While buildup in a pipeline can cause
transmittal slows or even plugging of the pipeline, cracks or flaws in the line
can be disastrous. A form of flow assurance for oil and gas pipelines and
flowlines, pipeline pigging ensures the line is running smoothly.
The maintenance tool, pipeline pigs
are introduced into the line via a pig trap, which includes a launcher and
receiver. Without interrupting flow, the pig is then forced through it by
product flow, or it can be towed by another device or cable. Usually
cylindrical or spherical, pigs sweep the line by scraping the sides of the
pipeline and pushing debris ahead. As the travel along the pipeline, there are
a number functions the pig can perform, from clearing the line to inspecting
the interior.
Foam pig Source:
www.pollypig.com
There are two main hypotheses for why the process is called "pipeline
pigging," although neither have been proved. One theory is that
"pig" stands for Pipeline Intervention Gadget. The other states that
a leather-bound pig was being sent through the pipeline, and while it passed,
the leather squeaked against the sides of the pipe, sounding like a squealing pig.
Engineers must consider a number of
criteria when selecting the proper pig for a pipeline. First, it's important to
define what task the pig will be performing. Also, size and operating
conditions are important to regard. Finally, pipeline layout is integral to consider
when choosing a pig.
Because every pipeline is different,
there is not a set schedule for pigging a line, although the quantity of debris
collected in a pipeline and the amount of wear and tear on it can increase the
frequency of pigging. Today, pipeline pigging is used during all phases of the
life of a pipeline.
Types of Pipeline Pigs
Although first used simply to clear
the line, the purpose of pipeline pigging has evolved with the development of
technologies. Utility pigs are inserted into the pipeline to remove unwanted
materials, such as wax, from the line. Inline inspection pigs can also be used
to examine the pipeline from the inside, and specialty pigs are used to plug
the line or isolate certain areas of the line. Lastly, gel pigs are a liquid
chemical pigging system.
Debris after piggingSource:
www.ppsa-online.com
Similar to cleaning your plumbing line, utility pigs are used to
clean the pipeline of debris or seal the line. Debris can accumulate during
construction, and the pipeline is pigged before production commences. Also,
debris can build up on the pipeline, and the utility pig is used to scrape it
away. Additionally, sealing pigs are used to remove liquids from the pipeline,
as well as serve as an interface between two different products within a
pipeline. Types of utility pigs include mandrel pigs, foam pigs, solid cast
pigs and spherical pigs.
Inspection pigs, also referred to as in-line inspection pigs or smart pigs, gather
information about the pipeline from within. . The type of information gathered
by smart pigs includes the pipeline diameter, curvature, bends, temperature and
pressure, as well as corrosion or metal loss. Inspection pigs utilize two
methods to gather information about the interior condition of the pipeline:
magnetic flux leakage (MFL) and ultrasonics (UT). MFL inspects the pipeline by
sending magnetic flux into the walls of the pipe, detecting leakage, corrosion,
or flaws in the pipeline. Ultrasonic inspection directly measures the thickness
of the pipe wall by using ultrasonic sounds to measure the amount of time it
takes an echo to return to the sensor
Specialty pigs, such as plugs, are
used to isolate a section of the pipeline for maintenance work to be performed.
The pig plug keeps the pipeline pressure in the line by stopping up the
pipeline on either side of where the remedial work is being done.
A combination of gelled liquids, gel
pigs can be used in conjunction with conventional pigs or by themselves.
Pumped through the pipeline, there are a number of uses for gel pigs, including
product separation, debris removal, hydrotesting, dewatering and condensate
removal, as well as removing a stuck pig.
Because there now exist multi-diameter
pipelines, dual and multi-diameter pigs have been developed, as well.
Source:
http://www.rigzone.com/training/insight.asp?insight_id=310&c_id=19. Accessed by 22-1-2014
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