Wind Turbine Foundations Research Articles

Complex High‐Cyclic Loading in an Accumulation Model for Sand

ABSTRACTExperimental evidence indicates that multidimensional cyclic loading of soils causes larger accumulation of deformations than equivalent one‐dimensional loading. The response of sand to high‐cyclic loading with 10,000 cycles and up to four‐dimensional stress paths (i.e., four independent oscillating components) is examined in 120 triaxial and hollow cylinder tests in this work to extend these findings. With increasing number of oscillating stress components, the accumulation of permanent strains tends to increase. It is demonstrated that the definition of the multidimensional strain amplitude incorporated in the high‐cycle accumulation (HCA) model can account for this. The validation of the HCA model for complex cyclic loading is complemented by the simulation of model tests on monopile foundations of offshore wind turbines subjected to multidirectional cyclic loading, for which the consideration of spatially variable cyclic loading with nonconstant load amplitudes in the HCA model is discussed. For this purpose, an extension of the HCA model considering multiple strain amplitudes is presented.

Coupling dynamic characteristics of installation vessel - Multi buckets jacket foundation lowering system

Precise control of the splash zone lowering process of the multi-bucket jacket foundation for an offshore wind turbine can save time during the entire construction period, thereby reducing construction and installation costs. This paper focuses on the “installation vessel - multi buckets jacket foundation” coupled dynamic system during the lowering process of the multi-bucket jacket foundation. Taking the three-bucket jacket foundation (TBJF) and the four-bucket jacket foundation (FBJF) as the research objects, four different sling layout types were proposed. The influence of the slings layout types on the motion response of the installation vessel and foundation and the slings' tension in this system were comparatively studied through numerical simulation. Research shows that through the lowering of Type 1 conditions, that is, each chord corresponds to a vertical sling and is directly connected to the crane's lifting point, the motion performance of the foundation and installation vessel can be reduced, ensuring the smooth progress of the lowering process of the structure. Under the same conditions, the FBJF has greater motion response due to the large wave force bearing area and large water plane area. Under different conditions, the DAF value and failure factor of the slings of the two structures during the lowering process all meet the specification requirements.

Bearing Behavior of Large-Diameter Monopile Foundations of Offshore Wind Turbines in Weathered Residual Soil Seabeds

The southeastern rock base sea area is the most abundant wind resource area, and it is also the mainstream construction site of offshore wind farms (OWFs) in China. The weathered residual soil is the main seabed component in the rock base area, which is the important bearing stratum of the offshore wind turbine foundation. Previous studies on the mechanical properties of seabed materials and bearing characteristics of the pile foundations in OWFs have mainly focused on the submarine soil-based seabed, resulting in a lack of direct reference for the construction of offshore wind power in the rocky seabed. Therefore, the mechanical properties of weathered residual soil and the bearing behaviors of monopile foundations are mainly investigated in this study. Firstly, dynamic triaxial tests are conducted on the weathered residual soil, and experiments analyze insight into the evolution law of the hysteresis curve, cumulative strain, and stiffness attenuation. Then, the horizontal loading behaviors of monopile foundations in residual soil are analyzed by numerical simulations; more critically, the service performances under wind and wave coupling loads are evaluated, which provide a direct theoretical basis for the construction and design of offshore wind turbine foundations in rock base seabeds.

A Study on the Mechanical Behavior of a Wind Turbine Foundation with a Constrained Structural Shear Connector

Aiming to solve the problems that a wind turbine foundation with a foundation pipe may suffer from grouting, where the concrete around the interface collapses and the interface disintegrates under a long-term wind load, a kind of wind turbine foundation with a constrained structural shear connector is proposed. In this article, the scaling model tests and a finite element simulation of a traditional stud foundation pipe, perforated steel shear connector foundation pipe, and three groups of constrained structural shear connector foundation pipes with different anchored depths are presented. The force transmission mechanism and damage mechanism of constrained structural shear connector wind turbine foundations are revealed, and the shear resistance of a constrained structural shear connector is analyzed. The influences of buried depth and other parameters on the mechanical properties of the shear connector are also investigated. The results show that the constrained structural shear connector has the advantages of stronger interfacial stiffness and significant force transfer and diffusion, and can more effectively connect the foundation pipe and concrete foundation to work together. It can give full play to the material advantages of concrete and reinforcements, and effectively improve the embedded stiffness and durability of concrete foundations. It can solve the problem of cracks in concrete caused by local pressure. At the same time, it is suggested that the diameter of the surrounding concrete should be in the range of 3 to 4 D, and the embedment depth of the stud should not be less than 0.4 D to give full play to the performance of the constrained structural shear connector.

