Recent developments in probabilistic pushover models for the reliability re-assessment of fixed offshore structures

Abstract

Re-assessment of existing fixed offshore structures in mature basins, to demonstrate they remain safe and reliable whilst optimising returns on the considerable investment, is a constant challenge for operators. This often requires the use of a probabilistic framework employing statistical techniques for environmental load predictions and nonlinear material/geometry models to determine the structural resistance. Stepping out, from code-based strength and loading recipes based on linear structural analysis and prescribed 100-year load events, to a probabilistic framework employing nonlinear structural models and a target annual failure probability, is not a straightforward task. This article explores the following aspects that engineers face when undertaking fixed platform re-assessments: (1) how to progress with an analysis when the capacity is limited by the initiation of tubular joint cracking and therefore susceptible to low cycle fatigue degradation; (2) how to determine system strength uncertainty for a given failure mode when each component has a varying contribution to the overall resistance; and (3) how to quantify the influence of gravity loading on system strength uncertainty.