API 653 Lap patch maximum size restrictions?
API 653 Lap patch maximum size restrictions?
(OP)
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".
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".





RE: API 653 Lap patch maximum size restrictions?
RE: API 653 Lap patch maximum size restrictions?
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
RE: API 653 Lap patch maximum size restrictions?
RE: API 653 Lap patch maximum size restrictions?
RE: API 653 Lap patch maximum size restrictions?
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?