Underwater habitats are a form of subsea technology. They are underwater structures in which people can live for extended periods and carry out most of the basic human functions of a 24-hour day, such as working, resting, eating, attending to personal hygiene and sleeping. In this context, 'habitat' is generally used in a narrow sense to mean the interior and immediate exterior of the structure and its fixtures, but not its surrounding marine environment. Most early underwater habitats lacked regenerative systems for air, water, food, electricity, and other resources. However, some underwater habitats allow for these resources to be delivered using pipes, or generated within the habitat, rather than manually delivered.
An underwater habitat has to meet the needs of human physiology and provide suitable environmental conditions, and the one which is most critical is breathing gas of suitable quality. Others concern the physical environment (pressure, temperature, light, humidity), the chemical environment (drinking water, food, waste products, toxins) and the biological environment (hazardous sea creatures, microorganisms, marine fungi). Much of the science covering underwater habitats and their technology designed to meet human requirements is shared with diving, diving bells, submersible vehicles and submarines, and spacecraft. It incorporates various developments used in other forms of subsea technology.
Numerous underwater habitats have been designed, built and used around the world since as early as the start of the 1960s, either by private individuals or by government agencies. They have been used almost exclusively for research and exploration, but in recent years, at least one underwater habitat has been provided for recreation and tourism. Research has been devoted particularly to the physiological processes and limits of breathing gases under pressure, for aquanaut, as well as astronaut training and for research on marine ecosystems.
Terminology and scope The term 'underwater habitat' is used for a range of applications, including some structures that are not exclusively underwater while operational, but all include a significant underwater component. There may be some overlap between underwater habitats and submersible vessels, and between structures which are completely submerged and those which have some part extending above the surface when in operation.
In 1970 G. Haux stated: At this point it must also be said that it is not easy to sharply define the term "underwater laboratory". One may argue whether Link's diving chamber which was used in the "Man-in-Sea I" project, may be called an underwater laboratory. But the Bentos 300, planned by the Soviets, is not so easy to classify as it has a certain ability to maneuver. Therefore, the possibility exists that this diving hull is classified elsewhere as a submersible. Well, a certain generosity can not hurt.
Comparison with surface based diving operations In an underwater habitat, observations can be carried out at any hour to study the behavior of both diurnal and nocturnal organisms. Habitats in shallow water can be used to accommodate divers from greater depths for a major portion of the decompression required. This principle was used in the project Conshelf II. Saturation dives provide the opportunity to dive with shorter intervals than possible from the surface, and risks associated with diving and ship operations at night can be minimized. In the habitat La Chalupa, 35% of all dives took place at night. To perform the same amount of useful work diving from the surface instead of from La Chalupa, an estimated eight hours of decompression time would have been necessary every day.
However, maintaining an underwater habitat is much more expensive and logistically difficult than diving from the surface. It also restricts the diving to a much more limited area.
Technical classification and description Architectural variations Floating The habitat is in the underwater hull of a floating structure. In the Sea Orbiter example this part should reach a depth of . The advantage of this type is horizontal mobility. Safety logistics are also easier due to immediate access to the atmosphere. Access shaft to the surface The habitat is accessible via a shaft to above the water surface. The depth of submersion is quite limited. However, normal atmospheric pressure can be maintained inside so that visitors do not have to undergo any decompression. This type is generally used inshore such as the underwater restaurant Ithaa in the Maldives or Red Sea Star (closed in 2012) in Eilat, Israel. Semi-autonomous Habitats of this type are accessible only by diving, but energy and breathing gas are supplied by an umbilical cable. Most stations are of this type, such as Aquarius, SEALAB I and II and Helgoland Autonomous The station has its own reserves of energy and breathing gas and is able to maneuver itself (at least in the vertical direction). This type is similar to submarines or atmospheric diving suits, but it avoids complete environmental separation. Examples of this type include Conshelf III and Bentos-300. Atmospheric pressure is a critical safety concern at all times.
