Seabed and deep dive submarines and submersible drones and sub hulls with steel and titanium alloys

There have been proposals to have a new class of seabed operating military submarines. They operate undetected on the ocean floor where hills and rocks reflect sonar and magnetic detection. The average depth of the ocean is about 14,000 feet.

The pressure on a submarine’s hull increases with depth, limiting the depth below the ocean surface at which it can operate. The water pressure increases by 44.45 pounds per square inch for every 100 feet of additional depth in salt water.

This would mean a submarine hull would need to withstand 622,000 psi to operate on the ocean floor and more like 900000 psi to have a safety margin.

The deepest diving military submarine was believed to be the Russian Alfa (4000 feet). Any seabed submarine would need to be able to handle triple the water pressure of the Alfa’s Titanium hull.

South Korea has a new steel, aluminum, nickel alloy that is inexpensive (ten times cheaper than titanium) and as strong as titanium. There are also developments which could make Titanium alloys inexpensive.

There are also new iron, aluminum, scandium alloys that light and very strong

Soviet Alfa nuclear submarine

All Soviet nuclear submarines, Project 705 used a double hull, where the internal hull withstands the pressure and the outer one protects it and provides an optimal hydrodynamic shape. However, unlike almost all other submarines, the hulls of the Lira had variable diameters. The shape is optimized for minimal active sonar signature and minimal water resistance and, although it complicated the design, it was essential for providing required maneuverability.

Apart from the prototypes, all six Project 705 and 705K submarines were built with titanium alloy hulls, which was revolutionary in submarine design at the time due to the cost of titanium and the technologies and equipment needed. The hull was designed for extreme depths, below the deep sound layer (at 1 km), but complete redesign of the plumbing and other inter-hull systems was delayed. According to some information, one of the submarines was tested on depths up to 1300 meters, but submerging to such depths and returning caused permanent damage to equipment, which in a few cycles would make the vessel very unreliable.

Submarine designers normally intend their creations to operate well away from the hull’s physical limits, imposing a safety margin that varies from country to country [1.5 in the USA, 1.75 in the UK, and 2.0 in Germany]. Typically a submarine will have three diving depths:

* a normal operating or “test” depth
* a safe excursion depth
* a crush or collapse depth

A submarine’s hull is normally constructed of steel, or exceptionally of titanium. Special High Yield [HY] steel alloys have been developed to increase the diving depth of submarines, although the improved depth performance of these alloys imposes a price of increased fabrication challenges. These special steels are denominated by their yield stress in thousands of pounds per square inch — thus HY-80 steel has a yield stress of 80,000 pounds per square inch [corresponding to a depth of 1,800 feet], HY-100 a a yield stress of 100,000 pounds per square inch [corresponding to a depth of 2,250 feet], and so on.

* During World War II, American fleet submarines normally operated at a depth of 200 feet, though in emergencies they would dive to a depth of 400 feet.
* Post-War American submarines, both conventional and nuclear, had improved designs and were constructed of improved materials [the equivalent of “HY-42”]. These boats had normal operating depths of some 700 feet, and a crush depth of 1100 feet.
* The Thresher, the first American submarine constructed of HY-80 steel, reportedly had a normal operating depth of 1,300 feet, roughly two-thirds the crush depth limit imposed by the HY-80 steel.
* The Seawolf, the first American submarine constructed of HY-100 steel, is officially claimed by the Navy to have a normal operating depth of “greater than 800 feet,” but based on the reported operating depth of the Thresher, it may be assumed that the normal operating depth of the Seawolf is roughly double the official figure.
* The Soviet Alfa submarines, constructed of titanium, reportedly had an operating depth of nearly 4,000 feet.

The Japanese Soryu submarine has HY-156 steel which could allow operation at 600 meters or 2000 feet depth

A special maraging heat treatment can create nickel-steel alloys that are 10% stronger and lighter than titanium. They were used in some 1-3 person vessels for reaching the bottom of the ocean. However, the maraging metal is more brittle and cannot handle multiple trips to the bottom.

Vessels that have made it to ocean bottom have been small and only went for a few hours at a time

On March 26, 2012, Cameron reached the bottom of the Challenger Deep, the deepest part of the Mariana Trench. The maximum depth recorded during this record-setting dive was 10,908 metres (35,787 ft). It was the fourth ever dive to the Challenger Deep and the second manned dive (with a maximum recorded depth slightly less than that of Trieste’s 1960 dive). It was the first solo dive and the first to spend a significant amount of time (three hours) exploring the bottom.

Several other vehicles are under development to reach the same deepest ocean depths. The groups developing them include:

Triton Submarines, a Florida based company that designs and manufactures private submarines, whose vehicle, Triton 36000/3, will carry a crew of three to the seabed in 120 minutes.

Virgin Oceanic, sponsored by Richard Branson’s Virgin Group, is developing a submersible designed by Graham Hawkes, DeepFlight Challenger, with which the solo pilot will take 140 minutes to reach the seabed. This project is on hold while they work on better technology.

DOER Marine, a San Francisco Bay Area based marine technology company established in 1992, that is developing a vehicle, Deepsearch (and Ocean Explorer HOV Unlimited), with some support from Google’s Eric Schmidt with which a crew of two or three will take 90 minutes to reach the seabed, as the program Deep Search.

SOURCES – Federation of American Scientists, NBC News, Wikipedia, gentle seas blog