I am looking to find a positively buoyant umbilical cable that would be compatible with our BlueROV2’s. Is this something that even exists?
Currently we use the Blue Robotics-produced Fathom tether, which is neutrally buoyant, however as I believe it is intended for salt water use, we find it sinks in potable water, which is where we do the majority of our work.
We have looked in the past at adding foams and floats to our umbilicals, and whilst this does keep the cables buoyant, it also causes other headaches in the forms of extra drag and things getting snagged.
Note - this problem mainly concerns our topside powered BlueROV2, our battery powered version doesn’t have this problem due to the nature of the umbilicals.
As a semantic point, I assume you’re actually referring to the High-Power Tether, used with the OTPS? Our Fathom Tether isn’t rated for high power throughput.
In our technical details the High-Power Tether is apparently slightly positively buoyant in salt water, and neutrally buoyant in fresh water, which is also what’s stated for our Fathom Tether options. If one floats and the other doesn’t in the same water, then perhaps the density ratio is slightly different, although they should be at least reasonably close.
Out of interest, is your water particularly warm or cold? The density of water depends on both what’s in it as well as the temperature. I’m also curious if there could be treatment chemicals in the water that make it less dense, but I’m not sure what that could be (I’m assuming it’s not full of alcohol…).
To your actual question, you may be able to get something more buoyant directly from a cable manufacturer, although given you’re putting high voltages through it with the topside supply I’d recommend being extra careful, given the potential risks if you get something poorly manufactured/isolated.
Perhaps others in the community have relevant experience/advice - I’m aware that @hscadden is experienced with high power subsea cables, and @Outland may have some ideas on higher buoyancy tether options for their system
I would have to say Google is your best friend on this one. A vast majority of the cables I deal with are in static installations or at worst cause in dynamic cable pathways but none of them are “tethers” … tow cables, that is a different story.
I understand your issue with attaching floats and the drag it can create … unless you have deep pockets and want to pay someone to make a custom cable for you, again Google. I do have some ideas on making what you are hunting for but the bundling houses I work with would require a minimum buy.
If you were interested in a solution and frankly had a market to get rid of what you didn’t need etc. I could assist in getting a custom cable made.
My mistake - you’re correct, I meant the High Power Tether.
The water we are operating in is usually between 10 and 15 degrees celcius (50-60F). The water has gone through an intense treatment process to eliminate foreign particles to get it ready to be sent to peoples’ homes. There is usually an above average level of chlorine in the water, so maybe that could be the issue?
I have emailed a couple of third party cable companies and will see what they say. I am a bit wary though due to the complex nature of whats inside the umbilical, and as you say, the potential risks that may occur with a poorly manufactured length.
@Slee We use a “neutrally buoyant” tether for our ROV’s. They are typically designed for use in seawater, so they will be very slightly negative in freshwater. However, it has been my experience, over the years, that manufacturing tolerances of the cables makes it very hard to hit the “neutrally buoyant” mark 100% of the time (many variables for tolerance stackups: wire diameter fluctuations, wire jacket, kevlar strength members, filler materials, outer jacket foam diameter, foam density, etc…then water density fluctuations affect the “neutral” as well). Our C-3400 cable is what we use on our ROV’s that draw a ton more power than the OTPS kit. The downside is that it is much more expensive than the BR tethers due to the added kevlar and copper. @buccaneer is in the UK and has both types, they may be able to show you some if you are nearby.
Thanks Harold, I agree - Google may be the answer, however we are not seeing much luck and our pockets aren’t ridiculously deep. I think adding floats will have to suffice. We have been doing that for a couple years now and it does work, I was just trying to see if there was a better alternative that wouldn’t break the bank.
