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OTO 0060/1999

Target levels for reliability-based assessment of offshore structures during design and operation.

Deals with approaches for establishing consistent target levels for structural reliability analysis with reference to ultimate and fatigue limit states of components and system in new and existing structures. The target failure probabilities should be referred to annual or service-life values. In principle the target level can be defined to reflect all hazards (eg loads) and failure modes as well as the different phases (in-place and temporary phases) for each relevant consequence (fatalities, pollution, loss of assets and the most severe criterion would then govern the decisions to be made. However it is proposed herein to allocate a certain portion of the allowable total risk (probability of global failure) to each group of hazards and each failure mode (eg ultimate or fatigue failure of a component), each phase (eg In-place condition) depending upon consequence of failure. It is emphasised that the target level should depend upon the reliability methodology applied, especially which uncertainties are accounted for; the cause and mode of failure; the consequences of failure as well as the expense and effort to reduce the risk of failure. It is also important to establish a reliability methodology, which is in accordance with design practice for well-known cases. Target failure probabilities given in general guidelines for structural reliability analysis should not be applied before they are justified for the relevant application.

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Updated 2010-03-19