Thursday, February 7, 2019

ROV Buoyancy

My first ROV uses expanded polypropylene foam and similar materials to adjust its buoyancy. The desired buoyancy for an ROV is just slightly positive. That way it will return to the surface on its own should there be a power or control failure (designing for a graceful failure). Too much positive buoyancy and the vertical thruster can't make the ROV descend.

Unfortunately, the foams I'm currently using are elastic and therefore compressed by the water pressure acting on them. Water pressure increases by 1 PSI for every 2.31 feet the ROV descends. I found that after descending five feet, the 2 PSI increase in water pressure is enough to make the ROV negatively buoyant. In other words, the foam is compressed to the point where it doesn't displace enough water to make the ROV positively buoyant. As a result, I can't make the ROV drive itself upward (I pull it out by its control cable). The solution is to find a buoyancy material that is hydrostatic, or is not effected by water pressure.

One solution is to placed a sealed PVC pipe on top the ROV. As long as the pipe doesn't leak, it will maintain the same buoyancy because it's volume won't change. Of course if the tube leaks, the ROV will lose buoyancy. The larger the pipe's diameter and longer it's length, the more buoyancy it provides. To help me out, I created the chart below from a  spreadsheet to calculate a sealed PVC pipe's buoyancy.

Another solution is to use a hydrostatic foam designed for ROVs. One such polyurethane foam is subsea foam or R-3300 by General Plastics

The foam us designed to be pressure resistant to a depth of 1,200 feet for their 25 pound per cubic foot (pcf), or R-3325. If the foam is sealed in fiberglass, it can operate to depths of 2,400 feet. Either case is significantly deeper than my little Seaperch will operate. It's even deeper than I hope my next ROV will operate at.

So I have ordered a sampler of seafoam from General Plastics that I plan to use on my Seaperch. The samples are 1/2" thick and 4" by 8" across (16 cubic inches). The foam will be bolted to the top of the ROV so that it's center of buoyancy is above its center of mass. They way the ROV won't flip over. The center of the subsea foam will be adjusted so that the center of buoyancy is directly above the center of mass. That way the ROV remains level. When it's level, the thrusters propel the ROV only vertically or horizontally.

To use subsea foam without wasting it, I created a spreadsheet to calculate the number of cubic inches of R-3300 an ROV will need to become neutrally buoyant. Here's what that spreadsheet looks like.

Just plug in the amount of buoyancyyou need and the spreadsheet calculates the volume of subsea foam you need.

The only thing I need to know is how much buoyancy my ROV has. To do this I weighed the Seaperch in the air and a second time submerged in water.

Dry weight of the Seaperch. This is how much a balloon needs to lift in order to carry the Seaperch.

The wet weight of the Seaperch. Eight ounces is how much the subsea foam needs to lift in order to float the Seaperch.

Based on needing eight ounces of buoyancy, my ROV will use either a 35 cm (14 inch) long 2" diameter PVC pipe or 17.13 cubic inches of R-3300 subsea foam. Since the largest piece of foam has a volume of 16 cubic inches, I will use a full sheet of R-3312 and 1.2 cubic inches of R-3315. Now I just need to know how I'm going to attach the subsea foam to the ROV frame.

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