Underway replenishment
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Underway replenishment, also called replenishment at sea (RAS), is a method of transferring fuel, munitions, and stores from a supply ship to a combat ship while underway.
Following some early development starting at about 1900 for transferring coal, the technique of underway replenishment was perfected by the United States Navy in the late 1930's and used extensively as a logistics support technique in the Pacific theatre of World War II. Since it allowed extended range and striking capability to naval task forces the technique was classified so that enemy nations could not duplicate it.
In the version of this technique called alongside connected replenishement (or CONREP), the supply ship sets and holds a base course, steaming at a speed of 12 to 16 knots. The unit to be refueled, reprovisioned, or rearmed takes station alongside (Romeo Corpen) at a distance of thirty yards or thereabouts. Shotlines are used to send messenger lines between the ships, and refueling and/or dry transfer rigs are then sent between the ships, using these messenger lines to pull the rigs. Though rarely practiced when helicopters are available, a person can also be transferred in a specially designed chair on a highline between the ships.
Because the ships are side-to-side, multiple transfer rigs can be set up between the two ships, so that more than one type of fuel, or a combination of fuel, dry stores, and munitions can be transferred from the supply ship to the receiving unit at the same time.
Because the supply ship is holding a set course it is possible to service two receiving ships at the same time, one to port and one to starboard.
In the U.S. Navy, aircraft carriers are always replenished from the port side of the supply ship—the starboard side of the carrier. The design of the aircraft carrier, with its overhanging flight deck, does not permit replenishment from the carrier's port side. Most other ships can receive replenishment from either side of the supply vessel.
Underway replenishment is a risky operation. The two or three ships running side-by-side at speed must hold to precisely the same course and speed for a long period of time while fuel and stores are being transferred. A slight steering error on the part of one of the ships could cause a collision between the ships.
Experienced and qualified helmsmen are required during the replenishment and the crew on the bridge must give their undivided attention to the ship's course and speed. The risk is tripled when a replenishment ship is servicing two ships at once, one on each side. Ships' crews practice and drill in a method called "emergency breakaway" that is used in the event that something goes wrong and one of the ships must leave the formation immediately.
A rarely used type of Underway Replenishment is astern fueling, where the fuel receiving ship follows directly behind the fuel supplying ship. The fuel supplying ship trails a hose in the water that the fuel receiving ship retrives and connects to receive fuel.
A third type of Underway Replenishment is Vertical Replenishment, or VERTREP. In VERTREP a helicopter lifts cargo from the supplying ship and drops it on the receiving ship. The advantage of this method is that the ships do not need to be close to each other, so there is little risk of collision. A disadvantage of this method is that the helicopters cannot carry loads that are as heavy as a connected replenishment rig can transfer, so heavy cargos cannot be transferred by VERTREP.
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