Practical considerations for numerical simulation of large-diameter monopiles for offshore wind turbines in overconsolidated sand: A case study of the VIBRO project in Cuxhaven, Germany

Monopiles are typically the preferred foundation for offshore wind turbines, which are becoming increasingly large. While 3D finite element modelling can aid the design process of monopile foundations, achieving reliable results requires precise constitutive soil models and input parameters derived from laboratory and field data, as well as careful consideration of soil sample preparation method. This study explores various soil preparation methods, including moist tamping, dry tamping, and air pluviation, along with two sample ratios (1 and 2) during triaxial tests. It uses an advanced soil model, the DeltaSand model, and a practical industry model, the Hardening Soil model with small strain (HSsmall). To evaluate model performance, the Vibro project performed in Cuxhaven, Germany, is used as a case study. Simulations using the DeltaSand model show good agreement with field data. The effects of soil preparation methods and sample ratios are more pronounced in HSsmall model simulations, in which better results are obtained when the model is calibrated using dry tamping data and a sample ratio of one.

Study on cementitious grout for scour protection of offshore wind turbine foundations

To study scour-resistant applicability, 6–10% mass ratio of polyurethane (PU) was added to sulphoaluminate cement (SAC) and ordinary Portland cement (OPC) grouts to generate OPCPU and SACPU. Sodium silicate (SS) with slurry volume ratio of 1:1–5:1 was added to SAC and OPC grouts to generate OPCSS and SACSS. The water-cement ratio (w/c) of grout was 0.8–1.5. Fresh-state properties and mechanical properties were investigated. Hydrated minerals and microstructures were analyzed, scouring tests were conducted considering key formulations. The results indicated that SACSS and OPCSS have a shorter setting time and higher strength, which makes them suitable as scour resistance materials. At the w/c of 1.0, OPCSS with a volume ratio of 3:1 and SACPU with 6% PU were selected for scour resistance tests. It showed that maximum scour depths and sediment transports were 7.0–12.2 and 1.1–1.2% as those without reinforcing conditions. The critical shear stress on the seabed under reinforcement is ∼103 times greater than the bed shear stress. This inhibits the occurrence of scouring. The study evaluated the applicability of cementitious reinforcement for scour resistance. This study analyzed material and mechanical properties, hydrated minerals, and microstructures, and conducted scour tests to optimize grouting material ratios for seabed scour protection, providing references for soil reinforcement and grouting protection.

Influence of tidal asymmetry on local scour near the offshore platform

An in-situ mooring measurement was conducted using a profiling sound of navigation and ranging (SONAR) system to reveal the influence of tidal asymmetry on the local scour near an artificial offshore platform, Southwest Offshore Windfarm Complex, Korea. During the mooring period, the flood tide was significantly dominated by positive tidal duration asymmetry and skewness. The lengths of scour hole were 9.8m and 5.1m along the flood and ebb directions, respectively. Flood-dominated currents intensified the horseshoe vortex, resulting in an approximately 10% deeper scour hole on the flood face than the ebb face. The periodic occurrence of bidirectional currents generated vortices that facilitated the mixing and redistribution of seabed sediments, forming a gentle slope within the scour hole. The average slope angles ranged from 14° to 16° on the flood face and 6° to 8° on the ebb face. Despite the strong influence of local scour, variations in scour depth (Ds) consistently remained around −2.15m, suggesting the exposure of the underlying consolidated sediment layer. During the intermediate period from spring to neap tide, the Ds on the lower slope of scour hole increased by 0.12m, while it decreased by 0.11m from neap to spring tide, suggesting that the net Ds gradually approached zero over the tidal cycle. These findings underscore the importance of understanding tidal impacts on the local scour morphology to enhance the stability and design of offshore wind turbine foundations.

