how i decide to replace top shell of a fuel tank according to API653 1

[thaierali]

dear all
i will try to make my question clearer > the issue is a conic roof tank with capacity 10000m3 . the roof is severely damaged by corroding > we decide to replace the roof plates with new ones with same thickness of the designed roof thickness is 5 mm > the problem is that the top part of the shell is croded down to 4 mm while the design thickness is 6 mm >
mu question is >>>if i go on with repalcing the roof plates only we will have a tank with roof plate thicker than the top shell >>>is that acceptable !!! and if it is not acceptable can i take this issue as a resonable reason to Take a decision to replace both roof and top sell plates

i hope i make my issue clear enough
thanks for your Patience

 

[JStephen]

Roof thicker than the shell is not a problem in itself, and some tanks may be built that way.
Evaluation of shell thickness is usually per API-653. Hoop stress due to product is very low at the top of the tank, and that's usually the major issue.
It may be cheaper to replace both upper shell and roof at the same time than replacing the roof now and then the upper shell in another couple of years.