Irregular Wave-Current Motion Response of a Lazy Wave Power Cable

Abstract This study investigates the dynamic response of a laboratory scale power cable model in a lazy wave configuration subjected to combined wave-current interactions. Experiments were conducted at the FloWave Ocean Energy Research Facility at the University of Edinburgh, where waves and currents were generated at incident angles of 0°, 90°, and 180° relative to the longitudinal axis of the cable. In still water the cable hung in the XZ plane with X horizontal and Z vertical. A Qualisys motion tracking system captured three-dimensional displacements along the cable. Test cases included irregular waves interacting with following currents, opposing currents, and waves oriented perpendicular to the current direction. Results show that the RMS displacement magnitudes of the cable were similar for currents aligned inline and opposite to the waves. However, distinct differences in dynamic response were observed when currents acted on the concave (upward curvature) and convex (downward curvature) regions of the lazy wave configuration. Peak cable responses occurred at the zero-crossing wave frequency in the absence of a current. However, the introduction of a current (0.3 and 0.5 m/s) caused frequency shifts and introduced an additional frequency component (Strouhal number 0.1–0.2) associated with vortex-induced vibration (VIV). This VIV-related frequency component varied spatially along the cable length, with distinct frequency shifts emerging dependent on the current direction. For the cases tested, the maximum RMS displacement in the X-direction was observed to be 0.25D (where, D is the cable’s diameter) when the current acted at 90° in alignment with the wave direction; whereas, in the Y-direction, it reached 0.38D when the current was perpendicular to the wave direction. It is observed that combined wave-current action results in a higher RMS response compared to cases with either wave or current alone.