Home » Blog » Seismic Analysis of Liquid-Storage Tanks

Seismic Analysis of Liquid-Storage Tanks

Water for drinking and firefighting, as well as petroleum, chemicals, and liquefied natural gas, are all stored in large-capacity ground-supported cylindrical tanks. For modern facilities, tank performance during strong ground shaking must be satisfactory. During previous earthquakes, tanks that were not properly constructed or detailed sustained significant damage. Steel storage tanks can be damaged in a variety of ways during an earthquake. The “elephant-foot” buckling of the tank wall can be caused by large axial compressive loads caused by beamlike bending of the tank wall. The roof and the top of the tank wall can be damaged by splattering liquid. The tank wall can be ruptured by high strains in the vicinity of inadequately specified foundation anchors. Base shear can reduce the amount of friction that causes the tank to slip.

In unanchored or partially anchored tanks, base elevating can destroy pipe connections that are unable to accommodate vertical displacements, rupture the plate-shell unction due to high joint stresses, and create uneven foundation settling.The hydrodynamics of liquids in rigid tanks resting on stiff foundations were the subject of the first analytical research. A portion of the liquid is shown to move in a long-period sloshing motion, while the rest travels firmly with the tank wall. The impulsive liquid – the last section of the liquid – accelerates at the same rate as the ground and contributes mostly to the base shear and overturning moment. The height of the free-surface waves, and thus the freeboard need, is determined by the sloshing liquid.

Later research revealed that the tank wall’s elasticity might lead the impulsive liquid to accelerate several times faster than the peak ground acceleration. As a result, assuming the tank is rigid, the base shear and overturning moments predicted can be nonconservative. Tanks supported on flexible foundations endure base translation and rocking, resulting in longer impulsive durations and overall better effective damping. These alterations may have a major impact on impulsive behaviour. Due to its long period of oscillation, the convective (or sloshing) response is practically insensitive to both the tank wall and foundation flexibility.

The above studies assumed that the tanks were completely moored at their base. Complete base anchorage is not always practical or cost-effective in practise. As a result, many tanks at their base are either unanchored or merely partially anchored. The seismic response of partially anchored and unanchored tanks supported on rigid foundations was investigated as a result of base raising. Base raising was found to lower the hydrodynamic forces in the tank while dramatically increasing the axial compressive stress in the tank wall.

According to additional research, base uplifting in tanks supported directly on flexible soil foundations does not result in a significant increase in axial compressive stress in the tank wall, but it can result in large foundation penetrations and several cycles of large plastic rotations at the plate boundary. Elephant-foot buckling damage is thus less likely in flexible-supported unanchored tanks, but uneven foundation settlement and fatigue rupture at the plate-shell junction are more likely

.In addition to the investigations mentioned above, a slew of other experimental and numerical studies have shed light on the seismic behaviour of tanks. The elastic analysis of fully anchored, rigidly supported tanks is the sole focus of this article. The impact of foundation flexibility and base raising on tank response could be significant.

Abstract Liquid storage tanks are critical components of liquid transmission and distribution systems, thus they must be constructed to handle a variety of loads. Seismic excitation is one of them. The purpose of this work is to determine the dynamic properties (e.g. natural frequencies and their related mode shapes of the tank and the liquid) as well as the seismic characteristics of a flexible circular vertical ground-supported liquid storage tank (e.g. hydrodynamic pressure distribution, base shear, overturning moment and maximum wave height). During seismic activity, the tank and the liquid have a unique interaction. It manifests itself as a tremor of the tank and its contents. One half of the liquid (lower) flows in lockstep with the structure, whereas the other (upper) part represents There is a sloshing effect on the free surface. The study also compares the obtained results from the seismic analysis computed using the FE approach (ANSYS) with analytical models (spring-mass and pendulum) and Eurocode 8 methodologies, respectively.

The refinements are based on a model proposed by Wozniak for the seismic analysis of fluid storage tanks, which is the basis for the API 650 design code. The adoption of API 650 guidelines has been observed to cause underestimating of maximum compressive stresses in the shell and overestimation of tensile stresses in the straps of anchored tanks in some circumstances. The stiffness of these straps in comparison to the shell has been proven to have a significant impact on the maximum compressive stresses that may be expected in the tank wall. The increased compressive shell forces in unanchored tanks are due to the limited mass of fluid that may be carried.

