Vortex-induced vibrations of lazy-wave cables under platform surge excitation

Surge motions of floating offshore platforms generate oscillatory fluid–structure interactions (FSIs) that can trigger vortex-induced vibrations (VIVs) in attached lazy-wave cables. Due to their segmented geometry, the resulting vibration response is inherently non-uniform and remains poorly understood. A fully coupled three-dimensional FSI model is developed to investigate the VIV response of a lazy-wave cable subjected to platform surge excitation under representative operating conditions. The results demonstrate that surge-induced VIV is characterized by pronounced multi-modal behavior with strong spatial heterogeneity. Under moderate excitation, the response is dominated by lower-order modes with localized higher-mode contributions, whereas intense excitation simultaneously activates multiple higher-order modes, giving rise to modal competition. Position–frequency energy analysis further reveals strong spatial heterogeneity of vibration energy in the frequency–space domain, with distinct modal responses governing different cable segments. Fatigue damage is governed not only by vibration amplitude but also by modal redistribution along the cable; damage hotspots are concentrated near geometric transition regions and migrate upward with increasing surge intensity. Despite their short duration, extreme conditions contribute disproportionately to cumulative fatigue owing to concurrent increases in both vibration amplitude and modal frequency. These findings provide a mechanistic basis for understanding surge-induced VIV in lazy-wave cables and its implications for fatigue assessment in floating offshore wind systems.