A Modelling Approach for the Seismic Fragility Assessment of Process Towers

Process towers, whose purpose includes executing operations of mass transfer such as stripping, distillation, or liquid-liquid extraction, are usually constructed as vertical steel cylindrical pressurised shells, supported on a cylindrical skirt. Seismic risk analysts in an industrial setting, especially in NaTech context, are often faced with the issue that structural vulnerability models are not always available for the various industrial facilities. Process towers, although widespread, are one of those cases. The objective of the study presented in the paper is to simulate the behaviour of such towers when subjected to earthquake-induced ground shaking, with the goal of numerically deriving seismic fragility models. To this end, as a case study, two towers were considered with the same height and internal diameter, but different operating pressures, which determines the thickness of the pressure vessel and, consequently, the configuration of the anchorage system. For each of the two models, multi-stripe dynamic analysis was conducted, using a selection of hazard-consistent ground motion records, on joystick models. The results of the dynamic analyses are presented and their use for the derivation of fragility functions for certain damage states is discussed. This modelling strategy can contribute to streamline the fragility evaluation of such structures, given the variety of geometries which appear in industrial plants.