The importance of asset integrity assurance of strategical equipment such as crude oil storage tanks (COS Tanks) is attributed to the possible impact on operation interruption, safety hazards, and cost of maintenance. This paper studies how the cyclic loading on tank shell affects tank integrity and tank life extension. This research study describes a case study of one million barrel capacity crude oil storage tank to identify the root cause of the non-uniform corrosion rates and material loss in tank shell. The tank is constructed from carbon steel material and experienced severe material losses in specific areas resulted to tank shell perforation and leak.
Computational Fluid Dynamics (CFD) simulation was conducted to study the anticipated movement of a floating roof of one million barrel capacity COS Tank under the routine tank operation. The pressure applied on floating roof due to fluid movement and the applied cyclic forces on guide pole were calculated. The cyclic forces acting on the guide pole were fused in 3D Non-linear finite element analysis (FEA) using ANSYS software in order to understand the influence of COS Tank filling and emptying cyclic process on tank structural integrity. The FEA model indicated the areas in the tank were high stresses and cyclic stresses develop during tank operation. The results of the FEA model were compared with a robotic ultrasonic testing (UT) data conducted on tank shell to correlate the locations of cyclic stresses to the tank shell corrosion mapping profile. The FEM analysis focuses on the stress distribution in tank shell, the locations of cyclic stresses, and the susceptible areas in tank shell for corrosion fatigue damage. The FEA results and corrosion mapping inspection results were in good agreement.
The current study identifies a hidden risk on crude oil storage tanks integrity, remnant life and fitness for service. The corrosion fatigue phenomenon due to the sour environment and cyclic loading on tank shell expedites corrosion rates and reduce COS tank operation life.