Johnstone, D., Rosen, J., Hourigan, D., Kilner, A.A., Sincock, P., and Carra, C., Paper Number OTCB-26285, Proceedings of Offshore Technology Conference Basil, Rio de Janiero, 27-29 October 2015.

Abstract

This paper details the application of a reliability approach to re-qualification and life extension of FPU moorings. Operators are including FPU moorings in their corporate asset integrity management strategy, driven by changing production requirements, the requirement to extend service life, or circumstances where the met-ocean Basis of Design has increased significantly over the life of the field. Reliability methods are gaining increasing acceptance as increased computing power allows large numbers of simulations to be undertaken using realistic fully coupled models previously validated against experiments. Reliability analysis offers considerable advantages over conventional deterministic return period design when applied to the re-qualification and life extension of FPU moorings, particularly with regard to re-qualification and life extension of in-place moorings. An application of the technique is illustrated by a case study of the life extension of an FPU mooring with design met-ocean conditions that have increased significantly over the life of the field. Discussion points include explicit identification of the most likely failure scenario versus uncertainty as to whether the worst design case has been identified and the potential for rapid reassessment of reliability for specific design conditions, such as a degraded mooring system or a system for which degradation is ongoing. The results demonstrate the significant advantages to the industry conferred by adopting reliability methods in the re-certification and life extension of existing FPU moorings, where conventional mooring code deterministic design methods, whilst adequate for original design purposes, lack sufficient fidelity to address the multi‐faceted issue of re-assessment of notionally marginal legacy systems. This technique is applicable to a wide range of FPUs requiring re-qualification or life extension, and will also provide insights to more efficient and reliable mooring design for new FPUs. For an existing, possibly degraded, mooring, application of these methods can demonstrate that the level of reliability of the system is still acceptable, while a conventional approach may produce an over-conservative indication that the mooring is non-compliant.

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