Seismic risk assessment of buried steel gas pipelines under seismic wave propagation based on fragility analysis

In this paper economic seismic risk analysis of API 5L buried steel gas pipelines is carried out through incremental dynamic analysis using an ensemble of 20 ground motion records. Multiple performance limit states for buried steel pipelines are proposed. Fragility curves of 15 pipeline models of various diameter to thickness (D/t) ratios, burial depth to diameter (H/D) ratios, assumed soil conditions and steel grades are plotted for damage states corresponding to the defined limit states. Expected global economic loss of 1 km length of the pipelines for a given peak ground velocity is obtained using the fragility curves, a pipeline repair ratio as well as costs of leakage repairing and break-induced pipe replacement. The mean annual frequency of exceedance of loss for the investigated pipelines is calculated using a seismic hazard curve and the expected annual loss (EAL) is obtained. It is concluded that in case of the pipelines with the D/t ratio greater than 39.9, the probabilities of failure for a given return period increase with increasing D/t ratio. It is obtained that the probabilities of failure for a given return period increase with increasing H/D ratio and shear modulus of the soil . The results show that the probabilities of failure decrease with increasing steel grade of the pipes. It is found that for a given return period, the global economic loss increases for an increase in the D/t, H/D and values. The results show that EAL of the pipelines decreases with increasing steel grade of the pipes and decreasing D/t ratio, H/D ratio as well as value. It is obtained that frequency of exceedance of a given loss value increases with an increase in D/t, H/D and values. It is noteworthy that the obtained results are valid only for Tehran, and the pipe and soil characteristics used in the study.