Giles Sparrow
1499

Submersibles

To explore the greatest depths of the ocean, normal submarines and submersibles are impractical - their flimsy hulls would be crushed by the enormous pressures of water at depths of up to several miles. When oceanographers need to descend to great depths, they use specially designed deep submersibles, of which there are only a few in the world. However the expense and danger involved in visiting the ocean depths in person means that most deep ocean exploration is carried out by unmanned submersibles called remote operated vehicles (ROVs) or autonomous underwater vehicles (AUVs).


Manned Submersibles

Although deep submersibles capable of descending to several miles may outwardly appear similar to the small submersibles used near the surface, their internal design is radically different. The pressurized part of any deep-sea vessel has to be made from a hollow sphere of strong metal such as steel or titanium. This may be built into a conventional-looking superstructure that carries the submarines ballast tanks, tools, and propulsion systems, but when the crew climb on board, they are confined within a small pressure sphere. A sphere is the only suitable design for an air-filled compartment capable of resisting deep sea pressures, because the ocean presses in from all sides equally, it resists crushing. Irregular shapes, conversely, have weak points that give way much more easily under pressure.

Deep submersibles usually carry a crew of two, plus an observer, typically, a scientist. Unlike normal submarines, they are not just used for transport. For scientific or commercial purposes, they need to be able to make measurements and collect samples from the water around them and the sea floor below. Submersibles therefore have to carry a variety of other instruments and attachments and usually include television and still cameras, lights, manipulator arms and external containers for collecting samples. To allow the crew a direct view of the scene outside, parts of the pressure sphere are often replaced with toughened glass or plastic. In order to maintain the pressure-resistant properties of the sphere, these windows have to be convex and follow the curve of the metal hull perfectly.

Apart from the pressure sphere, every part of the vessel's engines, tanks, and other equipment is either solid, or open to the sea, so it can flood without resistance. Normal submarines and submersibles also carry ballast tanks that are pumped full of air from onboard tanks or flooded with seawater to increase or decrease the vessel's buoyancy, though deep submersibles use gasoline instead of air. Gasoline is still considerably lighter than water, and so will force the submersible to rise, but because it is a liquid, it will help the ballast tanks to resist compression.


Development of Manned Deep Submersibles

Deep submersible designs are direct descendants of the first deep-ocean vehicle. This was the bathysphere, invented by U.S. oceanographer Charles William Beebe. A bathysphere was a simple hollow sphere with a thick steel hull, reinforced observation windows, and a crew hatch on the top. It was lowered from the surface by a cable attached to a ship, and was incapable of independent movement. Beebe used his bathysphere to set a series of records for deep descents into the ocean, culminating with a descent to over 900 meters (3000 feet) in 1932.

However, the early bathyspheres were very dangerous, as well as being unmaneuverable. With no buoyancy aids or propulsion, if the cable linking the sphere to the surface snapped, the occupants would be doomed. These problems, coupled with the clear advantages of a self-propelled vehicle, led the French explorer Auguste Piccard to develop the first maneuverable "bathyscaphes" from the late 1940s.

Piccard's early design for this deep submersible resembled a hot air balloon - a titanium pressure sphere was suspended from the bottom of a much larger system of ballast tanks. The entire vessel had "neutral buoyancy," so it could hang naturally in mid-ocean at any depth, and be maneuvered using a minimum of power. In 1960, Piccard and U.S. naval lieutenant Don Walsh descended to nearly 11 kilometers (36,000 feet) in the Marianas Trench, the deepest trench on the ocean floor, aboard the bathyscaph Trieste.

Modern deep submersibles are more compact than Piccard's bathyscaphes, with the pressure sphere now incorporated in a superstructure with the submersible's other systems. They still, however, owe a huge debt to these early vessels.


