**1. Introduction**

The impact of natural disasters such as earthquakes, hurricanes, and floods on chemical and process plants can be quite catastrophic due to the possibility of damage to process units and subsequent release of large quantity of hazardous chemicals which may lead to disastrous environmental pollution [1] or fires and explosions [2–8]. Such technological accidents, which involve release of chemical substances into the environment, due to the impact of natural hazards are also known as Na-techs [9]. Katrina and Harvey are the hurricanes, which hit the U.S. in 2005 and 2017, respectively, are the first and second costliest events in the U.S. history and considered as noteworthy examples for Na-techs and their substantial damage to the environment and the industry. Katrina (2005) caused extensive damage to chemical and process plants and resulted in release of 8 million gallons of oil, making it the second largest environmental pollution in the U.S after the Deep Horizon disaster in the Gulf of Mexico in 2010. Similarly, Harvey (2017) caused substantial damage to refineries and petrochemical plants in the Huston Greater Area, leading to release of 2000 tons of chemicals into the environment as of September 2017 [10–12]. **Table 1** includes a list of industrial plants that suffered direct damages during Hurricane Harvey [10].

Among the process units, storage tanks – and atmospheric storage tanks in particular – have reportedly been the most vulnerable type of industrial vessels during hurricanes both due to their thin shell, which makes them very susceptible to lateral forces exerted by wind and flood, and due their high volume-weight ratio, which makes them susceptible to buoyancy force exerted by flood and heavy rainfall as components of hurricanes [11–14]. During Harvey in 2017, a total of 47 incidents of tank damage was reported with a total spillage 2 million liters of gasoline, crude oil, and other chemicals [15].

Damage to storage tanks can cause considerable environmental pollutions. During Katrina, ruptured storage tanks released several millions of gallons of oil while during Harvey, more than two dozen storage tanks ruptured, spilling 550 cubic meters of crude oil and gasoline into surrounding areas [10]. **Figure 1** shows the release of crude oil from to storage tanks, which have been removed from their foundations due to flood forces during Hurricane Katrina (2005).

Environmental pollution, however important, is not the only concern with regard to Na-techs. Release of petrochemicals – which in most cases are highly flammable and explosive – from damaged storage tanks can readily result in fire or explosion particularly in the presence of a variety of ignition sources and hot surfaces in the chemical and process plants. Such fires and explosions can easily go out of control due to large quantities of released chemicals and vicinity of storage tanks, capable of resulting in a sequence of fires and explosions – known as domino effects [16, 17].


#### **Table 1.**

*Type of industrial plants damaged during the Hurricane Harvey [10].*

#### **Figure 1.**

*Release of crude oil due to displacement of storage tanks from their bases due to storm surges during Hurricane Katrina, 2005 [10].*

Compared to conventional technological accidents that are caused by random failures or human error, risk assessment and management of Na-techs are more challenging [9, 18] due to the presence of more uncertain parameters and interdependent and complex failure mechanisms. The uncertainty consists of aleatory uncertainty arising from the randomness of natural disasters, resistance and vulnerability of process vessels to combined forces – wind, flood, rainfall, lightning – during hurricanes, and epistemic uncertainty arising from lack of sufficiently accurate and large historical databases.

Probability theory has effectively been used to account for uncertainties arising from environmental and operational random variable involved in risk assessment of Na-techs [19–25]. Even given the probability of random variables (depth of flood inundation, speed of wind, etc.), conventional quantitative risk assessment (QRA) techniques such as fault tree analysis fall short in modeling interdependent failure modes and common root causes arising from concurrent impact of hurricane forces on process vessels.

Bayesian network (BN) is a probabilistic tool for handling uncertainty and modeling dependencies [26, 27]. Previous studies demonstrated that BN can effectively accommodate and handle a variety of probabilities in the form of point probabilities [28], probability distributions [29, 30], fuzzy probabilities [31], and imprecise probabilities [32, 33].

In the present chapter, we demonstrate how BN can be employed to assessing the vulnerability of process vessels to the forces of hurricanes. Section 2 reviews the constituent natural hazards in the event of a hurricane and the resulting failure modes with emphasis on atmospheric storage tanks as the most vulnerable type of process vessels. Section 3 reviews the fundamentals of BN. Section 4 demonstrates the application of BN to modeling, combining, and calculating the failure probabilities of an illustrative storage tank. The main outcomes of the study are summarized in Section 5.
