The cyclic behavior of clay significantly influences the dynamic response of offshore wind turbines (OWTs). This study presents a practical bounding surface model capable of describing both cyclic shakedown and cyclic degradation. The model is characterized by a simple theoretical framework and a limited number of parameters, and it has been numerically implemented in ABAQUS through a user-defined material (UMAT) subroutine. The yield surface remains fixed at the origin with isotropic hardening, while a movable projection center is introduced to capture cyclic hysteresis behavior. Cumulative plastic deviatoric strain is integrated into the plastic modulus to represent cyclic accumulation. Validation against undrained cyclic tests on three types of clay demonstrates its capability in reproducing stress-strain hysteresis, cyclic shakedown, and cyclic degradation. Additionally, its effectiveness in solving finite element boundary value problems is verified through centrifuge tests on large-diameter monopiles. Furthermore, the model is adopted to analyze the dynamic response of monopile OWTs under seismic loading. The results indicate that, compared to cyclic shakedown, cyclic degradation leads to a progressive reduction in soil stiffness, which diminishes acceleration amplification, increases settlement accumulation, and results in higher residual excess pore pressure with greater fluctuation. Despite its advantages, this model requires a priori specification of the sign of the plastic modulus parameter cd to capture either cyclic degradation or shakedown behavior. Furthermore, under undrained conditions, the model leads pstabilization of the effective stress path, which subsequently results in underestimation of the excess pore pressure.

The Augmented Kalman Filter (AKF) has been applied previously for input-state estimation of offshore wind turbines (OWT). However, the accuracy of the estimated results depend on the chosen model, for which various complexities exist, making this a challenging task. Two of which are the lack of information required to model the Rotor-Nacelle Assembly (RNA), and the high uncertainty associated with the soil-structure-interaction (SSI). Therefore, the primary focus of this work is to avoid these limitations by considering a suitable substructure which eliminates the need to model the RNA and the SSI, thus significantly reducing uncertainties. The substructure is obtained by 'cutting' the OWT at the top of the tower and at the ground level. To define the model, the resulting substructure then only requires geometries and material properties for the monopile and tower; information which is often known with greater certainty. A numerical case study is presented to investigate the accuracy of the proposed approach for input-state estimation of a 15 MW OWT. A series of commonly used setups involving accelerometers and inclinometers are used and the effects on the predicted fatigue life of the structure are discussed. Additionally, a simple approximation of the wave loading is considered to estimate and account for its contribution to the dynamics of the substructure. The proposed approach is shown to be an effective solution for input-state estimation of OWTs when the RNA or SSI are unknown or associated with significant uncertainty.

As a newly emerged solution for supporting the new generation of offshore wind turbines (OWTs), the pile-bucket foundation has received wide attention. However, little attention has been paid to the grouted connection that connects the monopile and bucket foundation. As the loadtransferring, yet vulnerable component, the fatigue mechanism of the grouted connection and its influence on the cyclic laterally-loaded response of OWT foundation are still not clear. In this study, a sophisticated three-dimensional (3D) finite element (FE) model of the pile-bucket foundation with grouted connection is constructed, which incorporates a hypoplastic clay model and the concrete damage plasticity (CDP) to consider the cyclic load effect on both soil and grout material. A modal analysis is first performed to verify the rationality of the proposed model. Then the influence of cyclic load frequency, load amplitude and stiffener arrangement on the accumulation of pile head displacement, stress distribution and crack development of the grouted connection is systematically analyzed. Results indicate that as load frequency approaches the eigen-frequency, the OWT structure tends to vibrate more intensively, leading to stress concentration and fatigue damage of the grouted material and rapid accumulation of the pile-head displacement. The influence of load amplitude on grout damage seems to be limited in the contact area in the simulated cases. Meanwhile, the installation of stiffeners slightly mitigates the pile head displacement accumulation, but also raises the risk of stress concentration and fatigue damage of the grouted connection. The numerical results reveal the load-transferring function and fatigue damage of the grouted connection, which could provide some reference for an optimized structure and dynamic design for the pile-bucket foundation under cyclic load.

The structural design of offshore wind turbines must account for numerous design load cases to capture various scenarios, including power production, parked conditions, and emergency or fault conditions under different environmental conditions. Given the stochastic nature of these external actions, deterministic analyses using characteristic values and safety factors, or Monte Carlo Simulations, are necessary. This process involves a large number of simulations, ranging from ten to a hundred thousand, to achieve a reliable and optimal structural design. To reduce computational complexity, practitioners can employ low-fidelity models where the soil-foundation system is either neglected or simplified using linear elastic models. However, medium to large cyclic soil-pile lateral displacements can induce soil hysteretic behaviour, potentially mitigating structural and foundation vibrations. A practical solution at the preliminary design stage entails using stiffness-proportional viscous damping to capture the damping generated by the soil-pile hysteresis. This paper investigates the efficacy of this simplified approach for the IEA 15 MW reference wind turbine on a large-diameter monopile foundation subjected to several operational and extreme wind speeds. The soil-pile interaction system is modelled through lateral and rotational springs in which a constant stiffness-proportional damping model is applied. The results indicate that the foundation damping generated by the nonlinear soil-pile interaction is significant and cannot be neglected. When fast analyses are required, the stiffness-proportional viscous damping model can be reasonably used to approximate the structural response of the wind turbine. This approach enhanced the accuracy of the computed responses, including the maximum bending moment at the mudline for ultimate limit design and damage equivalent loads for fatigue analysis, in comparison to methods that disregard foundation damping.

