Leakage from Tank bottom plate behind Anchor Lugs

[Nabeel Ahmed Qureshi]

At our site, one of an open to atmosphere water storage tank leaked from bottom plate when it was taken back into service after performing UT at sample locations from inside. All UT readings were well above PDT & based on UT results it was established that tank bottom plate is healthy.
Leakage occurred from tank bottom plate area behind the Anchoring bolt lug due to "External corrosion". Later investigations revealed that the leakage location was inaccessible (congested area b/w lug & anchor bolt) to perform UT from outside.Heavy mud/dust accumulation was also noticed inside other lugs of the tank which might became the Key factor for the above leakage. We also noticed that few Lugs were directly welded to the tank shell while in some there was a reinforcement pad b/w lug & the shell. Now can some one make a point about the recommended practice to be followed for welding lugs with tank shell either with reinforcing pad or without reinforcing pad to avoid such external corrosion.
Few pics of leakage are attached.

Water Tank Leakage.ppt

[ashfaqanwer]

Abnormal leakage. Must have gone something very wrong.
Have you undertaken any repair job during this inspection?

[arcpro]

Hello Nabeel,
Reinforcement pad has nothing to do with the external lug connection to shell.
You require reinforcement only where you provide any opening in a shell.
External corrosion can only be avoided by following best practices of external painting / coating of the shell.
Investigate the issue a bit more deeper that what activities were carried out on this tank. Call your inspectors and have their views also on what other inspection they made other than UT.

[mechcolor]

Anchor lugs are directly welded to the shell bottom course without any reinforcing.
The leakage which is shown in the attached figures indicate that some repair / modification job has not been undertaken properly at that particular spot.
Your idea of external corrosion at anchor lug location can only be true if you have taken this tank in service after years.
If you have some more pictures taken from the front side of leakage point, it would help me in answering you in detail.

[Nabeel Ahmed Qureshi]

Some more pics of the leakage after removal of Anchor bolt Lug

Tank Leakage.ppt

[ashfaqanwer]

It indicates poor coating protection causing corrosion and leading to a through and through leakage.

[arcpro]

Yes, the pictures are showing external corrosion which eventually resulted in complete metal loss.
Which repair method you have adopted? Patch work?

[ben]

It depicts poor inspection of tank before taking in service.
Bottom shell course is the most crucial one and needs to be inspected thoroughly.
Many a times, only a quality visual examination saves you from such situations.

[aterao]

1. It is a design flaw led under deposit corrosion failure. Who the hell can see, clean and paint that groove inside the foundation bolting lug? Normally for repair painting of existing facilities in a running plant do not have the liberty as it was done in project stage. Buddy, do not allow pocket for water retention. Close out the groove and think of other way to fasten the anchors..

2. Your painting scheme is wrong.. You have not coated the outdoor asset with UV protection coat(viz.PU).. I see chalking...

3. The welder who did the welding of the lug should look for some other jobs. See the ripples in his weld. No paint can perform with those kind of finish.. It will puncture the coating film..