Wind Turbine Foundations Research Articles

New hybrid foundation solutions for offshore wind turbines

This paper reviews latest developments of substructures for offshore wind turbines focusing on investigations and applications of hybrid foundations. Model tests and numerical analyses were used to simulate the loading of hybrid piles in sand. The results of pile-soil interaction were investigated to confirm the changes in soil stiffness around the hybrid monopile head. The mechanism and factors affecting the change in lateral stiffness of the hybrid foundation were explained by analysing p–y curves for M+H loading conditions in sand. Based on this research, a new shape of p–y curves for hybrid monopiles was established and a method for determining key parameters was proposed. The effectiveness of new p–y curves was verified by comparing back-calculated results with those from numerical simulations. The conducted tests confirmed that the hybrid monopile displacement is 30–50% smaller when compared to a standard monopile with similar dimensions. The gained experiences can be useful for designers and researchers to enhance the design of foundations for offshore wind turbines.

FDEM analysis of deep rock mass failure and its impact on horizontal bearing capacity in offshore wind turbine piles

Rock-socketed single piles are integral components of offshore wind turbine foundations in rock-based sea areas. Traditional numerical models have been employed to analyze the failure modes and lateral interactions between piles and the rock medium. However, these do not simulate the decline in the horizontal bearing capacity due to rock breakage, posing potential safety risks. In this study, the combined finite-discrete element method (FDEM) was used to simulate the deformation process of submarine rock masses transitioning from an intact to a fractured state. A notable innovation in the proposed approach is the consideration of the effects of the rock mass fracture and pile-rock cohesion on the load-bearing behavior of single-pile foundations. The FDEM model was validated through full-scale lateral load tests. The results showed a distinctive sequence in failure mechanisms, with the tensile failure of the rock mass around the pile at significant depths preceding the compression failure. Local failure at the contact interface significantly reduced the horizontal bearing capacity—a phenomenon that is inadequately represented by conventional finite element models. Furthermore, the cohesion element embedding method was introduced and used to simulate the influence of the tensile properties inherent in grouting materials on the failure strength and mode of the pile-rock interface. This study highlighted the importance of considering the nuanced effects of rock failure in the design of horizontally loaded piles. Failure to do so could lead to nonconservative designs and compromise the safety and efficacy of offshore wind turbine foundations. By pushing the boundaries of numerical modeling with the FDEM, the study aims to provide a more accurate and reliable framework for the design and assessment of rock-socketed single piles for offshore wind energy applications.

Foundations of onshore Wind turbines: current situation and trends

The commitments by the governments for the net zero emissions for the coming decades will be a huge challenge for the renewable sector all over the planet. Wind energy will play a significant role in this transition and scale-up, in both offshore and onshore, is a crucial step to move forward. The quick development of the new onshore wind turbine models in the race for the most powerful machine, the gradual increase in the hub height, and the unitary power of the new onshore wind turbines are producing modifications in different components of the turbines. This work presents a review of the onshore wind turbines foundations, especially about the different foundation design concepts available in the onshore business, the analysis of some of them and the trends.

A Study on the Scour Surrounding the Fixed Foundation of an Offshore Wind Turbine under Complex Waves, Tidal Currents, and Pile Vibration Conditions

Scouring around the fixed foundations of offshore wind turbines (OWTs) remains a pressing concern within the offshore wind industry, significantly jeopardizing OWT safety. Despite previous efforts, the current understanding of this phenomenon remains incomplete, as various factors related to offshore wind farms and OWT operations can exert intricate influences. To further explore this matter, this paper undertakes a new study to further understand the interplay between monopile vibrations, tidal currents, and sea waves, elucidating their combined impact on scour surrounding an OWT’s foundation. It was found that, besides pile vibration amplitude and frequency, the pile vibration direction can also notably influence the scour around the monopile. Specifically, the most pronounced scour mostly transpires when the monopile undergoes vibrations along the 135° direction relative to the inflow direction. The mildest scour mostly occurs when the pile vibrates along the 90° direction. Additionally, this study reveals that different types of waves, indicated by the ratio of water depth, H, to wavelength, L, also engender varying scour around the monopile. The influence of waves on scour increases with the decrease in the H/L ratio, implying that OWT foundations are more threatened by shallow water waves than by deep water waves.

