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In the United States Navy, an Oiler is a Combat Logistics ship that replenishes other ships with fuel and in some cases food, mail, ammunition and other necessities while at sea, in a process called Underway Replenishment or UNREP. Up through the Second World War Navy oilers used commercial tanker hulls, with the addition of UNREP gear, defensive guns, and military electronic and damage-control equipment; since the 1950s however they have been built from the keel up as specialized naval auxiliaries. They were previously classified as Fleet Oilers in the 20th Century; under the current MSC operation their full classification is listed as Fleet Replenishment Oilers. Since the 1960s the classification Transport Oiler (AOT) has applied to tankers which ship petroleum products to depots around the world, but do not engage in UNREP.
The first fleet oilers  were identified by the hull designation AO, which is still in use. Large, fast multifunction oilers which also provide ammunition and dry stores are identified as Fast Combat Support Ships (AOE), and mid-size ones Replenishment Oilers (AOR). The AOR designation is no longer in use. All of these oilers provide the combined services of the AO, AE, AFS and AK.
Note: tonnages are given in naval light/full load displacement
The Kanawha-class was a class of six ships commissioned between 1914 and 1921, of which the first three displaced 5,950/14,800 tons, and the next three (AO-4 to AO-6) displaced 5,723/14,500 tons. Until 1920 they were designated "Fueling Ship No. 1" etc. Cuyama conducted the first-ever refueling of another ship while underway and not stopped, in 1921. Three of these oilers were lost to enemy action during World War II.
Wartime acquisitions of civilian tankers. Arethusa was built in Britain 1893 as the SS Lucilene and was purchased in 1898, serving originally as a water carrier. Sara Thompson, 2690/5840 tons, was also British-built, in 1888 as the SS Gut Heil, and was purchased in 1917. Robert L. Barnes, a 1630/3850-ton Great Lakes tanker, was built in 1914 and purchased in 1918. With the advent of the Navy's new hull-numbering system in 1920 they were designated AO-7, AO-8 and AO-14.
In 1919 - '20 the U.S. Navy commissioned eight Patoka-class oilers of 5,422/16,800 tons displacement.
These were four 5450/14,500-ton tankers built to USSB Design 1128 between 1919 and 1921. Alameda was purchased from civilian service as the SS Alameda; the other three were built for the government.
The Cimarron-class was a class of 35 fast twin-screw oilers that began entering service in 1939. Four of the ships were converted into escort carriers (CVEs) in 1942, and two were lost in combat. The first 30 of these ships were of the U.S. Maritime Commission's Type T3-S2-A1 (7,256/24,830 tons displacement); the last five were of the very similar but slightly larger T3-S2-A3 type (7,423/25,480 tons), sometimes called the Mispillion class.
From 1964 through 1967, eight of the T3 type oilers were "jumboized". This jumboization was done by cutting the ships in two with cutting torches, then the aft section was pulled away, and new mid-body moved in and welded to the bows and sterns. After many other cutting and welding modifications a new long ship was created; the jumbos were known as the Ashtabula class.
In January 1942 the Navy moved to acquire two tankers then building for Standard Oil of New Jersey, the 5800/21,800 ton Esso Trenton and Esso Albany. These ships although not a Maritime Commission design were in fact very similar to the T2-A type commissioned as the Mattaponi class, and at 17+ knots were the fastest single-screw oilers in the Navy.
The second large oiler class built during World War II was the Kennebec-class. These 16 ships were of the single-screw MARAD type T2 (5580/21,000t, 16.5kt), larger T2-A (5880/21,750t, 16.5kt) and similar but somewhat slower T3-S-A1 (5630/21,000t, 15.3kt).
Gulf Oil's 1936 Gulf Dawn was requisitioned in April 1942, renamed Big Horn and nominally designated AO-45; in fact she was modified into a Q-ship, a U-boat decoy equipped with concealed guns. She was transferred in 1944 to the Coast Guard as USCGC Big Horn (WAO-124), then back to the Navy as a transport tanker in 1945.
SS George G. Henry had already served in the Navy in 1917-18 under her own name; as one of the few tankers to escape the Philippines in December 1941 and be available to the Allied fleet in Australia, she was recommissioned under an emergency bare-boat charter at Melbourne the following April. Her civilian master, a Naval Reserve officer, was placed on active duty and continued in command.
The third large oiler class built during World War II was the Suamico class. These 5730/21,880-ton oilers were of the MARAD Types T2-SE-A1, -A2 and -A3, differing from the Kennebecs principally in having turbo-electric drive, a consequence of a chronic shortage of reduction gearing. The Escambias had more powerful engines and were markedly faster than the others. 30 of these oilers were ordered, but three of them were canceled before their completion; two others were converted into water distillation ships (AW) and one into a water tanker. One of these oilers sank in 1947, and a second in MSC service in 1972. Some of the Escambias were later transferred to the US Army and used as mobile electric power plants in Vietnam.
The Navy requisitioned Standard Oil's 6000/24,100-ton Esso Columbia shortly after her launch in September 1942.
