API 653 - Repair of Tank Shell - Code Interpretation

[JPG1021]

Hello all! I just performed an "Out-of-Service" inspection of a Fuel Oil tank for one of my clients. The examination of the shell yielded some wall thickness measurements that are in violation of the calculated minimum required wall thickness for that course. I am familiar with making repairs according to the API 653 Standard but but repair is being slightly complicated by what may be an incorrect interpretation on my behalf:

First off, some background/design info on the tank...Nominal Diameter is 36.25-feet, Fill Height is 42-feet, Material is A283, Grade C (23,600psi, max. allowable stress), Joint Efficiency is 0.85 (according to Table 4.2, 1969 Basic Standard), Nominal Wall Thickness is 0.250-inches (1/4-inch).

This repair would be pretty simple and straight forward...Cut and roll a tombstone shaped (2-inch radius upper corners) piece of plate large enough to extend past the area of concern by at least 1-inch and attach it to the shell using a full fillet weld around the perimeter. However, my interpretation of articles 9.3.3.1 and 9.3.4.2 are giving me some trouble. They both indicate that I need to calculate the thickness of my repair plate using a joint efficiency of 0.35 and when I do that, I come up with a number that exceeds a 30% or 0.125-inch (1/8-inch) increase of the shell plate material, which is not allowed according to 9.3.3.2 and 9.3.4.3.

Any advice?

 

[Mito57]

My interpretation is is that you can't use a lap patch for your repair in this case. It does not meet the API 653 Code required conditions with your tank's parameters. You will have to cut the shell and install an inlay patch.