Cylindrical steel tank stress-strain state evaluation with operational loads taken into account

Abstract

Current regulations (in particular, GOST R 52910-2008 Vertical cylindrical steel tanks for oil and oil-products) don't specify a service limit for the tanks. The tank can be dismantled only on the basis of the results of diagnostic examination – both instrumental and calculating (for example, determination of strain-stress state of the tank and its structures). In the process of operation the tanks are exposed to a complex of stresses: static loads, low-cycle fatigue, snow and wind loads, temperatures and aggressive operational conditions. Besides, some geometrical imperfections are created. All this leads to substantial reduction of load bearing capacity, in-service reliability decrease and reduction of the tank structural durability. Numerical modeling methods, unlike the standard methods of strength and stability calculation, make it possible to create an adequate finite element model at the stage of development of a geometric model. The article describes a tank strain-stress state analysis with the help of ABAQUS software suite, taking into account operational loads. The strain-stress state analysis by finite element modeling method through the example of a vertical cylindrical steel tank showed nonuniform stress distribution in the shell, producing overstressed local areas which considerably increase with the wall thinning. The stress concentration zones which are located directly in the places of shell/nozzle junction as well as in the first and second ring weld area which are not considered in standard methods of calculation. Considering that this unit periodically works in filling and emptying mode, the shell metal experiences reversed loadings which can reach low-cycle area when used in local zones.