Undersea Station: Ballast

ballast-experimentKeeping an air filled structure on the sea-floor is more difficult than expected. Especially while descending and landing the station major difficulties occurred on previous stations. We can calculate about one kilo of weight (negative buoyancy) per litre of air inside the station (positive buoyancy). The dimensions are huge: imagine a space of 50 m² with a height of 2m, which equals 100 tons of counterweight. At the same time it has to be considered, that these weights have to be lifted again in the end of the mission. For ecological reasons we should find a way to leave the weights on the site and to use a material that would not harm the environment.

Our proposal for this material is simple sand that might be pumped from the seabed into the ballast tanks while on the surface. In the end of the mission we would just open the ballast tanks hatches and let the sand flow out. Therefore we made a little test by connecting two plastic bottles (zero buoyancy) to each other, filled one with air and the other with sand. The result was, that 850gr of wet sand (eastern Mediterranean) is necessary to keep 1 litre of air in balance (see image). Of course this result might differ according to composition of local sand in the site’s region. Using this concept means on the other hand that a space for ballast/sand release has to be considered below the station, which leads to an increasing height of the whole structure and therefore vulnerability for currents. Another difficulty might be the consistency of wet sand that is stored in a tank for a longer period of time (will it still flow out if necessary?).

station kugelIt was very interesting to see that ‘Conshelf II’ was kept down by putting huge lead bars on top of the station’s ‘feet’. As a result the full tension was concentrated on the joints between feet and legs as well as the connection between legs and station. Other stations used big pieces of concrete that the station was attached to. We came to the conclusion that the ballast tanks should be a full integrated part of the structure. ‘Conshelf III’ used the same idea: the counterweight tanks were completely attached to the structure and shaped like a ship’s hull to make transportation on the surface easier. In our example of the sphere draft (see image) the upper half would have been the living space while the lower half would have entirely served as tanks for counterweights (green). That means that the tension is maximally distributed to endless connecting points.

To calculate the amount of necessary ballast it is not only necessary to keep the structure on the sea floor, but to make it invulnerable to currents by adding an additional safety factor of ballast with a weight of for example +15% over the total air volume (the exact amount is subject for discussion). That means:

weight of station + weight of additional equipment + integrated ballast
+ variable ballast (e.g. sand)

=
amount of air volume + x% safety factor

Additional steel cables attached to rocks would ensure further fixture.

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