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Rogue wave detection & mitigation
Sun, 06/08/2008 - 12:15 — thebastidge
Obviously, big waves happen rarely, but often enough to be a concern. Questions:
- How long in advance can they be detected?
- How wide are they?
- How far do they travel before subsiding into more "normal" wave heights?
- Aside from the structural design to withstand them, what sort of mitigation measures can one take?
Some of these questions probably depend on your seastead's location, weather, and what the bottom looks like. But for the fourth point:
- Can they be detected with radar? (Probably yes) and can a computer system be automated to give an alarm when a radar signature matching a rogue wave is detected? This probably depends on question 1 and 2 above.
Background: This occured to me today because we have a warning system here in Baghdad which alerts us to incoming mortars and rockets by using radar. It usually gives us a few seconds warnign before impact, and is even zonal for predicted impact area.
- I would think a rogue wave exceeding our normal operating parameters would not likely form within meters of our position- more likely would require time to grow, and would proceed on a fairly linear scale of forward movement (I'm thinking a few kilometers an hour, probably not more than 20 or 30 Kph at most.)
- People are not capable of keeping such a look-out 24 hours/day, but automated systems are.
- Most responses would probably be limited to "hold on to something solid" or quickly secure yourself to it, but it's amazing how much a few seconds can matter in a dangerous situation.
- Eventhough a seastead is expected to be very stable, I would doubt there would be very many heavy, unsecured objects aboard. "Things get bolted down" should be the default response to anything new brought on board.
A radar wave detection
Rogue wave looks like it came out of nowhere
The rogue wave comes from the combination of other waves. It may be waves from 3 different directions that come together right next to a ship to make a big wave, and then the big wave disappears into its component waves. It is not like you can watch it coming on radar. Theoretically, if you could get real time data on all the ocean surface around you, and model the waves in a computer in faster than real time, you could tell where waves were going to combine to make a big rogue wave.
It doesn't seem entirely
It doesn't seem entirely certain that there are notprevious indications. Like many completely surprising events, I suspect that there are indicators which simply go unnoticed until it is too late. My point is that electronic sensors don't have to sleep. That is not to say they are infallible, but they either work or they don't work, they don't just let their attention wander at a criticial moment.
And a lot can be done with a few seconds warning in some emergencies.
This is my understanding as
This is my understanding as well. Rogue waves form quickly and break quickly - this is part of why they are dangerous. You cannot watch them coming. The way to deal with them is (to me, and the ocean engineers I've talked to) both obvious and trivial: you simply need to design for a higher expected maximum wave height than standard models imply.
Brute force
The brute force appraoch is usually obvious, but not necessariloy trivial.
I still submit that the phenomenon has not been studied and documented well enough to say that it is undetectable until the worst has already happened. The idea of tracking incoming artillery and shooting it out of the sky was science fiction not so long ago.
Rogue waves may be chaotic
Rogue waves may be chaotic and largely unpredictable; they "appear out of nowhere". OTOH non-linear dynamics may be able to model them. But they're much less predictable than an artillery shell or mortar round which follow newtonian physics pretty closely, show up on radar relatively long (many seconds) before they arrive, etc.
The obvious answer is to design for very tall waves.
Designing for very tall waves
OK so what is the problem with very tall waves?
It's probably somewhere between "Everything on my deck got drenched" to "Holy Shizzit the boat capsized and sank!"
On a spar the problem would seem to be the wave over-topping the spar and damaging its deck, possibly dumping water into the interior. It doesn't seem likely to actually capsize the spar, from the smaller moment of leverage above water vs the long lever below water. Even dumping water into the interior momentarily seems unlikely to sink it, though possibly causing damage, injury or loss of life. Maybe it could exert enough lateral force to crack or snap the spar? This last would not be solved by height, but by strength.
On a cantilevered spar/platform combo, we see essentially the same as above, with the additional possibility of damage to the platform, or the join of the platform and spar, but seemingly lesser possibility of the interior of the spar flooding simply due to being overtopped by a wave, because the platform would act like a lid that is mostly closed.
I don't think the possibility of a rogue wave justifies all the disadvantages of creating a spar platform that is beyond the reach of a rogue wave. Not just the economic cost and engineering difficulty of construction, but the disadvantages of being so far above the water in daily life.
For other types of platforms, floating islands, rube goldberg hamster habitats, whatever, I think you're better off spreading your stability out laterally and designing for some combination of strength, shape, and flexibility that allows for occasional waves overtopping the deck. Then you keep an operations crew or some type of automated monitoring going round the clock.
The problem with tall waves is lots of energy
One of the very sophisticated oil exploration platforms was sunk by water from a rogue wave breaking a small window and flooding the active bouyancy control system. All the people on it were lost. Does your platform have any windows? How many tons of water (really energy from moving water) can each window withstand?
http://en.wikipedia.org/wiki/Ocean_Ranger
http://en.wikipedia.org/wiki/Rogue_wave
Water is very dense. A large amount of moving water contains a very, very large amount of energy.
Lesson: Do not rely on
Lesson: Do not rely on active buoyancy control systems. And make any windows and doors as tough as the wall they sit in. Expect all of them to get hit by massive waves.
Rogue Wave Insurance Policy
A properly designed spar platform is unlikely to be sunk by a rogue wave. There can be a bunch of pretty expensive stuff up on top that be damaged by a rogue wave -- solar panels, wind generators, etc. One possible solution is to damage problem is to purchase a rogue wave insurance policy.
My latest designs expect the lower deck to encounter waves in bad weather. The solar cells and expensive stuff are placed higher on a very light weight platform.
It is all a trade-off.
