Overturning Stability - Floating Roof Tanks

[ship03]

Hello All,

I have an External floating roof tank design (Pontoon Type with single center deck) that i am looking to do overturning stability calcs for. Looking at API 650 11th Edition, Addendum 3: June2011.

Clause 5.11.2 states the tank must satisy both the uplift criterias....

0.6Mw + Mpi < MDL/1.5 + MDLR
Mw + Fp(Mpi) < (MDL + MF)/2 + MDLR

I would appreciate any feedback on the below queries....

1) I would expect that for an Atmospheric Tank with an External Floating Roof the value for Mpi = zero?

2) Are the formulas shown above only relevant to Cone Roof Tanks? If not how do we add the weight of the Floating roof with regards to MDL and MDLR, as these values in the code seem to be referring to weights associated with tanks that have fixed roof structures?

3) Why does the code not take into account any weight from the tank bottom plates with regards to wind loading/overturning stability?

Thanks in advance!

 

[JStephen]

I would assume that Mpi= zero, and neglect the weight of the floating roof, just check the tank like it was an open top tank.
A great deal of the overturning moment with the current methods is due to wind uplift on the roof, and eliminating that ought to add a great deal of calculated stability.

 

[ship03]

Thanks jStephen,

Any thoughts on why the code does not include the weight of the floor for the overturning?

 

[JStephen]

My guess would be, that without significant movement of the tank, you'd only get uplift resistance from the foot or two of bottom nearest the shell, and that was just considered a negligible amount.

 

[ship03]

Hmmm yes i see.....

This would also reinforce the theory to NOT include the weight of the floating roof in the stability calc, as it is supported/sat on the floor away from the shell and hence would not contribute to resisting any uplift.

Thanks for your comments and thoughts.