Underway replenishment or UNREP is a common and routine, but dangerous evolution conducted at sea. Ships can replenish ammunition, food, fuel, personnel, mail, spare parts, etc. Many transfers can be accomplished by helicopter, but refueling is must be accomplished by attaching hoses to another ship.

When refueling at sea is to be conducted, one ship is the delivering ship and the other the receiving ship. At the commencement of the evolution, the receiving ship approaches from astern and assumes station about 60 feet off the port side of the delivery ship.

A shot line is passed between the two ships, followed by cables and fuel hoses. The fuel hoses are inserted in the fueling trucks and the refueling commences.  All lines require constant tending - slacking off and hauling in - as the ships move apart and close in. It is common for smaller ships to refuel simultaneously from both sides of the supplier. Depending on the amount of fuel needed, refueling can take up to an hour for a destroyer and several hours for larger ships. As fuel is being transferred through the hoses, helicopters are usually busy flying pallets and containers of supplies onto the receiving ship's deck. This evolution is known as Vertical Replenishment, or VERTREP.

Winds and seas can drive the ships apart or push them together but the main reason two ship are drawn together is the suction that is created by the rush of water between the two ships. Contact is not uncommon although serious collisions are rare.