questions regarding repair =annular ring of a new bottom

[carthago]

we are intending to repare an API 650 storage tank.

The old tank bottom is leaking and the owner wants to add a second new bottom located say 150 mm from the old one.
The old bottom will be lined , a concrete or sand will fill in the space between the 2 bottoms.
we will add also means at this location to drain or check if they are leakages ...

My questions are:

1/- Does the new annualr ring needs to have the same thickneess as the old bottom ? or could it be thinner ??
( even if the old annular ring is in anot corrode and rests correctly on the ground)

2/-Does the new annualr ring has to protrude also from the lower shell and be about 2" from the edge ?
According to API it is so..and it has has to be welded on both sides (inside and outside of the shell )

3/- Can we not not weld this new annular ring only at the inside of the tank by providing fillet welds on top and botom of this ring


Thanks for your advices

 

[IFRs]

The new annular ring is usually not be thinner than the rest of the bottom. When there is a different thickness it is usually the annular ring that is thicker. API minimum thickness is 1/4" but corrosion allowance and other considerations may increase it's thickness (shell material, temperature, siesmic stability, etc). It absolutley must extend thorugh the tank shell and be welded on both inside and outside.