Artikel 6 Pipa Bawah Laut
Pipeline Pigging
What are pigs and pipeline pigging?
A pig is a device inserted into a pipeline which travels
freely through it, driven by the product flow to do a specific task within
the pipeline. These tasks fall into a number of different areas: (a)
Utility pigs which perform a function such as cleaning, separating products
in-line or dewatering the line; (b) Inline inspection pigs which are used
to provide information on the condition of the pipeline and the extent and
location of any problem (such as corrosion for example) and (c) special
duty pigs such as plugs for isolating pipelines.
Figure 1. Pigging (http://ppsa-online.com/_)
Figure 2. Pig launcher (http://smartpigs.net/how-they-work.html)
Why is it called “pigging”?
One theory is that two pipeliners were standing next to
a line when a pig went past. As the pig travelled down the line pushing
out debris, one of them made the comment that it sounded like a pig squealing.
The pig in question consisted of leather sheets stacked together on a steel
body. Without doubting the authenticity of the story, it does indicate that
these tools have been around for some time. Another theory is that PIG stands
for Pipeline Intervention Gadget.
What is the purpose of pigging?
Pipelines represent a considerable investment on behalf of
the operators and can often prove strategic to countries and governments. They
are generally accepted as being the most efficient method of transporting
fluids across distances. In order to protect these valuable investments,
maintenance must be done and pigging is one such maintenance tool.
During the construction of the line, pigs can be used to remove debris that
accumulates. Testing the pipeline involves hydro-testing and pigs are used
to fill the line with water and subsequently to dewater the line after the
successful test. During operation, pigs can be used to remove liquid hold-up
in the line, clean wax off the pipe wall or apply corrosion inhibitors for
example. They can work in conjunction with chemicals to clean pipeline from
various build-ups.
Inspection pigs are used to assess the remaining wall thickness and extent
of corrosion in the line, thus providing timely information for the operator
regarding the safety and operability of the line. Pigs (or more specifically)
plugs can be used to isolate the pipeline during a repair.
Smart pigs also are commonly used today to detect various elements
about the pipeline. As mentioned earlier, inspection pigs collect
information regarding temperature and pressure, corrosion/metal loss,
diameter, bends and curvature to verify pipeline integrity. Cracks, weld
defects, surface pitting and areas of crushing/deformation are often
measured through pigging as well.
Because of the variety of environments that pipelines are found in, including underground or underwater, and the construction materials of most pipelines, pigs must record their data internally until it can be retrieved by the user. Retrieving and monitoring where certain data was collected5 is done through surface instruments using audible, magnetic or gravimetric methods to record where the pig is at various points. The user then uses this information to map out defects to aid repair crews in maintenance.
Some new, “smart” pigs have GPS capabilities that can assist in mapping a pipeline. This helps maintenance crews save time and money by pin-pointing exactly where is a pipeline is run, instead of having to excavate a large area to reach a specific location in the line.
Because of the variety of environments that pipelines are found in, including underground or underwater, and the construction materials of most pipelines, pigs must record their data internally until it can be retrieved by the user. Retrieving and monitoring where certain data was collected5 is done through surface instruments using audible, magnetic or gravimetric methods to record where the pig is at various points. The user then uses this information to map out defects to aid repair crews in maintenance.
Some new, “smart” pigs have GPS capabilities that can assist in mapping a pipeline. This helps maintenance crews save time and money by pin-pointing exactly where is a pipeline is run, instead of having to excavate a large area to reach a specific location in the line.
Figure 3 Smart pigs (http://www.centralmaine.com/2013/03/31/pipeline-company-points-to-record-of-safety-in-tar-sands-oil-debate_2013-03-31/)
What inspection Techniques are there?
The main inspection methods that are used are MFL (Magnetic
Flux Leakage) and UT (Ultrasonics). MFL is an inferred method where a strong
magnetic flux is induced into the pipeline wall. Sensors then pick up any
leakage of this flux and the extent of this leakage indicates a flaw in the
pipe wall. For instance, internal material loss in the line will cause flux
leakage that will be picked up by the sensors. Defect libraries are built
up to distinguish one defect from another.
Ultrasonic inspection is a direct measurement of the thickness of the pipe
wall. A transducer emits a pulse of ultrasonic sound that travels at a
known speed. The time taken for the echo to return to the sensor is a
measurement of the thickness of the pipe wall. The technique needs a
liquid through which the pulse can travel. The presence of any gas will
affect the output.
How often should a pipeline be pigged?
Pigging frequency depends largely on the contents of the
pipeline. Some sales gas pipelines for example are normally never pigged.
This is since there is little by way of liquid to remove or debris / corrosion
products in the line. On the other hand, production oil lines can suffer from
wax deposition, which must be managed in order to allow production to continue.
It is difficult to give general guidance on this, as the pigging frequency must
be set for each specific pipeline. The general advice would be that a pig is
a valuable flow assurance tool and a decision should be reached with the operator
on the frequency of pigging based on the flow assurance analysis of the line and
in conjunction with the pigging specialists. Likewise, inspection intervals
should be based on discussions between integrity management and the pig vendors.
Sources:
Komentar
Posting Komentar