A review of scour impacting monopiles for offshore wind

Monopiles are important structures in offshore wind engineering, primarily serving as foundations for wind turbines. When installed on a sandy seabed, local scour around monopiles can reduce their capacity, and is a significant hazard to be addressed in the design process. This paper provides a review of research related to local scour around monopiles, focusing on the following aspects: (1) identifying flow structures related to local scour, (2) synthesizing existing data on equilibrium scour depth and scour time-scale, (3) summarizing technology for observing local scour through laboratory experiments, numerical simulations and field observations, and (4) outlining the existing challenges and research opportunities.

Sand deformation behavior around modified suction caissons during lateral loading and analytical method obtaining lateral bearing capacity

The modified suction caisson (MSC) is an innovative offshore wind turbine foundation. Model tests and numerical simulations were carried out to explore the sand deformation behavior around the MSC under lateral loading. The vertical displacements fitting method and the level bubble method were proposed obtaining the sand deformation range. Influences of the vertical load, loading eccentricity and external structure sizes were investigated. It was found that the shape of the sand surface deformation range in front of the caisson can be fitted by ellipse functions. In the limit state, the sand surface deformation ranges around both the MSC and the regular suction caisson (RSC) decrease with increasing loading eccentricity and vertical load. Besides, the sand surface deformation range in front of the MSC increases with increasing the external structure height. Three-dimensional shape of the sand deformation range in front of the MSC is dominated by the external structure height and the rotation center embedded depth. Based on the three-dimensional sand deformation behavior and limit equilibrium method, equations estimating the lateral bearing capacity of the MSC were proposed. The bearing capacities of MSCs obtained by using the proposed equations agree well with the test and numerical simulation results.

Behavior Analysis of a Bucket Foundation with an Offshore Wind Turbine during the In-Water Sinking Process

To enhance the safety of the in-water sinking operation for an integrated system, including a bucket foundation (BF), tower, and rotor nacelle assembly (RNA), in complex marine environmental conditions, a model test of in-water sinking for an offshore wind turbine and bucket foundation (OWT–BF) is conducted. The motion behavior of the OWT–BF and the sling tensions during the in-water sinking process are investigated, and the numerical method is validated through test results. The results demonstrate a positive correlation between the wave height and motion response of the OWT–BF. The most critical stage of the in-water sinking operation occurs when the top cover of the bucket is fully submerged, resulting from the substantial cross-sectional difference between the bucket base and the transition section. Furthermore, the closer the OWT–BF is to the seabed, the less it is affected by waves in terms of motion response. It is advisable to conduct the in-water sinking operation of the OWT–BF in sea states with wave heights below 1.5 m. Simultaneously, slings can efficiently control the motion response of the OWT–BF, thereby enhancing the safety of the sinking operation.

Static-dynamic damage mechanism and self-heating effect of a clean elastic polyurethane grouting material for trenchless rehabilitation under high stresses

Compared with traditional excavation rehabilitation, trenchless grouting rehabilitation technology has the advantages of fast, precise, and low-carbon, which has a broad application prospect in wind turbine foundation rehabilitation. In this work, a non-toxic, environment-friendly, fast-setting, high-strength, and high-toughness elastic polyurethane (PU) grouting material was proposed, and the static-dynamic damage mechanism and self-heating effect under high stress were studied by DMA, SEM, static-dynamic compression, and self-heating monitoring tests. The results show that the static compressive properties increased in a power function relationship with increasing strain rate. The dynamic compression fatigue process can be divided into adjustment stage (Stage I), viscoelastic development stage (Stage II) and failure stage (Stage III) or cyclic hardening stage (Stage III*). At 41.57 MPa and 36.95 MPa, fatigue damage parameters increased slowly in Stage I and Stage II, and sharply in Stage III. The stiffness damage, loss factor, cumulative strain, and strain rate increased by more than 100% in Stage III. Fatigue failure dominated by vertical/slanting through-cracks, shed debris, drum deformation, etc. was closely related to temperature and loading. The failure temperature about 59.1 °C, which was higher than the glass transition temperature. At 32.33 MPa, fatigue damage parameters changed slightly and then stabilized, showing exponential relationships with cycles. The max temperatures were less than 15 °C. The slight temperature rise facilitated the adjustment of structure and cyclic hardening under cyclic loading. This work can guide the further engineering application and fatigue durability evaluation of PU grouting materials.

