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Stress corrosion cracking led to pipeline explosion near Prince George in 2018

The Transportation Safety Board of Canada (TSB) has determined that stress corrosion cracking led to a pipeline rupture resulting in a fire near Prince George in 2018.

The resulting damage to the natural gas pipeline caused shortages across the province.

“Deficiencies in predicting the extent of cracking and a deferred inspection led to a hazard being undetected prior to the pipeline rupture,” according to a Transportation Safety Board report.

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 two-kilometre 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 four 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 Board 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.

“We know this incident has caused concerns and disrupted the lives of many people in the area. For that, we apologize,” said Michele Harradence, Senior Vice President and Chief Operations Officer, Gas Transmission and Midstream for Enbridge, in a news release. “We commit that we have learned from this incident and have taken steps to ensure the safety of our natural gas system.”

Since the Shelley incident, Enbridge has completed a comprehensive pipeline integrity program on its natural gas pipeline system in B.C. to prevent similar incidents from occurring and to significantly improve pipeline safety.

The program includes:

Enhanced Pipeline Inspections 

The entire T-South natural gas pipeline system has been inspected with the latest generation inline pipeline inspection tool. This tool has double the number of sensors than previous inspection tools and is significantly more accurate at assessing potential problems like stress corrosion cracking.  The T-South mainline system now is 100 per cent inspected by this tool.

Enhanced Maintenance Screening Criteria

In conjunction with these pipeline inspections, the company has implemented more comprehensive criteria to evaluate pipeline inspection data. This will identify potential risks earlier that may require monitoring and proactive maintenance work, according to the company.

Increased Integrity Digs

As a result of this enhanced screening criteria, Enbridge has increased the number of integrity dig inspections undertaken in a normal maintenance year. During an integrity dig, workers excavate a section of pipe to examine it further, validating its safety and undertaking proactive maintenance work if it is required. In 2019, Enbridge more than doubled the number of digs undertaken to validate the safety of our pipeline system compared to previous years.

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