Analyzing critical core components’ technology opportunities based on multilayer networks from a lifecycle perspective: A case study of offshore wind turbine foundation

Among renewable energy sources, offshore wind power has emerged as an important trend due to its more stable and powerful wind resources. Advances in the fundamental technology of offshore wind turbines are crucial for ensuring the long-term feasibility and reliability of wind power projects. Existing technology opportunity analysis (TOA) studies based on link prediction network analysis seldom consider the technical characteristics of the target component from different perspectives, such as lifecycle. In this study, we propose a framework to analyze technology opportunities for offshore wind turbine foundations. The method consists of four steps: 1) Collect patents related to the target component. 2) Use BERT to automatically layer patents into corresponding lifecycle stages. 3) Build a multilayer network centered on the target component. 4) Use link prediction to recommend technology opportunities throughout the lifecycle and at each stage of the target component. Using offshore wind power foundations as an example, the effectiveness of the method is verified. Our findings indicate that improvements in structural arrangements of foundations and anti-corrosion coatings are future technology opportunities for offshore wind power foundations. Multilayer network analysis based on link prediction from a lifecycle perspective provides a valuable reference for future TOA research and practice. Additionally, it offers enterprises insights into future technology opportunities at various lifecycle stages, presenting a new perspective for optimizing R&D resource allocation and market positioning based on these results.

Wind turbine foundation ring fatigue reliability analysis under wind-load using SCADA system

Under the action of wind load, the foundation ring of the fan will generate stress concentration and alternating stress, leading to fatigue failure. Based on the average wind speed obtained from the supervisory control and data acquisition (SCADA) system, the orthogonal expansion method of random pulsating wind field and the number theory point selection method are used to calculate and simulate the corresponding pulsating wind speed time series, and then calculate the wind speed time series and wind load time series. Based on the ABAQUS finite element software, a model of a 5 MW wind turbine is established. The modal analysis is used to obtain the modal vibration pattern and the intrinsic frequency to verify the reasonableness of the structural modeling. Subsequently, the wind load time series is applied to the wind turbine model to obtain the stress time series using instantaneous modal dynamic analysis (Modal dynamics). The stress time series at the location of maximum stress concentration in the foundation ring is extracted, and the stress time series with the same stress amplitude is obtained through conversion, which has Wiener characteristics. After obtaining the stress amplitude using the rainflow counting method, the fatigue reliability is calculated based on the structural fatigue reliability analysis method considering the threshold crossing duration of the random process. This research holds reference significance for the calculation of fatigue reliability under approximate working conditions.

Weakening characteristics of pile group foundations in clayey soil under vertical cyclic loading: Experimental and numerical investigation

Long-term cyclic loading can substantially reduce the bearing capacity of pile foundations, potentially compromising the structural integrity of offshore foundations. A small-scale model test system for vertical cyclic loading of pile foundations is independently designed. Model experiments and numerical simulations were conducted to investigate the weakening characteristics of four foundation types (i.e., slab foundation, single pile with cap, 2 × 2 pile group foundation, and 3 × 3 pile group foundation) in clayey soil under cyclic loading. The results of both methods demonstrate that 3 × 3 pile group foundations exhibit the greatest resistance to deformation, followed by 2 × 2 pile groups, single piles with a cap, and slab foundations under cyclic loading. Furthermore, a 3D numerical model was developed to simulate the real-world behavior of a pile group foundation for an offshore wind turbine. Results reveal that although the ultimate compressive bearing capacity decreases with higher cyclic loading levels, typical cyclic loads (0.1–0.2) encountered in practice do not greatly affect the long-term bearing capacity of the foundations. The proposed experimental and numerical simulation methods offer valuable insights into the weakening characteristics of pile group foundations under cyclic loading.

