Author(s): Dawei Guan
Linked Author(s):
Keywords: Local scour; Monopile foundations; Vibration; Combined wave-current conditions
Abstract: Among various foundation structures for offshore wind turbines, monopile foundations are widely adopted. The structure of wind turbines, under the combined effect of scouring and horizontal vibrational loads, may undergo reciprocating displacements, posing a threat to structural safety. By using physical and numerical modeling, this study aims to provide a comprehensive understanding of the mechanism of local scour around monopile foundations subjected to cyclic lateral vibration. Firstly, a three-dimensional experiment was developed to investigate the fundamental mechanism of soil-pile interaction around vibrating monopile foundations. The results show that increasing either vibration amplitudes or frequencies enhances subsidence and convective motions of the sand around the foundation. Additionally, the three-dimensional plastic deformation of the sand particles around vibrating monopile foundations is characterized by initial shear contraction followed by shear dilation. Furthermore, a flume experiment was conducted to explore the mechanism of local scour around vibrating monopile foundations under conditions of current only. The experimental results demonstrate that three major factors, namely vibration-induced sediment subsidence, vibration-induced sediment refill, and current-induced erosion, collectively influence the equilibrium scour depth around vibrating monopile foundations. Building upon the flume experiment, a numerical model was established to investigate the flow field around the structure and the influence of the vibrating foundation on the surrounding flow field. The numerical model reveals that the effect of vibration on the average flow field in front of the monopile is minor, whereas it significantly impacts the flow field behind the monopile. Additionally, vibration weakens the strength of the main vortex, leading to a decrease in the bed shear stress corresponding to the main vortex. Finally, a physical modeling experiment was proposed to investigate the local scour mechanism around vibrating monopile foundations under combined current-wave conditions. A new dimensionless parameter, undisturbed friction velocity-based Froude number Fr*, is developed to quantify the hydrodynamic intensities under combined wave-current conditions. Based on the Fr*, the hydrodynamic conditions can be classified into three regimes. The characteristic dimensions of the equilibrium local scour hole around vibrating monopile foundations develop differently in these three regimes. In weak hydrodynamic intensities, the characteristic dimensions of the equilibrium local scour hole are more prominent around vibrating monopile foundations.
Year: 2024