WASHINGTON (Sept. 24, 2019) — The National Transportation Safety Board determined Tuesday that Columbia Gas of Massachusetts’ deficiencies in management and oversight led to overpressurization of a natural gas distribution system which resulted in a series of fires and explosions in Merrimack Valley, Massachusetts.
The Sept. 13, 2018, accident occurred after high-pressure natural gas was released into a low-pressure gas distribution system in the northeast region 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.
Prior to 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. The contracted crew was working on a tie-in project that 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. Once 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.”
(This photo taken Sept. 13, 2018, is an aerial view of a burned-out home impacted by the natural gas explosion and fire in Merrimack Valley, Massachusetts. NTSB photo)
In the investigation
discussed Tuesday, the NTSB found Columbia Gas of Massachusetts should have first relocated the control lines to the newly installed 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 natural gas distribution system designed and operated without adequate overpressure protection contributed to the accident.
As result a of the investigation, the NTSB issued five new 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, Inc. Thirty-one states each received one safety recommendation.
These recommendations address safety issues including adequacy of natural gas regulations, project documentation, project management, risk assessment, emergency response and safety management systems.
The urgent safety recommendations issued earlier in the investigation are available at https://go.usa.gov/xVx73.
An abstract of the final report, which includes the findings, probable cause, and all safety recommendations, is available at https://go.usa.gov/xVdTY.
Links to other publicly released information about this investigation are available at https://go.usa.gov/xVpR5.