Explosives Safety 101: A Calculated Risk
Updated: Sep 1
Would you rather make a go/no-go decision on a new facility using a set table of distances meant to conservatively represent all theoretical scenarios (QD tables), or with an analysis that actually estimates who gets hurt, how badly, and how far away? That's the value of an explosives risk assessment.
The downside is that risk assessments require more input to create that more detailed output. Some forms of risk analyses require a significant amount of additional input related to building construction types, personnel numbers and schedules, as well as details of the explosive activity being performed. The results are extremely helpful but require an investment.
That’s where a Tier 1 Risk Analysis comes in. It’s a middle ground between rigid Quantity-Distance (QD) tables and extensive risk analyses. It doesn't replace a full Quantitative Risk Assessment (QRA), but it gives site planners and Commanders a fast, defensible first look at risk before committing time and resources to a more detailed study.
The Schreifer Group has just completed a contract with Naval Facilities Engineering Systems Command (NAVFAC) Mid-Atlantic (Partnered with Prime AE) to complete approximately twenty Tier 1 Risk analyses for US Navy piers at four installations. This has been a great exercise interacting with NAVFAC and Navy Ordnance Safety and Security Activity (NOSSA) to build out the process for generating and submitting these packages in bulk.
This article will share the background and lessons learned running Tier 1 analyses in the field.
Tier 1 vs. Tier 2a vs. Tier 2b
The DoW Munitions Risk Management Assessment (MRMA) process for explosives site plans recognizes multiple tiers of analyses. These are further defined in Department of War Explosives Safety Office (DWESO) Technical Paper (TP)-23.
Tier 1 – Qualitative Likelihood and Consequences. A rapid screening tool based on the DWESO Automated Safety Assessment Protocol – Explosives (ASAP-X) spreadsheet. This process estimates consequences using the six hazard zones described below.
Tier 2a – Qualitative Likelihood with Quantitative Consequences. A mid-level risk assessment intended to support a Commander's Risk Acceptance or Waiver Request. The consequence values are calculated for air blast, fragment/debris, and thermal effects using the physics-based models documented in DWESO TP-14.
Tier 2b – Quantitative Likelihood and Consequences. The most rigorous of the three, Tier 2b is a Formal Quantitative Risk Assessment (QRA) performed in accordance with DWESO TP-14. This is the only tier of analysis that can be submitted to the DWESO for a formal site plan approval. Anything less than this is considered a hybrid site plan and is accepted at the Service level.
Each tier builds on the last in terms of both rigor and the level of scrutiny it can withstand, so choosing the right tier for the decision at hand matters as much as running the analysis correctly.
Tier 1 and 2a have been incorporated into the Risk Based Explosives Safety Siting (RBESS) module of the DWESO Explosives Safety Siting (ESS) software.
What is a Tier 1 Risk Analysis?
A Tier 1 Risk Analysis within the ESS software identifies explosive consequences using predefined accident scenarios. It uses six hazard zones to visually estimate injury and building damage based on the Net Explosive Weight (NEW) and hazard division of each facility analyzed.
A typical QRA analysis utilizes the DWESO TP-14 equation for risk.
Risk = Likelihood (Pe) x Consequence (Pf|e) x Exposure (E)
A Tier 1 analysis makes the following simplifications to produce a quick estimate of risk.
The likelihood of an event is considered to be 1.0. In other words, like with QD, the analysis is “deterministic” assuming the event will happen or has happened.
The consequence portion is simplified in several ways.
The expected level of injury and building damage is interpreted from the written descriptions in Defense Explosive Safety Regulation (DESR) 6055.09 Section V1.E8.2.5 “Expected Consequences”. Results are numerically estimated rather than calculated based on specific variables.
Facility types and construction details are not accounted for at the donor or the exposed site.
Six hazard zones are defined based on common Hazard Division (HD) 1.1 K-factors. Results are shown by zone. There is no variation in results within a zone.
The exposure term is simplified by assuming the identified number of personnel at each exposed site are always there. In other words, the exposure term is assumed to be 1.0 if people are present and the number of personnel is the only factor that changes.
