Page 186 - CW E-Magazine (10-9-2024)

Special Report

able for an SCC condition assessment potential (CIP), pipeline current the material properties, and the resul-
can be summarized as follows: mapping, and soil resistivity mea- tant tolerable SCC size.
 Single SCC features that fail can be surements are conducted.
characterised entirely as to the failure 3. Direct examination – pipe is veri- Some analytical models allow for a
pressure, pipe material properties, fi ed for the above-ground survey determination of the mode of failure for
physical SCC dimensions, and inter- results and physical inspection of the SCC feature – a leak or a rupture. At
action. the pipe coating, pipe surface, and pressures associated with both normal
 A minimum failure pressure for soil electrolytes. operation and higher pressures for SCC
SCC features can be calculated 4. Post-assessment – results gathered in pressure testing, the metal ligament
for the remaining pipe segment by steps 1, 2, and 3 are integrated for an joining the tip of the SCC feature (in
assuming that the most severe feature overall integrity assessment, valida- the depth direction) and the inner pipe
has failed; however, this pressure is tion of the external ECDA process, wall will typically fail in a ductile man-
typically at or below the operating and determination of the interval ner, momentarily creating a through-
pressure. period to repeat the ECDA process. the-wall feature.
 The quantity, density, and location
of SCC features that do not fail Condition assessment of SCC severity Axial propagation of the resultant
within a pipe segment remains The severity of individual SCC fea- leak may or may not occur, depending
unknown. tures is defi ned by the calculated failure on the axial length of the leak, the
 Probably the largest source of pressure of that feature. The minimum applied stress, and the toughness of the
uncertainty is the assumed crack SCC failure pressure for a pipe seg- steel. However, axial propagation is
growth rate. However, errors also ment can be directly determined from typical of SCC features of the genera-
can be introduced from the calcu- the failure pressure of an SCC feature tion of pipe where SCC has been identi-
lations of the crack sizes. To be failing during an SCC pressure test or fi ed, resulting in a rupture as the normal
conservative, it is important not to in-service failure. mode of failure.
underestimate the size of the most
signifi cant surviving fl aw and not In addition, failure pressures can SCC mitigation methods
to overestimate the size of a critical be calculated from measurements There are many SCC prevention
fl aw at operating pressure(7, 8). obtained at an excavation of the pipe- methods available in the industry.
line where SCC has been documented However, the current article highlights
SCC condition assessment methodo- or from SCC ILI data where features the most popular methods(7, 8).
logies have been identifi ed (8).
External Corrosion Direct Assess- Coatings
ment (ECDA) methodology is men- SCC in association with other features Inadequate coating performance is
tioned in the NACE International stan- The accuracy of the failure pressure the central contributor to pipeline SCC
dard practice SP05021 for integrity calculation is dependent on the accu- susceptibility. The pipeline company
assessment of external corrosion threats racy of the calculation of stress, which should emphasize coating procedures so
on buried pipelines. This methodology infl uences the SCC feature. For SCC that future coating failure susceptibility,
can be used in combination or separately interacting with other integrity defects, especially disbondment, is minimized.
with NACE SP02042 on SCC direct the interacting defect should be con- When selecting the appropriate coat-
assessment methodology. sidered the primary integrity threat and ing material, a balance between perfor-
managed with the appropriate detection mance and application constraints must
The NACE ECDA protocol is a and mitigation methods for this defect. be considered. The pipeline company
four-step process: However, the discovery of SCC within must know the exposure conditions the
1. Pre-assessment – pipeline’s physi- these features may require an increase pipeline coating will face and the practi-
cal characteristics, operating his- in the risk of the interacting feature. cal challenges when applying a coating
tory, and prior inspections are for long-term performance.
documented. SCC failure pressure calculations
2. Indirect inspection – conducting There are several analytical models Coatings should possess the follow-
complementary above-ground fi eld available for determining SCC failure ing performance characteristics:
surveys like direct current voltage pressures. These methods rely on the
gradient (DCVG), close interval relationship between the applied stress,  Adhesion: Materials with good ad-

186 Chemical Weekly September 10, 2024

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