Trade-offs
Of course there are necessary trade-offs, but it's still good to acknowledge occasionally to keep it firmly in mind.
Designing to be able to occasionally overtopped by a wave and survive seems much more sensible than trying to design something that cannot be touched by waves. It's more likely to provide an economical and usable design, and more likely to incorporate a philosophy of graceful failure- not good to put all the eggs in one basket, i.e. "I will build this thing so tall a wave can't touch it" until of course, a wave comes along that is taller than your wildest dreams...
Spars without platforms
Spars without platforms would need to be massively larger in diameter to have anything like the useful cubic. Then they interact with waves more at the waterline.
There are examples of spars with platforms in existence, before we worry too much, lets look at the rtrack record of existing construction.
Larger, yes. I don´t know
Good points, but it's the
Good points, but it's the weight above water which must be countered with displacement. It's probably quite feasible to achieve much greater volume/weight ratios with the platform than with the spar, particularly of useful space.
The volume within the spar must take into account elevator and stair space (you need emergency backup to the elevator, but it's almost a guaranteed necessity to have an elevator in a structure 10 or more stories high.) In contrast, the platform is probably only 2-4 stories in height, and at least twice the diamter of the spar, I should think.
The platform (aside from a safety hull) doesn't really need to withstand much pressure, so it is much lighter, and is joisted rather than solid wall construction. It has to withstand the force of wind, but from what I've read of of high wind conditions, most damage comes from objects picked up by the wind, rather than just the wind itself, which is less of a problem for a single seastead on the open ocean. Wind damage is also mitigated somewhat by making the platform round or hexagonal.
I don't think the CG as a ratio of the length of the spar will be higher up on the cylinder. The platform is quite a bit of weight poised at the top, which moves the CG up higher, so no platform moves it down the length of the spar some distance. In absolute terms, the CG may be less distance below the surface because the overall spar is smaller and thus does not need the displacement to support a structure high out of the water. This also means that you are closer and more convenient to the water during normal operations, but you also have to have some arrangments similar to a semi-submersible, because waves WILL wash over you in some weather.
The volume will have about
Similar floorplan/purpose
Ah, but that is the point of a platform vs pure cylinder.
For these reasons, I think it is fair to say that the platform will NOT have the same density/volume as the cylinder. The force of an occasional wave beating on the outside of it, while important to design for, is not as critical as the strength of the submersed section.
On your last point, the missing weight (I still pick nits with you equating volume to weight. They have a relationship that is not necessarily one to one) means either the whole thing rides higher in the water, or you need less displacement below the wave zone to keep it from sinking. Less displacement means less ballast to keep it upright, all of which together means it can be shorter/smaller.
However, I think these savings are vastly overshadowed by the reduced comfort and usefulness of the space. all of it under water means no natural sunlight and lots of small decks rather than a few large ones. The smaller the deck area (as you note) the larger any inefficient use of space looms: like an elevator shaft which is 95% empty space and otherwise unusable except as displacement.
Well, we could speculate on
Platform strength
I'm not saying you don't design the platform to deal with waves. I'm saying that even though the outer shell of it needs to be strong, it encloses a greater volume, so the ration of heavy structural components to volume is not the same as the spar.
Suppose the platform is a hexagonal dome. It doesn't have to be a free-standing dome, it can have internal walls for support, and strength and still not have the density as the spar.
Well the difference in
"Tremendous force" would
"Tremendous force" would depend on how much of a wave we're talking about. You do have a valid point. However, let me offer three points in rebuttal:
A helipad is only useful if you really expect a helicopter to make trips to your seastead. It's quite probable that many seasteads will never see a helicopter. Particularly at more than 200 miles from the nearest land. Unless you also have Avgas available, because AFAIK the average commercial helicopter has (very roughly) 200 miles range. If you're clustering seasteads, it wouldn't be efficient for every one of them to have a helipad anyway. Another case where specialization advances civilization. Runways are just not likely on a Seastead any time soon, and both runways and helipads are completely irrelevant to this discussion of whether (from an engineering perspective) a spar should have a cantilevered platform or not; you've already done away with the platform, why worry about a runway or helipad on my domed concept? :)
Deck space is still useful under a dome. Look at sports arenas. It can largely be windows, because outside the Seastead, the only thing that could be blown into it by high winds would be very occasional birds. Safeco field in Seattle has a fairly large retractable roof (not a dome), from an engineering perspective it is feasible and commercially practical. Smaller seasteads might have a retractable dome along the lines of large astronomical observatories. Such a dome would mostly be a a protection against wind, and if it were mostly transparent, it would really change the micro-climate of the platform, essentially turning the whole thing into a greenhouse. It doesn't even have to be tightly sealed. It could have gaps and still make life more pleasent in cooler/stormier weather, as long as the gaps weren't placed in such a manner as to present a vulnerability to storm winds.
For example, a clamshell dome that splits across the middle, where the two halves hinge outward and the bottom edge of the retracted dome ends up underneath the edge of the platform. The upper edges of the dome provide a windbreak around the side of the platform. When closed, the dome covers the entire platform (not necessarily an opaque cover) and the bottom edge of the dome is at the bottom edge of the platform or slightly lower. Gaps exist between the outside edge of the platform and the inside edge of the dome, but in such an arrangement that gale-force winds still blow around rather than up and under the dome.
The top of the dome is mostly glass, so when it is retracted (normal operations), plenty of sunlight still reaches the side of the platform for hanging balconies and/or greenhouses. A central windmill is on a retractable mast (co-located with elevator shaft) for when you must raise the dome, but deploys when the dome is down.
Invisibility cloaks could take sting out of tsunamis
See also this crazy but brilliant idea from today's New Scientist: Taking the sting off tsunamis.