Another simplification made within ESS specifically for Tier 1 analyses is that separation distances are measured edge-to-edge between the Potential Explosion Site (PES) and the Exposed Site (ES). The measurements are taken from the edge-to-edge spatial analysis process already run within the software for the QD calculations.
The output helps support decisions for site planners and commanders long before a project reaches a formal risk acceptance or waiver process.
The Six Hazard Zones
A Tier 1 analysis divides the area around a PES into six concentric hazard zones; each tied to a specific consequence. Read more on K-Factors here.
Zone 1 – Barricaded Intermagazine Distance (IMD-B); K6
Zone 2 – Barricaded Intraline Distance (ILD-B): K9
Zone 3 – Unbarricaded Intermagazine Distance (IMD-B); K11
Zone 4 – Unbarricaded Intraline Distance (ILD-B): K18
Zone 5 - Public Traffic Route Distance (PTRD): K24/K30
Zone 6 - Inhabited Building Distance (IBD): K40/K50.
Plotting these zones over a site map gives planners and commanders an immediate, visual sense of who and what is exposed at a given NEW and hazard division, well before any detailed engineering analysis begins.
Data Required to Run a Tier 1 Risk Analysis
One of the advantages of a Tier 1 analysis is that most of the information needed is likely already in your ESS database. A few additional data points are required and make the results more realistic and useful:
Maximum number of people in each facility
Building height
Total cost, or replacement cost, of the facility
Lessons Learned
The process for risk analysis can be written out in a straightforward manner. But when implemented into a complex tool such as RBESS, a few quirks come up in the process. Here are a few issues that we encountered and the solutions we created jointly with NAVFAC and NOSSA.
Not all assets are picked up in the RBESS analysis. The automated features within RBESS attempt to make the Tier 1 analysis process easier for the user by automatically gathering potentially impacted facilities into a list. The user is then prompted to enter additional information for these facilities (e.g., personnel counts and replacement cost). However, the ESS facility type codes that were collected did not include some utilities, as well as some roads and parking lots. Close coordination with the local NAVFAC Explosives Safety Planner was required to ensure that facility type codes were updated in a manner that did not severely impact existing QD-based site plans.
Facility damage to parking lots. By default, RBESS asks for the replacement value of each facility and uses that value to estimate the total damage cost. This doesn’t directly apply to parking lots. In the event of an explosion, a parking lot not immediately adjacent to a large explosion would not be ripped up and broken apart. The “value” of the parking lot is not the pavement, but rather the vehicles and possibly light fixtures within the parking lot that would bear the brunt of the explosion. The replacement cost for parking lots was estimated to be the average value of the expected number of vehicles in that particular lot.
Accurate valuation of utilities. Utilities are represented in multiple ways within the GIS data in ESS. The segmentation of electrical, water or other utility lines can create a single segment that encompasses a large portion of the installation. Using the default method for RBESS, when any portion of a large utility falls within one of the hazard zones the replacement value of the entire utility is captured in the estimate. To solve this, the value of the utility was set as the fraction of the total asset that was encumbered by the hazard zone.
The key to a successful RBESS analysis is to coordinate closely with NAVFAC and NOSSA. Do not assume that the automated RBESS tool is accounting for everything in a perfect way. It’s a useful, fast-running model for quick estimates, but it still requires a qualified user and healthy dose of quality control. When approached with proper attention, the software can produce high-quality results.
Takeaways
A Tier 1 risk analysis is a quick, data-driven way to get an early read on explosives safety risk, but it's only as good as the data behind it. Make sure your facility data is current, don't overlook utilities, roads, and parking lots. Remember that a Tier 1 analysis is a screening tool, not a substitute for a Tier 2a risk acceptance package or a full Tier 2b QRA when the decision calls for it.
Contact The Schreifer Group
You can find more articles about explosives safety on our website (Explosives Safety Articles). If you are looking for assistance with explosives safety site planning, please visit our website to learn more about how The Schreifer Group can help. Our growing team of explosives safety SMEs are ready to support you.
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