Application of statistical analysis for the identification of critical bottom areas due to corrosion in atmospheric storage tanks

Equipment ageing containing hazardous substances is a criticality for the safe as leakages can give rise to serious accidental scenarios such as fires, explosions, and dispersions of chemicals into the environment. The attention towards this issue grows over the time and becomes significant when the equipment is reaching the end of its lifetime. Ageing management usually includes targeted and in-depth integrity controls performed at regular intervals. Atmospheric storage tanks of hydrocarbons are particularly critical as the control of the bottom integrity requires not only the service interruption, but also the need for the inspectors to stay long time in hazardous environments during the time required for the thickness measurements. One of the main causes of perforation is corrosion. The whole bottom is in any case subjected to significant stress, therefore, the thickness of the bottom plates must not be below a threshold value to avoid leakages. The aim of this work is to investigate the entire bottom of an atmospheric storage tank to identify the most susceptible locations with respect to the deterioration due to localised corrosion. The assessment of the probability of perforation of the bottom indicated the annular ring as the most critical. To achieve this objective, the latest inspection data have been used that allowed identifying the probabilities of leakage for the various homogeneous areas identified in the bottom. An estimate of the time to reach the critical thickness has also been given based on a recent approach that combines the extreme value theory of with the Bayes theorem.