Pressure modes Underwater habitats are designed to operate in two fundamental modes. Open to ambient pressure via a moon pool, meaning the air pressure inside the habitat equals underwater pressure at the same level, such as SEALAB. This makes entry and exit easy as there is no physical barrier other than the moon pool water surface. Living in ambient pressure habitats is a form of saturation diving, and return to the surface will require appropriate decompression. Closed to the sea by hatches, with internal air pressure less than ambient pressure and at or closer to atmospheric pressure; entry or exit to the sea requires passing through hatches and an airlock. Decompression may be necessary when entering the habitat after a dive. This would be done in the airlock.
A third or composite type has compartments of both types within the same habitat structure and connected via airlocks, such as Aquarius.
Components
Excursions An excursion is a visit to the environment outside the habitat. Diving excursions can be done on scuba or umbilical supply, and are limited upwards by decompression obligations while on the excursion, and downwards by decompression obligations while returning from the excursion.
Open circuit or rebreather scuba have the advantage of mobility, but it is critical to the safety of a saturation diver to be able to get back to the habitat, as surfacing directly from saturation is likely to cause severe and probably fatal decompression sickness. For this reason, in most of the programs, signs and guide lines are installed around the habitat in order to prevent divers from getting lost.
Umbilicals or airline hoses are safer, as the breathing gas supply is unlimited, and the hose is a guideline back to the habitat, but they restrict freedom of movement and can become tangled.
The horizontal extent of excursions is limited by the scuba air supply or the length of the umbilical. The distance above and below the level of the habitat are also limited and depend on the depth of the habitat and the associated saturation of the divers. The volume of the underwater environment available for excursions thus takes the shape of a vertical axis cylinder centred on the habitat.
As an example, in the Tektite I program, the habitat was located at a depth of . Excursions were limited vertically to a depth of (6.4 m above the habitat) and (12.8 m below the habitat level) and were horizontally limited to a distance of from the Habitat.
History The history of underwater habitats follows on from the previous development of diving bells and caissons, and as long exposure to a hyperbaric environment results in saturation of the body tissues with the ambient inert gases, it is also closely connected to the history of saturation diving. The original inspiration for the development of underwater habitats was the work of George F. Bond, who investigated the physiological and medical effects of hyperbaric saturation in the Genesis project between 1957 and 1963.
Edwin Albert Link started the Man-in-the-Sea project in 1962, which exposed divers to hyperbaric conditions underwater in a diving chamber, culminating in the first aquanaut, Robert Sténuit, spending over 24 hours at a depth of .
Also inspired by Genesis, Jacques-Yves Cousteau conducted the first Conshelf project in France in 1962 where two divers spent a week at a depth of , followed in 1963 by Conshelf II at for a month and for two weeks.
In June 1964, Robert Sténuit and Jon Lindberg spent 49 hours at 126m in Link's Man-in-the-Sea II project. The habitat was an inflatable structure called SPID.
This was followed by a series of underwater habitats where people stayed for several weeks at great depths. Sealab II had a usable area of , and was used at a depth of more than . Several countries built their own habitats at much the same time and mostly began experimenting in shallow waters. In Conshelf III six aquanauts lived for several weeks at a depth of . In Germany, the Helgoland UWL was the first habitat to be used in cold water, the Tektite stations were more spacious and technically more advanced. The most ambitious project was Sealab III, a rebuild of Sealab II, which was to be operated at . When one of the divers died in the preparatory phase due to human error, all similar projects of the United States Navy were terminated. Internationally, except for the La Chalupa Research Laboratory the large-scale projects were carried out, but not extended, so that the subsequent habitats were smaller and designed for shallower depths. The race for greater depths, longer missions and technical advances seemed to have come to an end.
For reasons such as lack of mobility, lack of self-sufficiency, shifting focus to space travel and transition to surface-based saturation systems, the interest in underwater habitats decreased, resulting in a noticeable decrease in major projects after 1970. In the mid eighties, the Aquarius habitat was built in the style of Sealab and Helgoland and is still in operation today.
Historical underwater habitats Man-in-the-Sea I and II
The first aquanaut was Robert Stenuit in the Man-in-the-Sea I project run by Edwin A. Link. On 6 September 1962, he spent 24 hours and 15 minutes at a depth of in a steel cylinder, doing several excursions. In June 1964 Stenuit and Jon Lindbergh spent 49 hours at a depth of in the Man-in-the-Sea II program. The habitat consisted of a submerged portable inflatable dwelling (SPID).
Conshelf I, II and III