Wind Turbine Foundations Research Articles

Horizontal Cyclic Bearing Characteristics of Bucket Foundation in Sand for Offshore Wind Turbines

During the service period, the offshore wind turbine foundation mainly bears the wind load from the upper structure and the periodic loads such as wave load and sea currents from the lower structure. Long-term cyclic loads have an important impact on the cumulative deformation, foundation stiffness changes, and horizontal ultimate bearing capacity of the offshore wind turbine bucket foundation. This paper conducts cyclic loading tests on mono-bucket foundations under unidirectional and multidirectional cyclic loading conditions based on the multidirectional intelligent cyclic loading system of offshore wind turbine foundations and analyzes the cumulative effects of the loading direction, vertical load, and drainage status on the mono-bucket foundation during the cyclic loading process, effects of rotation angle, cyclic stiffness, and monotonic bearing capacity of foundation after cycles. The research results show that multidirectional cyclic loading significantly reduces the cyclic cumulative rotation angle of the mono-bucket foundation, and the maximum reduction rate can reach more than 50%. After unidirectional and multidirectional cyclic loading, the bearing capacity of the bucket foundation can be increased by up to 30% and 20%, respectively. At the same time, this paper proposes calculation formulas for vertical load, number of cyclic loading, and normalized cumulative rotation and establishes a calculation method for vertical load and bearing capacity of the foundation after cycles.

Evaluation of Wind Power Plants from the Aspect of Earthquake Design

Türkiye in recent years, consequently, their share in the total generation of energy within the country is also increasing every year. Considering the cost of wind power plants, together with the targeted amount of energy generation, earthquake planning for these systems becomes crucial. Earthquakes are one of the principal risks of wind energy investments, especially when considering the earthquake hazard level in a large portion of the country. Under the present circumstances, the companies producing wind turbines conduct the analysis and planning they deem necessary and forward these to the investors in our country. The extent to which manufacturers consider earthquake parameters and analysis methods in the design of wind turbines for earthquake resistance is not well-defined. The lack of clarity makes it difficult for investors to accurately evaluate the suitability of wind turbine tower and foundation designs for the specific conditions of the country. This study evaluates earthquake design methods by looking into the regulations used in the design of wind turbines around the world. The magnitude of ground motion considered in the earthquake design, the method of analysis used, the load combinations on which the designs are based, and the criteria for the designs have been inspected. Moreover, the design documents for a wind turbine that is being constructed has been examined in detail, with the earthquake design stage being especially scrutinized. Assessments and recommendations have been made regarding the design specifications, analysis methods and criteria used in the analysis and design of the wind turbines currently being built in our country. Subsequently, obtained recommendations have been taken into consideration to propose an earthquake design procedure for the earthquake design of wind turbines.

Tidal Current with Sediment Transport Analysis and Wind Turbine Foundation Pile Scour Trend Studies on the Central Bohai Sea

Tidal Current with Sediment Transport Analysis and Wind Turbine Foundation Pile Scour Trend Studies on the Central Bohai Sea

CPTU-Based Offshore Wind Monopile Rigid Bearing Mechanism Analysis

With the development of the offshore wind industry in China, the amount of offshore wind turbines has increased rapidly. Large-diameter steel monopile foundations of offshore wind turbines have been widely adopted in China with lots of marine clay located. However, the conventional offshore wind monopile bearing capacity prediction from the American Petroleum Institute (API) based on the small-diameter flexible pile field test is inaccurate with the rigid mechanism of large-diameter monopile causing economically loss. The piezocone penetration test (CPTU) is a common marine in situ test to exactly acquire soil parameters. Therefore, a CPTU-based offshore wind monopile rigid mechanism inference method is proposed. A creative numerical offshore wind power monopile and CPTU combined model is established through COMSOL. A self-compiling parameter function is applied to soil modeling and an innovative mobile boundary function is created to simulate CPTU penetration. Through the model, real-time CPTU data can be acquired when monopile is applied with different horizontal loads. The peripile soil stress change can be timely detected by CPTU. Through CPTU data, the monopile rigid bearing mechanism is verified. A rigid rotation center is found at the 60% point of the inserted monopile. The method is an important foundation for the next step of monopile bearing capacity research.

