Wave buoys in current - experimental results and observations

Abstract

Wave buoys are used extensively for the characterisation of deployment locations for offshore technologies, where they are subjected to currents in addition to the ocean waves. It has been frequently observed that the measurements from wave buoys are affected by these currents, but the majority of deployed buoys cannot measure, or account for, this effect. Presented here are a series of experiments conducted at the FloWave Ocean Energy Research Facility, deploying a spherical wave buoy scale model with a simplified mooring in a series of combined wave-current sea states. The resulting openaccess dataset provides 6 degree of freedom buoy motion and force data (current only) in addition to wave gauge and acoustic doppler velocimeter measurements of the sea states. Experiments were conducted under a range of combined wave-current conditions with variables including velocity, wave period, and relative wave-current angle. It is observed that vortex induced motions (VIM) are significant and highly sensitive to the mooring configuration and current speed. Nevertheless, the wave-induced response and buoy motion amplitude is found to agree with linear wave-current theory predictions in most following wavecurrent conditions. This agreement is poorer in opposing conditions where larger surge motions than predicted are consistently observed. Wave buoy outputs in directional irregular seas were also found to closely match wave gauge outputs. If properly considered, it is suggested that the effect of the current on existing buoy technologies may be accounted for without hardware modification or additional sensing, but through updated analytical tools.