User:DM8954/Shipping Container Semi-Submersible Waterwalker Hybrid Design

From Seasteading
Jump to: navigation, search


The basis of this somewhat complex design is a semisubmersible that uses pressurized Shipping Containers as the primary means of buoyancy. The habitable space is suspended about 20 feet above mean sea level in order to avoid surface waves. Between the deep buoys and the habitable space are narrow supports, most likely made from coated/painted stainless steel, designed to support the structure above while minimizing the impact of waves on the structure through low surface area. This design alone doesn't have enough displacement in the water plane to maintain a stable position between the upper structure sinking to the surface and the deep buoys rising to the surface. To counteract this and to provide additional buoyancy, smaller spherical buoys, attached either to long posts or a system of cables, are added to the structure.


Shipping Containers are mass-produced, standardized units designed to transport a variety of goods via a variety of methods (truck, railroad, and ship). The abundance of these containers and their methods of production allow them to be both inexpensive and relatively durable. These units were not designed to resist the full pressure of ocean water, especially at any significant depth. Fortunately, in this application, since they would not need to be occupied, they could be filled with air at a pressure high enough to equalize with the pressure of the water at their intended depth. In this way, the container itself would not need to resist the crushing force of the sea water.

Surface waves on the ocean move in such a way that their vertical motion can only be felt a certain distance below the surface. This depth is approximately equal to 1/2 the wavelength of the wave. The lower in the water an object is, the less it will bob up and down. For this reason, I tried to place it at a considerable depth.

Sealeg Design

The first version of this hybrid seastead design utilize what I have become fond of calling 'sealegs'. [The version before hybridization would not have maintained proper depth easily.] These 'sealegs' were borrowed directly from Vincecate's Waterwalker design, in which 3 spherical buoys are attached to the ends of 3 posts to create a stable design, despite using surface buoyancy. I took one of these post-and-buoy units, added hydraulic shock absorbers, and attached multiple units to the bottom of the habitable space.


Dual-Buoy Tension System

The second major update to this design came when I got the idea for the dual-buoy system. The concept is described and illustrated below.

This is an animated illustration of how the Dual-Buoy tension system works.


In the illustration above, there are two spherical buoys attached to each other by a single cable, which is routed through a system of pulleys. In order for the system to work, there only really need to be 2 pulleys but the extra sets will be explained later. A single buoy will float to the surface and try to stay there. By connecting two buoys together on the same cable, in this way, each will pull toward the surface either until they reach the surface or until the cable is pulled tight. Since each has the same amount of buoyancy, once the cable is under tension, the forces will be pulling equally on both ends of the line. In this way they will always find equilibrium and an equal amount of upward force will be applied to both sides of the underwater frame. These natural forces, along with freedom of movement within the cables, will allow for an overall stabilizing effect even as waves pass by unequally.

Since these buoys are on cables, rather than posts, they have a greater freedom of motion. Even if a buoy is pushed to one side by a wave, the forces can only act in tension along the cable, so that the overall force will remain nearly straight up.

Two buoys in this system work well but any seastead design is likely to need more buoyancy than that of only two buoys and certainly all stable designs exist in more than 2 dimensions. So, multiple pairs of buoys would be needed to create enough lift and stability. The greatest balancing effect from a pair of buoys is when they are on opposite sides of the structure and on different parts of the wave. Two buoys next to each other will be on similar parts of the wave and only lift once side of the structure, making them act much more like a single buoy normally would. Another problem to consider is the fact that wave conditions at sea often change. If the two buoys are placed at the perfect distance to counteract a 15 foot wave, a 30 foot wave, or even a 7.5 foot wave could work against the system. [In actuality wavelength and not wave height are the important factors here but wave height is typically easier to visualize] For this reason I propose spacing buoy pairs at varying distances apart so that waves of all wavelengths can more easily be balanced.

Waves with longer wavelengths than the length of the frame that they are attached to will move the entire structure up and down, though there should be much less of a rocking motion in the process. This can be counteracted, to a certain extent, using water entrainment by the deep buoys. However, with larger wavelengths the depth of the water affected increases so that even the deep buoys would be moved by exceedingly large waves. This can actually be somewhat of an advantage because if a wave were large enough to impact the habitable space, you would not want water entrainment effect from preventing you from rising and falling with such a large wave.

Now, you may be wondering about the extra pulleys I mentioned. The 4 pulleys in the center of the system divert the cable along a longer path. All of the dual-buoy pairs would be routed to a movable ring placed roughly at the center of the frame. By moving this ring upward while attached to the buoy pairs, the path of all the cables would be lengthened simultaneously. This would force all the buoys deeper into the water at the same time. The effect of their combined buoyancy would raise the entire structure higher in the water. By raising and lowering this ring, you can have complete control over the draft of your structure and the height of your habitable space above the water. Since the system is based on cables under tension, you could raise or lower the craft at almost any height between the bottom of the habitable space and the deep buoys. Some caution should be used, however, since raising the center of gravity too far above the center of buoyancy will make it dangerously unstable. In calm water on the open ocean, you may want to lower the entire structure to be closer to the water. If you want to enter a port, you will need to raise the entire structure out of the water far enough so that the deep buoys have enough clearance in the shallower waters near shore. This would not be recommended in choppy water but additional stabilization through the use of sealegs might be a possible safety measure for such situations.

