PORTLAND — Greater Portland’s supplier of natural gas is set to begin a 14-year, $60 million project to replace or upgrade more than 100 miles of underground gas pipeline throughout Portland and Westbrook.

The replacement project is one of the largest of its kind ever undertaken in the Northeast, according to Unitil, the Hampton, N.H.-based company that operates the system. Unitil bought Northern Utilities in 2008.

Work is expected to start in early April in Portland’s Old Port. Construction also is set this year for the Veranda Street-Washington Avenue area. No work will be done during the winter.

Unitil says there are no imminent safety concerns with the network but some cast iron lines are more than a century old and must be replaced. They will be upgraded to plastic pipe.

As part of the project, Unitil will install 10,000 automatic shutoffs that can turn off gas to sections of the system in case of excess pressure buildup.

The project will modernize the area’s gas infrastructure and make it better able to meet the growing demand for natural gas in Maine, says Unitil. The work won’t expand the network in Greater Portland, but the new pipes will handle higher pressures, which will give more commercial customers access to gas.

The project was approved last year by the Maine Public Utilities Commission, which sought to balance the pace of replacement with the cost for Unitil’s 28,000 customers. The cost to ratepayers will be the subject of a rate case before the PUC later this year.

An average home customer who heats with natural gas and uses 110,000 cubic feet a year now pays $1,660, according to Unitil.

Wholesale natural-gas prices have dropped sharply, making the fuel an attractive choice for businesses and homeowners. Falling supply prices have reduced average rates for customers by 20 percent over the past two years, said Alec O’Meara, a Unitil spokesman.

Unitil’s upgrade is good for Maine, said state Rep. Jon Hinck, D-Portland. Hinck and other lawmakers from the area intervened in the PUC case last year, in an effort to expand the timeline and reduce the annual cost to ratepayers.

“I am pleased that legislator intervention, along with the public advocate, will make sure that this is done without a major rate shock,” Hinck said. “With natural gas prices moderating as a result of major domestic gas finds and new recovery techniques, Maine is well served by keeping the pipeline infrastructure in good shape.”

Unitil has selected New England Utility Constructors as the contractor for the project. Thirty to 40 workers are expected to be involved with the job.

The company says it will restore roadways, sidewalks and lawns after construction and will try to minimize disruptions to homes and businesses during excavation.

Staff Writer Tux Turkel can be contacted at 791-6462 or at: tturkel@pressherald.com