In industrial facilities, liquid-containing storage tanks are crucial structures. Because earthquake damage to liquid storage tanks can result in structural collapse, fires, and hazardous substance leaks, seismic fragility analysis has been used to help limit the effects of earthquakes. This research focuses on the seismic responses and fragility of liquid storage tanks in this regard. First, earthquake ground motion characteristics are a critical factor influencing the seismic fragility of structures; thus, this study uses real earthquake records from the target area, southeastern Korea, with earthquake characteristics estimated using the ratio of peak ground acceleration to peak ground velocity. Due to hydrodynamic pressures, when a liquid storage tank oscillates during an earthquake, additional forces can contact the tank wall. As a result, this research As a result, this research proposes a complex finite element (FE) model that accounts for the hydrodynamic effect of an oscillating liquid. Another benefit of a FE model like this is that precise structural reactions of complete wall shells may be calculated, which is not achievable with simplified lumped mass or surrogate models. Finally, for three crucial limit states: elastic buckling, elephant’s foot buckling, and steel yielding, probabilistic seismic demand models are developed. A seismic fragility analysis for a typical anchored steel liquid storage tank in Korea is performed using genuine earthquake ground motion records, a developed FE model, and limit states. A ring-stiffened model is also examined for comparison reasons in order to derive a seismic fragility curve. The results of the seismic fragility assessment reveal that elastic materials can withstand earthquakes.

The most vulnerable damage state, according to the seismic fragility evaluation, is elastic buckling. Elephant’s foot buckling and steel yielding, on the other hand, indicate relatively significant damage levels. Furthermore, ring stiffeners are found to reduce elastic buckling damage, although having no practical effect on elephant foot buckling and steel yielding at all ground motion intensities.

Industrial facilities rely heavily on liquid storage tanks. When a liquid-filled storage tank is subjected to seismic loads, it can be damaged and collapse, potentially leaking hazardous materials and creating massive fires. The 1964 Niigata earthquake in Japan, according to Persson and Lönnermark [1, caused fires that resulted in the loss of 97 tanks storing 1.1 million barrels of crude oil. The force of the 2003 Hokkaido earthquake was reportedly reported to have damaged or leaked 29 tanks. Three naphtha tanks and a crude oil tower were destroyed in Turkey as a result of the 1999 zmit earthquake.

As of 2018, there are 1189 industrial complexes in Korea, according to statistics provided from the Korea Industrial Complex Corporation [2], with 436 industrial complexes in southeastern Korea. Furthermore, earthquakes have happened often in the southern portion of Korea in recent years, putting these large industrial complexes at risk. As a result, the relevance of structural risk assessment has grown, and efforts to limit earthquake losses using seismic fragility assessments have continued [3, 4]. The structural instability of liquid storage tanks is the focus of this research.

The seismic fragility of structures is also influenced by the features of earthquake ground motions. Previous research [3–6] looked at the seismic fragility of structures subjected to various types of earthquake ground motions to see how ground motion features affected seismic fragility. These research’ findings revealed that earthquake damage to structures varied depending on ground motion parameters. Furthermore, genuine ground motion records from world-wide well-known earthquakes have been used in several research on seismic fragility evaluations utilising analytical models for an accurate seismic fragility study of structures. These records, however, may not accurately reflect the features of a certain place. Real ground motion data recorded in the target area (i.e., southeastern Korea) are gathered and employed in this study to estimate seismic fragility.

Additional stresses can hit the tank wall when a steel liquid storage tank is subjected to an earthquake due to hydrodynamic pressures. Convective and impulsive modes can be used to characterise the hydrodynamic response of a liquid in a tank. The upper section of the liquid exhibits a long period oscillation due to convective motion (commonly referred to as sloshing), whilst the lower part of the liquid travels in lockstep with the tank wall as if it were rigidly attached to the tank. Veletsos looked into the seismic consequences of a flexible liquid storage tank and discovered that hydrodynamic forces can have a bigger impact on a flexible tank than on a rigid one.

Also Check


Leave a Reply

Your email address will not be published.