The elderly tanker J. C. Donnell was acquired in January 1943 with the intent of using her as a floating storage tank at New Caledonia. When it turned out that concrete barges could fulfill that role, the briefly USS Pasig was returned to her owners in September. Her name was given to one of the Escambia class, AO-91. Sinclair Oil's Daniel Pierce was requisitioned in March 1943 and renamed USS Shikellamy (AO-90); in July however she was converted to a gasoline tanker and redesignated AOG-47.
The 11,600/38,000-ton Neosho-class oilers were the first streamlined oilers built for the U.S. Navy, the first oilers since World War 2 and the first designed from the outset to support jet operations. Six of these oilers were completed during 1954 and 55. "They were the first oilers designed specifically for underway replenishment. The final PROBE fueling device design was approved in 1965, consisting of a male fitting attached to the terminal end of a seven-inch hose". The Neoshos were also markedly larger than any previous USN oilers at over 650 feet in length (T6 class) with a capacity of 180,000 barrels of fuel.
The second Cimarron-class (see Cimarron-class fleet replenishment oiler) was a class of five fleet oilers that were commissioned in the early 1980s to replace older oilers constructed during World War II. Due to budget restrictions, these ships were constructed smaller than was actually needed, requiring them to be "jumboized" in the late 1980s and early 1990s. Displacement was 8200 tons as built and 11,650/36,800 after jumboization. However, to save expenses and in keeping with the Navy's move away from steam propulsion, these ships were decommissioned in the late 1990s and replaced by the diesel-powered Henry J. Kaiser-class oilers manned by the Military Sealift Command (MSC).
The Henry J. Kaiser-class is a class of fleet replenishment oilers for which construction began in August 1984. This class is composed of eighteen underway replenishment oilers which are operated by the Military Sealift Command to provide underway replenishment of fuel to Navy combat ships and jet fuel for aircraft and helicopters aboard aircraft carriers and surface warships. The Kaisers also have a limited capacity to supply ammunition, dry stores and refrigerated stores, although not as much as the AOEs and AORs; they do not have helicopter embarkation facilities. A Kaiser-class oiler operating in tandem with a Lewis & Clark-class AKE is considered to be the equivalent of one Supply-class AOE.
Two of this class were canceled and laid up incomplete, and a third transferred to the Chilean navy.
The 2012 30-Year Shipbuilding Plan calls for the Kaisers to be replaced by 17 double-hulled vessels under the T-AO(X) program. Bids have not yet been solicited so the design is undecided; T-AO(X) could be either a new design or a modification of the Lewis & Clark class.
The Sacramento-class was a class of four fast combat support ships that carried out the refueling, rearming, and resupplying the warships of the U.S. Navy on the oceans of the world - especially aircraft carrier task forces, which are inheritantly fast-moving groups of warships. To provide these fast support ships with their speed, they were built using (one-half each) the steam turbine propulsion plants of the incomplete Iowa-class battleships Illinois and Kentucky. At nearly 800 feet and 58,000 tons full load, the Sacramentos were the largest oilers ever to serve in the US Navy.
The fast supply ships combined the functions of a fleet oiler (AO), an ammunition ship (AE), and a refrigerated stores ship (AF) in one, as well as hangars and support facilities for three helicopters for Vertical Replenishment (VERTREP). "One-stop shopping", supplemented by VERTREP transfer of dry stores and ammunition, represented a signal decrease in the amount of time a deployed warship had to spend replenishing.
The Sacramentos were in service from 1964 to 2005. AOE-5 was canceled in 1968.
The four Sacramento-class supply ships were replaced by the four Supply-class ships commissioned between 1994 to 1998. All of these AOEs have been operated by the MSC since 2005. As with the Sacramentos, a fifth ship was canceled.
The first ship to carry the AOR-designation was the USS Conecuh, which was acquired as a war prize in 1946. She was the former German tanker Dithmarschen, and she served in the U.S. Navy from 1953 through 1956, where she was used to test the concept of the AOE/AOR.
The Wichita-class comprised seven 13,500/40,000-ton replenishment oilers that were used from the late 1960s through the mid-1990s. These ships, similar to but smaller and slower than the AOEs, though larger and faster than the Neoshos, were designed for rapid underway replenishment using both connected replenishment and vertical replenishment (supplies carried from ship to ship by helicopters). The ships could carry 160,000 barrels of petroleum fuel, 600 tons of munitions, 200 tons of dry stores, and 100 tons of refrigerated supplies. With the overall reduction in size in the U.S. Navy fleet, these ships were all decommissioned and stricken during the 1990s.
The T-AOT Transport Tankers are part of the Military Sealift Command's Sealift Program, carrying fuel for the Department of Defense. They are not intended to operate with the fleet or provide underway refueling, but to move fuel in support of military operations to ports and depots around the world; they are operated by civilian crews.
The Missions were Type T2-SE-A2 ships like the Navy's Escambias ordered by the Maritime Commission in 1943 as civilian-operated transport tankers. The original order was for thirty, but six were taken over by the Navy and commissioned as AO-91 to 96; on the other hand MarCom took over three canceled Navy oilers of the nearly identical T2-SE-A3 type. After operating under civilian charter during the late war and immediate postwar period, transporting fuel to the many US forces overseas, they were transferred to the Naval Transportation Service in 1947-48 and the new Military Sea Transportation Service in 1949.
