Guest Article from user Mike:
While talking with Mart about the funding of a simple underwater habitat for a first proof-of-concept, I wondered what would be the cheapest way to bring a subsea station in operation. In other words, what is the rock-bottom price for a simple solution that would allow at least two divers to spend a couple of days below the water surface?
No doubt, the cheapest way is to use something that already exists, such as an underwater cave. History is repeating, and the first aquanauts would live underwater in the same sort of ousing as the first men did a million years before on land. I have seen people dwelling in caverns on the mediterranean island of Menorca these days, and apparently the authorities ignored what they were doing. I have no idea how the public would think about the occupation of a subsurface cavern, and I leave it to others to discuss that. Although I like the idea in general, it is fairly off-topic and has many disadvantages. To show that the natural cave can be permanently isolated from the environment is somewhat ambitious. We can only hope that no crack will open all of a sudden at the roof of the cavern, which would allow the air to escape and the water to flood the cavern. But the cardinal point is that you cannot place the habitat wherever you like, nor can you move it elsewhere.
Coming from geological caverns, the next logical step is to build artificial caverns. First option the bionic approach, i.e. let mother nature do the job. The Calamar Park project has archived the W. Hilbertz’ paper about “Electrodeposition of Mineral in Sea Water” from July 1979. It is about crystallisation of the minerals that are dissolved in sea water, using electricity. More recent are experiments of smaller scale, still using electric currents, but using coral seedlings glued onto wire mesh. Hence, the simplest solution is to create a wire mesh hull with the shape of the future underwater habitat, connect a solar panel that floats on the water surface and then wait some decades until the coral saplings have grown to a massive structure, weighing a hundred tons. Then you fill the hollow coral reef with air (probably you have to put some cement first into the residual cracks) and you are done. This is probably the most ecological and cheapest way to erect an underwater station, but it has the same disadvantage as the cavern: It is not mobile. Plus, it takes far too long.
How about concrete? It is available almost everywhere in the world, fairly water resistant and comparably cheap. You might prepare concrete blocks on the land, then carry them by boat to the place of your choice and assemble them to an underwater igloo. The relative density of concrete is about 2, so you need as much volume of concrete as the volume of air contained in the station. To produce concrete from cement, you also need sand and water. Both are usually at hand at the coast, but unfortunately concrete does not like the salts from seawater: You need fresh water and sand from the inland. The street price for a cubic meter of fresh concrete (still liquid) depends on many things, but is roughly a hundred Euro, for example here (http://www.transportbeton-schuessler.de/preislisten.htm) or here (http://www.herkules-beton.de/preislisten). Having said that you need as many cubic meters of concrete as you have air in your habitat, you will pay round about 2,500 bucks (Euro and Dollar are almost the same now). How did I calculate that? The dimensions of a typical caravan are 6x2x2 meters, so 24 cubic meters. Let’s say 25 cubic meters to make sure it won’t be buoyant. But still, your concrete igloo is not mobile when assembled. A relocation, i.e. lifting it brick by brick to the water surface, moving (25 m³ @ relative density of 2 = 50 tons!) to the new location and then drown and re-assemble the bricks can easily exceed the costs of a new structure.
The effort to assembe 50 tons of concrete must not be underestimated. How would you try to move a concrete block of 1 ton underwater? Lifting is easy when using liftbags, but I am talking of horizontal movements for exact placement. A diver can tow another diver underwater, so 100 kg is a reasonable size for a concrete brick. But then you need 500 bricks to complete the 50 tons = 50,000 kg structure. Shall we assume that five bricks can be done per dive? So we add 100 dives with two divers, a total of 200 tank fillings, round-about 500 bucks. But the worst is probably to get the bricks from land to the habitat site, and that is additional cost of another 1,000 Euros.
