Offshore wind turbines, crucial for global electricity generation, face significant challenges from harsh marine conditions, including strong wind, waves, and uneven seabeds. To optimize the foundation solution, this study investigates the lateral performance of helical monopiles, comparing conventional monopiles under cyclic loading, with a focus on variations in pile configuration and soil conditions. Model-scale experiments were conducted with helical piles subjected to both monotonic and one-way cyclic loading conditions. Key variations in the study include three soil densities (Dr = 35 %, 55 %, and 75 %), along with different slope conditions (Flat, 1V:5H, 1V:3H, 1V:2H) and pile positions (c = 0Dp, 2.5Dp, 5Dp, 7.5Dp). Additionally, the effect of load amplitudes (xi b = 50 %, 40 %, and 30 %) applied at a frequency of 0.25Hz for over 1000 cycles was examined. Results showed that helical piles outperformed conventional monopiles, exhibiting up to 25 % higher lateral load capacity, 30 % less accumulated rotation, and 20 % greater cyclic stiffness, especially in dense soils. Furthermore, the analysis revealed that the performance of helical piles significantly improved when placed nearer to the slope crest and in denser soils. Numerical simulations using PLAXIS 3D confirmed these experimental findings, demonstrating that helical piles consistently maintain superior lateral resistance and cyclic performance under varying loading conditions and slope configurations. This study underscores the potential of helical piles to enhance the stability ad performance of offshore wind turbine foundations, offering a more robust and efficient alternative to monopile systems.
This study explores the performance of finned monopiles as an innovative foundation solution for Offshore Wind Turbines subjected to cyclic loading under varying seabed conditions. Traditional monopiles face challenges related to stability when installed on sloped terrains, which are common in offshore environments. To address this, the research investigates the effectiveness of rectangular fins attached along the monopile's length to improve lateral resistance and reduce accumulated rotation. Experimental and numerical analyses were conducted across different slope gradients (flat, 1V:5H, 1V:3H, 1V:2H), pile positions (0Dp, 2.5Dp, 5Dp, 7.5Dp), and soil densities (35%, 55%, 75%), applying cyclic loading at 0.25 Hz over 1000 cycles with lateral load amplitudes (xi b) of 30%, 40%, and 50%. This study is the first to investigate finned monopiles under cyclic loading on sloping seabed conditions, demonstrating a 30-60% improvement in lateral resistance by increasing the passive soil resistance by reducing the rotation compared to monopiles. This work addresses the challenges of Offshore Wind Turbine foundations in complex topographies. Numerical modeling using PLAXIS 3D closely aligned with experimental findings, confirming the effectiveness of finned monopiles in enhancing stability on sloped seabeds. These findings suggest that finned piles offer a robust foundation alternative for Offshore Wind Turbines, particularly in challenging environments with variable seabed topography.
Offshore wind turbines are often supported on monopiles and are always subjected to long-term cyclic loading during their service life. This cyclic loading induces changes in the damping, stiffness and permanent accumulated rotation of the monopile foundation. The main purpose of this paper is to investigate the effect of these three changes occurring simultaneously on the dynamic response and fatigue life of offshore wind turbines in sand. To this end, an integrated methodology is presented based on time-domain finite element model and small-scale model tests of rigid piles. Three states of the monopile foundation are selected and defined based on the operational time of the turbine. The results show that these three changes have a slight effect on the dynamic response of offshore wind turbines, but have a significant effect on the fatigue life. The fatigue life decreased from 23.3 years for the initial state to 20.99 years for the medium state and 19.45 years for the ultimate state, a decrease of 10% and 16.5%, respectively, indicating that these changes should be addressed in the design of the fatigue life calculation of offshore wind turbine structures. The systematic parametric analysis shows that soil damping has the greatest effect on the dynamic response and fatigue life, followed by soil stiffness, which is less affected by permanent accumulated rotation.