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Originally Posted by Robert Tulip
Quite a lively exchange on this topic is at http://climatecolab.org/web/guest/pl...essage_1349453
"Lively" is a hardly adequate adverb. Thanks for the link! One critique was the challenge associated with anchoring and maintaining a large-volume seawater bag; while I am skeptical of your proposal in general, I do not think this is a fatal flaw and would suggest an array of modular bags that can be individually brought to the surface for inspection and repair without interrupting overall pumping operations. I wish you the best in defending your proposal.

2. Here is a simple mathematical calculation that I hope someone can help me with.

I am looking at a tidal pump model based on use of a standard air balloon as the lifting component, as pictured. A 20 metre balloon contains 3800 cubic metres of air, and I assume this would be pressurised at average water depth of ten metres. For the weight, I propose to use sand, which has volume of 0.65 cubic metres per tonne. I want the formula to calculate how much sand is needed to give the balloon neutral buoyancy, fully submerged. Very grateful any advice.

Balloon Sand Tidal Pump Model.png

3. I have done the calculation myself, and it looks like the power of this apparatus is far bigger than I had imagined, nearly triple the estimate in my previous post.

Here are the numbers.

Balloon: Sphere of radius 10 m contains 3800 cubic metres of air, weighing 10 tonnes gross at average depth of 10 metres (2.6 kg/m3).

Sand: Cylinder of radius 40 m and height 1.3 m weighs 10,829 tonnes and has volume 7039 m3 (@0.65m3/t)

Combined apparatus volume of 10839 m3 = weight of displaced water, 10,839 tonnes.

This apparatus should maintain constant depth with the top of the balloon at the sea surface due to neutral buoyancy. That means that a bladder underneath the sand cylinder, fixed to both the ocean floor and the sand disk, will fill and empty with the tide. In a strong tidal location such as Australia’s North West Shelf, with tidal range of two metres, as a first approximation, not considering friction etc, this model will pump ten megalitres per tide, or 7 gigalitres per year.

By comparison, to pump 7 gigalitres in a year would take ten of these high volume irrigation pumps, working full time at 85,000 litres per hour.

Here is an updated sketch (not to scale).
Balloon Sand Tidal Pump Model.png

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Originally Posted by Robert Tulip
This apparatus should maintain constant depth with the top of the balloon at the sea surface due to neutral buoyancy. That means that a bladder underneath the sand cylinder, fixed to both the ocean floor and the sand disk, will fill and empty with the tide.
No, too much sand. If the sand/balloon system is neutrally buoyant, it will not rise back up with the tide to refill the bag. It would just remain fully submerged at the low-tide level.

Originally Posted by Robert Tulip
By comparison, to pump 7 gigalitres in a year would take ten of these high volume irrigation pumps, working full time at 85,000 litres per hour.
Those pumps have 50m of lift, counting friction. Your proposed system has <2m of lift, neglecting friction. One of those diesel engines (coupled to a higher-volume, lower pressure pump) could do the same job as your tidal pump, several times over.

5. Originally Posted by VQkr
No, too much sand. If the sand/balloon system is neutrally buoyant, it will not rise back up with the tide to refill the bag. It would just remain fully submerged at the low-tide level.
Okay, so if the sand volume is halved to 5000 tonnes, then the 3800m3 balloon would be mostly above water. Is that right? And how would you calculate the lifting power, ie how much it would sink on a rising tide when coupled to a bladder as shown?
Originally Posted by VQkr
Those pumps have 50m of lift, counting friction. Your proposed system has <2m of lift, neglecting friction. One of those diesel engines (coupled to a higher-volume, lower pressure pump) could do the same job as your tidal pump, several times over.
How do you calculate the lift? What I am looking at is a way of moving water around within the ocean, ie not using this method to raise water above the ocean surface.

Thanks.

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Originally Posted by Robert Tulip
Okay, so if the sand volume is halved to 5000 tonnes, then the 3800m3 balloon would be mostly above water. Is that right? And how would you calculate the lifting power, ie how much it would sink on a rising tide when coupled to a bladder as shown?
How do you calculate the lift? What I am looking at is a way of moving water around within the ocean, ie not using this method to raise water above the ocean surface.

Thanks.
When the sand/balloon assembly exerts force on the pump, this force is added to the weight of the sand and balloon. If low tide instantly went to high tide and the balloon went from halfway to (barely) fully submerged, the assembly would exert force on the pump, pulling it open and drawing the water in. As the balloon lifted out of the water, force would decrease until it reached the new steady state with the pump chamber fully expanded and no net force on the partially submerged balloon.

