SUB-MARINE: Modular Undersea Habitats

Overpopulation, specifically through overuse and ill-management of land, is becoming increasingly an issue for society. There is a lot of land on earth, but property prices and land prices are only shooting higher and higher. However, the surface of our planet is covered in nearly 70% oceans¹. That's a lot of real estate that has been overwhelmingly unexplored and unused to this point¹.

To hit two birds with one stone, a conclusion can be drawn to explore the possibility of human life in the oceans. This instructable seeks to research the viability of that point, by coming up with a possible undersea habitat. This habitat will explore this project as an engineering challenge, researching the viability of design and material properties, conducting hand calculations and simulations to ensure realistic and accurate expectations, and making conclusions based on the results.

I'm Levon, I'm 17, and this is my proposal for a housing solution for future generations.

Design Software:

• Autodesk Fusion

Structure:

This structure will be designed in 2 configurations: Light and Medium. More on this later.

Both Configurations:

• Marine Insulation
• Sheathing and Framing Supplies
• PEX water piping

Light Configuration

• 1" 316L Stainless Steel Sheeting
• Polycarbonate Dome - 1" Shell

Medium Configuration

• 3" 316L Stainless Steel Sheeting
• Polycarbonate Dome- 3" Shell

Life Support:

Both Configurations:

• Sea Turbine
• Reverse Osmosis Desalination System
• Hydroponics System
• Lavatory System
• Nitrogen and Oxygen Cylinders
• Electrolysis Fuel and Oxygen System

Light Configuration

• Air Supply Pipe
• Solar Panels
• Diving Module

With any engineering project, you must first start by identifying your constraints; what criteria your design must fall into, and what limitations and problems you need to address.

In terms of the structure itself, it needs to be incredibly robust to stand up to our first problem: Hydrostatic Pressure. As you go deeper into the water, the weight of all the water above you starts pressing down on you. This compounds a lot of pressure, which we will need to ensure our design can withstand.

In seawater, Corrosion also needs to be addressed. Traditional steels would oxidize and rust when put into contact with the potent corrosive agent that is saltwater. We will need to make sure our structure won't degrade due to corrosion.

For humans to live under the ocean, a variety of life support issues need to be remedied as well.

These include:

• Food & Sustenance
• Electricity
• Drinkable Water
• Breathable Air
• Quality of Life & Sanity

At pressures like those we will be facing in the deep-sea, complex designs are not the greatest of ideas. When so much load is on your design, minimizing possible failure points is a priority.

For this purpose, I chose to go with unibody-based designs. These are simple, thus strong, and allow for a variety of modularity to suit many different purposes.

If we analyze these NOAA bathymetric charts of the water depths around San Francisco², we can see that coastal waters here are <50m deep. Indeed, most coastal waters seldom reach deeper than 200 meters⁶.

Our Shallow configuration is built to be installed into these coastal waters, with simulation and verification up to 500-meter depths.

Our Medium configuration is simulated for deeper-sea installations, with calculated verification up to 3000-meter depths.

Why no Deep? As it turns out, engineering can take you incredibly far. But life support, and ensuring the viability of actually living at the bottom of the deepest oceans, comes with its own set of hurdles. More on this later.

How even do you check to see if a design can survive under tons of pressure?

You use the power of Mathematics :)

The simplified formula for calculating hydrostatic pressure, in units of pascals, is simply the product of the density of the fluid, the acceleration of gravity, and the depth of water.

For example:

This structure will be at a depth of 500m.

The density of seawater is ~1023kg/m^3.

The acceleration of gravity is ~9.81m/s^2.

Multiplying these together gets a pressure at this depth of 5,017,815 pascals. Converting this to PSI, as that is a unit of pressure we are more familiar with in everyday life, nets us 727.8 psi.

So, to test if the structure can survive at 500m depths, we can use the power of FEA simulation. By applying a pressure of 727.8 psi to the outer faces of our model, we will simulate the conditions at the bottom of the coastal ocean.

Luckily, Fusion has a built-in hydrostatic pressure simulator! To verify our mathematics with the actual simulation, I ran 2 simulations on the same model (shown in the images). One was a hydrostatic seawater simulation, with the surface of the ocean 500m above, and the other was a regular pressure simulation with our hand-calculated pressure of 730 psi.

