Abstract
There are an increasing number of older installations in use on the Norwegian Continental shelf. Inspections are a key issue in ensuring the safety of an older installation, and the inspection intervals, inspection methods and its reliability are clearly influencing the safety of the installations. Different approaches for updating inspection plans for older installations are considered in order to achieve decreased inspection intervals as the structure are ageing. The most promising method consists in increasing the rate of defects / crack initiation at the end of the expected lifetime. Different system aspects are considered incl. assessment of the acceptable annual probability of failure for one component dependent on the number of critical components. Information obtained from inspection of one component can be used not only to update the inspection plan for that component, but also for other nearby components. An example indicates that a high degree of correlation between the uncertain parameters in different components is needed in order to obtain substantial information which can be used in inspection planning. Fatigue failure of one (or more) components does not necessary imply total collapse of the structure. The importance of each component can be measured by the RIF (Residual Influence Factor) for each component and is illustrated by examples. When an installation becomes older then the number of fatigue critical components can be expected to increase. If the maximum acceptable annual system probability of failure is the same as for a new installation, this implies that the acceptable annual fatigue probability of failure decrease, which again implies more inspections.
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