Global hotspot concept-based first-order fatigue reliability analysis of offshore tubular steel joints

The introduction of high-strength materials has made fatigue damage a critical limit state in modern structural design. Structures subjected to frequent repetitive or dynamic loading are particularly vulnerable to fatigue failure. In this study, an offshore structure subjected to cyclic wave loading during its service life is evaluated, focusing on the reliability of multiplanar KT-joints under wave loading uncertainty. Using the Hasofer–Lind approach, the reliability index (β) and probability of failure (Pf) were analysed across various operational scenarios. The results indicate that while the structure satisfies basic reliability criteria under low repetitive loading, these requirements are not fulfilled as the number of cycles increases. Specifically, the target reliability index of 3.8, defined by Eurocode 1990, was achieved in many low-cycle scenarios; however, this threshold was consistently violated under high-frequency loading. The study concludes that since the probability of failure increases significantly under high repetitive loads, rigorous maintenance, reinforcement, and structural strengthening protocols must be implemented – especially towards the end of the structure’s service life – to ensure the safety of both the asset and the personnel.