Rogue wave detection & mitigation
June 8, 2008 at 12:15 pm #585
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.
June 21, 2008 at 8:30 pm #3285
- 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.
June 21, 2008 at 10:15 pm #3289
- A radar wave detection system seems easy enough to design, technologically. I can imagine some issues with calibrating the sensitivity though. Do you need to wait for a rougue wave to set it just right? Perhaps you can increase the wave height that triggers the alarm incrementally, during storms.
- In general, the problem with big waves is causing me to drift towards spars without any platform whatsoever. Cantilevered platforms that avoid all possible waves within some degree of reason need to be ridiculously high above the water.
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.June 22, 2008 at 1:47 am #3292
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.June 22, 2008 at 7:01 am #3294
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.June 22, 2008 at 8:11 am #3296
June 22, 2008 at 1:34 pm #3297
- Larger, yes. I don´t know about massively. All the volume above the water needs to be countered with displacement whether it´s in the shape of a platform or not. I agree the CG will be higher up with a cylindrical spar. The difference depends on how big the platform realistically can be. I have never seen any examples of existing or historical platforms with a spar to platform diameter ratio anywhere near some of the most prominent TSI examples. This is not to say that there is something wrong with them though.
- Brent Spar: smaller platform than TSI. Flip: smaller than TSI.
- Also, I think a seastead needs to have both lower risk and higher comfort than commercial platforms. Those who work at platforms today only do so because they get lots of money to endure high risks and harsh living. This is not going to fly if seasteading is to compete with land based life.
- Something inbetween might be the way to go; a conical part above the water would probably receive less impact from waves and still yield much internal space.
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.June 22, 2008 at 7:26 pm #3302
June 22, 2008 at 10:34 pm #3309
- The volume will have about the same average density if you use a similar floorplan or build for the same purpose. Ergo all the volume on top will need displacement. Volume = weight in this case, more or less. You are right about the extra room for stairs and lifts though. So there will be more volume .
- The cylindrical spar will pay a bigger penalty on a small scale as the lift etc will take up proportionally more room. With bigger structures (larger diameter spar) it won´t matter as much. I guess the cylinder would have to find a sweet spot somwhere between the smallest seasteads and those where the spar is big enough to enable placing a cantilevered platform out of reach of the waves for sure.
- If the platform is to be built light-weight it cannot be touched by the waves. You must build it to withstand the waves, or you must build it above them.
- On the CG I had the idea that the “missing volume” from the platform would have to be put on top of the cylinder – as more cylinder! – thus raising the CG. But this will probably have knock-on effects though so I´ll reserve judgement on this until some simulation or testing is done. This goes for everything else as well.
Ah, but that is the point of a platform vs pure cylinder.
- The platform gives you room for different purposes, and many of those purpose will simply be intended to give you space to work and breathe, not at the same density of activity/machinery as inside the spar.
- And if the platform is largely out of the way of waves, it doesn’t have the same stresses (particularly pressure) as the spar walls.
- The spar has to have tensile strength vertically, as well as compressive strength, vertical and lateral.
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.June 23, 2008 at 10:34 am #3316
June 23, 2008 at 12:10 pm #3319
- Well, we could speculate on about this but without actually testing or simulating it it´s hard to say anything with any certainty either way.
- I have to say that I disagree with your problem analysis though. It´s the occasional wave that hits a part of the structure that wasn´t designed to deal with this abuse that is the critical problem. The submerged parts, while under continous steady pressure, as well as under the influence by waves, will by neccesity be designed to cope with this. It´s when you start relying on the assumption that some building parts probably will never be hit by waves that you will get into trouble, IMHO.
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.June 23, 2008 at 9:04 pm #3325
June 24, 2008 at 8:51 am #3332
- Well the difference in structure requirements swing both ways. If a wave reaches the underside of the platform, effectively pushing up on a very large area against the mass of the entire spar and counterweight, there would be tremendous forces on the joint between the platform and the spar and the entire platform “floor”. Tremendous forces equals lots of material equals high weight. A cylindrical spar obviously never has to deal with this scenario.
- A dome with the convex side upwards? What about the deckspace? Runway? Heli pad? Tennis court?
“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:
- The higher the platform, the more attenuation of the wave; most of its force is expended getting there, rather than pushing against the underside.
- There’s also a balancing force in that the entire weight of the platform (even if it is less dense than the spar there is still a considerable weight) holding it in place.
- Part of it also depends on the shape of the under side. A shallow conical safety hull probably dissipates force better than a 90 degree joint between platform and spar.
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.September 30, 2008 at 12:03 am #3923
See also this crazy but brilliant idea from today’s New Scientist: Taking the sting off tsunamis.October 19, 2008 at 8:34 pm #3985
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.
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