Learning Forum

[Adrian Lim]

Hi,

I was running a pipeline buckling simulation and the results was able to give me a load multiplier.  The loads that I have applied are nodal pressures in the inner surface of the pipe and the operating temperature, however I was unable to verify the actual buckling temperature/pressure based on the load multiplier. Understand that the Buckling Load = Applied load * load multiplier, I could not get the critical buckling temperature/pressure and get the load multiplier to 1. 

So I was wondering what is the relationship between the load multiplier on the pressure and temperature if applied together?

[peteroznewman]

The load multiplier applies to the Temperature and the Pressure at the same time.

If you use only Pressure, what is the critical Pressure?

If you use only Temperature, what is the critical Temperature?

If you then apply the critical Temperature and critical Pressure together, what is the load multiplier?

[Adrian Lim]

Hi Peter,

If I were to find the critical pressure and temperature at the same time.

For example,

The applied loads are T = 100°C , P = 20MPa in the buckling section

Tcrit = 100 x 3.8853e-2 . Pcrit= 20MPa x 3.8853e-2

Would that be correct? Because when I tried this two and plug in the Pcrit and Tcrit, the value does not come close to 1. 

 

 

[peteroznewman]

Suppress the Pressure load in the buckling analysis.  What is your environmental temperature, is it 20 or 22 C?  Let’s say it is 20 C.  A Temperature of 100 C means a delta T of 80 C.  Try a temperature of 21C, which is a delta of 1 C.  Now run the buckling analysis.  What is the Load Multiplier (eigenvalue)?  Let’s assume the multiplier is 2.345.  Now set the temperature to 22.345 and run the buckling analysis. Do you get close to a 1.0 Load Muliplier?

Now suppress the Temperature load in the buckling analysis. Set the Pressure load to 1 MPa. Solve the buckling analysis. Get the first positive Load Multiplier for this analysis. Set the new Pressure in MPa to the value of the Load Multiplier. Do you get close to a 1.0 Load Multiplier?  Note that you must ignore any negative Load Multipliers since they represent a negative pressure in the pipe (partial vacuum) which causes bucking more easily than positive pressure.

This exercise shows you the size of each load to independently get to the onset of buckling.

I expect a very small temperature increase causes buckling while a very large positive pressure causes buckling.

[Adrian Lim]

 

Hi Peter,

Understood. But it would take out the pressure/temperature parameter when I’m performing the respective ones. Or when I get the critical parameter for both for example at 22.345°C, 2.235MPa, the pipe will buckle? On the other hand, when I suppress the temperature load, the pressure load multiplier that I got were all in negative. Did something went wrong?

 

[peteroznewman]

If all the Pressure load multipliers were negative, that means a straight pipe with fixed ends will not buckle with positive pressure. It will only buckle by negative pressure (partial vacuum). Positive pressure can cause the stress in the pipe to exceed allowable values, which is a failure unrelated to buckling.

Put the pressure only in the Static Structural model. You might find that increasing pressure will cause an increase in the Load Multipier on a Buckling analysis that only has a Temperature load.

[Adrian Lim]

Hi Peter,

Yeah I did try that with the pressure in structural and temperature in buckling. 

Thanks for the help so far!