Early-age-movement in grouted connections of offshore structures

To effectively combat climate change, transitioning from fossil fuels to renewable energy sources is essential. Offshore wind energy will play a key role in this transition. As future bottom-fixed offshore structures are planned for greater water depths and distances from shore, in addition to XXL monopiles, jacket foundations emerged as promising alternatives. Grouted connections, hybrid tube-in-tube connections with grout filler, are commonly used to create rigid joints between support structures and foundation piles. Thus they face unique challenges in offshore environments where undisturbed curing cannot be guaranteed. Relative movements between the steel components of the grouted connection can significantly influence bond and grout material properties. Existing knowledge in this specific field is based on a limited number of experimental studies conducted between 1978 and 1994 for the oil and gas industry. These studies primarily analysed axial relative movements using grout materials now considered outdated. Partially observed reductions in stiffness, load capacity, and fatigue strength have led to conservative guidelines for permissible relative movements, which often pose challenges to be met in current offshore projects. At the ame time, an urgent need for further research is already addressed within previously mentioned guidelines. Against this background, this dissertation developed a comprehensive experimental test program complemented by numerical analyses. The study investigates the impact of early-age movements on material and bond properties through real-scale segment tests and the load-bearing capacity of large-scale axially-loaded grouted connections. To ensure realistic application of early-age movements, typical offshore structures were evaluated using numerical seastate simulations. Lateral relative movements were identified as predominant, often measuring several millimetres. To replicate stresses and movements encountered by grouted connections during an installation, a novel experimental setup was designed, capable of simulating various types of relative movements under controlled boundary conditions. A special experimental control system was developed and implemented to account for realistic load redistributions caused by time-dependent stiffness variations considering connection and structural stiffness. Using a simplified mechanical model, it was demonstrated that these variations significantly affect stress levels in the grout material during the critical fluid-to-solid transition. Conducted segment tests were dismantled after 24 hours of early-age movement to analyse the material and bond properties within the grout interface. Key findings revealed a significant influence of movement direction (axial vs. lateral), displacement, and load amplitude, which caused localized structural damage or plas- ticization in the grout material near the moving steel component. Beyond standard strength evaluations, plasticizations were analysed using hysteresis evaluations and 3D scans, highlighting localized damage which conventional compression and flexural strength tests on extracted samples could not detect. Additional parameters, such as grout material, water-to-solid ratio, shear key height-to-spacing ratio, and ambient temperature, were investigated but deemed secondary as they indirectly influence the stiffness development and accordingly potential damage phenomena. To assess the impact of locally observed damage on the load-bearing capacity of axially-loaded grouted connections, identified primary parameters were analysed in a two-staged experimental approach using scaled cylindrical grouted connections. In the first stage, early movements were applied for 24 hours, followed by a 14-day curing period. The specimens were then tested for axial ultimate load capacity. Alongside early-age movement tests, three reference tests confirmed reproducible results with minimal scatter. The combination of conventional strain gauges and digital image correlation provided detailed insights into load transfer mechanisms in grouted connections with shear keys. Tests involving early movements revealed severe stiffness losses and load capacity reductions of up to 50 %. The predominant factors were the direction and the displacement amplitude of the applied early-age movement. Variations in load amplitude primarily affected plasticization and gap formation but were less significant for load capacity within the tested range. Generally, the observed load capacity reductions were smaller compared to earlier studies, which might be attributed not only to the grout material but also to the experimental design and test specimens. Due to the inhomogeneity of lateral relative movements along the grouted connection and circumference, a differentiation based on stress direction is strongly recommended. Finally, the experimental results were supplemented by numerical investigations covering the fluid and solid solid state of grout materials. Significant gaps in experimental methods for characterizing grout properties during this fluid-to-solid transition let to simplified approaches not able to consider detailed non-linear material behaviour within the transition phase. A comprehensive sensitivity study using finite element simulations of solid grouted connections – with non-linear contact and material law formulations – successfully replicated experimental results and expanded the existing knowledge base for modelling hybrid grout to steel connections. Numerical approaches also incorporated the effects of plasticizations caused by early-age movements, offering valuable insights for future applications.