API 653: Out-of-plane settlement for small diamenter tanks

[Skolde]

Hi all,

According to API 653, Annex B, for settlement measurements, a maximum spacing of 32 ft around the circumference must be applied. There must be at least 4 equally spaced diametrical measurements lines (8 measurement points).
When I have large diameter tanks, the above rule works good. You can adjust the no. of measurement points to have spacing below 32 ft. For example: 60 m tank, and 20 points (distance between points is 9.4 m).
With small diameter tanks, you can measure 8 points around the circumference, but the distance between the points will be very small. For example: 6 m tank, and 8 points (distance between points is 2.4 m). From API code, this would not be a problem as soon as the spacing is lower than 32 ft. But…

The permissible out-of-plane settlement (US units) is calculate as: S = (L^2 * Y * 11)/[2.(E . H)]

Small spacing (L in the above formula), will give a small critical deflection (S). Conclusion: it becomes more difficult to be within the permissible settlement.
What does it mean? Is the settlement really not ok? Is this approach not valid for small diameter tanks? Shall I take less measurement points? What is your opinion/experience with this issue?

Thanks and regards,

 

[gr2vessels]

Steve Braune has a contact detail in the end of his presentation, but others will contribute here for sure.

http://www.api.org/meetings/proceedings/upload/Braune_EdgeSettlement.pdf

Cheers,
gr2vessels

 

[JStephen]

Out-of-plane settlement is not the same as edge settlement, which is what that presentation is about.

I heard last week that Steve Braune had passed away, and that seems to be confirmed on the Georgia PE roster.

A lot of the difference seems to be due to the H/D ratio and not absolute diameter. If you compare a large tank and small tank of the same height, the allowable deflection will be much smaller on the small tank (although measured in a shorter span, too). But if you scale the height down on the second tank as well, the allowable settlement isn't that much different than on the larger tank.

 

[Skolde]

gr2vessels, thanks four your thread.
My condolences on the loss of Steve Braune.

JStephen,
When you change the D and H, you change indirectly the ratio L^2/H. When you say that “the allowable settlement isn't that much different than on the larger tank”, what actually happens is that you adjusted your tank to have the same (or very close) ratio L^2/H. Then you will have the same allowable settlement.

For low ratio L^2/H, the allowable settlement will be also low. Usually it happens for small tanks once L is linked to D and it has a square factor in the equation. Therefore I believe that when you don't follow the rule of 8 measurement points for small tanks, and use 4 points for example, you will have a larger distance L and a larger critical deflection.

What do you think about it?

Thank you

 

[FrCivEng]

We are talking about API 653 - Appendix B - § B22 Shell settlement evaluation . The max allowable S defined at § B32 is related to the shell out-of-plane deflection, calculated from levelling measurement, then from the uniform settlement, then from the planar tilt settlement, then from the differential or out-of-plane settlement ; please note that the out-of-plane deflection value, to be compared to the permissible S, is different from the out-of-plane settlement value .
On a small tank, S is reduced and this is not surprising considering the impact a significant differential settlement could have on the shell deflection, particularly on an earth foundation without RC annular ring which is usual for small tanks ; however, as it is easier to achieve the whole foundation levelling and compaction on a small tank, you should easily respect the permissible S condition