Author(s): Zishun Yao; Bruce W. Melville; Dawei Guan; Asaad Y. Shamseldin
Linked Author(s): Bruce W. Melville
Keywords: Sand motions; Vibrating monopile foundations; Backfilled sand; Saturated conditions; Dry conditions
Abstract: Monopile foundations of offshore wind turbines undergo continuous vibrations due to the cyclic lateral loads induced by waves and winds. To focus on the dynamics of sand behavior around vibrating foundations, researchers conduct two-dimensional experiments enabling convenient observation of sand motions and bed deformation around the structure, facilitating a better understanding of how the sand responds to the cyclic vibrations. Previous studies have investigated sand motions around vibrating monopile foundations under dry conditions and found sand convective and subsidence motions around them. However, two questions remain unclear: (1) what’s the influence of backfilled sand on the sand motions around vibrating monopile foundations? (2) whether the same convective and subsidence motions occur under saturated conditions and whether there are any differences in sand motions between dry and saturated conditions. Thus, this study with two experiments is presented. The first experiment was designed in two-dimensional conditions and different particle sizes of backfilling sand were used. As shown in Fig.1, the backfilling sand (blue color) is mixed evenly within the convective zone if the backfilling sand and original bed sand (red color) are of similar size. The relatively larger backfilling particles are prone to be concentrated in the upper part of the convective zone, while the relatively smaller backfilling particles usually migrate away from the vibrating monopile to the boundary of the convective zone. The experimental results indicate that backfilling sand with a larger particle size can effectively reduce the depth of the subsidence hole and mitigate the sand convective velocity around the vibrating monopile, compared with backfilling material of smaller or the same size as the original sand bed. The second experiment investigates sand motions around vibrating monopile foundations under dry and saturated conditions. The experimental results indicate that the compacted sand surrounding the structure reduces the vibration amplitude of the monopile. But the vibration amplitudes are more significantly attenuated under saturated conditions compared to dry conditions. Moreover, as shown in Fig.2, the convective motions of sand under the saturated condition are weaker than those under the dry condition, which suggests that the viscous dissipation effect may weaken the convective motions around the vibrating monopile foundations.
Year: 2023