Strength And Rigidness Analysis With Study Of Design Methods For Large Storage Tanks

Because of the shortage of petroleum resources and the price fluctuations of the oil worldwidely, many countries pay much attention to the strategic reserve of oil. Large storage tanks are the most ideal equipment for the large-scale storage of petroleum. The vertical cylindrical storage tank which has been used for nearly a hundred years, is the most common structure of large storage tanks. As both the structure and the loads are complex, the present design for the vertical cylindrical storage tank based on relevant codes is not so rational. In this dissertation, the mechanical properties and the structure design of the tank body and the floating roof were studied with concerntration on the strength, stability and seismic responce of the tank. The main contents and conclusions obtained are as follows:(1) About the design of the large storage tank wall The one-foot method and the variable-design-point method are the most common methods for the design of the tank wall. In addition, the optimal design based on the finite element analysis is more and more used in the design of large storage tanks. By applying these three design methods, two large storage tanks with a volume of 1×105m3 and 2×105m3 were designed and compared. Results indicate that the effects of the tank bottom and the lower tank wall on the upper tank wall are not well considered with the one-foot method and the variable-design-point method, but the optimal design based on the finite element analysis is very effective and rational. Besides, by studying the deformation form and stress distribution characteristics of the tank with a volume of 2×105m3, it is found that high stress occurs in the fillet weld region due to the structural change and the maximum stress occurs at the bottom of the inside fillet weld where the tank bottom is connected. The stress of the second tank wall is higher than it of other tank walls.(2) About the stability of large storage tanks The wind girders and the reinforcing rings are used to enhance the stiffness and the stability of the tank under wind loads. The design methods for the wind girders and the reinforcing rings in GB50341 and API650 were presented and the calculation formulas were derived to reveal the simplification regarding the magnitude and action zone of wind load and the mutual enhancement between various parts. Three-dimensional finite element models were built to study the wind pressure distribution around the tank and the mutual enhancement between various parts. Based on this model, a design approach of the reinforcing rings based on the minimum steel consumption was proposed and the formulas for the minimum section modulus and the maximum height of the unstiffened tank wall were given. For a given tank with the size and wind load in the rang considered in this dissertation, the steel consumption of reinforcing rings designed according to this method will not exceed 35% of the steel consumption according to API650.(3) About the design of the double-deck type floating roof The structural features, common loads and failure modes of the double-deck type floating roof were introduced. Considering the load redistribution when multiple loads are acted simultaneously, the calculation method of loads distribution in different conditions was presented with the two working conditions of "snow load+leak load" and "rain load+leak load" as examples. Regarding the strength and stability analysis of the floating roof, the sub-model method and the equivalent structure method are the common calculation methods. However, the whole model of the double-deck type floating roof is also needed in the two method and the simplification effect is not so effective. Based on the structural characteristics of double-deck type floating roof, a method named minimized region analysis method was proposed to obtain the limit load and the critical load of the floating roof. This method is easy to be operated and the whole model of the double-deck type floating roof is not needed. Besides, the size of the finite element model can be greatly reduced, the calculation efficiency can be remarbly increased and conservative results can be obtained.(4) About the analysis of large storage tanks under seismic excitation A reduced structure was constructed and tested to verify the finite element simulation of the large storage tanks under seismic excitation. Experimental results well agree with finite elemnt results simulated on the same reduced structure, which means that the model established in this dissertation for simulating the seismic excitation is effective and rational. With the finite element model, the seismic response characteristics of the real large storage tanks under three-dimensional seismic loads were studied. It is found that, for the tank and seismic load considered in this dissertation, the inertial force caused by the horizontal acceleration plays a greater role than the inertial force caused by medium sloshing in the deformation of the tank wall. The results also indicated that the medium sloshing is reduced by the floating roof in a great deal, however, the deformation of the tank wall can hardly be affected by the roof.