Our displacement figures were within 200mm of each other! How cool is that?!

(Don't worry about the excessive deformation, this was an intentional overstressing of the geometry for testing)

For the rest of the simulations, I'll use the built-in hydrostatic simulation function for convenience, as we've shown we get reasonably close results.

(This section was conducted through a bit of intuition and just going along with things, I have no formal engineering education yet. Results may be very off)

Having chosen the unibody modular design, we need to then figure out what shape we will be using for the cross-section. A variety of shapes can all be used, with different strengths and weaknesses, but there is no way to know without simulation.

To test the viability of various polygons, a solid model was made in Fusion of each shape.

• Each shape was a circumscribed polygon with a radius of 120"
• This polygon was offset outward a distance of 1"
• This was extruded to a length of 720"

For FEA Setup:

• The bottom face of each polygon was constrained
• Since this will be the face resting on the seafloor, I believe it is realistic to assume it will be relatively stationary.
• A Hydrostatic Pressure Load was applied to all other outer faces of the polygon, with the surface of the ocean being set to 500m above (this simulates the structure lying on the ocean floor 500m down).
• The model was meshed and results computed.

Results:

• Here we are looking for minimal displacement. The less a shape deforms, the stronger we can assume it to be. So, lower numbers are better.
• The triangle and square just fall apart hilariously, but from then on, the strength increases with more sides added to the shape. However, the displacement reaches its lowest at the 12-sided dodecagon, then begins to increase after.

Theory as to why:

• The pressure is a constant at these depths. So, the fewer faces these shapes have, the more share of the pressure each face has to take. I believe this is why the triangle and square deform so much. The 2 applicable faces of the triangle (the 3rd is constrained) each have to bear 1/2 of that pressure. And with the square, 1/3. This trend continues and is why the octagon behaves better than the hexagon (1/7 of the pressure, so less load, as opposed to the hexagon's 1/5.)
• This trend peaks at 12 faces, but then displacement increases afterward. Why?
• My working theory is that at that point, the shape becomes too flimsy and does not have enough structure in itself to stay together. This is very rough and unbased, so I would love to see some professional analysis on this.

So:

• The 12-sided unibody is the design that had the least displacement, so we will choose to go with that as our profile.

316L Stainless:

• Valued for its corrosion resistance, especially in chloride environments¹³. This is a good choice compared to traditional steels and stainless alloys such as 304. 316L's low carbon content allows for excellent performance in our deep-sea environment¹⁰.
• This can be readily sourced in plates/sheets up to 6" thick¹⁴.
• 316L can be welded using GTAW (TIG), and offers less pits and porosity than other forms of stainless steel¹⁰.

Polycarbonate:

Polycarbonate may not have the best stats in compressive strength¹⁵, but that is only one factor to consider when choosing a material. It has excellent transparency, chemical and corrosion resistance , abrasion resistance, and performs well in cold temperatures¹⁶ ¹⁷.

• Polycarbonate can be formed into large hemispheres such as the ones we will use, and can even be done locally in California¹¹, minimizing shipping costs from manufacturing our components overseas.

PEX Tubing (Water Systems):

PEX is a staple in modern construction. Its strengths include ease of installation as well as great cold-temperature performance¹² ¹⁸. Due to our environment at the bottom of the seafloor, conditions will be cold. This temperature stability will be a massive pro for our design.

Marine Insulation:

Due to those aforementioned cold temperatures, reaching near-freezing¹⁹, proper insulation and heating will be a must in these shelters. Due to the thermally conductive nature of our primary building material, stainless steel, a lot of our precious heat will be taken away by our environment. Marine Insulation also reduces vibrations and acoustic reverberations, helping with the quality of life for the occupants²⁰.

We saw in Step 5 that we can source our polycarbonate domes pre-made. However, these stainless polygonal prisms will be a challenge to find already manufactured. However, these can be fabricated ourselves.

