"The osmotic pressure Ps is given by van't Hoff equation:
Ps = c∙R∙T (1)
Where c is the ionic molar concentration, R = 0.082 (liter bar / degree mole) is the gas constant, and T is the absolute temperature in Kelvin units. T is equal to the Celsius temperature + 273.17. Thus, T = 300 K for 27o C. Typical ionic salt concentration of seawater is: c = 1.1 mole / liter, and the corresponding osmotic pressure is:
Psea = 1.1 x 0.082 x 300 = 27 bar."
http://urila.tripod.com/Seawater.htm
So 1000 or more feet under the water should be fairly cold, so we pick 275 K:
Psea = 1.1 x 0.082 x 275 = 25 bar.
A bar is equal to 33', so minimal depth would be 825'.
Next the formula deals with flow:
"The flow rate of water through the membrane Frate is given by:
Frate = Kf∙(Ppump - Ps) (2)
The membrane properties and its area determine the flow rate factor Kf. Ppump is the pressure generated by the pump and controlled by the pressure control valve. Ps is the osmotic pressure of the concentrated salt water in the module."
So, as I understand it a pipe somewhere around 850' deep would have enough pressure but as pipe filled up with fresh water this back pressure would eventually cause the flow to cease. The longer the pipe and thereby greater difference in pressure would push the fresh water up the pipe until at some point it would reach above the surface of the ocean- thereby not requiring any energy to pump up to the surface elevation. And making an even longer pipe, would increasing amount pressure at sea level.
So, in order words if you have a pipe to depth of 2013' [61atm- 896.7 psi], you have 896.7 psi on the ocean side of membrane and with 11.8 psi difference per 1000' have about 873 psi on the other side of membrane inside of pipe. And the amount pressure and flow at the top of pipe at the surface would depend upon the membrane properties and surface area of membrane.
I don't see any particular technical restraint on the size of the membrane [and by having it bigger, one increasing it's efficiency, and as it approached 100% in this regards you would get closer to the 11.8 psi per 1000' at the surface level of the ocean. One should also have the membrance facing horizontal, so that on the ocean side as there is increasing the salinity, the increasing water density causes the saltier water to fall towards bottom of the ocean.
And at 2000 to 3000' depth, I see little restraint on the diameter of the pipe, so with larger pipe one have more volume. For instance with pipe reaching depth of around 2000', the pipe could be as large 100' in diameter and be structural strong enough to withstand around 25 psi.
So, this a passive system, with no moving parts and no energy to make fresh water.