storage tank annular plat bottom corrosion

[scipion]

Dear all,

during internal inspection of a floating roof storage tank, we have detected external corrosion of annular plat of the floor

this corrosion is localised in the 100 mm inward from the shell and sems to be a goove corrosion with the remaining thickness of 2 mm in some areas

the legs of the weld of the shell to bottom is free from corrosion and the thickness of the projection of the annular plat outside the shell is 4 mm

according to API 653 table 4.5 the min thickness of the annular plat for our case is 0.17 in (stress < 24300 psi and thickness of first shell course < 0.75 in)

my question is:
Before changing the annular plat, it is tolerable to put the tank in service with a maximum fill hight of 6 meter (insted of original maximum fill hight 17 meter) corresponding to a stress of 7597 psi of the first shell course

what is the type of failure if there is any risk to put the tank in service

the tank characteristics are as follow:

hight: 17.16 meter
diameter:35.2 meter
thikness of first shell cours: 17,8 mm
bottom thickness: 6 mm
annular plat thickness: 9.5 mm
storage volum: 16620000 Litres
storage fluid: stabilized oil

 

[MJCronin]

A few questions:

- How does the tank bottom drain ? Does the slope of the bottom drain from the tank axis toward the shell ?

- Are there water drains on the tank shell ?

- Is there a history of water inside the tank ?

- Was there a corrosion resistant coating or lining on the floor of the tank ?

MJCronin
Sr. Process Engineer

 

[scipion]

HI MJCRONIN,
thank you for your reply
the water is drained via a sump and the slope decrease from the the center of the floor to the the shell
the water is drained every time (every 3 days)before starting to pump oil from the tank
the tank is coated with epoxy coating and protected using sacrificial anodes
i will just add that this corrosion is from the soil side because the infiltration of the water under the floor
the bottom experienced holes in the annular plat in the critical zone.

 

[ali.pk]

@scipion (Petroleum)(OP)

Please check whether your Tank bottom plates and annular bottom plates have been joined by a lap joint? If yes, then do the original design meet the requirements of Three Plate Lap Joint as per clause 5.1.5.4.1 and 5.1.5.4.2 of API 650?

I'm getting a feeling that such localized thinning in the critical area is due to the this particular reason and liquid is puddling during draw down.

Regards,

Asad Ali