Integrity Management for Lifetime Extension of Offshore Structures

Offshore structures are designed for safe operation until the end of a pre-defined design life. However, the value of a life extension beyond the original plan can be very large. Continued production from existing facilities has potentially lower cost and reduced carbon emissions compared to replacement of facilities. This is indeed the case for oil and gas production where additional near-field resources can be unlocked by tie-ins to existing facilities, but also for offshore wind projects where the energy production will be maintained over a longer time. Longer life will therefore increase the energy output and make the renewable energy more competitive. Integrity management for structures subjected to fatigue loading, and corrosion attacks, has traditionally been based on ‘safe life’ design principles. This design philosophy require that the integrity shall be maintained over a certain time interval under normal operational follow-up and without condition monitoring or integrity assessment during operation. The design life can be updated if more precise design data can be introduced, e.g. environmental loading conditions based on real data from operation. If the experienced loading is lower than assumed in design, the life can be somewhat extended. Lifetime extension beyond this will require a measure of the current condition of the structure as basis for an integrity assessment. This approach is termed ‘condition-based’ integrity management where we initially assume that the structure has infinite life if the condition can be monitored, and integrity maintained. The condition monitoring can either be continuous measurements or discrete inspections where the inspection interval is managed as part of the integrity assessment. As offshore structures are large, inspection, maintenance and repair are challenging tasks. The technology development required to support life extension and late life integrity management is therefore divided into three tasks: Improved inspection and condition monitoring technologiesImproved determination of the representativity of inspection results to assess overall integrity (including uninspected locations)Improved accuracy of fatigue calculations, both in terms of S-N and crack growth analysis. A major obstacle for improvement is the large variation in the current standards and recommendations for fatigue assessment. To ensure safe assessments that can be verified by independent parties, a common platform is needed. This platform should include aligned principles for all relevant technical aspects of the assessment. For fatigue assessment this includes variable amplitude loading, sequence effects, balance between initiation phase and propagation phase as well as environmental effects. For corrosion assessment the corrosion propagation model and the related environmental effects must be well defined.