Symmetry is commonly used to model pressure vessels. Symmetry boundary conditions provide numerical stability.
If the issue is numerical stability caused by small unbalanced nodal forces, you can try activating "weak springs" in the Mechanical UI. Mechanical will then add spring elements with small stiffnesses to the model at DOFs without sufficient restraint. The weak springs will carry the imbalanced nodal forces and prevent rigid body motion. You can use a reaction probe to check the amount of force developed in the weak springs. If it is very small, then they provided stability without affecting the overall model behavior. If it is large then, the model does not have sufficient restraint and you need to manually restrain additional DOFs.
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