1. HAZARD AND FRAGILITY ANALYSIS

2. 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

3. 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

4. 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

5. FRAGILITY CURVES 5

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. 6

7. 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

8. 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

9. 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

10. 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

11. 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