Dyneema rope suitability

[Topcat47]

Dyneema has a very high modulus of elasticity which means it stretches very little under load and consequently, when it fails, it does not whip. In this it is "safe" to use for the purpose mentioned. When it fails, it gives no warning. Examination of the Dyneema post failure can give a hint as to what the nature of the failure is. If the breaks occur randomly along the length, then the "cable" has probably been under specified for the task in hand although it may be that the diameter of the wheels over which is is running is too small for the diameter of the Dyneema. It may also be that the drum of the winch is at fault.

If the breaks are always close to the attachment point, then its may be that the manner of its construction is lacking in some way.

Wire and rope (including nylon rope) does stretch and more importantly when failure is imminent, it seldom occurs catastrophically and immediately. Careful examination of wire and rope can identify weakening by looking for broken strands which most commonly occur on the outside of the cable. This is not very useful with dyneema.

I would not use sheathed cable for this work.

We swapped Kevlar for Dyneema in a winch application back in the 70's as the Kevlar was severely degraded by UV light and the dyneema was not. As we were unfamiliar with the material at the time, we applied a high safety factor when specifying the cable and in three years, working within our machinery design parameters, we experience no failure of the Dyneema. This was in the early days of the material and since it has been adopted by yacht racing circles, it's high breaking stress has resulted in much thinner lines and higher load factors. It may be that in your application you are running too close to its UTS.

Coveman's post above is very relevant. The loads involved in breaking out an anchor are much higher than the load involved in raising it from the seabed.

 

[KREW2]

Agreed on the professional advice - but we can still discuss it. This is a forum, it’s what we do.

Exactly, but I am not in charge.
We have been doing mooring work for 15 years without a hitch using wire We are now trying to convert to Dyneema from wire.
Those in charge are getting reports, and advice from professionals.
There is a lot of knowledge and experience on here and I just wanted some opinions from others, however you will always get some ranters.

 

[Plum]

At our sailing club we have a team of about 10 who drive and operate a 38ft harbour launch that we use to lift and inspect moorings.
Lifting is done with a hydraulic winch which has a 16 ton wire cable. As the ground chain is pulled up over a chute on the bow with a maybe 500 to a 1000 kilo sinker there is considerable strain on the wire.
Some of the team, rightly, have concerns that should the wire part the whip back could cause sever injury to the winch operator. we have been doing this for 14 years and as of yet it has never failed.
Last January we decided to replace the wire with dyneema. In the three months to March we had at least four breaks, I was driving when 2 of them occurred. Now we have a debate among drivers as to the best way forward, wire or continue with the dyneema.
Those pro dyneema say the breaks were safe with no whipping back. Those pro wire say, when lifting weights of 1000+ kilos no break can be considered safe.
The pro dyneemas, some of who are committee members who do not operate the moorings boat, say it should not break because it also has a 16 ton breaking strain.
The argument from those pro wire is that, dyneema may have a good breaking strain but cannot cope with the shock loads that are put on it hauling huge weights and chain from the sea bed.
To me the solution is to move the winch handle so the operator can work from inside the front cabin, protected by the forward bulk head, but that seams too easy for some.

What is the rated max pulling load of your hydraulic winch? Suggest you lake a few 2 metre sections of the Dyneema line from one of the lengths that broke and get them tested on a tensile testing machine to check you are actually getting what the supplier is quoting.

Www.solocoastalsailing.co.uk

 

[fredrussell]

Wrong. This is safety critical, what if somebody follows advice that is dangerously wrong?

Well, lots of folks seem to be discussing it, despite you getting your knickers in a twist. I think OP is savvy enough to understand that none of us are experts in this area.

Exactly, but I am not in charge...
...Those in charge are getting reports, and advice from professionals.
There is a lot of knowledge and experience on here and I just wanted some opinions from others...

There you go, see?

 

[coveman]

Interesting video on wire v dyneema

 

[Zing]

You are asking the wrong question. The first question is why the cable snaps? I think it is probably very undersized. Like ten times.

 

[Neeves]

I'd contact the supplier of the dyneema and of the wire and ask their recommendation on what you should be using. That's a part of their marketing effort to ensure their products are used correctly - no supplier wants an accident on their components - even if it is nothing to do with the supplier. If they charge you for this support - find a new supplier. It sounds as if the wire might be sized correctly but if you 'guessed'[ at the size needed - maybe you have been lucky. You appear to have guessed at the size needed for the dyneema or there is a fault in your equipment (sharp edge somewhere). If your thought processes for the dyneema and wire are the same - then you have been lucky, so far, with the wire.