Predicting the Displacement Variation of Rehabilitated Foundation of Onshore Wind Turbines Using Machine Learning Models

The rehabilitation of wind turbine foundations after damage is increasingly common. However, limited research exists on the deformation of wind turbine foundations after rehabilitation. Artificial intelligence methods can be used to analyze future deformation state and predict post-rehabilitation deformation of foundations. This paper focuses on analyzing the stability of damaged wind turbine foundations after rehabilitation, as well as establishing and evaluating machine learning models. Specifically, Decision Tree (DT), Extreme Gradient Boosting (XGB), Support Vector Regression (SVR), and Long Short-Term Memory Network (LSTM) models are utilized to predict the vertical displacement of the rehabilitated foundation. Hence, the stability of the rehabilitated foundation is discussed in correlation with the measured wind speed, based on the foundation vertical displacement data. During the development of the machine learning model, the most suitable combination of hyperparameters is determined. The prediction performance of the SVR and LSTM models, which exhibit good performance, is compared to further evaluate their effectiveness. Furthermore, the models are analyzed and validated. The results indicate that the vertical displacements of the rehabilitated foundations gradually get close to a state of steady fluctuation over time. The SVR model is identified as the most effective in predicting the vertical displacements of wind turbine foundations after rehabilitation. This study aims to analyze and predict the vertical displacement of wind turbine foundations after rehabilitation based on extensive field monitoring data and powerful machine learning models.

Parametric study of catenary mooring system on floating wind turbine foundation dynamic response

The ability of the floating offshore wind turbine foundation to withstand harsh wind and wave conditions is intrinsically linked to the meticulousness employed in the design of the mooring system. Therefore, it is vital to conduct research to reveal the variations in sensitivity of new mooring materials parameters and the response characteristics of the foundation in the presence of intricate operational circumstances. A fully coupled time-domain dynamic analysis is performed via the commercial software AQWA to examine the VolturnUS-S semi-submersible foundation, specifically engineered to support the IEA 15 MW offshore reference wind turbine. The numerical simulation results indicate that synthetic fibers, specifically Aramid and high-modulus polyethylene, can replace the steel wire as mooring materials. At the same time, the mooring arrangement, length, diameter, and fairlead position influence foundation motions and mooring tensions, bringing valuable insight into the fluctuation patterns of these responses.

Scour Protection Measures for Offshore Wind Turbines: A Systematic Literature Review on Recent Developments

Offshore wind energy is considered as one of the most promising resources of clean and renewable energy to replace fossil fuels. Additionally, its cost is expected to be lower than onshore wind energy as the technology matures. Offshore wind turbines (OWTs) normally operate in harsh ocean environments, which could impact their structural integrity. Scour erosion around foundations of OWTs can substantially change the overall stiffness of these structures and shorten their lifetime. Currently, there are a limited number of studies on countermeasures and their engineering requirements for decreasing the scouring effect; this is due to their different hydraulic circumstances, such as their stability, reliability, and resistance capacity. To this end, advancements in scour protection measures in the offshore energy sector are evaluated in this paper through a thorough and critical review following the PRISMA systematic literature mapping approach. This includes 68 papers on scour protection and over 30 scour protection designs for various types of wind turbine foundations. Here, we aim to provide an overview of the latest scouring protection measures and their comprehensive assessment, as well as their prospects and future challenges. The findings of this study will provide key insights into scour protection measures for OWTs and will subsequently contribute to the future growth of the offshore renewable energy sector.

