In the United States, millions of miles of
pipeline carrying everything from water to crude oil. The pipe is vulnerable to
attack by internal and external corrosion, cracking, third party damage and
manufacturing flaws. If a pipeline carrying water springs a leak bursts, it can
be a problem but it usually doesn't harm the environment. However, if a
petroleum or chemical pipeline leaks, it can be a environmental disaster. More
information on recent US pipeline accidents can be found at the National Transportation Safety Board's Internet site,In an attempt to keep
pipelines operating safely, periodic inspections are performed to find flaws
and damage before they become cause for concern.
When a pipeline is built, inspection
personnel may use visual, X-ray, magnetic particle, ultrasonic and other
inspection methods to evaluate the welds and ensure that they are of high
quality. The image to the left show two NDT technicians setting up equipment to
perform an X-ray inspection of a pipe weld. These inspections are performed as
the pipeline is being constructed so gaining access the inspection area is not
problem. In some areas like Alaska, sections of pipeline are left above ground
like shown above, but in most areas they get buried. Once the pipe is buried,
it is undesirable to dig it up for any reason.
So, how do you
inspect a buried pipeline?
Have you ever felt the ground move under
your feet? If you're standing in New York City, it may be the subway train
passing by. However, if you're standing in the middle of a field in Kansas it
may be a pig passing under your feet. Huh??? Engineers have developed devices,
called pigs, that are sent through the buried pipe to perform inspections and
clean the pipe. If you're standing near a pipeline, vibrations can be felt as
these pigs move through the pipeline. The pigs are about the same diameter of
the pipe so they range in size from small to huge. The pigs are carried through
the pipe by the flow of the liquid or gas and can travel and perform
inspections over very large distances. They may be put into the pipe line on
one end and taken out at the other. The pigs carry a small computer to collect,
store and transmit the data for analysis. In 1997, a pig set a world record
when it completed a continuous inspection of the Trans Alaska crude oil
pipeline, covering a distance of 1,055 km in one run.
Pigs use several
nondestructive testing methods to perform the inspections. Most pigs use a
magnetic flux leakage method but some also use ultrasound to perform the
inspections. The pig shown to the left and below uses magnetic flux leakage. A
strong magnetic field is established in the pipe wall using either magnets or
by injecting electrical current into the steel. Damaged areas of the pipe can
not support as much magnetic flux as undamaged areas so magnetic flux leaks out
of the pipe wall at the damaged areas. An array of sensor around the
circumference of the pig detects the magnetic flux leakage and notes the area
of damage. Pigs that use ultrasound, have an array of transducers that emits a
high frequency sound pulse perpendicular to the pipe wall and receives echo
signals from the inner surface and the outer surface of the pipe. The tool
measures the time interval between the arrival of a reflected echos from inner
surface and outer surface to calculate the wall thickness.
On some pipelines it is easier to use remote
visual inspection equipment to assess the condition of the pipe. Robotic
crawlers of all shapes and sizes have been developed to navigate the pipe. The
video signal is typically fed to a truck where an operator reviews the images
and controls the robot.
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