Last year’s Merrimack Valley natural gas disaster north of Boston, which killed one man and destroyed dozens of structures, was the result of “engineering errors” by NiSource Inc. utility Columbia Gas of Massachusetts, according to federal officials.

The gas explosion on Sept. 13, 2018 rocked the Merrimack Valley, leading to a mass evacuation from Lawrence, Andover and North Andover, communities about a half hour north of Boston. The three communities near the New Hampshire border together have more than 146,000 residents.

The National Transportation Safety Board (NTSB) said Tuesday “deficiencies in management  and oversight led to overpressurization of a natural gas distribution system, which resulted in a series of fires and explosions” in the Merrimack Valley.

The accident occurred after high-pressure natural gas was released into a low-pressure gas distribution system in the northeastern part of the Merrimack Valley. One person was killed and 22 people, including three firefighters, were transported to area hospitals.

“The system over-pressure damaged 131 structures, including five homes that were destroyed,” NTSB noted.

Before the overpressurization event, a “Columbia Gas-contracted work crew, which included a Columbia Gas inspector, was performing a Columbia Gas-designed and approved pipe-replacement project at an intersection,” federal officials noted. The contracted crew was working on a tie-in project, which bypassed the existing cast iron line and directed gas pressure to a new plastic line.

The bypassed cast iron line was still connected to the primary regulator control lines, providing input to the gas pressure regulator used to control system pressure, investigators noted. When the contractor crews disconnected the cast iron line, the section containing the control lines began losing pressure. Since the gas regulators did not sense system pressure, they responded by opening fully, allowing high-pressure gas to enter the low-pressure system.

“Catastrophic tragedies like this should never happen,” said NTSB Chairman Robert Sumwalt. “When tackling major repair work that has the potential to impact a community, all precautions and planning should be considered and coordinated before, during and after the work has been done.”

During the course of the investigation, the NTSB found Columbia Gas of Massachusetts should have first relocated the control lines to the new plastic main line after the existing cast iron main line was replaced. The NTSB noted, as part of its probable cause, that a low-pressure gas distribution system designed and operated without adequate overpressure protection contributed to the accident.

As a result of the investigation, the NTSB issued five safety recommendations with two issued to the Pipeline and Hazardous Materials Safety Administration, one to the Commonwealth of Massachusetts Executive Office of Public Safety, and one to NiSource.  Thirty-one states each received one safety recommendation. 

The recommendations address safety issues including adequacy of gas regulations, project documentation, project management, risk assessment, emergency response and safety management systems.

An abstract is available of the final report, which includes the findings, probable cause and all safety recommendations.

In July, Columbia Gas of Massachusetts and NiSource agreed to settle class action lawsuits triggered by the explosion. According to NiSource, about $1 billion has been dedicated to address the needs of affected customers, residents and communities. Another $143 million was paid into a settlement fund for the class action settlement.