Impact risk assessment of gravity-based foundation of offshore wind turbine under tsunamis

Impact risk assessment of gravity-based foundation of offshore wind turbine under tsunamis

Life Cycle Assessment of Additively Manufactured Foundations for Ultratall Wind Turbine Towers

ABSTRACTWind energy production is rapidly growing in the United States and is expected to continue increasing as more and larger wind turbines are installed. To support these taller and heavier onshore turbines, new foundations must be designed and manufactured. One proposed method of reducing the total amount of concrete and steel in spread foundations is to utilize additive manufacturing to enable more material‐efficient designs. To compare these additively manufacturing‐enabled designs to conventional foundation designs, this study performs a life cycle impact assessment of four ultra‐tall wind turbine foundations: two foundations using 78‐MPa 3D printed stay‐in‐place concrete formwork cast with 35‐MPa ready‐mix concrete with reinforcements, and two conventional foundations cast entirely out of 35‐MPa concrete with reinforcements. The life cycle assessment investigates the environmental impacts of four different stages, including materials production, transportation, construction, and end‐of‐life. The materials production stage is found to dominate the life cycle results, contributing over 97% of the total CO2 emissions and over 88% of the fossil fuel depletion for each foundation. Compared to the conventional designs, the Short Flat Ribbed Beam foundation with 3D printed formwork has 22.4% lower CO2 emissions and 28.3% lower fossil fuel depletion than the Circular foundation, and 2.0% higher CO2 and 5.9% lower fossil fuel depletion compared to the Tapered foundation. Parametric studies indicate that reducing cement content and increasing recycled content in printed concrete can significantly reduce the overall life cycle impacts of the foundations.

Dynamic behavior of kaolin clay under bidirectional cyclic loading for suction bucket foundation

The stability of offshore wind turbine (OWT) foundations is largely influenced by the dynamic characteristics of marine clay. In offshore engineering, marine clays may exposed to bidirectional cyclic stresses. To empirically examine the influence of different cyclic stress magnitudes on the dynamic characteristics of marine clay at different depth, a sequence of dynamic triaxial tests is conducted on marine kaolin clay, encompassing diverse values of the cyclic stress ratio (CSR) and effective confining pressure (p'0). Some valuable inferences have been drawn: At low cyclic stress ratios (CSR≤0.3), the axial strain (εa), stiffness (k), and excess pore water pressure (EPWP) of the specimen exhibit logarithmic increase, logarithmic decrease, and logarithmic accumulation, respectively. Under high cyclic stress ratio conditions (CSR＞0.3), the axial strain increases linearly, the stiffness deteriorates linearly, and the logarithmic excess pore water pressure accumulates. After analyzing the dynamic characteristics data of kaolin clay, the correlation between excess pore water pressure and axial strain was established. By analyzing the vertical cyclic mechanism of suction bucket foundation, a prediction model for suction bucket foundation under vertical cyclic loading was established based on the dynamic characteristics of marine clay and the model was validated. The comparison between the validation outcomes and the experimental outcomes shows good agreement. This study provides certain guidance for the design of suction bucket foundations for OWTs.

Local Scour Mechanisms and Prediction Methods Around Offshore Wind Turbine Foundations: Insights and Future Directions

Local Scour Mechanisms and Prediction Methods Around Offshore Wind Turbine Foundations: Insights and Future Directions

