Page 85 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Background 4/63
Casings
Ground soil interface
Hot spots
Temperature
Humidity
Contaminants
Atmospheric Corrosion 0-10 pts Type, age, application of coating
A1 . Atmospheric exposures Fitness c--- Visual inspection age and results
A2. Atmospheric type Conditions Other inspection age and results
A3. Atmospheric coating
Flowstream conditions
Internal Corrosion 0-20 pts /
B1. Product corrosivity Upset conditions pH, solids, H,S, CO, MIC, etc.
Soil corrosivity -
B2. Internal protection \ Low-spot accumulatibns, equipment failure, etc.
Internal coating
Subsurface Corrosion 0-70 pts Operational measures
Monitoring
C1. Subsurface environment Resistivity, pH, moisture, carbonates, MIC, etc.
Mechanical corrosion Stress level, stress cycling, temperature, coating, CP, PH, etc
C2. Cathodic Drotection
Effectiveness
Interference potential \Test lead surveys, age, and results
C3. Coating Close spaced surveys, type, age, and results
Fitness \
\
Condition DC related
AC related
Shielding potential
Type, age, application of coating
Visual inspection age and results
Other inspection age and results
Figure 4.2 Assessing corrosion potential: sample of data used to score the corrosion index

tively rare failure mechanism for most pipelines. This is due to Presence of other buried metal
the normally slower atmospheric mechanisms and the fact that Potential for stray currents
most pipelines are predominantly buried and, hence, not Stress corrosion cracking potential
exposed to the atmosphere. The evaluator must determine if Spacing oftest leads
this is an appropriate weighting for her assessments. Inspections of rectifiers and interference bonds
Internal corrosion deals with the potential for corrosion orig- Frequency of test lead readings
inating within the pipeline. Assessment items include Frequency and type of coating inspections
Frequency and type of inspections of pipe wall
Product corrosivity Close interval surveys
Preventive actions. Use of internal inspection tools.

Internal corrosion is weighted as 20% of the total corrosion Subsurface corrosion is weighted as 70% of the total corro-
risk in the examples. This indicates that internal corrosion is sion threat in the examples of this chapter. For nonmetal lines,
often a more significant threat than atmospheric corrosion, but the evaluator may wish to adjust this weighting to better reflect
still a relatively rare failure mechanism for most pipelines. the actual hazards.
Nevertheless, some significant pipeline failures have been Note that corrosion threats are very situation specific. The
attributed to internal corrosion. The evaluator may wish to give weightings of the three corrosion types proposed here are
this category a different weighting in certain situations. thought to generally apply to many pipelines but might be ill-
Subsurface pipe corrosion is the most complicated ofthe cat- suited to others. Any ofthe corrosion types might lead to a fail-
egories, reflecting the complicated mechanisms underlying this ure under the right circumstances, even when weightings
type of corrosion. Among the items considered in this assess- suggest a relatively rare failure mechanism. The use of special
ment are a mix of attributes and preventions including: alerts or even conversions to absolute probability scales might
be appropriate, as is addressed in discussions of data analysis
Cathodic protection later in this text.
Pipeline coatings Especially in the case of buried metal, inspection for corro-
Soil corrosivity sion is commonly done by indirect methods. Direct inspection