wind pressure for Fitness-For-Service of API 650 tanks 1

[femman]

thread1452-357698

A related question regarding wind pressure. When tanks are emmpty, they are liable to blow in. Sure. More so if the shells are not within tolerances. In API 579-1, the Plastic Collapse verification indicates that the wind load case pressure be applied * a factor, but I wonder if this pressure is that mentioned in API 650 where it says "Design wind pressure (PWSand PWR) using design wind speed (V): The design wind pressure on shell (PWS) shall be 0.86 kPa (V/190) 2, ([18 lbf/ft2 ][V/120]2) on vertical projected areas of cylindrical surfaces."

It seems that this pressure distribution can account for overturning, but not for buckling of the plates inward, as can happen during hurricanes in empty tanks. Any ideas on what pressure distributions to use on the cylindrical and conical faces of the tank in a Finite Element simulation?

Thanks a lot!

 

[IFRs]

Shell buckling from wind loads is covered in section 5.9. I think API assumes a uniform distribution from top to bottom. Also see the footnote under 5.9.7 "R.V. McGrath, “Stability of API Standard 650 Tank Shells,” Proceedings of the American Petroleum Institute, Section III Refining, American Petroleum Institute, New York, 1963, Vol. 43, pp. 458 – 469"

 

[pdiculous963]

The pressure distribution listed in the Code (0.86 kPa (V/190) 2, ([18 lbf/ft2 ][V/120]2)) can be used for analysis of shell buckling using FEM. I have performed various FFS assessments using this distribution on storage tanks which are out of round to evaluate shell buckling.

 

[femman]

Thanks for your reply pdiculous963. I applied the loads as you describe, and in the absence of product in the tank, it's a critical loading condition.

In terms of seismic analysis, would you recommend applying the force and overturning moment from Annex E to a FEA model? If so, where would you apply thee force? Is there a way to capture the effect of sloshing in a quasi-static simulation?

Thanks in advance! Cheers,

 

[pdiculous963]

The equations in Appendix E can be used to compute the lateral force due to seismic, which can account for sloshing. This can then be run in a static simulation. This can then be applied as a gravity load on the tank walls to evaluate stability. You can apply the gravity from multiple angles to check the directional significance.

 

[femman]

Thanks a lot pdiculous963... that makes sense. Specifically speaking, would you find a new gravity vector (for both hydrostatic pressure and body force) so that the lateral forces equal a given value? Would you use the Total Design Base Shear for this equivalence? Wouldn't it be possible to estimate this new "gravity" from the

Ac -convective spectral accel parameter 0.30795E-01
Ai -impulsive spectral accel parameter 0.17143

?

Thanks in advance for your advice. Cheers,