Nonlinear instability and time-domain response of thin-walled UHPC towers under floating dynamic boundaries

The upscaling of floating offshore wind turbines (FOWTs) to 15-MW capacities requires ultra-flexible, thin-walled ultra-high performance concrete (UHPC) towers, posing critical challenges to nonlinear stability. Conventional decoupled design approaches fail to capture the coupled effects of material degradation and geometric nonlinearity under extreme dynamic loading. This study develops a cross-scale, non-intrusive mapping framework to investigate the time-domain nonlinear instability of a 15-MW UHPC tower. Results show that dynamic eccentricity activates a dual-softening mechanism, leading to an approximately 15% increase in the bending-moment amplification factor relative to the purely geometrically nonlinear model. The failure mode transitions from global bending to localized crushing, accompanied by a pronounced damage localization within a narrow leeward region. Parametric analysis identifies a marked nonlinear response transition point of 180, beyond which the interaction between material softening and second-order effects induces up to a 49% reduction in ultimate capacity. The instability limit is further characterized by the singularity of the global tangent stiffness matrix coupled with strain energy evolution. The proposed framework provides a physics-based criterion for stability assessment and anti-collapse design of ultra-flexible offshore towers.