There are many existing fixed offshore platforms, whose actual service life is about to exceed the original design life, that need to be maintained in production due to revised benefits in owner's economics. This is the main reason why a platform's reassessment is required in order to verify the residual structural capability of the platform.
Particularly, the structural performance shall be revised with detailed analyses relevant to the present fatigue and corrosion conditions, whose limit states are reassessed with respect to the original design safety margins but, more in general, the existing structure can be deemed fit-for-purpose when the risk of structural failure leading to unacceptable consequences is adequately low. Such a fitness for purpose, or required safety target, shall be therefore demonstrated for the specific site conditions and given operational requirements (such as the desired life extension) mainly considering, besides the detailed fatigue and corrosion analyses, that the original design target was ensured with respect to the dominant extreme environmental event, i.e. the wave, wind and current loading condition, whose return period (typically 100-yrs) is established with reference to the expected design life.
This paper describes how the required safety target can be related to the actual system capacity of the platform, measured by the residual strength reserve of the whole jacket evaluated by, e.g., a push-over analysis, and then introduced in a system reliability assessment capable to eventually determine the actual residual life of the structure and maximum return period of the extreme environmental loading that the platform is still capable to withstand.
Finally, the same approach can be used for a preliminary evaluation of the fitness for purpose of an existing platform to be reused and moved to a new site where the design conditions are different than the original design ones.
