Good Evening sirs,
what are the acceptance criteria for a tank vent?
During the construction, perhaps for a incorrect procedure welding of a stair support, we have suffered this distorsion.
Atmospheric or pressurized tank? Shape of tank? Size of tank versus size of distortion?
Maybe check Roark's to see if there are any calculations for plate stress with distortions. If there isn't, you may be able to figure out residual stress from the dent and then use that to figure out total plate stress while the tank is at normal/max operating conditions.