A Preliminary Study on Convective Motions of Sand Particles around Monopiles Subjected to Cyclic Lateral Loadings

Monopile foundations for wind turbines are subjected to millions of lateral cyclic loadings during their lifespan due to waves, winds and operations of the turbine motor. The vibration-induced sand particle movement around pile foundations can cause the densification of the seabed and alter the stiffness of foundation. The objective of this paper is to obtain an improved understanding of the mechanism of the sand particle motions around monopiles subjected to cyclic lateral loadings. To simplify the sand-monopile interaction process, the experiments were designed in 2D conditions and were conducted in a sand box (internal size: 0.40 m long, 0.12 m wide, and 0.55 m high) with a transparent vertical wall. The sand used in this study are uniform granular materials with d₅₀ = 0.48 mm and σ_g=1.1. The monopile model installed in the sand box is a 2D aluminum plate with a dimension of 10 mm wide, 100 mm long, and 750 mm high. The initial embedded depth of the monopile model was kept at 0.3 m for all the tests. The lateral cyclic loadings with different frequencies and force amplitudes were applied by an actuator on the top of the monopile model. During the experiments, the 2D convective motions of sand around the vibration monopile were recorded by a webcam with a filming speed of 30 fps. By using Particle Image Velocimetry (PIV) techniques, the velocity and displacement of sand particles at different stages of sand movements were calculated. The results show that the sand convective area and velocity around the monopile foundations are closed related to the frequency and force amplitude of the lateral cyclic loading exerted on the top of the monopile.

Key Words: Wind turbines, monopile, sand particles, convective motions, lateral cyclic loadings.