A tide is not, though, a step change followed by a decay curve. The balloon will always "lag" behind the tide a little bit because of the force being expended pushing on the pump; integrating this force over the distance traveled up and down by the balloon is the useful mechanical work you system can do. It will be smaller than half the buoyant force on the fully submerged balloon times the magnitude of the tide (the limiting case).

Lift (aka "head") is just another way of saying pressure. It is the distance a pressure would push liquid above its surroundings in a closed tube. 1 m of head in fresh water is equal to 9.81 kPa (10.1 kPa for sea water). Hydraulic power is (roughly; Google Bernoulli's principle for a more precise treatment) the flow rate times the change in pressure across the pump - this means you need to know the pressure as well as the flow rate to be able to estimate the work being done.

7. My Tidal Pump proposal has been selected as a finalist in the MIT Climate Collaboration Competition.

http://climatecolab.org/web/guest/pl...planId/1320162

Voting is open to the public until 13 September. Here are the judges' comments

"Combining ocean energy with algae production is an interesting topic. This revised proposal incorporates the judge comments well and brings forth an intelligible process for which algae farming can lead to a significantly positive impact on our environment. This proposal recognized the issue regarding commercial viability and does a good job at presenting possible solutions to that issue. The end product focus and using locally provided energy to generate the end products are welcome considerations.

Scaling this technology would appear to be a large challenge. More can be said though about commercial viability than what was presented. While it was noted that several entities will need to partner up in order to make this project more feasible, specifics would have been welcomed.For example, Australia was mentioned as a possible location for this project. What entities in Australia would help further this proposal? Any possible policy concerns for the Australian people/government? What possible scientific entity in Australia or elsewhere would take charge of the testing of the pump? I believe some of the portions of this proposal were too broad and specific mentions of groups or financiers would have made this proposal stronger. Think: who would have a legitimate stake in a project like this? Also, more discussion about the impact or safeguards for the pump in deep ocean waters would have been welcomed. What safeguards would be in place to present boats or sea life or people from interfering with the pump or being harmed by it. I understand that more research and tests need to be done, but there seems to be no mention of this in the proposal."

8. My Tidal Pump invention won Judge's Choice for the MIT Energy-Water Nexus Competition.

I will present it at the Crowds and Climate Conference in Boston this Tuesday October 5-6.

http://climatecolab.org/web/guest/pl...planId/1320162

9. Congrats, Bob !!!!!

10. Oh hell yeah congrats!

Hey! Where is everybody else? The man did something here!

11. I will give a seminar on my Tidal Pump at the University of Queensland Sustainable Mining Institute.

Poster for my seminar on December 17 is at http://rtulip.net/yahoo_site_admin/a....342163331.pdf

I will have two days in Brisbane and hope to meet interested people, including regarding the Great Barrier Reef. Here is a link to the event.
http://www.smi.uq.edu.au/events/smi-...gae-production

Tidal Pumping for Large Scale Ocean Based Algae Production
PUBLIC SEMINAR Date: Thursday 17 December 2015
Time: 10:00am-11:00am
Location: Level 4 Seminar Room, Sir James Foots Building (47A)
Sustainable Minerals Institute, The University of Queensland
Presenter: Mr Robert Tulip
Assistant Director, Energy and Resources, Department of Foreign Affairs & Trade

Summary
Innovative technology suited for rapid deployment could help prevent dangerous warming and acidification, using methods that if scaled up globally could remove more carbon from the air and sea than total emissions. Large scale ocean based algae production can use carbon dioxide as a useful commodity input. Carbon can be mined to make fuel, food, feed, fabric, fertilizer, bitumen, construction material and other goods. Algae farming can protect and enhance biodiversity while providing sustainability at scale, utilising the space, energy and resources of the world oceans to make carbon capture and storage a profitable industry instead of a cost.

The Tidal Pump and other components of the proposed algae system are at schematic proof of concept stage. Recent MIT recognition provided an important validation for the concept, with the Tidal Pump winning the 2015 MIT Climate Colab Energy-Water Nexus competition. Partners are now sought to assess feasibility. If proven, ocean based algae production could become a major new industry for Northern Australia, contributing to global security priorities in climate, energy, ecology and food.