Influence of soil hysteretic damping on lateral response of offshore wind turbine monopile in sandy soil

Damping plays an important role in the design of offshore wind turbine structures. The hysteretic damping of the seabed soil represents the energy dissipation caused by the soil-particle interaction and the nonlinear behavior of the soil under cyclic loading. However, the effect of sand damping on the lateral response of the monopile foundation of an offshore wind turbine is still unclear. In this paper, the effect of soil hysteretic damping on the lateral dynamic response of a monopile foundation in a sandy seabed is investigated using a subplastic soil constitutive model. The constitutive model response at the foundation level is verified by comparing the monotonic and cyclic responses of the monopile with the results of the 1g model test. The results show that when soil hysteretic damping is present in the monopile-soil system, the energy dissipation in the soil reduces the stress accumulation in the soil, resulting in a reduction in the bending moment and horizontal displacement of the monopile, compared with the case without soil hysteretic damping. The results are crucial for optimizing the monolithic design of offshore wind turbine structures.

How Will Concrete Piles for Offshore Wind Power Be Damaged Under Seawater Erosion? Insights from a Chemical-Damage Coupling Meshless Method.

Based on the background of the continuously rising global demand for clean energy, offshore wind power, as an important form of renewable energy utilization, is booming. However, the pile foundations of offshore wind turbines are subject to long-term erosion in the harsh marine environment, and the problem of corrosion damage is prominent, which seriously threatens the safe and stable operation of the wind power system. In view of this, a meshless numerical simulation method based on smoothed particle hydrodynamics (SPH) and a method for generating the concrete meso-structures are developed. Concrete pile foundation models with different aggregate contents, particle sizes, and ion concentration diffusion coefficients are established to simulate the corrosion damage processes under various conditions. The rationality of the numerical algorithm is verified by a typical example. The results show that the increase in the aggregate percentage gradually reduces the diffusion rate of chemical ions, and the early damage development also slows down. However, as time goes, the damage will still accumulate continuously; when the aggregate particle size increases, the ion diffusion becomes more difficult, the damage initiation is delayed, and the early damage is concentrated around the large aggregates. The increase in the ion diffusion coefficient significantly accelerates the ion diffusion process, promotes the earlier and faster development of damage, and significantly deepens the damage degree. The research results contribute to a deeper understanding of the corrosion damage mechanisms of pile foundations and providing important theoretical support for optimizing the durability design of pile foundations. It is of great significance for ensuring the safe operation of offshore wind power facilities, prolonging the service life, reducing maintenance costs, and promoting the sustainable development of offshore wind power.

Analytical Model and Optimization of Joint Systems in Modular Precast Foundations for Onshore Wind Turbines

This study presents a comprehensive design and calculation methodology for prefabricated expansion foundations of onshore wind turbines, addressing the critical need for modular construction while ensuring structural integrity. The proposed approach encompasses both overall structural performance verification and connection joint design, considering the stiffness reduction effects of modularization. A novel punching shear capacity checking method is developed specifically for assembled expansion foundations, incorporating joint weakening factors. The research establishes equilibrium equations and verification formulas for splice joints under different loading conditions, demonstrating that the shear amplification factor at joints varies with local compressive stress. The analytical results demonstrate several key findings: (1) the derived maximum joint shear formula shows excellent agreement with finite element results across various load conditions (R2 = 0.99906); (2) the optimized shear key configuration increases the joint’s load-bearing capacity by 35% compared to conventional designs; and (3) the developed Python-based calculation system reduces the design time by 60% while maintaining accuracy within 5% of detailed FEM analysis. These quantitative outcomes validate the effectiveness of the proposed methodology and provide practical guidelines for implementing modular construction in wind energy infrastructure.

Effective Stress-Based Numerical Method for Predicting Large-Diameter Monopile Response to Various Lateral Cyclic Loadings

Extreme marine environmental cyclic loading significantly affects the serviceability of monopiles applied for the foundation of offshore wind turbines (OWTs). Existing research has primarily used p-y methods or total stress-based models to investigate the behavior of monopile–marine clay systems, overlooking the pore pressure development in subsea clay. Studies on the effective stress-based behavior of clay under various lateral cyclic loading conditions are limited. This paper presents an effective stress-based 3D finite element numerical method developed to predict key behaviors of pile–clay systems, including permanent pile rotation under cyclic loading, pile bending moment, and the evolution of pore pressure in subsea clay. The model is verified by contrasting the simulations results to centrifuge experimental results. Cyclic lateral loading is divided into average cyclic load and amplitude of cyclic load to investigate their impacts on the pile–clay system response. The research findings offer insights for the design of large-diameter monopiles under complex cyclic loading conditions.