Dynamic response analysis of a semi-submersible floating wind turbine based on different coupling methods

This article compares and analyzes the overall performance of offshore floating wind turbines using fully coupled and semi-coupled methods considering aerodynamic loads, as well as decoupled methods without considering aerodynamic loads. The aim is to study the impact of different coupling methods on the dynamic response of floating wind turbines and provide a reference for selecting more convenient calculation methods for the preliminary design of floating wind turbines. Based on the open-source software OpenFAST and the commercial hydrodynamic software AQWA, the OC4-DeepCwind semi-submersible offshore floating wind turbine foundation and the NREL 5 MW reference wind turbine model are selected as the research objects. The natural period of the floating structure is validated, and then the power generation and shutdown conditions of the floating wind turbine are selected as the calculation conditions. The motion responses of the floating structure and mooring tension of the floating wind turbine are calculated in the time domain, and the calculation results are statistically analyzed. The research results show that the decoupled analysis method without considering aerodynamic loads significantly underestimates the dynamic response of the floating wind turbine, and the semi-coupled method is more conservative than the fully coupled method. Therefore, in the absence of conditions for fully coupled analysis, the semi-coupled method can be used as a substitute analysis method in the preliminary design stage.

Experimental Investigation on the Motion Characteristics of Air-Floating Tripod Bucket Foundation during Free Floating

In recent years, multi-bucket foundations have been studied and gradually adopted in engineering practices as a novel foundation for offshore wind turbines within a range of water depth of 30 to 50 m. This study investigated the motion characteristics of air-floating tripod bucket foundation (AFTBF) through a series of experiments during free air-floating. The experimental results show that the surge force appears to be the most important factor influencing pitch moment and motion, whether it is a change in water depth or a draft for AFTBF. The maximum amplitudes of surge acceleration and pitch angle show a trend of increasing with narrower spacing and decreasing with wider spacing, while the heave acceleration shows an opposite trend. The added mass and damping of heave motion for AFTBF increase with shallower water due to the increasing pressure difference between the inside and bottom of bucket foundation. The shallower the water depth and the larger the draft, the longer the resonance period of heave.

Scour Effect on the Lateral Bearing Behaviour of Monopiles Considering Different Slenderness Ratios

Scour leads to the loss of soil around monopile foundations for offshore wind turbines, which affects their structural safety. In this paper, the effect of scour on the lateral behaviour of monopiles was extensively investigated using finite element analysis, and calibration and comparison were undertaken using centrifuge tests. Piles with three slenderness ratios, i.e., 3, 5 and 8, were studied by keeping the diameter constant and varying the embedment length. Three scour types (local narrow, local wide and global) and four scour depths (0.5D, 1D, 1.5D and 2D; D signifies the pile diameter) were considered in this investigation. The results indicate that the lateral resistance of the pile is the greatest in the case of local narrow scour, followed by that in the cases of local wide scour and global scour. When the scour depth is larger than 1D, the influence of the scour type on the pile lateral bearing behaviour is insignificant. The influence of the scour type and scour depth on the pile lateral bearing behaviour is broadly similar for piles with slenderness ratios of 3, 5 and 8. However, the piles featured with smaller embedment lengths show a larger decrease rate in their lateral capacity, which means the effect of scour should cause more concern on small slenderness ratio monopiles.

Offshore wind farm foundations as artificial reefs: The devil is in the detail

Climate change and global biodiversity loss call for clean energy production systems with minimised ecological impacts. Offshore wind energy production will become one of the main uses of global marine spaces within next decades. Offshore wind turbine foundations can function as artificial reefs but it is unknown if these capabilities apply to different foundation types. We collected field data on Atlantic cod (Gadus morhua), a species under pressure in the southern North Sea, around three foundation types to assess these capabilities. Catch rates showed that monopile foundations with rock protection on the seabed were able to attract significantly more fish than monopile foundations with sandbag protection and jacket foundations. Fish densities varied on small scales meaning that reef effects were spatially restricted. This implies that offshore wind energy production can be used as tool to combine climate change mitigation with local biodiversity conservation but that a consideration of the wind farm design is required.

Simplified modelling of full-strain-range non-linearity of cyclically loaded undrained clays

