'Clump weight' system to reduce tether drag

#1

Hello everyone,

I’m new to this community so I thought i’d first introduce my self. My name is Zac and I have recently started a PhD project which will be involving the OpenROV.

While looking at some literature I came across this simple idea, not sure if the idea has already been mentioned but thought it would be worth sharing. It basically involves using a buoy, weight and a rope to secure the tether at a certain depth to reduce the tether drag and improve maneuverability at depth.

Has anyone tried this? Or do you think it would be useful?

Unfortunately as i’m a new user I cant post images, but find the image in the following link:
http://postimg.org/image/ly6yw3wv3/

#2

Hey Zac,

Welcome to the community. First off you can attach an image with the upload button found in the window that shows up when you write a forum post. It looks like a flat bar with a vertical arrow above it.

Second, I know at least at OpenROV HQ we have tested using a clump weight a couple times. However, the system that we have used was been different than the one that you posted. Our tether was actually broken up into two sections, the “free tether” was the normal ROV tether, while the “ROV tether” was replaced with speaker wire with a clump weight attached between the “free tether” and the “ROV tether”. This was done because we found that with a clump weight attached to the normal ROV tether it had a tenancy to try to untwist itself.

Personally, I’m not sure if you really need a separate clump weight rope. You should be able to mount the clump weight on the same tether as the ROV if you do it correctly. But, give it a shot, this should all be relatively easy to setup and test. Also don’t forget to post your results and the forums or put your expedition on OpenExplorer.com, that’s what this community is really all about.

@Walt_Holm could provide more details on this subject, since he actually designed the clump weight system that we used on the ROV. I’m just paraphrasing what I know about this system, secondhand.

Drop/Clump Weight
#3

Hi Zac:

I use a clump weight on the tether any time I’m operating deeper than 50m or so. We’ve taken experimental versions of OpenROV down to 280m in Lake Tahoe, and it wouldn’t work at all without some form of clump weight.

As Patrick said, you can basically skip the “clump weight rope” that is shown on your diagram, and attach the weight (~1-2 pounds) directly on the tether. I’ve been evolving the design over time, and the last iteration of the design seemed to work pretty well. The downline was made of speaker wire, in this case 24AWG rather than the 26AWG tether provided by the kit. The heavier wire is stronger, and can better carry the load of the clump weight. Speaker wire won’t try to untwist when you put the load of the clump weight on it.

From the clump weight, run 15-30m of neutrally-buoyant tether (you can find it in our store) to the ROV. Where the clump weight mounts, attach a small strobe light to aid in orientation at depth.

At some point I’ll post something more complete, with pictures, but this should give you an idea.

-W

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Manufacturing a shell for deep dives at 1000m+
#4

Interesting, thanks for your responses. I think the diagram I posted was meant for heavier, deeper setups where the tether is not appropriate to hold tension, it definitely makes sense to do it the way you guys have suggested, it’s much simpler. 280m? wow that’s very deep. Did you document that expedition anywhere?

#5

Hi all !!
Opening again this post because trying to drive and dive the trident about 80 MTS deep water in ocean is difficult, very difficult ,almost for me…
Has anybody more information about a clump with the rov?
How it works? Has the rov enought power for push it or needs to be small one?
Will test it…but before if is possible would like your opinion!

Thanks🤗

#6

Hi,

What they are talking about here is a well known problem in the subsea construction world. That is why heavy duty work ROV’s use TMS (tether management system). This a basically a bell with a tether winch on it. This allows the rov to dock to the TMS subsea and the havier re-inforced umbilical brings the ROV topside.
The problem with drag is due to the weight of the ROV is to low. The drag on the tether is due to the area of the tether is to large relative to the wheigt pulling down and the current. So either have a larger weight down or reduce the area of the tether.

I would try to do it with a weight that has ha hole trough it (maybe a shackle)witha rope holding it. The rope should be about as long as the depth you want to operate on. The I would run the tether through the weight and you will be free to maneuver the ROV with the shackle effectively being your TMS. The rope would be your “umbilical”.

PS
Make sure to pull the umbilical at the same pace as the tether to avoid exceeding the weight limitations of the tether. If you only pull the tether the weight of the shackle/clump
Weight will be added to the the load placed on the tether.

You can also calculate drag pretty easily. See link below:
http://www.rov.org/rov_design_drag.cfm

Stian
Subsea Engineer

#7

Interesting. How far from the ROV would you attach the shackle/ weight?
-p-

#8

G’day petr.myska,

It depends on how much current exists at your target depth and how far from the clump weight you need Trident to go. If you can maneuver your vessel so that the clump weight hangs relatively motionless near your target (or follows behind Trident for line surveys) then you might get away with a short length of tether between the clump weight and Trident…just enough for Trident to explore the target area with a bit of slack. If you need to go further away from the clump weight then a longer length will be needed. There is a good diagram of a typical clump weight setup on Page 211 of this document.

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#9

This is my design. A heavy cage that carries the ROV to depth and then unspools lightweight tether to allow the ROV to roam. Similar to the Nautilus system but theirs doesn’t allow the ROV to dock with it. Why waste battery power driving the ROV down and back up?

#10

Thank you!
Are you familiar with the Trident ROV tether? Any idea on how to attach the clump weight to the tether without jeopardizing its integrity? I understand the weight is being suspended on its own cable/ line, which carries the weight. Still I assume there has to be some firm bond in between the clump weight and the tether of the ROV.

#11

Yes petr.myskr,

You can find information on the tether in other threads in this forum (including Kaare’s splicing thread Tether length options - splicing and the drop/clump weight thread Drop/Clump Weight ) by searching with keywords.

In general however, the tether incorporates kevlar fibres or similar stranding for strength so you should be able to lift small clump weights directly so long as you maintain strain relief and prevent sharp bends in the tether.

I have been taping short lengths of HDPE tube around the tether with electrical tape at the clump weight location and using zip ties to secure the clump weight to the tube. I see other people using coiled heavy gauge stainless steel wire wrapped around the tether and this is probably better in some ways. There are likely to be purpose made connectors for this type of thing as well…I will look for them when I run out of HDPE tubes and electrical tape.

For heavier clump weights to suit stronger currents you will need to include the lifting line for the clump weight as mentioned. In this case the same strain relief connection can be used to prevent sharp bends in the tether.

In my applications a couple of wraps of electrical tape around the tether at each end of the HDPE tube is adeqaute to resist the axial tension created by drag in the upper section of tether and/or Trident pulling against the clump weight on the lower section.

#12

If you are familiar with fishing, this system is called a downrigger. It has a large weight (10+ lbs) on a steel cable attached tot he boat. It has a clip to attach a line (fishing or tether). When a fish strikes, the tension will pull the line out of the clip so you can fight the fish. If you replace the clip with a shackle or d-ring the tether can slide though, you can use the downrigger to go to depth, and then start exploring.