Repair of lamination in plate corner 1

[KiwiTanks]

We have a tank due to come out of service shortly for an internal inspection. During the external inspection (and NDT) we found that there was a lamination in the corner of one of the plates - ie above a horizintal weld, and to the right of a vertical.

Although it is up in the 2nd-to-top strake, it is regularly below the product level, and has been identified as in need of repair.

What is the best way to replace/repair this section of the tank?
a) Should the entire weld Tee be removed and replaced (as in the patch on the right hand side of API653 Figure 9-1),
b) Should the 2nd-to-top strake plates be removed and replaced as per the section top left on the same diagram, or
c) Remove just the corner of the plate, replacing only the problem section of plate, with a 3 square corner 1 rounded corner plate, and re-weld the vert and hori welds as if they were the originals?

 

[Bonswa]

How large of an area is the lamination?
What's the nominal thickness?
What's the depth of the lamination?
Is there any internal or external corrosion going on?

If it must be repaired, use option A. It's simple. Just make sure to keep your weld spacings that are identified in that same figure.

 

[KiwiTanks]

Going from memory here - the lamination is in 6mm plate, with an area of about 300mm square. The lamination comes to the surface and up to about 2mm deep. There may be minor internal corrosion, but nothing significant at this stage.

 

[Bonswa]

I'll be honest with you. I wouldn't repair this using an insert plate. Reason being is this...

Even with the 2mm deep lamination, you still have 4mm of shell thickness that's left over. it is most probably still above t-min for that course. I say that because you siad that it's in the 2nd to top course. I'm not sure how high the tank is. You may want to see if that 4mm thickness is below t-min or not. If it's not, then you dont need to install an insert plate. at MOST, you can grind down the entire area until the lamination is completely gone, IF YOU WANT. You're talking about a 300mm x 300mm square or approx. 1ft x 1ft.

If you dont have any significant corrosion, and that 4mm is still above t-min for that course, there's no need for an insert plate.

One more thing...
Which surface is the lamination open to? internal/external?

 

[KiwiTanks]

I'm pretty sure it's external.
I'm going to go back to the NDT company for a copy of the results as I can't find mine easily - as I said I'm just going from memory from the external insepection performed a while ago.

 

[Bonswa]

oh ok...

you should just leave it alone. Like i said, at most, while it's out of service, just grind it down, PT afterwards, and paint over it once it's gone. Do some UT in that area to get the lowest thickness and make sure you're still above t-min.

honestly, though, i would leave it be.

 

[Bonswa]

Just to follow up on this... The manufacturer actually has lamination tolerances for new construction. It is possible, and most probable, that the lamination was there the entire time. Below is the citation from API-650. hope this helps.

6.2.4
a. The Manufacturer shall visually inspect all edges of shell and roof plates before installing the plates in the tank or before inserting a nozzle into the plate to determine if laminations are present. If a lamination is visually detected, the Manufacturer shall ultrasonically test the area to determine the extent of the laminations and shall reject the plate or make repairs in accordance
with 6.2.4b.

b. For laminations found not exceeding 75 mm (3 in.) in length or 25 mm (1 in.) in depth, repairs may be made by edge gouging and rewelding to seal the lamination. The Manufacturer shall submit the edge repair procedure for Purchaser acceptance prior to the start of fabrication. For laminations exceeding these limits, the Manufacturer shall either reject the plate or repair the plate by
entirely removing the lamination. Before making such repairs the Manufacturer shall document the extent of the lamination and submit a case-specific repair procedure for Purchaser approval.

 

[KiwiTanks]

Thanks Bonswa,

I have found a copy of the report, which showws it is up to 2.5mm deep, but may be corroding inside the lamination. Grind and paint sounds like the best option, as it is high up ion the tank, in a low stress area. The main lamination is only 40mm x 50mm, but the surrounding plate also has some defects.

The worry with the tanks here in NZ is that some of them have been built to unknown codes, and sometimes by cowboys. The welds may never have been checked, and often the plate was whatever they could get their hand on at the time.

Check out the welding and the lamination in the attached photo....

 

[Bonswa]

You're most probably allowed to go down to 2.5mm for that course. If that's the case, then you're still going to be above t-min for that course.

you got it! just grind it out, PT afterwards, you'll be fine.

 

[chicopee]

If remaining shell plate material has a thickness greater than 50% of original thickness, do a weld overlay and grind smooth.

 

[MJCronin]

That seam welding is'nt really beautiful either....