Seasteading: Structure Designs

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Seasteading: Structure Designs

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Structure Designs

{This section feels a bit incomplete. We've had to leave out a lot of engineering information, as there is neither space for us to describe everything in detail, nor time for us to finish all plans to completion now. But we still want to hit all the major points, so let us know if we've missed something that seems crucial to you. }

There is no one correct design for a seastead, since the best choice depends on your goals, budget, and location. For this reason we'll give a broad overview of the many types of structures which have been suggested and some of the necessary tradeoffs. Then we'll delve into detail on our favorite design, the spar platform.

The most important design criterion is that the seastead be safe in the harsh ocean environment, with its wind, waves, and currents. Thus all of these designs will need to use some of our wave avoidance techniques. Besides safety, the structure must be reasonably cost-effective, or it will never be built. Cost may well be the main barrier which has prevented people from becoming seasteaders. Its also important that the design scale well to different sizes, so that we can apply our incremental approach. Dynamic Geography tells us that physical modularity is also very desirable.

Seastead designs break down into three rough categories, depending on their location relative to the water's surface.

Underwater

While there are some neat benefits to an undersea structure, there are a large number of disadvantages and engineering challenges as well.

Note that the positive scenic aspect can be achieved just by having a small undersea portion of a structure which is mostly above the water. (Although many seastead locations will just not be very scenic). On net, it does not seem worthwhile to start with an undersea structure. Those interested in more ideas about living underwater should see [Fisher1985, pp. 64-73]

On The Water

This category of designs consists of structures which float directly on the ocean's surface. Since waves are dangerous, these methods will need to somehow avoid them. We'll start with options based on modifying existing designs, and move on to more novel ideas.

Sailboat Fleet

While we have not found any published literature on this concept, it is unlikely that we are the first to think of it. The concept is that a group of like minded people could purchase a bunch of sailboats different sizes and costs and sail around the ocean together. The standard self-sufficiency technologies described later would be used to provide electricity, fresh water, etc.

There are several advantages to this idea. Obviously sailboats are a mature technology, with a large number of types, repair facilities, books and so forth available. They are extremely mobile using renewable energy, and can thus travel all over the world living in "endless summer". While they are not built for extremely rough seas, they are mobile enough that with advance planning they should be able to avoid such situations. And a fleet of sailboats is both modular and scalable. They can even grow some food, as described in Sailing the Farm [Neumeyer1982]. Unfortunately, there are serious drawbacks.

Boats tend to be built from expensive materials, and are costly to build and maintain. They are optimized for movement, so they have have small deck areas (tough for solar panels and greenhouses) and cramped interiors (tough for living in). Nor are they particularly comfortable in significant waves. The marketing/publicity angle is more difficult because it doesn't seem like a new way of life. We cover these problems in more detail in the FAQ question Why not just buy a boat?.

There are definitely some nice aspects to a fleet of sailboats, such as not having to design a new structure. It would be a relatively easy way to start living on the water, since there are people already doing it. But boats are designed to travel from place to place across the ocean, not to live in. The difference between a sailboat and a seastead is like that between a house and a car. Sure, you can live in a van or RV, but its just not that comfortable. The residents would be more wandering nomads than permanent settlers. There is nothing wrong with this lifestyle, but its not what we think of as true seasteading.

Big Boat

Many people have suggested that rather than designing a unique structure, seasteaders could just purchase and retrofit a large used vessel such as an oil tanker or cargo ship. Again, the standard self-sufficiency technologies would be used to provide infrastructure. As evidence that big boats make decent floating cities, we need only look at the cruise ship industry. One way to think of this concept is as a low-budget, do-it-yourself cruise ship.

Obtaining a used boat would reduce costs, and it would already contain many useful systems like propulsion and navigation. The propulsion could be used occasionally, although the ship would mostly drift to conserve fuel. Large vessels are less responsive to the waves than small ones and so more comfortable to be on during storms, as well as safer.

Unfortunately, large boats are not exactly scalable, so it would take a sizeable initial group to purchase one. They are also not reconfigurable or modular (although an assembly of multiple ships would be). They have many of the same drawbacks as sailboats, like limited solar area and not seeming like a new way of life. We've basically traded modularity and low starting cost for seaworthiness and some interior volume, without really gaining any ground. Like the sailboat fleet, the idea certainly has some merit, but we don't think its the most promising option.

Some of these issues can be addressed with a hybrid combination of a cargo ship and some of the platform types described below. The cargo ship would take the materials to some remote island or atoll, and the colony would be deployed. In the event of political problems, bad weather, or simple boredom, the colonists would load everything back onto the boat and move someplace new.

Simple Platform

In waters that are naturally calm or somehow protected, there are many simple systems that can be used to turn water into land. Each consists of some sort of buoyant foundation on which to put whatever structure is desired. While we'll be recommending a different approach, this one is quite promising as well. In an area without large waves, it would be quite cost-effective, and should be strongly considered as an alternate design.

2 Liter Bottles

One of the simplest systems was suggested by Wayne Gramlich in his original seasteading paper [Gramlich1999]. It utilizes plastic 2-liter beverage bottles, which are extremely common, incredibly cheap, and resistant to sea water. These bottles can be banded together into hexagonal grids of 7 bottles each. The grids are then stacked and layered to form a buoyant lattice. Alternatively, one can use Rich Sowa's method of filling nets with the bottles. Some sort of rigid surface then needs to be placed on top of the flotation.

Inverted Cylinder

Another simple technique is to have an inverted cylinder, open at the bottom, containing air. This idea was used by Sea Structures Inc. for their SeaCell system [SSI]. A disadvantage with open containers is that as depth increases, the air is compressed and displacement goes down. This flotation is cheap to manufacture, and can be stacked for easy transport. Again, some sort of rigid platform needs to sit on these cells.

Concrete Slabs

Yet another simple method is concrete slabs such as those manufactured by IMF [IMF]. These are hollow boxes of reinforced concrete, with enough buoyancy from the interior airspace to support the concrete as well as a structure. IMF's designs include shock-absorbing connectors, integrated structural cleats and pile rings, and integrated utilities. Because the structures are monolithic and sealed, they cannot take on water and are unsinkable unless broken. And ferrocement is cheap. Most floating homes in the USA nowadays are built on such slabs. They'd be fairly easy to connect to one another, and small ones could be easily built onboard. We think this is the most promising aquatory technology for protected waters.

