There are numerous instances where an Aboveground Storage Tank (AST) must be elevated for tasks such as foundation repair, relocation or reorientation.
AST bottom supported using cable during the lift stage
Over the years, inspection technology has advanced, enabling us to generate 3D spatial mappings of ASTs. Additionally, engineering evaluations using Finite Element Analysis (FEA) have become prevalent, with clients specifically requesting FEA alongside traditional hand calculations.
We have collaborated with Washington-based O&G company AR Watson on several occasions to execute AST lift and relocation projects. One such instance involved an owner needing to demolish an existing tank and replace it with a new one. To expedite the process, the new tank was constructed in an adjacent field while the old tank was demolished, and a new foundation was constructed. Subsequently, the new tank was elevated and relocated to the former tank’s site.
The relocation process began with lifting the tank using airbags, followed by supporting it on wooden cribs around the circumference — in this case, at four locations with 5 feet by 5 feet areas. The tank bottom was designed for continuous support on the ground, but lifting it introduced the challenge of large bottom deflection due to the lack of continuous support, particularly as the tank diameter increased. For better visualization, imagine supporting a thin sheet of paper at the corners with your fingertips, causing significant sagging in the center. To mitigate this issue, cables were affixed at multiple points from the tank bottom to the roof near the tank shell, transferring the weight of the bottom to the shell (see Figure 1).
Our proprietary software calculated the requisite number of cables and their placement to ensure the bottom deflection and stress remained within acceptable limits, also ensuring the shell did not buckle due to the additional weight. We conducted FEA simulations to validate the engineering design. The design also required looking at wind loads as high-wind forces can cause sliding or overturning.
Deflection contour comparison between laser scan and FEA results
For this project, we conducted multiple laser scans at different stages: before the lift, after supporting on cribs and after the final placement of the tank. We compared the deflection with the FEA results. The FEA accurately predicted the deflected contour, as shown in Figure 2. The deflection values did not precisely match due to field variables such as pretension in the cables and the exact location of the cable supports on the bottom. However, the deflections were very similar.
Another noteworthy observation was that the shell distortions present before the lift were no longer evident after lifting and replacing the tank on the new foundation. This indicates that the lifting process facilitated the shell in achieving a better shape, resolving out-of-roundness present from the original construction.
The project concluded that the FEA model accurately predicted the true deflection of the tank during the lifting and relocation.
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