Clear-water scour at monopile foundations subjected to cyclic lateral loads in different directions

Monopile foundations have been widely used to support offshore wind turbines in shallow waters. Currentinduced local scour at monopile foundations, as a universal problem threatening the structure safety and the operational efficiency of the turbines, has been extensively studied in recent decades by flume experiments and numerical models. Recent studies reported that the scour process around monopile foundations is affected by cyclic lateral loads induced by waves, currents, and winds. However, the scour mechanism around monopile foundations subjected to cyclic lateral loads in different directions remains unclear, necessitating further investigation. This paper presents an experimental study investigating the clear-water scour characteristics around monopile foundations under cyclic lateral loads applied parallel and perpendicular to the flow direction at a relatively low flow intensity. The experimental results reveal that when the amplitudes of the cyclic lateral loads exceed 4% of the monopile diameter at the monopile's top, the equilibrium scour depths at monopile foundations subjected to cyclic lateral loads perpendicular to the flow direction are generally larger than those under cyclic lateral loads parallel to the flow direction. Sour holes extent more significantly in the direction of cyclic lateral loads rather than the unload direction, and the slope angles in the load directions are gentler than those in the unload directions. Furthermore, based on the experimental data, an empirical equation is proposed to predict the current-induced equilibrium scour depth at cyclically vibrating monopile foundations in different directions.