The National Transportation Safety Board (NTSB) issued what it said were five "urgent safety recommendations" based on investigators' findings in the ongoing investigation of the Sept. 13, 2018 series of explosions and fires in the Merrimack Valley region of Massachusetts.

The accident killed one person and sent at least 21 others to area hospitals.

Four of the recommendations were issued to NiSource Inc., parent company of Columbia Gas of Massachusetts, which owns and operates the natural gas distribution system involved in the accident. These recommendations seek:

  • Revision to the engineering plan and constructability review process across all of NiSource subsidiaries to ensure all applicable departments review construction documents for accuracy, completeness, and correctness, and that documents or plans be sealed by a professional engineer prior to work commencing.
  • A review of all of NiSource records and documentation of natural gas systems to ensure they are traceable, reliable, and complete.
  • Application of a "management of change process" to adequately identify system threats that could result in a common mode failure.
  • Development and implementation of control procedures during modifications to gas mains to mitigate risks identified during management of change operations, with gas main pressures continually monitored during modifications and assets placed at critical locations to immediately shut down the system if abnormal operations are detected.

A fifth safety recommendation was issued to the Commonwealth of Massachusetts, seeking the elimination of the professional engineer licensure exemption for public utility work and a requirement for a professional engineer’s seal on public utility engineering drawings. (Read the NTSB recommendations report.)

Timeline of Events

On Sept. 13 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, Mass. The system overpressure damaged 131 structures, including at least five 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.

(Read "Two Alarms, Then Fires and Blasts, NTSB Says.")

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

Replacement Projects

Prior to the accident, Columbia Gas had a plan consisting of multiple projects to replace 7,595 feet of low-pressure, existing cast-iron and plastic natural gas main with 4,845 feet of low-pressure and high-pressure plastic gas main on South Union Street and neighboring streets.

damage done to a home in North Andover due to gas explosion on Sept. 13, 2018. Credit: Wikimedia and whoisjohngaltdamage done to a home in North Andover due to gas explosion on Sept. 13, 2018. Credit: Wikimedia and whoisjohngaltNTSB said that on September 13, prior to the overpressure event, a Columbia Gas-contracted work crew, which included a Columbia Gas inspector, executed one of the Columbia Gas-designed and -approved pipe-replacement projects at the intersection of South Union Street and Salem Street in South Lawrence.

The project was to install a plastic distribution main and abandon in place 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 being abandoned, the section containing the regulator-sensing lines began losing pressure.

As the pressure in the abandoned distribution main dropped to about 0.25 inches of water column (about 0.01 psig), the regulators responded by opening further, increasing pressure in the distribution system. The regulators opened completely when they no longer sensed system pressure, allowing the full flow of high-pressure gas to release into the distribution system supplying the neighborhood.

As a result, natural gas was delivered to customers at a pressure well above the maximum-allowable operating pressure which led to the ignition of fires and explosions in homes.

(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: NTSBTwo Alarms

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.

NTSB said that following company protocol, at 4:06 p.m., the Columbia Gas 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. In response, three technicians were dispatched to perform field checks on 14 regulators. Columbia Gas shut down the regulator at issue by about 4:30 p.m. The critical valves of the natural gas distribution system were closed by 7:24 p.m.

Beginning about midnight, crews made up 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.

Engineering Omissions

In the safety recommendation report, the NTSB said that omissions in the engineering work package and construction documentation for the project were discovered during its investigators’ review. These omissions allegedly were not identified by the Columbia Gas constructability review. While the engineering design package for one project--known as the South Union Street project--underwent a constructability review, NTSB investigators said that they found the review did not identify the impact on pressure regulation and control. The Columbia Gas field engineer who developed the engineering plans told NTSB investigators he developed them without reviewing engineering drawings that documented the regulator-sensing lines.

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: NTSBThe NTSB said that it believes a comprehensive constructability review, which would require all departments to review each project, along with the seal of approval from a professional engineer, would likely have identified the omission of the regulator-sensing lines, thereby preventing the error that led to the accident. At present, neither Massachusetts nor Columbia Gas policy require a registered professional engineer to develop or review public utility engineering plans, NTSB said.

The NTSB said it further believes it is critical for an engineer with appropriate qualifications and experience to review engineering plans for a gas company, if not develop them. Massachusetts’ exemption for the requirement of professional engineer licensure to perform “industrial” and public utility work foreclose an opportunity to detect this design oversight.

Columbia Gas engineering plans used during the construction work did not document the location of regulator-sensing lines, NTSB said. The NTSB said it believes had accurate alignment sheets with comprehensive system information been available and used, engineers and work crews would have been able to identify the regulator-sensing lines and ensured their relocation prior to abandoning the pipeline main.

NTSB investigators also determined that had Columbia Gas adequately performed management of change and placed personnel at critical points along the system, Columbia Gas could have immediately addressed the issue and mitigated the consequences of the event.