Making an Island/Construction Guides

Floating Fishnet
First of all, it would be better to construct these on land and then transport them to the water.

Materials:
 * Soil
 * Fishnets
 * Empty bottled water containers and/or empty drink bottles.
 * Make sure the bottles are closed and air tight; preferably plastic.
 * Do not employ glass bottles, they will break!

Here's how to calculate how many bottles you'd need for your island:
 * Calculate how much weight in soil you'll have for the island (in kilograms).
 * Calculate how much weight in other facilities you'll have for the island (in kilograms).
 * Calculate the maximum number of people you expect on the island, and multiply that number by 70 kg.
 * Add the above numbers, altogether, and add about an extra 5~15% of that for safety.
 * The resulting number is the total number of liters in bottles that you'll require. Divide the number by the bottle size (in litres) that you'll be using to get how many of them you need.
 * 250 mL = 0.25 L; 500 mL = 0.5 L

Steps:
 * 1) Spread the fishnet over a flat area where plants are able to grow into the soil but not the ground underneath (construct on top of something like concrete).
 * 2) Figure out how you want the empty bottles to lay in the fishnet.  You could try to tie them to the fishnet or just lay them on their sides (if you think the roots of the plants you're going to be using in the next step will secure them in the soil).
 * 3) Cover with soil and plant plants as hopefully they will trap the bottles in their roots. You will have to determine the depth of the soil you want. If it's too thin your weight won't be dispersed enough and you may fall through. If it's too thick it may simply sink.

Modular Construction
It is envisaged that the island will be produced in a modular way, with sections able to be added as required. Modules can be either free floating and anchored to the seabed, possibly using an existing sea mount as the anchor point, or built up from the seabed itself.

Modules can be any shape and design but the easiest shape both to deploy and built is the rectangular platform. Hexagon modules should also be considered for additional reasons. The prime one being increase of volume for a given surface area.

It is suggested that modules should be able to be raised and lowered in the water to allow the modules to be moved as needed. A module completely empty of water has less inertia and will be much easier to control for a tug or other propulsion system. A module half full of water for example will be much more stable as long as that water is prevented from moving from side to side (this is the same principle used for ship-board stabilizers and fuel tanks in cars.) The easiest way to do this is to provide baffles in the buoyancy tanks that prevent fluids from moving from one side of the tank to another.

One design that may be viable is that of a box, solid at all sides except the base which will be covered with either mesh, if an internal lift bag is to be used or solid of all sides with valves able to control airflow into and out of the tank. It is suggested that the tank be divided into several sections to allow for redundancy if a tank should become damaged. It is also suggested that the valves should be designed in such a way that if they fail there is a backup valve able to stop it failing in a dangerous way.

The biggest risk for any island is the destructive force of a storm. A floating island as suggested above should be able to cope with such a storm by increasing the amount of water in it buoyancy tanks. In doing so it will become heavier and sit lower in the water. It will also have less cross-sectional surface area to be pushed against by the wind. The only real problem will be the waves. Wave energy decreases with the depth below the surface. The depth being dependent on the wavelength of the wave. In calmer water the sub-surface effects of waves will be minimal, but during a storm wave-length increases and therefore sub-surface effects do also.

One possibility for a floating island which will be difficult in practice would be to literally sink the island during a storm and re-float it after it has passed. This would probably be a last resort option as it would require a lot of disruption unless the modules were designed for this action.

Compartmentalization of the buoyancy tanks of a module should allow greater tolerance of water ingress by limiting center of gravity shifts through the structure and thus making the module more stable. This can be proved in a little thought experiment. If we have a spherical module, unlikely but it makes it easier to demonstrate the basic idea, with 25% of its volume filled with water. Then there is nothing to stop the whole module from spinning in any direction about its center of gravity. If we now divide this sphere into many little compartments and fill the lowest 25% of these with water then we essentially have a module that bears a close resemblance to a weeble in terms of mass distribution. Each time it is pushed from its stable position it will tend to return to this equilibrium point. This is called pendulum stability. For every subdivision we create inside the module we have a set of trade offs. With every division we tend to the ideal module with a fixed center of gravity, but its a case of diminishing returns since while initial divisions will have great effect as you keep adding more you will reduce the effect and add more mass to the structure, also cutting down usable buoyancy tank volume. This isn't very scientific but instinct tells me that the ideal number of subdivisions is between 3 and 5 giving us somewhere between 9 and 25 separate buoyancy tanks, but this will depend upon application.

