API 650 - Calculation of Thickness by the Variable Design Point Method

Dear Members,

I want to design tank as per API 650 and our client request to design by the variable design point method. I have information as below

Material : SA 283 Gr. C

Nominal Diameter of Tank, D = 8.84m

Design Liquid Level, H = 4.9m

Specific Gravity, G = 1

Corrosion Allowance, CA =2

Allowable Design Stress, Sd = 137MPa ( From Table 3.2)

Allowable Design for HT, St = 154MPa ( From Table 3.2)

As per clause 3.6.4.1, this method can be used if L/H < or = 1000/6 (SI unit). and from data above, method can be used.

refer to cluase 3.6.4.3 preliminary bottom-course thickness for design and hydrotest, shall be calculate first as per formula in clause 3.6.3.2.

so, i got tpd = 3.45mm

while refer to clause 3.6.4.4, i got design bottom-course thickness t1d=3.61mm and refer to note that for design condition, t1d need not be greater than tpd, but i got opposite.

Could any one explain to me regarding this matter.

Thank you.

Faizol