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Abstract In the design of offshore wind farms the simulated dynamic response of the wind turbine structure includes loading from turbulent wind. The International Electrotechnical Commission (IEC) standard for wind turbine design recommends both the Mann spectral tensor model and the Kaimal spectral model combined with an exponential coherence formulation. These models give deviating wind loads. This study compares these two models to a large eddy simulations model and a model based on offshore wind measurements. The comparisons are performed for three situations, covering unstable, neutral and stable atmospheric conditions. The impact of the differences in the wind fields on the quasi‐static response of a large bottom‐fixed wind turbine is investigated. The findings are supported by an assessment of the impact of individual wind characteristics on the turbine responses. The wind model based on measurements causes high tower bottom and blade root flapwise bending moments due to a high wind load at very low frequencies. Low and negative horizontal coherence is obtained using the Mann spectral tensor model. This causes relatively large yaw moments as compared to the results using the other wind models. The largest differences in response are seen in the stable situation. We furthermore show that the quasi‐static wind load has great impact on the total damage equivalent moments of the structure. From the results, we conclude that in the design of large offshore wind turbines one should carefully consider the structure of the turbulent wind. Further, longer simulations than recommended by the standards should be used to reduce uncertainty in estimated response.

Abstract Maintenance is essential in keeping wind energy assets operating efficiently. With the development of advanced condition monitoring, diagnostics and prognostics, condition‐based maintenance has attracted much attention in the offshore wind industry in recent years. This paper models various maintenance activities and their impacts on the degradation and performance of offshore wind turbine components. An integrated maintenance strategy of corrective maintenance, imperfect time‐based preventive maintenance and condition‐based maintenance is proposed and compared with other traditional maintenance strategies. A maintenance simulation programme is developed to simulate the degradation and maintenance of offshore wind turbines and estimate their performance. A case study on a 10‐MW offshore wind turbine (OWT) is presented to analyse the performance of different maintenance strategies. The simulation results reveal that the proposed strategy not only reduces the total maintenance cost but also improves the energy generation by reducing the total downtime and expected energy not supplied. Furthermore, the proposed maintenance strategy is optimised to find the best degradation threshold and balance the trade‐off between the use of condition‐based maintenance and other maintenance activities.

With the large number of offshore wind farms put into operation in China, the adoption of reasonable and effective foundation scour protection measures has an important impact on the safety and stability of wind turbine structure. The in-situ sludge solidification protection scheme and construction technology for scour protection of monopile foundation were studied and proposed based on an offshore wind farm with silt sediment. The laboratory tests of silt with different moisture content and different amount of curing agent were carried out. And the in-situ sludge solidification field trials for scour protection were completed by using two schemes of "curing before pile driving" and "pile driving before curing". The strength test of solidified soil samples taken on site and the scanning test of the scour protection effect around the foundation show that the implementation of in-situ sludge solidification for monopile foundation scour protection of offshore wind turbines is effective. [ABSTRACT FROM AUTHOR]

Exposed to the cyclic action of wind and waves, offshore wind structures are subject to fatigue deterioration processes throughout their operational life, therefore constituting a structural failure risk. In order to control the risk of adverse events, physics-based deterioration models, which often contain significant uncertainties, can be updated with information collected from inspections, thus enabling decision-makers to dictate more optimal and informed maintenance interventions. The identified decision rules are, however, influenced by the deterioration model and failure criterion specified in the formulation of the pre-posterior decision-making problem. In this paper, fatigue failure criteria are integrated with Bayesian networks and Markov decision processes. The proposed methodology is implemented in the numerical experiments, specified with various crack growth models and failure criteria, for the optimal management of an offshore wind structural detail under fatigue deterioration. Within the experiments, the crack propagation, structural reliability estimates, and the optimal policies derived through heuristics and partially observable Markov decision processes (POMDPs) are thoroughly analysed, demonstrating the capability of failure assessment diagram to model the structural redundancy in offshore wind substructures, as well as the adaptability of POMDP policies. [ABSTRACT FROM AUTHOR]

Abstract This study compares the dynamic behaviour of a conventional and a compact gearbox for the DTU 10 MW wind turbine supported on a monopile offshore structure. The conventional gearbox configuration is composed of two planetary epicyclic stages and one parallel stage, while the compact gearbox configuration consists of a fixed planetary stage and a differential compound epicyclic stage. The design methodology for these two gearboxes is described, and the final gearbox specifications show a lighter weight and smaller volume for the compact gearbox design compared to the conventional one. Computational gearbox models are established using the multi‐body system dynamic analysis method. A decoupled approach is employed for the gearbox load effect analysis. Comparisons of the dynamic behaviour between these two gearboxes are conducted under pure torque load cases, tangential pin position error conditions and non‐torque load cases. The results demonstrate that the compact gearbox has better dynamic performance under different torque load cases and is more robust to withstand the effects of manufacturing errors and rotor non‐torque loads compared to the conventional gearbox. It is believed that the proposed compact gearbox concept is promising and would be a good alternative for multi‐megawatt floating wind turbines, although challenging with respect to the design and operation complexity.

