R EVIEW OF HSE’ S P ROTECTION B ASED A PPROACH 1 Overall Conclusions

The findings of the review are summarised in Table 5.2.

The terminology used in relation to protection-based analysis is not well defined. Similarly, for a given type of installation, the reasons for resorting to a protection-based analysis, and the reasons for the selection of the particular event (or events) chosen to define the LUP zones are generally not well explained or documented.

As further research and development is performed, protection-based analysis may be replaced by risk analysis for some types of installation. In particular, adoption of fatality as the harm criterion (in the form of TROD) instead of dangerous dose would deal with some of the objections to using risk analysis in particular cases. However, even if risk analysis methods for all types of MHI were available, there may continue to be other reasons justifying the use of protection-based analysis. A protection-based analysis could still be appropriate where:

• the Hazardous Substances Consent documents contain insufficient information for a risk analysis;

• the surrounding population density and demand on land-use are low; and,

• a protection-based analysis would generate similar results (in terms of the sizes of LUP zones and the advice given) to those from a risk analysis.

5.2.2 Recommendations

The review of HSE’s protection-based approach resulted in the following recommendations:

5. the terminology used in relation to protection-based analysis

(‘worthwhile’ protection, ‘unlikely but foreseeable’, etc.) is better defined; 6. the relationship between the two levels of protection stated in the aim

and the three LUP zones defined by the analysis is described;

7. HSE develops internal guidance for Inspectors on selecting events for use in protection-based analyses;

8. where protection-based analysis is used for installations of a given type, the reasons for adopting a protection-based approach, together with the rationale for selecting the event or events used in the analysis, are documented in such a form that the information could be released to interested parties outside HSE as required; and,

9. HSE continues to perform research into risk analysis methods so that some of the reasons for using a protection-based approach can be resolved.

With regard to the specific protection-based approach used for bulk LPG storage, it was noted that the choice of event for the analysis was influenced by the frequency of the BLEVE event currently assumed by HSE. It is recommended that:

10. the estimate of this frequency is updated and, depending on the outcome of this revision, that the selection of the event used in protection-based analyses of bulk LPG installations is revisited.

Table 5.2 Summary of Review Findings - Protection-Based Analysis

Review Area Specific Subjects Review Findings

General Considerations Aims and Terminology The terminology used is not well defined. It is not clear how the stated aims (which refer to two levels of protection) relate to the three zones used for LUP. The objections to use of protection-based analysis as stated in the original Risk Criteria Document remain valid (62)_.

Reasons for Using a Protection-Based Approach The general reasons stated for using a protection-based analysis are considered to be appropriate, considering the current limitations of risk analysis techniques. However, it is not always clear specifically which reasons have been used to justify use of protection-based analysis in a given case. Some (but not all) of the reasons for using a protection-based approach may be eliminated as research into risk-based analysis methods progresses.

Selection of Event Event selection is a critical step. However, there is no guidance for Inspectors on how to select a suitable event when performing a protection-based analysis of an

installation of a type that has not been considered before. The justification for the selection of events for a given type of site is not always documented.

Combination of Protection-Based Analysis Results The current approach of ‘merging’ overlapping protection- based zones does not take into account the potential for increased levels of risk in the regions where the zones overlap. This problem could be avoided by use of risk analysis instead of protection-based analysis.

Analysis Approach for Bulk LPG Storage Level of Harm The level of harm arising from many of the major accidents at a bulk LPG establishment would be considerably worse than a ‘dangerous dose’, justifying the use of a protection-based approach. However, this objection could be dealt with by using a risk-based analysis with fatality as the harm criterion.

Review Area Specific Subjects Review Findings

Uncertainty in Risk Analysis Another reason for the adoption of a protection-based approach is that there is considerable uncertainty associated with the likelihood of some of the events that may occur, particularly the probability of ignition of flammable clouds and the frequency of a BLEVE (see below). Further research in these areas is recommended. It may be that, as an interim position, protection-based analysis is retained, but with the use of an event other than the BLEVE, to generate the LUP zones.

BLEVE Frequency The BLEVE frequency is not used within the analysis, but influences the selection of BLEVE as the event to be modelled. Estimates of BLEVE frequency in the technical literature vary widely, with a significant discrepancy between estimates based on historical data and those calculated using analytical techniques. The reasons for this discrepancy are not clear. Further research is recommended.

ENVIRONMENTAL RESOURCES MANAGEMENT HSE0016072AUGUST 2004

5.3 REVIEW OF ASSUMPTIONS USED