Deflecting Falling Ice

[mikeh]

Due to a broken steam line, ice forms at the top of a stack. It would cost million $ to shut plant down to repair. It may cost >$50,000 if large pieces of ice land on underlying equipment.

Assuming that the ice weighs about 500kg and fall for a height of 100.0m, what is the resulting force on any structure built to protect the underlying equipment? How is this force affected by deflecting the ice with a sloped surface.

Any information regarding the behavior of falling ice (i.e energy/momentum transfer/absorption due to the ice breafing) and any corresponding guidance on the design of the structure would be helpful.

 

[flamby]

The force can be computed if you know how quickly you want to bring the moving ice to a halt.

Force=rate of change of momentum =m*(v2-v1)/t

m=mass 500 Kg as per your specification

v2=final velocity=0.0 m/sec for dead stop or (-)ve value for rebound velocity

v1= velocity at strike, m/sec = sqrt(2*9.81*H)
where H is height, 100 m as you said

t= time, sec within with the change is done from v1 to v2 (1 sec? 2 sec? Or just a fraction of it? )

Now the million $ question is what value is to be taken for 't'? If the surface can act as a spring and a shock absorber, you can use a higher value for t and bring down the force. But if it is a tough surface with hard fixity, t goes less adding to your worries. So try to use a softening of the surface and insert some cushioning by way of adding fluid, dashpot, springs etc.

Hope it helps.

 

[OzzWald]

This problem, with its wording and all, sound straight out of a textbook.

 

[dlew]

mikeh,

I do not have experience in protection structures against falling ice, but the followings are general comments that could apply to structures subject to impact energy :

A 500-kg mass falling 100-m produces a very high energy that need to be absorbed. I do not know what would be the energy absorption of the ice block, but I assume it would be at least 50% of the total energy. In the design of docks, the ship is assumed to absorb 50% of the energy, and the dock and fenders are designed for the other 50%.

I think that sharp edges on the top of the protection structure that help to break the ice block would decrease the energy transmitted.

The force to be taken by the protection structure depends on how much the structure displaces when hit by the ice block. The more it displaces, the smaller the force per the formula: Design energy = Force x displacement in the direction of the force.

A sloping top (say 45 degrees) will help to reduce the force on the structure. The reaction force would be normal to the sloping surface, and could be decomposed in vertical and horizontal components. The structure would displace both, vertically and horizontally reducing these forces accordingly.

Hope this will be of some help. Good Luck.


AEF

 

[flamby]

Some alternatives that you can try:

1. Hydraulic Boom: This is extensively used in water-reservoirs to prevent flowing debris from entering the intakes. This is nothing but a sort of floating (pontoon) bridge with net which allows water to pass through but not any solid flowing debris. You have arrangements to remove the solids periodically.

2. Rock Nets: This are strong wire fabrics with elastic fixing to catch the rolling rocks on steep mountains to prevent downstream structures.
You can find plenty of manufacturers of these if you fire a query in google.

See which is suitable in your case. I dont know your general arrangement, but I think one of these two will satisfy your requirements.