Impact Risk Assessment of Gravity-Based Foundation of Offshore Wind Turbine Under Tsunamis

As an important renewable engineering structure, offshore wind turbine foundation is threatened by tsunami bore. A three-dimensional numerical model of dam-break wave tsunami is established and validated based on computational fluid dynamics to study the impacting effects of gravity-based foundation (GBF) of offshore wind turbines under tsunamis. The impacting process of the small, middle, and large tsunami on the GBF is numerically simulated, and the load effects characteristics of the GBF of the wind turbine at different stages of the tsunami impact process are analyzed. The research results show that the GBF is subjected to two peak impact forces under tsunami-impacting action. The tsunami bore has a frontal impact on the GBF, and the force peak of the flow direction is the largest, which has the greatest impact on the foundation of the wind turbine. Specifically, the middle tsunami exerts a maximum frontal impact force of 16.8[Formula: see text]N, with a peak dynamic pressure of 678.2 Pa, and a maximum base moment of 1.24[Formula: see text]N[Formula: see text]⋅[Formula: see text]m. Besides, the variation on the base moment in the incident flow direction of the incoming is larger than that in the transverse direction. The research provides valuable insights into the tsunami-resistant GBF, offering recommendations and guidelines for ensuring the long-term stability of offshore wind turbines in tsunami-prone regions.

Upper bound solution of the vertical bearing capacity of the pile-bucket composite foundation of offshore wind turbines

Pile-bucket composite foundations are regarded as a promising solution for offshore wind power infrastructure. However, accurately assessing their vertical ultimate bearing capacity remains a technical challenge. Therefore, establishing an upper bound solution of the ultimate bearing capacity of the composite foundations is of significant importance for their promotion and application. This study begins with small-scale model tests, using Abaqus for modeling based on the relevant test conditions. Next, following model validation, the vertical bearing capacity of the composite foundations under different H/D ratios and the corresponding soil failure modes are investigated. According to the upper limit method and the ultimate equilibrium theory, the kinematic velocity field is subsequently constructed to derive the upper bound solution of the vertical bearing capacity. Finally, the effectiveness and accuracy of the proposed theoretical upper bound solution are verified against the results of model tests, and this study hopes to provide a reference for the future design of vertical bearing capacity of pile-bucket composite foundations.

Experimental investigation of ice resistance in jacket foundations for large offshore wind turbines

ABSTRACT Each winter, the Bohai Sea in China freezes, forming a thick ice layer that significantly affects the safety of ship and offshore structures. Offshore wind turbines, particularly high-rise structures, are especially vulnerable to collapse under the pressure of sea ice. This paper presents an experimental study on the ice resistance of the steel jacket foundation of a 10 MW offshore wind turbine. The study examines the failure modes, hysteresis curves, stiffness, and energy dissipation capacity of the jacket foundation under ice loading. Results indicate that the branch tube joints within the sea ice zone are prone to fracture, leading to a reduction in the overall stiffness and load-bearing capacity of the foundation. These findings offer a valuable experimental basis for improving the ice resistance performance of offshore wind turbines and enhancing their structural safety in ice-prone regions.

Experimental Investigation of Nonlinear Forces on a Monopile Offshore Wind Turbine Foundation Under Directionally Spread Waves

Abstract Accurate prediction of nonlinear wave loading is crucial for designing marine and offshore structures, yet it remains a challenging task. Prior research has primarily focused on unidirectional extreme sea states, revealing that linear loading cannot accurately represent the total wave forces acting on offshore wind turbine foundations, with significant contributions from high-order harmonics. This study broadens the scope to include multidirectional and bidirectional wave interactions with monopile offshore wind turbine foundations. We use a phase-based harmonic separation method to isolate harmonic components in the presence of complex wave scenarios. This approach allows for the clear delineation of individual harmonics from the total wave force by controlling the phase of incident-focused waves. Remarkably, this method is effective even with multidirectional and bidirectional spreading. The clean separation of individual harmonics enables the estimation of contributions from each harmonic. Our findings are in line with previous research, showing that nonlinear loading can constitute up to 40% of the total under certain wave conditions. We have also observed that wider wave spreading reduces nonlinear high-order harmonics, and unidirectional waves induce the most severe nonlinear forces. These insights emphasize the importance of accounting for high-order nonlinear wave loading in offshore structure design.

