WASHINGTON, D.C. - The following are the findings reached by National Transportation Safety Board in their investigation into the Sept. 13, 2018 gas disaster in the Merrimack Valley communities of Andover, North Andover and Lawrence. 

1. None of the following were factors in this accident: the training and qualification of the construction crew, the use of alcohol or other drugs, or the condition and operability of the regulators at the Winthrop Avenue (Lawrence) regulator station.

2. The multiple over-pressurization accidents investigated by the NTSB over the past 50 years demonstrate that low -pressure natural gas distribution systems that use only sensing lines and regulators as the means to detect and prevent over-pressurization are not optimal to prevent over-pressurization accidents.

3. A comprehensive and formal risk assessment, such as a failure modes and effects analysis, would have identified the human error that caused the redundant regulators to open and pressurize the system.

4. Columbia Gas of Massachusetts’ inadequate planning, documentation, and record keeping processes led to the omission of the relocation of the sensing lines for the South Union Street project .

5. The abandonment of the cast iron main without first relocating the sensing lines led to the system overpressurization, fires, and explosions.

6. The delay between the development of the initial project work order and its execution had no impact on this accident.

7. The Columbia Gas of Massachusetts constructability review process was not sufficiently robust to detect the omission of a work order to relocate the sensing lines.

8. NiSource’s engineering risk management processes were deficient.

9. Requiring a licensed professional engineer to stamp plans would illustrate that the plans had been approved by an accredited professional with the requisite skills, knowledge and experience to provide a comprehensive review .

10. The municipal public safety answering points had available and ready resources to handle the large number of distress calls requesting emergency services.

11. The field radio communications used across fire departments on Sept. 13 lacked adequate interoperability and availability to ensure that emergency responders had efficient means of interdepartmental and intradepartmental communications. 

​12. The communications issues during the Sept. 13 over-pressurization illustrate the need for emergency planning for a multi-jurisdictional response.

13. The Columbia Gas of Massachusetts incident commander faced multiple competing priorities, such as communicating with affected municipalities, updating the emergency responders, and shutting down the natural gas distribution system, which adversely affected his ability to complete his tasks in a timely manner.

14. Columbia Gas of Massachusetts was not adequately prepared with the resources necessary to assist emergency management services with the response to the over-pressurization.

Probable Cause

The NTSB determines that the probable cause of the over-pressurization of the natural gas distribution system and the resulting fires and explosions was Columbia Gas of Massachusetts’ weak engineering management that did not adequately plan, review, sequence, and oversee the construction project that led to the abandonment of a cast iron main without first relocating regulator sensing lines to the new polyethylene main. Contributing to the accident was a low-pressure natural gas distribution system designed and operated without adequate overpressure protection.

Source: National Transportation Safety Board. 


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