Sunoco Pipeline LP resumed service Tuesday on the Mariner East 1 natural gas liquids pipeline after a three-month shutdown ordered by regulators when sinkholes reappeared in Chester County.

The company, a subsidiary of Energy Transfer Partners (ETP) of Dallas, restarted the 350-mile, 8-inch diameter pipeline on the condition that it regularly monitor the pipeline and perform geophysical tests in the right-of-way area behind Lisa Drive in West Whiteland Township. The shutdown came less than a year after sinkholes first developed, prompting an initial two-month shutdown last year.

The 88-year-old pipeline, which formerly carried such refined products as heating oil, was repurposed in 2014 to transport Marcellus Shale gas liquids such as propane and ethane across Pennsylvania to ETP’s Marcus Hook Industrial Complex. Sunoco expanded the pipeline’s right-of-way to accommodate two new, larger pipelines, whose construction was linked to the emergence of the sinkholes.

Energy Transfer said it worked closely with Pennsylvania Public Utility Commission investigators during the shutdown, which confirmed the pipeline’s integrity. “The investigation also confirmed that at no time was Mariner East 1 ever destabilized in this area,” the company said in a statement.

The PUC said that “strain gauges” installed along the pipeline last year have not revealed any movement of the pipeline “before, during or after the subsidence event.” Sunoco remotely monitors the gauges from its operations center.

Sunoco notified the PUC on Friday of its intent to restart the pipeline in 72 hours.

The $5.1 billion Mariner East project is a key link in the state’s effort to promote Pennsylvania shale-gas development, but its construction has provoked opposition from residents, and aroused fears about pipelines transporting highly volatile liquid fuel near homes, schools, and nursing homes.

In recent months, Pennsylvania Attorney General Josh Shapiro and the district attorneys in Chester and Delaware Counties have announced criminal investigations of Sunoco’s conduct during construction.

Several West Whiteland residents whose backyards have been ripped up for more than a year by the pipeline mishaps settled a lawsuit earlier this month with Sunoco for undisclosed terms.

Sunoco in early April agreed to pay a $200,000 fine and conduct a “remaining life study” to determine the forecasted retirement age of Mariner East 1 pipeline to settle a complaint with state regulators stemming from a 2017 leak of natural gas liquids in Berks County.

Sunoco in December started service on its new Mariner East 2 project, a 20-inch diameter pipe that mostly follows the route of the older pipeline. The three-month shutdown of the older Mariner East 1 curtailed, but did not halt Sunoco’s ability to transport products to Marcus Hook.

Sunoco expects to complete the Mariner project by the end of 2020, including a third pipeline, the 16-inch diameter Mariner East 2X. The three pipelines will carry up to 675,000 barrels a day of ethane, propane, and butane to Marcus Hook, or more than three times the volume that was processed at the Delaware River site before the oil refinery closed at the end of 2011.