API 650- 1-ft Design Method vs. Variable Design Point Method

[TarikY]

For API 650 tank shell thickness calculation, what are the reasons why someone would use the 1-ft Design Method over the Variable Design Point Method? I know that API 650 5.6.3.1 states that the 1-ft method shall not be used for tanks larger than 61 m (200ft) in diameter. If I have a 50 ft diameter tank, why should I favor one method over the other?

 

[Geoff13]

The 1-foot method usually results in thinner required thicknesses for "small" tanks, and variable point is thinner for "large" tanks. There is no particular size where this switch takes place.

Having said that I would guess that your tank is "small", but since minimum thickness (clause 5.6.1.1) will govern many of your upper rings the savings will be minimal. Try both methods on your tank and see the difference for yourself.

 

[HTURKAK]

5.6.3.1 states that the 1-ft method shall not be used for tanks larger than 61 m (200ft) in diameter. If I have a 50 ft diameter tank, why should I favor one method over the other?

I did not see your point.. you have 50 ft dia tank and you do not want 1- foot method is that true?
API 650 is STANDARD for Welded Tanks for Oil Storage . The rules for SHELL DESIGN ;

- 1 foot method = if D ≤ 61 m and check for minimum thickness as Geoff13 stated..

- Variable-Design-Point Method = if D › 61 and, (L/H) ≤ (1000 /6), and ( This method may only be used when the Purchaser has not specified that the 1-foot method be used ).. and check for minimum thickness as per 5.6.1.1

- Calculation of Thickness by Elastic Analysis = if L/H is greater than 1000/6 ...and check for minimum thickness as per 5.6.1.1

Other codes ,e.g. BS EN 14015 only use 1-foot method and does not allow Variable-Design-Point Method.

 

[IFRs]

Why would you use VDPM? Per API 650 5.6.4, using VDPM "normally provides a reduction in shell-course thicknesses". That's the reason. For your tank you will see very little if any benefit from using the VDPM but feel free to run the calculations to see for yourself. VDPM is a good tool to be familiar with. As mentioned above, you can use the VDPM only if the purchaser allows it, and you must also check the minimum API thicknesses.

 

[JStephen]

The reason to use 1-foot instead of VDPM: Simplicity, especially if doing manual calculations.
I think part of the motivation behind VDPM was to allow larger tanks to be built within the maximum shell thickness limitations, which is not a factor for most tanks.