Robert is an international development professional who has worked for AusAID and then DFAT since 1989, managing Australian aid programs and policy in sectors including mining, water, forestry, climate, research, governance, health, finance and infrastructure. Building on this multi-disciplinary experience, he developed ideas for ocean based algae production which recently won a global competition on the Energy-Water Nexus at the Massachusetts Institute of Technology, for a proposed method for tidal pumping. He has a Master of Arts Honours degree from Macquarie University for a thesis in ethics and a Graduate Diploma in Foreign Affairs and Trade from Monash/ANU.

12. ^ Cool, Robert!

13. Hi, That's the kind of applied technology that makes us proud ! I know you shall do very well .
Continued success in all ways.
Best regards,
Dan

14. Hi Robert , belated congratulations for the win at MIT. Hopefully some of the Innovation Nation funding will come your way. I'd be interested to see how a decent sized prototype handles the big tides up north.

15. Congratulations indeed but this proposal needs a lot of work. Ok now it is in a lifting tide, but the ballon is in water with pressure increasing vertically so it will not stay that shape at all. The air inside must be at approx constant pressure while the water pressure is max at the bottom, so the balloon will try to pancake at the surface, stretching its equator. The float therefore needs work, then you can pump, as drawn the balloon will just deform with the tide, and if literally an air type balloon will burst. Robert , well done but you need help. I hope you get it before trying your idea as sketched.

16. Originally Posted by VQkr
"Lively" is a hardly adequate adverb. Thanks for the link! One critique was the challenge associated with anchoring and maintaining a large-volume seawater bag; while I am skeptical of your proposal in general, I do not think this is a fatal flaw and would suggest an array of modular bags that can be individually brought to the surface for inspection and repair without interrupting overall pumping operations. I wish you the best in defending your proposal.
Agreed, the big balloon is a howler, use for first trials large fenders already produced.

17. Originally Posted by profloater
Congratulations indeed but this proposal needs a lot of work. Ok now it is in a lifting tide, but the ballon is in water with pressure increasing vertically so it will not stay that shape at all. The air inside must be at approx constant pressure while the water pressure is max at the bottom, so the balloon will try to pancake at the surface, stretching its equator. The float therefore needs work, then you can pump, as drawn the balloon will just deform with the tide, and if literally an air type balloon will burst. Robert , well done but you need help. I hope you get it before trying your idea as sketched.
Thanks very much Profloater. I am not an engineer, so I am looking at this idea as a way to spur investigation of what would actually work, for example as a way to pump deep ocean water on to coral atolls to protect them from acid and heat and enable them to continue to grow with possible rising sea level instead of suffering the impending death by bleaching.

My interest is to get conversation happening about how best to use tide to pump water at sea, starting with conceptual discussion, of which I have still had very little beyond the brief comments here, which I appreciate a lot.

I don't understand why the balloon would pancake if it was inside a net. The air in the balloon provides the lifting power, while the structural strength is from the ropes around the balloon. I have been trying to build a small prototype and am steadily finding out the details that need to be addressed, such as needing a wide enough i/o pipe to address water friction, and needing a way to translate the up and down motion directly into in and out water movement instead of allowing the bellows walls to collapse out to the side. Also I think that the overall power would be significantly increased if an air pump fills the balloon on a rising tide and empties it on the falling tide. Materials would be addressed by lab tests and then hopefully in sheltered bays to completely address all safety and efficacy issues in an orderly way.

I made a youtube video just to show how much a balloon can lift in water, showing a plastic container holding sand and a metal weight. Sorry it is a bit slow - 2 minutes https://www.youtube.com/watch?v=78-DdT4h_Tc

And here is one showing the weight falling down when the balloon pops. https://www.youtube.com/watch?v=gKhlAcWdyig

I am finding it much harder than I expected to build a model with a bellows and pipe under the weight.

18. Originally Posted by Robert Tulip
Thanks very much Profloater. I am not an engineer, so I am looking at this idea as a way to spur investigation of what would actually work, for example as a way to pump deep ocean water on to coral atolls to protect them from acid and heat and enable them to continue to grow with possible rising sea level instead of suffering the impending death by bleaching.

My interest is to get conversation happening about how best to use tide to pump water at sea, starting with conceptual discussion, of which I have still had very little beyond the brief comments here, which I appreciate a lot.

I don't understand why the balloon would pancake if it was inside a net. The air in the balloon provides the lifting power, while the structural strength is from the ropes around the balloon. I have been trying to build a small prototype and am steadily finding out the details that need to be addressed, such as needing a wide enough i/o pipe to address water friction, and needing a way to translate the up and down motion directly into in and out water movement instead of allowing the bellows walls to collapse out to the side. Also I think that the overall power would be significantly increased if an air pump fills the balloon on a rising tide and empties it on the falling tide. Materials would be addressed by lab tests and then hopefully in sheltered bays to completely address all safety and efficacy issues in an orderly way.

