**4. Detailed analyses**

*3.2.1.3. Category 3 combinations of unrelated hazards*

hazards due to a common cause, from [3].

138 Probabilistic Modeling in System Engineering

tions with unrelated events is not possible or very unlikely.

Hazards that occur independently of each other have no common cause and are unrelated. The simultaneous occurrence is in general highly unlikely and is therefore investigated on an international level mainly for hazards of longer duration. In the example of the hydrological hazards not screened out qualitatively (B2, B3, B4, B6a, B8, B9a) a broad majority of combina-

**Figure 3.** Correlations of the hydrological hazards B2, B3, B4, B6a, B8, and B9a not screened out qualitatively with other

The plant model for risk assessment of the facility under consideration needs to be extended by taking into account those hazards and hazard combinations remaining after screening. It has to be analyzed, which structural elements, plant operational components, or even complete systems maybe impaired in their required function (so-called initiating events, IEs). That also requires to extend the original list of risk-relevant functional unavailabilities, the so-called basic events (BEs) in the plant model by those ones related to the hazards and hazard combinations to be considered as well as by the corresponding failure dependencies. This requires another two analytical steps:


As provided in more detail in [1, 3], the hazard equipment list for a single hazard *Hk* covers the entire number *j* of structures, systems, and components *SSCj* identified to be vulnerable to *Hk* and for which their failure contributes to the risk induced by *Hk :*

$$\mathcal{H}\_{\mathrm{k}}\mathrm{EL} = \left\{ \mathbf{S}\mathbf{S}\mathbf{C}\_{\mathrm{1}^\vee}, \dots, \mathbf{S}\mathbf{S}\mathbf{C}\_{\mathrm{m}} \right\}\_{\mathrm{HK}}.$$

In order to quantify the failure probabilities of the remaining structures, systems, and components vulnerable to the hazard *Hk*, information from the facility being analyzed such as technical reliability of systems and components and other factors affecting the hazard-induced scenarios like human reliability in case of actions (e.g., for the remaining hydrological hazards B2, B3, and B4 and their combinations, temporary flood protection measures) have to be taken in a predefined period to prevent damage.

The model extension also needs to take into account any countermeasures for preventing a risk-significant impact to the facility or mitigating the consequences of the hazards such that

**Figure 4.** Extension of the model of the facility being analyzed for probabilistic risk assessment of hazards, adapted

Natural Hazards: Systematic Assessment of Their Contribution to Risk and Their Consequences

http://dx.doi.org/10.5772/intechopen.76503

141

A typical example for preventive countermeasures is the timely implementation of rotatable bulkheads or stop logs as temporary means for protecting water ingress in case of flooding hazards. An example for mitigative measures in case of flooding events is the use of portable equipment to remove water from buildings with systems or components needed for safe operation of the facility such that their required function will not be inadmissibly impaired. In this context, the time and flooding scenario-dependent success paths including the manual actions to be taken have to be included in the probabilistic plant model considering also the human factor adequately in the corresponding HRA (human reliability analysis) model. As a

The systematic assessment of natural hazards including their contribution to risk and their consequences such as physical and operational impacts on critical infrastructures is still of

result, additional end states (damage states) of the fault trees can be determined.

the damage to the facility remains non-negligible.

**5. Conclusions and outlook**

from [3].

In a further analytical step, the dependencies among the failure characteristics of the vulnerable structures, systems, and components need to be investigated. Each dependency in this list *Hk DL = {D1 ,…, Dn }Hk* is characterized by a triple *Dk = {Ak ,Sk ,ck }* of parameters, which include the set of dependent structures, systems, and components *Sk* , the common characteristics of the elements of *Sk* (e.g., water level as cause for a flooding hazard-induced dependency) *Ak* , and a correlation factor *ck* for the dependency strength. The hazard equipment lists and hazard dependency lists need to be generated based on the corresponding parameters to be estimated and are used for the qualitative plant model extension. For adequately modeling the dependencies between the structures, systems, and components and/or the hazards impact, the fault trees of the analytical risk analysis model need to be modified and multiplied for the different hazards to be considered. In addition, new elements of the fault trees have to be specified (see also [5]) within the database representing a probabilistic model of a plant system.

A schematic overview of the approach for the plant model extension by hazards is given in **Figure 4**.

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**Figure 4.** Extension of the model of the facility being analyzed for probabilistic risk assessment of hazards, adapted from [3].

The model extension also needs to take into account any countermeasures for preventing a risk-significant impact to the facility or mitigating the consequences of the hazards such that the damage to the facility remains non-negligible.

A typical example for preventive countermeasures is the timely implementation of rotatable bulkheads or stop logs as temporary means for protecting water ingress in case of flooding hazards. An example for mitigative measures in case of flooding events is the use of portable equipment to remove water from buildings with systems or components needed for safe operation of the facility such that their required function will not be inadmissibly impaired. In this context, the time and flooding scenario-dependent success paths including the manual actions to be taken have to be included in the probabilistic plant model considering also the human factor adequately in the corresponding HRA (human reliability analysis) model. As a result, additional end states (damage states) of the fault trees can be determined.
