PORTLAND — The largest natural gas provider in southern Maine is replacing more than 100 miles of pipeline in Portland and neighboring Westbrook as part of a 14-year, $60 million project to modernize the system.

The project, which begins in April, involves replacing cast-iron pipes — some of which are more than 100 years old — with state-of-the-art plastic pipe, Unitil Corp. said Monday.

The undertaking does not call for extending pipeline to areas that now don’t have access to natural gas. But the upgraded pipes will be able to handle increased volumes of gas, allowing Unitil to offer gas service to additional customers on existing routes, said spokesman Alec O’Meara.

“This project is to modernize our system. It’s going to make our system ready for growth in the 21st century,” O’Meara said at a press conference in Portland. “It’s going to make it more reliable. It’s going to allow us to have greater (gas) pressure in the pipeline.”

Unitil, which is based in Hampton, N.H., has about 28,000 customers in Maine, making it the largest natural gas supplier in the state. Customers are primarily located along the southern coast and in the Lewiston area.

The company has hired New England Utility Constructors Inc. to dig up the old pipes and replace them with new pipes. The project will be done in stages across Portland and Westbrook on pipeline routes that now serve 17,000 customers. Construction costs will be paid for by ratepayers, O’Meara said.

O’Meara said there are no imminent safety concerns with the existing pipes, and that the project simply upgrades the system now in place. Plastic pipe is more durable and flexible than cast iron and has an indefinite life expectancy, he said.

New England Utility Constructors expects to use 30 to 40 workers during construction, which will result in some traffic delays and detours on roads where there is construction.

When construction gets under way, it will be the largest natural gas pipe-replacement project in the Northeast, O’Meara said.