Mission San Francisco sank in a collision in 1957, and Mission San Miguel was lost after striking a reef several months later. Three Mission-class ships were later converted to Missile Range Instrumentation Ships and played a role in the space program: Mission San Fernando became USNS Vanguard (T-AGM-19), Mission De Pala became USNS Redstone (T-AGM-20), and Mission San Juan became USNS Mercury (T-AGM-21).
Mission Santa Ynez, scrapped in 2010, was the last survivor of the over 500 T2 tankers built during World War II.
Five Type T2-SE-A1 tankers were transferred to the USSR under Lend-Lease and four returned to the United States in 1948-49, making them part of the extended Suamico family. The Maritime Administration replaced the wrecked Donbass (ex-Beacon Rock) with her sister Sappa Creek.
The Maumee-class was a class of four 7184/32,950 ton T5-S-12a fleet oilers that were in service from the mid-1950s through the mid-1980s. These were the first tankers built specifically for the Military Sea Transport Service. The ships were not designed for underway replenishment (refueling ships at sea), but rather, they were made to carry bulk quantities of petroleum products, such as fuel oil, diesel fuel, and aviation fuel, to American and allied military forces overseas.
At some time after the loss of USNS Potomac (T-AO-150) in 1961, the three survivors were reclassified as "transport oiler"s (AOT).
At the time of the 1956 Suez Crisis the Military Sea Transportation Service purchased twelve additional T2-SE-A1 merchant tankers, making them belated members of the vast Suamico class. Their naval service was temporary; with the strain on US tanker capacity easing in late 1957 the twelve were transferred to Maritime Administration custody and struck.
A T5-S-RM2a tanker, American Explorer was laid down in 1957, intended to be the world's first nuclear-powered tanker, but construction costs ballooned; the MSTS, using funds left over from the construction of the Maumees, funded her completion with a conventional steam plant. American Explorer gained some notoriety in 2008 as a stricken hulk awaiting scrapping when she broke her moorings during Hurricane Gustav and collided with New Orleans' Florida Avenue Bridge.
By 1970 the MSTS, now renamed the Military Sealift Command, was operating an aging tanker fleet comprising largely WW2-built ships, which were wearing out. With limited budgets the MSC hit upon a build-and-charter program, under which new tankers would be built for private ownership but chartered to the MSC for twenty years. These nine new tankers were the Sealift class, which were intended to replace the T2s; their size was kept relatively small (587', 6786/34,000t) for access to smaller ports and shallower anchorages. They served from 1974 to 1995.
The Maumee-class Potomac (T-AO-150) suffered a catastrophic fire in 1961 which however left her after section and machinery largely undamaged; this portion was purchased by Keystone Tankships and mated to a new bow and midbody to create the SS Shenandoah in 1964. After serving under charter for the MSTS/MSC for several years, Shenandoah was acquired by the Navy in 1976 and transferred to MSC ownership under her old name. She was the first ship equipped with an offshore petroleum discharge system (OPDS), allowing her to supply fuel to forces ashore by pumping it directly over the beach instead of having to deliver it in a port.
In parallel with its build/charter operation of the Sealift class, the MSC in the 1970s obtained by a similar arrangement four larger T5-class tankers built for Falcon Shipping.
The five T5 Champion-class tankers have double hulls and are ice-strengthened for protection against damage during missions in extreme climates. They were built by the American Ship Building Company of Tampa, Fla., for Ocean Product Tankers of Houston, Texas, for long-term time charter to MSC, and entered service in 1985-87. These tankers embark on many unique missions including refueling the National Science Foundation in Antarctica and Thule Air Base in Greenland. In 2003 the MSC purchased four of the five outright, making them United States Naval Ships. Matthiesen is equipped for UNREP.
In the 1980s MSC acquired several other merchant tankers for service in the Ready Reserve Force and/or Pre-Positioning Fleet. American Osprey, Mount Washington, Chesapeake and Petersburg are OPDS ships.
U.S. Navy oilers were traditionally named for rivers and streams with Native American names- USS Neosho, Monongahela, Neches, etc.
Then, for the combined oiler, ammunition, and food replenishment ships (AOE), the names of cities (traditionally cruiser names) were used - USS Detroit, Camden, etc. - and for the similar but smaller AORs city/river pairs with Native American names were used- USS Kalamazoo, Wichita, Savannah, Wabash, Roanoke, etc.
The first nine ships of the newest class of oilers were named for noted ship designers and builders - USS Henry J. Kaiser, Joshua Humphreys, etc.- before returning to the traditional river names.
There are no U.S. Navy museum ships dedicated specifically to oilers. There is one model of an oiler that has been on display at the Defense Logistics Agency, in Fort Belvoir, Virginia. She is the USS Tamalpais (AO-96), named for a creek on a hill above Sausalito, California. In promoting the creation of an all 18 feet of the model can be seen.
The following is a list of tanker or cargo type hulls:
These perform Underway Replenishment. The first two are oilers; the others are dry cargo ships.