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, 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.