Hydro Test Exemption with Door Sheet Cut to Bottom

[DSTankee]

API 653 repair scope to an asphalt storage tank includes a bottom replacement and some nozzle relocation (the shell surrounding the nozzles were cut so that the nozzles would be installed as inserts maintaining the same centerline elevations with respect to the new bottom). As there was heater coils in the tank, a door sheet was removed for ease of access and removal or coils while removing and replacing the tank bottom. After the door sheet and nozzles were removed we were advised that water will not be available for hydro test.

At this point we can no longer satisfy 12.3.3.3.8 "Door sheets shall comply with the requirements of this standard for shell plate installation, except they shall not extend to or intersect the bottom-to-shell joint." 12.3.3.7 allows for a fitness for service evaluation to be utilized for exemption as well. Is there a established standard practice listing acceptance criteria for FFS in this case?

Typically this would be achieved by an reviewing the existing and new material MTRs, establishing acceptable WPSs, developing an ITP, and calculating the shell and shell-to-bottom stresses. In the past I've seen the formulas in API 650 5.6.3.2 used for checking shell stress. Unless The tank is designed for elevated temperature what reference (aside from performing FEA) could be used to evaluate the the shell-to-bottom stress?

 

[HTURKAK]

I looked to this post a few times. I have API 653 5t h ED. Probably item 12.3.3.3.8 at a new edition.
My points,

- shell insert plate and door sheet welds by API 653 12.2.1.6 , vertical and horizontal shell joints as required ,
12.3.2.3.5, specific hydrostatic test exemption requirement.

- API 650 and 653 suggests formulas for hoop stress of the shell .What is the reason to evaluate the shell-to-bottom stress? I would like to remind (5.6.5 Calculation of Thickness by Elastic Analysis
For tanks where L/H is greater than 1000/6 (2 in USC units), the selection of shell thicknesses shall be based on an
elastic analysis that shows the calculated circumferential shell stresses to be below the allowable stresses given in
Table 5.2a and Table 5.2b. The boundary conditions for the analysis shall assume a fully plastic moment caused by
yielding of the plate beneath the shell and zero radial growth

- The following snap is copy and paste from Exxon Mobile doc. ( Tank Inspect,Repair, Modification & Test MP 14-P-08 )

5. Inspection and Testing of New Tank Bottom and Shell Welds
...
· All tank bottom welds shall be tested using vacuum test equipment with a vacuum of 250 to 350 mm Hg (10 to 14 in Hg). Alternatively, a helium leak test may be performed if the procedure is submitted to and approved by The Company.
· Shear wave (or angled-beam technique) ultrasonic inspection shall also be used to examine all butt-welded joints in annular plates. (The shear wave technique is not useful on "lapped" joints using fillet welds.)
· A magnetic particle test or liquid-penetrant test shall be used on welds around sumps, corner joints and new or relocated nozzles.
· Prior to completing the exterior weld, the interior bottom-to-shell weld shall be tested with the oil-penetrant and vacuum box tests.
· The contractor shall fully radiograph all doorsheet welds except the area of the shell-to-bottom weld.







Use it up, wear it out;
Make it do, or do without.

NEW ENGLAND MAXIM

 

[DSTankee]

@HTURKAK, I suppose the lingering question is what is the API committee's intent for paragraph 12.3.3.3.8 restricting the verts of the door sheet from extending to the bottom when no such restriction is place on butt welded inserts or shell replacement at the lowest shell course. Or am I missing something?

 

[HTURKAK]

The paragraph 12.3.3.3.8 does not exists at API 653 5th edition. But you stated "Door sheets shall comply with the requirements of this standard for shell plate installation, except they shall not extend to or intersect the bottom-to-shell joint." and this clause at 12.3.2.3.8 at API 653 5th edition.

Referring to your first post , the repair scope includes bottom replacement. The common practice for bottom replacement is shell is slotted completely 100-150mm above the bottom .
The new bottom is placed on spacer material ( sand or concrete ). In this case , the restriction of the verts of the door sheet from extending to the existing bottom will not be concern and no need to extend to shell-to-bottom weld. ( Pls look Figure 9.5—Details for Door Sheet in Butt Welds Shell Seam Tank—Tank-Vertical Seam Offset )

May be i am missing something. If you can provide some sketches and the full clause 12.3.3.3.8 , you may get better responds..




Use it up, wear it out;
Make it do, or do without.

NEW ENGLAND MAXIM

 

[DSTankee]

@HTURKAK, I have API 653 5th Edition, Addendum 2 (May 2020). Below is a link to the section 12 hydro static testing exemption requirements.

I am familiar with the practice of slotting the shell that you mentioned. Whereas we have also install leak detection and liners above the existing bottom, placed sand or even concrete with grooves leading to leak detection ports at the shell, before installing a new bottom into the slotted shell. In this particular scope, this was not the case. The existing bottom was cut at the upper toe of the cornerweld (minimizing a reduction in usable capacity) and removed. The new bottom was placed on the existing foundation.


[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1706815022/tips/Pages_from_API_653_Fifth_Edition_May_2020_-_Hydrostatic_Testing_Exemptions_kczjkf.pdf[/url]

 

[HTURKAK]

@DSTankee ,

I just saw your last post and looked to the addendum you have posted.
In this particular case, since the tank operates at elevated temperatures , bottom of first course will be more prone to fatigue failure. Because , cutting at the upper toe of the cornerweld will increase the brittleness. My suggestion would be to speak with a welding expert.
The conn. of shell to bottom always a headache and there is no simple solution. The hoop stress at bottom approaches to zero but the bending moment will reach to maximum.

The following figure from Structural Shell Analysis (by Johan Blaauwendraad,Jeroen H. Hoefakker )

-

Use it up, wear it out;
Make it do, or do without.

NEW ENGLAND MAXIM

 

[IFRs]

Hydrotest is required if the work is a major repair.
There is a maximum length of corner weld you can replace and not be considered a major repair.
Major repairs also include removing and replacing a section of shell that exceeds 12" in any direction.
An insert nozzle that goes to the bottom will always need an exemption because the minimum width or height is more than 12" and it disturbs enough of the corner weld.
A door sheet even though it is more than 12" in any direction that does not extend to the bottom is granted an exception to this rule and does not need a hydrotest provided additional NDE is done.

In your situation a hydrotest is required because you replaced the entire corner weld. The door sheet only adds to the requirement for a hydrotest.

Since you are doing major repairs and a hydrotest is required, the only way to get a hydro exemption despite the major repairs you are doing is a FFS evaluation. The tank engineer doing your hydro exemption will be able to tell you what analysis is needed.

 

[DSTankee]

@HTURKAK & @IFRs, Thank you both for you feedback.