Recovering research equipment after 6months with BlueROV2

Hello,
I am a dyslexic marine ecologist and newbie to the ROV/tech world, and this is my first post here so I apologize if I am not using the right vocab and spelling to give context for my question/problem.
Does anyone here have experience or tips using the BluerRov2(BR2) to find and recover gear that they have put out and left in the sea for a while?
I will be studying deep-water kelp habitats in the Galapagos (50-70m deep) for my PhD thesis, and am be using the BR2 (BlueRov2 +heavy configuration + griper) to do video surveys. To assess long-term temperature fluctuation in this ecosystem I also am hoping to leave ~4 hobo temperature loggers on the seafloor (~48-52m deep), attached to cement mourning blocks, and I recover them again ~7 months later. Deploying these seems relatively straightforward: my sites are on a knoll 40km offshore so from a boat I plan to lower the 2 moorings, each with the temp loggers and a bright marker buoys and take the GPS position of where they were lowered. We cannot leave buoys floating in the first 6m of the water column in the Galapagos due to marine traffic (and its a local rule), so my plan is to leave the buoys floating ~10-14m above the seafloor (~40-35m deep) so that will be visible to ROV and accessible to Scuba divers that are limited to a max depth of 40m.

After ~7 months we can go back to the GPS points where they were the moorings were lowered, and then use the BR2 to find them - hoping that the buoys will be conspicuous enough to see them (visibility here varies between 5-15m). Once located we will use the gripper to hold on to the buoy rope and hold the position. A team of divers will descend following the ROV tether line down to the buoy and tie a rope to it, and then ascend which to bring it to the surface (doing appropriate safety stops and all the precautions given ROV in water). BR2 will let go of the buoy and retreat as soon as divers are close. From the boat with rope in hand will use a winch to bring up the moorings with the temperature loggers+ buoys (with my precious data!)

My main questions to you (community members and experts) are:
A) how likely/challenging will it be to find these loggers again using a BR2 (considering I have no USBL or Sonar)?
B) Do any of you have done anything like this at these depths?
C) Is Anyone doing anything like this but with help of better tech: e.g. some kind of acoustic pinger that can be attached to the mooring with loggers and the ROV has some kind of receptor or sonar that can detect it?

I welcome suggestions, feedback and questions you may have for this (crazy?!) plan I propose here, which I hope will enable me to collect temperature data that will be super interesting to understand if and how climate change-driven marine heatwaves affect these mesophotic habitats and kelp communities

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Hi @Salome, welcome to the forum! :slight_smile:

Sounds like an interesting (and hopefully fun) project!

I’d say that depends on how accurate the initial position record is, and the visibility and current strength at the time of recovery combined with the skilfulness of the operator(s).

It’s not something I’ve personally done at those depths.

This retrieval example from @btrue may be worth a look, although your use-case is quite a bit deeper so will need lighting from the ROV to see what you’re looking for.

That may be possible, although depending on the constraints of the use-case it could be quite challenging to achieve. Hardware wise it would require at minimum a transmitter at the bottom (with the loggers) and a receiver on the ROV or the boat, but the most efficient and practical retrieval approaches would likely make use of both transmitting and receiving on each end.

Behaviour wise the main approaches I can think of are:

  1. bottom transmitter transmits at regular intervals, and is searched for and detected by the moving receiver
    • too frequently could be wasteful of power, and may cause confusion for other vessels that come by if they have a sonar sensor with a similar frequency, and could potentially be disruptive to nearby marine life
    • too infrequently could make searching and finding slow and challenging
  2. bottom receiver waits for a transmission, and when it detects one it transmits back so it can be detected (resulting in faster ping-ponging as the sensors get closer)

Beyond that, if you know you’ll only be searching after a set amount of time, or within a particular time period during each day, then that could be used to help save power by turning off the bottom sonar while it’s not needed (albeit potentially with some extra risk, because if your control computer somehow loses track of time it may change to a range that’s misaligned with the search time).

Technologically speaking that kind of use-case could be achieved with a couple of Ping Sonars being controlled by some Arduinos or Raspberry Pi boards or similar, although it would require custom firmware on the sonars to support the non-standard usage. At the moment we don’t provide firmware source code (i.e. that would allow adding custom messages and/or logic), so unless that changes custom firmware would need to come from us.

The ~30 degree beamwidth on the Ping Sonar means detection would be possible within a radius of about 10m for an ROV 35-40m above the bottom transmitter :slight_smile:

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Why not use a topside(boat) depth sounder to locate the mooring buoys? Design the mooring buoys to give a strong return when pinged from topside.

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Thank you @EliotBR for all the info and feedback. @model14 appreciate your suggestion, which sounds a bit easier as the speed boat we will be on does have depth sounds. I will look into a what material/size of mooring buoys are needed that can give a strong return, please if you have any ideas or suggestions do share :slight_smile:
Cheers,

@Salome Just to add a little bit since Eliot mentioned my video
In my limited experience, so far the ROV saying of “depth is just a number as long as you stay above the implosion depth” is accurate. The only difference between shallow dives and deep dives is the tether drag and less ambient light. The Blue Robotics tether is positively buoyant in freshwater until it gets exposed to water for a few times within short period (before it completely dries).

Here’s an example at 250’ when the tether ‘aggressively’ pulling my ROV backwards at 17:40:

I would need to give the ROV constant thruster input just to hold position.