Pneumatically Stabilized Platform

One interesting possibility is the PSP designed by Float Incorporated [FloatInc]. It consists of a number of inverted hollow cylinders, as described earlier, but with a clever addition. Imagine what happens as a wave rolls under these cylinders. The water in each cylinder moves up and down, and the air pressure in the trapped airspace changes. In a PSP, these spaces are connected through pneumatic lines and valves, so that these pressure changes result in air moving between cells. This dampens the waves and distributes their force so as to reduce peak load on the structure. If air turbines are attached to these lines, it becomes a wave-powered electricity generator.

The PSP has some characteristics of a platform (it can support loads) and some of a breakwater (it attenuates waves). It is built out of concrete, our favorite construction material, so its relatively cheap. Its very modular and fairly reconfigurable. Cost estimates are $5M - $7.5M an acre ($115-$160/ft² in the open ocean. However the inventors have not been able to find a major purchaser, so this is an unproven technology. We have some concerns about the design's ability to withstand (or block) severe storms, with waves large enough to wash over the edge of the platform. Still, it is a promising system.

Cargo Containers

The ideal seastead technology is safe, inexpensive, and modular. Here we'll consider whether structures built out of converted freight containers qualify.

The shipping industry has been revolutionized by these containers. Freight containers can be moved between trucks, ocean freighters and trains without requiring that the container contents be changed. Their popularity has made them cheap and plentiful. Used 40 ft freight containers can sometimes be obtained for less than a thousand dollars. A similar alternative is large propane tanks, which are much stronger because they're built to hold pressurized gas. They cost several thousand dollars (used), but this may be worthwhile for safety.

Container	Length (ft'in")	Width (ft'in")	Height (ft'in")	Volume (ft³)	TARE (lb)	Payload (lb)	Max. Gross (lb)
20' Dry	19'10.5"	8'	8'6"	1,173	5,160	47,740	52,900
40' Dry	40'	8'	8'6"	2,391	8,730	58,470	67,200
40' Hi Cube	40'	8'	9'6"	2,692	9,150	58,050	67,200
48' Domestic Dry	48'	8'6"	9'6"	3,463	9,700	57,500	67,200
53' Domestic Dry	53'	8'6"	9'6"	3,830	10,280	56,920	67,200

Since freight containers are not designed to float, some effort must be expended to convert them. The basic concept is to weld all holes and vents shut, along with the access doors, and to install a seaworthy access port. It must also be sandblasted and coated with seaworthy paints. It may need some structural reinforcement, as the corrugated steel skin is not meant to withstand much force.

Once the container is seaworthy it is ballasted on one end to force it into a vertical orientation with 1/2 to 2/3 of the container submerged below the water line. This reduces interaction with waves. In a storm with large waves, the structure will basically move up and down with the waves with relatively little rocking motion. Because its small enough to bob, it doesn't absorb much wave energy. In addtion, by submerging a significant fraction of the structure below the water line, there is less swaying due to high winds. This is similar to Marc Piolenc's spar buoy. In a severe storm, the occupants of a container will definitely be pushed around. However, as long as everything is properly secured inside the container, about the worst that will happen is a severe case of sea sickness.

Even though the freight container should be relatively safe in pretty severe weather, it is still prudent to plan on situating freight container seasteads in areas that do not often experience severe weather. It is further prudent to have a means of moving a freight container seastead out of the way of an approaching severe storm. A basic outboard motor should provide the means to move a modest distance even though a freight container is hardly shaped for optimum traversal through water.

One nice characteristic of this design is that it can be easily purchased, stored on inexpensive property during conversion, converted, and then shipped off to an ocean deployment location. Freight containers are designed to be moved around, so it is relatively easy and inexpensive to do so. Ballasting may need to weight until the final site, as it will make the container heavy and unbalanced.

Since its oriented vertically, we can divide the container into floors. Assuming a 40 foot container on end with approximately 8' ceilings yields 5 floors. The bottom floor will be partially occupied with ballast, so it should really be thought of as a cramped storage compartment rather than a full livable floor. Using a 48' container provides an additional floor and a 53' container might provide two additional floors. Since the dimensions are 8' by 9.5', each floor is 76 ft². This is not luxurious, but for some people it will be adequate. The total area of 300 ft² actually compares favorably with the floor area of a sea worthy sail boat. For more space, multiple containers can be welded together into larger units.

Giving the limited top deck area, we need some creative solutions to provide an adequate supply of food, water, and power. Just like the larger structures we'll propose later, there is no reason why a container seastead can't have a cantilevered upper platform to provide additional solar area. During bad weather, anything kept up here can be stored safely inside. Another simple solution is to tether inexpensive inflatable floats to the seastead. These could support solar distillers, PV panels, and small greenhouses. Again, in bad weather these are deflated and brought inside.

The primary reason to think about freight containers is to propose alternatives that further lower the cost of bootstrapping seasteading into existance. Will a freight container seastead be as safe in severe weather as one of its larger cousins? Almost certainly not. However, it is probably safe enought that it can seriously be considered as a potential start. As the seastead community gets larger, the need for this design may well diminish as people switch to structures designed specifically for seasteading. Thus, freight container seasteads should really be thought of as a bridging technology between what is available now versus what we can build eventually. Alternatively, they may continue on as low-income housing, much like trailer homes on land.

Breakwaters

A simple example of a breakwater is any island or reef, which acts as a natural barrier for its lee shore. Artificial breakwaters can be seen surrounding the entrances to any marina, usually consisting of concrete piers or piles of rock.

The advantage of using a breakwater is that it eliminates all the problems caused by waves. Structures become much cheaper, safer, and easier to expand, seaplanes can land and cargo is easier to offload. But to do this, you must dissipate the tremendous energy found in ocean waves, and do it continuously, for years on end, even during severe storms. If the breakwater fails, suddenly your structures must face waves they were not designed for, which may be disastrous. We'll outline a number of the different methods that could be used to build such breakwaters. Later we'll explore the question of when this is the best way to deal with waves.

Natural Breakwaters

Any landmass which reaches close to or above sea level acts as a natural breakwater. Rock is tough stuff, and it takes quite awhile for the ocean to grind it into sand. There are basically two options: we can shelter by a large landmass (which will almost certainly be inhabited), or a small one. Large landmasses have political difficulties, as we will be fairly close to existing nations. It is difficult to be protected on all sides yet still be in international waters. Still, there are some possibilities if we are willing to accept moderate waves, such as seas like the Mediterranean.

Smaller breakwaters include atolls, reefs, and seamounts. An atoll is a special class of island that is formed when the ocean has worn a volcanic peak down to a roughly circular shape. As a result, they basically consist of a breakwater surrounding a calm lagoon. Because so many islands are volcanic in origin, atolls are quite common, and many are uninhabited. One of the more famous is the Bikini island atoll in the Marshalls, where the US conducted nuclear testing [Bikini]. These lagoons range in size from tens of thousands of acres down to almost nothing.

Yachtsmen, encountering unexpected storms, have weathered gale-force winds by anchoring in such lagoons, so atolls definitely act as a wave barrier [Fisher1985, p. 52]. Unfortunately, the fact that atolls contain land means that they are all claimed by an existing country. While an abandoned atoll could doubtless be used for awhile before anyone noticed, our goal as seasteaders is to create a stable way of life. We want to be pioneers, not outcasts. This renders claimed atolls unsuitable for frequent use.

The obvious solution is to look for submerged atolls or reefs, which do not count legally as land. After all, a breakwater does not need to extend above water to provide significant protection, it need only come close. These submerged reefs and rocks, formerly only known as hazards to navigation, can be used to protect our new way of life from the elements. While the Minerva incident indicates that nations do not always respect these rules, our chances are much better if we follow them.

Unfortunately, suitable geographic features are likely to be rare. Any rock above water creates a zone of 3-24nm around it of sovereign waters. So we need a reef which is not within that distance of any above-water reef. It can't be too far below water, or it won't be a useful breakwater. So we need an area where the reef comes quite close to the surface, yet never rises above it, and the odds are against this happening.

An additional advantage to such natural breakwaters (if we can find them) is that they provide for cheap and easy anchoring. Also, they are likely to have pretty underwater scenery. A disadvantage is that the colony is tied to one physical location, which means that it cannot easily avoid political problems, move with the seasons, etc.

An alternative to finding a submerged breakwater is to be close enough to some appropriate landmass that it can be used for shelter during severe storms. While the waters would be legally controlled by another nation, the use would only be occasional and its unlikely that anyone would be paying attention. Still, satellite photos could be used later as part of some legal maneuver. In an emergency this solution is fine, since any court is more forgiving than Davy Jones Locker, but it seems a poor idea to depend on it.

Artificial Breakwaters

Artificial breakwaters have a long history of use to protect harbors, marinas, and coastlines. There are numerous breakwater designs, and they are fairly simple in principle, so we won't cover them in detail. Most rely on big pieces of concrete, although there are many alternative methods. Most designs are meant to rest on the seafloor, or at least be tethered to it. While this is fine for shallow water (perhaps on a seamount or reef), it won't work in any significant depth. Hence we need a floating breakwater.

Ocean waves can be very large, hence a traditional design would need to be very large. Rather than absorbing all the energy, perhaps we can simply get it to dissipate harmlessly. This may sound difficult, but this can be seen on any beach with a wave break. The incoming waves, reaching shallow water, begin to pile up. They reach an unsustainable height, form the familiar whitecaps, and break, collapsing on themselves. Only a gentle wash reaches the shore. The soothing sounds and pretty patterns on the sand are all that remain of the wave's energy.

This effect could be simulated by submerging a long triangular breakwater. As waves reach it, they will pile up and eventually break. This breakwater does not need to be particularly strong, because this aikido-like method never takes the brunt of the force. Still, it will need to be be quite large, and will not be cheap or easy to build.

Any non-anchored breakwater will be steadily pushed by the waves towards the center colony, so the two must be strongly connected. Many breakwater designs such as the simple concrete wall, the aikido breakwater, and the PSP could be used in such a configuration.

Above The Water

The final option for avoiding waves is to place our structures above water level using pillars made of steel or concrete. Many permanent marine structures, such as oil platforms, use this technique. These "spars" present little cross-sectional area, so that waves pass through without imparting much energy. The extra engineering problems posed by spars are more than balanced by not having to endure the bashing of waves.

Pillar Platform

If the water is shallow enough, the pillars can rest directly on the sea floor. Thus there is no movement due to currents, wind, and waves. Unfortunately, pillars are not well-suited to dynamic geography, and they have all the political dangers of a fixed location. While oil rigs can have pillars as deep as 3000 ft., they also have budgets in the billions, and it will take much shallower waters for a pillared seastead to be cost-effective.

The most impressive example of this type of structure is the massive Troll A gas platform, located in the North Sea off the coast of Norway. Only a handful of skyscrapers and oil rags are taller, and it became the tallest structure ever moved over the face of the earth when it was towed 174 nautical miles to its operating location. It is built from ferrocement, our material of choice.

Statistics

The deck and pillars were built separately and united while floating in a fjord. Norske Shell describes the process:

The design, assembly, and towing of this platform validate a number of our design features which you'll see later. Hence we don't need to demonstrate that such structures are possible, but merely that they can be built cost-effectively.

Tension Leg Platform

Another option is to have pillars which are buoyant and floating, but anchored to the seafloor with tensioned lines. These lines prevent vertical movement, but allow for some horizontal motion. Unlike fixed pillars, a TLP can be detached and moved to a new location. They can operate as deep as 7,000 feet, although the tensioned lines are very expensive.

Semisubmersible Barge

A seagoing, self-propelled barge that rides at anchor, stands on partially submerged vertical legs on submerged pontoons, and serves as living quarters and a base of operations in offshore drilling. [AHDE4]

This is a standard barge design for places where good weather is infrequent. It has a much lower response to waves than a normal ship, because the waves sweep through between the columns. This allows it to operate in rougher conditions. The disadvantages are related to the weight balance required for stability. The topside cargo capacity is much lower than a ship, because too much weight causes stability problems (the barge becomes topheavy and tips). It needs sophisticated ballast controls, like a submarine, which adds expense compared to a ship.

