THE EFFECT OF LONG-TERM CORROSION ON DYNAMIC BEHAVIOUR OF UNANCHORED STEEL CYLINDRICAL TANKS

In this paper, long-term corrosion effects on dynamic characteristics of unanchored steel cylindrical tanks are investigated. The corrosion effect is considered as thinning of the upper and lower parts of the wall over time. Dynamic analyses are performed on three different models with different wall thicknesses and height to diameter ratios of 0.4, 0.63 and 0.95 using ANSYS finite element software. In the analyses, the tank’s liquid is considered to be crude oil with a level at 90% of the height of the tank. The tank’s base is unanchored and without any constraint and is in contact with the foundation through the effect of its weight. For determining the tank dynamic characteristics, including natural frequencies and the corresponding mode shapes, the models are analyzed using linear modal analysis. Comparing the natural frequencies of the corroding unanchored tanks with those of the non-corroded tanks reveals reduction in the stiffness of the tanks due to thinning effects of corrosion. The results of investigations also indicate substantial influence of long-term corrosion on the mode shapes of the tank. As the tank ages, its mode shapes change to completely different forms. Also, comparing the results of numerical natural frequencies with those of the coderecommended approximate methods shows that the latter approximate solution for natural frequencies presents reasonable results for the short tank (H/D = 0.4). However it errs considerably as the height of the tank is increased.