Underway Replenishment (UNREP)
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.