However, a seastead doesn't have large load requirements, so this is not as much of a problem. This closely resembles our preferred design.

Floating Spar Platform

The problem with the pillared platform is that it is immobile, and in deep water it requires very long, expensive pillars. The problem with the semisubmersible is that much of its flotation is close to water level, and its platform is not high enough to avoid all waves. So it escapes some wave force, but not enough. The logical solution is to make tall thin legs, like a pillared platform, but to have them resting on submerged buoyancy, like a semisubmersible. We call this a floating spar platform.

The simplest version is Marc Piolenc's spar buoy concept. This consists of a vertical cylinder ballasted at one end. Essentially, the structure is all spar and nothing else. The ballast must be considerable in order to make the structure float vertically, especially if a substantial portion of the spar is above water. The spar is a suitable design for weak building materials such as seacrete, which cannot handle cantilevered loads. { Picture }

There are some disadvantages to this system, however. Solar area is very important for PV panels, growing food, heating water, etc, and the tip of the spar doesn't give us much. We don't have much living volume either, just what's inside the spar. So its natural to stick a platform on top of the spar, to get a lot more solar area, and make that platform several levels high to get more volume.

Unfortunately, this makes the structure a little more topheavy. Now we need more flotation and ballast to compensate. As we add them to the bottom of the spar, it begins to get very long. The point of the spar is to present a thin front to the waves. This means that once you get below the bottom of the waves, its not really necessary to use a spar shape. We can simply widen out into a larger flotation chamber.

Combine these observations, and you get our preferred seastead design, which looks somewhat like a dumbbell, possibly with multiple spars:

Having several pillars is necessary for large seasteads, since you can only project the platform out so far. But for smaller platforms, its easier to just have one spar, since the cantilevering is not as expensive as the multiple spars and connections. This also allows for more modularity. We can build multiple single-spar platforms, and assemble them together to get a multi-spar system.

SWATH

When the same concept for avoiding waves is applied to boat design, the result is a Small Waterplane Area Twin Hull, or SWATH. As you can see from the picture of the Frederic G. Creed on the right, such boats have two submerged, torpedo-like hulls with hydrodynamic struts above them. So the drag mostly consists of laminar flow along these hulls, rather than drag from waves at the waterplane (there is no waterplane). This makes the hull a little slower in calm water, but much more stable in heavy seas [SWATH].

While a monohull version was patented in 1880 and a SWATH design in 1946, the first ships were not built until the late 1960's and early 1970's. Although there are only about 50 worldwide, several are notable. For example, in 1992 Radisson built the Diamond, a 20,000 ton SWATH cruise ship. The design gives this 350-guest ship much less rolling motion than other cruise ships of similar size. While it looks somewhat like a catamaran above the water, below the water the Diamond features the same torpedo-like pontoons as the Creed.

In 1993, the public learned about the US Navy's Sea Shadow, a futuristic-looking A-Frame SWATH vessel which had previously only been operated at night. This 160-foot long stealth ship was manufactured by Lockheed-Martin in Redwood City, CA as a test platform for various technologies. It features a low radar signature, and while only capable of 14 knots can operate in extremely rough conditions.

Used Oil Platform

As we've mentioned, oil platforms are an excellent example of a pillared marine structure built to withstand the battering of the ocean. A number of people have suggested that rather than building some unique new structure, a group simply find an old oil platform and use it. While platforms are expensive to build, there is not much reason to charge a lot for an old one. In fact, the group could even be paid to dispose of it. There are thousands of oil platforms, and disposing of them safely is required and costly.

There are obviously both advantages and disadvantages to this technique. You get the building material much more cheaply. However, its part of a structure that was not intended as a permanent residence, so considerable retrofitting will be necessary. Having exceeded its expected lifetime, there are likely to be structural concerns. For example, sacrificial coatings for biofouling will be worn through and sacrificial anodes will be dissolved. Further investigation by experienced marine engineers is necessary to determine whether this is a feasible option.

Design Issues

When evaluating these designs, there are several issues which come up frequently.

Fixed-Position vs. Free-Floating

Our main concern with fixed-locations is political insecurity. If some country claims that you don't have the right to be there, the colony is screwed, as relocating is likely to be very expensive. Colonists may not be willing to move there in the first place because of this risk. A free-floating design can always just move on. When you're talking about an expensive capital outlay (analagous to a house plus part ownership of the local utility company), the residenst are going to find that level of security invaluable.

One could point out that the same pair of options is true today in normal countries, yet we usually choose to live in buildings rather than RVs because the political inflexibility is worth the extra room. But at sea, the tradeoffs are quite different, because the cost of moving buildings is so low. It is possible to have a house-like amount of space with car-like flexibility. Additionally, the potential dangers are more serious because of the uncertain political position of seasteads. Hence the fixed strategy is much less attractive.

Admittedly, there are advantages to a fixed location. One can discover and exploit local resources. Building costs will be cheaper. The colony can establish trade routes, and lay fiber optic lines. More long-term planning based on weather and local resources becomes possible. Pollution and bad waste practices are less likely. For these reasons, some groups may choose this route. However, we see political freedom as the fundamental motivation for seasteading, and freedom of movement is an important part of getting and keeping political freedom.

Breakwaters vs. Pillars

Besides the underwater solution, which we find unattractive, there are three basic ways to deal with waves. We can avoid them geographically (doldrums), stop them with breakwaters or rise above them with pillars. We'll focus here on comparing the latter two methods.

The crucial difference lies in how the two methods scale. A breakwater for ocean waves must be massive, it can't be built on a small scale. On the other hand, if you think of the breakwater as forming the perimeter of a circle with the colony inside, you can see that as the circle grows, the size of the breakwater grows with r, while the enclosed area grows with r². This means that the ratio (how much breakwater is needed per unit area) falls with 1/r, which is a huge economy of scale. So for large colonies, breakwaters will be cheap, for tiny colonies they'll be tremendously expensive.