If I understand correctly the failure of the dyneema has been in a section of 'clean' rope and its been in the same place each time. This suggests, to me, that somewhere within your system there is an issue with part of the equipment, pulley block (its not how you attach or join components and nothing to do with splicing etc). It should not be difficult to work out which parts of the equipment have the failed section in contact - that's the key.

There are many suppliers of dyneema but I'd go to someone with a focus on lifting, Bunzl are big here (but I'm not sure its a focus for them in the UK) give them the problem and see what they recommend. Dyneema for lifting is not unusual - you are not entering new and novel technology.


Someone has already made the point - there is no, or little yield, in dyneema. If your lifting barge is subject to a snatch load when the line is already under tension the snatch load could be enormous - do you have a load cell within the system (they are pretty cheap) with an indication of continuous loads developed, so you need the data recorded in some way or other.


A final comment - the fact you are asking here might suggest there is no expertise within your team. If you have inadequate expertise and are guessing then maybe you have been very, very lucky. I'd not take the risk - next time it might be different. When I'm testing 2/3,000kg I go to professionals (Bunzl Safety), I've seen equipment 'we' use fail well inside the advertised specification (because the manufacturer tested incorrectly) - I trust professionals not amateurs purporting to be experts.


Jonathan

 

[Laser310]

think it is probably very undersized. Like ten times.

...and it's probably undersized because something about the geometry of the system, or the dynamical effects,is not understood and is causing them to grossly underestimate the actual loads.

 

[coveman]

...and it's probably undersized because something about the geometry of the system, or the dynamical effects,is not understood and is causing them to grossly underestimate the actual loads.

...and it's probably undersized because something about the geometry of the system, or the dynamical effects,is not understood and is causing them to grossly underestimate the actual loads.

It would be important that there is nothing that hinders the rope which would add to the strain. A good bow roller for the rope comes to mind but that also needs to be man enough to take the size of the chain - anything to reduce friction.

 

[KREW2]

It would be important that there is nothing that hinders the rope which would add to the strain. A good bow roller for the rope comes to mind but that also needs to be man enough to take the size of the chain - anything to reduce friction.

The dyneema is not taken over the bow roller. The riser is pulled inboard and hooked up, then it has a clear straight run to the winch drum. the only point of friction is the roller guide on the winch.

 

[KREW2]

Dyneema has a very high modulus of elasticity which means it stretches very little under load and consequently, when it fails, it does not whip. In this it is "safe" to use for the purpose mentioned. When it fails, it gives no warning. Examination of the Dyneema post failure can give a hint as to what the nature of the failure is. If the breaks occur randomly along the length, then the "cable" has probably been under specified for the task in hand although it may be that the diameter of the wheels over which is is running is too small for the diameter of the Dyneema. It may also be that the drum of the winch is at fault.

If the breaks are always close to the attachment point, then its may be that the manner of its construction is lacking in some way.

Wire and rope (including nylon rope) does stretch and more importantly when failure is imminent, it seldom occurs catastrophically and immediately. Careful examination of wire and rope can identify weakening by looking for broken strands which most commonly occur on the outside of the cable. This is not very useful with dyneema.

I would not use sheathed cable for this work.

We swapped Kevlar for Dyneema in a winch application back in the 70's as the Kevlar was severely degraded by UV light and the dyneema was not. As we were unfamiliar with the material at the time, we applied a high safety factor when specifying the cable and in three years, working within our machinery design parameters, we experience no failure of the Dyneema. This was in the early days of the material and since it has been adopted by yacht racing circles, it's high breaking stress has resulted in much thinner lines and higher load factors. It may be that in your application you are running too close to its UTS.

Coveman's post above is very relevant. The loads involved in breaking out an anchor are much higher than the load involved in raising it from the seabed.

 

[KREW2]

A recap

We have been using wire for 15 years with no incident. The general committee along with the club H&S officer, and a couple of drivers decided they would like to swap it for dyneema, so a team was appointed to do trials. They started last January, and I went along as driver on a couple. Unfortunately Covid put paid to all that. We were hoping to start again this January, but it looks as though that is not going to happen.

Thanks to all those who posted some useful info, and I will be copying their posts and sending them to one of the team.

