NTSB releases preliminary report on Merrimack Valley gas disaster

LAWRENCE, MASS. (WHDH) - The National Transportation Safety Board has released its preliminary report of last month’s Merrimack Valley gas disaster.

The report details how on Sept. 13, a series of explosions and fires occurred after high-pressure natural gas was released into a low-pressure gas distribution system owned and operated by Columbia Gas of Massachusetts.

The report found the incident damaged 131 structures in Lawrence, Andover and North Andover, including at least five homes that were destroyed.

Prior to the over-pressure incident, a Columbia Gas-contracted work crew, which included a Columbia Gas inspector, was performing a Columbia Gas-designed and approved pipe-replacement project at the intersection of South Union and Salem streets in South Lawrence.

According to the report, the contracted crew was working on a tie-in project of a new plastic distribution main and the abandonment of a cast-iron distribution main and when the crews disconnected the distribution main that was going to be abandoned, the section containing the sensing lines began losing pressure.

“As the pressure in the abandoned distribution main dropped about 0.25 inches of water column (about 0.01 psig), the regulators responded by opening further, increasing pressure in the distribution system,” the report reads. “Since the regulators no longer sensed system pressure they fully opened allowing the full flow of high-pressure gas to be released into the distribution system supplying the neighborhood, exceeding the maximum allowable pressure.”

Minutes before the fires and explosions occurred, the Columbia Gas monitoring center in Columbus, Ohio, received two high-pressure alarms for the South Lawrence gas pressure system: one at 4:04 p.m. and the other at 4:05 p.m. But the report found the monitoring center had no control capability to close or open valves; its only capability was to monitor pressures on the distribution system and advise field technicians.

At 4:06 p.m., the Columbia Gas controller reported the high-pressure event to the Meters and Regulations group in Lawrence, five minutes before a local resident made the first 911 call to Lawrence emergency services.

Columbia Gas shut down the impacted regulator by about 4:30 p.m. The critical valves of the involved natural gas distribution system were closed by 7:24 p.m. All meters were shut off by the following morning.

“The NTSB’s investigation into this accident is ongoing,” the report reads. “Future investigative issues include the coordination between the emergency responders and Columbia Gas; an analysis of the engineering work package preparation and execution, including the design documentation; and a review of construction packages for constructability and safety.”

One person was killed and 21 others, including two firefighters, were hospitalized with injuries sustained in the disaster.

Click Here to view the complete report.

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