Something unusual in the system. What is the magnitude of stress in MPa? Usually waterhammer will not excite shell modes of vibration. If the frequency is very high then, beam models will not be a valid approach to check the stresses. If the frequency is very high it will be just like AIV where the frequency ranges from 500Hz to 2000 Hz.

Anyway, I'm also confused with the physics in this case. Please correct my statement if I have made wrong ones.

Any other opinion is highly appreciated.

Cheers!!!