An unintentional release of 165 MMcf of natural gas that burst into flames and damaged a portion of the Leach XPress pipeline in West Virginia, burning more than $437,000 worth of fuel, was likely caused by a landslide, Columbia Gas Transmission LLC (TCO) said in an incident report submitted to the the Pipeline and Hazardous Materials Safety Administration (PHMSA).

Most of the pipeline has been down since the incident in early June for repairs. The report, submitted late last month, said the TCO control room received a low pressure alarm at 4:16 a.m. ET on June 7, when personnel were dispatched to investigate. About an hour later, a 14.35 mile segment of the 1.5 Bcf/d, 36-inch Leach XPress line in Marshall County running from upstream valve LEX-500 to the downstream LEX-600 valve was shut-in.

The gas caught fire and exploded, ejecting 83 feet of the system from the ditch into the right-of-way, scorching nearby trees and vegetation in the rural area near milepost 20.6 about seven miles from Moundsville. There were no injuries. The actual cause has not been determined, and it’s unclear how long the ongoing investigation might take, a PHMSA spokesperson said.

In a notice of proposed safety order (NOPSO) issued this week, PHMSA instructed TCO to submit a root cause failure analysis and a remedial work plan within 90 days of receiving a final safety order. “As a result of the investigation, it appears conditions exist on your pipeline system that pose an integrity risk to public safety, property or the environment,” the NOPSO said.

The order said TCO parent TransCanada Corp. has identified six other points along the pipeline with similar geological conditions, where steep slopes and indications of slips exist, posing the risk of additional failures. The company has already done minor repair work and grading at the failure site to address the issue, PHMSA said.

The affected segment was built last year. Leach XPress came online in January to expand TCO and move Appalachian natural gas primarily to the Southeast and Gulf Coast via an interconnect with the broader system and TransCanada’s Rayne XPress, which entered service last year.

TCO said in an update Thursday Leach’s Stagecoach meter has again returned to service, restoring limited capacity. After acknowledging last week that “weather in the region has continued to create challenging conditions during the remediation process,” the company also said it expects repairs to be complete and service to return on the full line by Sunday (July 15), pending PHMSA’s approval.

TransCanada has 30 days from the time it received the July 9 NOPSO to submit a written response.

The order proposes several actions to address the potential risks associated with continued operations, including mechanical and metallurgical testing and analysis of the failed section; installing strain gauges in the vicinity of the failure; hydrostatic testing of any new pipe installed; and reviewing past records.

While the explosion periodically stirred up the Appalachian spot market in the days after it occurred, most of the gas has been rerouted from TCO onto other pipes, mitigating the effects on production in the region.