Simple, stable, modular, spar type interlocking platforms


#21

I’ve been tinkering with an idea for a modified mini-semi-sub. Deck space ~24ft x 24ft, submerged pods in the 36ft range. Room on deck for a small, 18ft Airstream trailer as housing, greenhouse/grow area, and a bit of lounging space. This will eventually be the guest cabin, as it were, once I get my main hull done and floated-out.

Failed semi-subs are few and far between. Welding and corrosion are issues of QC and maintenance, only for metals. My goal is Geopolymer, as a permanent, durable structure.


(Garrett Herschleb) #22

That would be really cool to have a small stand-alone. You would have to have mitigation for the top of a rogue wave to catch you. The airstream could probably take it, especially if secured well. If you’re high enough and you only catch the tip, it’s not that much water. I’d put in some tough walls for the green house just in case.
I’ve also been thinking more about rogue waves in general. They are measured peak to trough. In my vast experience (sarc), it appears that one component of a rogue is that the trough unusually low, in addition to the peak being unusually high. If your structure doesn’t bob up and down so much, it won’t react enough to the trough to drop low enough to be caught by the peak. Anybody know more about that?


#23

Generally speaking, offshore platforms are moored in tension, pulled down against their buoyancy. Look at Tension Leg Platforms, to get a better idea of what I’m talking about.

Often, they are ballasted for stanility, when moving, and partially flooded to drop lower, while setting the mooring depth, then they blow the water back out, to achieve a very high tension on the mooring lines, using dynamic tension of buoyancy, against the hold-down capacity of suction anchors, or other forms of anchoring.


(Matias Volco) #24

If Simple and Interlocking is desireable consider starting with only two points of connection, like Pelamis or SeaTrain

small ramforms can serve as bifurcations


(Garrett Herschleb) #25

Thanks for the idea Matias. I love how ideas here can cross-pollinate!


#26

Perhaps PGS would sell you a REAL Ramform. They have several in dry storage, at this time… Get a floating city that can actually handle rough weather, not just some pretty pictures…


(Larry G) #27

Which requires a certain minimum size. If I have a platform with 50 ft of draft. and I get a wave that drops 26 feet, I am no longer in tension. if I lose tension, and snap to the top of my tension anchor, I am exerting more force than it can take.

So there’s a minimum size in order to effectively moor in tension.


Boat/seastead survival in a hurricane
#28

Yes and no… Legs are much like piers. Matter of bracing ans sizing for expected design stress loads.

26 ft trough would indicate about a 52 ft wave. With the NOAA data, it should be easy enough to spcify lengths based on recordedax wave heights, with additional length for Rogue waves. I forget the calculations, at the moment, but eyes are dilated after eye-exam, and have a headache as a result…


(Larry G) #29

My point being that the draft of the tension leg platform, regardless of depth beneath the platform, has to be related to the expected wave height. If wave drop exceeds the draft point where the platform is positively buoyant, then you have unacceptable movement. This means there is a minimum size/draft of a tension leg platform.

Doesn’t matter the depth, if the wave height exceeds the buoyancy line in a negative direction, you could still get a wave drop that exceeds the balance of buoyancy vs tension. Once you lose tension, you have a good chance of exceeding design tolerances on the tension anchor/line on the upward rebound, not to mention uneven vertical acceleration. At a very minimum you get an abrupt deceleration at the top (abrupt jerk to a stop.)

In my previous example above, I over-simplified, but the idea is that I get at least one foot of play for the platform to vertically accelerate upward before being stopped by tension. X tons moving a foot/second doesn’t like to stop abruptly.

A more precise example would have included details about total weight, vs total volume and determined where the buoyancy line is in terms of draft. Bottom line, if your water line drops low enough to loose the tension on the platform legs, you get unwanted movement and abrupt deceleration again back at the top, which is dangerous to platform and occupants. Thus the minimum draft/buoyancy ratio of the platform is dictated by the wave height design parameter. You have to have enough draft volume to maintain buoyancy without moving vertically at the maximum expected wave drop, ad enough freeboard to be above water at the maximum expected wave height. Thus there’s a minimum hull draft for a tension leg platform under given conditions.


(system) closed #30

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