Shear-Wave Velocity Data Assists Modeling for Offshore Windfarm Development

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 35354, “Pseudo-3D Underwater Multichannel Analysis of Surface Waves for Offshore Windfarm Development,” by Stephan Unterseh and Luis McArthur, TotalEnergies, and Edouard Mouton, SismOcean. The paper has not been peer reviewed. Copyright 2024 Offshore Technology Conference. _ In the course of a research and development (R&D) project dedicated to quantitative ground modeling, underwater multichannel analysis surface-wave (UMASW) data were acquired within an offshore windfarm area currently in development in the North Sea. The goal of the project described in the complete paper was to test the added value of the shear wave velocity (Vs) interpreted from the UMASW, particularly as a propagation tool of geotechnical information. While UMASW, together with seismic refraction, is regularly used to characterize the ground for burial or trenching purposes, it appears also to bring valuable design parameters for offshore wind turbine foundations. Introduction To investigate the value of Vs in the performance of quantitative ground modeling studies, an internal R&D initiative was launched aimed at acquiring Vs data and the associated uncertainties of foundation depth of an offshore windfarm currently in development. These data will be tested at a later stage for the propagation of geotechnical parameters used in foundation design. The acquisition consisted of obtaining Vs data through UMASW methodologies around two distinct geotechnical boreholes and along a corridor tying the two areas. The UMASW commonly is used in combination with seismic refraction to perform burial assessment analysis of cable or pipeline or to characterize ground conditions before performance of horizontal deviated drilling. Thus, the data acquisition did not require extensive hardware development. A unique pseudo-3D processing technique nevertheless was implemented to allow for 3D data visualization and ground characterization. Site Conditions The selected site lies in the North Sea in water depths ranging from 20 to 30 m. The area was first investigated with standard geophysical techniques. Preliminary geotechnical investigation included 6-m-deep vibrocores and 60-m-deep drilled boreholes, with alternating cone penetration tests and push sampling and coring. P-S logging also was performed at key geotechnical locations to gather 1D primary wave velocities (Vp) and Vs. The seabed generally consists of a thin cover of Holocene dense sand, occasionally with boulders, overlying quaternary subglacial formation deposits, including moraine, gravelly sand, and clays of variable thicknesses. The bedrock is generally Jurassic and includes dense sand; hard clay; weak claystone; and, locally, strong limestone bands. The test locations were selected within the coverage of the geophysical data set in areas where variable lithologies occur and where the seabed was found free of boulders and outcrops because these are detrimental to UMASW acquisition. The lithology of the two boreholes performed within the test area is indicated in Table 1 of the complete paper.

Experimental study on local scour for large-diameter mono-column composite bucket foundation for offshore wind turbines

The issue of seabed scour around offshore wind turbine foundations is a significant safety concern. In this study, local scour around a specific type of foundation, known as the large-diameter Mono-Column Composite Bucket Foundation (MCCBF) with six connectors, has been investigated through physical experiments. The study involved various flow conditions, including unidirectional flows, tidal flows, and combinations of regular waves and currents. It has been found that under similar flow intensity and orientation angles, unidirectional flow tends to cause deeper scour depths, but with a more limited extent compared to scour induced by tidal flows. The disparity in maximum scour depth between tidal and unidirectional flows decreases as flow velocity increases. Under regular wave-current interaction, both the scour depth and the scour extent are larger than those induced by unidirectional flow. The presence of lee-wake vortices and streamline contractions near the connectors results in larger scour depths on either side of the foundation, especially when the orientation angle is set to 0°. Different methods for predicting scour depth of the MCCBF were applied and discussed.

Numerical study on the flow characteristics of an integrated fish cage based on the monopile offshore wind turbine foundation

The integration of net cages and offshore wind turbines can effectively develop and utilize marine resources and is one of the important ways of intensive use of the sea. The numerical simulation of the hydrodynamics of an integrated fish cage based on the monopile offshore wind turbine foundation in currents is carried out. The realizable k-ε model is adopted and the porous media model is used to simulate the net. To verify the validity of the numerical model, the numerical result is compared with the data of the corresponding literature. The results show that the numerical method is effective. On this basis, the flow field distribution results of the integrated structure at different velocities are calculated. At the center of the downstream zone of the integrated structure, the time-average velocity is lower than 80 % of the initial velocity. The effects of the net solidity and the cage draught on the flow field characteristics of the integrated structure are analyzed. The results show that the increase of net solidity will significantly reduce the velocity inside the net cage and in the downstream zone of the cage. As the net solidity increases from 0.14 to 0.32, the average velocity attenuation inside the net cage increased by 13.9 %. The change of cage draught will affect the velocity distribution in the horizontal and vertical directions. When the cage draught increases, the velocity on both sides in the downstream zone of the integrated structure increases more than 5 %. It is expected that the results of this study can provide data support for the design of integrated structure of net cages and offshore wind turbines.

