🎓 Lesson 8
D5
FMEA Fundamentals: Severity, Occurrence, Detection
FMEA is a step-by-step method to spot potential failures in a blasting design before they happen, and rank how serious, likely, and detectable each one is.
🎯 Learning Objectives
- ✓ Explain the meaning and scoring logic of Severity, Occurrence, and Detection scales in FMEA
- ✓ Calculate Risk Priority Numbers (RPNs) for blast-related failure modes using standardized rating tables
- ✓ Analyze and prioritize at least three blast design failure modes (e.g., flyrock, oversize, ground vibration exceedance) using RPN ranking
- ✓ Apply industry-standard FMEA rating scales (e.g., SMEC or MSHA-aligned 1–10 scales) to real blast planning scenarios
- ✓ Design mitigation actions for high-RPN failure modes by adjusting burden, spacing, or delay timing
📖 Why This Matters
In open-pit blasting, a single undetected failure—like misfired holes causing flyrock or excessive vibration damaging nearby infrastructure—can lead to fatalities, regulatory penalties, and production delays. FMEA transforms subjective 'what-if' concerns into quantifiable, prioritized risks. Mining companies like Rio Tinto and BHP require formal FMEA documentation for all high-risk blasts (>500 kg ANFO), making it not just best practice—but a contractual and compliance necessity.
📘 Core Principles
FMEA rests on three interdependent dimensions: Severity measures the worst credible consequence of a failure mode (e.g., fatality = 10; minor equipment damage = 2); Occurrence estimates the likelihood of that failure occurring *before* controls are applied (e.g., poor stemming = high occurrence in wet holes); Detection reflects how likely existing controls (e.g., pre-blast hole inspection, seismograph monitoring) are to catch the failure *before* detonation. Crucially, these ratings are *not* probabilistic but ordinal—based on expert consensus using calibrated scales. The RPN is a prioritization tool—not a risk probability—and must be interpreted alongside qualitative judgment: a low-RPN item with Severity=10 (e.g., catastrophic slope failure) may still demand immediate action regardless of O or D.
📐 Risk Priority Number (RPN)
The RPN aggregates the three FMEA ratings into a single numerical priority index. While not a true probability metric, it enables comparative ranking across failure modes. Scoring must follow documented, auditable rating criteria—not intuition—to ensure consistency across teams and audits.
Risk Priority Number (RPN)
RPN = S × O × DNumerical index used to prioritize failure modes for mitigation based on severity, occurrence, and detection ratings.
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| S | Severity Rating | unitless (ordinal scale 1–10) | Score representing worst-case consequence of failure mode |
| O | Occurrence Rating | unitless (ordinal scale 1–10) | Score representing inherent likelihood of failure mode occurring before controls |
| D | Detection Rating | unitless (ordinal scale 1–10) | Score representing likelihood existing controls will detect failure before detonation |
Typical Ranges:
Low-risk blast (small quarry, stable rock): RPN < 40
Medium-risk blast (large pit, fractured ground): RPN 40–120
High-risk blast (near infrastructure, steep slopes): RPN > 120
💡 Worked Example
Problem: During pre-blast FMEA for a limestone quarry bench blast, the team identifies 'inadequate buffer zone leading to flyrock beyond exclusion zone' as a failure mode. Using the SMEC 1–10 scale: Severity = 8 (serious injury possible), Occurrence = 4 (occasional—due to variable rock cover), Detection = 3 (moderate—visual inspection + drone survey used). Calculate RPN and interpret.
1.
Step 1: Confirm rating sources — SMEC Blast Safety Guide Table 4.2 defines S=8 as 'Major injury or significant environmental release', O=4 as 'Occurs occasionally (1 in 10–100 blasts)', D=3 as 'Controls detect failure >75% of time'.
2.
Step 2: Multiply ratings: RPN = S × O × D = 8 × 4 × 3 = 96.
3.
Step 3: Compare to threshold: SMEC recommends immediate review for RPN ≥ 80 *or* S ≥ 7 regardless of RPN. Since both apply, this failure mode is Tier 1 priority.
Answer:
The RPN is 96, which exceeds the SMEC action threshold of 80. Combined with Severity=8, this triggers mandatory redesign—e.g., increasing minimum buffer distance from 150 m to 220 m and adding real-time radar monitoring.
🏗️ Real-World Application
At Newmont’s Ahafo Mine (Ghana), an FMEA identified 'delay sequence error causing simultaneous detonation of adjacent rows' as a high-RPN failure mode (S=9, O=5, D=4 → RPN=180). Root cause analysis revealed outdated blast software failing to flag incompatible delay intervals. Mitigation included: (1) upgrading to BlastLogic™ with embedded FMEA checklists, (2) requiring dual-signoff on delay files by blaster and engineer, and (3) installing wireless node-based initiation with post-detonation delay verification. Post-implementation, no delay-related incidents occurred over 24 months—validating FMEA’s predictive value.