فهرست مطالب h. showkati

Thin-walled steel cylindrical shells are industrial structures that play an important role in the storage of petroleum and refineries, potable water supply, and fire extinguishing systems. Steel storage cylindrical shells are manufactured with variable thicknesses in height for economic reasons. Each steel cylindrical shell structure is made of several individual cylindrical parts of constant thickness. These shells with fixed roofs are subjected to axial pressure due to wind effect and snow accumulating on the roof. The cylindrical shells are also subjected to external pressure due to wind load and/or vacuum load when the containing liquid is discharged. The combination of axial compression and external pressure may lead to the failure of the shell structures. In this paper, two experimental studies were performed to investigate the effect of initial buckling during the axial compressive preloading or at the external pressure phase on the buckling behavior of cylindrical shells with stepwise wall thickness under combined loading of axial compressive preloading and external pressure. The results showed that the buckling capacity decreased under axial preloading and external pressure when the initial buckling occurred during the application of axial compressive preloading. Also, more deformations and buckling waves formed in the thinner individual section of cylindrical shell and also failure occurred faster and the structure became unstable faster, when initial buckling occurred during the axial compressive preloading. Theoretical relationships, in which geometric imperfection is considered, were employed to predict the buckling load of cylindrical shells with variable thicknesses subjected to combined loading of axial compression and external pressure. One of these two relationships shows a closer correlation with experimental results. Also, the results showed that the buckling behavior of cylindrical shells was very sensitive to the applied axial compressive preloading; therefore, as the axial compressive preloading increases, the quasi-empirical theoretical relationships become more conservative.

The pipeline in service may be subjected to complicated loads (including lateral, axial, vertical loads and hydrostatic pressure in addition to internal pressure) when crossing complex geohazard regions. In this study, two kind of loads that cloud be more fundamental are numerically investigated using finite element method. The loads imposed on pipelines depend on the pipe content and the environment that the pipeline is passing through. Axial compression can arise within pipelines from thermal loads arising from hot hydrocarbon passage from offshore oil wells to an onshore station or can arise from anchor forces acting on pipelines and External pressure can arise within pipelines from hydrostatic pressure, sudden valve closures, and pump failures. It is very important to select suitable geometric imperfection form to exact investigation behavior of pipelines and mechanism of failures. In order to verification response of numerical analyses, one of the experimental results is compared with numerical result and concluded that there is a good agreement between results. Meanwhile, the effect of the eccentric axial compression, pipe diameter to wall thickness ratio (D/t) on the buckling external pressure are studied. The interaction between the axial load and external pressure was graphically demonstrated and compared for different geometrical ratios through numerical analysis. During analysis, the eccentric axial compression load in the pipe was primarily induced and maintained constant less than its capacity. Subsequently, the uniform peripheral pressure was gradually increased until failure, and, besides, the response of some specimens was separately investigated under pure external pressure and axial compression load. It was found that the D/t ratio is the decisive parameter to specify the buckling behavior of steel pipelines and type of created failure mode subjected to axial compression. Some significant conclusions were drawn based on extensive parametric studies. The buckling external pressure reduces with the increase of pre-axial compression and diameter to thickness ratio.

Cylindrical steel tanks have been used to store various materials and fluids. From a geometrical point of view, steel tanks have a very low thickness compared to the other two dimensions and thus are classified as thin-walled structures. Nowadays, the use of thin-walled structures is very popular. The main reason for this is the low weight and high strength of such structures. In order to reduce the weight and cost of the structure, cylindrical steel tanks can be made with variable thicknesses in height. Changing the thickness of the tank's wall has been considered to reduce weight and economic requirements. Settlement is one of the important issues that must be considered for reservoirs. In general, foundation settlement occurs under the tanks’ walls due to special soil properties, which are divided into three general components: Uniform, Tilt, and Local settlement. In the meantime, local settlement has the greatest impact on the tank’s shell, although it has the lowest value. This component can cause large radial displacements, shell buckling, and even tank’s failure. In this study, 17 steel cylindrical tanks with or without a reinforcing ring on the upper part were modeled in ABAQUS software and placed under local settlement at the edge of their floor. The values of settlement, equivalent buckling load, and their radial deformation were compared to each other. S4R elements (a four-node element with reduced curvature double integral) were used to mesh the tanks. The results of nonlinear analysis indicate that buckling occurs at the lower part of the shell. Moreover, the local settlement causes severe buckling at the site and perpendicular direction of the settlement. Appropriate and acceptable behavior of tanks with variable thickness in comparison with constant thickness and the hardening ring can have positive effects on the buckling behavior of tanks.

