Stiffener Circ. Stress at Tip

[rockeo]

I am designing a vessel that is 100 ft long (tan to tan) and 16 ft wide (diameter). This vessel has 23 stiffening rings and demands for more than three saddles. I have tweaked a lot with the stiffener sizes, materials and number of saddles (four) to remove all exceeding stresses but now only one remains, "Stiffener circumference stress at tip". I get rid of it by adding one more saddle but that comes out to be a non-optimal solution, since the intent is to pass the design with as low a material as possible. Any tips how can I get rid of this stress?

 

[TGS4]

A tad bit o'information would help us help you. Start with your Code of Construction. Design conditions, temperature and pressure (both internal and external), contents, etc.

 

[rockeo]

Code: ASME Sec Viii Div-1
Design temperature internal: 50 degree celsius
design temperature external: 50 degree celsius
Internal design pressure: 50 kPa
external design pressure: 40 kPa
Internal corrosion allowance: 1.5 mm
Content: gasoline, specific gravity 0.739
Hydrotest : as per Ug-99(b)
Material: SA-36 (both for elliptical heads and shell)
No wind code applies since the vessel is buried.
Seismic code: UBC 1997, zone 2A, Importance factor 1, seismic coefficient ca 0.22, cv 0.32

 

[rockeo]

Software: PV Elite 2019
And Stiffener shape that I have been using is L (weak axis out).

 

[r6155]

Use only 2 saddle supports.
This is not ASME VIII Div 1 storage tank.

 

[JStephen]

Design of low-pressure tanks on saddles becomes a problem due to the thin shell required. And that is a very large horizontal tank.
When you say the tank is "buried", does that just mean below grade? Or is it actually backfilled around the tank?
Regardless, look up Zick's original paper on saddle design (assuming you're using the Zick method via software) and see if that sheds any light on the problem. Also, check other discussions in the vessel handbooks, as some of the factors used have been adjusted by other people.
Is that circumferential stress at the tip of a stiffener or at the horn of the saddle? (I would think stiffeners would be all around and wouldn't have a "tip")
It may be cheaper to furnish that storage capacity as two or more smaller tanks.

 

[rockeo]

Check the attached file. I have managed to lower the tip stress by changing the type of stiffener ring. But the longitudinal stress at midspan is only addressed by the number of saddles and their locations.
It is backfilled around the tank.

 

[JStephen]

Based on that printout, it sounds like the program is calculating bending/hoop stresses at the inside of a stiffener and at the outside of a stiffener, and the latter case is shown as the "tip". If that's the case, adjusting stiffener dimensions/configuration would be one fix.

 

[r6155]

See rules and regulations for underground storage tank.

Regards

 

[TGS4]

See EEMUA Bulletin 190.

 

[Geoff13]

I know this isn't your question, but I'd be very nervous about having 4 saddles.

With 2 saddles the distribution is easy (50% each), but 4 will depend on the long term differential settlement, and it's pretty much guaranteed to not be 25% each. It sounds like your stresses are already near the allowables, so how much room is there for an increase in the per saddle loads?

I used 4 saddles once for a horizontal hydrotest of a vertical pressure vessel. The field sent me frequent survey results both as they filled and during the hold. Fortunately they had done sufficient ground improvement prior to the test that it went without a hitch. The settlement differences were within what I had allowed for. For a permanent vessel, how do you convince the Owner to monitor the future settlements, how do they do that once it's buried and how do they correct them if they get outside your tolerance?

No reply is needed. Just some things for you to consider.