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.

High Draft

• Examples: FLIP, spar platform
• Rationale: improved stability
• Mechanism: The mechanism by which a spar gains its exceptional stability is twofold
  • 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.
  • Loading: wave phenomena are biased towards the surface of the water (effects fall off exponentially with depth). Hence, 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.
• 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)

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.
  • With respect to 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.
  • With respect to heave: the upward forcing effect of the water and its waves is averaged out over a larger area, meaning the net heaving force
• 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. Compare the roll and pitch motions of a ship; because of its elongated shape, it would much sooner roll than pitch.
  • 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.

Minimize waterplane area

• Examples: spar platforms, semisubmersibles, clubstead, FLIP
• Rationale: minimize interactions with the waves.
• Mechanism: Floating vessels derive their stable position on the water from the fact that moving them up and down changes the volume of displaced water. If there is little waterplane area, this coupling is weak. This means the waves will have relatively little effect. On the other hand, the total flotation has to come from somewhere; as can be seen from any of the examples, this implies that the flotation is located somewhere below the waterline. This necessarily implies a medium to high draft, which has significant drawbacks (see: Design requirements/incrementalism/draft). Roll-stability can derive from any source; through the use of very deep ballast (FLIP, Spar), or wide footprint (semi-sub), or a combination of both (clubstead)
• Drawbacks:
  • Only works up to a given waveheight. How big of an air-gap do you design for? Being relatively unaffected by waves up to 10m is great; but how will you handle the 5m of a 15m wave that will hit your platform?
  • Poorly compatible with small scale designs