API 653 Lap patch maximum size restrictions?

[uncletank]

We are currently performing an assessment of a tank which has been repaired at some stage in the past using a lap patch that covers the full circumference of the first strake/course. The lap patch has been fillet welded directly to the floor projection and to the first strake. The height of the patch is 10in. The total patch is made up of a number of plates - these are joined with butt welds.

The tank properties are as follows:
Tank Height = 56’
Tank Diameter = 84’
Safe filling height = 47.583’
Product is Diesel.
First strake thickness (originally) = 14mm
Patch thickness = 12mm.

This patch obviously does not comply with API 653 due to the size limitations and the thickness of the first strake

I cannot really see any major problem with this detail except if there are locations where butt welds in the shell are close to butt welds in the lap patch.

I am thinking the restrictions in API 653 are due to brittle fracture considerations.

My questions are as follows:

1) Has anyone encountered this kind of thing before, and if so how was the patch removed or accepted?

2) Why does API 653 put a limit on the maximum patch size, and restrict the use of lap patches in strakes exceeding a thickness of 1/2".

 

[IFRs]

I've seen this type of patch before, sometimes called a "belly band". It was done either before codes prohibited it, in ignorance of the code or in violation of the code. If yours is a water tank, the downside risks are low. But if your tank stores hazardous or regulated liquids I'd think your risk is too high to let it go. Last month removed a similar patch by removing and replacing the entire bottom course of steel up to the first round seam.

 

[uncletank]

Thanks very much for your reply IFRs,

Could you explain a little why the risk is so high? (Looking at it from a purely structural point of view it seems that installing a "belly band" would add strength to the bottom strake).

I really want to understand why there is a maximum size limitation on the patch size to API 653, and the effect it has on a tank to exceed this size. (i.e. will it cause the tank to fail etc).

Thanks for your response in advance

 

[IFRs]

By "risk" I meant having to defend yourself and your company in the event of a failure when the judge says "why did you make a non-code repair?" or "if you knew the tank was out of code, why did you use it?". API 653 is a good code to hide behind - if you follow the code and there is still a failure, you and your company will have a much easier time of surviving financially. I know this did not answer your question. It may be that the API committee felt that if the tank needed that large a patch then it really needed to be fixed properly. I'll gtry to dig through my notes and see if I can find anything. In the meantime perhaps one of the many smarter folks than me will chime in.

 

[uncletank]

I appreciate your replies and advice, Thanks IFRs

 

[KiwiTanks]

I understand that the code doesn't allow this, but is there any technical reason for not allowing this type of arrangement?

For example in a seismic event will it promote buckling in the area above the band to a greater extent than if it weren't there? or will the additional heat from welding on the band reduce the srength in the shell plate?

I have seen a few tanks with this type of repair, and highlighted it as non-compliance to the API653 code, but from an engineering perspective is there anything wrong with it?