ship-to-ship (STS) transfer operation is the transfer of cargo between seagoing ships positioned alongside each other, either while stationary or underway. Cargoes typically transferred via STS methods include
crude oil,
liquefied gas (
LPG or
LNG), bulk cargo, and
petroleum products. The nomenclature STS transfer should be used in reference to techniques used by civilian merchant vessels, as differentiated from
underway replenishment which is the term used by the US Navy for similar, but usually far more complicated, operations between naval vessels while underway.