Local scour of composite cylindrical wind turbine foundation on fine sand seabed under combined waves and current

The composite cylindrical wind turbine foundation is characterized by its large-diameter cylindrical base, which offers superior anti-overturning capability, and it is widely used in the soft soil seabed of Jiangsu, China. Due to its complex structural form, the local scour under combined waves and current significantly differs from that of monopile foundations. However, research on the scour characteristics specific to composite cylindrical wind turbine foundations remains scarce. A numerical model for local scour of wind turbine foundations was established in this study, which was verified with the field-measured scour data. A series of numerical simulations of local scour depths for composite cylindrical wind turbine foundations under various water depths and wave-current combinations were conducted. The simulation results indicate that the wake vortex shedding caused by the complex structural form leads to the local scour around the composite cylindrical wind turbine foundation; the normalized scour depth increases with the Keulegan-Carpenter number and the relative current strength; when the relative current strength is greater than 0.6, the influence of the Keulegan-Carpenter number on scour depth tends to be weakened; similarly, as the Keulegan-Carpenter number increases, the effect of the relative current strength on scour depth gradually diminishes. A scour equation of the composite cylindrical wind turbine foundation is suggested to predict the local scour in fine sand bed under waves and current.

Centrifugal Test Study on the Vertical Uplift Capacity of Single-Cylinder Foundation in High-Sensitivity Marine Soil

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.

Numerical investigation on rotation accumulation and natural frequency degradation for offshore wind turbine in clays

Prolonged lateral cyclic loading leads to soil stiffness degradation around offshore wind turbine (OWT) foundations, which reduces the system's natural frequency and increases the accumulation of foundation rotation angle. Proper evaluation of natural frequency and rotation angle is crucial for the design of OWT foundations. This study develops a two-stages numerical approach to calculate the fundamental frequency of OWT systems considering the foundation stiffness degradation by integrating a stiffness degradation model of soft clays with a simplified three-spring model. Subsequently, it investigates the evolution of accumulated rotation and natural frequency for three foundation types—monopile, monopod bucket, and hybrid monopile-bucket—throughout their service life. It is observed that the hybrid foundation shows the smallest rotation in the first cycle, attributable to its relatively high initial stiffness compared to the other two foundations with the same steel consumption. However, it also exhibits the highest rate of cumulative rotation growth. At the same load level, the monopod bucket foundation exhibits the largest cumulative rotation angle due to its lower bearing capacity, and the degradation of natural frequency is most pronounced for monopod bucket OWT. For all three foundation configurations, increasing either the pile diameter or the bucket diameter is the most effective approach to reduce the cumulative rotation angle and improve natural frequency degradation, while maintaining the same steel consumption. These findings should be considered in the design of OWT foundations.

New insights into local scour characteristics around vibrating monopiles under combined wave-current conditions

Cyclic lateral vibrations on monopile foundations of offshore wind turbines throughout their service life may affect the local scour around the foundations. While previous studies have explored local scour around vibrating monopile foundations under current-only conditions, the local scour characteristics around these foundations under combined wave-current conditions remains unclear. This experimental study presents new insights into local scour characteristics around vibrating monopile foundations under combined wave-current conditions. A novel dimensionless parameter, the friction velocity-based Froude number (Fr∗), is proposed to quantify the effect of hydrodynamic intensity on the scour process. Based on the Fr∗ values obtained in the experiment, three distinct regimes were identified to characterize the impact of vibrations on the scouring process: In Regime 1 (Fr∗ < 0.055), scour dimensions are more pronounced, while in Regime 3 (Fr∗ > 0.06), they are less significant compared to the equilibrium scour hole around static monopile foundations. In the transitional phase, Regime 2 (0.055 <Fr∗ < 0.06), scour dimensions fluctuate by up to 10.9% relative to static conditions. Additionally, an estimation equation for equilibrium scour depth based on Fr∗ is developed, demonstrating good prediction accuracy and broad applicability. These insights advance the understanding of scour features and provide a valuable reference for predicting scour depth around vibrating monopile foundations in diverse hydrodynamic environments.