Constitutive modelling of cyclically loaded undrained clay is of significant importance for various branches of geotechnical engineering exemplified by offshore wind turbine (OWT) foundation design and prevention and mitigation of earthquake hazards. Fine-grained soils can display non-linear stress-strain relations from small (10-5) to relatively large (10-1) strain levels. This full-strain-range non-linearity can remarkably affect the cyclic response. Effective stress-based constitutive models have achieved great success in modelling clayey soils, whereas they can be overly complex for practicing engineers, in particular, when considering non-monotonic loading and full-strain-range non-linearity. This paper explores the possibility of modelling, in simplified manner, the full-strain-range nonlinearity of cyclically loaded undrained clay, which can directly utilize outcomes of in situ site exploration and routine laboratory tests. For this purpose, we idealize soils under undrained conditions as single-phase materials governed by total stress. Considering that most current total stress-based models are proposed for metals and may be limited to capture the non-linear stress-strain relations of soil, a novel generalized non-linear (GNL) hardening law is proposed that can describe versatile stress-strain relations of undrained clays. Bounding surface and a mapping rule considering update of projection centre are introduced to reflect the influences of maximum past stress history and recent stress history, respectively, on the stress-strain nonlinearity. The simplified constitutive model is first validated at the element level by simulating monotonic and cyclic loading laboratory tests on undrained clays. Later, the proposed soil model is applied to the finite element analyses of OWT pile foundation subjected to cyclic lateral loading.

Residual strength of corroded ring-stiffened cylinder structures under external hydrostatic pressure

Corrosion damage to submarine pressure hulls is a crucial consideration during the structural design phase because it diminishes the structural strength of a submarine, reducing its operational depth and posing a severe risk to the life of the crew. This necessitates using a validated analytical method to estimate the residual strength and assess the safety of corrosion-affected pressure hulls. This paper presents an experimental and numerical investigation of the residual strength of corroded ring-stiffened cylinders, which are typical components of submarine pressure hulls, the main legs of semi-submersibles, and Floating Offshore Wind Turbine (FOWT) foundations. Four steel ring-stiffened cylinder models were fabricated: two remained intact, whereas the other two were artificially corroded by machining. Hydrostatic collapse tests were conducted in a pressure chamber. To evaluate the effect of initial shape imperfections caused by welding fabrication on the strength of the structure, the initial shape imperfections were measured using a three-dimensional coordinate measuring machine. The experimental results illustrate a significant reduction in the residual strengths of the two damaged models compared with those of the intact models. The collapse processes simulated using the Abaqus software package are presented, demonstrating a close agreement between the experimental results and numerical predictions.

Single pass laser vacuum welding of thick steel plates using electromagnetic support

The increasing demand for renewable energy produced by offshore wind turbines goes along with an increased demand in the production of offshore wind turbine foundations, so called “monopiles”, which are made by joining thick metal sheets. The industrial standard of multi-layer submerged arc welding (SAW) for joining of thick metal sheets is the current bottleneck in the production of monopiles. A possible increase in productivity by the implementation of high-power laser welding in a newly developed mobile vacuum chamber (MoVac) and an electromagnetic root support is the subject of this study. Single run butt welds are performed in flat position on S355 mild steel of thicknesses up to 80 mm using a disc laser system with 1030 nm wavelength and a maximum output of 60 kW. The laser optic is fixed on the MoVac-System which is held and manipulated by an articulated robot.

Offshore floating wind turbine foundation revolution enabled by fiber-reinforced polymer (FRP) reinforced cementitious materials

<p>Offshore floating wind turbines (OFWTs) are gaining popularity due to their superior wind energy capture and minimal visual impact. However, traditional steel support foundations for OFWTs are plagued by corrosion issues. This article proposes the use of Fiber-Reinforced Polymer (FRP) reinforced Ultra-High Performance Concrete (UHPC) composites, referred to as FRU composites, for OFWT foundations. Durability assessment of FRU plates under simulated marine environment is conducted based on accelerated aging tests on FRU plates. Scanning electron microscope (SEM) analyses are conducted to explore the fracture surface and interface between FRP and UHPC matrix. A series of tests are conducted and the test results of the FRU elements are summarized in this article. Strength design methodologies for FRU elements under various loadings are established based on summary of existing studies. Hydrodynamic analyses and comparative studies between FRU and steel OFWTs reveal that FRU OFWTs demonstrate improved stability and reduced motion responses under combined wind-wave-current loading conditions. The successful development of FRU composites is anticipated to revolutionize the OFWT industry by offering durable and cost-effective foundation options.</p>

Ice-Induced Vibrational Response of Single-Pile Offshore Wind-Turbine Foundations

Ice-Induced Vibrational Response of Single-Pile Offshore Wind-Turbine Foundations