In its investigation report, the Transportation Safety Board of Canada (TSB) found that stress corrosion cracking led to a pipeline rupture resulting in a fire near Prince George, British Columbia in 2018.
Deficiencies in predicting the extent of cracking and a deferred inspection led to a hazard being undetected prior to the pipeline rupture.
On 9 October 2018, a 36-inch natural gas pipeline operated by Westcoast Energy Inc. ruptured about 13 km northeast of Prince George. Following the rupture, the natural gas being transported was released and ignited, resulting in a fire. There were no injuries; however, 125 people within a 2 km radius of the occurrence location were evacuated as a precaution.
The investigation found that the pipeline ruptured due to stress corrosion cracks on the outside surface of the pipe; and that the polyethylene tape coating applied to the exterior surface of the pipe as a measure to protect it from corrosion deteriorated over time.
This allowed soil moisture to come into contact with the pipe surface, leading to corrosion and cracking. Growing and merging over time, the cracks reduced the load-bearing capacity of the pipeline at normal operating pressures.
The pipeline operator had a stress corrosion cracking hazard management plan in place for this pipeline. However, the extent of the existing cracking on the segment of pipe that ruptured was not identified. The model used to predict crack growth did not take into account all potential uncertainties in the predicted crack growth.
This resulted in cracks growing at higher rates than the model predicted. Additionally, an inspection of this pipeline segment scheduled for 2017 was deferred until the fall of 2018. As such, the existing cracks remained undetected.
Although emergency response activities were successful in mitigating the impacts of the occurrence, the investigation determined that during periodic emergency response exercises in the 4 years prior to the occurrence, not all nearby communities and operators of nearby pipelines were included.
If pipeline emergency response exercises are not conducted periodically with all potentially affected stakeholders, gaps in emergency response plans may not be identified, thereby increasing the risk that all parties will not be sufficiently prepared to respond to a pipeline emergency.
Following the occurrence, the TSB issued a Pipeline Safety Advisory Letter to Westcoast Energy Inc. regarding the management of stress corrosion cracking of susceptible pipelines. The letter suggested that the operator may wish to review its stress corrosion cracking management practices, including inspection intervals, to mitigate the risks associated with polyethylene tape-coated pipe. In response to the letter, the operator revised several aspects of its inspection practices and integrity management program for this pipeline.
The National Energy BoardFootnote1 restricted operating pressures until it approved the engineering assessments submitted by the operator to ensure safe operation of the relevant segments of the pipeline. The regulator also conducted field inspections to ensure regulatory requirements were met. ■