API 653 "Chime" Projection outside of bottom weld 1

[dforce]

Hello,

Does anyone know the reasoning behind API653 11th ed 4.4.5.7 referring to the projection of the external chime outside of the shell-to-bottom weld being at least 3/8" ? It also says it must be at least 0.1" thick.

I'm asking since we have external corrosion on one tank leaving a chime of only 1/4" and now it looks like we must replace the bottom plates in this area. (Tank floor is butt welded without annular plates). I would like to know what purpose the 3/8" projection serves.

Thanks
D

 

[Bonswa]

Here's my thought process on this one:

-When you can visually see heavy corrosion on the bottom side of a chime, you would normally gather a good bit of U.T. data in that area as well as the rest of the chime.
-The minimum thickness on a tank with no leak detection or liner is 0.100"
-The standard U.T. probe (1/4" diameter) will not allow you to get close enough to the shell to bottom weld. The 3/8" projection is just a cushion.
-If the U.T. readings show less than 0.100" at 3/8", then there's really no way of telling wether it get's better or worse as you get to the bottom side within the shell to bottom weld. You have to assume that it's the same thickness within the shell to bottom weld.

-If this is the case though, shouldn't you be allowed to go down to 0.050" at 3/8" on a tank with a liner and leak detection?

That's just what I think. It's an assumed thickness for the underside of the tank within the shell to bottom weld.
Also, if you're down to 0.100" due to top side corrosion, it shouldn't require plate replacement as long as you mitigate the corrosion by coating it.

Someone else chime in if you disagree.