Failures of Pipelines
| Gas and oil pipelines have
established an impressive safety record over the years. However,
failures have occurred for an assortment of reasons. Some of the causes
of failure are identified in this commentary.
Since the 1940s, all of the oil and gas
transmission lines have been built by welding. In general,
American Petroleum Institute (API) 5L specification steels are used in
pipelines. Pipeline wall thicknesses are established on the pressure
in the line and on the allowable hoop stress levels for the material.
The allowable stress levels for gas pipelines vary based on the location of
the pipeline and are regulated by the U.S. Department of Transportation
(DOT). |
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Pipelines are pressure tested in addition to nondestructive testing prior to
being put into service. Normally, pipelines are hydrostatically
stressed to levels above their working pressure and near their specified minimum
yield strength. This pressure is held for several hours to ensure that the
pipeline does not have defects that may cause failure in use. This proof
test of pipelines provides an additional level of confidence that is not
found in many other structures.
Pipeline Failure Analysis
Some of the causes of pipeline failures are listed below:
- Mechanical damage
- Fatigue cracks
- Material defects
- Weld cracks
- Incomplete fusion
- Improper repair welds
- Incomplete penetration
- External or internal corrosion
- Hydrogen blistering
Mechanical damage normally consists of gouges and dents. They generally
are created by excavation or handling equipment during construction.
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