What is Supporting the Annular Outer Ring at Bottom Draw-off Drains per API 650 Appendix O?

[yamoffathoo]

A 40ft x 32ft high diesel storage tank at my plant has provided excellent service since 1978 with some minor pitting on the underside of the floor plates, which have been patched.

Recently, localized thinning below 0.1" of the 0.25" outer annular ring (chine) adjacent to the 12" wide concrete cut-out for the 3" bottom draw-off pipe has raised some concerns.

Appendix O of API 650 indicates (to me) that the loss of support and torsional resistance of the chine and critical zone over the cut-out is provided by the draw-off pipe and structural bracing that is welded to the underside of the middle of the chine.

In my opinion, localized thinning of the chine at the edge of the cut-out should not require a major alteration to replace this section of bottom plate.

Has anyone performed a Fitness for Service Assessment for localized chine thinning at cut-outs and if so, is there a hand calculation that can quantify the extent or span of allowable chine thinning in this area rather than a parametric study using FEA?

 

[HTURKAK]

I read three times your post and still i am not sure i understand your thread. However , this is not the scope of API 650 . You should look API 653 (Tank Inspection, Repair, Alteration,and Reconstruction )..

 

[Dhurjati Sen]

1. Is the corrosion external only?
2. Did you carry internal inspection of the tank after the thinning was noticed?
If the answer to 1 is yes, then find out the reason. Possible causes could be rain water accumulation, presence of acidous gases nearby, etc.
If the answer to 2 is no, then carry it out asap.

 

[yamoffathoo]

HTURKAK, Instead of a ring wall with an "opening", this tank has a 5 foot thick reinforced concrete slab, which is why I call the missing concrete a "cut-out". Figure O.1 of API 650 indicates a beam bridging the ring wall "opening" located inside the tank, however my tank does not have this member. If you agree that the annular ring cannot support the shell where it bridges the 12" wide cut out, and there is no internal bridging beam, then you should agree that shell support and torsional resistance of the annular ring is provided by the draw-off pipe and structural bracing that is welded to the underside of the middle of the chine. I would compare the vertical "support plate" to a keel on a sail boat that prevents rotation by pushing against the water, which would be the 3"nps draw-off pipe that is welded to the bottom plate.
If this is the case, then localized thinning of the outer edge of the annular ring that is suspended over cut-out would have negligible effect on preventing rotation of the shell.
I suppose FEA is the only way to demonstrate this by showing compliance with API 650 Design.
I would run the 'as built' model to benchmark stresses, then increase cut-out width and reduce outer annular ring radial length to determine the response in shell stress.

 

[yamoffathoo]

Dhurjati Sen, As described in my post there is "some minor pitting on the underside of the floor plates, which have been patched", which includes the bottom plate over and adjacent to the "cut-out" area. The reason for external annular ring thinning is organic growth in soil caught in the grout cracks that are exposed to sunlight and rain. The vegetation was cleaned out before the photo was taken.

 

[yamoffathoo]

HTURKAK and Dhurjati Sen, I have attached the Drawing for discussion. I fitted a polynomial curve to the dimensions in API 650 Table O.1 and most drawing dimensions agreed when extrapolated to 3"nps.

 

[HTURKAK]

If you agree that the annular ring cannot support the shell where it bridges the 12" wide cut out, and there is no internal bridging beam, then you should agree that shell support and torsional resistance of the annular ring is provided by the draw-off pipe and structural bracing

I disagree with this interpretation. However you can assume ( model with FEM and see ) the shall can easily support the internal bridging beam .
My points ,
- Perform inspection of the tank as per API 653,
- If the remaining thickness of annular plate at exterior , 0.15 in, this is acceptable .( API 653 suggests greater than 0.10 in. )
- Repair as necessary as per API 653.

 

[yamoffathoo]

HTURKAK, to you points:
- My tank has no bridging beam
- API 653 Inspection has been performed
- The recommended API 653 repair involves a "major alteration" involving jacking up the tank, cutting out the entire bottom plate covering the 36" x 12" cut-out including the 3"nps draw-off piping and supports, then sliding in a 42" x 18" replacement bottom plate complete with 3"nps draw-off piping and supports and welding it all back to the shell perimeter and joining it to the existing floor plate, probably with a massive patch and lap welds.
- The replacement 3"nps draw-off piping and supports will likely need to be modified to be in complaince with Annex O of API 653.
- All this for localized thinning of the outer annular plate thickness at the edges of the cut-out, where it is essentially performing no structural function?

 

[Dhurjati Sen]

"All this for a localized thinning of the outer annular plate....."
YES as yours is a mounted tank.
The onus of this scenario lies on Civil Maintenance & Inspection Department as the vegetation issue took quite some time (maybe months) to cause such thinning all the while those guys were sleeping!
However, as you have already externally patched the annular plate you are TEMPORARILY ok.
Mark this as a temporary repair in the history card of the tank and carry out frequent inspection of the repaired portion.
Also make a note that API 653 repair needs to be carried out EVENTUALLY.
Cheers!

 

[yamoffathoo]

Question for any Storage Tank Stress Analyst out there: "Do you know the basis for the limiting dimensions in API 650 Annex O Table O.1a or b?

 

[yamoffathoo]

A consult with the tank manufacturer revealed that this cutout should have been filled with grout before the original hydrostatic test back in 1978 !
That's quite a testimony to the conservatism in API 650.