Development of double-variable seismic fragility functions for oil refinery piping systems

Seismic risk assessment of piping systems, as a group of vulnerable facilities in oil refineries, is mostly based on the single-variable fragility curves. However, it is well-known that the fragility curves, developed based on a single intensity measure (IM), are not much reliable. For increasing the confidence level of seismic risk assessment of piping systems, it was tried, in this study, to develop double-variable fragility functions by using peak ground acceleration (PGA) and peak ground velocity (PGV) together as the IMs. For this purpose, the piping system of the ISOMAX Unit of Tehran oil refinery was considered, and modeled by a powerful finite element analysis program under various loadings, including gravity, pressure and seismic loads. For seismic analyses 157 set of three-component earthquake records were employed, with PGA and PGV values varying respectively from around 0.1 g–0.6 g and 10 cm/s to 60 cm/s. By using the nonlinear time histories analyses results, two single-IM fragility curves and one double-IM fragility surface were developed based on the probability of exceedance of the maximum created stress, considered as the ‘damage index’, from the allowable stress. The results indicate that using PGA and PGV jointly, as the IMs in the development of fragility functions, provides more reliable vulnerability estimations. For example, the single-IM fragility function gives, for PGA = 0.2 g, a probability of exceedance of 75%, while by using the double-IM fragility function this probability may change from 30% for PGV = 10 cm/s to 95% for PGV = 60 cm/s.