Evaluation of existing shell patches 1

[Orakel]

Hi All,

I have a question regarding evaluation of existing patches installed on shell courses, during an inspection according API653. As mentioned in API653 section 9.3.1 'These requirements may be used
to evaluate existing lapped patch shell repairs; however, the plate thickness limits need not apply.' Where the requirements are explained in detail in the rest of section 9.3.

My question is: which plate thickness limits do not apply? The shell plate where the patch plate is installed on, the patch thickness itself or both?

 

[HTURKAK]

Pls read clause ( 9.3 Shell Repairs Using Lap-welded Patch Plates ) 9.3.1.1 through 9.3.1.10 .
A job specific INSPECTION AND MAINTENANCE PROGRAM should be prepared. Refer to ANNEX C for inspection checklist.

 

[Orakel]

Pls read clause ( 9.3 Shell Repairs Using Lap-welded Patch Plates ) 9.3.1.1 through 9.3.1.10 .
A job specific INSPECTION AND MAINTENANCE PROGRAM should be prepared. Refer to ANNEX C for inspection checklist.

Thanks HTURKAK, I do understand the conditions outlined in 9.3.1.1 through 9.3.1.10. I don't want to install a patch, but let's say I'm mister API653 inspector and I inspect an AST at a client and I see a square patch with rounded corners of 10x10 in. with a thickness of 3/4 in. , installed on a shell plate of 3/4 in. original construction. Client informs me the patch is installed around 1980.

This is not according clause 9.3.1.2 due to thickness exceeds 1/2 in., but because it is an evaluation of an existing patch, the plate thickness limits need not apply according 9.3.1. If I interpreted this correctly, this patch is OK according API653?

Many thanks

 

[IFRs]

I don't think it is your job to tell the owner if the patch is OK, but rather the results of your inspection and calculations. I would tell them the condition of the patch, the stress in the weld around the patch, the integrity of the weld, all the conditions in API 653 that the patch meets (weld spacing, etc) and the parts of API 653 the patch does not satisfy.

I would examine the patch, weld and shell within 3" of the weld, determine the stress in the shell at that point and the reason for the patch (is there a hole, etc). It is also important to determine the service (water, acid, poison, etc), the age of the tank and patch, the operating conditions (liquid level, temperature, etc) and the site conditions (exposure to seismic, extreme winds, etc). Then I would present this to the owner with the information that current API 653 would not allow you to make that patch, all in writing. If they ask your opinion, tell them how comfortable or uncomfortable you are considering all the information.

The risk of failure is what you are trying to establish and that risk is the owner's to bear, not yours. I have no sympathy for an owner experiencing a failure when they could have spent a little money to fix their tank in such a manner that leaves no doubt. Unfortunately, when a tank fails, it is often mostly the public and the environment that suffers while the owner walks away without liability or social conscience.

Sorry for the rant...

 

[Orakel]

IFRs, thanks for sharing your view on this. I agree that the thickness is not the only aspect to look at and other factors must be considered as well.

True, the tank owner wil be responsible at the end of the day, but correct interpretation of the inspection code (where owner's decisions are based on) would be the job of the inspector.

 

[HTURKAK]

I do not look everyday to the web.. IMO,

- Your interpretation ; of the old patch 10x10 in. with a thickness of 3/4 in. should be OK acc. to wording but the Shell Repairs Using Lap-welded Patch Plates is defined for thk . less than 1/2 in.

- Pls look 9.2 ( Removal and Replacement of Shell Plate Material ) for shell thk . 1/2 in or more.

 

[Geoff13]

Client informs me the patch is installed around 1980.

The rules in API 653 discussing lap welded patch plates were added in the 3rd Edition (Dec 2001).  Even the 1st Edition didn't come out until Jan 1991, so there were no API tank repair rules in 1980.

... because it is an evaluation of an existing patch, the plate thickness limits need not apply according 9.3.1. If I interpreted this correctly, this patch is OK according API653?

I don't see any words in API 653 that state the 1/2" limit doesn't apply to pre-existing patches.  I'm not sure how you are interpreting that this patch is "OK according to API 653".

The 3/4" thickness would indicate you have a fully stressed shell in operation.  A double lap welded patch has only 0.70 joint efficiency (9.3.2.3), and a single lap welded patch has only 0.35 joint efficiency (9.3.3.1).  These are pretty standard joint efficiencies so I can't understand how a 3/4" patch was justified to repair a 3/4" shell plate, even without API rules.  With this in mind, I'd lean towards replacing this with a proper butt-welded repair, even after 40 years, but I don't believe that's the Inspector's decision to make.  Or are you the "storage tank engineer" and the Owner is expecting more than just inspection results?

