Neutrally buoyant tether


#1

Hello all,

I contacted a subsea cable supplier (Elmeridge Cables Ltd, they have been very helpful) in the UK and asked if they could supply a neutrally bouyant tether for the Open ROV.

I specified a single twisted pair, 24 AWG, neutrally buoyant, yellow colour sheath, with as small an outer diameter possible.

This is what they came back with:


M.O.Q is 2000mtrs
Spec
Neutrally Buoyant 1 x 2 x 24AWG
conductor: tinned copper class 6
Tape: non woven tape to hold the conductors in place
Pulling Element: open Braid yarn
Jacket: bright Yellow PUR
O.D 6.80mm
Cable Weight 37.5kg/KM

Price 1.81 GBP per/mtr


I have posted this here so that everyone can see this and have a chance to offer some feedback. As the minimum order quantity is 2000 m (there are, of course, substantial costs in providing a bespoke production run of cable, hence the M.O.Q.) a group purchase could be an option if we have enough interest.

Tether management is one of the most important aspects of piloting a ROV and a neutrally buoyant tether, with a smooth profile (floats can add bouyancy but they also encourage entanglement and create tether handling issues.) would have many advantages.

Of course, like anything, there are downsides with a larger tether spool required, more weight during transport, increased tether drag subsea, inherant stiffness of the tether - which could put a turning moment on the ROV etc.

I thought I would post this here so that the Open ROV community can give their opinions. Any feedback is appreciated.

One thing I forgot to ask them is the minimum bend radius, which will have implications for our spool sizes. I can find this out if there is interest.

Cheers guys and happy ROVing!
Chris


#2

Hi Chris

I will be looking for something similar in due course, i have made an enquiry on similar specification from “Hesheng Cable” in China - will let you know if i get any positive response from them.

Cost might not work out much less on a custom product once freight and import duty are factored in though!

kind regards
stuart


#3

Hi Stuart,

apologies for the delayed reply - I have just seen your reply now. It also looks as though the OpenROV team have seen the value of such a tether as it is now for sale as an experimental option.

Good stuff!

Best regards,
Chris


#4

What about the following low-tech solution?

Use heatshrink tubes, e.g. the transparent ones found in the OpenROV kit. It should be rather easy to calucate, or just experiment (e.g. 10 cm of heatshrink for each 1m of cable). Shrink it at both ends and let the air be trapped in the middle.

Some colored marks can be also inserted for the purpose of cable measurement.


#5

Hi nb1,

that would be great idea but the compressibility of the air means it wouldn’t work. At the surface you could have positively buoyant tether, but as you dive, even if the heat shrink was perfectly sealed, the air would compress and the bouyancy provided would decrease - making ballasting the ROV pretty much impossible.

The PU coating on commercial tethers compresses negligibly.

I suppose this could be a soultion for a specific task at a specific depth though - if you were willing to rig the ROV for the task it could be useful and the ROV could be ballasted for the depth of the task…

Cheers,
Chris


#6

Hi Chris,

We can manufacture and supply the neutrally buoyant cable as below:

Spec.
1x2x24AWG
Tinned copper stranded wires
Kevlar fiber filling(50kgf)
PE insulation
PUR foam sheath
OD 4.5mm
MOQ 1000mtrs

Price is 1.5 USD/meter

Best wishes
Eric