Bulge of Bottom plate in a water tank

[brpillai]

Hi Experts,
I am faced with a problem of restoration of a Water tank, designed conforming to API 650, built in sand pad foundation. The tank is 48 m dia and 16 m height having cone roof supported by Shell.
The tank was ready for water filling when a bulge of the bottom plate was noticed. These bulges seem to have been created due to the peripheral settlement of the foundation.
While analyzing the issue, it is noted that PI 650 does not have any tolerance to the bottom flatness. While checking other related codes API 653 Section 3.3 provides a formula BB=0.37R, where BB ( inch) is the allowable bulge and R is the Radius of the inscribed circle in the bulged area or local depression, in ft. while applying this formula, for about 4 locations, the value of BB is almost double, say for 75mm actual vs 40mm as per API653.
The Figure B-9-Localized Tank Bottom Settlement Limits for Single Pass Welds, whereas we have done multipass welding at these locations.

I would also like here to mention that as the tank foundation is on the sand pad, with Cathodic protection anode mesh below and with HDPE liner, having all bottom and annular plates welded it is almost impossible to lift the tank for repair.

I would like to hear Experts view whether for this Water tank, is it safe to go ahead, for taking the tank into service.
Thanking you in advance and eagerly waiting for responses and comments, which could help me in overcoming this issue.

Bhaskaran Pillai

 

[LittleInch]

It's not for us to say.

See this post which is one of my favourites as it introduced the concept of the "Fat guy test".

Drilling the plate and filling underneath is quite common.

https://www.eng-tips.com/viewthread.cfm?qid=352287

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[brpillai]

Dear LittleInch,
Thanks for the concept of Fat guy Test and filling the void, if the Fat guy test shows empty space.
Regards

 

[Dhurjati Sen]

Dear brpillai,

If you do not drill the plate, then the other option is to cut out the bottom plate at the bulged location, do localized ramming and re-weld with a larger size plate.

Carry out LPI of the fillet weld joints, followed by a vacuum box test of the welds.

Seal weld the air connection location followed by LPI.

As the owner, you can waive hydro-test of the tank.

Regards.

DHURJATI SEN

https://www.nace.org/people/dhurjatisen

 

[chicopee]

Are the bulges close to the tank wall where peripheral settlement has been detected and if so do you show edge settlement as depicted in fig B-5 of of API Std 653?