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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.

CONREP involves two processes, refueling and re-supply. In fueling at sea (FAS), fuel is pumped from the delivering ship, such as a fast combat support ship (AOE). Other replenishment ships such as the combat stores ship (AFS) and the ammunition ship (AE) can deliver fuel, but their primary mission is the delivery of dry cargo by methods referred to replenishment at sea (RAS).

CONREP uses a wire highline between the delivering and receiving ships that uses a Standard Tensioned Replenishment Alongside Method (STREAM) rig. The two ships steam side-by-side and the hoses and lines used to transfer fuel, ammunition, supplies, and personnel connect the ships. The STREAM rig is preferred over other connected replenishment methods since it permits greater ships separation. The delivery ship may have a receiving ship on both sides at the same time.

Some navies conduct CONREP with the receiving ship steaming astern of the delivery ship. There are several factors in favor of replenishment with the ships alongside each other instead of having the receiving ship astern. First, by replenishing alongside, the delivery ship, can service two ships at once, with multiple replenishment stations to each ship. Second, by replenishing alongside rather than astern, the whole formation of ships can maintain greater speed, up to 16 knots instead of the 7-8 knot maximum for astern refueling. Lastly, by replenishing alongside, both fuel and dry cargo can be transferred, instead of being limited to fuel only. Astern fueling does have a place in the replenishment plan, but it is generally limited to a tanker in convoy refueling the convoy escorts.

When using the STREAM rig for FAS operations a tensioned span high wire is suspended between the two ships. A series of hose saddles are attached to the span wire by trolleys. The transfer hoses are then suspended in between the saddles. The receiving end of the hose rig is tipped with a coupling. A variety of fueling couplings may be used to ensure compatibility between the delivery and receiving ships. The most common is a probe-fueling coupling. The probe may be used in the transfer of either DFM or JP-5 products. The probe itself has a latching mechanism that holds it in the receiver by spring force. The receiver is mounted on the receiving ship by a swivel arm. The swivel arm allows the receiver to move throughout the full working range of the receiving station, ensuring proper alignment prevents the probe from unseating. The probe assembly will unseat from the receiver when a 2,500 lb. line pull is applied. The receiver also has a manual release lever, which is the desired way to release the probe upon completion of the fuel transfer.

During RAS the STREAM transfer rig utilizes a tensioned wire highline suspended between two ships. The exact type of STREAM rig is dependent on the kind of cargo. In all rigs, cargo to be transferred is connected to a trolley, which rides on the highline. The trolley is moved between the ships by inhaul and outhaul winches located on the delivery ship. When using a STREAM rig with all tensioned wires, the wire rope outhaul is fair led through a Standard Underway Replenishment Fixture (SURF) block and attached to the outboard side of the trolley. The SURF is located on the receiving ship. A ram tensioner, located on the delivery ship, applies highline tension ensuring constant load support regardless of ship separation or motion. However, if ship separation becomes too great, the amount of wire on the winch drum may be exceeded. A stream rig can handle loads up to 8,750 lbs. under ideal conditions.

An UNREP consists of two or more ships, one of which will be designated the "guide" ship, usually the ship delivering cargo, but in a two-ship replenishment this may be changed. From the ship handling aspect, the responsibility of the guide ship is to maintain steady course (by gyro) and speed (by engine). The other ship(s) are referred to as "approach" ship(s), and their job is to come to station alongside the guide and maintain that station throughout the replenishment. The goal of the approach ship is to come alongside the guide, with sending and receiving stations aligned, at a lateral separation of about 160 feet, and then maintain that station throughout the replenishment.

The first step in an UNREP, from the operations and ship handling standpoint, is to coordinate a rendezvous time and position. While this is being done, additional information such as fuel quantities required and fueling stations and fittings available will also be exchanged and coordinated. Selecting a good rendezvous position, one with plenty of open water that is acceptable to all ships' operational requirements, often requires some compromise of less urgent requirements in favor of more important considerations. If either ship has other pressing commitments, the replenishment course and speed (Romeo Corpen) may also be a subject for discussion during the planning and coordination stages.

Once the receiving, also referred to as "customer" or "approach,"ship rendezvous with the delivery or "guide")ship, the next task, if not already accomplished, is to agree on a Romeo Corpen. Normal speed for auxiliary ship replenishments will be 12-14 knots. Selecting the replenishment course can be more of a challenge, depending on sea state. Replenishments are routinely conducted in sea state 4; however, with highly skilled personnel on both ships, they can successfully be conducted in sea state 5. A rule of thumb is that if the guide ship is able to remain within 1 degree of base course, the replenishment is a definite "go". If the guide is yawing 1.5 degrees, it is a judgment call based on skill and experience, as well as operational necessity. If the guide is yawing as much as 2 degrees on either side of base course, it is probably not possible to conduct a replenishment safely. Replenishments will normally be conducted on a Romeo Corpen that best satisfies both ships' follow on commitments, but in extreme conditions, the sea state will determine the course and whether the replenishment is even possible. Quartering seas are the worst possible situation from a ship handling standpoint.

