This post was written by George Petrie, Director of Engineering at The Seasteading Institute, in response to a number of posts that have appeared in our forums promoting designs for fully submerged seasteads.

The Seasteading Institute as a Forum for Discussion

The Institute is pleased to provide a forum for enthusiasts to discuss all aspects of seasteading and to explore all avenues for advancing the goals of the organization, and we encourage the open exchange of ideas on a wide range of topics. Moreover, we do not cast ourselves as sole arbiters of the merits of various ideas that are put forth.

Notwithstanding our commitment to this freedom of expression, we do feel it is appropriate to express our views on the establishment of underwater, or submersible seasteads.

In brief, the Institute neither encourages nor endorses the idea of establishing communities whose primary living accommodations are beneath the surface of the sea. We have adopted this view primarily because of concerns for safety, but also based on considerations of feasibility and compatibility with our vision of community building. Further discussion of these issues is provided below.

Safety Issues

As we have noted many times in our research reports and web postings, the sea is a harsh mistress that can develop powerful wind and wave forces capable of inflicting devastating damage to vessels and structures. The hope of escaping these destructive forces seems to be a prime reason for advocating sub-surface seasteads. However, we feel it is important to underscore some of the more worrisome issues with the idea.

While it is possible to escape the full force of an ocean storm by submerging beneath the waves, doing so is not nearly as simple as some in the forum suggest. In deep water, waves produce an orbital motion of water particles that diminishes exponentially with depth, roughly in proportion to wave length (1). This orbital motion of water particles beneath the sea surface causes a corresponding variation in pressure that can cause objects on (or under) the surface to move up and down or side to side as each wave passes.

Because they diminish with depth, the pressure variations associated with a surface wave are no longer perceptible at depths exceeding about a half-wavelength below the surface. As an example, consider an ocean wave with a period (time between successive crests) of about 14 seconds and a wavelength of 300m, or about 1,000 feet; waves of this length are expected about 10 percent of the time in a typical open ocean location. To completely escape the sub-surface pressure variations from such a wave, one would have to be more than 500 feet below the surface. At a depth of 100 feet, the pressure variation with each passing wave would be about 50% of that occurring at the surface; at a depth of 50 feet, the pressure variation would be nearly 75% of that occurring at the surface. For shorter wavelengths, the attenuation occurs at shallower depths; for a 500-foot (150m) wavelength, 50% attenuation occurs at a depth of about 50 feet below the surface.

These pressure variations can cause a submerged body to heave, pitch and roll. In the presence of a severe ocean storm, these motions can be significant. Moreover, these pressure variations can increase the structural loads on the hull, and are particularly worrisome because of the additional loads imposed on windows and doorways that are often envisioned for submerged seasteads.

The take-away from the above discussion is that one must be substantially deeper than just a few feet below the surface to comfortably ride out a storm. A small submersible would likely be tumbled about rather severely at depths less than 50 to 100 feet. Even larger vessels like military submarines have to dive deeper than 100 feet to ride comfortably in stormy seas.

Obviously, the technology to go this deep exists, as submarines and other submersible vessels have demonstrated for more than a century. However, the technology needed to cope with forces under the sea is every bit as sophisticated as that required for oil rigs, passenger ships and other vessels designed for operation on the surface of the sea. For more information, one can refer to Rules for Building and Classing Underwater Vehicles, Systems and Hyperbaric Facilities, 2012, published by the American Bureau of Shipping. Topics include requirements for design, testing, materials, ventilation, life support and a host of related safety issues. The document is available as a free download at the ABS website.
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(1) Wave properties are somewhat different in shallow water and/or near shore so anecdotal experiences in those situations may differ from what we describe above.

Feasibility

Numerous forum posts claim that submersible seasteads can enjoy the best of both worlds; they can rise to the surface and allow residents to enjoy sunshine and fresh air on vaguely conceived platforms without having to abide by the principles of Archimedes, which dictate fundamental requirements for buoyancy and stability. In reality, they will have to contend with the same issues of stability and reserve buoyancy as any other vessel, including docking with a supply vessel, landing a helicopter, or finding some other means of logistical support.

Equally pervasive is the false notion that submersible seasteads can be made cheaply. The reality is that submerged vessels must have sufficiently sophisticated and reliable ballast systems that will allow them to accurately maintain their target depth. For a vessel on the surface, floating is relatively easy, because there is reserve buoyancy in the hull; if weight is added or buoyancy is lost (due to a leak), the hull can continue to float, albeit at a deeper waterline. But for a vessel below the surface, there is no reserve buoyancy; if it springs a leak or simply wants to return to the surface, it must eject water from its ballast tanks or otherwise increase its buoyancy.

To be sure, people have sailed around the world on small sailboats and crossed oceans in rowboats or rafts. But these were expeditions taken on by adventurers, not proponents of an alternative lifestyle. The fact that one might successfully utilize a submersible as a novelty or a tourist attraction does not mean it is feasible to use as a permanent seastead any more than dirigibles or hot-air balloons validate the idea that humans can build castles in the sky.

Compatibility with the Institutes Vision of Seasteading

Notwithstanding the ambitions of the Atlantica Undersea Colony, the totality of human existence has shown that human beings prefer (and are biologically adapted) to live on the surface of the earth, where they are free to enjoy sunshine and fresh air.

While we respect the rights of others to embrace their own views, the Institute chooses to allocate its finite resources towards furthering the establishment of floating communities, in the belief that these will ultimately be the preference of an overwhelming majority of potential seastead residents.