The number of ships exporting Marcellus Shale natural-gas liquids out of Philadelphia's port quintupled in 2016, according to the Maritime Exchange, reflecting an increased flow of propane and other materials through Sunoco Logistics Partners' Marcus Hook terminal.

Exports of the natural-gas liquids, mostly ethane, increased to 34 vessels last year, up from six ships in 2015. The volumes are likely to increase this year if Sunoco completes a scheduled third-quarter expansion of its Mariner East pipeline system that would increase its capacity from 70,000 barrels a day to 345,000 barrels.

Maritime Exchange officials expect the annual traffic of ethane vessels will eventually exceed 100 ships.