In the past ten years, there have been a few oil storage tank annular plate failures due to soil side corrosion and fatigue loading (filling and emptying) which has led to large spills. The corrosion tends to be localized in a groove fashion and the size of the flaw could vary from 3 feet to 12 feet in the circumferential direction. The location of the corrosion is also where the highest bending stress will occur during filling and emptying of tanks. See Figures 1 and 2 for the location of the failure on the annular plate.
Since the location of failure is under the tank, the corrosion flaws cannot be detected easily from the outside of the tank. It can only be detected if an internal inspection is done which would require the tank to be emptied and cleaned and this is very expensive. However, with UT shear wave technology there is a possibility of identifying these kinds of flaws from the outside but accurate sizing of these flaws is not achievable. Due to this ambiguity, a risk-based approach is recommended to determine which flaws can be left behind and which would need more inspection and may require repair.
For this risk-based approach development, finite element method was used to determine limiting flaw sizes. Various sizes of tanks and flaws were modeled in 2-D axisymmetric as well as 3-D finite element to determine stresses and fatigue life. Figure 3 shows a typical axisymmetric model used for the study.
Figure 4 shows the maximum von Mises stress in the thinned area with increasing width of the local thin area. From the plot it is seen that the stresses decrease when the thinned area gets larger and then it plateaus off.
From the 2-D and 3-D finite element stress analysis, the conclusions were as follows:
From the study a risk-based approach combined with external inspection was developed. This helped provide significant cost saving versus internal inspection while giving the Owner confidence in a reliable and safe operation.
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Nadarajah ("Ranjan") Chithranjan's career has spanned over 18 years of worldwide involvement in petrochemical industries for ExxonMobil Research and Engineering. Prior to joining Becht Engineering, he worked as a fixed equipment specialist at ExxonMobil Research and Engineering and he has worked in more than a dozen countries worldwide to solve complex mechanical plant problems as well as mechanical support for large scale projects. He has extensive knowledge in pressure vessels, piping, and storage tanks, design and maintenance codes.
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 ckness: 9.5 mm
storage volum: 16620000 Litres
storage fluid: stabilized oil