A 3-D modelling of monopile behaviour under laterally applied loading

Deploying higher-capacity offshore wind turbines to meet the growing energy demand poses a significant challenge in designing their foundations. Monopiles currently constitute 80% of the foundation installations for these turbines. This study utilizes a nonlinear three-dimensional (3D) finite element model to explore the behavior of monopiles underpinning a five megawatt wind turbine under horizontal loads. The findings reveal that the performance of monopiles is influenced by the strength of the soil and the ratio of pile depth of embedment to diameter (L/D). Examination of flexural bending profiles at/close to failure loads demonstrates the flexible behavior of monopiles, even with a low L/D ratio. The L/D ratio exhibits varying degrees of impact on the normalized ultimate lateral capacity of monopiles, with a notable effect observed in soft clays, resulting in an increase of up to five times for L/D ratios ranging from 3.33 to 13.33. Stiff clays show comparatively lesser effects. Under serviceability loading, an increase in the L/D ratio leads to a 3-6% rise in the maximum flexural moment of monopiles, while the maximum shear force experiences a decrease of 20-30%. Furthermore, a significant reduction, up to five-fold, in the maximum tower tip displacements and rotations at the mudline is observed with an increasing L/D ratio. However, this reduction is more pronounced for higher foundation rigidity. Flexibility of Monopiles: The examination of flexural bending profiles near failure loads reveals the flexible behavior of monopiles, even with a low L/D ratio. These findings challenge conventional assumptions about the rigidity of monopiles and underscores the need for a deeper understanding of their structural behavior.Impact of L/D Ratio: The study demonstrates that the L/D ratio has a significant impact on the normalized ultimate lateral capacity of monopiles, particularly in soft clays, where increasing the L/D ratio can lead to a substantial increase in lateral capacity, potentially up to five times.Serviceability Loading Effects: Under serviceability loading conditions, increasing the L/D ratio results in a rise in the maximum flexural moment of monopiles and a decrease in the maximum shear force. This suggests that optimizing the L/D ratio can enhance the structural performance of monopiles, especially in terms of mitigating shear forces.Reduction in Tower Tip Displacements: An important finding is the significant reduction in maximum tower tip displacements and rotations at the mudline with an increasing L/D ratio. This reduction, which can be up to five-fold, highlights the potential for improved stability and reduced structural stresses with higher L/D ratios, particularly in environments with higher foundation rigidity.Overall, this study offers valuable insights into optimizing the design and performance of monopile foundations for offshore wind turbines, particularly in the context of deploying higher-capacity turbines to meet growing energy demands.