[idea] Concrete Modular Submersible Structures

[Théodose]

Hello everyone,

I'm both a young engineer in Advanced Material Science and a Sunday armchair inventor. Since I like to assess unconventional approaches to engineering, I have read treads about potential use of concrete for submarine applications. It drove me to give it a try, so I would like to present the preliminary results of my thoughts and calculations.

I know well that this kind of subject is quite controversial on this forum because of numerous unfounded claims, so I'll try to stick to conservative methods and reviewed data. Also please keep in mind that :
1) my proposition is only theoretical, since I don't have the means and funds to do even small-scale experiments
2) I only work in the metric system and I'm not very familiar with the imperial units.
3) Unless specified, all my calculations are based on static homogeneous loadings.

Three axis have guided my approach to this problem of concrete submarine structures for civilian use :
- The low tensile strength of reinforced concrete limits it to lower depths than high-strength steels and the maximum depth with natural visibility is around 50m, so I decided to limit the service and rupture depths to respectively 50m and 150m.
- Concrete submarines is only a potential niche market, as well as sub-surface or sea-standing habitats... But by using a few base modules, a niche market purchaser can buy, furbish and then assembly the needed modules to create the structure tailored to its specifications.
- One factor that have plagued the previous projects is the large weight of the completed submarine, that greatly restricts the number of shipyards available and makes land transport prohibitive.
So I try to keep each module small and light enough to be transported in a ISO 20ft container (internal 5.758 * 2.352 * 2.385 m / max load 28200 kg) in the same way as a tanktainer.

The first image illustrates the basic modules that can be assembled end to end with bolts to form the wanted structure. Internal frames, slid along the four "rails" inside the tube and locked in place by girders, allow to flexibly equip a module with standard racks like in the ISS. The second image is mainly about the hatch variants, all kept the same size in order to have only one standardized hull opening, and two external systems fitted to the hull that I can think of.

I'll welcome any constructive criticism from your part and hope that this proposition isn't redundant or too far-fetched.

Seems to be the first proper approach here on that issue.

[Submarine Tom]

Well, you are a brave individual to delve into these waters.

I think your crush depth should be five times your service depth.

-Tom

[Théodose]

I think to see your point, Tom : increasing the crush depth to 250m makes possible to operate "safely" over the most part of the continental shelf. I just need to recalculate the thickness needed to achieve this specification.

By the way, I forgot to mention in the first post that all my maths for the moment are done with homogeneous static loadings. I still need to find S-N or Wohler curves corresponding to the kind of reinforced concrete I'm using, so I can determinate the maximum amount of stress permitted by the number of diving cycles envisioned.

As a first order approximation, I'll use the following formula applicable to statistically pressurized tubes with a R/e ratio greater than 10 :
Smax = dP*R/e => e = dP*R/Smax, where Smax is the maximum stress supported by the material before rupture (MPa) dP is the pressure differential (MPa) R and e the radius and thickness (m)
Here we have e = 2.5*1.125/35 = 0.08 m or 8 cm (well into the R/e>10 criterion) so the initial thickness of 10 cm should be enough to withstand the compressive stress due to an immersion at 250 m.

The reason why I chose such a thickness in the first place, even if the result of my calculation for a 150 m depth was only 5 cm, is that the EN 206-1 norm recommend to keep at least 30 mm between the surface of the concrete and the rebar in the case of a structure permanently immersed in salt water.

[BATAAN]

Thanks for your sensible approach to this design problem. It seems to me that concrete structures are perfectly viable for underwater use. The problem always arises at the hatches, electrical conduits, valves etc due to stress risers and corrosion. If one take steel out of the equation and substitutes say, titanium like the Russian subs, much changes.

[mydauphin]

Aye, the concrete submarine resurfaces its ugly periscope.
Concrete submarine

check out link above , everything you wanted to know about concrete subs but where afraid to ask....

I am a Marine Contractor. I build seawalls of iron and concrete all day long for many years. I know a little on building concrete structures that are exposed to seawater all the time. Here is my advice.
1. Build it from highest psi concrete you can 5000 psi and higher
2. Add fibers to mix
3. Add DCI to mix which corrosion controller
4. Used galvanize rebars or at least epoxy coated rebars
5. The whole thing needs rebars everywhere in a cage.
6. Paint it
7. Allow for stress cracks, I don't know how in submarine
8. Pour all at one time for maximum strength
9. Keep it wet during curing
10. Allow over 30 days to cure
11. Don't stand under it...


Besides hatches and thru-hulls other problem all subs have is cycle stress. This determines lifespan of airplanes and submarines. This contraction and expansion will kill any material overtime. Concrete is not a good flexible material for this.

[BATAAN]

In California high-end floating homes on cast concrete barges have been built for many years now. mydauphin's remarks are 100 percent on for concrete structures exposed to salt water.
The cycle stress problem is very real. Every time you stress something you create micro-faults in the form of tiny cracks. Since the sub is loaded in compression this is different than an aircraft, so a study model and several thousand loading cycles would answer these questions maybe.

[rwatson]

A much more rigourous approach to the concept, someone who actually considers 'stress cycles' !

The question that springs to mind is the longtitudional challenge. Are your envisaging being able to join two or more hull modules together, and if so, how do you see the engineering to provide lengthways strength ? Some modular hulls use tensioned cables or rods to achieve this.

[BATAAN]

Maybe some sort of interlock cast into the tube ends that would engage in a hemispherical terminal end unit or another tube. This would need a bulkhead obviously.

[Submarine Tom]

Bulkheads are good.

[BATAAN]

5x crush depth to safe working depth ratio same as specs for cranes, i.e. collapse strength is 5x allowed working load. Damm things still fall down.

[mydauphin]

I think concrete would be really good for a permenant undersea structure but have my doubts about it enduring cycling. I would build it in carbon-graphite/epoxy.

[MatthewDS]

@mydauphin, I have been toying with something similar to what you describe (as a mental exercise only) It seems to me that an hull made of an epoxy resin mixed with fibers and a silica sand aggregate would be very strong, but would also be flexible enough to withstand cycling.

[BATAAN]

Advanced experimental aircraft these days are using titanium cloth in their laminates.

[srimes]

I've considered using fiberglass pipe for a sub. But only because I see it all the time and wonder what I could use it for.