ROVs and AUVs

Since the 1970s, computer technology has allowed the development of sophisticated uncrewed vehicles that are capable of collecting scientific data and carrying out many of the more mundane tasks involved in deep-sea exploration. Remote operated vehicles are controlled by instructions from an operator in a submersible or, more frequently, a command vessel on the surface, while autonomous vehicles are capable of making basic decisions independently - they can be released into the sea (for example to carry out a survey), and recovered days or even weeks later.

These robotic explorers come in a huge variety of designs, each specifically tailored to a certain task. Some are capable of floating freely and propelling themselves about in the water, but many are bottom-reliant or structure-reliant; they may have caterpillar tracks for crawling along the seabed, or hydraulic rams for climbing along undersea pipes. They may be dragged through the water by a tow-line from a ship, or self-propelled.

The most versatile type of ROV is the free-swimming tethered vehicle, which typically contains a set of instruments, engines, and manipulators mounted on a framework around 2 meters (6.6 feet) long. Buoyancy tanks filled with lightweight foam counteract the weight of the rest of the vehicle, giving it neutral buoyancy so that it can stop and float at any depth without using power. This type of vehicle often carries television cameras, robot arms and containers for taking samples of rock or marine organisms. It is linked to its controller on board a command ship by a tether, which relays television pictures and other data from the vehicle, and returns instructions from the control console. Some free-swimming ROVs are untethered, and information is transmitted to and from the command ship by radio or acoustic waves.


Uses and Applications of Deep-sea Vehicles

Deep-sea vehicles of all sorts have a wide variety of uses, and crewed submersibles are often used in conjunction with ROVs or AUVs, usually in systems where the robot vehicle is controlled by an operator on board the submersible.

Both submersibles and robots, originally used for the commercial purpose of inspecting oil rigs, have revolutionized scientists' understanding of the deep seas and the ocean bed. Using them, oceanographers have discovered life around deep sea vents, studied undersea volcanoes and strange fish from the deep abysses, and surveyed the ocean floor to understand how it has moved over time.

But today, most deep-sea technology is used for commercial purposes, linked mainly to the discovery and exploitation of undersea resources such as oil and minerals. As well as carrying out this kind of surveying work, submersibles and ROVs equipped with tools for cutting and welding are used for the undersea construction projects that extract resources once they have been found. Once building work is complete, ROVs can be used to inspect and maintain undersea structures, with occasional repair work by crewed submersibles. Many structure-reliant ROVs are ring-shaped machines designed to crawl along underwater pipelines or piles and survey them for damage.

Because most exploitation of the sea is concentrated around the continental shelves in relatively shallow waters, most commercial submersibles and ROVs are not built to withstand the enormous stresses of descents to thousands of meters. One exception are underwater cable-laying ROVs, which crawl along the seabed (or are dragged by a towline from the surface) excavating a trench, laying a cable into it, and filling in the hole.

Another important use of submersibles and ROVs is in search and salvage work after aircraft crashes or shipping disasters. ROVs were deployed to identify and recover parts of the Space Shuttle Challenger after its explosion in 1986, and the 1985 discovery of the Titanic was only possible with the aid of the Woods Hole Oceanographic Institute's deep submersible Alvin.


Further Reading

Ballard, Robert D. The Discovery of the Titanic. Toronto: Madison Publishing Inc., 1995.

Ferris Smith, P (Ed.) Underwater Photography: Scientific and Engineering Applications. New York: Van Nostrand Reinhold, 1984.

Society for Underwater Exploration. Advances in Underwater Inspection and Maintenance. Boston: Graham and Trotman, 1990.

Society for Underwater Exploration. Diverless and Deepwater Technology. Boston: Graham and Trotman, 1989.

Society for Underwater Exploration. Submersible Technology. Boston: Graham and Trotman, 1986.

Society for Underwater Exploration. Submersible Technology: Adapting to Change. Boston: Graham and Trotman, 1988.

Related Topics

Autonomous Underwater Vehicle, Deep-sea Exploration