Crews failed to relocate pressure-sensing lines from an old cast-iron distribution main during a mid-September Columbia Gas pipeline replacement, igniting a series of explosions and fires north of Boston, according to a preliminary report by federal investigators. The mistake resulted in one death, 25 injuries and 131 damaged structures in Lawrence, Andover and North Andover on Sept. 13.

The contracted work in South Lawrence being overseen by a Columbia Gas inspector involved tying in a plastic distribution main and retiring an early-1900s cast-iron, low-pressure distribution main. The National Transportation Safety Board report notes the work package did not specify the location of sensing lines or require the relocation that would have ensured the regulators were sensing actual 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,” the report says. “As the pressure … dropped about 0.25 inches of water column (about 0.01 psig), the regulators responded by opening further, increasing pressure in the distribution system.”