Dual-buoy Design


The zig-zag lines in the dual-buoys above, represent buoy pairs that are floating on the surface with enough slack in the cables so that they are not currently contributing lift. 60% of the buoys are still supporting the structure in this configuration and for these wave conditions.

Version 3 Design

The third major addition to the design came from the idea of retractable/collapsible stability extension armatures. Since part of the buoyancy and stability of the structure comes from the surface buoys, a simple way to increase their stabilizing effect is to extend them further from the center of the structure.

Click for larger (readable) versions of the drawings below.

Click for larger version!

In case the plan view is too cluttered, the multiple armatures radiating from a single point are just the various possible positions of each armature so you can get an idea of the possible configurations and the range this adds to the base of the structure.

Click for larger version!

I'm not sure if the 'concrete hinge' has been invented yet, or even if it's been proven to to work properly, but the idea is to use concrete under compression (it's greatest strength) in a system that should not require any lubrication. Each piece is designed to 'roll' past every other part in such a way that nothing needs to slide and friction is no longer an inhibiting factor. Spheres might cause some grinding as it rolls around the circular track, so conical cylinders might be used in an alternative design.

Another addition to this version of the design is the reinforced concrete shell. This adds strength to the shipping containers (likely enough to no longer need positive pressure) as well as entraining more water, protecting the steel shipping containers from corrosion, adding lateral support, and serving as additional ballast to keep the entire structure balanced and stable. In addition, the concrete may serve as a suitable surface for marine life to safely attach to in order to form a small artificial reef, for additional food (crab, lobster, eel?) and as a fish aggregator.

The dual-buoy lines will be run through conduits embedded in this concrete shell so that they don't get tangled up or damaged.

Future changes to the design may include replacing the steel monopoles with a concrete lattice structure or Honeycomb Tower (as suggested/drawn by Liam Hassett). The addition of Wave Propulsion Louvers may also be detailed.

Requirements Analysis


  1. Safety. The near-neutral buoyancy of the structure, deep buoys, habitable space, and stationary equipment, combined with the addition of additional buoyancy in the form of surface buoys and a sealable lower deck provide a good measure of safety.
  2. Comfort.
    1. Tests need to be done to determine just how stable this design will be. In my opinion, the potential for wave decoupling seems high and the height to base width ratio is reasonable even before the stability armatures are added, which improve this factor considerably. Between the low surface cross section, high buoyancy, large water entrainment area, and the potential for passive stabilization via dual-buoys, the system seems to have potential for a very smooth ride.
    2. Ample sunlight and lots of open space make for a comfortable environment. Moving much of the equipment to the central core and pushing living space to the exterior walls would allow for maximum light, views, and privacy.
  3. Cost. ~{cost per square foot estimates, explain use of low-tech solutions & existing mass-production components, link to further calculations below.}~
  4. Pretty. The Habitable space can be arranged almost like any building built on land. The main limitations are that the lower deck(s) should be sealable to a water-tight condition for additional safety in emergency situations and that support columns cannot be moved very far from design specifications (which is also true in many limited site conditions on land). Land-based construction techniques and standards can be used for most of the construction and, as a result, nearly any architectural or decorative design can be accommodated to some extent.
  5. Modular. This particular design is best suited at the current size or larger. (I could foresee 1 smaller iteration but the benefits would be limited) Multiple units of this size could conceivably be connected together at a distance no closer than about 20 feet on the short ends or 15 feet on the long sides. At a distance of 40 feet apart, the stability extension armatures could remain fully deployed and natural corridors/canals of about 20 feet wide would be formed by the surface buoys.
  6. Cargo. While it would be difficult to intentionally destabilize the system so that it moves in harmony with a regular ship in high wave conditions, the wide base should allow equipment and supplies to be hoisted up on one side, without pushing the entire structure off balance.


These can be weakened if necessary to achieve/optimize the absolute requirements.

