Bathtub Ring Corrosion on Crude Tank with Geodome

[apigui]

Hey all,

One of the projects I'm working on:

A large crude tank was found to have a roughly 30'' wide band of corrosion on the interior of the top shell (~60' diameter). From inspection reports, it didn't look too bad so we just planned on putting lap patches as needed.

However, once the area was sandblasted it turns out that the corrosion was much more severe than expected. There are large areas of corrosion throughout this 'band' that would not meet the minimum thickness required to support the geodome.

API 653 has a maximum size of lap patches so I'm pretty sure we cannot just butt weld lap patches throughout the entire course of the shell. A partial solution to alleviate the compressive load of the geodome would be to add vertical stiffeners from the top windgirder (located on top course) to the intermediate windgirder. However, we are still looking for solutions regarding patching the areas of corrosion while staying compliant.

The solution must be cost and time effective..we cannot just replace the entire top shell course as we have business drivers, and the tank needs to be back in service by mid January.

Any thoughts?

 

[JStephen]

I would say there are two issues, one of insuring the integrity of the tank, and one of complying with the tank standards.
On the former issue, lowering the working level below the corroded area and possibly adding the stiffeners as mentioned would work.
On the latter issue, I would think you'd need to replace shell plate, either the whole ring or a strip around the tank. I'm not sure how practical it would be to insert a 3' wide strip into an existing shell without removing the roof, etc.
As I recall, API-650 doesn't include any allowable stress for non-seismic shell compression, so you might want to revisit that calculation some. API-620 does include allowable compression for cylinders. You might be able to use average thickness instead of minimum thickness or similar approaches to show adequacy.