Swamp weights for buoyancy control

[Engrinmaking]

Hi,

I am trying to calculate the concrete swamp weight spacings for natural gas pipelines.
I do not know why a normal force balance is not giving me a logical answer.
Can someone share the method I can use to come up with these spacing using following data:

Pipe OD: NPS 4 , 114mm
Pipe WT: 6.02mm (standard wall)
Mass of swamp weight: 102 kg
Medium water: 1000 kg/m3

I understand length of the pipe would be ideal but can we do calculations in per meter?
Will really appreciate the help. I have to make a table for all the way up to NPS 48.

Thank you

 

[zdas04]

Your data is missing a couple of key elements. First, what is the weight per unit length of the pipe empty? Second, how long are the weights (if each 102 kg of weight is 20 m long then it isn't going to work). Third, what is the density of the weight.

With that you can calculate a weight of a length (I would use 10 joints of pipe) of pipe with the weights on it. Then you could calculate the volume of the pipe plus the volume of the weights. Finally you calculate the weight of your representative length, calculate the weight of the water it displaces--if the pipe and weights weighs less than half the weight of the displaced water then you have enough weights (with a 100% safety factor). Add weights until the math works and then spread them evenly along the submerged pipe. It really is that easy.

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[dhengr]

David:
One of us is thinking bass-acwards here. Is it possible that you were thinking right and typing left? Shouldn’t it be that if the pipe and weights, on average and over a typical length, weighs twice as much as the displaced water, you have a FoS of 2, or a 100% safety factor? Or, your statement might be rewritten something like this... ‘--if the pipe and weights weigh (s) twice (less than half) the weight of the displaced water then you have enough weights (with a 100% safety factor).’ Seems you were thinking about supporting something with buoyancy, not holding it down.

Engrinmaking:
You should space the counter weights evenly and close enough together so as not to cause any bending problems with your empty pipe.

 

[LittleInch]

"I do not know why a normal force balance is not giving me a logical answer. "

Neither do we if you don't share it...

Buoyancy control is fairly basic physics.

Basically calculate outside volume of pipe per metre or per 10m (including coatings etc). The calculate uplift force assuming water usually. = A

Now calculate downwards force = weight of pipe plus contents =B

If A >B then the pipe floats.

Calculate effective weight of your block in water, so the same thing - weight of your block in air - volume of your block in water.

102 kg looks a very precise number and rather big for a 4" pipe.

There are many other issues to contemplate including over doing it and having the pipe sink into the sodden mass or creating local high bending forces / shear forces.

For this size pipe have you considered screw anchors or similar? Or a uniform concrete coating, or just a bit more steel?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.