U.S. safety regulators proposed a long-awaited rule that would require pipeline operators to install technology to isolate lines following a rupture and to swiftly respond to the incidents.

The rule became public on Feb. 5, more than eight years after the pipeline safety bill mandating the rule became law. Its official publication on Feb. 6 will bring the U.S. Pipeline and Hazardous Materials Safety Administration, or PHMSA, one step closer to closing out a rulemaking backlog now going on a decade old.

The rule is meant to address operational failures and technical shortcomings that exacerbated two of the defining pipeline disasters of the previous decade: the deadly 2010 PG&E Corp. natural gas transmission line explosion in San Bruno, Calif., and the massive 2010 Enbridge Inc. oil spill in Marshall, Mich.

In San Bruno, it took PG&E Corp. crews 95 minutes to close mainline valves on the pipeline, hamstringing emergency responders as free-flowing gas fed the blaze. Enbridge took 18 hours to confirm the rupture in Marshall, allowing 800,000 gallons of oil to flow into the Kalamazoo River.

PHMSA's proposed rule aims to prevent similar failures in the future by pairing issues raised in National Transportation Safety Board recommendations and congressional mandates from the Pipeline Safety, Regulatory Certainty, and Job Creation Act of 2011.

It would first require pipeline operators to install valves that can be closed remotely or shut off automatically when an incident occurs. The valve requirement would apply to newly constructed gas transmission lines and hazardous liquid pipelines, as well as "entirely replaced" lines, six inches in diameter or greater. Operators entirely replace a line when they replace two or more contiguous miles of pipeline, according to the proposed rule.

It would also require operators to declare a rupture within 10 minutes after the initial indication or notification of an incident and isolate the line within 40 minutes of identifying the failure.

The notice of proposed rulemaking gives stakeholders 60 days to file written comments once it is published in the Federal Register. PHMSA estimates the rule will cost operators $2.5 million to $3.1 million annually to implement.

The rule would apply to certain pipelines that run through high-consequence areas, or densely populated areas where an incident is more likely to cause death, injury and property damage.

Under the proposal, operators could use alternative technology equivalent to remote-control valves and automatic shutoff valves in some cases. It would also allow operators to install manual shutoff valves in places where remote-control or automated valves are not "economically, technically or operationally" feasible, provided the operator ensures staff are in place to close the valve within 40 minutes of a rupture.

The rupture identification and response provision would also define what constitutes a rupture, establish response procedures and require operators to "immediately and directly" notify lead public safety officials.

PHMSA declined to require operators to install leak detection equipment on gas transmission and distribution lines, as the National Transportation Safety Board recommended in 2011, saying it is more challenging to feasibly detect small leaks. The agency did require pressure monitoring equipment at certain points on gas transmission and hazardous liquid lines, which could help detect leaks, according to PHMSA.

PHMSA completed three rules last fall, including the first part of its three-part gas transmission and gathering regulatory overhaul, and recently finalized its Safety of Underground Natural Gas Storage Facilities to send to the Federal Register for publication.