Mega house boat... kinda

Hey, we are playing with an idea of using HDPE sheets welded together to form a giant flat boat.
The idea is to use it as a platform for houses, essentially creating a floating city, or maybe some other sea based industry - somewhere down the road anyway.

The design calls for the forces of the water from the underside to be counteracted evenly and entirely by the weight of what ever is placed on it. Meanwhile the forces from the side should be counteracted by a dike like ring structure.
Thus it should be possible to have a relatively thin membrane, saving on material costs for expensive HDPE:


Does this idea hold? Could you press the underside so far into the water as to get below the waves, which would instead "just" crash into the stronger sides?

(You can see our idea in detail here: http://floathaven.com/2011/07/the-solution-to-rising-waters-wednesday-blog/)

 

You may want to discuss this concept with an engineer, to address the obvious issues.

 

My concern (as a retired scientist, rather than anyone with competence in ship design) would be with the bending, twisting and wracking forces the structure would have to withstand. Getting the base of the structure below the wave affected zone (which means getting down below around 20m or more, depending on where you intend placing these things) doesn't change this, as the wave height around the edges of the structure will be asymmetric, creating forces that will want to bend and twist the whole structure.

I believe that the base will need to be made rigid, either with substantial bracing or by being built as compartmented honeycomb structure, in order to withstand these bending and twisting forces. This may not be a bad thing from the perspective of the dwellings built on the deck, as it would give you somewhere to run services, place storage tanks for water, foul drainage, fuel etc.

 

This seems to be poorly thought out from beginning to end. But the idea of using a thin membrane to ensure people's lives and property...

 

Realpra
Do some googling on the floating housing in Finland. The only viable material I can see for this kind of building is concrete.

Lurvio

 

That is true, but the twisting of the structure will not necessarily mean damage if it is built of a flexible material like steel or HDPE.
If you look at large container freighters in big waves they will also twist slightly.
With structures of this size you simply can't have complete rigidity.

I like the idea of using a honeycomb foundation, but maybe it could be limited to where the point stress are located so that it doesn't have to span the entire structure?
As I said I don't believe its possible to make the entire structure rigid - only capable of handling the flexing.

I haven't heard of many big concrete ships around though I have heard of ferro cement boats.
I don't know exactly, but I assume there is a reason for that mainly how heavy concrete is and how it cracks at tension.

What I have heard of though is larger fibre glass boats with a honeycomb stiffened bottom and fibre glass may be an alternative to HDPE.

 

If you design the structure to flex then you will be committed to a limited lifetime, and a regular inspection regime, because of fatigue. It depends on your planned design life, but don't assume that using a fatigue-resistant material like HDPE means that it is immune from fatigue cracking; sooner or later it will start to fail at the points subject to the greatest flexure, with those areas with the combination of the greatest deflection amplitude and frequency failing first.

There are ship designers on this forum and they are better placed than I to comment on fatigue failure from wave-induced flexure, but I believe that it is one of the most significant reasons for very large ships having a relatively short working life, measured in just a few tens of years.

Concrete would work well, and there is a lot of evidence to support the longevity of large floating or semi-submersible concrete structures. Floating concrete docks were built as long ago as WWII and still litter the coast of France (they were towed over the Channel to provide instant docks to support the Allied invasion - google "Mulberry Harbour" for more info on them)). I suspect that having a lot of mass relatively low down would also make the structure ride more comfortably, by acting to limit wave-induced motion to some degree, or at least lower the pitch/roll frequency. It'd also be cheaper, I suspect, to build in reinforced concrete and certainly it would lend itself to modular construction, which might ease the logistics of assembling and "launching" the thing.

 

I have read the wiki article already, those same mulberry harbours sank after only 8 months (due to waves, not bombs).
It's evidence all right, but it doesn't put concrete in a good light and I think the cost is more or less the same if HDPE is not cheaper.

Were will you find more tension than at sea? It's exactly what concrete is bad at.

 

Bear in mind that they were designed to sink! Plus they were intended to have a working lifetime of only a couple of months.

There are examples in the oil industry of large floating or semi-submersible concrete structures, too, plus concrete sea defences and tidal barrages, plus things like offshore wind turbine foundation caissons have to withstand wave action and tidal effects for many tens of years..

Bear in mind that what you're looking at here is using a material, HDPE, that has a very low Young's Modulus of only 0.8 GPa so is inherently very poor at resisting flexure. Concrete has a Young's Modulus of around 30GPa, so is massively stiffer.

Strength is a bit of a moot point, because you're dealing with a stiffness-limited structure here, so anything stiff enough to form a stable foundation for the buildings (where allowable local deflection will probably be very small, maybe just a few mm) will almost certainly be massively over the required tensile/compressive strength requirements.

Reinforced concrete also has a high tensile strength, determined principally by the tensile strength of the steel reinforcement, so of the order of 250MPa (yield, around 400MPa ultimate). HDPE has an ultimate tensile strength of just 15MPa.

Whichever way you look at it HDPE is just not a good structural material for large-scale applications like this, as it is relatively weak and lacks stiffness when compared to other, much cheaper, materials.

 

I think this is what you want:

http://www.treehugger.com/files/2005/11/tired_of_living.php

 

Floating island for houses

My two bits on the subject:

1. You have to put an incredibly rigid structure and platform on the water to prevent twisting and moving if for no other reason then for comfort.
( Imagine everything in the cupboard moving around... due to wave action.)

2. The mass of any structure like this should be immense to overcome the action of the waves.

3. There should be a ballast way below water line to stabilize the island.
You must have the Cg way below waterline!

4. Anchoring is a must with an adjustable length chain or some other means to ensure being washed away or blown away, yet maintaning the level defference at all times.

5. Concrete island with a void close to the top for buoyancy might be OK, but it has been known that the re bars rust away even at close proximiti to salt water... so maybe an epoxy cement concrete mix...?

I question the merit of building islands like these... have we ran out of land space?

What do you do for water supply, sewage lines, gas and electricity, commuting to and from, where do you park your boat and car?

 

Realpra; There were many hundreds of ships built of concrete during WW2. They were called Liberty Ships among other names. Some of the reasons for using concrete was cost, quick build, shortage of steel that had been diverted to other uses such as military tanks and guns, and the concrete was less likely to attract magnetic mines.

 

The first true ferro cement ships, where built in the early and mid 19th century, some still surviving as lightships today! Concrete is quite viable and a hell of a lot less costly the HDPE.

 

Of course, you may as well start out researching all the other discussions on the topic of sea borne living.

eg

http://www.boatdesign.net/forums/boat-design/floating-homes-communities-36446.html


you might like also to search for 'concrete submarines'

 

Have you ever read any Buckminster Fuller?

Here's one on Floating Cities.

If you haven't read Fuller before -- I'd recommend Ideas and Integrities to get a background on his reasoning process.

Interesting stuff.