So far, I did not mention the pressure level inside the low cost underwater habitats. The answer is straightforward, because it is ambient pressure, i.e. the pressure at the seawater depth where the station is placed. There is no easy way for an engineer to proof that any of these constructions can withstand inner underpressure / exterior overpressure of some atmospheres. In other words, no do-it-yourself subsea station can be normobaric and cheap at the same time. But, if you want normobaric pressure, a second hand pressure vessel is the cheapest way to get hold of a structure that is able to bear exterior overpressure of some atmospheres. Yes, there is a second hand market for pressure vessels, mainly from the chemical and petroleum industry. See here (http://www.behaelter-kg.de) or here (http://www.sielmann.de). Depending on the steel grade and the condition, the prices vary between 25,000 and 50,000 Euros on the caravan scale of size. The bottom line of the price is dictated by the scrap value you can obtain for the steel the vessels are made of. While many vessels are designed for over 5 bar internal overpressure, they are either not certified for external overpressure at all or only for 1 bar (also called “full vacuum”). A vessel that is designed for full vacuum can be used to a depth of 10 meters maximum, no more. That’s not enough for our aquanauts. If you put it a little deeper, you do that on your own risk and sooner or later it will collapse. If you were very lucky, you could find a second hand vessel that is strong enough to withstand some atmospheres overpressure, such that it might be placed in interesting depths of 20 to 40 meters. But even in that case you lack the formal proof. It is not obvious that a former chemical reactor is a safe habitat for human beings. By experience, authorities and insurance companies often ask for these details. Provided that the original documentation of the manufacturer is at hand, a formal recalculation done by a notified engineer might be successful. That way you get the required certificates for official approval. Depending on the difficulty, this recalculation will cost another 1,500 to 3,000 Euros. Usually, the opening to inspect the vessel is not wide enough to allow a fully geared diver to pass, so the inspection opening has to be modified at extra costs. To draw a conclusion, an underwater habitat at normobaric pressure level is always way more expensive than a the one at ambient pressure. In other words, pressure vessels are not what we are looking for, because we want the cheapest solution.
So we leave the pressure vessels behind, but let‘s stick to steel as a material for construction. The price per ton is much higher compared to concrete, but you need much less of it. It is quite tolerant to mechanical stress, it is a commodity compared to fiber materials (fiber glass, carbon fiber etc.) and repairs can be done almost everywhere in the world. I admit that building a habitat from standard raw material, such as I-profiles, band steel and wire requires a proper factory. But I am not talking of building a steel structure, because I think of using a steel structure that is available everywhere in the world, with the same size, the same quality and design: the sea cargo container.
Despite it is called standard container, there is a variety of standardized models circulating around the world: 20 feet (about 6m) or 40 feet length (about 12m), standard (2.6m) or extra height (2.9m). Depending on the size, the weight of an empty container varies between 2.2 and 4.1 tons. To get an idea of the dimensions, visit Wikipedia (https://en.wikipedia.org/wiki/Intermodal_container) or just google (e.g. http://www.containex.de/de/produkte/seecontainer). But even the smallest 20 feet containers (aka TEU) are of the size of a caravan, so let us assume a 20 feet container to start with.
Transport on the ground: The smaller ones can moved by a mid size truck and, when empty, be lifted by its on-board crane. Transport from A to B is as easy as can be, for every-one in the business knows how to handle a standard container, on road, on rail or on a cargo ship. A standard container has openings for the hook of a crane, as well as for the bars of a fork-lift. (Reference: https://www.containerbasis.de/container-abladen)
Heavy loads: A standard cargo container is designed to carry heavy loads. The maximum allowed freight weight is 28,480 kg per container, and the standard requires that at least six fully loaded containers can be stacked on top of each other (in reality it is up to nine). Sea containers also have openings for the attachment of clamps, so you can assemble them to larger modules. When you follow this link (http://www.containex.de/-/m/images/ctx/pdf-ctx/technische-beschreibungen/technische-beschreibung-seecontainer.ashx), you can also find technical drawings of the container at the end of the PDF document.
But do sea containers resist the conditions under water? First of all, if the container was built according to the criteria of the design standard, and it is used under conditions that do not exceed the design criteria, the container will withstand. When immersed and filled with air, the container is buoyant equal to 33 tons. Its own weight is approximately 3 tons, so net buoyancy is 30 tons. That is a little more than the 28,480 kg maximum allowed load. In the water, the force will act in upward direction to the sea surface. Hence, in order to make the operation conditions comparable to the design criteria, you must turn the container upside down – the floor should be reinforced in relation to the roof.
Second, the container must be kept immersed, so we need to load it with 33 tons. For a moment, let us assume to use lead weights. Since the container was designed to carry five more fully loaded containers, we are allowed to put the lead weights on the four corners in full alignment with the ISO standard. Actually, we do not put the weight on top, but on the bottom, since we have flipped the container upside-down. But think of a container stack on land. The lowest container is squeezed between the ground and the upper container like in a bench vice. Here, the mechanical forces attack only at the corner points and not on the full area, like the buoyant force.
When we use the container as a divers’ bell with a free opening to the water at the lower side (the words “upside” and “downside” become confusing when speaking of an object that is flipped around), the container is under internal overpressure. As you remember from your SCUBA diving lessons, the water pressure increases by 1 bar with every 10 meters of depth. In consequence, when an object is 2 meters high, the water pressure between the upper and the lower edge differs by 0.2 bar. There might be a need to structurally reinforce the container a little bit – engineers need to check that. I do not expect a big deal, just a few sand bags on top and a belt made of some I-profiles. The good news is that the (european) law does not consider 0.2 bar as significant overpressure, the minimum threshold is 0.5 bar. Although the container is under internal overpressure, the european law does not consider it as a pressure vessel.
