Page 187 - CW E-Magazine (10-9-2024)

Special Report

hesive properties are less likely the reliability of the pipeline increases of safely removing near-critical axial
to be affected by the mechanical dramatically. defects, such as SCC, from both natural
action of soils, which expand and gas and liquid hydrocarbon pipelines.
contract during periods of wet/dry Cathodic Protection (CP) Usually, pipeline companies use hydro-
or freeze/thaw cycles. They will be CP criteria and procedures should static retesting for a variety of reasons,
better able to resist the effects of conform to applicable regulations and including:
water vapour transmission through standards in NACE Standard RPO169 To qualify a section of the pipe-

the coating. or the Canadian Gas Association (CGA) line for higher maximum operating
Recommended Practice, OCC-1. pressures.
Low water permeability: Water va- To qualify a section of pipeline for a
pour transmission into the coating The recommended polarized or change of service.
may result in coating disbondment “off” potential range for the operation of To confirm the integrity of a section

where less than optimal coating CP systems is between -850 mV and – of the pipeline from potential time-
adhesion is present. 1100 mV. It is recommended to avoid dependent threats such as corrosion
potentials more negative than -1100 mV (both external and internal), SCC,
Effective electrical insulation: because they may promote hydrogen construction damage, and manufac-
Coatings that provide good electri- generation, which can lead to turing defects.
cal insulation perform well if coat- hydrogen embrittlement of the pipe
ing adhesion is good. However, if material, blistering and ultimately Since pipeline integrity is much
the coating disbonds, and therefore shielding the coating materials. more likely to be affected by longitudi-
shields the CP current, isolated cor- nally oriented defects than by circum-
rosion cells form, which causes The 100 mV shift criterion, outlined ferentially oriented defects, hydrostatic
high levels of localized corrosion. in both NACE Standard RP0169 and testing is one of the best ways to dem-
OCC-1, may promote potential in the onstrate the integrity of a pipeline. End-
Abrasion and impact resistance: cracking range for high-pH SCC. There- users need to examine their specific situ-
Pipe contact with mechanical fore, this criterion should be applied ation when developing a hydrostatic
equipment or rocks could create with caution when associated with high- retest procedure.
coating damage. Good abrasion and pH SCC conducive environments.
impact resistance will minimize this The following factors should be
damage. Pipe-to-soil potentials will be mea- considered when developing a hydro-
sured with a Cu/CuSO4 half-cell elec- static retest procedure:
Temperature effects/sufficiently trode (or alternative with the appro- Test appropriateness;
ductile: Coatings must remain suf- priate potential adjustment). The CP Level of safety factor to establish in

ficiently ductile to resist cracking in currents will instantaneously be inter- the pipeline segment;
the range of temperatures expected rupted on the protected structure. All Test pressure level;

to be encountered during pipe bend- measurements must be free of induced Actual yield strength test;

ing, handling, installation, and the AC or DC interference from nearby Mill defects and test pressure;

pipeline’s operational life. foreign structures or foreign CP systems. Retest frequency; and
Safety and safety factors.
Retention of mechanical/physical Seasonal variations can also influ-
properties: Over time, a coating’s ence ground conductivity by variation Repairs
properties (like tensile, hardness, in soil moisture content. The corro- Repairs play a crucial role in the
elongation) may change while in sion personnel charged with adjusting mitigation of SCC defects. Pipe or
service. For example, polyethylene and monitoring individual CP systems pipelines containing SCC defects can
tapes have been observed to stretch should modify these potential criteria to be repaired using one or more of the
over time, and old Fusion Bonded account for these variables. acceptable repair methods given in the
Epoxy coatings have been known to CSA Z662 code.
become brittle. Hydrostatic retesting
Through operating experience and Grinding and buffing
If the coating selection contains research, hydrostatic retesting has been SCC cracks should not be removed
the characteristics mentioned above, shown to be a very effective means by grinding on a high-pressure pipe.

Chemical Weekly September 10, 2024 187

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