REASSESSMENT OF OFFSHORE STEEL STRUCTURES: SHAKEDOWN AND CYCLIC NONLINEAR FEM ANALYSES

The design conditions for a platform may change during the services life due to: - Increased topside (operational) loading. - Extended life beyond the design service life. - Re-appraisal of the environmental loading. - Reduced load-carrying capacity due to damages, cracks or corrosion. The consequence of these events may be that first member failure occurs for an environmental load smaller than the revised design load. If the ULS design criterion is to be met at any time during operation in the conventional way, the structure may either have to be strengthened, or the functional loads must be reduced. This may be a very expensive task. On the other hand, it is unlikely that the structure will collapse for a single design wave action, due to the inherent strength reserves that can be demonstrated. In 1988, Shell published a progressive procedure for re-assessment (Stewart et al, 1988) utilising non-linear static pushover analyses for structures whose integrity could not be demonstrated using conventional linear methods. This paper suggests how these more common pushover analyses can be extended: Instead of analysing the structure under single wave action, the structure is subjected to a series of waves, to account for the cyclic nature of the loading. A methodology, complementing the previous pushover procedures, is outlined for integrity assessment of structures under revised 'design' conditions. The methodology is based on the following simple observations: - Most offshore structures have a significant reserve strength, and a significant potential for redistribution of internal forces. - The dominating loading on offshore structures is a repeated, variable loading (wave loads). Nonlinear shakedown analyses is employed in order to investigate if the structure is likely to fail from the repetitive action of the waves, or if the structure remains stable.