Application of F–N curves in API RP 752 building siting studies

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Highlights

  • Quantitative risk-based method is applied for occupied building siting evaluations.

  • Primary focus is impacts of vapor cloud explosions.

  • Pressure–impulse curves are used to predict building damage from explosion events.

  • Building damage vs. occupant fatality relationships are applied to generate F–N curves.

  • Result is a risk-based measure of building occupant fatalities due to explosions.

Abstract

An explosion overpressure analysis is a routine part of compliance with the American Petroleum Institute (API) Recommended Practice (RP) 752 and 753. A basic consequence analysis often attempts to demonstrate a few scenarios that may appear to have the most extreme consequences; however, these scenarios may also have extremely low likelihoods of occurrence. To have a better understanding of how likely as well as how extreme a consequence may be, risk-based analysis is often required. One method used for this is an overpressure exceedance analysis which uses overpressure results from many potential explosion scenarios coupled with the probability of each scenario. Pressure–impulse curves can make better use of the explosion model's outputs (pressure and impulse) in order to predict building damage and possible impacts to personnel. Using this type of analysis with a building damage–fatality relationship, an F–N style curve can be created which shows the cumulative frequency vs. the number of potential fatalities. Generation of F–N curves can help to better define the risks to building occupants, provide an additional means of evaluating a building's acceptability, and can serve as a part of a quantitative risk analysis (QRA) for facility personnel. This paper will discuss the method used to predict the probabilities of building occupant fatalities for use in an F–N curve, as well as the benefits and potential problems with this type of analysis.

Introduction

In a series of papers by the authors, the topic of building siting studies has been evaluated with the use of overpressure exceedance curves (Marx & Werts, 2013) and also with an extended analysis using pressure–impulse (P–I) curves (Marx & Werts, 2012). These methodologies were presented to offer means for analysis beyond that of a simple worst-case consequence analysis when evaluating the effects of vapor cloud explosions on occupied buildings in or near a petrochemical process facility. These methodologies provide a risk-based method to comply with API RP 752 and/or 753.

In some situations, the risk-based evaluation of a building may show that a building can sustain significant damage due to overpressures that are predicted to occur at frequencies which may be considered acceptable (e.g., 1.0 × 10−4 per year). Although the damage sustained by buildings is always a concern, the greater concern is the potential impact on building occupants. While many evaluation techniques have focused on the damage to the building itself, this paper extends that type of evaluation to a probabilistic prediction of building occupant fatalities. The chosen measure of risk for these results is the F–N curve, a measure of societal risk, which requires the analysis to develop a relationship between building damage and occupant fatalities. A particular challenge in generating an F–N curve of this nature is the probability of fatality for a given number of building occupants; a methodology for this calculation is explained in this paper.

Section snippets

Risk-based explosion modeling

The first step in producing a measure of building occupant fatalities is to model the explosion events. The model employed in this work is the Quest model for estimating flame speeds (QMEFS) (Melton & Marx, March 2009), an adaptation and extension of the Baker-Strehlow-Tang (BST) model. The model provides modeling flexibility by allowing the user to specify the defining parameters in more detail than the BST model, avoiding the step changes inherent in the typical low/medium/high categories.

Example analysis

Consider a small hydrocarbon processing facility with eight small equipment groups. The layout of this fictional facility is shown in Fig. 1. In the figure, the facility's four occupied buildings are labeled and the PESs (regions of congestion due to process equipment) are shown as dashed-outline polygons. The PESs have various properties of confinement and congestion due to the obstacle density and restriction of flame expansion that is present in each particular area. All the flammable

Overpressure exceedance curves

To construct an OEC, the list of potential explosion events is evaluated in reference to one specific location (typically the closest point on the building to the process areas). The explosion model is run for each event, and the resulting overpressure at that point is calculated. For any building, multiple points may need to be evaluated in order to account for shorter distances between the different sides of the building and the various PESs that exist around it.

The construction of an OEC

Building damage estimates from P–I curves

Overpressure exceedance curves demonstrate the overpressure impacts following vapor cloud explosions, without taking into account the corresponding impulse. While overpressure-only methods may provide the characteristic measure of potential building damage, a more complete description of the explosion's blast wave effects involves both the peak overpressure and the corresponding impulse.

The next potential step in blast wave evaluation is to also include the impulse corresponding to each

Determining fatality probabilities

The first factor to be considered when constructing an F–N curve is the amount of time a certain worker population spends inside a building. Because the same number of people are not inside the building 24 hours per day (various shifts, worker groups, and working patterns), a conditional probability is assigned to each population to account for occupancy. The frequency of each explosion event that a worker shift may be exposed to is based on annual equipment failure rates which are further

Evaluation

The three methods of determining fatalities due to explosion overpressure for building occupants presented in this paper each have a set of assumptions or conditions built into them. For this reason, it is worth evaluating the differences between the three methods.

One of the issues with the relationship from the CCPS book is that the data for serious injury or fatality versus overpressure is based on building damage data from earthquakes. While this may seem like a straight-forward comparison,

Conclusions

The methodology presented in this paper provides a means to apply vapor cloud explosion predictions to a building, accounting for its specific population. In order to produce an F–N curve for building occupants, two things are needed:

  • A set of predictions relating explosion impacts to building damage; and,

  • Some measure of the building response in the form of occupant fatalities.

The prediction of building impacts following explosion overpressure can be accomplished with the use of published

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