04.05: Analysis type influence on the durability prognosis for a steel tank corroded shell

Abstract

ABSTRACTAn original approach to the evaluation of remaining service time of a corroded on the ground steel shell of a tank used to store liquid petroleum products is presented and discussed. A typical steel tank equipped with floating roof is analysed. The limit state for such structure is determined by the tensile hoop stress at the completely filled tank reaching the bearing capacity of steel plating. It is assumed, that in the foreseeable future the tank will not be modernized nor strengthened in any manner. The service mode of the tank will not change as well. The future progress of corrosion is forecast by the extrapolation of trends, which have appeared until now, and identified during the cyclical obligatory examinations of the tank technical condition. Detailed map of the corrosion reach and intensity determined in this way is superimposed over the numerical tank model, replicating its real imperfect geometry measured by geodetic methods. In this paper two alternative methods of analysis are compared. The first one is geometrically nonlinear (GNA), while the second one is geometrically linear (LA). The obtained results are compared with the analogous results obtained under the assumption of perfect tank geometry. It turns out that the quantitative differences with respect to the forecast tank shell life, due only to the numerical approach recommended for analysis, are substantially bigger than those generated by monotonically progressing, but locally varying corrosion progress. In the case considered in this paper, the linear analysis (LA) yields the most restrictive estimate of authoritative hoop stresses. The application of nonlinear analysis (GNA) proves to be the less restrictive approach in this field, as it takes into the account the smoothing of possible local folds in the deformed shell, and thus as a consequence making it closer to the shell with perfect geometry.