PRELIMINARY REPORT

Pipeline

Over-pressure of a Columbia Gas of Massachusetts

Low-pressure Natural Gas Distribution System

Merrimack Valley, Massachusetts

September 13, 2018

PLD18MR003

The information in this report is preliminary and will be either

supplemented or corrected during the course of the investigation.

On September 13, 2018, about 4:00 p.m. eastern daylight time, a series of explosions and 

fires occurred after high-pressure natural gas was released into a low-pressure gas distribution system 

in the northeast region of the Merrimack Valley in the state of Massachusetts.

1 (See figure 1.) The 

distribution system was owned and operated by Columbia Gas of Massachusetts (Columbia Gas), a 

subsidiary of NiSource. The system over-pressure damaged 131 structures, including at least 5 homes 

that were destroyed in the city of Lawrence and the towns of Andover and North Andover. Most of 

the damage was a result of structure fires ignited by gas-fueled appliances. Several structures were 

destroyed by natural gas explosions. One person was killed and at least 21 individuals, including 

2 firefighters, were transported to the hospital. Seven other firefighters received minor injuries. 

 1 Title 49 Code of Federal Regulations (CFR) 192.3 defines a low-pressure distribution system as “a distribution 

system in which the gas pressure in the main is substantially the same as the pressure provided to the customer.” The 

Massachusetts Natural Gas Pipeline Safety Code, Title 220 Code of Massachusetts Regulations (CMR) 101.06, further 

defines a low-pressure distribution system “as any system in which the gas pressure in the main is equal to or less than 

two psig (pounds per square inch gauge).”
2

Figure 1. Area impacted by accident depicting structures that were damaged.
3

Fire departments from the three municipalities were dispatched to address the multiple fires 

and explosions. (See figures 2 and 3.) First responders initiated the state’s fire mobilization plan, that 

included mutual aid from other districts in Massachusetts, New Hampshire, and Maine. 

Figure 2. Aerial view of damaged home where a fatality occurred after the chimney collapsed on 

vehicle in the driveway.

Figure 3. Aerial view of burned-out home impacted by the event.

Emergency responders asked residents to evacuate from the impacted area to four evacuation 

centers. 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 
4

passenger railroad operations were suspended. Columbia Gas also shut down the low-pressure, 

natural gas distribution system.

2

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, unlike high-pressure gas distribution systems that 

place an individual regulator to reduce pressure at each customer service.3 Each of the regulator 

stations reduced the pressure from about 75-pounds per square inch gauge (psig) natural gas main 

pipeline to 12 inches of water column (about 0.5 psig) for delivery to customers. 

Prior 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 a nearby intersection (South Union Street and Salem Street) in South Lawrence. 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. 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. 

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. 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. 

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. In light of this accident, Columbia Gas implemented a safety 

stand-down for all employees who perform work related to low-pressure natural gas systems for 

NiSource subsidiaries.

4

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. 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 accordingly. Following company protocol, at 4:06 p.m., the Columbia Gas 

 2 Columbia Gas delivers natural gas to about 320,000 natural gas customers in southeastern Massachusetts, including 

the Merrimack Valley. 3 Title 49 CFR 192.3 defines a high-pressure distribution system as “a distribution system in which the gas pressure 

in the main is higher than the pressure provided to the customer.” The Massachusetts Natural Gas Pipeline Safety Code, 

220 CMR 101.06, further defines a high-pressure distribution system “as a system in which the pressure in the main is 

greater than 60 psig, but equal to or less than 200 psig.” 4 A safety stand-down is an organized break from work by which employers hold discussions with their employees. 

They are often taken as occasions to discuss safety topics while also reinforcing the organization’s policies regarding 

safety in general.
5

controller reported the high-pressure event to the Meters and Regulations group in Lawrence. A local 

resident made the first 9-1-1 call to Lawrence emergency services at 4:11 p.m.

Columbia Gas shut down the regulator at issue by about 4:30 p.m. The critical valves of the 

involved natural gas distribution system were closed by 7:24 p.m. Beginning about midnight, crews 

consisting of two Columbia Gas technicians escorted by two emergency response personnel began 

shutting off the meters at each house to isolate the homes from the natural gas distribution system. 

All meters were shut off by the following morning. 

National Transportation Safety Board (NTSB) Chairman Robert Sumwalt and Member 

Jennifer Homendy led a 19-member NTSB team on scene. Parties to the investigation include

NiSource, Columbia Gas, Pipeline and Hazardous Materials Safety Administration, Massachusetts 

State Police, and Massachusetts Department of Public Utilities. In addition to the on-scene work, 

investigators interviewed multiple people including two Columbia Gas Supervisory Control and Data 

Acquisition controllers from Columbus, Ohio. 

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. 

The NTSB’s investigation into this accident is ongoing. 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.