I'm with IFRs on being very clear with your inspection information, but being very careful to leave the decision to the Owner.

 

[Orakel]

The rules in API 653 discussing lap welded patch plates were added in the 3rd Edition (Dec 2001). Even the 1st Edition didn't come out until Jan 1991, so there were no API tank repair rules in 1980.

Correct. But you can evaluate existing patches under 9.3.1.

I don't see any words in API 653 that state the 1/2" limit doesn't apply to pre-existing patches. I'm not sure how you are interpreting that this patch is "OK according to API 653".

My interpretation comes from the last sentence in clause 9.3.1:
Lapped patch shell repairs are an acceptable form of repair for butt-welded, lap-welded, and riveted tank
shells, under the conditions outlined in 9.3.2, 9.3.3, and 9.3.4; only when specified by the owner/operator. In addition, the repair details shall comply with the requirements of 9.3.1.1 through 9.3.1.10. These repairs are permanent repairs subject to an ongoing inspection and maintenance program. These requirements may be used to evaluate existing lapped patch shell repairs; however, the plate thickness limits need not apply

An existing patch of 10x10 in. patch with a thickness of 3/4 in. fulfills the dimensional requirements of an existing patch. If it would be a new patch, yet to be installed, it does not fulfills the API653 requirements due to the thickness is over 1/2 in.

An existing patch of 80 in. wide and 50 in. high with a thickness of 3/4 in. does not fulfill the dimensional requirements of an existing patch. While the thickness is OK as plate thickness limits need not apply, the vertical and horizontal dimension exceed the max. allowed as per API653. If it would be a new patch, yet to be installed, it exceeds the max. allowable of the LxW and the thickness is >1/2 in.

FYI, these are just examples. A lot of reports of tanks with patches are coming across my desk and I try to 'judge' them as much as possible according API653. It is just the part 'plate thickness limits need not apply' sentence which got me thinking on how to interpreted this.

I agree with you that, if I report a non-compliant patch is present on a tank shell, the ultimate decision is to the Owner in conjunction with a storage tank engineer on how to proceed.

 

[Geoff13]

These requirements may be used to evaluate existing lapped patch shell repairs; however, the plate thickness limits need not apply

My apologies.  I somehow missed these words. The Code does indeed allow you to ignore the 1/2" thickness limit for pre-existing lapped shell patches.

 

[IFRs]

Your review should include a calculation of the required vs actual stength of the lap patch, using 70% joint efficiency if it is welded inside and outside or 35% if just on the outside.
I generally find that lap patches are often not possible on butt welded tanks because most of the original strength was used and there is not much excess shell plate thickness, however for lap-welded and riveted tanks there is excess shell plate thickness from the original calcs using lap or riveted joint efficiencies.

In any case we are curious as to how this winds up being resolved!

 

[MJCronin]

I would like to see some pictures and examples of acceptable and non-acceptable patches on storage tanks ...

What type of errors and non-acceptable patches are found most often ?

Does welding an exterior patch often destroy the tank internal coating system ?

MJCronin
Sr. Process Engineer

 

[Ericapi]

[/I would like to see some pictures and examples of acceptable and non-acceptable patches on storage tanks ...

What type of errors and non-acceptable patches are found most often ?

Does welding an exterior patch often destroy the tank internal coating system ? ]

There is no limit to the design (or rather lack thereof), locations and type of patches slapped upon a tank for a repair. Most common are patches installed too close to existing welds, no matter vertical seam, girth seam, nozzles. The square "no radius corners" patch is the one that grates worse for no more particular reason than that is it harder for a welder to weld and one would think that the radius corner would e an organic solution to making the job easier. (rant over)

Welding an exterior patch does burn/melt an existing internal coating, if applied.

Mr. Eric
API 510 & 653

 

[Orakel]

OK, so here is a picture of an example of a patch installed on the lower shell course, used to close former shell openings of 2 in.
I don't have pictures of the internal side, but the inside is welded as well.

This patch LxWxt = 1200x200x25 mm, installed on a shell thickness of 19.4mm In inches: ~ 47 ¹/₄" x 7 ⁷/₈" x 1", installed on a ³/₄" thick shell plate.

Because 'plate thickness limits need not apply' on existing patches, this plate is acceptable (except for the lack of rounded corners).
A new patch is not allowed due to shell thickness > 1/2 in. An insert would be required.

 

[IFRs]

Did you calculate the required thickness of the patch using 70% joint efficiency?
Did you check the weld spacing to the corner weld or what looks like a shell butt weld on the right?