Pillars, on the other hand, have no such economy of scale. If you want to build another unit of area, you need the same amount of pillar as before. However, each individual pillar is fairly small, a small platform only needs one, and their cost is not out of reach of a new project. Thus pillars are well suited to our incremental approach. Still, one of the great things about the ocean is that there is plenty of room. So the high marginal cost of pillars is somewhat unfortunate, in that it makes using this space expensive.

This suggests to us that for small, initial seasteads, the pillar is a much better method. As the communities grow and become city-sized, they will reach a point where breakwaters become cost-effective. So while breakwaters are not suited to the initial stage which we're focusing on, they will be a crucial way to bring down costs later, and let us expand into all that cheap, unused real estate.

There are several other factors to note. Breakwaters can be used to obtain energy while damping waves. It is easiest to build them in fixed locations, which is problematic as described above. However it is possible to build a floating breakwater with a rigid connection to the colony, so the problem is avoidable.

Another worry is that by fencing off a fixed area, breakwaters may lead to a static geography. That is, if a single government has control of the area, they are a monopoly. Because of the economies of scale, residents can't just leave and start their own colony cheaply. This is not insoluble, but it is something to be aware of.

Modularity

The level of modularity in our designs varies greatly. Sailboat fleets, for example, are quite modular. Individual boats can leave or shuffle around as they wish. A huge cargo ship, on the other hand, is monolithic. Modularity is important for two reasons.

The first is dynamic geography. In order for DG to function, small groups of residents must be able to move their personal space. We think this is crucially important to making this new way of life better than the old. Hence we are skeptical of fixed-geography designs.

The second reason is to allow for incrementalism. A modular structure is likely to be amenable to an incremental approach where one module at a time is built. We think this approach is vital to actually making a floating city happen. Thus designs which require a large initial capital outlay do not seem promising.

New or Used

Just like a car buyer, seasteaders must ponder this age-old conundrum: lay out the dough for something new, customized, with a long life ahead of it, or thriftily convert someone else's throwaway. While we tend towards the new approach, the issue is certainly not clear cut. A new, strange-looking seastead will feel to the world like a different way of life. This helps our political ends, as well as marketing appeal to prospective residents. It can be designed specifically for a permanent, comfortable, settled ocean life - which is not true of any existing structure. On the other hand, there are many large boats and oil platforms which are no longer useful for their original purpose, and can be bought quite cheaply considering the materials which have gone into them. Avoiding design work may lead to an imperfect solution, but at least it gets there sooner.

What tilts the balance for us is the surprisingly low cost of the structural portion of a seastead. (Renewable energy technologies are more expensive, but they would be needed in any design). Because designing and building a seastead is not that expensive, we see less reason to go the cheap route.

Materials

Our preferred material is ferrocement, which is cement reinforced with iron rebar. However, a large number of previous ventures have proposed using an interesting material called seacrete. Our skepticism of this substance bears explaining.

{ Should we also explain other engr. mat. considerations in the marine environment? ie stainless, coatings, sacrificial anodes, plastics... }

Seacrete

Professor Wolf Hilbertz came up with the fascinating idea to create a material by submerging an electrified wire mesh in seawater. Minerals are drawn out of the water by the current, and a cement-like substance is slowly grown by accretion. Eric Lee [Lee] has done a very complete analysis of the use of seacrete as a marine construction material.

The number of 4.2 lbs / kWhr (1.9 kg/kWhr) cited for seacrete energy requirements in places like [Savage1992], if correct, would make it quite efficient. Unfortunately, this figure has two serious flaws. First, it is based on a single experiment [Hilbertz1979]. Second, it is off by a factor of 42 due to a computation error, as Eric Lee has demonstrated. Rather than integrating power over time to get energy, the power used was taken as the energy. The process took 42 hours, hence the error. In fact, at maximum theoretical efficiency the rate is only 1 kg/kWh, and practical efficiences are much less than this. Hilbertz's published experiment produced only 0.046 kg/kWhr. At this rate, the energy alone costs well over an order of magnitude more than just buying cement.

There are additional problems. The major power loss is resistive heating of the forming seacrete. This is because the electricity has to get from the mesh through the seacrete to the seawater, and the seacrete is not a very good conductor. So of course the thicker the seacrete gets, the worse these losses will be. If you want to make structural walls for a sea colony, this is a definite problem. You can reduce resistance using a 3d wire mesh, but such meshes drastically increase the cost.

Because you are trying to replace such a cheap material (ferrocement), it doesn't take much to make seament uneconomical. In fact, there are some ways in which the ocean is the worst place to use seament. Its a place where energy is expensive and transportation is cheap. Using seacrete instead of importing cement is choosing to use energy instead of transportation - a poor tradeoff.

Marc Piolenc has suggested one interesting way of making seament worthwhile. You could set up a structure and some renewable energy scavenger in a remote place, then leave it for years to do its work. Even though the process is inefficent, you can replace efficiency with time if your source of energy is the kind which keeps on producing.

However attractive the idea of turning seawater into cement, seacrete appears to be a poor choice as a construction material. In practice, it is probably easier to use boring concrete and steel to build economical marine structures.

Ferrocement

Our building material of choice is reinforced concrete, also known as ferrocement. It is a composite of two materials: steel rebar and concrete (which is made from cement and gravel). The steel has a very high tensile strength (its hard to break by pulling), and the concrete has very high compressive strength (its hard to crush). The combination is a material which is strong under many loads. Since its mostly made out of rocks (which are plentiful) its extremely cheap.

In the image you can see the framing system used for ferrocement construction. First a rebar mesh is built, then concrete can be troweled on, or forms can be built and the cement poured in. One advantage is that once a set of forms has been built, they can be re-used many times. Also, since this is an extremely common building material, there are a huge number of ferrocement books, supplies, consultants, and contractors.

