
o Allow five minutes for gases
and mists to clear after opening: keep clear.
o Ensure closure of pig trap
door during pigging operation.
viii.
Provide hydrocarbon detectors in the area
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