We will go through a worked example for the rough design of a cubic floating module.

If we require a rectangular module that has a surface of 5 meters by 5 meters and can take a payload of 4 tons in addition to its own mass then we need to calculate the mass of the structure, the mass of the water inside the structure and how much water we need to displace to get it to float.

The volume of the structure is given by the height x width x length for a rectangular object. We'll use the height as 3 meters for now, we can always go back and change it later.

This gives us a volume of 75m3.

If we assume that we will be using seacrete or concrete as a basis for the structure then the calculations are as follows. Steps will be similar but with different figures for other materials.

If we use a wall thickness of 0.1 meters then the volume of the walls is given by (height x width x thickness x 2) + (width x length x thickness x2) + (height x length x thickness x 2)

or

(3x5x0.1x2) + (5x5x0.1x2) + (3x5x0.1x2) = volume 3      +      5      +      3       = 11m3

The mass of the walls will be given by volume of walls x density

If we use the mass of sea water as 1020kg/m3, air as 1kg/m3 and concrete as 2750kg/m3

For concrete this would be

11 x 2750kg/m3 = 30250kg

If we add to this the mass of air inside the structure we get an overall figure of 30325kg

The volume of water the structure will displace if fully submerged is 75m3 (The volume inside the structure) + 11m3 (The volume of the walls) for a total of 86m3 The mass of water it will displace is 86m3 x density of water

or 86m3 x 1020kg/m3 = 87720kg

Since this is greater than the mass of the structure it should float and should be able to take a payload of 57395kg before it would begin to sink. However unless you distributed the load carefully the structure is more likely to fail before you get to this point.

So far we have neglected to include subdivision of the tanks into the calculations. If we wish to calculate the extra mass required to subdivide the buoyancy tanks we do the following.

Decide how much we will subdivide then tank by.

For example to divide it into 9 sub-tanks we would require 4 extra panels. 2 in the lengthwise direction and 2 in the widthwise direction. Volume of these panels can be calculated as follows.

(w x h x thickness x #panels) + (l x h x thickness x #panels) = total additional volume. for our example previously this would give an additional 6m3 of volume (which needs to also come off the usable volume of the tanks when filled with air) which has a mass of 6m3 x 2750kg/m3 = 16500kg

We can do this technique for more complex structures as long as the masses of the structure and water displaced is known or can be calculated.

Anchors
There are many techniques used for anchoring items to the sea bed. The simplest technique is simply to drop a large mass onto the sea bed and tether an object to it. However, this solution is neither a particularly elegant solution nor is it efficient in terms of raw materials.

One technique, which has gained popularity with structural designers in recent years, is the idea of dropping a large bell onto the sea floor and pumping water from it. While water is being removed, the bell will fill with sand, either by sucking up a large mass of sand into itself, thereby causing its mass to increase, or by burying itself in the seabed, causing an increase in static friction of the anchor. Which condition occurs will depend upon the design of the anchor and the amount of suction provided upon installation. It is unknown whether the first situation would have long term durability.

Biorock
The first step is to build a steel frame out of metal. (sources of metal can be rebar, thick gauge wire, tie wire, chicken wire, hardware cloth etc.) To create solid structures, the spacing between terminals should be kept to a minimum. (The reef structures currently being built feature spacing of half a foot or more) 1/4 to 1/2 inch might prove most advantageous for an island. Keep the anodes (positive terminal) around 4-6 inches away from the cathode (negative terminal) screen. the metal structure can be anchored in a shallow part of the ocean, powered by floating solar panels. Placing the anodes half a foot from the sides of the island the limestone will grow around the outside then later the insides will fill in. To increase the size, the anodes will be moved farther from the sides of the island.