While it is not as popular as steel and fiberglass, ferrocement has a long history of use in the marine environment. It does require some special treatment, however. Over time, structural stresses create small cracks in cement. While this is not a problem on land, in the marine environment saltwater can seep in and corrode the rebar. For this reason, all surfaces will need to be carefully sealed. In our case, this means two layers of sealant. First will be Ashford formula, which makes it very difficult for water to penetrate the concrete. However, pressure over time will cause water to slowly seep through this, so an additional coat of epoxy will be used on the outer surface. Internal surfaces will only be exposed to humidity, not pressure or direct water, so the Ashford formula alone is sufficient.

An alternate possibility is to use a treatment like Xypex, which fills the pores and capillary tracts of concrete with impermeable crystals. Also, fibers can be added to the cement mix to add extra strength against cracking. Whatever system is used, it will be important to check occasionally to ensure that corrosion has not occured. The easiest way to do this is with a commercial device which examines the electrical resistance of the rebar lattice.

Cement Lite

For interior, non-load-bearing surfaces, the full strength of ferrocement is not required. There are several alternate concrete formulations which are not suitable for holding up buildings, but feature the same durability and convenience. There are two basic ways of making them.

The first is to bulk up ordinary cement by adding foam. This can greatly reduce the cost per unit volume when strength is not a priority. One method is to add 1-20 pounds of powdered aluminum per ton of concrete to the mix. The alumnimum foams up into gas, and can double the volume of the resulting product [TechTopics2000]. Another technique is to generate foam from soap such as dishwashing liquid. This can be done with a cheap homemade device and an air compressor, and the result also has about half the density of ordinary cement. Plans for such a device can be found at [Pelagic], and a picture of Wavyhill's experiments is shown at right.

The second method is to substitute a weaker, lighter, cheaper material for the gravel constituent of concrete. One example is "papercrete", which is a form of concrete using paper instead of gravel. The paper is mixed with sand, cement, and water to form a material which can withstand 300 psi and has an insulation value up to R-2. While papercrete holds its shape and is reasonably strong even when wet, it is not suitable for very wet environments (like exteriors or bathrooms) as it soaks up water quickly and dries slowly. When free paper trash is available, papercrete is even cheaper than ordinary unreinforced cement [MotherEarth2000].

Shotcrete

While concrete is normally poured into forms, it can also be sprayed into place using a mixture known as shotcrete. This can make construction much easier. For example, it is a key part of the construction technique used to build monolithic domes like the Eye Of The Storm mentioned earlier. These structures are cleverly made by inflating a large plastic form, building a rebar lattice, and then spraying on shotcrete. This avoids the need to build house-sized forms, since the inflated plastic provides the shape. As we'll describe later, similar techniques may help us build seasteads without a shipyard.

Our Design's Details

Our preferred design for initial seasteads is the free-floating dumbbell. Some of its advantages:

While we feel that the spar platform is the best combination of safety and cost-effectiveness, others may disagree. Some people are quite emphatic that this is too expensive and sailboat fleets, cargo ships, or dirt-cheap concrete slabs are the way to go. If that describes you, don't worry. As you've seen in Ocean Environment, we discuss many topics of general interest for any ocean-worthy floating home, and this will continue to be true as we talk about infrastructure and further issues.

Flotation

On the right you can see the seastead hull with the top and outside wall removed. The central spar is inserted into the center. The torroidal floatation hull is divided into several equal sized compartments. Each compartment has a fixed amount of relatively dense ballast (e.g. lead, steel) represented as brown. In addition, ocean water is used as a variable ballast which is pumped in and out to change the displacement; this is represented by the light blue in the diagram above. The hull is made from ferrocement.

As an additional safety factor, the seastead will have additional floation right below the bottom of the living platform. This may take the form of cheap flotation like 2-liter bottles, or the platform itself may be designed as a hull. If, for some reason, the submerged flotation or spar should entirely fail, the seastead would sink only to the bottom of the living platform. During construction, and certain engineering operations involving the spar, we may lower the seastead so its resting on this safety hull. This requires calm weather, since the living platform is not designed to take substantial waves.

While our current plans call for a symmetrical hull, another option would be to make the hull more oblong in shape. During transportation, the seastead would be oriented to move towards the narrow end, thus reducing drag. The hull would also act as a keel.

Ballast

Material	Weight (g/cm³)
Water	1.0
Concrete	2.3
Iron, Steel	7.0-8.0
Lead	11.3
DU	19

A large quantity of ballast is required in order to make the structure stable (as we explain later). This is what the yellow area in the flotation hull diagram represents. This ballast needs to be very dense, because only weight in excess of the weight of water counts towards its effect - and water is pretty heavy stuff. The weights of some potential ballasting materials are shown on the right.

As we can see, concrete is just not dense enough. Almost half its weight does not count underwater. Iron, steel, and lead are all of good ballast density. Ballast is actually one of the biggest structural costs of a seastead, because our design needs such huge quantities (200 tons for even a small coaststead). Since all we care about is weight and density, we should use scrap. We want to minimize our cost per pound of a material with the appropriate density. Lead is several times more expensive than scrap iron and steel, so is a worse material, although it does have the advantage that scrap can be easily melted into ingots.

One interesting possibility is depleted uranium (DU). It is extremely dense, and sealed into concrete its tiny amount of radioactivity would not be dangerous. Water stops what little radiation it has extremely quickly. Since its expensive to dispose of, we might possibly even get paid to take it, which would be a big cost savings. It would also be a "swords to plowshares" conversion - using a byproduct of the weapons industry to start a new way of life. However, not everyone may see it that way, and there could be negative political consequences, so using DU should be considered carefully.

Reader W.E. Johns suggests that if the seastead's power systems use conventional lead-acid batteries, the battery bank itself could form part of the ballast. This makes the weight a benefit rather than a cost. On the other hand, it adds distance and thus transmission loss, as well as making maintenance more difficult.

Spar

The main criteria that go into designing the spar are its height and the total amount of weight it must support. The choice of the height depends upon the anticipated worst case waves expect to hit the tower during the expected lifetime of the spar. It is potentially a matter of life and death that our structure be safe, but given that constraint, we wish it to be economical.

At right is a cutaway view of the spar, which is compartmentalized up its length to provide a number of individual water tight compartments. If any compartment springs a leak, the remaining compartments will continue to provide bouyancy. Additionally there is an access ladder that runs the length of the spar with water tight doors between the compartments. Thus, somebody can go all the way down to the flotation hull from inside the spar.