Once a Romeo Corpen is agreed upon and the guide ship is steady on that course and speed, the receiving ship's next task is to come to waiting station. The duty of the guide ship is to steer the agreed upon course and maintain a constant engine speed. Both ships will have gear tested and stations manned to at least the same standard used for sea details at arrival and departure from port.

The purpose of waiting station is threefold. First, it improves the efficiency of the operation by having the approach ship begin coming alongside from a fairly close station, shorter approach times mean less waiting around on deck. Second, it provides the approach ship an opportunity to gauge the guide ship's course and speed accurately. Thirdly, it gives everyone on the bridge, an opportunity to acclimate to being at such close proximity to another ship. All ship handling on the approach ship side is relative to what the guide ship is doing, so matching course and speed is critical. A waiting station is usually 600 yards astern the guide ship, just outside the guide ship's wake on the appropriate side, with about 100 feet of open water between the approach ship's side and the guide's wake. Ships normally spend at least ten minutes in waiting station, and may spend 30 minutes to an hour if one arrives early.

When the guide ship is ready to receive the customer ship alongside, she'll indicate that by hauling up the Romeo flag on the appropriate side. At that time or whenever ready, the customer ship will commence her approach alongside the guide. The approach ship indicates the commencement of her "approach" by also hauling up the Romeo flag on the appropriate side.

An UNREP demands the very best of helmsmanship from both the guide and approach ships since, as the two ships close each other, the hydrodynamic forces will both change and increase noticeably. At a replenishment speed of 12 knots, a one-degree course variation will move the ship 20 feet sideways per minute. The best separation alongside during the replenishment depends on a number of factors, but is controlled by wanting to ensure the safest separation while keeping the probes seated. For surface combatants, 140-160 feet seems to work well. Larger ships seem to favor 160-180 feet. Carriers are especially challenging because of the flight deck overhang, but by the time the separation increases to 200 feet, they are probably at the point of unseating the probes.

To commence the approach and begin closing the guide, all that is required of the approach ship is to increase engine speed by 4-5 knots. While closing the distance to the guide ship, the lateral separation between ships deserves some attention; however, if the approach ship has established good waiting station, it is likely that nothing more than minor course corrections will be required until alongside.

When about one ship length astern of the guide, the approach ship reduces speed to 1-2 knots above base speed. From this point until alongside and settled in position, matching speed will be the conning officer's primary concern.

As the approach ship's bow crosses the guide ship's stern, the approach ship rings up an engine order to match base speed. Before reducing to base speed the conning officer should ensure that there is enough momentum to pass through the pressure wave generated by the guide ship and carry the approach into station. From this point forward, engine orders to bring the ship into position and match speed are made almost entirely by eye, keeping in mind the base speed determined while in waiting station.

As soon as the approach ship reaches adequate position, a shot line is sent for the phone and distance (P&D) line, which is marked every 20 feet by a flag. Once the P&D line is across, the job of maintaining separation becomes much easier, since constant "eyeballing" is no longer required. The P&D line also provides for bridge-to-bridge communications via the sound powered phone line. Once alongside, the shot lines for the replenishment stations can be sent over, the messenger hauled across, with span wire and hoses following. The team on deck and in the pump room are then ready to commence cargo transfer.

Upon completion of cargo transfer, the team on deck will begin sending back or retrieving the replenishment rigs. At this time, a prime concern from the ship handling standpoint is to maintain station and not begin drifting away from the guide. Lines can become fouled, and the added distance will put more span wire in the water. Once all lines are clear of the other ship, the approach ship can begin opening the guide. This is probably the easiest part of replenishment ship handling and can be accomplished by ordering a 2-3 degree course change away from the guide and increasing speed 2-3 knots. As those changes begin to take effect, and with the ship’s a safe distance apart and opening gradually, the UNREP process may be repeated with another ship as desired.

UNREP involves an extended period of time where two ships are in close proximity while at relatively high speeds. Any problem at all, either external to the ships or internal to one or more of the ships, may require an immediate and timely disengagement. The Captain of either ship can initiate an emergency breakaway procedure if there is a maneuvering problem or an unsafe situation is developing. An emergency breakaway follows the same procedures as a normal breakaway, but all steps are expedited as much as possible.

