HAZARD AND FRAGILITY ANALYSIS

HAZARD AND FRAGILITY ANALYSIS Most analysis methods for external events are special purpose However, there is a general-purpose method: Based on hazard and fragility analysis Mainly used for earthquakes and severe winds (hurricanes and tornadoes) 2

HAZARD AND FRAGILITY ANALYSIS Why does a general-purpose method work here? You can define a single scalar metric of severity (peak wind speed, peak ground acceleration) Hazard curve: How likely any level of severity is to be exceeded Fragility curve: How likely any given item is to fail Conditional on a given level of severity 3

HAZARD CURVES Lower and upper curves are confidence bounds However, this picture is not fully realistic: In real life, we also don’t know the slope! Links and Notes: Fullwood R.R. (1999), Probabilistic Safety Assessment in the Chemical and Nuclear Industries, Butterworth-Heineman, Chapter 5 4

FRAGILITY CURVES 5

FRAGILITY ESTIMATION We can do fragility estimation with shake tables: But shake tables ignore many characteristics Earthquakes vary not only by: Peak ground acceleration But also by: Direction of motion Duration Frequency of vibration (spectral composition) Magnitude of displacement Etc. 6

FRAGILITY ESTIMATION We can do fragility estimation with shake tables: But shake tables ignore many characteristics Earthquakes vary not only by: Peak ground acceleration But also by: Direction of motion Duration Frequency of vibration (spectral composition) Magnitude of displacement Etc. 7

DEPENDENCE Different components: E.g., tanks versus pipes North-south versus east-west orientation May be vulnerable to different characteristics For a given peak ground acceleration: Similar components will tend to fail in the same earthquakes Different components will tend to fail in different earthquakes 8

DEPENDENCE Component failures are typically assumed to be conditionally independent: For a given peak ground acceleration This assumption is: Non-conservative when components are similar Conservative when components are quite different Simplifies the fragility model: Can just input fragilities into system fault trees But ignores important earthquake characteristics 9

INTEGRATING HAZARD AND FRAGILITY Differentiating the hazard curve yields a probability density function for peak ground acceleration Fragilities are weighted by how likely they are Individual component fragilities could be replaced by the results of a fault tree or event tree: To estimate likelihood of system failure For a given peak ground acceleration 10

IMPLICATIONS FOR PLANT DESIGN Interconnections are normally advantageous: Compensate for individual component failures But they can be disadvantageous in earthquakes: Pipe break in one train can drain the other train Compromise: Interconnection is installed But normally locked closed 11