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.

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.

The quest for clean, renewable energy resources has given a global rise in offshore wind turbine (OWT) construction. As OWTs are more exposed to harsh environmental conditions, the dynamic behavior of OWTs with jacket support structures under critical loading scenarios is crucial yet least understood, which becomes more convoluted with the consideration of soil-structure interaction (SSI) effects. In addition, the seismic characteristics of such systems heavily depend on the excitation characteristics like frequency content, a feature that is still ambiguous. This research aims to examine the influence of seismic frequency contents on the dynamic characteristics and damage modes of jacket-supported OWT systems including SSI effects. The numerical model is established and validated based on a previous study, which ensures the accuracy of the numerical modeling framework. Upon validation, extensive numerical analyses are performed under earthquakes with varying frequency contents. Results reveal the relationship among the ground motion frequency, SSI, and the dynamic and damage behavior of jacket-supported OWTs, offering important insights for the improved seismic design and analysis of jacket-supported OWTs.

A critical investigation of three constitutive models for clay by means of analyses of a sophisticated laboratory testing program and of centrifuge tests on monopiles in clay subjected to (cyclic) lateral loading is presented. Constitutive models of varying complexity, namely the basic Modified Cam Clay model, the hypoplastic model with Intergranular Strain (known as Clay hypoplasticity model) and the more recently proposed anisotropic visco-ISA model, are considered. From the simulations of the centrifuge tests with monotonic loading it is concluded that all three constitutive models give satisfactory results if a proper calibration of constitutive model parameters and proper initialisation of state variables is ensured. In the case of cyclic loading, the AVISA model is found to perform superior to the hypoplastic model with Intergranular Strain.

Offshore wind turbines (OWTs) empoly various foundation types, among which Jacket-type offshore wind turbines (JOWTs) are often used in shallow waters with challenging soil conditions due to their lattice framework foundations and multiple anchoring points. However, prolonged exposure to harsh marine environments (e.g. storms) and age-related degradation issues like corrosion, fatigue cracking, and mechanical damage increases failure risks. To address these issues, this paper introduces a Digital Healthcare Engineering (DHE) framework, which provides a proactive strategy for enhancing the safety and sustainability of JOWTs: (1) Real-time health monitoring using IoT; (2) Data transmission via advanced communication technologies; (3) Analytics and simulations using digital twins; (4) AI-powered diagnostics and recommendations; as well as (5) Predictive analysis for maintenance planning. The paper reviews recent technological advances that support each DHE module, assesses the framework's feasibility. Additionally, a prototype DHE system is proposed to enable continuous, early fault detection, and health assessment.

This study investigates the impact of monopile dimensions on the seismic response of offshore wind turbines (OWTs), considering soil-water-structure interaction (SWSI). During earthquakes, OWTs may experience critical conditions, leading to operational shutdowns due to structural responses such as high acceleration, permanent rotation, and displacement. Key factors influencing seismic response include monopile diameter, driven length, and hybrid monopile designs, which are the main focus of this research. Eight physical models were constructed: one baseline model, two with modified monopile diameters, two with altered driven lengths, and three hybrid monopile models featuring shallow wheels of varying diameters. These models were tested under 1g conditions and on a shaking table with nine sinusoidal motions at three frequencies and three amplitudes, simulating saturated soil conditions. The pore water pressure generation, soil and superstructure acceleration, and displacement were monitored during each test. The results show that increasing the monopile's driven length reduces the superstructure's cumulative displacement and improves overall seismic performance. Moreover, increasing the monopile diameter or adding shallow wheels to create a hybrid monopile increases pore water pressure, which in turn results in greater cumulative displacement of the OWT.

Wind energy offers significant advantages over fossil fuels, including extensive energy storage and environmental sustainability. Offshore wind turbines serve as the primary technology for harnessing offshore wind power. However, the corrosive effects of the marine environment pose serious threats to their safety and stability. This paper provides a comprehensive overview of corrosion issues affecting steel pipe pile infrastructure, focusing on the following key aspects: (1) Differentiating corrosion mechanisms under various environmental conditions, (2) analyzing the comprehensive corrosion response, particularly the changes in mechanical properties of the pile-soil interface and the bearing capacity of steel pile foundations, (3) summarizing the patterns and trends in corrosion processes to offer theoretical insights for engineering design, and (4) reviewing commonly employed corrosion prevention methods and their respective applicability in relation to specific corrosion mechanisms and responses.

Offshore wind power is a new type of clean energy with broad development prospects. Accurate analysis of the uplift capacity of offshore wind turbine foundations is a crucial prerequisite for ensuring the safe operation of wind turbines under complex hydrodynamic conditions. However, current research on the uplift capacity of suction caissons often neglects the high-sensitivity characteristics of marine soils. Therefore, this paper first employs the freeze-thaw cycling procedure to prepare high-sensitivity saturated clay. Subsequently, a single-tube foundation for wind turbines is constructed within a centrifuge through a penetration approach. Ten sets of centrifuge model tests with vertical cyclic pullout are conducted. Through comparative analysis, this study explores the pullout capacity and its variation patterns of suction caisson foundations in clay with different sensitivities under cyclic loading. This research indicates the following: (1) The preparation of high-sensitivity soil through the freeze-thaw procedure is reliable; (2) the uplift capacity of suction caissons in high-sensitivity soil rapidly decreases with increasing numbers of cyclic loads and then tends to stabilize. The cumulative displacement rate of suction caissons in high-sensitivity soil is fast, and the total number of pressure-pullout cycles required to reach non-cumulative displacement is significantly smaller than that in low-sensitivity soil; (3) the vertical cyclic loading times and stiffness evolution patterns of single-tube foundations, considering the influence of sensitivity, have been analyzed. It was found that the secant stiffness exhibits a logarithmic function relationship with both the number of cycles and sensitivity. The findings of this study provide assistance and support for the design of suction caissons in high-sensitivity soils.

Offshore wind turbines (OWTs) often operate in complex marine environments, where they are not only subjected to wind and wave loads, but also adversely affected by scour. Therefore, it is of great significance to explore the effect of scour on the dynamic responses of OWTs under external loads to ensure structural safety, improve performance, and extend service life. In this study, a comprehensive numerical model of a 5-MW OWT, including tower, monopile, and soil-structure interaction (SSI) systems, is established by using ABAQUS platform. Aerodynamic loads is generated using blade element momentum, while wave loads is generated using the P-M spectral. The depth of scour is obtained based on on-site measured data. A comparative analysis is conducted between fixed foundations and SSI systems when conducting dynamic response analysis of OWTs under wave loads. Subsequently, the effect of scour on dynamic responses of OWTs under aerodynamic and wave loads is investigated. Results demonstrate that SSI can significantly influence the natural frequency and dynamic responses of the OWT. Therefore, it is essential to thoroughly consider SSI when evaluating the dynamic response of the OWT with local scour. The tower-top displacement and acceleration of the OWT with show a significant increasing trend compared to the non-scoured OWT. An increase in scour depth leads to higher maximum stress and stress amplitude in the steel monopile, which could potentially cause fatigue issues and should be given due attention.