[Llarry]

It's inefficient for warships to pull into port every time they need fuel, weapons, food or general cargo and so the procedure of underway replenishment was developed over the years.

The U.S. Navy has three distinct types of ships devoted to underway replenishment (unrep, as it is called), all operated by the Military Sealift Command (MSC) and crewed mostly by civilian mariners, who are U.S. government employees:
-- Fast Combat Support Ships (T-AOEs). There are just two T-AOEs active. Two others were retired early as an economy measure. These are large and fast multiproduct ships that can supply fuel and weapons to ships at sea.
-- Oilers, also referred to as Fleet Oilers (T-AOs). T-AOs of two classes are active, including a number of ships put into service in the 1980s and 1990s, as well as a new class put into service beginning in 2022. The T-AOs primarily supply fuel to the fleet.
-- Ammunition cargo ships (T-AKE). These 14 ships entered service in 2006-2012 and supply dry cargo as well as a limited amount of fuel, refrigerated food stores, etc. Generally speaking, a T-AO and a T-AKE together are considered to have the capability of a T-AOE, except for speed.

The ships:
1) USNS Supply and USNS Arctic are both T-AOEs. They have the same propulsion system as a destroyer: 4 gas turbines with 105,000 hp, providing a max speed of 26 knots. They can carry 177,000 barrels (bbl) of Diesel and aviation fuel and over 2,000 tons of ammunition. They have two large hangars and operate two MH-60S helicopters for vertical replenishment (vertrep) duties. They can transfer fuel to two ships (one on each side) simultaneously. They were originally commissioned Navy-manned ships with self-defense weapons and large crews; they were transferred to the MSC as an economy measure. They have crews of 160 civilian mariners (civmars) and 29 Navy personnel.
2) The USNS Henry J. Kaiser (T-AO 187) class is the the older of the two oiler classes in service, having entered service in 1986-1996. The15 ships of this class are being retired as newer oilers join the MSC. They carry 180,000 bbl of fuels. While they have a helicopter deck, they do not have assigned aircraft.
3) The USNS John Lewis (T-AO 205) class is currently being built to replace the Kaisers; only several have entered service with more under construction. Like the earlier T-AOs they are good for 20 knots with Diesel power, which does not allow them to keep up with carrier strike groups. They have a fuel capacity of 162,000 bbl and have a helo deck, but no assigned aircraft. The ships have a crew of 99 civmars.
4) The USNS Lewis and Clark (T-AKE 1) class of ammunition cargo ships supplements the oilers with dry and refrigerated stores, weapons and also carry a limited amount (18,000 bbl) of fuels. Like the T-AOEs, they have helo decks and hangars and normally operate two MH-60S helicopters for vertrep. They have a Diesel-electric propulsion system and a max speed of 20 knots. The crews have 124 civmars and 11 Navy personnel.

The T-AOEs were built to military standards, while the others were built to civilian standards that are not as sturdy. However, all ships reserve space, weight and power for self-defense weapons; in a wartime environment they would presumably be supplemented with additional Navy crew to operate these weapons.

A general note on Military Sealift Command ships: MSC ships are prefixed as United States Naval Ship (USNS) and have stripes on the stack that identify them as MSC. Commissioned Navy ships are prefixed as United States Ships (USS) and do not have the stripes.