Formulas in Roark's Formulas for Stress and Strain.
You can approach this in a simplistic way as a plate with simply supported edge.
Or you could calculate edge moments and rotation/deflection in the cylinder as well, which should give you a little better idea of stresses in the welds.
With pressure that high, thickness or weld size may be unreasonable.
If this is part of a pressure vessel, use formulas and detailing from the pressure vessel codes.