🎓 Lesson 16
D5
Bow-Tie Diagram Construction: Threats, Controls, and Recovery
A bow-tie diagram is a visual tool that shows how a dangerous event can start, what can stop it from happening, and what can be done if it does happen.
🎯 Learning Objectives
- ✓ Analyze a mining blasting scenario to identify credible threats and potential consequences using bow-tie logic
- ✓ Design and label at least three effective preventive and three mitigative barriers for a specified hazardous event
- ✓ Evaluate barrier effectiveness using industry-defined criteria (e.g., CCPS Barrier Effectiveness Rating)
- ✓ Explain how barrier degradation or failure contributes to escalation paths in real-world incidents
📖 Why This Matters
In open-pit mining, a single misaligned blast can trigger rockfall, flyrock injury, or dust-induced respiratory hazards — but these outcomes aren’t inevitable. Bow-tie diagrams help engineers *see* the full risk story: not just ‘what could go wrong’, but *how* we stop it before it starts, and *how* we limit harm if it does. They’re required in major mining EHS management systems (e.g., Rio Tinto’s Risk Excellence Framework) and are central to incident investigations under MSHA Part 46/47 compliance.
📘 Core Principles
The bow-tie structure consists of five core elements: (1) Threats (initiating causes), (2) Preventive Barriers (controls that reduce likelihood), (3) The Central Hazardous Event (e.g., ‘uncontrolled blast overpressure’), (4) Consequences (outcomes like injury or equipment damage), and (5) Mitigative Barriers (controls that reduce severity). Barriers must be Specific, Measurable, Achievable, Relevant, and Time-bound (SMART) and classified by type: physical (e.g., blast mats), procedural (e.g., pre-blast clearance checklist), technical (e.g., seismograph monitoring), or human (e.g., certified blaster authorization). Escalation factors — such as poor maintenance, inadequate training, or bypassed interlocks — explain why barriers fail and are mapped as ‘barrier degradation paths’.
📐 Barrier Effectiveness Rating (BER)
The Barrier Effectiveness Rating quantifies how reliably a barrier performs its intended function. It combines frequency of use, reliability, independence, and detectability into a single ordinal score used in CCPS and IEC 61511-aligned assessments.
Barrier Effectiveness Rating (BER)
BER = F + R + I + DOrdinal score quantifying barrier performance across four dimensions; used to prioritize barrier assurance activities.
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| F | Frequency of Use | dimensionless | How often the barrier is relied upon (e.g., Daily = 3, Weekly = 2, Per-shift = 4) |
| R | Reliability | dimensionless | Historical success rate of barrier function (e.g., 95–100% = 4, 90–94% = 3) |
| I | Independence | dimensionless | Degree to which barrier operates without shared components/failure modes (Yes = 2, Partial = 1, No = 0) |
| D | Detectability | dimensionless | Time to detect barrier failure (≤1 min = 3, 1–5 min = 2, >5 min = 1) |
Typical Ranges:
High-integrity technical barrier (e.g., blast seismograph auto-shutdown): 12–16
Procedural barrier with periodic verification (e.g., pre-blast checklist): 7–11
💡 Worked Example
Problem: A blast exclusion zone barrier relies on a handheld radio check (used daily), has 92% historical reliability, is independent of other barriers, and its failure is detectable within 2 minutes during pre-blast briefing. Assign BER using CCPS Table 5.3 (2018).
1.
Step 1: Frequency = Daily → Score = 3
2.
Step 2: Reliability = 92% → Score = 3 (90–95%)
3.
Step 3: Independence = Yes → Score = 2
4.
Step 4: Detectability = <5 min → Score = 2
5.
Step 5: Sum = 3 + 3 + 2 + 2 = 10 → BER = Level 3 (‘Good’; range 9–12)
Answer:
The result is BER = 10, which falls within the ‘Good’ effectiveness range (9–12) per CCPS Guidelines (2018), indicating this barrier is suitable for high-consequence prevention but requires quarterly verification.
🏗️ Real-World Application
At the BHP Olympic Dam copper-uranium mine (South Australia), a 2021 near-miss involved flyrock breaching the exclusion zone due to undetected water saturation in a blast hole. The bow-tie analysis revealed: Threat = ‘inadequate moisture assessment’; Preventive barrier = ‘pre-blast geotechnical scan + moisture probe protocol’ (BER = 7 → ‘Fair’); Gap = no verification that probes were calibrated weekly. Consequence = ‘worker struck by rock fragment’; Mitigative barrier = ‘hard-hat PPE + designated shelter’ (BER = 11 → ‘Good’). Post-incident, the barrier was upgraded to include automated moisture logging with alarm integration — raising BER to 13 (‘Excellent’).
📋 Case Connection
📋 Ammonia Refrigeration System HAZOP & LOPA Integration at Midwest Food Processing Plant
Unplanned releases during maintenance due to undocumented isolation points and missing P&IDs
📋 Ethylene Oxide Sterilization Facility QRA and Bow-Tie Implementation
Regulatory pushback on offsite risk due to proximity to residential area (≤500 m)
📋 Offshore LNG Transfer System Fault Tree Analysis and SIS Architecture Optimization
High consequence of LNG spill + ignition in congested maritime corridor; existing SIS used single-channel logic