For decades now it has been possible to wield sea power without a navy, and the current situation in the Strait of Hormuz is demonstrating just how vulnerable ordinary shipping is to modern missiles and drones — which got me wondering about the practicality of a submersible oil tanker:
In the early ’70’s there was great interest in economically transporting oil from the large oil finds in the Arctic to the markets in the U.S. and Europe. Either pipelines or marine systems seemed feasible. But, bringing the oil out by submarine tanker — on a year-round basis — appeared to be the most cost-effective approach. Consequently a design study of an Arctic submarine tanker was conducted by General Dynamics’ Electric Boat Division to demonstrate the practicality of this approach.
Though this project never materialized, the evident value of such a submarine tanker for refueling oil-burning surface ships in wartime has kept this concept alive. A battle group of nonnuclear powered carriers and escorts, capable of being refueled from a submerged tanker — on any course and at relatively high speed — would greatly increase transit speeds while ensuring a vital underway replenishment capability, particularly in a conventional war environment of enemy ocean surveillance satellites and enemy long range cruise missiles.
The submarine tanker designed by Electric Boat was most economically sized to carry 250,000 deadweight tons of oil. With a length of 1,000 feet, an 80 foot draft, a submerged displacement of 360,000 tons, an operating depth of 1,000 feet and a sustained speed or 18 knots, this giant submarine could transit efficiently under the Arctic ice, through the restrictions in the Northwest Passage and readily avoid icebergs in Davis Strait.
Since this tanker could and probably would load its oil from a bottom loading pad, its total cycle of operations could be secure from enemy observation. Although designed for peacetime commercial use, it could be considered an asset to be activated as a naval auxiliary in wartime. Thus, an enemy campaign against such a vital element in U.S. logistics should have little chance of being successful. With the U.S. advocating a “forward offensive maritime strategy,” the security or its critical refueling elements “under the gun” of enemy homeland defenses even moreso emphasizes the submarine tanker solution.
When the attractiveness of this submerged commercial tanker for wartime naval operations became evident, a further design study for the underwater refueling system was conducted. A probe and drogue system similar to that used for aircraft refueling from tanker aircraft was shown to be feasible — the submarine positioning itself under the surface ship and pumping oil up through ·its telescopic probe into a bottom drogue on the surface ship. The safety factor in this method of refueling was particularly good because of the stability of the submarine under all sea conditions and the little movement of a surface ship drogue, positioned at its center of flotation.
The vessel is essentially a large, rectangular tanker-like ship hull with the long internal cylindrical pressure-resisting hull, usually associated with a submarine, centered within the outer rectangular hull. The central hull contains the living and control spaces, pumps and auxiliaries, and the propulsion machinery. Except for the free flooding ends of the ship, the remainder is filled with oil cargo in the loaded condition and sea water in the ballasted condition. The variable cargo tanks on either side are provided to compensate for the difference between density of sea water and the oil.
The propulsion is by twin screws driven by steam turbines. Steam is supplied by a pressurized water reactor, similar in design to those presently in use for commercial electric power generation. The nuclear steam supply system produces steam for the two propulsion trains, each plant developing 37.500 SHP at the propeller for a total of 75,000 SHP. The sustained sea speed would be 18 knots.
By the end of World War II, the Germans were using “milk cow” submarines in this role — but submarines have come a long way since then. In particular, modern submarines travel more efficiently while submerged, not less, because they’re designed primarily for undersea travel, where they encounter no wave-making resistance.
They also encounter no air, which is why “true” submarines only became practical with the advent of nuclear power. But there are non-nuclear forms of air-independent propulsion (AIP), like fuel cells:
Fuel cells are not new. They have undergone significant technological improvements from when they were first considered for submarine propulsion by Germany in the 1950s. The principle of producing power is straightforward; hydrogen and oxygen gas react to produce water and an electrical current. It is the reverse process of electrolysis, where a current is sent through liquid water to split the bonds between the oxygen and hydrogen atoms. Through engineering optimization, enough electrical power can be harnessed from this reaction to power a variety of loads. Current uses include cars, buses, remote cell phone towers, and forklifts. The German Navy already has a hydrogen fuel cell–powered submarine class, the Type 212, first launched in 2005, and variants it sells abroad to countries such as Italy and Singapore.
The Gotland-class submarine, a Swedish boat, is the most prominent example of the extreme stealth of non-nuclear AIP submarines. During a joint wargaming exercise in 2005, it tactically sank the USS Ronald Reagan (CVN-76) several times. It was virtually undetectable by all available antisubmarine efforts.
While powered by a Stirling engine, the concept and application of the Gotland-class AIP system are the same as for others. Stirling engines and other forms of non-nuclear AIP, while quieter than nuclear, are louder and less efficient than fuel cells. There are no mechanical parts in the main fuel cell system such as in combustion driven engines. Fuel cells offer the lowest noise levels because almost no sound is produced by an electro-chemical reaction. The only components in the engine room that could contribute to the sound signature are the compressors and pumps for fuel, water, and cooling.
Yet, cooling requirements for fuel cells are much lower than combustion and nuclear because of the low operating temperature of 100°C for proton-exchange membrane fuel cells. Conversely, nuclear-powered submarines need extensive cooling and vibrational dampening because of high operating temperatures, requiring several large coolant pumps and bulky, complicated mechanical systems such as steam turbines and reduction gears.
In addition to the fuel cells, there are advanced lithium-ion batteries on board AIP vessels that can power the electric motor at higher speeds with no loss of acoustic fidelity. Without the nuclear reactor, there also is a smaller infrared heat signature and no radiological trace. There is a significant stealth advantage to fuel cells that lowers the detectable range of the vessel.
Fuel cell AIP submarines do not have the nominally infinite endurance of nuclear-powered submarines; however, they can remain underwater for much longer than alternative AIP options such as closed-cycle diesel generator, Stirling, and MESMA (a French steam turbine). Fuel cells are significantly more efficient than diesel engines, thus requiring less oxygen fuel per kWh of energy produced. Diesel-electric boats have a max underwater time of a couple of days because of battery limits. Fuel cell AIPs can last weeks underwater and have a range of up to 2,000 nautical miles. Further, by forward deploying these vessels in ports that are close to their respective operating areas, the ratio of time on station to transit and refueling time is increased.
They wouldn’t be immune to mines and underwater drones, of course, but one thing at a time.
The finest young battalion and company officers that S.L.A. Marshall knew, he explains (in The Soldier’s Load and the Mobilty of a Nation) were men of this type: 