The spar is hollow for several reasons. First, we do want to have some flotation area at the waterline, otherwise the seastead will be totally unstable in terms of vertical height, and difficult to maintain at a desired height. Also, having some of the flotation be in the spar moves the center of buoyancy up, which makes it easier to get the center of gravity under it for stability. Of course, the downside of spar flotation is additional sensitivity to waves in both vertical and horizontal motion.

It is probably desirable to have at least a small habitable underwater room with viewports. The lowest section of the spar would be an appropriate place, and in clear water the view would be well worth the additional cost. Lower spar sections can also be used to store rarely used items. Some of the upper spar sections, well above the water level, can be used as regular rooms.

The upper and lower joins between the spar and the hulls must to be able to handle the necessary stresses. One option is stress risers. More elegant is to have a smooth transition, so that the spar simply widens and blends into the bottom of the platform and the top of the flotation hull, like a champagne glass. { Need Picture }.

Superstructure

The living platform is basically a building sitting on top of the spar. Since the purpose of the spar is to keep any wave crest from ever reaching the tower, the construction of the platform is fairly straightforward. It can be built out of wood like a house, although in the marine environment the wood needs to be water-sealed and regularly inspected. We prefer various forms of concrete, especially the lightweight ones mentioned earlier.

There is not much to say about decks, except the top deck (the "roof"). Since the easiest way to collect water is to capture and collect rain, the top level will be plumbed for this purpose. Things which need sunlight, like solar panels and greenhouses, will be located here.

An intriguing alternative way to build the top decks is to hang them from a central mast. Buckminster Fuller pioneered this design, which was used in his Dymaxion house and other designs. It requires less materials, since the platform does not have to be cantilevered or as stiff. And a lighter platform means we need less ballast and flotation, so its a big win. While this is worth considering, it may be a bit experimental for early models.

Shape

There are a number of different shapes the platform can take. its important to consider how they will interact when multiple steads are joined. The simplest shape is a box where the levels are all the same:

However, this results in most of the interior space not getting any sun. This is especially true when you connect multiple steads - now only the top deck gets sunlight, because the exterior windows look right into your neighbors window. One way to get interior sunlight would be something like:

But this still doesn't help much unless the sun is close to overhead. The key is to add reflected sunlight through a wine-glass design like:

Not only can light reach all the levels from above, but sunrise/sunset and light reflected off the water can reach the levels from below. This remains true even when steads are joined together. This design is more visually appealing as well. The area at the center is ideal for communal space. The platform increases its solar area. A downside is that it may raise the center of gravity by making the floors at higher levels larger. However, these effects are small because we're already pretty far from C_g, so the changes are relatively small.

Another option is to use a pyramidal shape, as in Buckminster Fuller's Triton City design. The problem is that the inner core of the pyramid is a large area without sunlight. Also while exterior units get sunlight during at least half the day, they aren't getting reflected sunlight. It does keep center of gravity down. This has the disadvantage of a smaller top deck, which means either less greenhouses and solar panels, or distributing them among floors, which is a lot more effort.

Mast

We think it desirable to have a mast as part of the design. After all, its a proven bit of nautical technology. We can put decorative items such as a flag on there, and a crows nest for a good view. In the modern age, the crows nest can even have a webcam to spare us the long climb. This might be an appropriate place to mount wind generators. And finally, it enables a useful additional structure:

Skin

We can hang a hemispherical skin of transparent plastic from the mast, covering the seastead's top deck. This has a number of advantages:

This skin has a lot of sail area but not much strength, which means that it is vulnerable to high winds. During hurricanes, it will need to be removed. Also, the skin means that we can't have a runway or helipad on the top deck. For smaller prototype seasteads like Baystead, this is not a problem, as they don't really need such facilities. However, larger platforms will need some transportation infrastructure. We can use their size to have both - make part of the deck a solarium, and put the runway on the uncovered area.

Runway / Helipad

Larger seasteads will need an STOL runway or a helipad, as described later under transportation. Helipads are fairly small, but runways take up a fair bit of area. Even large structures like Seastead Lite will only be able to accomodate Short Take-Off and Landing aircraft (STOL). Not until many spar platforms are connected, with their runways lined up, or breakwaters are used, will normal aircraft be able to land onboard.

Because the runway takes up a lot of area, we may wish to use an elevated runway which is partially transparent to light (ie made out of a grating). This would let us put crops underneath it. (Or this might be more complex than its worth.)

Misc

Dock

All seasteads will probably want to have a floating dock. This would be loosely connected to the central pillar, but able to rise with the tides and as the seastead's waterline changes. A very simple design is simply an annulus around the pillar, unconnected. During bad storms, the dock could be hoisted all the way up to the bottom of the main platform to avoid the waves. Otherwise, the waves will be repeatedly slamming the dock against the central column. It might be possible to use some sort of rollers so that the dock could move vertically, but still be braced against the waves.

Several people with nautical experience have expressed are serious worries about transferring any substantial cargo from such a dock. While the seastead will be steady, the dock and boat will be moving quite a bit in anything but glassy seas. This makes moving things, even with cranes or hoists, very difficult. While non-surface transfer via submarines and helicopters avoids this problem, it is also very expensive.

The best option is probably a small breakwater. It would protect a small area from waves in a particular direction, might well be temporary, and only meant to handle smaller waves. In calm weather, it would be deployed to facilitate cargo transfer. The rest of the time it would either be submerged or hoisted. A v-shaped pair of walls extending from the spar should do the trick, preferably fairly narrow. Cargo should also be packed in small containers, and off-loaded only in calm conditions. This issue is a disadvantage to the pillar design, as a full-sized breakwater would of course create a calm cargo loading area.

Elevator

The seastead will need some way of getting large objects from sea-level up to platform level. Some sort of hoist or crane is probably the way to go. We may want to locate it close to the center so as to put minimum torsional stress on the structure.

Design Issues

There are a number of things we must take into account as part of this design and that people wonder about.

Bobbing

The question we are probably asked the most is: will the platform bob in the waves like a boat? The answer is a clear "no", although to explain why we'll have to delve into some of the physics of flotation.

