Off-Topic I know, but Pneumatic Breakwaters?

[percyff]

I have heard of Pneumatic Breakwaters some time ago, and I have found (via Google) some references always to definitions or in encyclopedias.

It consists of a leaky pipe laid on the seafloor outside a harbour say, and then when needed compressed air is introduced and a curtain of bubbles rises and disrupts the cyclic motion of the waves. There is passage for large ships, although smaller may sink!

It seems to me an easy method of protection for a harbour or other structure, while having minimum cost to install and run.

I cannot find any reference to any being used or the results of any tests.

Anyone help?

[D'ARTOIS]

It fits some explanation why all in a sudden ships have disappeared in the past. Certain gas-inclusions aggregated by submarine volcanic activity,may, or have caused enormous foamlayers in some parts of the ocean, and ships trapped by suc a layer have sunk due to this natural occurrence.
Air/foam does not support any substance so the actual result is that a vessel drops to the bottom and is swallowed by the elements when the activity ends.

Certain areas in the north atlantic seems to have this kind of activity and I have heard that also around the northern Scottish isles is also a kind of maelstrom that is extremely dangerous for small ships.

[jehardiman]

Originally Posted by percyff

It seems to me an easy method of protection for a harbour or other structure, while having minimum cost to install and run.

Air curtian breakwaters are very expensive and impractical in the sizes needed to be effective. They have a huge energy requirement to produce the high volume high pressure air needed for any sort of useable harbor size. Nice idea for a swimming pool, but too expensive in real applications.

[percyff]

Can you advise of any studies or tests that show this?

The web seems devoid of hard data.

[jehardiman]

It simple engineering. Lets say I have a harbor I need to protect wath a breakwater 10 m deep and 100 m long. In order to reduce the wave height by 2 I need to reduce the wave energy by 4, This implies that I need to reduce the mass of the water 4. Also, in order for the mass spring to work, the width of the lower mass volume needs to greater than 1/4 the wave length (check the laterial velocity curve of a gravity wave) so for a 5 second wave, this means the bubble screen needs to be 9.7 m wide. This gives a needed volume of air as 100*9.7*.75 = 727.5 m^3 per second at 2 atm (note that bubbles rise at ~1m/sec). This equates to 4,442 MMcfd (million cubic ft day) with a power requirement of 42 hp per MMcfd for a total power rquirement of 186,564 hp per day to keep it going. This is the same as a power requirement of 10,364 kW/hr. Allowing $0.09/kW/hr gives a day cost of $22,387, or a yearly cost of $8.2 million dollars. And we haven't even considered the sunk costs of the piping, compressors, or motors and maintainence. I can dump a lot of rock for 8 million dollars!

[cyclops]

I like rocks more than a unlimited supply of wind from the Army Engineers.

[percyff]

Thanks Jehardiman.

I had a bit more luck using "air curtain" breakwaters as a search string.

I live near Cowes on the Isle of Wight, and Cowes is vulnerable to a northerly gale blowing for a few days. The likelyhood of this condition is low. The prevailing wind is south-westerly. The harbour mouth faces north and is approximately 300 m from the end of the existing 1936 breakwater to the logical position on the western Cowes side. Water depth varies from 1m to about 4m at low water and 5m to 8m at high water. As a temporary breakwater that never impedes water traffic I would guess that it would be needed for about 3-5 days a year. The effectiveness during that time would save moving many craft upriver and damage to pontoons and erosion.

While I am studying your calculations [jehardiman] I am sure you have erred on the high side. Theere would never be a point in using this in place of a rock breakwater. The purpose surely is to place a temporary breakwater in a shipping channel. The breakwater can be switched on and off quickly, and can have a gap at the centre for small craft. Reducing the wave height to a half seems a reasonable thing to do.

From a discussion on WWII - http://www.ibiblio.org/hyperwar/USA/...s1-7.html#cn10

"One of the more novel solutions suggested was the creation of an "air breakwater." By the use of pipes on the ocean floor this scheme proposed to maintain a curtain of air bubbles which theoretically would interrupt the wave action and thus provide smooth waters inshore of the pipe. This idea was actually not new either. Studies along this line had been carried out in the United States forty years before, and both Russian and U.S. engineers had conducted model experiments since 1933, although without conclusive results. The bubble breakwater would have required such large power and compressor installations that it was impractical for breakwaters on the scale envisaged, and the idea was discarded as infeasible early in September 1943.

To use instead of the Mulberry Harbours to protect the north French coast during and after D-Day.

Any references of trials or tests would still be appreciated.

[safewalrus]

Still prefer rocks, they can't breakdown! But for a 'door'? now there's a thought!

[marshmat]

The island ferry near my house has a bubbler track to keep its winter run clear. Different purpose, but the same concept. There is a visible reduction in waves where the bubbler runs, when it's on, but it's not very significant. I do think Jehardiman's numbers are slightly on the high side, but he does get the point across that this is a very energy-intensive way of reducing waves. As a gate, perhaps; but for temporary structures I think a floating steel breakwater is probably much cheaper and more effective. For permanent, well, rock of course!