On August 1, 2019, a 30-inch Enbridge natural gas pipeline collapsed near Danville, Kentucky. The gas ignited, the pipeline exploded, 30 acres of land were set on fire, 14 homes were damaged, five homes were completely destroyed and one person was killed.
The rift released about 101.5 million cubic feet of natural gas, which ignited. The blast launched a 33.2-foot section of pipeline that landed about 481 feet away. More than 75 people had to be evacuated and six others were injured.
This week, the National Transportation Safety Board said the accident could be traced to a manufacturing defect and ineffective cathodic protection. Cathodic protection is an electrochemical process used to prevent corrosion on buried pipelines where the applied coating has been damaged and the bare pipeline metal is exposed to the soil.
Investigators said a pre-existing manufacturing defect known as a hard spot, combined with a deteriorated pipeline coating and ineffective cathodic protection, led to hydrogen-induced cracking on the pipe’s outer surface.
According to the NTSB, Enbridge’s integrity management program could not accurately assess the health of the pipeline or assess risk from interacting threats. Enbridge underestimated the risk of vulnerabilities because its processes and procedures were not consistent with industry knowledge.
Indeed, Enbridge had increased the cathodic protection voltages on the affected pipeline segment to compensate for the increased external corrosion. Buried steel pipelines corrode due to moisture and groundwater in the ground.
The NTSB issued six safety recommendations, three to Enbridge and three to the Pipeline and Hazardous Materials Safety Administration. The recommendations address incomplete assessment of risks caused by a change in gas flow direction, limitations in data analysis associated with the use of in-line inspection tools, incomplete assessment of threats and threat interactions, and missed opportunities in training and requalification practices.
The pipe was manufactured in 1957 to the then-current American Petroleum Institute Pipeline Standards. In the 1950’s, the standards did not specify negative criteria for a hard point. By today’s standards, the hard spot would have been rejected due to its size and hardness.