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@Salome The best advice I can give you is to locate the buoys over a seabed that you can distinguish with the depth sounder/fathometer. Air, as in a fish bladder, gives a good return, as does hard material that reflects the sound. Styrofoam, as you may be using for the buoy absorbs the sounder ping and gives a poor return. When you have your instruments in place try and chart the surrounding bottom and get some pictures from the sounder/fathometer screen. Don’t be mistaken, what you are trying to accomplish will be difficult! I have a lot of fishing experience trying to return to the same location using GPS at 100-150 feet depth, and it can be quite difficult. Before your drop those instruments give it a lot of thought. Give us your ideas and we can help you ferret the best ones out.

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Hey @Salome, I’ve done quite a bit of this sort of thing and am reasonably experienced with BlueROV. While I love the latter (makes fish ecology surveys at depth a real possibility for us), I wouldn’t use it for your purposes. A few people have suggested using the boat’s depth sounder to look for the buoy which can work. In the depths you are considering I would use the sounder in conjunction with a very good GPS mark/waypoint (worth investing in a decent unit). Once you’ve turned up on site and located the mark, I would drop a shot line to the bottom so that you a) have a surface visual reference of where the position is and b) have a line that you can descend on. Is there any reason why you want your buoy so deep? If the depth limit for legal/marine traffic purposes is >6m, then personally I would have your float system as shallow as possible, at say 8-10m. You can then descend the shot line to ~12-15m and conduct a search pattern (circular, attached to shot line) looking for the bouy and or the line to the logger on the bottom. You could use the BlueROV for this part of the job but I would still have that float as shallow as possible, conditions dependent. And of course, all of this is dependent on conditions: if you have strong currents at your surface site it’s going to be very difficult to make any of the suggestions here work! In regard to your Question C about alternative tech - if you have the funds you could look at acoustic release options but they do tend to be quite pricey. All in all, I think it’s very doable and I’m interested to see the results of your research! Good luck!!

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I was looking through my old phone pictures and I found some pictures from that dive and it reminded me of this post. I know this isn’t very useful to most people, but I think it help explain the large tether tension other than the usual dragging through the water. It will bump this topic on the recent posts if nothing else. The positive buoyancy of the tether creates a lot of tension that increases with ROV depth. I don’t think that excess floating tether has a real effect on ROV handling.

All 150m (492’) deployed to get me to 250’. I think the reason why I stopped at 17:40 in that video was because I took this picture.

It still floats even after being exposed to a “high” water pressure of 250’.

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Thanks, @bjcresswell , your suggestion of using shot line for a surface visual reference and descend line is such a helpful tip that did not occur to me. Do you have any resources or suggestions for the best setup (type of anchor weights, rope, and float you use) for a place where surface currents are between 1-3 knots down to 10 m depth, to won’t get dragged away (or at least hold position for the first 30 minutes!)

No worries @Salome! I’m not sure I know of any reference manuals that go into the details of site setup and relocation, but I work and have worked with a number of organisations (Australia) and the collective approach is pretty similar across them all. With 1-3knots current you should be ok with the shot line approach, although 50m depth is probably the deepest you’d want to go (imagine how much the shot weight can drift off the mark as it descends through the water column). Specifically:

Shot/weight: Because of the depth I’d use as heavy a weight as possible. You can use an anchor as your shot if you’re worried about dragging, however anchors tend to drift more on the way down so I’d use something very heavy and compact (for deeper stuff I’ve used solid steel bars ~10kg, very unlikely to drag if you’ve just got a lightweight float on the surface and go down quickly through the water column. If you’re short on options then a bunch of diveweights cinched and locked in on a dive belt would work alright.

Line: Any marine rope would be fine but don’t go too thick (drag). You’ll want to use a line that is probably ~50% longer than the max depth of your site location, so at 50m you’d need 75m of line, but as you get more experience with your study site you’d be able to fine tune that (you actually want as short a line as possible that isn’t going to drag the surface float underwater).

Float: Literally anything would be fine for your purposes I think. If you were going to experience strong currents then I would say to use a higher buoyancy float to prevent getting dragged under but I imagine you wouldn’t even be attempting this in high currents anyway (and as an extra point I would advise planning your fieldwork around slack water (high/low) or any other known periodicity of currents, where possible).

Final observations: all the above is just to get you accurately onto your site(s). It won’t work without a very good GPS mark (you’ll just be dropping your shot into some random spot and setting yourself up for failure!) and as other people have noted you can combine with the depth sounder to help orientate yourself to the benthos. Once you’re on your mark, I would then motor up current/wind and drop anchor so that your boat “hangs” back near the surface buoy and from there you can roll with the rest of your site location activity, either ROV or diver. As others have observed it’s going to be challenging and might be worth practicing in some shallow/benign environment before you go live with the real thing.

Hope that helps and good luck.

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Thanks for sharing your thoughts with me Ben, all of this is really useful and will help my planning. Cheers

You should look a bit more into acoustic release system. So call pop-up boys. With these system you past near your mooring with a boat and send specific ping in the water. The mooring will release a boys that will move up to the surface. Then you have to find it and pull on the rope to retrieve your equipment. These device are made to last and operate on internal batteries. Not fun like driving a BR but straigth forward en efficient. You just need to make sure the pinger can listen at the desired depth.