Shaping of Shell Plates

[Gustavo Barros]

Hello guys,

In the API 650 Section 6 - Fabrication, there's a Figure (6-1) that provides criteria for shaping of plates to the curvature of the tank prior to installation in the tank, when they are required or not, depending on the thickness of the shell plate and the tank diameter.

I would like to know if somebody has more information about the consequences of the plastic and elastic effects on the steel plate, as well as the behavior of the stresses on the plate, when they are previously subjected to shaping or if they are made concurrently in the field.

Thanks in advance

 

[JStephen]

The consequence are primarily cost of fabrication vs ease of erection.
If you roll a plate, you stress it past the yield point, and have residual stresses of varying levels.
If you don't roll it, you have some level of elastic stresses as fabricated. Service stresses plus those elastic stresses may exceed the yield point, in which case you have a stress situation similar to the rolled plate.
I've not seen any ill effects due to stress in either case.
If you know the exact shape of the stress-strain curve, you can calculate what the stresses either way. But that stress isn't considered in the design.
Also note that a flat plate right out of the mill may have residual stresses as well.