Impact Loads - Navigation Piles 3

[Myoho]

Hi, I’m designing some navigation piles (free ended single laterally loaded piles) to protect a water inlet structure for a water treatment works. The inlet is constructed on the River Thames (UK). The design impact load from a vessel on the river is 100 tonnes at 2.5 meters/second. However I’m not quite sure to convert that force into kN. Any ideas please!

 

[Panars]

These types of piles are also called dolphin piles. You can search for the design of dolphin piles to find some information.

The short answer to your question is that you don't convert the impact load to a force. The kinetic energy of the vessel is absorbed by the dolphin. So you determine the load-deflection behaviour of the dolphin and calculate the energy absorbed by taking the area under the load-deflection curve. At the point where this area equals the kinetic energy of the vessel is the deflection of the dolphin when the vessel has come to rest.

 

[himat42]

Myoho

You could looked at the following publications

1. Ship collision with bridges
IASBE Structural Engineering Documents 4, 1993
2. Bridge and Pier Protective Systems and Devices
By: K,N, Derucher and C.P. Heins
Marcel Dekker, Inc, NY,NY 1979

 

[Myoho]

Thanks guys. Think Im getting there. I work as a consultant and there is no way I charge the client the time I spent researching this. Im almost there now but I'll be more careful accepting bespoke jobs next time.

 

[JimKSF]

You can always try it this way.

Pick a pile size. Determine the distance from the impact point to the point of fixity for the pile. (Absent geotechnical guidance, say 6 diameters below the mudline, assuming mud.)

Determine the load required to cause failure of the pile and the deflection of the pile at that point. (As you will define it. Permanent deflection is probably not desirable so keep it elastic.)

The energy the pile absorbs (assuming straight line load/deflection, likely for an elastic range monopile) is 1/2 x Reaction x Deflection. (The area under the load-deflection curve.)

Check the ship energy.

Energy= Mass x Velocity^2 (There are other factors for berthing such as added mass of the water being pulled, eccentricity, etc., but these tend in the aggregate to decrease your imparted energy so this will be conservative.)

If the pile does not have excess energy absorbing capacity at its maximum elastic deflection, try a bigger pile until it does.

Jim

 

[JimKSF]

Oh, I forgot to mention-- You actually DO want your pile to fail before the hull of the ship in all cases, but especially for a navigation pile.

I was speaking in terms of a very low velocity berthing situation when I said to keep going to a bigger pile until you get the energy you need. (That's where my calc's have been lately.) In your case, that would also mean a reaction much larger than the ships would find pleasant.

You can increase the energy absorbing capacity by installing fendering on the pile. This will also help to reduce any damage to the vessel in case of a collison, but the pile, with or without fendering, has to be the sacrificial component of that collision.

For a case such as yours with single, vertical piles, "donut" fenders might work well.

Trelleborg supplies fenders, but they also give good guidance on ships and berthing energies.



Jim

 

[JimKSF]

And of course I just caught an error in my first post.

The ship's energy is 1/2 MxV^2.

oops

 

[Myoho]

Thanks Jim, Another valuable post. Much appreciated. Ive pretty much designed the Navigation piles now and it is obvious they are going to require a fender of some sort to absorb some of the ships energy.