A Cost-Effective Approach to Explosion Consequence Modeling
Failure to identify and mitigate explosion hazards is a persistent cause of industrial accidents, impacting sites ranging from fertilizer storage facilities to refineries. While some explosion consequence assessments are performed to comply with regulatory requirements or industry-specific process safety standards, in many cases the potential for loss of life and property damage is overlooked until a tragedy occurs. Explosion Consequence Modeling (ECM) can provide detailed information about the possible effects of a blast, and can be used to identify and design mitigation measures. The two traditional ECM techniques have been: 1) basic phenomenological models that are very cost-effective but provide very simplistic results, and 2) computational fluid dynamics (CFD) techniques which provide great detail but are time consuming and can be prohibitively expensive. Many facilities are in need of a detailed ECM analysis to identify and mitigate explosion hazards, but require a moderate approach that can provide detailed damage and injury predictions without the high costs of CFD modeling.
For these facilities, the Explosive Damage Assessment Model (ExDAM) can provide many of the benefits of CFD modeling with a much smaller commitment of time and money. Originally developed by Dr. Frank Tatom and marketed as part of Trinity Consultants’ BREEZE® Software, ExDAM expands on the basic phenomenological model concept by adding the ability to model damage and injury while accounting for the structural details of buildings as well as the shielding effect of structures and equipment on the area behind them. Thus, ExDAM provides the safety benefits of a detailed ECM study to a much wider range of facilities at risk from either conventional high explosive (HE) materials or vapor cloud explosions (VCE). Facilities around the world have used this approach to evaluate and prepare for both potential industrial explosions and potential terrorist attacks. ExDAM has been applied to a broad range of facilities, including chemical/petrochemical, military, chemical storage, oil and gas exploration, ammunition manufacturing, rocket assembly, and stadiums/public buildings.
ExDAM Explosion Models – History and Computational Methodology
ExDAM provides a phenomenological method to predict damage and injury levels for open-air explosions. Beginning in the mid-1980s, organizations involved in the development of this method included the Strategic Defense Command, U.S. Army Corps of Engineers, Southwest Research Institute, Facility Army System Safety (FASS) Office, the Naval Civil Engineering Laboratory, Engineering Analysis, Inc. (EAI), and BREEZE Software / Trinity Consultants, Inc.
Developed from the Nuclear Damage Assessment Model (NDAM) and the Enhanced Nuclear Damage Assessment Model (ENDAM), ExDAM currently includes two modules: HExDAM, used to model conventional high explosive materials, and VExDAM, used to model vapor cloud explosions.
These modules use the following steps to produce a detailed assessment of injury and damage:
- Traditional phenomenological models, such as the Van den Berg Multi-Energy Model, are used to model a basic open-air blast wave (before the blast wave encounters obstacles, buildings, etc.).
- Shielding effects of structures, equipment, and people are incorporated. The shielding algorithm uses the actual design of buildings to provide more detailed results. For example, a blast wave may destroy window glass and wood-frame walls in the model, allowing persons standing behind the window or wood-frame wall to receive the full effect of the blast, but leave a concrete wall intact, shielding people behind that wall. ExDAM models these effects, which allows evaluation of existing structures as well as possible mitigation measures such as blast hardening of structures and addition of protective berms or walls.
- The damage and injury from the blast wave is calculated. Damage effects are calculated based on the material type (e.g. glass versus wood-frame construction versus cinder block construction). Injuries are determined based on the ExDAM human body model, which is composed of 28 total body components and 19 different body component types (e.g. various bones, organs, etc.). The model accounts for the fact that different materials/body parts are sensitive to different aspects of a blast wave. For example, ear drums are sensitive to the peak overpressure value, while long bone injuries are caused more by dynamic pressure. These calculations are performed in the background – the model typically presents damage and injury results to the user in a straightforward visual format (slight, moderate, or severe damage/injury for each building component or body part), though the associated numerical data (such as incident overpressure on each structure component) can also be viewed.
- If desired, the prediction of injuries from secondary fragmentation (e.g., flying glass from a broken window) can be added via the HEXFRAG module.
ExDAM Project Development Process
Project development in ExDAM is relatively simple, requiring only three steps:
- Block structures and people are created and each structure block is assigned a material.
- An explosion location and its relevant characteristics are specified. For high explosives, a mass of TNT equivalent is specified. For vapor clouds a mass (or volume), fuel type (e.g. methane, ethylene) and atmospheric conditions (e.g. temperature, pressure) are specified for each sub-cloud sphere (see yellow spheres in Figure 1).
- A pressure/impulse (P-I) sample grid is specified with fixed resolutions in XYZ directions. A P-I value will be computed for each sample grid point.
Scalability, Applicability, and Limitations of ExDAM Analyses
ExDAM can be applied to a wide range of spatial scales, from near-field effects of an explosion on a single nearby structure to effects on a large area with numerous structures. The primary limitation of ExDAM lies in the model’s reliance on P-I relationships, with modifications to account for shielding effects and to allow damage and injury calculations. ExDAM does not explicitly model flame front or blast wave propagation, and does not account for channeling or reflection (other than interactions with the ground). As with other multi-energy models, some details of a VCE, such as the degree of confinement and congestion and the fuel’s characteristic flame speed, can be accounted for by an explosion strength adjustment factor, but fluid flow is not explicitly modeled as in a CFD application. The model is, however, suitable for a large variety of applications, and has been used to model situations ranging in spatial scale from a subway car to large petrochemical facilities and entire towns.
