API RP2A, 21st edition recognizes several consequence-based levels of platform design or assessment. L-l gives critical new platforms a level of reliability almost comparable to continuously manned platforms in the North Sea, and the economic impact of their failure risk essentially vanishes. L-2 is used for manned-evacuated platform, gives a finite but still low level of risk based on economic trade-offs; it has served the Gulf of Mexico industry well for three decades. Re-assessment criteria are lower in terms of metocean loading, and accept more risk, because upgrading platforms in place is an order of magnitude more expensive than adding a little beef to a new-build. Assessment level A-l is for well-maintained platforms built to "100-year" metocean criteria of the 1960s, but still serving vital functions, e.g. pipeline hubs, where the business interruption risk they pose to upstream producers can be considerable. A-2 simply provides for personnel safety in case a sudden (but weak) hurricane precludes evacuation; economic consequences are left for the operator to evaluate. Levels L-3 and A-3 are reserved for unmanned structures whose collapse does not even have significant economic consequence. It is presumed that high-reliability SCSSVs will prevent pollution in any case.The existing A-2 level is substantially weaker than L-2. It is suitable for production platforms on stripper-well status under the philosophy of "Produce until it falls down," which makes more sense than arbitrarily abandoning the productionprematurely. However, the risk of an old A-2 platform (or a cluster of them in the same field) failing before another decade is out has been estimated to be as high as 25% to 45%. This puts structural risk right up there with reservoir and price uncertainties, deserving consideration in the venture evaluation stage. Mitigation strategies, e.g. plugging depleted wells while they are still upright, also deserve consideration. This paper will examine these structural risks in light of existing standards, based on earlier criteria studies. The API-2 INT series of Interim standards, currently under development based on HEAT (Hurricane Evaluation and Assessment Team) work, will be the subject of future presentations.
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