Investigation of Oil Reservoir Vibration under the Impact of Earthquake in Proper and Corrosion-Occurred Tanks

Abstract

Steel cylindrical reservoirs had the highest utilization in the field of oil source storage and petrochemical in the recent years due to their significant importance. These types of reservoirs are conventionally made with steady and floating roofs. Long term erosion agents have destructive impacts on dynamic features of these reservoirs. Results of numerical researches demonstrate that the internal corrosion of reservoir walls as a constant dependent to time, which is made due to the connection of fluids and chemical interactions with the internal wall of reservoir causes the corrosion of inside the reservoir wall and reduction in wall stiffness. In this research dynamic behavior of three models of steel cylindrical reservoirs (with the same height to diameter proportion) containing fluid is modeled using ANSYS software by applying the finite element method. In this modeling, features of a cylindrical reservoir containing 0.9 height of liquid is used which its fluid is considered to be incompressible and viscose. First Modal and Harmonic analyses are used to evaluate the natural frequency and formed mode-shapes in the tank-fluid system. These models are compared and verified with the similar and current experimental formulas. Next by applying corrosion on one of the tanks, dynamic features are evaluated using the software. Then some mass is applied in the place of corrosion to the walls of reservoir with proper boundary conditions in several steps, which in each step, the changes in the added mass are considered using the transient analysis and finally the natural frequency is evaluated. This cycle continued until the frequency of reservoir having added mass has equaled to the frequency of corrosion reservoir which this method is called equivalent mass. In the end, the added mass to the reservoir is formulated using mathematical techniques. Based on the analytical results we found that corrosion or long term erosion have remarkable influences on natural frequency, mode-shapes of structures and its vibration.