Quantitative risk assessments (QRAs) for offshore oil and gas exploration and production assets are commonly used in the industry to demonstrate that risks are within tolerable limits. The results of these QRAs are included in an installation’s Operations Safety Case, which is prepared in response to the requirements of Regulation 7 of the Offshore Installations (Safety Case) Regulations 2005 (SI 2005/3117). These results provide part of the demonstration that the risks are managed to ALARP and are also used as input into the Justification for Continued Operations. A limitation of these assessments, however, is that a realistic comparison between assets is often not possible since methods, input data and assumptions vary from one QRA to another, in some cases quite widely. These variations may include the use of differing leak frequency databases, differing parts count definitions, ignition models, and fatality and impairment probabilities. In addition, QRAs do not focus on potential asset, or environmental consequences which are of significant interest to operators and equity partners.

This paper presents a methodology for assessing Major Incident Frequencies to enable a realistic comparison, on a like for like basis, of a range of asset types, which have been assessed using differing QRA methodologies. The methodology presented within this paper has been applied to various assets including fixed manned and unmanned installations, FPSO’s and FSO’s. The results indicate that the methodology developed is sufficiently robust to allow a meaningful comparison to be made between different types of assets, and provides an improved estimation of the overall major incident risk frequency compared with that which is typically presented in installation specific safety cases.

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