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Seismic vulnerability assessment and minimization of seismic risk of fuels storage tanks

Tsipianitis Alexandros

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Summary

Large-scale storage tanks are used worldwide for the safe storing of various liquids. Rectangular and cylindrical, concrete or steel, storage tanks are constructed for storing water, hazardous chemicals, liquefied natural gas (LNG) and oil. In general, there is a variety of tank types regarding the liquid content, the shape and their position (i.e., underground, above-ground, elevated). On the other hand, many such tanks have been constructed in areas with high seismicity. Several seismic events have shown that potential damages of these critical infrastructures can cause leakages, explosions and fires. In order to avoid serious environmental and socio-economic consequences, the optimum aseismic design of liquid storage tanks is of paramount importance. In addition, it should be noted that liquid storage tanks, due to their important role in serving basic needs, should remain functional even after a severe earthquake.Cylindrical steel tanks are widely used for storing oil and LNG. During an earthquake, liquid storage tanks exhibit different seismic behavior compared to ordinary structures (i.e., buildings) due to dynamic liquid-tank-soil interaction phenomena. More specifically, liquid storage tanks are subjected to inertial earthquake loads and hydrodynamic pressures. The main damages of liquid storage tanks are related to buckling phenomena of tank walls, typically in the form of "elephant-foot" and "diamond-shape" buckling types, while roof damages can be caused due to sloshing. In addition, kinematic type distress (i.e., liquefaction phenomena in coastal areas) can also cause problems to liquid storage tanks and related industrial facilities. Several norms have been proposed for the aseismic design of liquid storage tanks (such as Eurocode 8 in Europe, IITK-GSDMA in India, API-650 in USA, among others). The basic aim of these codes -in the contemporary framework of performance-based design- is the avoidance of severe damages even for extreme seismic events, due to the adverse on the population and the environment.Simple measures for the seismic protection of liquid storage tanks include increased tank wall thickness and stiffening rings for the avoidance of buckling phenomena. In addition, base-isolation schemes are implemented, in which isolators are installed between the base of the superstructure and the foundation. Generally, the foundation type of storage tanks depends on various parameters (e.g., soil characteristics, topography, tank construction type, use of anchors, etc.). In any case, ground deformations should be limited. Many researchers have studied the seismic response of liquid storage tanks, investigating the hydrodynamic tank-liquid interaction. The majority of these studies have considered the structure fixed at its base, which is not always a realistic representation. In general, the realistic and reliable assessment of seismic vulnerability and the minimization of seismic risk of such critical facilities consists an important and continuously developing research field.Consequently, in this doctoral thesis -based on an extended literature review- soil-tank-liquid interaction phenomena have been elaborately examined, via efficient numerical tools and finite element models. In this way, the dynamic response and seismic vulnerability of large-scale tanks have been assessed. Moreover, efficient seismic protection measures, based on seismic isolation and supplemental damping, have been proposed utilizing advanced optimization methods. Conclusively, the seismic vulnerability and the minimization of seismic risk of liquid storage tanks consist the main contributions of the present doctoral dissertation.More specifically, the seismic vulnerability of liquid storage tanks isolated by various types of friction isolators has been studied in terms of isolators maximum displacement capacity. The tanks are base-isolated via Single Friction Pendulum Bearings (SFPB), Triple Friction Pendulum Bearings (TFPB), as well as Quintuple Friction Pendulum Bearings (QFPB). In addition, the impact of global damping on the seismic vulnerability of base-isolated storage tanks has been examined. More specifically, the “damping leakage” phenomenon has been extensively investigated in liquid storage tanks isolated by SFPB or TFPB. According to relevant studies, if global damping is not simulated in an appropriate manner, then “damping leakage” can significantly affect the results.Another goal of this doctoral research is the systematic investigation of the dynamic interaction soil-tank. It is a very complex phenomenon that needs extra attention in any problem in structural dynamics, especially for critical infrastructure such as liquid storage tanks. Accordingly, various parameters were examined, such as the friction coefficient at the interface between tank base and foundation, the tank slenderness ratio, the liquid filling percentage, etc. The current study has also been focused on the seismic vulnerability assessment of base-isolated storage tanks with supplemental viscous dampers. More specifically, a storage tank with SFPB devices and supplemental linear viscous dampers has been used to investigate the impact of additional damping percentage on the vulnerability of the isolators, the superstructure accelerations and damper hysteretic curves.In addition, a significant part of this doctoral research has been devoted in establishing novel optimum design formulations of base-isolated storage tanks. The first approach examined the isolators optimization of base-isolated tanks using standard, enhanced and hybrid evolutionary optimization algorithms. The second methodology constitutes a combined approach aiming to concurrently optimize both isolators sizing parameters and placement utilizing Cuckoo Search (CS) optimization algorithm. Lastly, an efficient multi-objective optimization formulation is proposed for base-isolated liquid storage tanks with supplemental viscous dampers.

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