Pipeline Failure Case Report: TSB P22H0023 – External Corrosion Rupture
1. What happened​
​
On 7 April 2022, the NPS 8 Simonette Lateral pipeline operated by NOVA Gas
Transmission Ltd. ruptured near kilometre post 19.764 while transporting sweet natural gas at an
operating pressure of about 5074 kPa. Approximately 3.75 million cubic metres of gas were
released and immediately ignited, producing a fire that burned for roughly four hours until
upstream and downstream valves were manually closed. The rupture created an eighteen-metre
by thirteen-metre crater, damaged an out-of-service four-inch pipeline located above it, burned
close to twelve thousand square metres of vegetation, and temporarily interrupted supply to one
customer. No injuries were reported.
2. What was in place before the failure​
​
The pipeline used field-applied polyethylene tape coating installed in 1969. Corrosion
control depended on an impressed-current cathodic protection system designed to maintain
“on-potentials” more negative than −950 mV. The line was also electrically bonded to a nearby
four-inch pipeline to limit stray-current interactions. Integrity management included a
magnetic-flux-leakage in-line inspection conducted in 2016, which did not identify metal-loss
features near the eventual rupture location. Annual CP surveys and diagnostic tests were
performed, although the critical electrical bond was not monitored monthly as required by
internal procedures due to limited technician availability.
3. Why it failed​
​
The rupture was caused by severe external corrosion that developed under deteriorated
polyethylene tape. Laboratory analysis showed large areas of coating disbondment and bare steel
exposure, confirming that the tape had lost adhesion over much of the pipe surface. These
disbonded regions created shielded micro-environments that trapped moisture and prevented
cathodic-protection current from reaching the steel. CP survey values in the years before failure
ranged from −849 mV to −718 mV, which did not meet the −950 mV criterion and indicate
insufficient polarization. The electrical bond between the two pipelines had also become loose,
allowing stray current to leave the NGTL pipe and creating anodic zones that further accelerated
corrosion. By the time of failure, metallurgical examination showed only 0.49 mm of wall
thickness remaining at the rupture origin, compared with the original 3.96 mm. The resulting
burst capacity of about 4683 kPa was below the operating pressure.
4. What should be done​
​
The coating deterioration played a major role in the failure, as laboratory results showed
25 to 35 percent bare steel and up to 75 percent disbondment. Modern bonded coatings such as
fusion-bonded epoxy or multi-layer polyethylene should be used in any future rehabilitation,
since these systems prevent the shielding effects that allow corrosion to persist beneath the tape.
The CP readings near the rupture consistently failed to reach the −950 mV protection
criterion, showing that the exposed steel was under-protected. When CP values fall outside the
required range, field excavation should be mandatory so coating condition and wall loss can be
verified directly. Additional anodes or higher rectifier output may also be required in segments
with elevated current demand.
The loose bond at the pipeline crossing allowed stray-current discharge, which intensified
corrosion at the damaged coating. Critical electrical bonds should therefore be inspected at the
prescribed frequency, and remote monitoring should be added where feasible to ensure
continuous electrical continuity.
Stray-current interaction between the NGTL line and the nearby four-inch pipeline
clearly contributed to metal loss at the crossing. At locations where multiple pipelines share a
corridor, interference testing should be carried out regularly so that current flow paths are
understood and CP settings can be adjusted to maintain stable protection levels.
Reference
Transportation Safety Board of Canada. Pipeline Transportation Safety Investigation Report
P22H0023: Pipeline Rupture and Fire – NOVA Gas Transmission Ltd., 8-inch Simonette Lateral
Natural Gas Pipeline, Near Fox Creek, Alberta, 07 April 2022. Released 09 January 2024.
Pipeline transportation safety investigation report P22H0023 - Transportation Safety Board of
Canada