I made a youtube video just to show how much a balloon can lift in water, showing a plastic container holding sand and a metal weight. Sorry it is a bit slow - 2 minutes https://www.youtube.com/watch?v=78-DdT4h_Tc

And here is one showing the weight falling down when the balloon pops. https://www.youtube.com/watch?v=gKhlAcWdyig

I am finding it much harder than I expected to build a model with a bellows and pipe under the weight.
A small model might work but at full size you have pressure on the outside of the balloon. Roughly one atmosphere of pressure every 10 metres down. If the balloon is close to atmospheric pressure inside, then the skin at the bottom at ten m deep has about a ton per square foot (sorry to mix units) force pushing it upwards. That's the force you want but a flexible balloon will just move upwards locally, distorted. You need a bottom rigid plate or lots of ties to the fabric to hold that force. In other words a special bag, not anything like a hot air balloon. You can get very big fenders as used on ships, they are strong and will work as floats for trials or indeed they may be right for operation. My other point was that the tides are sideways motion so you need to allow for that, or indeed use it to recover a pumping force.

19. Suppose you used a fender 5 m diameter by 15 m long, that's a buoyancy of about 200 tons in round numbers.
Secure it to the floor ten (?) m down by ties as in your sketch to a larger ring of sand bags to withstand the sideways forces while allowing vertical movement. This means long diagonal ties like springers on a moored boat. Now fix the float to your pump bladder or bladders, using modular design as VQkr suggested. I imagine each pump is a rigid disc with cylinder flexing sides and rigid disc base with sand bags, (separate bags from the float mooring) each one is about 2 m high 2 m stroke, so might pump at 5 m diameter, say 30 tons of water per tidal cycle. I don't know the force needed over that 6 hour stroke, depends on pipe sizes, but wave action is the design criterion. Waves at say 4per minute will generate much more than the tides in that set up! Of course there are calm seas sometimes! Interesting project.
Last edited by profloater; 2015-Dec-10 at 12:43 PM. Reason: Typo

20. As soon as you go deep, as I pointed out a long time ago here, the tidal movement is sideways and not vertical. The pumping scheme is more difficult.

21. Originally Posted by profloater
A small model might work but at full size you have pressure on the outside of the balloon. Roughly one atmosphere of pressure every 10 metres down. If the balloon is close to atmospheric pressure inside, then the skin at the bottom at ten m deep has about a ton per square foot (sorry to mix units) force pushing it upwards. That's the force you want but a flexible balloon will just move upwards locally, distorted. You need a bottom rigid plate or lots of ties to the fabric to hold that force. In other words a special bag, not anything like a hot air balloon. You can get very big fenders as used on ships, they are strong and will work as floats for trials or indeed they may be right for operation. My other point was that the tides are sideways motion so you need to allow for that, or indeed use it to recover a pumping force.
My understanding of these questions is as follows. The air pressure in the balloon will be determined by the water pressure, so for every ten metres depth the air inside will have an extra atmosphere of pressure. All that means is that the pressure inside the balloon will be higher at the bottom than at the top, the same pounds per square inch as the water immediately outside, so 20 psi at 10m. Ropes fixed to the weight go over the balloon so there will be minimal stretching or deformation on a rising tide. Use of horizontal tidal current energy is important, and is already the main driver of tidal electricity systems, so can also be used for pumping. Use of ship fenders could be practical for models. I will be very interested to see if your views on the balloon prove correct. Thanks

22. Order of Kilopi
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Even the snarks here seem impressed
http://wattsupwiththat.com/2015/12/1...nergy-storage/

We have something similar in my state--but using rock
http://www.powersouth.com/mcintosh_p...sed_air_energy

23. Originally Posted by Robert Tulip
My understanding of these questions is as follows. The air pressure in the balloon will be determined by the water pressure, so for every ten metres depth the air inside will have an extra atmosphere of pressure. All that means is that the pressure inside the balloon will be higher at the bottom than at the top, the same pounds per square inch as the water immediately outside, so 20 psi at 10m. Ropes fixed to the weight go over the balloon so there will be minimal stretching or deformation on a rising tide. Use of horizontal tidal current energy is important, and is already the main driver of tidal electricity systems, so can also be used for pumping. Use of ship fenders could be practical for models. I will be very interested to see if your views on the balloon prove correct. Thanks
Oh no, the air is 800 times less dense than the water so its pressure is about constant and if it were to retain its shape would have to equal the deepest water with stress in the fabric at the top, the stress in a lightweight fabric would just burst it. I am afraid the sketch of an air type balloon in water is just not possible.

