Page 164 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

142 CORROSION CAUSES

program may protect against these environmental conditions and permit continued
operation of the pipeline. On the basis of severity and density of the stress corrosion
cracks, however, pipe replacement may be the most economical option.
Replacement/rehabilitation decisions involve several considerations. These
considerations are terrain conditions, expected or required life, excess capacity
and throughput requirements, internal versus external corrosion, and so on. A
comprehensive list of considerations for pipeline rehabilitation is given below (13).
1. The location of the pipeline is important for repair considerations. A pipe in
swampy clay would exclude recoating versus a repair option. On the contrary,
prairies are conducive to recoating, with firm footing for the equipment and
good accessibility.

If the expected life of a section of pipeline is relatively short, the pipeline operator
must decide whether recoating and repair would extend the life of the pipe section
to match the rest of the pipeline. If not, replacing the pipe section may be the best
solution.

3.8.7 Corrosion-Related Failures
If corrosion is allowed to continue unabated, the integrity of the pipeline will even-
tually be compromised. In other words, the pipeline will fail. Depending on the flaw
size, pipeline material properties, and the pipeline pressure, the failure refers to either
a leak or a rupture. Typically, rupture of a high-pressure natural gas pipeline results in
the release of so much stored energy (compressed gas) that the pipeline is blown out
of the ground. A leak results when the flaw penetrates the pipeline but is not of suffi-
cient size to cause a rupture. The leaks in natural gas pipelines are detected by either
periodic inspections or third-party reporting and are repaired without any significant
event. However, leaks can result in substantial problems if not detected promptly. Nat-
ural gas leaks can fill enclosed or confined spaces, and if an ignition source is present,
explosions and/or fires can result, causing significant property damage and injuries
or deaths. For natural gas leaks or ruptures, the immediate environmental impact is
minimal.
A liquid pipeline has less stored energy than a gas pipeline, and a rupture does
not cause an explosion. However, an explosion can occur on ignition of an explosive
product. In the case of a hazardous liquid product pipeline, the environmental impact
can be as serious as an explosion. The risk of an oil leak from the Trans-Alaska
Pipeline System has continued to be the primary driver for the aggressive corrosion
prevention and inspection program maintained by the operator. Of major concern is
the risk of oil leakage into water streams and thereby contaminating water supplies.
Corrosion-induced pipeline failures can result in the following:

1. Loss of product
2. Property damage
3. Personal injury or death