Design Features

Seastead design features

Many designs for ocean going structures exists, and many have been suggested for the explicit purpose of seasteading. There is significant overlap between these different designs: often, these differences can be viewed as mere variations along a continuous spectrum of some parameter.

This section is an attempt at identifying the main features or characteristics a design might employ to meet the challenges of providing a safe and comfortable piece of real-estate. In this way, we can look at seastead concepts in a more systematic way, avoid reinventing the wheel, and quickly get a qualitative feel for the properties of a given design. Not all designs can be perfectly categorized in such a way, but it creates some order in the chaos nonetheless.

Wave-evasion

Waves Waves are strongest near the surface of the water. To be precise, wave effects fall off exponentially with depth. Trying to minimize interaction with this region offers opportunities for minimizing structral motions due to waves, and hence attaining acceptable comfort levels.

Fully submerged

• Examples: submarines, underwater hotels
• Rationale: Evade the waves
• Mechanism: Due to the exponential decay of wave-disturbances with depth, being completely submerged offers an opportunity for nearly completely evading the waves.
• Drawbacks:
  • Existing implementations are very expensive. Even though costs might be reigned in significantly, researching this technology to habitable perfection would require a lot of time and money.
  • Lack of daylight, horrible failure mode

Semi-submerged

• Examples: semi-submersibles, clubstead, FLIP, some spar platforms
• Rationale: minimize water-plane area, thus interactions with the waves.
• Mechanism: Given that we want to avoid the region near the water surface, and that we do not want to locate our real-estate under water, we can separate the flotation from the real-estate, and bridge the gap with a relatively slender construction, which interacts minimally with the waves. This seperation between real-estate and flotation/ballast necessarily implies a higher draft than a normal floating hull would.
• Drawbacks:
  • What air-gap do you choose, and how to avoid catastrophe in case it is exceeded?
  • Poorly compatible with small scale designs. A 20m airgap is hard to fit into a structure that is supposed to be small, especially considering the volume of underwater construction needed to obtain stability.

High Draft

• Examples: FLIP, spar
• Rationale: extend down towards deep stationary water
• Mechanism: The mechanism by which a spar gains its stability is twofold
  • Loading: Since wave-phenomena are mostly confined to the surface, a deep spar is resting largely in stationary waters, and is only being pushed around at the top. The pressure fluctuations that drive heave motions hardly make it to the bottom at all, hence the heave-inducing vertical force fluctuations are small.
  • Intertial: its high mass makes it hard to move, and its elongated shape makes it hard to roll over. Because the ballast is so deep, the center of gravity is strictly below the center of buoyancy, which gives it unconditional stability.
• Drawbacks: High draft. This complicates deployment in deep waters, and rules out operation in shallow waters. It is not clear that the concept scales down to a more incremental size. It has never been done before (spar: 200m-ish, FLIP 100m-ish), and its operating mechanism suggests it will lose its stability properties once scaled down further.

Indirect displacement

• Examples: Pneumatically Stabilized Platform
• Rationale: cushion wave forces
• Mechanism: Anything near the surface of the water will be moved around by waves with hardly stoppable force. Instead of rigidly connecting the platform with something floating in the waves, one can connect them indirectly by means a spring of sorts. A practical implementation of this seemingly complicated concept is found in the Pneumatically Stabilized Platform. By floating a cylinder with an open bottom end in the water, the entrapped compressible air will naturally act as a spring between the water surface and the top-end of the cylinder.
• Drawbacks:
  • still an experimental idea, that has never been applied and tested in a real application.
  • What air-gap do you choose, and how to avoid catastrophe in case it is exceeded?
  • Good stability demands a large platform.

Wave-Averaging

Big Footprint

• Examples: cruiseship, clubstead, pontoon
• Rationale: increased stability by averaging out wave effects over a long span
• Mechanism: 'being big' is the proven method for increasing stability out on the ocean. There are two attractive aspects to having a big footprint, pertaining to roll and heave.
  • Roll: a wider structure has a more favorable metacentric height: any attempt to roll it over results in a large restoring force, which leads to smaller rolling motions. Compare the roll and pitch motions of a ship; because of its elongated shape, it would much sooner roll than pitch.
  • Heave: the upward forcing effect of the water and its waves is averaged out over a larger area, meaning the net heaving motions are reduced.
• Drawbacks:
  • A big footprint implies a big structure. In order for size to start to matter against oceanic waves, quite some size is needed. 20m is still small in ocean waves.
  • Bigger means more fragile. The bigger a structure is, the bigger forces it can bring down upon itself. Driftwood doesnt break in a storm; boats do. Big boats need to get out of the way in big storms, or they run a risk of catastrophic damage (reference miguels presentation).

Sparse Footprint

• Examples: Catamaran, Minifloat, WaterWalker
• Rationale: benefit of a large effective footprint, with minimum material use.
• Mechanism: essentially the same arguments that apply to a big footprint: A wider structure is more resistant to rolling motions. Instead of having one big hull, connecting a few small hulls by trusses spanning the same area, has roughly the same stability benefits, while being much more scale-friendly.
• Drawbacks: not as modular as it seems. One can rigidly connect three units in a triangle without any problems, but growing this structure further brings back the fragility problems of a big structure.

Add-ons

Heave plates

• Examples: spar platforms, semi-submersibles, clubstead
• Rationale: increase the difficulty of moving the structure up and down.
• Mechanism: heave plates increase the amount of water that needs to move in order for the structure to be able to move. This increases the added mass, which improves overall performance. As opposed to minimizing waterplane area or other strategies to minimizing coupling with the wavy surface waters, this techniques aims to increase coupling with the relatively stationary deep waters. More in general, the underwater geometry can be optimized for coupling with deep stationary water.
• Drawbacks: need to be located in relatively deep waters, less affected by wave motion. Adding them at the surface would only increase the tendency of your structure to follow the waves.

Fins

• Mechanism

Conclusions

Most design features aimed at providing stability in oceanic waves fail to scale down well. The most promising among these is the Sparse Footprint concept.