The afternoon commute was just getting under way on Thursday, Sept. 13, 2018, when a series of explosions and fires tore through Boston's northeastern suburbs after high-pressure natural gas was released into a low-pressure gas distribution system.

Hundreds of miles away, a warning of a system anomaly was received at a Columbia Gas monitoring center in Columbus, Ohio. Operators at the center received two high-pressure alarms for the South Lawrence, Massachusetts, gas pressure system: one at 4:04 p.m. and the other at 4:05 p.m.

(Click to enlarge) Map showing the extent of the area affected by the September 13 natural gas accident. Source: NTSB(Click to enlarge) Map showing the extent of the area affected by the September 13 natural gas accident. Source: NTSBBut the monitoring center had no control capability either to close or open valves. Its only capability was to monitor distribution system pressures and advise field technicians. So at 4:06 p.m., a Columbia Gas controller reported the high-pressure event to the Meters and Regulations group in Lawrence. Five minutes later, a local resident made the first 9-1-1 call to Lawrence emergency services as gas-fueled fires and explosions erupted.

One person was killed and at least 21 people, including two firefighters, were transported to the hospital as a result of the accident. Seven other firefighters received minor injuries.

The timeline was put together by incident investigators from the National Transportation Safety Board (NTSB), which arrived at the scene shortly after the accident. The NTSB released the timeline on October 11.

According to NTSB investigators, Columbia Gas shut down one gas pressure regulator by about 4:30 p.m. Critical gas distribution system valves were closed by 7:24 p.m. And beginning about midnight, crews that included two Columbia Gas technicians escorted by emergency response personnel began shutting off meters to isolate individual houses from the natural gas distribution system. All meters in the affected area were shut off by the following morning.

In addition to the NTSB, parties to the investigation include NiSource, Columbia Gas, the Pipeline and Hazardous Materials Safety Administration, the Massachusetts State Police and the Massachusetts Department of Public Utilities. Investigators reportedly already have interviewed multiple people including Columbia Gas Supervisory Control and Data Acquisition controllers in Ohio.

The distribution system is owned and operated by Columbia Gas of Massachusetts, a unit of NiSource. The system over-pressure damaged 131 structures, including at least five homes that were destroyed in the city of Lawrence and the towns of Andover and North Andover. The NTSB said that most of the damage was a result of structure fires ignited by gas-fueled appliances. Several structures were destroyed by natural gas explosions.

At the request of emergency management officials, National Grid shut down electrical power in the affected area to remove a source of ignition. Nearby roads were closed, and freight and passenger railroad operations were suspended. Columbia Gas also shut down the low-pressure, natural gas distribution system.

Pipe-replacement Project

The NTSB said that the cast-iron, low-pressure distribution system was installed in the early 1900s and had been partially improved with both steel and plastic pipe upgrades since the 1950s. The low-pressure distribution system in the affected area relied on 14 regulator stations to control gas at the required pressure into structures serviced by the system. The design differs from high-pressure gas distribution systems that place an individual regulator to reduce pressure at each customer service. Each of the regulator stations reduced the pressure from about 75 pounds per square inch gauge (psig) on the natural gas main pipeline to 12 inches of water column (about 0.5 psig) for delivery to customers.

Aerial view of a home destroyed by fire after the September 13 accident. Source: NTSBAerial view of a home destroyed by fire after the September 13 accident. Source: NTSBPrior to the over-pressure 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 in South Lawrence. The crew was working on a tie-in project of a new plastic distribution main and the abandonment of a cast-iron distribution main, the NTSB investigation said.

The distribution main that was abandoned still had the regulator sensing lines that were used to detect pressure in the distribution system and provide input to the regulators to control the system pressure. Once the contractor crews disconnected the distribution main that was going to be abandoned, the section containing the sensing lines began losing pressure, the NTSB said.

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. Because the regulators no longer sensed system pressure, they fully opened. This allowed the full flow of high-pressure gas to be released into the distribution system supplying the neighborhood, exceeding maximum allowable pressure.

The NTSB said that Columbia Gas developed and approved the work package executed on the day of the accident. The work package did not account for the location of the sensing lines or require their relocation to ensure the regulators were sensing actual system pressure. The work was performed in accordance with steps laid out in the work package.

Safety Stand-down

After the accident, Columbia Gas implemented a safety stand-down for all employees who perform work related to low-pressure natural gas systems for NiSource business units.

According to Columbia Gas, all cast iron and bare steel piping in the affected neighborhoods will be replaced due to system integrity concerns. The new system will consist of high-pressure plastic mains with regulators at each service meter to reduce the line pressure from the main to the required pressure. The Massachusetts Department of Public Utilities is providing oversight of the new system installation to ensure that all work complies with state and federal safety regulations.