Boats float because their hulls are filled with air. This air is lighter than the water, so it has buoyancy. As a boat settles into the water, more and more air is below the waterline, so it has more and more buoyancy. Eventually the buoyancy is equal to the weight, and it is stable. This has the nice effect that if you increase the weight on a boat, it will just drop a little lower until the extra buoyancy has compensated.

Now consider the effect of waves. When the waves go up, the water line has gone up, so the boats displacement has increased, so it has more buoyancy, so it lifts. When the waves go down, the water line has gone down, so the boat has less float and drops until things equalize. It is this large difference in buoyancy caused by a small difference in water level that makes boats rock up and down so much.

On a seastead, however, things are quite different. Most of our flotation is submerged way below the surface. So as the waves bob up and down, the displacement hardly changes:

Boats also move laterally in the waves, for a different reason. While boats have narrow bows, they have long sides. The impact of the waves against that large surface transfers energy. But a seastead only has that central column which presents a small front in all directions. Thus it gets less lateral forces than a boat.

Stability

There is another matter of physics we have to worry about, and that is stability, ie whether the seastead floats upright or falls over. In order to be stable, when there is a shift in weight, a restoring force needs to act to counterbalance it. When a boat begins to lean, it dips farther into the water on that side. This increases the buoyancy on that side, providing a restoring force. So unless it tips so far that the edge goes underwater, it will reach an equilibrium. [Fay1991, Chapter 2 - Static Equilibrium]

Unfortunately, the same submerged flotation that gives seasteaders a comfortable ride means they lack such a restoring force from their flotation. If an unballasted seastead began tilting sideways, it would just fall over. It is naturally unstable, because the heavy top wants to fall and the light bottom wants to rise. Here is a brief and approximate explanation of the physics of seastead stability:

There are two important points, the center of buoyancy C_b and the center of gravity C_g. The center of buoyancy, on the left, is the center of mass (basically the two-dimensional average) of the part of the seastead which provides buoyancy. This area is shaded, and consists of the airspace below the water line, both in the spar and in the hull. The center of gravity, on the right, is the center of mass of the solid parts of the seastead, such as the greyed-in top platforms, the structures walls, and the ballast at the bottom. We can model a floating object as a rod connecting these two points. The C_g feels the effects of gravity and wants to go down. The C_b feels the effects of flotation and wants to go up. Depending on which is higher, you get one of these two configurations.

You can probably see now why we need the ballast. The long spar platform has substantial mass way up high (the living area) which must be offset by mass way down low. It is also aided by having a significant part of its buoyancy be in the spar area, which moves C_b up. The design must calculate these factors so as to achieve the stable configuration.

The lower the ballast is located, the more it pulls down the center of gravity. Currently we have it at the bottom of the flotation chamber. Another possibility would be to have the pillar extend farther down, and have a seperate ballast area. { picture} This would add spar length, but require less ballast to achieve the same stability. It would also add the worry that a structural failure between the ballast and flotation would cause a loss of ballast only, thus making the structure rotate unpleasantly to the horizontal. (Whereas if ballast and flotation are lost together, the platform simply drops down onto the safety hull.)

Tilting

Another frequent question is how much the seastead will tilt in strong winds, such as from a hurricane. Despite what most people think, the answer is "very little". This is nonintuitive because our intuition is based on boats, particularly sailboats, which are very light and have a lot of surface area. Thus they heel over quite easily. But air is really quite thin, while concrete is quite heavy.

Consider a standard concrete block with one side sealed. Now visualize it floating in a pool along with a little model sailboat. Imagine trying to tilt the concrete block by blowing air on it. Its an almost impossible job, because the thing is so heavy. The long lever arm of our pillar does give the wind a mechanical advantage, which may be as high as 5-10 (technically its the ratio between our righting arm, which is the distance between the center of gravity and center of buoyancy, and the winds lever arm, which is the distance from the center of gravity to the surface it is pushing on, which is the top platform). However, as the platform tilts, the righting force increases, so it is unlikely to tilt much. { should we diagram this? } Note that this slight tilt will only be for an anchored seastead. An unanchored seastead will simply be pushed.

For those who prefer less technical explanations, the children's tale aboout the Three Little Pigs tells us all we need to know. The houses made of straw and sticks were easily blown down. The house made of brick didn't move. End of story.

Multiple Platforms

Part of the modular nature of this design is that individual seasteads can be connected to form larger units. If the platform is made in a hexagon, it can be packed in a nice hexagonal grid. This has the disadvantage that platforms must be the same size, since hexagons cannot be subdivided or composed into hexagons (as reader Otto Drachen pointed out). Squares might be a better shape from this standpoint. However, with loose enough attachments, hexagons do tile, as a group of 7 looks much like a jagged-edge hexagon. A large hexagon can be fit in at the cost of some triangular gaps.

The interconnections must be somewhat flexible because of the dynamic forces operating on the seasteads. For example, as waves pass through, there will be continuous small fluctuations in the relative buoyancy of platforms. On a longer time scale, differences in weight onboard may cause platforms to want to rise or fall relative to each other. Good buoyancy control is very important because these differences will put great stress on the joints. A particularly tricky problem is mooring interconnected platforms, especially in areas with changing tidal currents.

Note that if the spars are small relative to the gaps between the platforms, our flotation design allows for a clever way of removing a single platform from a large collection of seasteads. Disconnect it, then reduce its flotation a little so that the platform drops beneath the level of the other platforms. Now it can be towed out between the spars, under the other platforms { 3D animation? }. The extra space between the edges of the platform and the forest of spars through which it travels is the same as the distance between adjacent platforms (the size of the interconnecting joint), minus the radius of the spars. So we'd best tow slowly. Still, its quite nice for modularity and dynamic geography for platforms to be able to leave without needing special gaps in the grid, or for other platforms to move out of the way.

It seems likely that standard utility interconnects will be developed to allow infrastructure sharing between platforms. A benefit of multiple platforms is longer runways, to allow larger, non-STOL planes to land.

Height	472m
Expected Lifetime	50-70 years
Total Weight	1,050,000 metric tonnes
Deck Area	8,670 m²
Water Depth	303m