Structural Integrity of Typhoon Damaged Structures

Abstract

This paper concerns the integrity reassessment of offshore wellhead platforms damaged by a typhoon, the damage consisting of cracking at joints between the conductor support frames and the main jacket structures. These structures were reassessed to establish the stability and growth rates of the cracks, and the effect of the cracks on the overall stability of the conductor frames and the structures as a whole. The structures were reanalysed using models that included joint flexibility which had a significant effect on load levels at the critical crack locations. Non-linear ultimate load analyses were carried out at selected joints in order to develop plastic collapse values for a fracture mechanics assessment of crack fracture criticality and growth rates. A series of analyses was carried out to investigate the potential progressive failure of the conductor frame supports and the effect of this on overall structural stability. Remedial measures were considered in detail including crack grinding, chord grouting, member removal and clamping. The results of the reassessment showed that the analysis including joint flexibility predicted more accurately the crack growth rates. The flexibility considerably relieved member end moments albeit increasing mid span moments and deflections. The progressive collapse analysis showed that a rapid failure in the event of a complete member severance is unlikely and that therefore a planned programme of inspection and remedial grinding can be instituted. This paper describes the overall assessment strategy and the detailed assessment methods used. It demonstrates the useful role of non-linear finite element analysis and fracture mechanics techniques in minimising the need for expensive underwater structural repairs.