IFR atmospheric storage tanks - double deck vs single deck 1

[racp12]

For an atmospheric storage tank, capacity 170 Mbls, internal diameter 44 m, fluid methanol, it is required internal floating roof, full contact with the fluid. Code is API 650 11th ed.
My concern is regarding the type of IFR. Single or double deck?

 

[racp12]

I have found very useful the following page:

http://www.astanks.com/EN/Floating_roof_EN.html