Thin-walled steel storage tanks are used for an extensive array of engineering applications such as urban resource water, petroleum industry, and nuclear power plants for storing variety of liquids, e.g. water, gas, and petrochemicals. The ratio of the radius to the thickness of these tanks is about 5 00to 0000, and the height to radius ratio is about 0.5 to 0.0.The support beneath the tank may lose the bearing capacity in some areas, with the primary reason for this being geometrical misfits, poor soil conditions, ununiformed applied load. Additionally, foundation unevenness, change of ground topology, and foundation subsidence beneath a steel tank can all lead to the development of support settlement of tanks, with tank support settlement being variously categorised as uniform settlement, tilting, and local settlement. After compacting the substrate, these tanks are supported on a concrete foundation using a concrete ring. Thin-walled steel tanks are used in various industries to store oil and gas condensate. These tanks are supported on a concrete foundation by compacting the substrate. This foundation may lose its load-bearing capacity and settle in a part of the reservoir foundation due to non-uniform geometry or improper soil compaction under the foundation or uneven distribution of load applied to the foundation. Possible settlements are including a uniform, tilt of plan and non-uniform. Non-uniform local settlement is one of the most common and destructive types tanks settlement. In this study, the effect of local settlement on the buckling capacity in the tank with and without stiffening ring was investigated in laboratory and numerical. The results showed that the stiffening ring has an effective role in increasing the parameters of toughness, maximum equivalent load and initial stiffness. It increases the toughness by 98%, the maximum equivalent load by 58% and the initial stiffness by 101%. The presence of stiffening ring also plays a very important role in controlling radial displacements. As by preventing the radial displacement of the upper edge increases the buckling and post-buckling capacity of the tanks.

Due to their attractive mechanical properties such as higher strength-to-weight and stiffness-to-weight ratios, instead of conventional isotropic materials, Fiber Reinforced Polymer (FRP) cylindrical shells are being increasingly used in various engineering applications such as aerospace, oil tanks, liquid storage vessels, and silos. Cylindrical shells may experience axial compression load due to primary loading conditions. For a shell with a closed roof, axial compression load is caused by the weight of the roof. In addition, many shells under other loading conditions can also induce either symmetrical or unsymmetrical axial compression loads. The buckling and failure analysis of cylindrical shells made of composite materials is a complex task when compared to the cylindrical shells made of isotropic materials. There are two main failure modes in the composite cylindrical shells subjected to axial compression load, one associated with material strength of the cylinder wall and the other with buckling of the cylinder wall. This study deals with the behavior of chopped Glass Fiber Reinforced Polymer (GFRP) cylindrical shells subjected to axial compression load with an experimental and numerical procedure. Three specimens were used with the same R/t ratio. In the laboratory, the axial compression load was applied by a vertical hydraulic jack. In order to measure the deformation of the cylindrical specimens during the loading, four Linear Voltage Differential Transformers (LVDTs) were used. The nonlinear static analysis method (Riks) using the ABAQUS/Standard software has been employed, considering the boundary and geometric imperfections. For modelling the cylindrical specimens shells, the four-node quadrilateral element (S4R) was used. The critical load and failure mode were determined using Hashin's failure criteria. The effects of the L/R ratio on these shells are examined. Results indicate that the failure mode of these specimens was material failure because of high thickness-to-radius ratio. In addition the stiffness of the cylindrical shell specimens decreases with increasing the height. The comparison between the experimental results and Finite Element Analysis (EFA) exhibited acceptable adaptation.