API 650 - Tank stiffening ring calculations

[EmilianoS87]

I have encountered a standard interpretation issue that has raised some doubts. According to API 650, the following is stated:

"5.9.1.3 All tanks shall be stiffened by intermediate wind girders when required by 5.9.6. Vacuum stiffeners, when required by Annex V, may be considered to also act as wind girders when the spacing and stiffeners meet all other requirements of both this section and Annex V."

In my case, the shell stiffness must be reinforced to meet external pressure requirements specified in Annex V, but wind girders are not required under 5.9.6. Specifically, the condition in 5.9.6.1 states that H1H_1H1 is greater than the transformed height of the tank shell HTSH_{TS}HTS, meaning wind girders are not mandated.

To comply with Annex V, I have added stiffeners that act as vacuum stiffeners. However, my question is as follows:
Since wind girders are not required in this case, do I still need to comply with the section modulus and other requirements outlined in 5.9.6.6?

Thanks for your help.

 

[IFRs]

You have to satisfy all sections of API 650 that apply to your tank, and all load cases. If section 5.9 is satisfied without additional stiffeners you have complied with that section. If you are designing an Annex V tank then you need to satisfy that Annex. If your tank needed stiffeners per section 5.9 and Annex V apparently you are allowed to use the Annex V stiffeners to help comply with section 5.9.