Tank annular plate projection length 1

[Surefer]

Hi,
We are currently changing a tank bottom plate.The annular plates has been cut as per 50mm projection from tank shell plate.But after welding, projection of annular plate remained 42mm.(Maybe becuse of some mistakes as well)
API 650 5.4.2 states (Bottom plates of sufficient size shall be ordered so that, when trimmed, at least a 50 mm (2 in.) width will
project outside the shell or meet requirements given in 5.1.5.7 d whichever is greater.) And API653 also refers API650 for this issue.
What should I do? Can I prove by a calculation that 42mm is sufficient for my tank?

Regards
Sureyya

 

[blawn]

par.5.1.5.7 d "The bottom or annular plates shall be sufficient to provide a minimum 13mm (1/2 inch) from the toe of the fillet weld referenced in 5.1.5.7 c to the outside edge of the bottom or annular plates."
The above paragraph states that minimum of 13mm should be provided but paragraph 5.4.2 uses "whichever is greater "at the end of its sentence.Confusing though,but I think your case still acceptable so long as it will not fall below 13mm.
If you can perform calculations to prove it is sufficient,then go for it.

 

[JStephen]

I don't think you can necessarily "prove by calculation" that 42 mm is adequate because there's not any calculation "proving" that 50mm is required in the first place. It's one of those rules that gets established by committee and not because 50mm is good and 49mm will make the tank fall down.

 

[Surefer]

Thank you for your valuable comments.
There is one study which examines the "effect of increased bottom plate projection for tanks with rigid ring wall foundations".In this study, stress analysis had been made at the junction of shell and bottom. I thought that we can inspire from it if calculation needed. Hence I thought that if I can prove by calculation.
I am fully agree with 42mm is enough but client satisfaction is more important as you know. I am looking for a base for it.

 

[IFRs]

I'd be interested in seeing that study - can you post or email it?
Some analysis assume that the stiffness of the corner weld is greater than that of the plate that extends past it and the excess material will bend about the point where an extension of the outside fillet weld intersects the underside of the floor plate. This would suggest that material well past the corner weld was non-critical once you get beyond this point. Unfortunately 2" [50mm] is the code minimum for projection outside the shell and there is no wiggle room here. I don't think you will find any easy calculation but if necessary you can model it in FEA. Boundary and loading conditions are important to model correctly as well as the stiffness of the shell and annular plate and the corner weld. Thermal and hydrostatic loading superimposed on dead, wind and seismic loads certainly can make this complicated.

Note: if the projection is 50mm past the shell but less in some areas because there is a repad or thickened insert you may be OK.

Possibly you may just have to replace the annular plates and call it a lesson learned.

 

[JStephen]

It is my understanding that the assumption of a plastic hinge in the bottom plate in API-650 assumes a plastic hinge both inside and outside the shell, and maintaining a lip long enough to develop that hinge is the only reason I am aware of for having the 2" extension. That would only matter for larger shells built to the variable design point method or using shell analysis theory. I think I had looked into this sometime back, and if I remember right, older API, and some of the other standards had 1" past the weld, newer API has 2" past the shell, API-12D has no minimum at all. For most tanks, I don't see this as being a stress issue at all, it's just a detailing issue similar to the minimum lap length, etc.