A series of natural gas explosions and fires that rocked three Massachusetts communities in September killing one man and damaging 131 structures may have been caused by gaps in construction work orders for the old system that were prepared by NiSource Inc. subsidiary Columbia Gas of Massachusetts, according to the National Transportation Safety Board (NTSB).

Investigators made their initial findings Thursday regarding the incident that occurred at about 4 p.m. ET on Sept 13, when “high-pressure natural gas was released into a low-pressure gas distribution system” in the northeast region of the Merrimack Valley.

“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,” investigators said.

“Most of the damage was a result of structure fires ignited by gas-fueled appliances. Several structures were destroyed by natural gas explosions.” One man was killed and least 21 people, including two firefighters, were transported to the hospital. Seven other firefighters received minor injuries.

According to NTSB, the response effort was immense. Four evacuation centers were set up as residents were asked to evacuate. 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, while freight and passenger railroad operations were suspended. Columbia Gas also shut down the low-pressure gas distribution system.

“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,” investigators noted.

“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 address.

Each of the regulator stations reduced the pressure from about 75 pounds per square inch gauge (psig) in the natural gas main pipeline to 12 inches of water column (about 0.5 psig) for delivery to customers, NTSB said.

“Before the overpressure 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 St. and Salem St.) in South Lawrence,” NTSB said.

Contractors were “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.”

Pressure in the abandoned distribution main began dropping to about 0.25 inches of water column (about 0.01 psig), and the regulators responded “by opening further, increasing pressure in the distribution system,” the NTSB said.

“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,” investigators said. However, the work orders “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.”

Following the incident, Columbia Gas implemented a safety stand-down for all employees performing work related to low-pressure natural gas systems for NiSource subsidiaries.

In the minutes before the system erupted, the Columbia Gas monitoring center in Columbus, OH, received two high-pressure alarms for the South Lawrence gas pressure system, with one at 4:04 p.m. and the other at 4:05 p.m., the report noted.

However, 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.”

At 4:06 p.m., following company protocol, the Columbia Gas controller reported the high-pressure event to the Meters and Regulations group in Lawrence, MA. A local resident made the first 9-1-1 call to Lawrence emergency services at 4:11 p.m., according to investigators.

Columbia Gas shut down the regulator at issue at around 4:30 p.m. The critical valves of the involved gas distribution system were closed by 7:24 p.m. At close to midnight, crews that included two Columbia Gas technicians and two emergency response personnel began shutting off the meters at each house to isolate the homes from the gas distribution system. All meters were shut off by the following morning, NTSB said.

Columbia Gas is planning to replace all cast iron and bare steel piping in the affected neighborhoods because of system integrity concerns, with high-pressure plastic mains installed with regulators at each service meter to reduce the line pressure from the main to the required pressure.

The Massachusetts Department of Public Utilities (DPU) is providing oversight of the new system installation to ensure that all work complies with state and federal safety regulations.

NTSB said its investigation 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.”

Parties to the investigation include NiSource, Columbia Gas, the Pipeline and Hazardous Materials Safety Administration, Massachusetts State Police and DPU.

In response to the incident, Massachusetts regulators plan to hire an independent evaluator to examine the safety of the state’s entire gas distribution system.

National Grid also has been caught in the crossfire. The utility last Monday was ordered to halt all nonessential work in the state after excess gas was “inadvertently” introduced into pipelines, raising concerns of a Merrimack Valley repeat. Following the incident, National Grid shut off gas to about 300 homes in Woburn, which is about 11 miles north of Boston.

National Grid supplies power and gas for more than 20 million people in New York, Massachusetts and Rhode Island and is the largest gas distributor in the Northeast.