There is a naval tradition of playing "breakaway" music over the 1MC public address system of each ship as the ship’s are breaking way. Ships often have a signature tune or the captain may pick one. Some examples are "Wichita Lineman" for the oiler USS Wichita; "Thanks for the Memories" by the prepositioning ship USNS Bob Hope", and  the theme from the movie The Final Countdown by the aircraft carrier USS Nimitz (CVN-68), which was featured in the film. During the late 1980’s USS Bainbridge (CGN-25) played “Bad to the Bone.”

Ship Terminology

Control Ship – The ship that acts a guide for the UNREP and is responsible for maintaining a steady speed and course.

Delivery (Control) Ship – It is normally the control ship. It contains items to be delivered and provides lines, rigs, etc. used for delivery.

Approach Ship – The ship that is accepting the delivery. It makes its approach alongside the delivery ship and keeps station on the control (delivery) ship.

Receiving ship – It is normally the approach ship. It receives lines, rigs, etc. and item to be delivered from the delivery ship.

Transfer Station - A predestinated area aboard each ship where the UNREP rig is located and hooked up.

Replenishment Course - A predetermined course determined by the delivery ship that will permit ships to maintain course with a minimum of stress on rigs, etc. with considerations given to the mission of the battle group and the condition of the seas.

Replenishment Speed – The speed maintained during the actual UNREP operation; usually between 12 and 16 knots. It is determined by wind and sea conditions and is set by the delivery ship.

Underway Replenishment Group (URG) – The group is comprised of ships to be unrepped and the delivery ships. A Officer in Tactical Command (OTC ) in charge of the group.

Waiting station - An area approximately 2,000 yards aft of the delivery ship where other ships wait for replenishment.

Lifeguard station – A ship positioned 1,000 yards astern of the delivery ship that will rescue any personnel that may fall overboard during the UNREP.

Standby station – A location on the approach side and 300 to 500 yards astern of the delivery ship where the receiving ship waits to begin the UNREP.

Equipment Terminology

Inhaul/Outhaul line - A line used to recover any piece of gear such as a trolley block. The vessel providing the gear retains the inhaul and sends the outhaul to the other ship. It consists of two wire whips connected at a trolley block.

Messenger - 800 feet of continuous graduated manila or nylon line used to bring the UNREP rig aboard.

Winch - The primary source of power for cargo handling and replenishment at sea rigs. It does all the inhaul/outhaul work.

Line throwing gun - Usually a M-14 rifle that fires a projectile from the delivery ship to the receiving ship carrying a light nylon line to be used to pull in a heavier line. The exception to this is when delivery is to an aboard aircraft carrier. In this case, the carrier (receiving ship) delivers the projectile to the delivery ship to prevent a projectile from the delivery ship from striking an aircraft.

Bolo - A nylon shot line with a padded lead weight that is thrown by a seaman to the other ship to use to pull in a heavier line. It is used in place of a line-throwing gun.

P&D Line – A Phone and Distance line comprised of a salt and pepper (two lines, one white, one black) phone line  for use  during the UNREP. The line has different colored flags attached to it at intervals to indicate to the bridge how far apart the ships are during the UNREP. The flags are colored green (00 feet), red (20 feet), yellow (40 feet), blue (60 feet), white (80 feet), repeating out to 300 feet separated by 20-foot increments. At night, chemical lights (in clusters of three) mark the 60, 100, 140, and 180-foot markers.

Fair-Lead Block - Usually a snatch block located at an area where an obstruction is to be bypassed.

Snatch Block - A single sheave block with a hinged strap that can be opened and the bight of a line inserted.

King Post - One of a pair of short, strong uprights used to support cargo booms and UNREP rigs. 1 mainstay and 2 back-stays where delivery ship's stations are located. Most new combatants have the king post located on the helo-deck or fantail.

Sampson Post - Same as King post, except permanently mounted.

Riding Lines - Four inch manila lines about 45 to 60 feet long that are used for hogging to prevent double heads from popping out due to weight.

Tie-Down Lines – Lines used to secure various rigs and hoses.

Easing Out Line - A line with a bight on one end that is thrown over the hook on a delivery hose, and then run back to and secured to a cleat. It is used to retrieve the hose during high line operations.

Contour Lights - Lights used to show the contour of the delivery ship. Two blue lights are shown by the control (delivery) ship during the approach and while the receiving ship is alongside. If the control ship is over 600 feet in length, a third blue light is used. Six red lights are displayed in a horizontal line along the deck edge or on a level with the highest obstruction outboard of the receiving stations landing or work area.

Whips - Wires 1/2 or 3/4 inches in diameter with a minimum length of 450 feet that are used for heaving in or slacking off.

Hose Saddles - Devices used on hose rigs to keep the 7-inch hose from kinking. The Type A hose saddle is 19 inches long and is used with a single hose rig. The Type B hose saddle is 32 inches long and is used with the upper hose on the two hose rig.

