Vertical pressure vessels and storage tanks are among vital structures that may undergo significant degradation over their life span. The current study investigates fitness-for-service (FFS) of steel methanol tank. The tank is composed of six courses of plating along the height of the tank. Upon inspection, significant corrosion and deep pitting was identified at various locations in the tank with severe and extensive metal loss near the bottom of the tank. The aqueous chloride environment coupled with tank operating scheme is responsible for the corrosive damage. The mechanism for the concentrated metal loss in the tank bottom and the decreasing impact proportional to elevation is the same. This study discusses the probable corrosion mechanism focusing on the tank components that may limit the remaining life of the tank and recommendations to address the limiting components in the tank. Due to the severity of the deterioration, detailed Level Ⅲ FFS analysis per API 579, which requires detailed 3-D nonlinear finite element analysis, has been performed to gauge the remaining life of the tank. The results of the analysis indicate that the likely failure mode is buckling in shell course 2. Repair recommendations have been made, the repair strategy was to add reinforcing steel to the deteriorated shell course bands that are susceptible to buckling, which consisted of steel plating seal welded to the existing cylinder. A parametric study to determine the corrosion thresholds and minimum tank thickness required to preclude buckling under normal operating conditions has been performed.
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