The two drivers doing the trials talked to the rope suppliers for sizes and suitability of dyneema for the work it will be doing.
I had an interesting talk last night with one of the drivers involved in the testing.
He has contacted the manufacturer, done some research and thinks it may be the drum size on the winch. Apparently there is a minimum size that the drum should be. The guidance is a ratio, (which I cannot remember) is to do with the diameter of the dyneema and the diameter of the drum it goes round. He assures me our drum falls short of the minimum size.
He explained it to me, and my understanding is, if the drum is too small as it winds on it creates stress at the top and compression at the bottom which can cause strands to fracture. This weakens the rope over time and reduces its ability to cope with snatch, and shock loads, thus the break.

 

[penberth3]

....Apparently there is a minimum size that the drum should be. The guidance is a ratio, (which I cannot remember) is to do with the diameter of the dyneema and the diameter of the drum it goes round. He assures me our drum falls short of the minimum size....

True for any type of rope. Didn't anybody know about that?

 

[KREW2]

True for any type of rope. Didn't anybody know about that?

The driver who phoned me last night said it was always a concern of his and he had brought it up at meetings. He said he would like to see a much larger winch installed capable of taking a larger size dyneema. Hopefully those that went ahead anyway will now listen to him in future.

Having said that all trials were done with safety in mind, and when the breaks occurred the dyneema just flopped on the deck and the retaining hooks prevented the chain from falling back any more than 18 inches.

 

[coveman]

The driver who phoned me last night said it was always a concern of his and he had brought it up at meetings. He said he would like to see a much larger winch installed capable of taking a larger size dyneema. Hopefully those that went ahead anyway will now listen to him in future.

Having said that all trials were done with safety in mind, and when the breaks occurred the dyneema just flopped on the deck and the retaining hooks prevented the chain from falling back any more than 18 inches.

It would certainly seem that dyneema would be the safest option if you can decide on the correct winch size.

 

[TLouth7]

I think you actually want a slightly springy lifting line, to put a steady, controlled force on it, which doesn't go off the scale if the barge moves a few inches.

This is an incredibly bad idea. The entire appeal of using dyneema is that it is not stretchy, so there is minimal elastic potential energy to be released when the line snaps.

 

[Dogone]

This is an incredibly bad idea. The entire appeal of using dyneema is that it is not stretchy, so there is minimal elastic potential energy to be released when the line snaps.

Actually it is not necessarily a bad idea. You will massively reduce peak loads if you permit stretch. If loads are unchanging then dyneema is OK, but it is so stiff if you use it instead of nylon on a mooring then peak loads will go up hugely. You need to design this so it never, ever snaps.

 

[rgarside]

The axial load-elongation of dyneema rope approaches the same order of magnitude as steel wire rope [after the initial bedding-in stretch has taken place]. So in that respect the behaviour of the wire rope and dyneema will not be too different, and the dynamic loads associated with breaking out a sinker should be similar. So the change in performance of the rope should not be associated with differences in axial stiffness.

The more benign behaviour of the dyneema when it breaks is due to the much lower amount of potential energy released, this is due to the much lower mass/unit length of the dyneema compared with steel wire rope despite the amount of strain being comparable. [16mm SWR around 1.2 Kg/m, 16mm D12 dyneema around 0.06 Kg/m]

OP says the failure takes place around 2 feet back from the hook - if the line is spliced, this might be around the end of the buried tail. If the rope is of the correct size to give a suitable safety factor, the splicing method should be investigated to see if it can be improved.

OP hasn't indicated the actual diameter of wire rope and dyneema they have used. Do they actually give comparable safety factors?

The diameter of the winch drum should be suitable, if too small it will in time give rise to progressive damage to the rope [whether it be wire or dyneema rope], however, I guess at the moment the dyneema is breaking for other reasons before this causes enough damage to be a problem. The bending diameter / rope diameter ranges usually used for steel wire ropes and for dyneema ropes are similar.

Are there are no sources of chafe between the hook on the end of the dyneema and the winch?

 

[KREW2]

On reflection the rope might have failed about 4/6 feet back from the hook. I remember one of the crew wanted to tie it together, but I wasn't having that.
The only course of friction is the roller guide on the winch.
I don't know what diameter it was, I was told it had a 15 ton breaking strain. Googling shows that to be 12mm.

 

[coveman]

I realise that the wire/rope does not go over the bow roller, but is the bow roller large enough to take the rising/possibly ground chain without friction - I am presuming there is one?? A 90 degree pull over the bow without an adequate roller would,I should think, add considerably to the force required to winch the chain aboard. It would seem to me that there are 3 checks to be made - size of winch,size of rope,adequate bow roller.