Page 163 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

CORROSION OF UNDERGROUND PIPELINES 141

1. Severe corrosion damage of a pipeline not properly cathodically protected
2. Severe coating deterioration leading to increased CP requirements.
3. SCC along a large area of the pipeline.

3.8.3 Pipeline Integrity Management Programs
Pipeline integrity management programs are used by pipeline operators to determine
the locations in which corrosion defects present a threat to safe operation. Repairs at
these locations can vary from the installation of a reinforcing sleeve to the implemen-
tation of a large-scale pipe rehabilitation or replacement program. In the case of local-
ized corrosion flaws, the repairs might involve composite sleeves, full-encirclement
steel sleeves, or replacement of a pipe segment. For local flaws, decisions regarding
the repair process can be usually handled by company procedures and criteria. In
the case of large-scale corrosion and/or coating deterioration problems, the replace-
ment/rehabilitation decision must take into account both operational and economic
factors.

3.8.4 In-line Inspections
In-line inspections (ILIs) are widely used to generate a profile of defects present in
a pipeline. The high-resolution UT and MFL ILI tools may be used to determine the
geometry and the orientation of corrosion defects. These routine inspections may be
used to determine the number and the locations of near-critical flaws that should be
immediately examined such as a dig program to verify the flaw followed by repair.
By using appropriate corrosion growth models, predictions can be made on future
dig/repair/and/or reinspection requirements for the ILI-inspected line. If the density
of corrosion defects is high or the potential exists for continued increase in dig/repair
frequency, the affected pipe section may be repaired or replaced.

3.8.5 Aging Coating
Another concern related to corrosion assessment is the cost of maintaining the
required level of CP. The effectiveness of the CP system can be verified using
corrosion surveys. An increased number of coating defects requires an increased
amount of CP current. This is accomplished by increasing current output of the
impressed current rectifiers, installing impressed current rectifiers at more locations
along the pipeline, or installing additional sacrificial anodes. Coating defects can be
identified by conventional potential surveys or by specific coating defect surveys
and verified by direct visual inspection (dig program). Under certain circumstances,
coatings fail in a manner that makes assessment of the corrosion condition of pipe
through conventional surveying methods difficult. Aging coating and the associated
defects can make the continuous need for CP upgrading uneconomical.

3.8.6 Stress Corrosion Cracking
The presence of extensive SCC may qualify a pipeline for replacement or rehabili-
tation. As SCC depends on unique environmental conditions, a large-scale recoating