An accurate and ready-to-use approach for the estimation of impedance functions of regular pile groups for offshore wind turbine foundations. Vertical and rocking components

The interaction factor superposition method in the determination of the vertical and rocking impedances of pile groups for jacket-supported Offshore Wind Turbines is studied. The vertical and rocking group dynamic impedances obtained by this method are compared with those computed by a previously developed continuum model. The influence of the main parameters that define the problem is also studied. In order to propose an alternative calculation solution, some expressions dependent of these parameters are fitted from an extensive selection of jacket pile group configurations. These expressions allow to estimate the vertical flexibility of the source pile and the interaction factor between a pair of piles. Subsequently, applying elastic superposition, the group impedance functions are determined. Results show that the interaction factor superposition method correctly reproduces the vertical and rocking group dynamic impedances. Therefore, for these regular pile groups with large spacing ratios, the assumption of only considering the direct interaction between pile pairs leads to accurate results. Results obtained with the proposed fitted expressions generally conduct to a good estimation, properly reproducing the variation produced by the influence of the different parameters. Thus, this superposition method and the proposed expressions can be employed to quickly and simply reproduce the vertical soil–structure interaction of this type of structures.

Optimum design of wind turbine foundation according to rebar detailing

This research proposes a Mixed Integer Linear Programming model to assist structural engineers in the reinforcement detailing phase of circular foundations, with a specific focus on wind tower foundations, aiming to optimise the design process and, consequently, reduce the costs associated with constructing these elements. After analysing the internal forces in circular foundations, the designer must define the calculation sections to determine the required area of steel and select the diameter of the steel bars and their positioning in the base. This determination is often based on the designer's experience, which can lead to choices that unnecessarily increase costs, thus justifying the creation of a model to optimise this process. The proposed model consists of three phases: the first involves a non-linear regression to obtain an equation that represents the bending moments at any point in the foundation; the second applies a linear quadratic model to choose the ideal layout of the calculation sections; and the third uses a binary integer linear model to determine the ideal diameter of the bars and their consequent positioning. This search for optimality must comply with the restrictions imposed by normative criteria, including maximum bar lengths and rebar lap splices, maintaining good constructability based on standardisation. Validation tests were conducted with confirmed cases using the proposed model, resulting in an average reduction of 10.72 % in the value of the rebar weight due to the model's second stage and a decrease of 12.28 % in the best case during the final step. It can, therefore, be concluded that the proposed model is viable for application in circular foundation projects, leading to savings in the total weight of steel used in the foundations.

Numerical modeling of seismic behavior of Caisson foundation for Offshore Wind Turbines taking into account wave and marine currents forces

It is crucial to simulate the seismic behavior of offshore wind turbines, especially when dealing with foundations on non-cohesive soil. There is a risk of liquefaction occurring, which highlights the need to obtain values of excess pore water pressure. In this study, we created a three-dimensional model of a caisson foundation for an offshore wind turbine on loose sandy soil from the Syrian coast. The Mohr-Coulomb constitutive plasticity model was used to analyze two scenarios. The first scenario involved applying wind and earthquake loads, while the second scenario included marine currents and wave loads in addition to the wind and earthquake loads. We used Coupled acoustic-structural medium analysis after confirming its effectiveness on the soil through comparison with simulation results of the FLAC3D program from a previous study. The numerical modeling results indicated that it is possible to use Coupled acoustic-structural analysis in soil and water modeling. The study monitored the values of excess pore water pressure and found that liquefaction occurred in the soil due to the earthquake. The analysis also highlighted the importance of considering wave and marine currents loads in analyzing these structures. While these factors had a slight impact on excess pore pressure values, they significantly affected the directions and values of displacements.

Experimental investigation on dynamic remediation with solidified soil for scour around offshore wind turbine foundations

Scour occurs inevitably around monopiles of offshore wind turbines (OWTs), negatively impacting the operation of the turbines. For the purpose of ensuring the long-term safety and stability of the foundations, solidified soil which provides higher scour resistance can be used to remediate scour considering its technical convenience. In this study, flume tests were conducted under unidirectional flow conditions to investigate the dynamic remediation of the local scour hole, which was applied to different scour development processes, using solidified soil. Various remediation schemes were simulated by controlling the design elevation of the solidified soil layers to investigate their performance under different conditions. Meanwhile, the phenomenon and characteristics of edge scour during remediation were also analyzed. Results show that different remediation elevations and initiation would lead to varying degrees of edge scour. Early remediation costs less solidified soil but results in severe edge scour, while late remediation needs more materials and experiences milder edge scour. In addition, an evaluation method assessing solidified soil remediation was established. This method was utilized to evaluate the comprehensive benefit and edge scour risk for different remediation schemes.