Mechanisms of fatigue degradation process in axially loaded grouted connections under submerged conditions

Grouted connections in offshore support structures bond overlapping steel tubes using ultra-high strength grout. These connections, used in jacket structures, face dynamic pulsating loads and submerged conditions. Current design guidelines offer simplified S-N curves that do not reflect realistic conditions for offshore wind turbines. This study introduces a new S-N curve for axially loaded grouted connections under submerged conditions, based on large-scale high-cycle fatigue experiments. It also analyses the degradation behaviour of cyclic axially loaded grouted connections to identify critical phase changes: stable, incremental, and progressive. The study examines the impact of different load levels on degradation, focusing on displacement between the pile and sleeve. Lower load levels result in longer stable and progressive phases. As load levels increase, the stable phase shortens and eventually disappears at the highest load levels. Damage within the grout material is categorised into crack-driven and crushing-driven patterns, with the latter linked to longer lifetimes. Even at the lowest load levels, minor damage occurs, precluding an endurance limit. This understanding of degradation mechanisms supports the assessment of grouted connections using monitoring systems.