But anyway, the best that can be said about sea containers is yet to come – the price. As a commodity, containers are pretty cheap. New 20 feet containers cost about 2,500 Euros, while second hand containers are almost half the price. So this is only a twentieth of the price for a pressure vessel.
Sea containers have to be water tight to protect the freight goods from rough sea. But that does not mean that containers were gas-tight, nor corrosion resistant. The ISO standard also prescribes the paint and coating to reduce corrosion, but these means were never meant to protect the material when completely immersed in salt water for a long period. With regard to the price of a bare container, it is probably not beneficial to spend lots of money for better coating and extra layers of ship paint. Precautions to slow down the speed of the corrosion process are a better investment, such as anodic protection and an internal lining. The latter is also needed to gas tighten the container body. As a common observation around shipwrecks, the deterioration of a wreck increases with the number of divers who visit the wreck. And that is not only because they break things, but mainly from the oxygen in the bubbles they leave inside the wreck. Hence, the lining has two purposes: To keep the air inside and the oxygen of the air away from the steel. A quick internet check (http://www.wunschplane.de/html/preisrechner.html) provides an estimated price of 750 Euros for a canvas cover made from 200g/m² polyethylene, including the labour to cut and weld it to a tent-like shape with the internal measures of the sea container.
Another interesting point is how do we keep the container immersed? The current market price for lead is around 1,000 Euros per ton. Using lead to neutralise the buoyancy of a 30 m³ net volume submarine habitat would cost us 30,000 Euros just for the material, freight excluded. That’s way more expensive than the habitat as such, so it is economically rubbish to use lead. A simple solution would be to place the station below a rock, or – much better – an underwater arch. The difficulty with this idea is similar to the approach to use natural caverns for underwater habitats: You are not free to place the station wherever you want and you are likely to earn protests.
However, large rocks are much more often, and in particular waters such as the Mediterranean Sea they are abundant. In most cases, their largest diameter is not at the very lower end but an arm length above the ground. In this case, the only job to do is to tie a thick rope around the rock at ground level, like a belt. Then the sea container can be attached to the belt with multiple lines. When adjusted properly, the sea container is floating on top of the rock, only held by the anchor lines. The line length will have to be adjusted properly such that there will be sufficient space left to allow divers to leave the station via the bottom exit.
But what can we if the station is supposed to be placed on sandy bottom? As we have learned, containers are comparably cheap. So an obvious third way to anchor a container station is by means of a second container, which serves as an anchor to the habitat container. When you look-up the apparent relative density of sand, you will find values of around 1.6, depending on diameter distribution of the sand particles. Quartz as bulk material has a relative density of 2.4 instead. As sand consists of quartz, the difference in the density comes from the air between the sand grains: One third (1.6 over 2.4) of the content is just air. A sand bag immersed in water would have a relative density of 1.9 (2/3 * 2.4 + 1/3*1). Therefore, a container that is completely filled with sand would has nearly the ability to anchor a second container of the same size, filled with air. It is straightforward that the remaining gap of approximately 3 tons negative buoyancy can be achieved with little effort, such as concrete blocks, sand bags, smaller rocks and so on.
So far, so good. But then Mart told me the story about a ship wreck that was intentionally placed at a convenient depth for SCUBA divers. But the first winter storms that came hit it hard and displaced it over tenths of meters. We do not want that to happen with our habitat, even though it is cheap. So first thing to think of is to evacuate the station when a storm is rising. Second is to flood the station. Yes, correct: we must flood the station to make it more resistant to shocks, more heavy in terms of inertia and buoyancy. But can we recover the station once it has been flooded? Remember that the container must be flooded to drown and fix it at the bottom. The inner canvas liner that will later form a some sort of balloon around the aquanauts is not unfolded, and the space between the container and the liner is filled with sea water. After completion of the mounting work, we will pump the air into the canvas liner. In case of a storm, we almost reverse what we did for the installation: Simply pump out most of the air – say 80 to 90 percent. Any equipment that remains inside must be boxed, and the canvas will fold around it like a vacuum pack. Sure, that procedure would not guarantee the survival of the habitat in a perfect storm, but it will defintely increase the odds.
Let’s summarise for a conclusion of what has been stated so far: Sea containers can be used with little modification for both as habitat structure as well as anchor weight. Using two containers and a custom tailored canvas cover as an inner liner, a bare underwater habitat would cost around 3,500 Euros, including the ropes, but excluding shipping (given that sand is for free).