The National Transportation Safety Board has issued its preliminary report on the Sept. 13 gas explosions in the Merrimack Valley region of Massachusetts.
The report confirms that overpressurized natural gas lines were the source of the deadly explosions and fires in Lawrence, Andover and North Andover, and details the chain of events that led to the disaster, which killed one person, injured 25 others and damaged 131 structures.
Thursday's preliminary report says Columbia Gas workers failed to account for the location of critical gas pressure sensors in Lawrence, which caused high-pressure gas to flood a low-pressure distribution system at excessive levels.
Read the full report below:
The NTSB's preliminary report said that Columbia Gas' monitoring center in Ohio received two high-pressure alarms for the South Lawrence gas pressure system at 4:04 and 4:05 p.m., more than five minutes before Lawrence emergency services received the first 911 call from a resident and almost a full 30 minutes before Columbia workers in Massachusetts were able to shut down the regulator causing the problem.
Prior to the over-pressurization event, the report says a Columbia Gas-contracted work crew was performing a pipe replacement project at the intersection of South Union and Salem streets in 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.
Once the contractor disconnected the distribution main that was going to be abandoned, the section containing the regulator sensing lines used to detect and control pressure began losing pressure.
As the pressure dropped, the regulators responded by opening further, increasing pressure in the distribution system, the report says. 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.
According to the report, Columbia Gas developed the work package, which did not account for the relocation of the sensing lines or require their relocation to make sure the regulators were sensing actual system pressure.
The NTSB said its investigation into the explosions is ongoing.
Future investigative issues include the coordination between Columbia Gas and emergency responders, an analysis of the work package preparation and execution and a safety review of construction packages.
Columbia Gas has deployed thousands of workers to replace 45 miles of aging pipeline and gradually restore service. The company says all customers should have gas by Nov. 19.
Joe Hamrock, president of NiSource, Columbia Gas' parent company, said the company can't comment on the incident's cause until the investigation is complete.