Double-Bottom Crude Oil Storage Tanks 4

[Inchtain]

Hi,

Out of my curiosity, does API recommend or require the use of double bottoms for a leaking Crude Oil Storage Tank (Tank in-service), when the whole corroded bottom replacement is foreseen?

In my opinion, it will be more economical to replace the Tank floor with the addition of geomembrane underneath. Do you agree?

Which other alternatives than can be adopted for the plan from your experiences.

TANK GENERAL DATA:

Construction Code API 650 Appendix F, Edition 2001
Design Pressure/Vacuum 3.45 kPag/0.5 kPag (51 mmH2O)
Working Pressure (kPag) 0.9 ÷ 1.5
Contained Fluid/Blanketing Gas: API 38 Crude Oil / Nitrogen
Max. Operating Temperature (°C) 90
Actual Working Temperature (°C) 57
Dimensions (m) Height 14.6 - Diameter 37.2
Nominal Capacity (15,900 m³ )(100.000 barrels)
Material Bottom: ASTM A 283 Gr. C
Annular & Shell: SA A 516 Gr. 70
Roof: S235 J0
Roof Structure: S275 J0
Corrosion Allowance (mm) 1.6
Bottom/Annular/Roof Plates Thickness (mm): 8 / 10 / 7
Shell Plates Thickness (mm)
1st course 14
2nd course 13
3rd course 11
4th course & up 10

Other question is that concerned with the annular plate under the cleaning door, why it has high thickness and when it should be replaced and what rules to follow?

Thanks and regards,

 

[IFRs]

If the tank is known to leak, it must be taken out of service immediately, without question or delay. All pollution must be remediated without delay. Those should be your first priorities.

The tank owner has a responsibility to the community to provide a secure place to store the product, with design, materials, execution, monitoring and maintenance to guarantee containment.

API does not care if you use one, two or three bottoms, with or without a liner, sand or concrete spacer, etc. If you install a bottom, API has guidelines for you. The decision is based on local conditions (your tank bottom rotted away after 23 years), service, performance and most importantly the clean water / clean air regulations that you are required to comply with.

Economy of replacement should be only one factor in the design of a new bottom.

 

[Inchtain]

Dear IFS,

I agree with you and the leakage is put under close monitoring with mitigation strategy in place until the return of the other stand-by Tank into service which is currently under process of repairing.

With consideration to the environment, we are planning for maintenance scenarios and urgent requestion of repair materials (plates), based on worse predictable defects.

Any hint or advice could be very helpful

 

[Geoff13]

Other question is that concerned with the annular plate under the cleaning door, why it has high thickness and when it should be replaced and what rules to follow?

I assume this is a flush-type cleanout (FTCO) fitting per API 650 Clause 5.7.7. I find the easiest source for the FTCO dimensions and thicknesses are Tables 5.9 and 5.10. They list the required uncorroded thickness, so if corrosion has reduced it below this value it must be repaired. I would use weld repair to fill the pits before considering replacement. If you try to replace the thickened bottom plate you would have to consider if post-weld heat treatment would be required, as PWHT is required for new FTCO assemblies. API does not explain which welds they feel need PWHT, so you would have to make an engineering judgment.

If you add a second, leak containment bottom it does not need to be thickened under the FTCO as the reinforcing requirements have been met in the main FTCO bottom plate.

 

[IFRs]

Since you seem to be removing the entire bottom to dig out and remediate the contaminated soil under the bottom, going back with a single bottom over a liner and sand or concrete pad seems to be a good option. API does not mandate a double bottom.

Without additional tank / foundation / soils / corrosion protection details and understanding your risk profile as well as the environmental world you live in it is hard to know if a double bottom is warranted or practical.

Did the tank have or are you adding cathodic protection?
Which foundation type from API 650 Annex B are you using?
Which undertank leak detection system from API Annex I are you looking at?

Regarding the flush cleanout, if operations and maintenance can do without it, take the opportunity to get rid of it and install a shell insert patch plate. If not, evaluate and re-use if possible. If you are re-using it, the annular ring may need to be wider in that area for weld spacing. If the thickeed bottom has corroded badly and needs to be replaced, I think you may need to remove the entire assembly, repair it, post weld heat treat it and reinstall it.

 

[Inchtain]

Hi,

Thanks for raising interesting notes and opinions.

Here below additional information about the Tank and location:

The Field is at a place rich in archaeological significance, where attention must be given to ensure protection of both large scale (carvings and monuments), and smaller pervasive ones (Lithic scatter) as per the recommendations of UNESCO experts report of August 1999.

ENVIRONMENTAL DATA
 Maximum ambient temperature +55 ºC (131 ºF)
 Maximum steel temperature in sunshine 80 ºC (176 ºF)
 Minimum ambient temperature -5 ºC
 Average relative humidity 30 %
 Maximum wind speed (design) 45 m/s
 Maximum rainfall (design) 30 mm/d
 Altitude above sea level 850 m
 Airborne dust Fraction as high as 42% of particles less than 2 micron

I have attached the foundation details of the Tank.

Moreover, the Tank has 56 anchor bolts around the external perimeter.

Thank is internally lined and has sacrificial anode and we will upgrade the CP Protection by adding an external Cathodic Protection System in the form of Impressed Current.

The current minor leak was discovered on the running Crude Oil Storage Tank from the leak detection lines in the concrete base area.

Regrads,

 

[Inchtain]

Hi again,

I forget to show you the Company recommended double bottom design.

I am not really convinced about it and I have doubts for the reliability of the design, especially whether it is compliant with API.

Please have a look and let's us discuss.

Regards,

 

[IFRs]

This design does not comply with many aspects of API 650 or 653.
The upper bottom must go through the tank shell.
API does not include the "rectangular bar profile" and is not clear what its material or function is besides being a corrosion accellerator
Self drilling screws are typically leaks and corrosion locations, if SS they may not comply with API 650 material requirements
The "support plate" is not in API and it is not clear what its material or function is besides being a corrosion accellerator
The welded wire mesh has no real function as the concrete will crack no matter how much reinforcement you add, and the "distancer"s are likely to tear into the liner.
The concrete may inhibit impressed current cathodic protection.

Overall I would rate this as a zero and besides not being in compliance with API 650 it will likely damage the tank shell because it is welded to the tank shell, provides significant resistance to shell expansion under liquid loading and will tear the weld shell and apart at the weld joint.

I urge you to use an API compliant design and review API 650 Annex B and I.
Attached is but one arrangement that may give you some ideas.