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.

The tetrapod jacket-supported offshore wind turbine is subjected to marine environmental loads, resulting in monotonic and cyclic lateral-compression-tension interaction behavior of the pile-soil system. Although the excellent applicability that has been demonstrated by three-dimensional numerical simulation for aiding the revelation of the mechanism of jacket foundation-soil interaction, a significant challenge remains in accurately reflecting the nonlinear stress-strain relationship and cyclic behavior of the soil, and others. Finite element numerical models are therefore established for laterally loaded tetrapod jacket pile foundations in this study, and a bounding surface model is adopted to simulate the elastoplastic characteristics and cyclic ratchet effect of the soil. Subsequently, a parametric analysis is conducted on different net spacings and aspect ratios of the jacket base-piles to investigate the pile deformation characteristics, bearing mechanisms, evolution of pile-soil interaction, and the internal force development under monotonic and cyclic conditions, respectively. The results indicate that under monotonic loading, the pile deformation pattern transitions from a flexible pile mode to a rigid rotational deformation mode as the aspect ratio decreases. Under cyclic loading, attention should be paid to the asynchronous accumulation of axial forces within the base-piles and its impact on overall bearing performance.

Drag embedded anchors (DEA) are widely used in offshore engineering. The anchor foundations are installed in the seabed through the drag force applied by the mooring line and provide holding capacity to marine structures. Offshore wind farms in Taiwan are located in active earthquake zones, where a considerable amount of sandy soil at the upper layer of seabed results in a high potential for soil liquefaction. Since DEA are a promising option for floating wind turbines, this study conducted a shaking table test on two 1/30-scale anchors in medium dense sand to investigate the dynamic behavior of DEA during earthquakes and after excess pore water pressure dissipation. The test results reveal no significant impact on the orientation of the anchors, which could be due to the uplift force from the excess pore water pressure acting on the fluke. After the excess pore water pressure dissipates, the soil density increases, and the fluke angle becomes favorable, thus increasing the anchor's holding capacity when subjected to additional drag.

The accelerating climate crisis has intensified global efforts to develop renewable energy, with offshore wind power emerging as a key solution due to its vast potential and low environmental impact. However, the stability of offshore wind turbines (OWTs) is increasingly compromised by extreme storm events, such as typhoons, which induce strong winds, large wave loads, and seabed liquefaction. While extensive research has been conducted on monopile foundations, most studies focus either on horizontal loads or seabed responses in isolation, lacking a systematic analysis of the coupled pile-soil interaction in extreme storm conditions. This study develops a pilesoil interaction model incorporating pore pressure response to evaluate the stability of monopile and seabed under extreme storm loads. The model is validated using seabed pore pressure models under wave action and monopile response models under cyclic loading. The model is applied to the stability analysis of monopiles at the Cangnan offshore wind farm, where extreme storm loads are quantified using buoy measurement data and incorporated into the model to calculate the responses of both monopiles and seabeds. The results show that the monopile displacement reaches its maximum at the wave crest, and the displacement and moment of the monopile are positively correlated with wave height and negatively correlated with wave length and period. Although changes in wave parameters do not affect the failure mode of the soil, they influence the magnitude and distribution of pore pressure around the pile. The findings provide critical insights into offshore wind turbine foundation stability, offering a scientific basis for improving design strategies to enhance resilience against extreme weather events.

Damping plays a crucial role in the design of offshore wind turbine (OWT) monopile foundations. The soil damping of the monopile-soil system (MSS) represents the energy dissipation mechanism arising from the interaction between the pile and the soil. It is typically derived by back-calculating from the overall damping measured in the entire OWT structure. However, few studies have independently examined the soil damping in MSS, and the impact of key parameters such as pile diameter, pile embedded depth, cyclic load amplitude, and load eccentricity on the variation of soil damping in MSS remains unclear. This paper introduces an elastoplastic-damage constitutive model for the numerical simulation of the damping ratio variation in seabed soil and MSS. The model is implemented in ABAQUS software and validated against cyclic triaxial tests on stiff clay soil. On this basis, a three-dimensional finite element sensitivity study was conducted to elucidate the effect of these key parameters on the MSS damping ratio. The results of the study reveal that the MSS damping ratio exhibits a nonlinear and asymmetric trend as the loading cycles increase. The MSS damping ratio decreases with increasing pile diameter and embedded depth but increases with increasing lateral cyclic load amplitude and load eccentricity from the mudline.

Monopiles are the most popular type of foundation for offshore wind turbines. Although capturing the effects of cyclic loading is critical to the design of monopiles, there is no recommended approach in the main design standards and no consensus in the literature as to how this can be achieved. Hence, this paper presents the step-by-step methodology and validation of a new cyclic model in sand and clay. The model consists of the degradation of monotonic soil reaction curves based on soil cyclic contour diagrams. The model is found to provide a very satisfactory match with the PISA field tests in Dunkirk dense marine sand and Cowden stiff glacial till.