Dear users,

While I am calculating the thickness of plates for tanks, I have realized that, there is a rule in Appendix E . Normally we design the thicknesses of annular,bottom and shell plates as per section 5 of API650. As per this, the thickness of annular plate is enough if it is bigger than the bottom plate thickness (or equal) .

But as per Appendix E , there is a rule for annular plate thickness related with shell plate thickness
E.6.2.1.1.2 , ( API 650 , 11 th )
The thickness, ta, corresponding with the final wa in Equations E.6.2.1.1.1-1 and E.6.2.1.1.1-2 shall not exceed the first shell course thickness, ts, less the shell corrosion allowance.
ta: thickness of annular plate excluding C.A.
ts: thickness of first shell course including C.A.

Now , My question, What if ;
bottom plate thickness is 9 mm (including C.A) ,
bottom C.A : 3 mm ,
annular C.A : 3 mm,
shell plate C.A : 1,5 mm ,

than, shell plate thickness should be at least 8 mm. Am I wrong ? ( by the way , according to the design section of API650, shell thickness is only related with tank diameter. and it is given as 5 mm for my tank diameters.)

I kindly request your comments.

Thanks in advance .

Regards,

E.6.2.1.1.2 "Annular Ring Requirements" is under E.6.2 RESISTANCE TO DESIGN LOADS/ E.6.2.1.1 Self-Anchored, so I'd like to consider it is a specific rule for Self-Anchored Tanks.

In my understanding, the main concern in this case is to allow an seismic uplift that will not damage the tank, and API formulas are based on a model with two plastic hinges- first one under the shell, the second one at a distance L1 inward the tank.
API is focused on an "L" calculation as necessary width of annular bottom, but must be sure there the plastic hinges exist.
For this reason it makes sense to ask for E.6.2.1.1.2 requirements as limiting the annular bottom thickness and providing the conditions for the "plastic hinge".

API says "In thickening the bottom annulus, the intent is not to force a thickening of the lowest shell course, thereby inducing an abrupt thickness change in the shell, but rather to impose a limit on the bottom annulus thickness based on the shell design.".

My opinion- when they are particular cases (as the Client would ask for thicker Annular plate and wants to have also the tank qualified under the calculation under Appendix E), you should thicker the shell just to comply with Appendix E "Self-anchored tanks" requirements, but you still have the alternative to anchor the tank and to escape this rule.

Again in my opinion, it is hard to estimate what will be the piping consequences of uplift- see E.7.3.1 values, so also for this point of view it is safe to have mechanical anchors - the API 650 trend solution....

Best regards.

PS. I've attached a sketch from the original work that is the basis of that calculation.

Dear Mariog ,
I apologize for my late answer..
Thank you very much for your comments. It was very helpful to me.
I am avoiding of anchoring tank,unless it is necessary and no other way out. So I have designed as per Appendix E requirements and thickened the lowest shell course.

Thanks again for your answer.

Regards,
Caner