forces exerted by water

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This topic contains 12 replies, has 3 voices, and was last updated by  Anonymous 1 year, 9 months ago.

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• #22868

Anonymous

Does anyone have data on the forces of water in a large ocean wave, as it rises up, as the portions of it move in a circular path? If it isn’t a breaking wave, and it’s 20ft from crest to trough, what is it’s maximum push, and in what direction?

#22869

A few years ago (before the Draupner event) naval engineering was calculating with the “linear wave model” this model predicted a maximum height of 15m a “non breaking” behavior of waves in open ocean. This means that you NEVER would encounter a signifficant water mass moving forward and therefore pushing in a specific (horizontal) direction as it happens in breaking waves shore near. What a waterfront of just 5m moving forward horizontally does to a structure you have seen in the helicopter pictures of the japanese tsunami. The nonlinear model predicts this kind of waterfront as a condition that can happen 4 times a year in a 30m version at any open ocean location. The experience of the Bremen and Caledonian Star who lived it to tell the story shows that a modern, well built, expedition style, ship taking this over a strong 16m high bow can (barley and with damage) survive it. Calculate 1 cubic meter of water with 1 ton of mass, take the speed “trainspeed” and depending the size make it 5000 tons of water or more moving (low numbers) – so the impact is more or less compareable to the impact of a liquid train. When this comes at you make sure your structure is at least a concrete shell of bunker quality. A fragile dutch style house on a floating platform (Delta Sync) will not cut it to protect you. See pictures of floating oceanic concrete shell structures here: http://www.pinterest.com/wellmer/oceanic-concrete-shell-building/
Dig more into the concept of building Draupner Wave impact safe structures on the watersurface here: http://concretesubmarine.activeboard.com/f544141/concrete-surface-floating-solutions/

#22874

Anonymous

Obviously, i did not make my question clear enough in terms you understand. Let me try again…

Lets say i have one of those submerged concrete ball seasteads that works like you say it will, and all is happy and wonderful. Then i decide to anchor 100 miles offshore, in an appropriate place, and everything is happy and wonderful, except i cannot pick up my favorite radio station anymore, because i am 100 miles away from it. I need a pole with an antenna on top. So i stick a 20ft/6m pole, 4inches/10cm diameter, with a radio antenna on top to the roof of my concrete submarine ball. The next day a 13ft/4m wave washes over the concrete ball submarine, and the submarine is all happy and wonderful. What forces were on my antenna pole?

#22875

Steelposts sticking out of the water will in general recieve a force on their diameter that will not go beyond their breaking point – if it would not be so – Draupner would be gone by now. As the wave is arbitray – the force is arbitrary and you can not put exact numbers on it nor model it exactly – that is the meaning of arbitrary. But oil rig practice shows that steeltubes sticking out do fine over many years – in general.

#22876

Anonymous

That is not true. If it were true then the spiral ladder around one of the legs of Diamond Shoals light tower would not be missing from the wave zone. See the left-most leg on http://upload.wikimedia.org/wikipedia/commons/5/5a/Diamond_shoal_light.jpg

#22877

i said in general – that is the engineering problem – there is nothing you can build – including oil tankers, and drill rigs – that is freak wave safe – except a submarine – because it operates outside the impact zone.

#22878

Anonymous

I did not ask about freak waves, i asked about the forces of a normal wave. Any water moving against the pole, what is the force in pounds against the pole, as the water in the wave moves in a circular motion around the pole?

#22879

OCEANOPOLIS
Participant

The force of the wave is directly proportional to the amplitude and the frequency of the wave, in general.

#22880

Anonymous

And what is that in pounds per foot of pole?

#22881

OCEANOPOLIS
Participant

I don’t think there is a formula here,… like this 20 ft wave in 60 kt of wind is X lbs/square foot…If it broke the pole, the wave was too strong and the pole too weak. A tanker’s hull thickness is usually 3 cm of steel and an extreme wave could cut through it like butter. Now, a pole might fair better, since it will cut the wave. For a flat surface (like the topsides of a hull) it’s a different story.

#22882

Anonymous

There is a formula, there’s maybe a dozen formulas, none of which make sense, they give results in newtons, have pressure in torr, etc.. And none tell me what the WATER in the wave is doing, a wave is not a lateral movement of water, we should all know by now that the water in a wave moves up, down, to and fro, in a circular motion. This naturally means in a pole long enough to extend vertically thru the entire wave, the water is moving in many directions as the water level goes from trough to peak to trough. I don’t have data on that movement to plug into the formulas, even if the formulas made sense to me.

The issue isn’t so much about the pole breaking, as much as it’s movement and fatigue cracking. A 3 inch diameter length of tubing will not start crushing until it get a load over 15 tons on it, if it is held perfectly vertical. But if it is deflected to one side, it will fold up under much less weight. And if it is bent far enough, and bent repetitively enough times, it will fail with *no weight* on it.

I am guessing my only option is to build it and sit on it thru a hurricane and see if it fails.

#22883

OCEANOPOLIS
Participant

What on Earth are you trying to built that requires such in depth knowledge of that wave force if I might ask….

#22884

Anonymous

Virtually anything involving a pole in the water, if you don’t want the pole bent, broken, needs to be designed to stay intact, right? Anything from an antenna on a submarine, to a pole you drive into the lake bottom to keep the seastead more localised than an anchor, or any form of tower, really. If you look at the Diamond Shoals tower, there’s several spans of poles missing at the wave zone of that tower, and they all vanished the year it was auctioned. While that is an extreme idea of a “pole”, being those legs are ~44 inches diameter, it would matter to know what a minimum diameter pole could replace the missing parts. It’s a balancing act, the smaller poles have less wave resistance, but are also less stiff.

I don’t suppose you’d want to make a rope harness to drag a 3″ diameter pole (pvc pipe, electrical EMT, whatever) behind a boat and measure it’s drag at various knots/mph with a fish scale? Or a 4″ diameter? Anything? The pole would need to stay below the surface to be most accurate. It would not be the same as being vertical in a wave, but it would be a starting point.

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