Hazard Studies & Risk Assessment

Human Reliability Analysis (HRA)

Quantified human-error probability for LOPA, QRA, and safety-case credit defence

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What this study delivers

Human Reliability
Analysis (HRA)

HRA quantifies Human Error Probabilities (HEPs) for safety-critical tasks — typically in the range 10⁻¹ to 10⁻⁵ — to anchor LOPA operator credit, QRA initiating-event frequency, and Bow-Tie human-barrier effectiveness. Methods cluster into three generations: First-generation behavioural (THERP NUREG/CR-1278, 1983) with detailed task decomposition; Second-generation cognitive (CREAM 1998, ATHEANA NUREG-1842) integrating Common Performance Conditions and contextual factors; and Third-generation Bayesian / contextual (Petro-HRA — IFE NORWAY 2017, IDHEAS-G NUREG-2199 2021) tailored to oil-and-gas and modern control rooms. HSE Human Factors guidance (HSG48), CCPS Human Factors Methods (2007, rev. 2024 in draft), and ISO 11064 control-room ergonomics frame the discipline. Common audit failures: indiscriminate use of HEART nominal probabilities without proper PSF / EPC selection, double-counting operator credit between LOPA and SIS, and ignoring time-pressure and stress effects in upset scenarios.

Human Reliability Analysis (HRA) — Overview
Study execution

How the study is executed

A structured, facilitated process — from scope definition through close-out — producing defensible, actionable outputs.

Safety-Critical Task Analysis (SCTA)

Identify safety-critical tasks from HAZOP, LOPA, operating procedures, and incident history; rank by consequence severity and frequency of demand; define HRA scope and method selection basis; agree boundary between HRA-scope and nominal operating-procedure design.

Hierarchical Task Analysis (HTA)

Decompose each safety-critical task to atomic action steps using HTA; document goals, sub-goals, operations, and plans; identify decision points, information requirements, feedback loops, and time constraints; provide structured task description for HEP calculation input.

Method Selection & HEP Calculation

Select method per task type and complexity — HEART for screening, THERP (NUREG/CR-1278) for detailed decomposition, CREAM for cognitive tasks, Petro-HRA (IFE 2017) for process-industry context, IDHEAS-G for nuclear-grade; calculate nominal HEPs per method tables.

PSF / EPC Calibration & Error Mode Analysis

Calibrate Performance Shaping Factors (THERP) or Error Producing Conditions (HEART) to plant context — time pressure, stress, training level, HMI quality, procedure clarity, communication quality, environmental conditions; distinguish Error of Omission (EOO) vs Error of Commission (EOC).

Dependence Treatment & Recovery Factor Analysis

Apply THERP dependence modelling (zero to complete) to sequential operator actions; calculate combined task HEP accounting for complete dependence; compute recovery factors for actions that can be corrected by a subsequent check; prevent operator double-credit against LOPA SIF.

LOPA Integration & Design Recommendations

Integrate calibrated HEPs into LOPA operator-IPL credit and QRA initiating-event frequency; issue Bow-Tie human-barrier effectiveness scores; produce HMI, alarm rationalisation, and procedure improvement action register; provide competency and refresher-training prescription ranked by HEP priority.

Human Reliability Analysis (HRA) — Scope
Study scope

What the study covers in full

Safety-Critical Task Analysis (SCTA) with criticality ranking and HRA scope definition
Hierarchical Task Analysis (HTA) decomposition to atomic action level
Method selection — HEART for screening, THERP for detail, CREAM / Petro-HRA for cognitive, IDHEAS for nuclear-grade
Performance Shaping Factor (PSF) calibration — time pressure, stress, training, HMI quality, complexity
Error of Commission (EOC) versus Error of Omission (EOO) treatment
Dependence treatment per THERP — preventing operator double-credit
Recovery factor calculation for sequential operator actions
Integration with LOPA — HEP feeding initiating-event frequency or operator-IPL credit
Bow-Tie human-barrier effectiveness scoring
HMI, alarm rationalisation, and procedure redesign recommendations driven by PSF findings
Why it matters

Outcomes of Human Reliability Analysis (HRA)

Human Error Quantification Accuracy
  • Quantifies the human contribution that drives 60–80% of major-accident causal chains
  • Surfaces over-credited operator IPLs that fail under realistic time pressure and stress
  • Drives HMI, procedure, and alarm-system redesign at points that actually matter
  • Anchors competency programmes against the highest-HEP tasks
IEC 61511 / COMAH Human-Factor Defence
  • Satisfies IEC 61511 Cl.9 human-factor requirement for LOPA operator credit
  • Withstands HSE / CSB / OSHA NEP scrutiny of operator-action claims
  • Provides COMAH safety-case human-factor demonstration
  • Aligns with ISO 11064 control-room design and HMI ergonomics
HMI, Alarm & Procedure Improvement
  • Drives DCS HMI graphics design per ISA-101 toward situation awareness
  • Targets alarm rationalisation per ISA-18.2 to genuine cognitive bottlenecks
  • Anchors operating procedure rewrite — particularly abnormal and emergency chapters
  • Informs shift-handover protocol design and fatigue-risk management
LOPA Credit Accuracy & Training ROI
  • Prevents the LOPA over-credit pattern that drives later SIS retrofit
  • Reduces near-miss frequency through realistic procedure and HMI design
  • Targets training spend to the 10–20% of tasks with the highest HEPs
  • Trims underwriter loadings where human-factor maturity is documented
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