Force from a 13-tonne chain falling onto a 5m round sling in seawater

[BasileDiver]

Hello everyone!

I am a commercial diver working in the Northsea. I happened to be very curious but unfortunately I forgot bits and pieces from my formal education and that stops me from answering my question.
I don’t think that this question will be a big problem for you and that is the reason I joined the forum.

I have two chains hanging from a ship. The plan is to secure one to the other and then cut the first one and then recover it. We are planing to secure the first chain with a 5m round sling to the second chain.
My question is: What will be the force applied by the falling chain to my round sling, and will it resist to it.

mass of the chain: 13 000kg.
Fluid surrounding the chain: sea water.
length of the round sling: 5m
Drag Coefficient of the chain: unknown.
Distance between the two chains: they are so close that we can consider that the fall will be vertical all the way.

Any idea?

 

[Baluncore]

Welcome to PF.
I assume the two chains are next to each other attached to the ship above the water line.
We need more information.
1. What height above the water line are they attached to the ship ?
2. What are the lengths of the chains ?
3. How far down each individual chain will you attach the sling ?
4. Is the mass of the chain: 13 000 kg each, or is that total of two.

 

[BasileDiver]

The chains are next to each other yes, almost touching. Everything is happening underwater. To put it like that: the saw is underwater so the cut is underwater as well.

We don’t have the exact length of the chain, but the scale on the winch measured around 13 tonnes from the cutting point.

Each attachement points will be at the same depth. The fall should not exceed the length of the sling (if it doesn’t break). The sling will be attached on the eye just under the cut. The other end should be attached on the other chain at the same depth.

13000kg is the mass of the chain that will be cut, from the cutting point.

Does this help you?

 

[Baluncore]

Each attachement points will be at the same depth. The fall should not exceed the length of the sling (if it doesn’t break). The sling will be attached on the eye just under the cut. The other end should be attached on the other chain at the same depth.

If you attach the sling to the top link of the chain to remain, and to half a sling below on the chain to be cut, the length of chain will fall for only one link maximum, with the upper few links of the cut chain inverting. Is that possible to arrange?

Does a "5m round sling" have a total circumference of 10 m if laid out in a circle ?

 

[Lnewqban]

Hello everyone!
...
We are planing to secure the first chain with a 5m round sling to the second chain...

Welcome!

Since the sling seems to be the weakest link in this situation, resisting the yank of the second falling chain (as far as I can understand your description), how much can you tell us about it?

Do you know some specifications for that particular sling, such as load bearing material, load capacity, safety factor, number of strands, type of wrap (if any), minimum allowed bending, certification, etc.?

 

[256bits]

they are so close that we can consider that the fall will be vertical all the way.

I am not so sure that can be considered absolutely factual.
Length of chain is important.
As well as the viscous drag upon chain links ( which would or could be estimated somehow ).

Once the cut is made, the whole dropping chain does not fall immediately as one complete unit.
The first top link falls, then the first and the second, then the first second and third and so on as the support for each link farther down is removed. I can imagine some of these links will rotate with respect to one another due to the different velocities of fall of each link.
Depending on the length of the chain, the top link could be arrested from falling by the sling before the last bottom link in the chain has even begun to fall, or even ones farther up from the bottom link.