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:
- The width of the flaw is critical. The smaller the width the higher the stress and it will take lesser fatigue cycles to fail.
- The calculated maximum von Mises stress on flaws which are 12” or larger in the circumferential direction is approximately the same. Flaw sizes less than 6” circumferential length will result in stresses which are smaller in comparison.
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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