PORTLAND — Greater Portland’s natural gas supplier is set to begin a 14-year, $60 million project to replace or upgrade more than 100 miles of underground  natural 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 take place over the winter.

Unitil says there are no imminent safety concerns with the existing network, but that some cast iron lines are more than a century old and need to be replaced. They will be upgraded to plastic pipe. As part of the project, 10,000 automatic shutoffs will be installed that can turn off gas to sections of the system in the event 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, according to Unitil. While the work won’t expand the network in Greater Portland, 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 impact on Unitil’s 28,000 customers. The cost to ratepayers has yet to be determined; it will be the subject of a rate case at the PUC later this year.

Currently, an average home customer that heats with natural gas and uses 1,100 ccf a year pays $1,660, according to Unitil.

The project is taking place during a period in which wholesale natural gas prices have dropped sharply, making the fuel an attractive choice for businesses and homeowners. Falling supply prices have reduced overall average rates for customers by 20 percent over the past two years, according to Alec O’Meara, a Unitil spokesman.

Unitil’s upgrade is good for Maine, according to Rep. Jon Hinck, D-Portland. Hinck and other area lawmakers intervened in the PUC case last year, in an effort to expand the timeline and reduce the annual financial 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 infrasture in good shape.”

Unitil has selected New England Utility Constructors to be the contractor for the project. Between 30 and 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.

Residents looking for the latest information on the project can call a toll-free hotline at 866-821-4386, or go to a website dedicated to the project.