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THE SUBMARINE 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 again~t 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.

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The outer rectangular hull is longitudinally framed over transverse web frames and bulkheads not unlike a conventional tanker. The ship is divided to provide four main cargo tanks, a port and starboard wing tank, and a port and starboard center tank. The central pressure resisting hull is a ring-stiffened cylinder, 50 ft. in diameter.

A typical cross-section through the hull of the tanker is shown in Figure (2), the left view depicting the loaded condition submerged. The entire rectangular hull comprising the main cargo tanks, is filled with oil as are the four (4) variable pressure-resisting cargo tanks. All of the oil in the main cargo tanks would be at the ambient pressure of the outside sea water in this operating condition. The oil, being less dense than sea water, has a buoyant force, therefore the vessel must be heavy enough to maintain and operate at neutral buoyancy when fully loaded. This weight is largely in the hull-steel and pressure-resisting structures and enables the ship to get to the operating depth without paying a heavy price in fixed ballast for it. The right view depicts the “in ballast” condition submerged. The main cargo tanks are filled with sea water. The four variable cargo tanks are carried empty and at one atmosphere of pressure to support the weight of the ship. Briefly, the added buoyancy of these four tanks is necessary to support the weight of the ship when in the ballasted condition. It should be noted that, even though the same weight is carried, not as many barrels of sea water are carried as there is cargo oil.

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A simplified explanation can be used to show how oil and sea water of various densities along with their differences can be compensated for, to make the submerged weight of the tanker, with or without cargo, equal to the weight of the water it displaces — making it neutrally buoyant.

Submerging and surfacing operations are accomplished by taking on or expelling sea water from main ballast tanks just as the earliest submarines did — as a matter of fact, in 1900 before the Wright Brothers flew at Kitty Hawk.

Controlling attitude and depth is through the use or bow and stern planes not unlike the familiar control surfaces or WW II diesel-electric submarines.

The shipboard cargo handling system for the submarine tanker is a self-compensating system. With this system, the ship’s cargo tanks are always full or oil or full or water or some combination of the two.

This type of system offers a number of advantages, among them: it allows the tanker to be loaded or off-loaded at a submerged terminal facility; if surface facilities are used, it allows the tanker to dive immediately upon leaving the ice-free facility area; it eliminates oil vapors in the cargo tanks, thus reducing the explosion hazards commonly associated with the handling of oil cargoes; it reduces corrosion of cargo tank structures; and it tends to eliminate fatigue-stress on the surface ship-like structures which are caused by loading and unloading alternate combinations of tanks.

Perhaps the most important advantage is the high potential for this system to prevent oil contamination or the sea.

Figure (3) is a schematic diagram of the cargo system. On arrival at the loading terminal, the main cargo tanks are full of sea water. During the loading operation, cargo oil is forced into the top of each tank simultaneously by pumps at the terminal. The oil displaces the ballast water in the tanks, forcing ballast water from the bottom of the tanks to the sea. When the oil water interface approaches the discharge line, loading will be slowed and the ballast water passed into the expansion tank to allow separation to take place. Discharge to the sea is through a separate line at the tank bottom. An oil-water separator is indicated for the use in the final topping-off process, should large scale testing indicate the need.

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The crew was sized from a manning analysis. Based on the functions to be performed, thirtynine men would operate this tanker — but accommodations for 49 were provided to include cadets and trainees.

For piloting in confined waters of straits and sounds, some method of determining the ship’s position very precisely with respect to shoals, under ice ridging, icebergs and other obstructions, must be considered. An acoustic system using today’s advanced technology is logically used for this function. Figure (4) illustrates the various types of sonar apparatus that would be used and the type of information they would relate to the submarine operator. This equipment is essential in determining a safe path in the vertical dimension.

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Both surface and subsea loading were studied and it was concluded that subsea is clearly superior because the hostile environment is not challenged throughout the life of the system. Figure (5) illustrates one of the submerged loading concepts. The loading pad would be built in a temperate area and towed up to the loading terminal location and submerged. It would then be piped to the beach with offshore pipelines similar to those in use in offshore producing areas.

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The most practical loading technique is to bring the vessel down on the pad in a fixed location. maintaining slight negative buoyancy during the operation.

Ultimately, after the 250,000 ton tanker scheme was found wanting for customers, a 100,000 ton submarine tanker design was proposed to the Navy for fleet use. This was seriously considered, but a limited budget, which then was focused on a new attack submarine project, swept the fleet submarine tanker concept under the rug.

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