Icing problem for a fishing line towing an underwater probe (Arctic mission)

[MDLU]

Dear all,

- Our research team is looking to deploy an underwater towed probe (measure conductivity, temperature, depth for climate change research) in the Arctic (the towing boat in a previous mission,

https://www.nanuq2020.eu/).

- Probe is towed from the boat with a ~300m fishing line (at depths 0-100m).
- The line operates for continuous 12-hour-long daily missions over several months.
- Probe design highly dependent on fishing line: Power Pro 150lb is by far the best line we could find on the market for our design
- The line is a braid of Honeywell Spectra fibers (very well tested,

https://linghunt.com/101Spearfishin...-spectra-fiber-capability-guide-brochure.pdf)

- The Power Pro 150lb is 0.56mm in diameter (keeping diameter around this value is important)
- Arctic water temperature is around -2C and the air temperature ~-15C. Line operates mostly underwater (-2C) but if towed from the boat, first few meters of the line will be exposed to air (-15C).

POTENTIAL PROBLEM AND POTENTIAL SOLUTION
- When I asked Power Pro about icing problems (they have a line for ice operations but too weak/small for us)
they say: "small water drops can get trapped between fibers and freezing creates small sharp particles that cut the fibers when the line bends under tension".
That's why the coated with PTFE the other line.
- We couldn't find any ice resistant line with the strength we need (>150lb breaking tension) and the PowerPro person suggested we look into PTFE coating the line to prevent water entering between fibers

QUESTIONS FOR YOU Before we consider coating the PowerPro 150lb with PTFE as a solution:
- Have you heard of such icing problem before (is it a real problem we could encounter?) and are you aware of any solutions that other marine engineers use?
- Suggestions for unconventional solutions we could try (maybe keep entire cable completely underwater all the time)?
- Are you aware of fishing lines in our design range (breaking strength and line diameter) that can resist to icing conditions?

Thank you,
Dr. Matteo Di Luca

 

[EdStainless]

Perhaps you could saturate the exposed portion with a fluoro-polymer oil such as Krytox.
I don't know how great of a risk it is since the line will never be static.
Under continuous motion it may not have a chance to freeze solid enough to cause problems.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[zeusfaber]

If it's going to be a real problem, a relatively cheap solution could be to lead the line from the winch to somewhere a metre or so beneath the surface through a hose fed with a steady stream of warmed seawater. Keep the winch drum enclosed and warm too. As well as ensuring that water in the line never freezes, the hose will offer a bit of protection against damage from floating debris.

A.

 

[GregLocock]

Both the above make sense. Mine would be to use a header line for the first few metres with different characteristics, such as chain. Or simply treble the line and loosely plait it for the first few metres, this will bend and flex and rub and generally discourage the formation of crystals.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[TugboatEng]

Spectra and steel are diametrically interchangeable. You could replace the line with steel rope and that would likely eliminate your ice problem but introduce a corrosion problem. The low cost of steel would possibly permit a regular replacement interval to mitigate corrosion issues.

When towing the line remains straight and taught. Is it failing under tow or during deployment/recovery? High performance rope lacks elasticity and is weakened around bends. Another possibility is to use a heater to melt ice before the line reaches any bends. That small diameter wouldn't require much power.

Krytox is a good idea but anything that makes the line hydrophobic would likely help in the same manner. Spectra is a UHMWPE fiber so you want an oil that has strong capillary action on a polyethylene surface.

Greg also has good input, freezing is a surface problem so you don't need to treat your entire string.

 

[MDLU]

EdStainless, Zeusfaber, Greg Locock and TugboatEng,

Thank you for all your suggested solutions (steel*, Krytox**, warm water**, underwater line/chain***). As I consider them,
the solution could be a combination of them. I will get back to you with the final solution (field results much later).

Best,
Matteo

* Steel would be the easiest, but the probe + the 300m long cable needs to be buoyant to resurface if the cable were to break near the boat.
Hence high buoyancy for the probe (say 10% of weight) -> high lift/drag ratio (e.g. an extra >0.8-1) to reach required depth at the lowest operational speed (1 m/s, lift needs to fight buoyancy force).
Since the probe is practically a large cylinder with small wings attached, getting the extra L/D>1 is not easy without elongated wings (structural complexity).
A neutrally buoyant line results in much less stringent L/D requirements.

** The probe needs to be loaded on the boat daily (battery charging, data download) and also
resurface every once in a while to acquire gps coordinates and geotag the data.
During deployment/recovery the line is exposed so Krytox, warm water and chain may not be able to solve the problem.

*** Along the chain/underwater line ideas, a potential solution is: boat - underwater winch (say 0.5m below water surface) - PowerPRO 150lb (298m) - ice proof cable (5m) - robot
During robot recovery on the deck only a portion of the ice proof cable is exposed to air (-15C) while the PowerPro stays underwater.
When robot resurface for gps acquisition only the ice proof cable is potentially exposed to air.
This solution may not solve the problem of operating the robot at or near the surface (for gps) with 2-3m high waves.
In that case, I may have to revisit steel or drop gps/near surface operations.

 

[romke]

If I understand the Spectra documentation well, the line is coated with a PTFE coating that has hydrophobic properties - which in my view means "water repellent". That would mean that as the coating is not damaged by for example floating ice, water in the fluid state would not adhere to the line and that when towed in the part of the line above sea level will possibly still have some waterdroplets on it but they will more or less move down under gravity. The speed that the droplets move down most likely depends on the outside temperature and the rate at which tiny water parts will solidify. If indeed crystals are formed they might well damage the coating and then possibly cause other harm. The best way to find out is to set up a experiment where the end of the line is in cooled seawater for a time long enough to attain the water temperature and then rolled in a environment with low temperature and appropriate humidity to see whether crystals are formed and what then subsequently happens.

The most simple way to prevent damage by ice crystals is to use a corrosion preventative that puts a rather thick layer around the line. Unfortunately those products are based mostly on mineral oil containing a rust preventative. That may not meet ecological requirements in this case. However, there are developments in the composition of these substances that are more ecologically friendly,based on natural esters or chemically modified esters that do not immediately break down in water but are nontoxic to the aquatic environment when they finally do. My suggestion is that you talk to firms that develop wire rope lubricants. They might possibly be able to assist you finding a suitable solution with your specific application.

 

[MDLU]

Thanks romke,

an icing test with the POWERPRO is a very good suggestion and indeed chemicals would have to be nontoxic to the aquatic environment.

The "captain" of the mission is finally back from his sailing trip and it looks like he wants the line to be fully submerged (towing from below water surface)
to avoid floating algae and ice. If we go with this solution, we may not have an icing problem on the POWERPRO after all (perhaps just use an ice resistant piece of line
just to onboard and offboard the probe).

The ideas suggested have been very helpful. Thank you and I will keep you posted on what we end up doing.

Best,
M

 

[Compositepro]

Weights or floats can be added to the line to get the buoyancy you want.

 

[MDLU]

Thanks Compositepro, you are right, we could add some buoyancy on the line.

I will let you know what we end up doing.
Best,
M