Probabilistic failure assessment diagram with reliability update

Abstract

Offshore structures are susceptible to fatigue cracks due to the dynamic loading they endure from the marine environment, such as wave-induced forces. The initiation and propagation of fatigue cracks can significantly compromise the structural capacity of offshore installations, affecting their fitness-for-service. For structures with cracks, failure often manifests through fracture and/or plastic collapse. The Failure Assessment Diagram (FAD) methodology is specifically designed to evaluate the interaction between fracture and plastic collapse, as recognised in industry standards such as BS 7910. While a considerable number of studies have explored this area, many have been confined to either deterministic analysis or probabilistic analysis without considering crack growth, hindering the capability of FAD to seamlessly support structural integrity management which demands a life-cycle approach. This paper seeks to address this research gap by introducing a structural reliability analysis methodology grounded in FAD principles. The methodology developed herein comprises three primary components: crack growth analysis, probabilistic FAD, and reliability updating. The first two components work in conjunction to compute the structural reliability, with the limit state function based on FAD formulation. Reliability updating is facilitated through conditional probability, enabling the incorporation of crack sizing data from inspections so that preserving FAD’s utility for post-inspection engineering critical assessment. An illustrative example is presented to demonstrate the application of the methodology on a tubular T-joint, including a comparison with evaluations based on critical crack size. Finally, avenues for future research and developments in this field are outlined.