Research on the ice-resistance performance of conical wind turbine foundation in brash ice fields

Due to the significantly higher wind energy density in high-latitude sea areas compared to other regions, the construction of offshore wind farms is shifting towards these areas. However, in high-latitude sea areas, large amounts of sea ice float on the surface during winter, making ice loads a critical factor affecting the structural safety of wind turbines. For monopile flexible wind turbines, installing an ice-breaking cone near the waterline is an effective measure to withstand the threat of sea ice. To clarify the ice resistance performance of conical wind turbine foundations in brash ice zones, this study proposes a coupled Computational Fluid Dynamics-Discrete Element Method (CFD-DEM). This method is used to calculate the ice load results for a cylindrical riser, and the results were found to be in good agreement with the ice load experiment results from China University of Petroleum's wave flume (CUPWF). This validation confirms the rationality of the method. Taking a 5 MW monopile wind turbine installed with an ice-breaking cone as the research object, the time history of dynamic ice loads and the displacement response of ice-induced vibration during the interaction between the wind turbine and brash ice are simulated. Considering the water level variations caused by tides, the differences in ice resistance performance between the upright cone and the inverted cone of the ice-breaking cone are analyzed. Finally, the impact of changes in cone angle on the ice resistance performance of the wind turbine foundation is determined. The results indicate that under the impact of brash ice, the upright cone of the ice-breaking cone has a higher ice resistance capacity compared to the inverted cone. With changes in cone angle, the trends of ice loads in the horizontal and vertical directions vary oppositely. However, the overall ice resistance capacity of the wind turbine foundation decreases as the cone angle increases. This study can provide a reference for the rational design of conical wind turbines in cold regions.

Seismic response of the composite bucket foundation supported offshore wind turbines under marine environmental loadings

Many offshore wind turbine (OWT) foundations are situated in earthquake-prone areas, where complex environmental loads intensified the seismic responses of the OWTs. In response to this situation, the composite bucket foundations is proposed and extensively applied due to its reliable installation and high seismic resistance. To analyze the seismic response of composite bucket foundations subjected to environmental loads, refined numerical models of the 6.45 MW OWT system are established, and parametric analyses are conducted, considering the influence of seismic frequency contents and intensities, the soil layer distributions and wind velocities. The results show that the composite bucket foundations undergo cumulative rotational deformation around a point biased toward the windward side of the foundation under the seismic and environmental loads. The increasing mean wind velocity significantly enlarges the displacement response envelope, while has limited influence of the acceleration response envelope of OWTs. Increasing the bucket diameter enhances the deformation resistance of the OWT, while the efficiency of this improvement diminishes as the diameter increases. Greater bucket skirt length also enhances its deformation resistance, but may lead to increased lateral acceleration response of the OWT.

Dynamic response of OWT tripod pile foundation in clay considering scour

Tripod-supported offshore wind turbine (OWT) foundation must withstand wind, wave, and seismic loads. In addition, it may be threatened by scouring during its service life, which could affect its bearing capacity and the overall OWT dynamic response. This study develops a three-dimensional numerical model of OWT tripod pile foundation to investigate scour effects. The numerical model incorporates a simplified single bounding surface model to simulate the dynamic stress–strain response of clay soil. The results revealed that scouring can reduce the first-order natural frequency of tripod pile-OWT system; however, the reduction usually does not lead to resonance within the 1P frequency range. Nonetheless, as the scour depth increases under wind and wave loads, the horizontal displacement of the wind turbine structure increases monotonously; however, its peak acceleration increases initially then decreases slightly as the scour depth continues to increase. Under seismic loading only, the OWT response exhibits complex evolution pattern as the scour depth increases. On the other hand, the foundation horizontal displacement and rotation angle increase significantly under combined wind-wave-seismic loads when the scouring depth reaches three times the pile diameter. Finally, the scouring depth has a relatively small impact on the foundation vertical displacement.