24. Originally Posted by profloater
Oh no, the air is 800 times less dense than the water so its pressure is about constant and if it were to retain its shape would have to equal the deepest water with stress in the fabric at the top, the stress in a lightweight fabric would just burst it. I am afraid the sketch of an air type balloon in water is just not possible.
Yes, I understand that marine fabrics are much tougher than an air balloon. I was just using that as a shape, not to suggest that an actual air balloon could operate in the sea. A ten metre high balloon in the sea would have air pressure of about 15 pounds per square inch as I understand it, so the fabric at a minimum would need to withstand that, plus all the extra strains of maritime operation, assuming calm water location only, and short life span due to barnacles etc.

Just considering the physical pressure, why would it need stronger fabric than a balloon in the air with internal pressure of 20 psi? At what depth would a rubber balloon rated to 20 psi burst?

25. Originally Posted by Robert Tulip
Yes, I understand that marine fabrics are much tougher than an air balloon. I was just using that as a shape, not to suggest that an actual air balloon could operate in the sea. A ten metre high balloon in the sea would have air pressure of about 15 pounds per square inch as I understand it, so the fabric at a minimum would need to withstand that, plus all the extra strains of maritime operation, assuming calm water location only, and short life span due to barnacles etc.

Just considering the physical pressure, why would it need stronger fabric than a balloon in the air with internal pressure of 20 psi? At what depth would a rubber balloon rated to 20 psi burst?
OK you use tougher fabric and then you pressurise to the deepest water, so at 10m that is about 15 psi or about one ton per square foot. Now the size of your cylinder is limited by the tear strength of that fabric, ie the tensile strength. A cylinder radius r m has an area pi r^2 m^2 so for example 2.5m radius gives a force of about 200 tons on the 25 feet of top ring, 8 tons per foot. No wonder those fender things I mentioned are built like tanks, to coin a phrase.

Lets assume you build or buy some floats like that, they need to be rigid or under tidal lift they will just deform losing some or all of the lift. The 10m depth is significant, although you could revise your floats to be surface floats, like a boat, then inflated floats would work.

26. Originally Posted by profloater
OK you use tougher fabric and then you pressurise to the deepest water, so at 10m that is about 15 psi or about one ton per square foot. Now the size of your cylinder is limited by the tear strength of that fabric, ie the tensile strength. A cylinder radius r m has an area pi r^2 m^2 so for example 2.5m radius gives a force of about 200 tons on the 25 feet of top ring, 8 tons per foot. No wonder those fender things I mentioned are built like tanks, to coin a phrase.

Lets assume you build or buy some floats like that, they need to be rigid or under tidal lift they will just deform losing some or all of the lift. The 10m depth is significant, although you could revise your floats to be surface floats, like a boat, then inflated floats would work.
In boating, a fender is a bumper used to absorb the kinetic energy of a boat or vessel berthing against a jetty, quay wall or other vessel. Fenders are used to prevent damage to boats, vessels and berthing structures.

Those stresses on a cylinder are very different from a round balloon used only for flotation and pumping, protected from shipping and storm impacts.

I don't think a near-spherical balloon would deform as you suggest, when it is tethered by ropes from its apex in a spoke array to a heavy disc weight below it which provides near neutral buoyancy, as shown in the Tidal Pump seminar poster diagram.

As comparators for pressure and stress, a standard car tire has pressure of three atmospheres, equal to depth of twenty metres, while a bicycle tire has pressure equal to about 70 metres deep, as I understand it.

For this ten metre balloon in the sea, the need would be for thicker fabric at the top, rated to 20 psi, such as recycled tires, while the bottom of the balloon could be made of light fabric, and could be open at the base, like a hot air balloon, since the water pressure outside the balloon at the base is equal to the air pressure inside removing any deformation pressure at the base.

27. Originally Posted by Robert Tulip
In boating, a fender is a bumper used to absorb the kinetic energy of a boat or vessel berthing against a jetty, quay wall or other vessel. Fenders are used to prevent damage to boats, vessels and berthing structures.

Those stresses on a cylinder are very different from a round balloon used only for flotation and pumping, protected from shipping and storm impacts.

