Seasteads can be classified by three levels of mobility:
Seasteads which are built on a fixed foundation (sealand), or anchored is a way that is part of the design (Tension Leg Platform). In general, these types of designs are considered neither practical (depth) nor desirable (Dynamic Geography)
Global position is kept either by virtue of geograpical location (gyre), or by means of anchoring. In case relocation is desired, specialized equipment (tugboats) are hired. Position relative to neighbors is best kept by means of connections between individual seasteads.
The seastead has its own propulsion systems. They should be at least powerful enough to maintain a time-averaged position, and powerful enough to avoid collisions with nearby seasteads.
Physics of propulsion
(wave-making resistance might be considerable for blunt objects too. Simple CFD software would be good to have. TODO: check if the free version of delftship covers this, once the website is back up.)
The dependence on velocity is quadratic, which means slow movement is more economical.
The drag coefficient appearing in the equation is a semi-empirical number. For blunt bodies, it is roughly equal to unity, and for streamlines bodies, it can be arbitrarily low. Therefore, the energy required to move or maintain position for a platform is much higher than for a ship of equivalent tonnage.
Economies of scale
It should be noted that mobility on the water is subject to strong economies of scale (Fuel consumption as function of size) For large seasteads, the cost of fuel is likely to be small compared to other expenses (see: flotel data). For seasteads on the small side of the scale, these figures are probably very different, especially if the design under consideration does not optimize for weight and hull shape, the way boats do. At present, little information is known on the subject; existing data generally does not deal with the hull shapes and low speeds we are interested in; getting good estimates is the subject of present research.
On the other hand wind and solar work much better for small seasteads. For example the average little sailboat today spends nothing on fuel and big ships use huge amounts. So it may be that moving small seasteads is much cheaper per person.
To get a rough estimate, the figures in this article are useful. A 3250m3 blunt object is dragged at an estimated speed of 0.5 knots, requiring 12.4HP or ~10kW of electricity input, or 36MJ/h. If said electricity was generated by a diesel generator, that would amount to 3L of diesel an hour at 1/3 efficiency, or two tons of diesel fuel a month.
The displaced volume is in the same range as some of the designs under consideration, which intend to house several families. 0.5 knots might not be enough; finding a place with currents limited to that speed should be possible, but that does not include wind-induced loads, which are likely to be dominant under most circumstances.
Some data from the clubstead report contains useful information:
The thrusters are powered by electric diesel generators. Two 2MW Diesel generators provide the platform with utility and propulsion power. The Diesel generators are marine type generators. According to industrial data, such generators consume about 0.2st of fuel per MWhr of energy produced.
Consumption per year (MWhr/year) Utility 3780, Propulsion 7155
Fuel Consumption per year (st/year) Utility 748 Propulsion 1417
The propulsion power reflects the amount of energy required to move the ClubStead at 2knots during a quarter of the time.
(40MJ/L, 40GJ/st, 11MWh/st, minus efficiencies, 5MWh/st, or 0.2st/MWh. ~40% efficiency.)
Note that even for this modest amount of mobility, the propulsion-bill is twice the utility-bill. This is rather significant, as off-grid energy, however generated, is expensive.
Clubstead has a total displacement of roughly 19000mt.
A structure like clubstead has a displacement of about 70mt pp.
Comparison with a simple viscous drag calculation:
Frontal underwater area: 41x75 = 4000sqft per leg. 1600m2 total. At a speed of two knots, or 1m/s, and Cd=1, that corresponds to 1/2*1000*1*1^2*1600, or ~1MN, and 1MW of power. Running that year-round would cosume 9000MWhr; 25% of the time 2250MWhr. Efficiency factors will not cover the factor three difference; it appears they used a higher drag coefficient, or included adverse wind-loads or currents, or wave-making drag is significant in this case. Unfortunately, they do not explain the assumptions and steps that go into their result.
The costs of the various options strongly depend on a variety of factors. In shallow coastal waters, anchoring is cheapest by far, and there is relatively little room to drift around, so some form of station keeping is required.
In the open ocean, anchoring may be prohibitively expensive, and in many locations, a low-powered system might suffice to lazily maintain a position within some large region.
A seastead continuously operating at cruise speed is certainly not economically viable on the small scale, not economically viable for platforms of any kind, and probably expensive even for large scale shipsteads. Besides the fuel costs, continuous mobility around the globe might not be as good as it sounds at first. While nice for vacationing or retired people, most jobs are dependent on being tied into existing economic and social networks. It is assumed that a good measure of stability in geographic location or region is necessary for economic viability.
Mobility at a station-keeping level is already expensive on a big platform like clubstead. The smallest seastead we envision is about 1/5th the size of clubstead, accomodating 50 people and displacing 5000mt of water. Station keeping efficiency should be comparable, perhaps somewhat worse. The biggest uncertainty is in the encountered currents and winds, and political reasons for having to relocate. Under generous assumptions, station-keeping will be a small expense relative to the utility bill, under conservative assumptions (otherwise available locations have strong winds and currents), station keeping without anchoring may be problematic for platforms.
For legal reasons, permanently mooring within the EEZ or continental shelf dramatically extends the claims the host nation can lay on the seastead under international law, as it becomes a 'permanent installation'.
There are various scenarios with impact on mobility:
- Strike a deal with the host nation, not to regard anchored seasteads as permanent installations; or establish a precedent to that effect. This would allow seastead operation in shallow waters, where anchoring is no problem.
- The seasteads are self propulsed, to avoid being classified as a permanant installations. As noted, the economics of this are questionable.
- Find locations, just outside the EEZ and continental shelf, where anchoring is affordable. Unclear if such locations exist, and if they do, these locations will be far from land.
None of these have guaranteed succes though. The safest route is thus:
- When operating within the EEZ, anchors are used, and these legal compromises are accepted as a necessary transitionary stage
- When moving out of the EEZ to international waters, there is more choice of low-current locations, and plenty of room to drift around.