27 September 2008

Can You Build A Seastead to Withstand Class V?

* Category 1 -- Winds 74-95 mph
* Category 2 -- Winds 96-110 mph
* Category 3 -- Winds 111-130 mph
* Category 4 -- Winds 131-155 mph
* Category 5 -- Winds over 155 mph _Source
Hurricane waves are not the only wave danger to a seastead. Rogue waves and other types of storm waves can sometimes capsize large ships without warning. An open-ocean seastead will be exposed to constant destructive forces from the elements. The map above from Sea Friends shows common wave heights in meters.
Water waves can store or dissipate much energy. Like other waves (alternating electric currents, e.g.), a wave's energy is proportional to the square of its height (potential). Thus a 3m high wave has 3x3=9 times more energy than a 1m high wave. When fine-weather waves of about 1m height pound on the beach, they dissipate an average of 10kW (ten one-bar heaters) per metre of beach or the power of a small car at full throttle, every five metres. (Ref Douglas L Inman in Oceanography, the last frontier, 1974). Attempts to harness the energy from waves have failed because they require large structures over large areas and these structures should be capable of surviving storm conditions with energies hundreds of times larger than they were designed to capture. _SeaFriends

Tsunamis are not a significant hazard to seasteads:
Many people think of the tsunami as the most fearsome wave, but that's a landlubber's perspective. Generally driven by earthquakes, tsunamis are often unnoticeable in the deep ocean, where they have extremely long wavelengths and low wave heights (several meters at most, usually much less).

As this wave reaches a continental shelf, it piles up, becoming shorter and higher. Only then will it resemble the monsters of legend -

....Scientists used to dismiss...tales of unusually large [[Rogue ]] waves as mere folklore, like monsters or mermaids. But with the proliferation of oil and gas platforms, some of which record wave data, accumulated observations have finally led to mainstream acceptance of this seafaring "myth" [Lawton2001]. And recent data from the European Space Agency's ERS satellites has not only re-confirmed the existence of these waves, but indicated that they may be fairly common. Researchers with the MaxWave project computer-analyzed satellite photos from a three-week period in 2001 during which two ships were hit by 30m rogues. They found "ten individual giant waves around the globe above 25 metres in height." [ESA2004].

These rogue waves are the real dangers in open water. Towering above their neighbors, they are unstable and break quickly, thus containing tremendous power. They sometimes come unexpectedly from a different direction than the prevailing swell, which adds to the surprise and danger. Rogues have been known to ravage coastlines as well, sometimes coming out of calm seas to sweep away unsuspecting victims. Emergency services have warned beachgoers in some areas to be aware of this danger [RogueWarning]. _Seasteading Book
The circular array breakwater illustrated above is described better at Brian Wang's site. It is designed to protect a structure from a "tsunami", but might also protect from more practical and realistic wave hazards as well.The triangular shaped floating breakwater above was designed a hundred years ago or so, to simulate coastal underwater terrain that causes waves to break on shore. A floating ring surrounding a seastead, with a similar cross-sectional area, would provide protection against some wave hazard, providing the breakwater could be properly secured in relation to the seastead.The floating flat plate breakwater above is capable of reducing the height of waves passing over it. It was designed to protect coastal aquaculture projects. It would also have to be well-secured in relation to structures it is meant to protect.

A seastead will want to utilise wave energy via transduction from cyclical mechanical energy to other forms of energy--such as electrical, hydraulic, pneumatic, constant rotational mechanical, linear mechanical, heat, etc. When energy from waves exceeds the ability of a seastead's energy conversion systems, a fallback to dissipation and deflection of the energies is necessary. For example, the hull of a ship performs mostly deflection (with minimal dissipation), to maximise progress through the water. A fixed breakwater along shore performs mainly dissipation, along with deflection. Such breakwaters are made of significant mass, and built to withstand incredible energies. Floating breakwaters must be much less dense and massive, requiring more ingenuity on the part of designers, if the structures are to hold up over time.

You wish to build what is essentially a floating barrier reef around your seastead which is capable of drastically reducing wave hazard while generating energy, and not presenting a hazard to the seastead in and of itself (by breaking its mooring).

Do you have any ideas? The seastead itself may very well be built of pre-stressed concrete, capable of withstanding significant wave and wind stress along with wide temperature extremes. Such construction is being developed for offshore oil and gas drilling, and for large offshore cryogenic storage containers (PDF). Such structures will be very tough, but they will last much longer if protected from both constant day in day out pounding of waves, as well as the more extreme storm and rogue waves.

What material will you use to build your floating breakwater? Bucky Fuller suggested a multi-layered membranous structure filled with seawater, with internal baffles that allowed moving seawater to dissipate large amounts of energy against itself--inside the breakwater. Using seawater itself as part of the structure of a floating breakwater is resourceful, and devising internal channels that cause moving currents of seawater to oppose each other, dissipating wave energy, is also clever. Even if the exact design is not copied, the ideas involved may prove seminal.

This is not a trivial project. One must first understand the forces one is up against, before one can plan rationally. In the open sea, the wind and wave energy can be not only unimaginably intense, but also relentless. Open water seasteading, like the next level, is not for the easily intimidated, nor for the careless.

A reminder: 1st Seasteading Conference October 10, 2008

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Blogger Will Brown said...

It's an interesting problem, and complex at several layers simultanously.

If the seastead is to be permanently located on some undersea feature, then it risks eventual loss of soverignty to some relatively nearby already established land-based political entity. Some degree of mobility results in the structure being classified as a vessel instead, which is subject to it's own catagory of international regulation and vulnerability.

All that aside, the question of how a boundary screen would be stably mounted relative to a seastead is only half the challenge. How does one go about stablilising such a shield against a general trend of force (wind, tides and currents), the relatively predictable - in the short term, at least - transient extreme weather event and the completely unpredictable (as to both extent and direction) "rogue wave" event?

I expect the research into these questions will have practical value for protection of estuary systems and harbors, but I suspect their seastead applications will always prove too expensive to be viable in any extensive open water application. A ship will almost certainly prove cheaper/safer to construct and operate instead.

One possible application that does come to mind is that of a shallow-water arcology. Such a structure is designed to remain submerged at some minimal depth below that of lowest swell trough or tide. The wave and storm effects are so greatly reduced at even a shallow depth that such a protective structure could be targetted to protect by means of channelling the surge rather than deflecting it, I think. A structure that can be raised to the surface at need (surface conditions permitting) would likely prove necessary, but that's a comparatively straight-forward engineering exercise.

Anybody have any thoughts on what types of suitable construction materials might be the most inhibitive to the start of sea-life growth and corrosion?

Saturday, 27 September, 2008  

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