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Tether buoyancy near ROV

Hello everybody,

I are thinking about mounting small buoyancy foam (from BlueRobotics) on the tether - ROV side, like 50 - 100 cm away from ROV (not sure yet), so that the tether doesn’t get stuck or scratch on concrete weights that have sharp edges or overgrown old ropes during the inspection (up to few KMs long pipelines).

The purpose would be that part of tether near the ROV keeps a bit higher underwater then the ROV, so the operation is safer. The balance, in my opinion should be between the amount of needed buoyancy to keep the tether up (so this has a purpose) whilist it is tense with the surface boat, and so it doesn’t unbalance the ROV to the point where it is eating trough batteries due to poor balancing.

I hope someone thinks about that and we can find some solution, ask questions and give ideas, so maybe we come up with something.

Cheers,
Mateo

Hi @mateov,

This depends primarily on the following factors:

  • tether buoyancy (dependent on tether density, and water density in the operating environment)
  • ROV buoyancy (dependent on ROV mass + external volume, and water density)
  • thruster strength relative to ROV external forces (gravity, buoyancy, drag)

Note that if your tether is already positively buoyant in the water then it will tend to hang up from the ROV, whereas if it’s negatively buoyant it will be more inclined to hang down behind the ROV. Adding pieces of buoyancy foam can help pull it up higher near the ROV, but also adds some additional drag, and something that’s easier for other things to catch on.

If you’re doing inspections up to a few kms long, presumably you’re not using our tether, so do you have some idea of the buoyancy of the tether you’re using in the water you’re operating in? :slight_smile:

Hi @EliotBR, first of to say we do use BlueRobotics tether (old 300m PE and slim 100m PU). So we do have all info available :smiley:

ROV is fairly well balanced, without this tether modification, and I’m having hard time figuring out (even harder to calculate it) what impact would it have on the ROV and would it have an impact on tether.

We commonly work from surface boat, that is how we manage to get long inspections, and the forces on the tether really vary in those conditions (we sometimes pull relatively hard on the tether, sometimes it is free in the water - and everything in between). That is the reason I’m trying to wrap my head around this.

Just to show an example of what can be the problem during inspection:


Footage from BR camera, you see how there is marine growth going 3-4 times above the pipeline. And tether can really get you into problems around stuff like this…

Also, we operate more in saltwater, this is footage from saltwater, so tether here is positive (does go up) but in this inspection where we go a lot up and down (to manouver around objects that are in the way and still see the object of interest) the tether is all over the place - we dragged it on these overgrown parts so we were a lot slower…

Glad to start a discussion :slight_smile:

Makes sense, not sure why I assumed the ROV had to make the full trip alone - I’ve probably been reading too much about fibre optic tethers and the like lately…

It’s a bit hard to see that from the image, but I understand your point.

As a side note, I’d recommend moving that large black component out of the view of the camera if possible, because it’s likely making it significantly harder for the camera to adjust its exposure correctly. If that’s not possible then I’d suggest you try adjusting the Camera Settings through the web interface, and set up a profile that works well at your operating depths :slight_smile:

For now, I cropped out the black part so I could adjust the photo to see it more clearly. We’re considering adding some similar processing to the QGC live display to improve visibility, but if there’s a constant large black portion of the image that kind of processing likely wouldn’t help very much.

This sounds like it’s perhaps mostly a tether management issue. Ideally the tether shouldn’t get close enough to the marine growth to get caught on it, so your ROV should either stay above the level of the marine growth, stay off to the side, or the tether should be pulled more taught when passing over marine growth so that it can’t droop back down afterwards and catch on it. That’s understandably easier to monitor if you have an additional camera pointing back towards your tether, but it’s perhaps also possible to turn around and check the tether is free when you pass over something spiky.

If you add a buoyancy block to the tether that could lift it higher behind the ROV, but would also provide another component of the tether for things to get stuck on. My intuition is that if you add enough foam to make a significant difference to the tether orientation while the ROV is in motion then that will also have a non-negligible effect on the ROV balancing, which you may need to compensate for with weight distributed towards the back. I imagine the slim tether would have less of that because it wouldn’t require as much foam to lift a bit, but you can always do some tests with your two different tether types, and different foam amounts and distances from the ROV, and see how much of an effect each option has :slight_smile:

Any news on BlueRobotics implementation of fiber tether? :grin: hehe

That side note and camera settings look amazing! Thank you very much for that input, the adjusted photo looks really good. The “black thing” is actually our middle camera, and since it is rated for 150m it is quite bulky housing. We will try to move it out of the way for sure and check your suggestion more in-depth!

We try to get the footage of inspection elements as good as possible, that is why we often need to go closer, and that is why we are trying to develop some kind of solution that would keep the inspection at same level whilst keeping us as safe as possible from tether getting stuck.

Completely agree on that one, maybe the effect would be okay but battery life will drop so in the end it is the worse combination, due to more power consumption in stabilize/depth hold.

Also agree on that one, do you have any info if someone has done tests like that?
Also to mention we like to work on our small boat, so TMS for the moment is out of the question, but feel free to link something if you think it would be useful.

Thank you Eliot!

We’re working on several product developments at the moment, but as far as I’m aware that’s not one of them. From what I understand that’s only required by quite specialised use-cases, so it’s more likely in the realm of our distributors :slight_smile:

Glad I suggested it then. Fair enough, hope it helps!

Fair enough - it can be quite a difficult challenge to manage.

Unfortunately it’s not something I’ve come across being done. It could possibly be simulated, but that’s unfortunately not super easy to set up - I imagine at this stage it’s likely easier to test experimentally, unless you already have a precise model of your ROV’s mass+buoyancy distribution in CAD.

“Tether management” can be done by hand, it’s just not as easy as an automated option could be. I haven’t worked with an automated TMS before or looked much into it, so unfortunately don’t have any easy resource links.

No worries - glad to be helpful, and to spur some interesting conversation :slight_smile: