Using an Airlift for Wreck Excavation at 60m (196ft)

[divingdave]

I have searched high low for information on using airlifts at 60 msw but have not found anything. Anybody here have experience using an Airlift at this depth. I need to find out what size compressor I would need for this depth? I have a fairly large diving compressor onboard and wonder if this would cut it. The wooden wreck and cargo is buried under sand and silt. Diving on CCR so can do 30-60 minute bottom times at 60m and wondering if this would be an effective method for removing sediment.


I can put the boat on a four point anchor and hold over the site and have bought a couple of hundred metres of LP hosing to supply the Airlift from the compressor but not tried it at this depth. Can anyone point me in the direction of airlift tables, comparing depth, supply gas pressures and hose diameters?

Any help would be greatly appreciated.

 

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[SADS 669]

Best of luck with the lift at that depth, you might be better blowing than sucking, perhaps have a secondary blow pipe to drive the overburden away....... As long as you aren't jetting you shouldn't loose any artifacts downstream. Very slow but it's the way I would do it.

 

[pvet7521]

Better off with a water dredge , air compresses water does not you would need a commercial compressor mounted on a barge to get sufficient air down to that depth.with water You can either jet or dredge .

 

[divingdave]

I also have airbanks but cannot find tables of what flow of air, pressure and volume would be required. Anyone know where I can find this?

It would be anchored to sand screws, I thought the the expansion of the gas bubbles from that depth to the surface would provide massive suction.

 

[SADS 669]

The only two things I can suggest Dave is to contact either the navy or commercial diver schools . If they can't help, it's unlikely anyone can..... Best of luck. If you are in UK let me know I can help with a contact there......

 

[ARC]

Years ago I built / invented a dual tank trickle system with clear acylic 2" tubes and at 90 feet worked for awile. but at that depth your time would be limited at best.
I would suggest using torpedo sled (DPV) to blow sand into current stream.

 

[Sommy]

At your proposed depth you will need a compressor capable of maintaining 125psi (100psi for the depth and a few more to get the air out of the tube). Your flow rate will need to be large(55ft^3 per minute @ 100psi if using a 20' tube and 8" diameter). This is calculated by taking 75% of the tube volume (an educated guess) and assuming your air will rise at 60 ft per minute, so you need to refill the tube three times a minute, while maintaining the pressure. Yes the numbers are simplified, but close enough for you to get an idea of what you will be needing. Yes you will see about a 13-19% expansion in the air over those 20ft(100psi down to 90psi, depending on where your air enters your tube), I figured the volume of the air at the top of the tube. If you want to have a large fudge factor, use 100% of the tube volume (28ft^3 * 3 refills per minute = 84 ft^3 per minute minus the gas expansion = 73ft^3 per minute)...I am assuming I missed something (like how fast air bubbles rise in water)...but it starts you in the right direction.
The airlift would look like a mini-volcano on the surface. I would recommend a dredge, it would be much easier to maneuver at that depth and your outlet for debris can be behind you or as long as you can make it.

 

[Jason in Enid]

The suction power you get from an air-lift depends on the length of the suction hose. You could play with different diameters and lengths of hoses with the best compressor you can get, but like was said above, I don't think this is the route you want to go.

Suctions dredges are something I know very well and would recommend here. You don't need a giant motor or pump, but you do need something designed for gold dredging because they use a pump with both high pressure and volume. You could buy some fire dept hose to be your pressure hose from the boat to your underwater site. This pressure hose then connects to a venturi jet tube (again, source this from gold dredging supply). then you need a exhaust hose to direct the debris away, and a suction hose with nozzle to do the work.

 

[Whitt]

http://www.techdivetools.com/dig/dredges.html

 

[Jason in Enid]

Diving Equipment Specialties

That link does show the type of items needed, but certainly not all of that, and the price is absolutely rediculous.

Honda GX200 & HP400 Pump Combo($1195)
4" Power Jet ($266)
3" FV ($115)
4" Swivel Tip ($195)
195 Series Suction Hose 4" ($11.55/ft.)
195 Series Suction Hose 3" ($7.35/ft.)

2-INCH-X-100-FEET-PIN-LUG

DISCHARGE HOSE WITH PIN LUG

$56.76

So there is everything you need, (most of it the exact same item sold in the link) for just over $2000. I made the assumption of 40 feet of 4 inch suction and discharge hose, 20 for the suction, 20 on the discharge. Also, you don't need that monstrosity of the stainless steel arm-mount-suction-thingy. You will have to make your own mounting frame for the pump, but that is a few bucks in tube steel and a kid with a stick welder.

Grey items from proline, discharge hose from universal hose and fitting.

 

[Whitt]

For what it's worth, I'm not trying to sell anything here (they aren't my dredges), just trying to post useful information and contribute to the thread

 

[Jason in Enid]

For what it's worth, I'm not trying to sell anything here (they aren't my dredges), just trying to post useful information and contribute to the thread

I didn't think you trying to sell them, I was just trying to let the OP know what the costs should be.

 

[divingdave]

Thank you gentlemen for your replies.

I have looked at dredges and believe that they have there own problems at that depth with friction due the length of the hose. Plus the water pressure at that depth is 7 bar (100psi), so although the pumps are high volume they struggle with the water pressure at these depths. The ones posted above are only 80psi and one is just rated to 40 psi, so I don't think they would work at that depth, correct me if I am wrong? I would also have to anchor directly over the site and we have a spell of rough weather.

Blowers would reduce visibility to zero as the seabed is a combination of silt and sand, so I like that the airlift helps improve visibility.

I have built an airlift like the one below, but only tested it at 30 m (99ft) so far



With rough weather I lowered airbanks with 30,000 litres (1000 cft) to the seafloor so we didn't have to moor directly over the site and seemed to be very effective. With the same design running up from 60 m (200ft) will it require less air due to the air expanding 7 times the volume on its way to the surface? I am guessing that it will require less air as the length of the hose is now double? And I imagine the suction will be quite intense with a pipe this long :-)

The bottom of the lift is anchored to the seabed with sand screws and then 60 m of pipping is held with lift bags near the surface, the working end is on a flexible pipe so can be easily maneuvered around. I hoping that by increasing the length of the pipe it will require less gas to be pumped and more suction. One way is just to go and do it and learn by your own mistakes but any input on this would be very helpful.

Any suggestions or comments would be greatly appreciated.

 

[SADS 669]

I hope you are successful, best of luck, sounds like you will be spending a lot of time at that depth, ( teaching a grandmother to suck eggs no doubt but take care with Deco etc)

 

[Jason in Enid]

Thank you gentlemen for your replies.

I have looked at dredges and believe that they have there own problems at that depth with friction due the length of the hose. Plus the water pressure at that depth is 7 bar (100psi), so although the pumps are high volume they struggle with the water pressure at these depths. The ones posted above are only 80psi and one is just rated to 40 psi, so I don't think they would work at that depth, correct me if I am wrong? I would also have to anchor directly over the site and we have a spell of rough weather.

You are thinking about pressure backwards. If you were trying to use it at a height ABOVE the pump, then you would lose pressure and working ability. The fact that this is below water, you are NOT LOSING PRESSURE. There is not a sufficient amount of friction loss in 250 feet of 2 inch hose to worry about. You would not have to be anchored any closer than using an airline for a lift. You can add as much pressure hose as you need. The biggest problem with gold dredging is that you have to lift all that material back up to the surface, including cobbles up to about 90% the diameter of your suction hose. Depth has an effect there. Just clearing overburden at an underwater site, you aren't lifting, you are just moving it laterally. This is why some miners use submersible dredges, you can use a far smaller pump if you are keeping everything underwater.