Quasi-probabilistic Human Reliability Assessment (QHRA)

InterAction of Bath Ltd

Quasi-probabilistic Human Reliability Assessment (QHRA) resulted from a series of trials on representative tasks and it will be integrated within the overarching standard on ship safety analysis, SSP 96.

This article briefly describes QHRA's structure and the programme of testing and validation that resulted in its final form.

QHRA is a tool for assessing human reliability in naval vessels
QHRA is a tool for assessing human reliability in naval vessels

Structure of QHRA

In QHRA the analyst first builds a fault tree for a particular hazard associated with human error, for example "Hatch #4 not closed". The hazard may be an entry in a larger fault tree that primarily addresses hardware failures - such fault trees are typical of those produced by the Quasi-Probabilistic Safety Analysis described in SSP 96.

In building the fault tree the analyst identifies task failures that could lead singly or in combination to the top event - in this case "Hatch #4 not closed". Task failures are identified from of the set of procedures relevant to closing the hatch. It is assumed that all tasks have procedures. By reviewing procedures all relevant task failures can be identified for a particular hazard associated with human error.

Once the fault tree is constructed, the analyst is in a position to quantify the human error base events (HEBEs) in the fault tree, i.e. the task failures at the lowest level in the tree.

QHRA works in three passes of incremental detail and decreasing pessimism. If the system does not satisfy the safety criteria at Pass 1, the analyst continues to Pass 2, and similarly to Pass 3, as necessary.

Pass 1

In the first pass all task failures are assumed to occur with a probability of 1. All HEBEs therefore have a probability of 1. The analyst determines the acceptability of the system's safety by comparing it against safety criteria. If it is outside the criteria, the analysis continues to Pass 2.

Pass 2

Pessimism is more limited in Pass 2. Beneath AND gates, the most likely HEBE is quantified from a set of tables in the appendix to QHRA, the other(s) being set to 1. Beneath OR gates, all HEBEs are quantified, again by recourse to the tables. The probability of the top event is calculated and compared to the system's safety criteria.

Pass 3

Still less pessimism is assumed in Pass 3. All HEBEs are quantified and the possibility of error recovery is considered. Results are compared to the safety criteria and a judgement is made on the acceptability of the risk posed by the system.

Defining features of QHRA

Like the Human Error Assessment and Reduction Technique, QHRA requires the analyst to select generic probabilities from tables, matching the character of the reference task to exemplars.

But QHRA is unique in applying a three-pass approach to analysing human reliability. The underlying rationale for the phased approach is that analysts are able to allocate time and effort more efficiently by assessing in detail only those systems shown early in the analysis to be less safe.

By including dependency and error recovery, the technique attempts to move closer the reality of ship operating environments where subsystems are often closely linked and where the probability of error recovery varies considerably.

Trials of QHRA

InterAction of Bath conducted a series of user trials with QHRA. In the first programme of trials, users applied the methodology to two human hazards typical of ship operations. The trials served to check the usability and internal consistency of QHRA.

After suitable modifications, QHRA was applied to two hazards present in weapon embarkation. Involving a number of analysts and resulting in quantitative data, these trials allowed us to compare QHRA's output with that of HEART to check for concordance and reliability.

InterAction of Bath - experts in human reliability assessment

As the reliability of hardware and software in military operations has risen steadily, attention has turned to the role of human reliability in assuring safety.

InterAction of Bath is a Human Factors consultancy specialising in human reliability assessment (HRA), operability studies and safety case development. Since 1993 we have worked with some of the UK's largest defence companies, including Devonport Royal Dockyard, Alstom, BAE SYSTEMS, Weir Strachan and Henshaw, and VT Group. Outside the defence sector, we have had the pleasure of working with large civil companies, including Halcrow Group, Royal Mail and Rotork UK.

We have a high level of expertise in managing all stages of safety assessments and safety cases according to military standards, particularly JSP 520 and Def Stan 00-56 - the standards usually mandated by the Defence Procurement Agency.

High-profile accidents have reinforced the need to proof systems against operator error and improve human reliability. In pursuit of this goal, InterAction of Bath is pleased to offer its services to the defence sector.

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