Underway Replenishment (UNREP)

To carry out the Navy's mission effectively, fleet units must be capable of remaining at sea for prolonged periods of time, possibly in areas of the world where friendly re-supply ports are not available, and remain fully ready to carry out any assigned tasks. Military Sealift Command (MSC) ships are equipped to replenish combatants underway with fuel, ammunition, provisions, and spare parts.

The first significant underway replenishment (UNREP) operation at sea was with the collier USS Marcellus and the Navy warship USS Massachusetts in 1899. Since this first UNREP, many methods for transferring cargo have been tried. The two major methods of transferring dry cargo used today are via vertical replenishment (VERTREP) and connected replenishment (CONREP). These two methods may be used singly or at the same time.

VERTREP is carried out by helicopters with the ships in close proximity, or miles apart, depending on the tactical situation and the amount of cargo to be transferred. The helicopters transfer cargo using a sling similar to a net that is suspended from a hook on the underside of the copter. One or more helicopters continuously shuttle back and forth between the delivery ship and the receiving ship. There may be more than one ship receiving cargo at one time.