  1. Free Floating. This structure is completely free floating an requires no deep-sea mooring.
  2. Scalable. This design has about 14,400 sq.ft. of interior space, including a 4800 sq.ft. greenhouse across the entire top deck. Depending on the amount of space required for storage and life support equipment, there could be between 4,500 sq.ft. and 9,000 sq.ft. reserved for office and living space for the occupants. The same design could be expanded horizontally by increments of roughly (2)1/4 sized bays in either direction along the primary axes. The limit of expansion is limited to one's ability to sufficiently handle bending forces along the entire length or width of the structure. It is also possible to scale up in height using taller towers, or other vertical structures (honeycomb/lattice concrete forms) with low wave interaction. Making the structure taller will also increase the minimum draft, potentially making port calls much more difficult or even impossible.
  3. Standards. The construction of the habitable spaces can utilize regular land-based construction methods and, therefore, could follow building codes (recommended use of maximum hurricane and earthquake safety factors). [Note: I'm not familiar with marine safety standards.]
  4. Mobile - Low wave interaction means that this design should have the ability to be moved by standard diesel or electric propulsion, as well as kites, sea anchors, and (potentially) my Wave Propulsion Louver design.
  5. Draft. The draft of this craft is considerable when deployed on the open sea but can be rapidly adjusted by changing the length of the surface buoy lines. The ability to dock in a harbor should be possible in the range of 20 feet of draft but only at the cost of raising the entire structure height. This means bridges may become a problem and boarding the craft while at dock will require a custom design for climbing up to the habitable space, which could be as high as 60 feet above the dock in this configuration.

Explicit Non-Requirements

  1. Self-sufficiency.
    1. Energy - A single large wind turbine at the top of the central support is expected to be the primary energy source. In order to reserve maximum solar area for a greenhouse, solar panels would have to be added only as an additional support system on the perimeter of the vessel. Wave energy is also an option that could be explored, as long as it doesn't diminish the primary advantages of the design.
    2. Water - Primarily, reverse osmosis desalinization 'water makers' would be used for fresh water self-sufficiency. These can be run purely on electricity, which should be plentiful with a properly designed electrical system.
    3. Food - Fishing will be a primary source of protein on the open ocean. To supplement that, a large greenhouse will cover the entire upper deck of the seastead. With proper crop rotation, this should be able to provide a limited supply of a large variety of fresh fruits and vegetables year-round, for 4-16 people. Complete food self-sufficiency will not be possible with this system because there would not be enough land for wheat (flour) or any animals. One could survive on a diet provided by such a system but a complete and balanced diet would likely require outside supplies.
  2. Defense. Defense has not explicitly been taken into account in this design but large caliber machine guns and water cannons could be mounted on the mid-deck 'porch' to be used along with hand-held firearms to repel a small-scale hostile incursion.
  3. Green. Most, if not all, technologies on board are intended to be sustainable and non-polluting, whenever possible. No claims of carbon neutrality or other 'green' labels could be applied until all elements of the system are designed and analyzed.
  4. Land vs Sea construction. Initial construction of the deep buoys and most of the structure would need to be built on land. A majority of the habitable portion could be built, using traditional construction methods, while already at sea. In the long term, seasteads of this design could also be built at sea using converted barges because of the relatively small dimensions of the main structure.

Cost Estimate Calculations

[Note: The following are cost estimates from version 1 of the hybrid design. The removal of the sealegs and the addition of the concrete shell will change prices considerably. Designing with reinforced concrete instead of pre-engineered steel monopoles could potentially reduce material costs drastically. I intend to create a chart to better illustrate the variations in cost based on different options and to improve readability.]

My cost estimates so far seem to imply that the structure (shipping containers, sealegs, monopoles, & even wind turbine system) would only add up to about $18-$29 per square foot (labor & material transport not included). The habitable space above could be built almost like a regular house rated for hurricane force winds. The construction methods and materials would generally be the same except for the use of steel construction... which is actually already gaining some prevalence in larger homes and in areas with higher strength requirements to resist earthquakes or hurricanes. Making the lower two decks of the habitable space water tight (to the point of floating, not just drenching splashes) would increase costs considerably. With normal construction ranging between $150-$350 per square foot, it's no more a premium than buying a prime piece of property to build your dream home on.

Just to show you how I'm figuring my cost estimates at this point:

Air-tight shipping container with hull coating, internal reinforcement, and pressurization valve: $3,500 x 12 = $42,000 or $7,000 x 12 = $84,000

SeaLeg (part of a WaterWalker) with leg, buoy, hinge, and cable: $650 x 18 = $11,700 or $1,125x 18 = $20,250

Monopole supports: $17,000 x 9 = $153,000 or $27,500 x 8 +$35,000 x 1 = $255,000

Wind Turbine System @15kw: $50,000 x 1 = $50,000

[before house = $256,700 - $409,050(18-$29/sq.ft.)]

Housing @ $200/sq.ft. for 3 floors @4800sq.ft.ea.: $200 x 14,400 = $2,880,000

$3,136,700 = $3.2mil total ($218/sq.ft.) $3,289,050 = $3.3mil total ($228/sq.ft.)

Housing @ $350/sq.ft. for 3 floors @4800sq.ft.ea.: $350 x 14,400 = $5,040,000

$5,296,700 = $5.3mil total ($368/sq.ft.) $5,449,050 = $5.5mil total ($378/sq.ft.)

These figures are very rough estimates and I tend to lean toward overestimation because cost overruns are more likely and unknown costs tend to add up as the budget is refined.

On top of these costs would be waste processing systems, backup generators, hydrogen/oxygen/nitrogen generators (for energy storage and/or scuba), furnishings, gardening seeds and tools, and various other heavy equipment and storage containers I haven't considered yet.