doubt with background repair according to API 653 1

[Andres Romero]

Hello, I work for a client who wants to comply with part 9 of API 653 to repair a bottom that has corrosion on the bottom.

In principle we had considered installing a May 24" patch, but API 653 does not allow it in the critical zone. So we have had to divide it into 4 patches so that it can comply with figure 13 of API 653.

In my opinion, I think it is better to have installed a patch larger than 24", even if it does not comply with the requirements of API 653, than to install 4 patches very close to each other in order to comply with the maximum patch size and minimum distances between welds.

The damage mechanism is corrosion due to pitting located at the bottom. There is no widespread corrosion.

Why can't I install the patch larger than 24"?

What do you think is the best repair if the client does not want to replace the sheet metal?

I attach a photo of the situation of the patches.

 

[HTURKAK]

Consider new plate rather than patch plates.
...
9.10.1.2.4 If more extensive repairs are required within the critical zone than those listed in 9.10.1.2, the bottom
plate welded to the shell shall be cut out and a new plate shall be installed. Weld spacing requirements shall be in
accordance with 9.10.2.4, and API 650, Section 5.1.5.4 and Section 5.1.5.5. The shell-to-bottom weld shall be
removed and replaced for a minimum distance of 12 in. on each side of the new bottom plate.
..

He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[Andres Romero]

Hello HTURKAK, I already know what the code says. But I want to understand why?
I think that in the case of very localized corrosion in the lower part of the bottom, it is better to install a large patch, even if it does not comply with API 653, than to install those 4 patches that do comply with the recommendations of API 653.
If it were generalized corrosion, yes I agree not to exceed the patch dimensions in the critical area, but since there are 4 very close and localized areas, I do not understand why a patch with a dimension greater than 24" cannot be placed.

 

[HTURKAK]

- Refer to Figure 9.13 NOTE 2 ( Minimum distance between two welded-on patch plates in the critical zone will be one half of the patch width )

- Your opinion to install a patch larger than 24" at critical zone is not acceptable at critical zone .Refer to Figure 9.13 NOTE 3 .The reasons are , to minimize the stresses due to metal temperature and provide shell to bottom joint flexibility . ( the theory of shell design assumes boundary conditions for the analysis fully plastic moment shell develop caused by
yielding of the plate beneath the shell , so the thick. of bottom plates less than the shell thk. at first course ).

- IMO , you are free not to comply API 653 requirements if NON STANDARD repair is acceptable. If you want repair acc. to API 653, strictly follow the standard.

He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[UseTJ]

This almost looks like you could group the patches better. Hard to know without a tape in hand. 24" is max width of patch. The three left side of photo patches almost like you could fit 24" width and something longer like 36" inward toward center of tank. It may cover more than one corroded area and will help minimize weld spacing issues.

You could also have an FEA performed and install an oversized patch. You may want to avoid new plate if it's going to push you into hydrostatic testing.

You mentioned no widespread corrosion, however you also have existing patches in the area you need more repair. Could have been poorly placed initially and missed overlap/additional indications or you have a local area of concern. Potentially worth investigating.

Edit: another option is have them reduce their interval. Not a perfect solution but another option. Can they take credit for additional safeguards? What is the diameter of the tank? Or will they internally coat? You may be able to reduce the MRT or corrosion rate if the correct details are in order

 

[IFRs]

The intent of the standard is to provide an owner a clear path to maintaining their expensive capital equipment, knowing that the interval between inspections can be long, corrosion is not perfectly predictable, external and service conditions can change and risk is never zero but can be lowered to an acceptable level for all stakeholders.

There has to be some limit to what you can do and still have a reasonable expectation of low risk to the owner and the public. The limits are based on hard science and math but may also include good engineering judgement and for sure consensus of the committee. I'm sure the limits were determined after very careful consideration of a large group of very smart and experienced folks with expertise in many areas.

When you cross the limit, you are on your own, and any future owner may be at increased risk. If you want to base your repairs on your opinion that's fine as long as the owner agrees to shoulder the present and future risk. It is unlikely that you could get an engineer to PE stamp a repair that was not in accordance with the standard, but that may not be an issue for you.

It looks like repair #4 partially crosses an existing lap patch, which I think is not in the standard. It looks like repair #2 totally surrounds an existing lap patch, also not in the standard.

Unfortunately there is no simple calculation disclosed in the standard that you can compare your situation to (assuming you know everything about the existing and future conditions). You may be able to validate your opinion with FEA or other analysis, then agree with the owner and take responsibility that spending more money to fix it per the standard is not needed.

Sometimes there are grey areas that benefit from a closer look and careful evaluation, but at some point you just recommend that money be spent and everyone can sleep soundly knowing that they took the the safe and documented approach.

 

[Andres Romero]

Hello, patch 4 was installed incorrectly. The weld was less than 150 mm from the casing so it had to be a stone type. Patch 2 covered one corrosion and now had to cover another nearby one. Since the distance to the welds could not be met, it had to be removed and made larger. They had to be designed like this to comply with API 653. I am well aware of the API 653 recommendations regarding bottom and casing repairs, but sometimes I think that installing these 4 patches is worse than having installed 1 slightly larger one, although It would have slightly exceeded the maximum size recommended by API 653.