Trolley - Connected to hose saddles and rides the span wire. It is used to bring the hose over from the delivery ship.

Ram tensioner – A hydraulic device used to keep a constant strain on the span wire. It consists of a ram cylinder, accumulator cylinder, air flasks, and an indicator assembly.

STREAM (Standard Transfer REplenishment Alongside Method ) - There are two basic STREAM rigs, the surf and concord. Both are equipped with two hauling winches and are used for transferring cargo and ammunition.

STREAM Cargo Drop Reel – A device that lowers the load from the tensioned highline allowing the STREAM rigs to be used by ships having only fixed pad eyes, a pendant station, or support legs. It is provided by the delivery ship and is attached to the STREAM trolley.

STREAM Sliding Pad Eye - Raises and lowers the attachment pad eye, bringing the rig down to the deck.

STREAM Support Leg - Combines the features of a fixed pad eye and pendant receiving station. It is usually installed on aircraft carriers.

End Fitting - Any one of numerous fittings used for rig conversion.

Traveling Surf or Surf Block- An all tensioned wire rig with the highline and inhaul/outhaul lines being tended by winches on the delivery ship.

Star Assembly - An all tensioned wire rig with the highline and the inhaul and outhaul lines being tended by winches in the delivery ship. It is a bell shaped assembly which is bolted to the traveling surf.

Pendant Receiving Station – It is being phased out of the Navy.

Span Wire/Highline – A 3/4 inch diameter galvanized steel wire that supports the UNREP rigs.

Probe/ROBB Coupling – A device used to receive fuel. The combined quick release (ROBB) coupling and valve consist of a female and a male end. The male end, rigged on the receiving ship, is the slightly tapered tube with a flange at one end. Despite the name, the ROBB coupling does not qualify as a quick release device because uncoupling is virtually impossible when the fitting is under strain. Any strain must be taken by the riding line, and, to connect or disconnect, the ends must be lined up perfectly. To provide for emergency breakaway, a breakable spool is inserted between the receiving ship's manifold and the male end. Only U.S. ships are fitted with the ROBB coupling.

Personnel Terminology

Safety Observer (on both the rig and the bridge) - Looks for unsafe practices during operations. Wears a white jersey and a white helmet with a green cross.

Rig captain – The person in overall charge of the UNREP detail. Wears a yellow jersey and a yellow helmet.

Riggers – They connect and tend tag-lines, prepare for breakaway, and disconnect the rig. Wears a blue jersey and a blue helmet.

Signalman - Receives orders from the rig captain and transmits them to the other ship. Wears a green jersey and a green helmet.

Corpsman - Maintains watch on station to provide first aid in the event of an injury. Wears a white jersey with a red cross and a white helmet with a red cross.

Gunner's Mate - Operates line-throwing rifle. Wears a red jersey and a red helmet.

Winch operator - Maintains even tension on the STREAM line. Wears a brown jersey and a brown helmet.

Hand held whistle signals between the delivery and receiving stations when using a line-throwing rifle.

One blast - Prepare to fire.

Two blasts - All clear to fire.

Three blasts - Completion of firing.

Visual flag hoists displayed by delivery ship or receiving ship - The flags of a hoist are always read from the top down. When two or more are flying, they are read from outboard to inboard or from forward to aft. During UNREP, the hoists are displayed on the yardarm toward the rigged ship.

Romeo at Dip – Romeo flag is located 3/4 way up toward the point of the hoist. On the control ship, it means, "I am steady on course and speed and am prepared to receive you alongside on side indicated." On the approaching ship, it means, "I am ready to come alongside.”

Romeo Close Up - Romeo flag is at the top of the hoist, touching the point of the hoist or as high as it will go. On the control ship, it means, "I am ready for your approach.” On the approach ship, it means, "I am commencing my approach.”

Romeo Hauled Down – This means the first messenger is in hand for controlling and receiving ship.

Prep at Dip - Prep flag is located 3/4 way up toward the point of the hoist. On the receiving ship, it means, “I expect to disengage in 15 minutes.”

Prep Close up - Prep flag is at the top of the hoist, touching the point of the hoist or as high as it will go. On the receiving ship, it means, “Replenishment completed and I am disengaging at final station.”

Prep Hauled Down - On the receiving ship, it means all lines are clear.

Bravo at Dip - Bravo flag is located 3/4 way up toward the point of the hoist. On the delivery ship, it means, "I have temporarily stopped supplying.” On the receiving ship, it means, "I have temporarily stopped receiving.”

Bravo close up - Bravo flag is at the top of the hoist, touching the point of the hoist or as high as it will go. On both ships, it means fuel or explosives are being transferred.

Bravo hauled down - On both ships, it means delivery is complete.