I don't think a near-spherical balloon would deform as you suggest, when it is tethered by ropes from its apex in a spoke array to a heavy disc weight below it which provides near neutral buoyancy, as shown in the Tidal Pump seminar poster diagram.

As comparators for pressure and stress, a standard car tire has pressure of three atmospheres, equal to depth of twenty metres, while a bicycle tire has pressure equal to about 70 metres deep, as I understand it.

For this ten metre balloon in the sea, the need would be for thicker fabric at the top, rated to 20 psi, such as recycled tires, while the bottom of the balloon could be made of light fabric, and could be open at the base, like a hot air balloon, since the water pressure outside the balloon at the base is equal to the air pressure inside removing any deformation pressure at the base.
You must take account of the scale effects. A pressure in a tyre exerts forces on the fabric. If you increase the scale the forces obviously rise at the same pressure. Well actually it seems that is not obvious but it is the case. Take a spherical balloon example, You can work out the stress at the equator by multiplying the pressure by the area of that circle, and of course that are goes up with R squared. So the fabric burst forces at a given pressure rise with R squared while the length of fabric at that equator rise with R.

In order to remain in shape the internal balloon pressure must be equal to or greater than the deepest water pressure, or being fabric it will deform. That means at the top there is a big internal bursting pressure and skin forces rising with scale. It is not just pressure but the skin stresses that you must consider. If you google images of "big boat fenders" you will see what I suggest. Those things are built for ocean environment and they can be used as floats. They are very tough for all the reasons above.

28. Originally Posted by profloater
You must take account of the scale effects. A pressure in a tyre exerts forces on the fabric. If you increase the scale the forces obviously rise at the same pressure. Well actually it seems that is not obvious but it is the case. Take a spherical balloon example, You can work out the stress at the equator by multiplying the pressure by the area of that circle, and of course that are goes up with R squared. So the fabric burst forces at a given pressure rise with R squared while the length of fabric at that equator rise with R.
I expect you are right here, but my assumption was that the burst force (meaning the strength of fabric required to prevent burst) would go up with the cube of the radius rather than the square, meaning that as the balloon scales up the fabric strength required would be much greater. This could be tested with a five litre party balloon to see at what depth it bursts in a diving pool, and then try the same experiment with different size balloons, such as a sausage balloon, variously inside light and heavy bags.
Originally Posted by profloater
In order to remain in shape the internal balloon pressure must be equal to or greater than the deepest water pressure, or being fabric it will deform. That means at the top there is a big internal bursting pressure and skin forces rising with scale. It is not just pressure but the skin stresses that you must consider. If you google images of "big boat fenders" you will see what I suggest. Those things are built for ocean environment and they can be used as floats. They are very tough for all the reasons above.
But fenders are mainly designed for use at the surface, and to protect against physical impacts. This need here is quite different on both those counts. I am thinking the optimal shape might be a bell jar formed by a hemisphere on top of a cylinder, or maybe an elliptic paraboloid with an open bottom.

29. Originally Posted by Robert Tulip
I expect you are right here, but my assumption was that the burst force (meaning the strength of fabric required to prevent burst) would go up with the cube of the radius rather than the square, meaning that as the balloon scales up the fabric strength required would be much greater. This could be tested with a five litre party balloon to see at what depth it bursts in a diving pool, and then try the same experiment with different size balloons, such as a sausage balloon, variously inside light and heavy bags.But fenders are mainly designed for use at the surface, and to protect against physical impacts. This need here is quite different on both those counts. I am thinking the optimal shape might be a bell jar formed by a hemisphere on top of a cylinder, or maybe an elliptic paraboloid with an open bottom.
The fabric tear force goes up with the radius because the total force rises with the square but the linear length rises with the radius. But that scale effect is important obviously. I only mentioned fenders because they are in production already, and they do float, marine ready etc. It's always more expensive to start from scratch if somebody has productionised a suitable product.

The optimal shape is found from the design of the attachment points, in the case of fenders they also come with attachment points. Otherwise, like submarines you use spheres and cylinders.
I am not saying fenders are the only way to go, but I would look at them first personally, and I have worked in related problems. The other advice I would offer is not to bother with small scale except in the sense of limited pumping, you must try your idea at the depth you want in the tide you want, because otherwise scale effects will lead you astray. In other words, make a pumping bladder and get a float and try it out at depth from a boat. It can be a small set up but go for the right depth so you establish the right way to tether and establish your pumping effectiveness.