By using multi-axis plasma cutting tables to cut bevels on our stainless sheets²¹, we can cut these angled plates and then TIG weld them to each other. To ensure quality control and consistent welds, we can employ a robotic TIG welding system on rails to minimize the reliance on human labor. These shelters will be manufacturable without having to make dozens of human welders to be in these steel tubes for hours on end.

The junctions on these shelters can similarly be welded, and stainless steel bolts with rubber gaskets will be used to attach the polycarbonate domes to the main bodies.

Air:

This is going to be one of the most challenging factors to work around. Contrary to popular belief, oxygen isn't the only gas we need to breathe. The air we breathe is around 78% nitrogen and only about 22% oxygen. So relying on pure oxygen systems will not work as a long-term solution.

Now, oxygen can be harvested from seawater via electrolysis, by using electricity to separate filtered water molecules into their respective hydrogen and oxygen atoms. The hydrogen can be harvested as a fuel, and the oxygen for life support and breathing. We will include such a system in our Medium Configurations as a backup.

Unfortunately for our project, too much exposure to pure oxygen long-term can result in some serious health issues and oxygen poisoning²⁴.

The issue with building a deep configuration of our habitat is that there is no reliable way to source nitrogen in the deep ocean. We breathe around 2000 gallons of air per day²⁵ or about 270 cubic feet. Doing some napkin math leads us to calculate that we need about 211 cubic feet of nitrogen and 59 cubic feet of oxygen per day. Now, there is just no way to source this amount of nitrogen in the ocean. Seawater contains trace amounts of it, but we would have to cycle thousands and thousands of gallons to harvest just a fraction of the nitrogen we need.

So, the best solution with existing technology goes as such:

• Equip the Light Configuration with an air pipe leading up to the surface of the water to intake atmospheric air.
• Equip both Medium and Light with refillable cylinders of oxygen and nitrogen; for medium, as the primary source of air, and for light, as a backup.
• For example, if a medium habitat is designed as a research station for 2-week missions for a group of 3 scientists, here's how many 300 cubic foot nitrogen and oxygen cylinders⁸ ²⁶ we would need to utilize:
• 14 days x 3 scientists each breathing 270 cubic feet of air per day = 11340 cubic feet of gas. Add 10% for safety, and we are left with about 12,500 cubic feet worth of air needed. Using the 78-22 ratio of nitrogen and oxygen, and assuming we have 300 cubic foot cylinders, we'd need 33 cylinders of nitrogen and 10 cylinders of oxygen.

This leads to some constraints with implementing these habitats. The light configurations can be full homes, apartments, and hotels but just will have pipes leading up to the surface.

However, the medium will have to only serve as a temporary habitat, say, for scientific missions. Its stores of nitrogen and oxygen will be replenished by visiting submarines.

So we could definitely build a habitat to survive at the absolute bottom of Challenger Deep, but it would have the same limits as the Medium Configuration. Except that getting expeditions to it to replenish its gas stores would be exponentially more complex.

Water:

Water is going to be the opposite situation. Water will be the resource we will be harnessing for drinking water, sewage systems, electricity, growing food, you name it.

We have incredible amounts of water, and, as a result, electricity in this environment, so we will be leaning on them for a lot of our life support.

Desalination systems such as the one above will help us with filtering seawater to source water for drinking and irrigation. Reverse Osmosis is used in some home water filters to ensure ultra-pure water, so the water quality will be good enough for our uses.

Electricity:

Tidal turbines have been an effective technology for years now, and work to create electricity from the moving currents of the ocean. As the currents are an unlimited resource, we will be able to source a lot of electricity to power whatever we may need, for both configurations.

For the light configuration, a solar array will also be attached to a module that will float on the surface of the water. This type of solar configuration is sometimes referred to as "floatovoltaics", a play on photovoltaics²⁸. This surface module can also house the air supply pipes. As the habitat will be deep under the water, sunlight will be scattered and won't properly reach the habitat itself. For this reason, putting solar panels on the habitat itself will not work the best.

Using renewable energy will allow these habitats to be more self-sufficient and cleaner for the environment.

Food:

Hydroponics systems allow many plants and crops to be grown without the use of soil or sunlight. Nutrient-rich water is used instead of soil, allowing us to harness our vast supply of water to support our food system. The nutrients can be brought down via submarine every once in a while and will be supplemented by crops that can fix the water supply, as well as harvesting some trace amounts from the seawater.

