Temporary Refuge Impairment Frequency (TRIF) for Normally Unattended Installations (NUI)



This SPC provides guidance for inspectors on the approach to take to determine whether duty holders have addressed the actions required by the Offshore Installations (Safety Case) Regulations 2005 (SI 2005/3117) and the Assessment Principles for Offshore Safety Case Assessment. This SPC replaces SPC/Enforcement./117, which has been withdrawn.


It has become apparent that some duty holders and/or their QRA consultants have been calculating TR impairment frequency (TRIF) for NUIs on a yearly basis, then factoring the frequency for the level of occupancy of the installation (ie number of days/hours the NUI is attended).

This factored frequency has then been quoted in the safety case with units of 'per year (/yr)' rather than per number of days occupied. This can have the effect of masking the fact that the TRIF may be in the intolerable region.

While it is valid to base the calculation Individual Risk on exposure time it is not valid to calculate TRIF based on the time personnel spend in the TR. TRIF should be calculated and quoted in safety cases on a yearly basis (ie /yr).


If inspectors find that factoring has been included, and the un-factored TRIF is found to be at or near the intolerability threshold (generally accepted as 1 x 10-3/yr), the duty holder should be asked to carry out a review of the TRIF calculation, TR definition, structure and protection philosophy. Risk reduction measures should be identified and implemented as appropriate to ensure that risk of impairment is as low as is reasonably practicable.

A recalculation of TRIF based on a less conservative approach may be acceptable. However, any removal or reduction in conservatism, or change in approach must be fully documented, justified and explained in the revised safety case. It is not acceptable for duty holders to merely re-calculate TRIF downwards with no justification.

If technical support is required OSD3.2 Fire, Explosion and Risk Assessment should be consulted.

Further information

For further advice relating to this SPC contact - OSD3.2

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Updated 2020-12-09