Page 555 - Corrosion Engineering Principles and Practice

518 C h a p t e r 1 2 C o r r o s i o n a s a R i s k 519

to be identified. The remaining strength of the corroded areas of the
pipeline should also be determined.
Once the site has been exposed, corrosion monitoring devices
(e.g., coupons, electrical resistance probes) can be installed to identify
and monitor corrosivity in the pipeline. Inline inspection results may
also provide information to assess the downstream conditions of the
pipe. Once regions most susceptible to corrosion are free of damage, it
is then considered that pipeline integrity to a large extent is assured.
Postassessment
Postassessment determines the effectiveness of ICDA for dry-gas
transmission lines based on the correlation between detected corrosion
obtained during detailed examination and ICDA predicted regions.
Reassessment intervals are determined depending on the remaining
life of the pipe and rate of corrosion growth in the pipe.
12.7.3 Hydrostatic Testing
Hydrostatic testing is an effective verification method for pipelines.
This technique is used to conduct strength tests on new pipes while
in the manufacturing process, as well as at the completion of pipeline
installation in the field prior to being placed in service. Hydrostatic
testing is also used, at times, for integrity assurance after a pipeline is
in operation. Hydrostatic tests are generally the preferred integrity
assessment method when the pipeline is not capable of being
internally inspected or if defects are suspected that may not be
detectable by ILI tools.
The hydrostatic test establishes the pressure carrying capacity of
a pipeline and may identify defects that could affect integrity during
operation. Testing is done to a pressure that is greater than the normal
operating pressure of the pipeline. This provides a margin of safety.
The test stresses the pipeline to a predetermined percentage (e.g., 93
to 100 percent) of its specified minimum yield strength (SMYS)
generally for eight hours. If SCC is suspected, the test pressure may
be increased to 100 percent of SMYS or higher for 30 minutes to an
hour. Axial flaws such as SCC, longitudinal seam cracking, selective
seam corrosion, long narrow axial (channel) corrosion and axial
gouges, which are difficult to detect with magnetic flux leakage (MFL)
pigs, are better detected with a hydrostatic test.
Although hydrostatic testing is an effective method for validating
near-term integrity, it provides limited information about the status
of integrity threats, creates environmental problems related to water
treatment, and results in potentially significant downtime, affecting
deliveries.
12.7.4 In-Line Inspection
In-line inspection (ILI) tools, also commonly called “smart” or
“intelligent pigs,” are cylinder-shaped electronic devices used by the
pipeline industry to detect loss of metal and in some cases deformations