If a Light Configuration is not installed too deep, we can also employ fishing as a food source.

Quality of Life:

This may be one of the most important categories. Living in a metal tube for a long time can be rough on people, so we will be sure to do our best to address this.

• Circadian Rhythms, the bodily systems that regulate your sleep-wake cycle, are thrown off by lack of sunlight. We will use artificial sunlight panels that will change in brightness and color temperature throughout the day to curb this issue²⁹.
• Being able to look outside is incredibly important, so we prioritize having a polycarbonate clear dome at the end of every structure in this habitat, to make sure our inhabitants are never too far away from a view.
• Our hydroponics system will grow crops to provide a balanced diet. Lentils and beans can be grown for protein, potatoes for vitamins, a vast array of veggies for nutrients, herbs, etc.
• Airflow system around the habitat, circulating air and scrubbing CO2 to ensure a fresh and clean atmosphere.
• Electric heating coils around the insulation of the habitat to maintain a comfortable temperature and use the electricity for which we have an abundant supply.

Here are some images of the Design process of the interior of the habitat. These are modeled, then a section view is used to show the inside layers. Then the Fusion local Renderer is used to give a professional render of the structure's skeleton.

In the Section View you can see the 316L Stainless Steel Outer Layer, the white Marine Insulation, the electric heating coils, as well as the inner sheathing.

This is a mockup of the surface module for the Light Configuration. This is a floating solar panel array and an air intake for the habitat. This air intake will be connected to some pipes leading down underwater.

These are some mockups of the air, desalination, and hydroponics systems in a Light Configuration habitat.

• The gas cylinders can be stored under the floor. Because our shape is cylindrical in nature, adding a flat floor eliminates some space from the overall room. However, this underfloor area can be used as storage space.
• These are modeled and rendered locally in Fusion.

Because of the modularity and freedom of this design, there is no "final product", per se. However, here are some possible ways this can be configured and designed. This is an example of a 4-Module Light Configuration final habitat. These have simulation verification under 500m of hydrostatic pressure, with a factor of safety of 8+ and a maximum of 31mm of displacement.

• Share insight on the following questions: What if extreme environment habitats embraced their unique surroundings to enhance human well-being? What did you learn through this process that you could apply to addressing a problem of the built environment in your own community? 

This challenge has been an excellent lesson in being thorough. Through researching and planning out this project, I learned an incredible amount about engineering and the ocean, and how they can go together. Due to the unique attributes of the ocean, such as an endless amount of water, we can take advantage for other systems. This water can be used for food, drinking, energy. The surface of the ocean has no shortage of sun, so solar panels can be installed for energy as well. These habitats have to fight with the ocean a good amount, having to be designed to withstand extreme pressures and corrosion. However, they also benefit from the natural resources around them.

In my community, sunlight and heat are available in large amounts. I've already gotten ideas for possible projects to work on to take advantage of these, including vertical greenhouses, solar-powered everything, and harnessing that heat in heat sinks to create possible solar ovens.

Finally, using your natural environment and resources to grow and improve is a useful life lesson. If you truly take advantage of all the opportunities and resources you've got, you can achieve great things.

I've learned a ton throughout this project, and I'd like to extend a thank you for getting to participate!

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2- https://pubs.usgs.gov/publication/ofr20151068

3- https://upload.wikimedia.org/wikipedia/commons/2/24/AYool_topography_15min.png

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22- https://thgautomation.com/systems-tig/

23- https://singularityhub.com/2021/04/30/this-powerful-tidal-turbine-will-power-2000-homes-in-the-uk/

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25- https://www.lung.org/blog/how-your-lungs-work

26- https://gascylindersource.com/shop/oxygen-cylinders/300-cuft-steel-oxygen-cylinder/

27- https://grow-it-led.com/what-is-a-hydroponics-system/

28- https://www.nbcnews.com/mach/science/floating-solar-farms-how-floatovoltaics-could-provide-power-without-taking-ncna969091

29- https://www.coelux.com/