Risk Based Inspection Planning of Ageing Structures

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. INTRODUCTION The number of older installations is increasing in the Norwegian Continental shelf. The regulation of these older installations is a part of the Petroleum Safety Authority’s responsibility, with the intention of ensuring that also older installations can be regarded as safe. Inspections, maintenance and repair of the installations are a key issue in ensuring the safety of older installation, and the inspection intervals, inspection methods and its reliability are clearly influencing the knowledge of the safety of the installations. Development of a risk based inspection approach for aging platforms taking into account that for aging platforms several small cracks can be expected – implying an increased risk. Reliability and Risk Based Inspection (RBI) planning for offshore structures have been an area of high practical interest over the last decades, especially within inspection planning for welded connections subject to fatigue crack growth in fixed steel offshore platforms. The basic assumption made in RBI planning is that a Bayesian approach can be used. This implies that probabilities of failure can be updated in a consistent way when new information (from inspections) becomes available. Further, the RBI approach for inspection planning is often based on the assumption that at all future inspections no cracks are detected. If a crack is detected then a new inspection plan should be developed. The Bayesian approach and the no-crack detection assumption imply that the inspection time intervals usually become longer and longer with time. It is noted that RBI planning for future inspections are based on predefined ‘decision rules’ on repair / maintenance when inspection results become available. Further, inspection planning based on the RBI approach implies that single fatigue critical components are considered, one at the time, but with the acceptable reliability level assessed based on the consequence for the whole structure in case of fatigue failure of single components. Examples and information on reliability-based inspection and maintenance planning can be found in a number of papers, e.g. Madsen, Sørensen & Olesen [1], Skjong [2], Sørensen, Faber, Rackwitz & Thoft-Christensen [3], Ersdal [4], Sørensen, Straub & Faber [5], Moan [6], Straub & Faber [7], Faber, Sørensen Tychsen & Straub [8] and PIA [9].