Creation and maintenance of fleet auxiliary finds its origins through the time-tested and sound principles of war, conceptualised in the British Defence Doctrine. The doctrine propounds offensive action in a practical way in which the commander seeks to gain advantage, sustain momentum and seize initiative and economy of effort to judiciously exploit the manpower, material and time in relation to the achievement of objectives; and finally sustainability of the force to generate the means by which it’s fighting power and freedom of action are maintained.

A new dimension to these principles were added in the Soviet Union and Russian doctrines, which lay due emphasis on ‘capability’ to ensure full use of various means and capabilities of battle to achieve victory; destruction of enemy throughout the entire depth of their deployment; and finally the logistics for restoration of reserves and combat capability for the modern, fast paced battlefield.

Incorporation of these vital tenets prompted development of suitable platforms for sustenance of the fleet and to equip them with longer reach, flexibility and freedom of action over a longer span of operational deployment. Thus the method of transferring fuel, munitions and stores from one ship to another came to be known as replenishment at sea (RAS).

In the first two decades of the 20th century, the US Navy evolved the innovative procedure for under way replenishment (UNREP) with the supply ship and receiver ship coming along side each other at full stop. This method, however, had serious limitations and could be practised only during calm conditions and good sea states. Therefore, methods had to be developed for replenishment while ships were actually under way, so as not to impede the operational tasks. Innovative methods had to be developed to augment a logistics support technique which facilitated extended range and striking capability to naval task forces.

Alongside connected replenishment (CONREP) is a standard method of transferring liquids such as fuel and fresh water, along with ammunition and break bulk goods. The supplying ship holds a steady course and speed, generally between 12 and 16 knots. The receiving ship then comes alongside the supplier at a distance of approximately 30 metres. A gun line, pneumatic line thrower, or shot line is fired from the supplier, which is used to pull across a messenger line. This line is used to pull across other equipment such as a distance line, phone line, and the transfer rig lines. Because of the relative position of the ships, it is possible for some ships to set up multiple transfer rigs, allowing for faster transfer or the transfer of multiple types of stores. Additionally, many replenishment ships are set up to service two receivers at one time, with one being replenished on each side.

Alongside CONREP is a risky operation, as two or three ships running side-by-side at a particular speed, must hold to precisely the same course and speed for a long period of time. Moreover, the hydrodynamics of two ships running close together cause suction between them. A slight steering error on the part of one of the ships could cause a collision, or part the transfer lines and fuel hoses. For this reason, 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 increased when a replenishment ship is servicing two ships at once. In case of an emergency, the crews practise emergency breakaway procedures, where the ships separate in less-than-optimal situations. Although the ships will be saved from collision, it is possible to lose stores, as the ships may not be able to finish the current transfer.

Astern fuelling: The earliest type of replenishment, rarely used today, is astern fuelling. In this method, the receiving ship follows directly behind the supplying ship. The fuel-supplying ship throws a marker buoy into the sea and the receiving ship takes station with it. Then the delivering ship trails a hose in the water which the fuel-receiving ship retrieves and connects to. This method is more limited as only one transfer rig can be set up. However, it is safer as a slight course error will not cause a collision. This however was the method used by the Soviet Navy for many decades after the end of World War II.

Vertical replenishment: A third type of under way replenishment is vertical replenishment (Vertrep). In this method, a helicopter lifts cargo from the supplying ship and lowers it to the receiving ship. The main advantage of this method is that the ships do not need to be close to each other and so there is little risk of collision. Vertrep is also used to supplement and speed stores transfer between ships conducting Conrep. However, the maximum load and transfer of speeds are both limited by the capacity of the helicopter.