In formulating a modeling analysis, time constraints and user needs typically dictate one of two strategies for creating buildings and people in the model environment. For large-scale analyses or a high-level overview of a scenario, each building can simply be represented as one block or a few blocks using one of ExDAM’s whole-structure material categories.
Examples of these include:
- Multistory Reinforced Concrete Building with Concrete Walls
- Multistory Steel-Frame Office, Earthquake Resistant
- Building with Medium Weight Pre-Fab Metal, Lightweight Walls and Roof
- Building with Tilt-up Concrete Wall, Lightweight Roof
- Building with Reinforced Concrete, 25 cm Walls, Reinforced Concrete Roof
- Building with Reinforced Masonry, 20 cm Walls, Light Roof
- Industrial Building with Heavy Frame (Steel or Concrete), Unreinforced Masonry Walls
- Residential Building with Wood/Steel Stud Wall, Lightweight Joist or Truss Roof
- Residential Building with Multistory Wall-Bearing, Brick Apartment House
- Residential Building with Wood Frame, House Type
This type of classification allows a large number of structures to be quickly created in the model when the structures are sufficiently distant from the explosion point as to experience a relatively uniform impact across their entire surface and/or when only a high-level damage assessment is required.
When explosions are located close to a building, such that the incident pressures/impulses of the blast wave on different parts of the structure vary significantly, or when a high level of detail is desired in the damage assessment, structures can be re-created in detail, with each of the different materials composing a structure represented separately (e.g. column, beam, wall panel, floor panel, door, window). For this type of ‘micro-level’ structure damage assessment, standard structure-component materials can be used such as:
- Brick Wall Panel, 20 or 30 cm, Non-Reinforced
- Concrete or Cinder-Block Wall Panels, Non-Reinforced
- Glass Windows: Large/Small
- Steel (Corrugated) Plating
- Wood Siding Panels, Standard House Construction
Experienced users, familiar with material vulnerability variations, will often use the standard whole-structure materials for structure component blocks such as columns, beams, and wall/floor panels.
ExDAM has been used to model a wide range of HE and VCE cases. Two example case-study projects are described below:
- An industrial plant siting analysis involving multiple potential VCE scenarios
- An urban multi-story building external VCE scenario
Example 1: Industrial Plant Siting Analysis
Explosion consequence modeling at industrial and military facilities has historically been the most common use of ExDAM. In these scenarios, one or more explosive material storage and/or processing locations are known. By modeling worst-case explosion scenarios across the facility, worst-case peak overpressures/impulses and subsequent worst-case damage and injury levels can be estimated facility-wide. In this scenario, modeling was performed to determine the potential VCE hazard posed to existing process and office buildings, as well as a safe location for siting of a future portable building (as per API 753 ). Model execution required approximately one hour on a laptop computer for a full-facility model run. Model results predicted broken windows and minor structural damage to exposed faces of several permanent structures. Predicted injuries to personnel were slight for personnel in shielded locations, but broken bones and other injuries were predicted for personnel in poorly shielded locations such as near windows. An area for safe placement of portable buildings was defined based on areas with low modeled overpressure. The location of this area was dictated primarily by distance from explosion sources and shielding from permanent structures. A plot plan illustrating some of the shielding effects, structure damage, and occupant injury is shown in Figure 2.
Creation and execution of this modeling scenario required approximately two days of time from an experienced user. The process was expedited by features such as 3-D CAD import tools and the program’s intuitive 3-D interface.
Check out our video on Petroleum Plant Siting Analysis to learn more about BREEZE ExDAM.
Example 2: External VCE Event Near Urban Multi-Story Building
Another common use of the ExDAM model is to assess the potential for damage or injury in a particular structure of interest from an explosion or to reconstruct a past incident. Often, these analyses are performed in the homeland security field to model the effects of high explosives. Real-world examples performed by ExDAM’s original developer, Dr. Frank Tatom, include an analysis of the 1996 Oklahoma City Bombing and a consequence analysis of a hypothetical bombing attack on a major U.S. college football stadium. Consequence analysis for a structure can also be performed for a vapor cloud scenario, as in this case.
The study scenario is a propane vapor cloud explosion in close proximity to multi-story hotel/apartment buildings. Vertical cross sections of the buildings and floor plans of each floor were imported into ExDAM and used to define the building structures (Figure 7). Tools within the program, such as the ability to create only one upper floor and then to copy/paste that to create the other levels, reduced the time required to create the structure. Building construction was primarily wood frame. Figure 3 shows the results of this modeling analysis, including predicted building damage and 3-D isosurfaces of predicted overpressure.
Check out our video on Building Structure Demonstrations to learn how BREEZE ExDAM computes damage/injury levels and simulates blast waves from an external explosion.
The BREEZE ExDAM model provides an intermediate complexity solution which is well-suited to modeling explosions of both vapor clouds and high explosives in environments where effects such as reflection and channeling are not expected to have a significant effect, but where shielding caused by structures and people may have an effect.
The model is relatively simple to use and understand in comparison to approaches such as CFD. Thus, while in no way a replacement for applications in which CFD modeling is required, ExDAM is accessible to a wider audience of safety professionals, first responders, etc. than CFD modeling. The ability of the model to correlate predicted overpressure and impulse into predictions of injury and damage adds to the potential utility of the model for these groups – impacts are translated into terms that can be readily understood by those without formal training in interpretation of overpressure and impulse data. However, some expertise is required to properly use the model, particularly when modeling vapor cloud explosions as these can be more difficult to quantify.