Dear Linus,

I'm not able to clarify your points, but I would highlight some aspects.

First I think you made two calculations having different seismic spectra: S0= 0.28g and S0=0.36g.

Second, regarding Dynamic Liquid Hoop Forces/ stress evaluated by API/650 E.6.1.4, you may note that there are consistent differences between API 650 Basic 11th edition and API 650/ Addendum1 and Addendum 2.

Comparing the formulas, you may note that in API 650 Basic 11th, the calculated values were 4 times bigger than in corrected formulas- Add 1 and 2.
More specific, today USC formulas coefficients have been changed from 18 to 4.5, from 11 to 2.77, from 5.54 to 1.39 and from 3.9 to 0.98.

Hope these remarks would help you.
Regards,