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. • The interaction factor superposition method for jacket pile groups is studied. • This method is employed to estimate the vertical and rocking impedance functions. • The influence of the main geometrical and material parameters is analysed. • Expressions for the interaction factors incorporating these parameters are proposed. • The superposition method and the proposed expressions lead to a correct estimation. [ABSTRACT FROM AUTHOR]

Assessing the dynamic performance of offshore wind turbines (OWTs) is crucial for their safe operation and maintenance. To evaluate the structural dynamic performance, traditional methods rely on the reanalysis of damage models, leading to the lack of the real-time capability. Therefore, this paper presents a structural dynamic performance assessment technology based on the root morphology model to address such issue. The approach is developed to synergize data fusion-based and environmental condition classification-based dynamic performance assessment technologies. The data fusion-based method, using optimized VMD for denoising and ARMA models for free response signals, enables real-time dynamic performance assessment by tracking changes in AR coefficients over time. In parallel, the environmental condition classification-based method employs wind speed and wave height monitoring data to classify structural response data into multiple healthy sub-databases. Subsequently, Extenics is applied to compute the correlation between the data to be evaluated and the performance state levels based on the classified data, providing a precise evaluation of the structural dynamic performance. Finally, these two technologies are integrated through root morphology models, which fully consider structural vibration response data and environmental monitoring data. To verify the real-time and reliability of the proposed method, field study has been carried out on a 4 MW monopile OWT during the passage of Typhoon 'In-fa'. Results indicate that the root morphology model-based assessment technique effectively evaluates the impact of typhoons on the dynamic performance of OWTs. Further, a rose diagram for the dynamic performance assessment of OWTs is drawn to achieve real-time evaluation of OWT dynamic performance, which has certain engineering significance to guarantee the safe operation of OWTs and reduce the cost of operation and maintenance. • Root morphology model is developed to integrates different assessment outcomes. • Pearson distance is proposed to quantify the assessment result based on environmental condition classification. • Structural response data and environmental data are both considered in the assessment progress. • Field test has been carried out to verify the proposed method. [ABSTRACT FROM AUTHOR]

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. • A new dimensionless parameter, the undisturbed friction velocity-based Froude number Fr ∗, has been developed to quantify the hydrodynamic intensities under combined wave-current conditions. • Three distinct regimes around vibrating monopile foundations are identified based on the Fr∗ number: Compared to static monopile foundations, the characteristic dimensions of equilibrium scour holes around vibrating monopile foundations are larger in Regime 1 and smaller in Regime 3. • An empirical equation based on Fr ∗ for estimating the equilibrium scour depth around vibrating monopile foundations under combined wave-current conditions has been established. [ABSTRACT FROM AUTHOR]

The soil-structure interaction (SSI), which presents intricate nonlinearities and inevitably varies throughout the structural lifetime, is critical for the structural performance of monopile-supported offshore wind turbines (OWTs). Since its direct monitoring is challenging, a new time domain model updating method for support condition identification of monopile-supported OWTs is proposed in this study. A distributed spring-dashpot model incorporating nonlinear stiffness and damping is used to simulate the SSI. Time-domain responses of the OWT are used to construct the objective function for model updating. Vibration tests on a scaled model of the DTU 10 MW OWT are performed in the laboratory to verify the proposed method. The model updating results based on the test data show that the mean squared error between simulated and measured responses is less than 0.011, significantly smaller than the results obtained by using the traditional frequency-domain model updating methods. These findings demonstrated that the proposed distributed spring-dashpot model can accurately capture the complex nonlinearity in SSI and that the proposed time-domain model updating method can be used to identify the support conditions of monopile-supported OWTs. The methodology is expected to contribute significantly to enhancing the efficiency of the operation and maintanence for OWTs. • Time domain model updating-based approach for OWT support condition identification; •The nonlinear stiffness and damping of the OWT foundation at different depths are considered by a distributed spring-dashpot model; •Description of soil-structure interaction using minimal parameters for support condition identification; •The extreme values of the time domain data and the corresponding time are used to establish the objective function. [ABSTRACT FROM AUTHOR]

As large wind farms are now often operating far from the shore, remote condition monitoring and condition prognostics become necessary to avoid excessive operation and maintenance costs while ensuring reliable operation. Corrosion, and in particular uniform corrosion, is a leading cause of failure for Offshore Wind Turbine (OWT) structures due to the harsh and highly corrosive environmental conditions in which they operate. This paper reviews the state-of-the-art in corrosion mechanism and models, corrosion monitoring and corrosion prognostics with a view on the applicability to OWT structures. Moreover, we discuss research challenges and open issues as well strategic directions for future research and development of cost-effective solutions for corrosion monitoring and prognostics for OWT structures. In particular, we point out the suitability of non-destructive autonomous corrosion monitoring systems based on ultrasound measurements, combined with hybrid prognosis methods based on Bayesian Filtering and corrosion empirical models.

A dynamic model of the mutual interdependency between humans and machines in offshore wind farms (OWFs) is developed in this paper. The model emphasises the importance of having early indicators to dynamically moderate human behaviour and the need to account for both human and physical subsystems and their mutual interactions for optimal asset management practices. This work simulates three distinct scenarios for: (1) examining the interconnectedness of technical and human dynamics and their implications on error and failure; (2) assessing the impact of production loss on human and organisational behaviour; and (3) evaluating human error probability as an early warning sign of production loss. The findings suggest that adopting an appropriate maintenance culture and considering human error likelihood and production rate in decision-making leads to optimising production and mitigating the risks associated with human error. The research highlights the significance of comprehending the complex interactions between human and machine factors in the operation and maintenance (O&M) of offshore wind turbines (OWTs). The proposed dynamic model helps organisations identify the underlying causes of errors, allowing them to improve their maintenance strategies. • Developing a new model for human-turbine coupling in O&M. • Adopting the system dynamics for OWT maintenance. • Considering Human-asset mutual dependency. • Addressing root causes via feedback loops. [ABSTRACT FROM AUTHOR]

Offshore wind energy has become a key strategy in the global transition to sustainable and renewable energy. This emphasis has significantly propelled the advancement of offshore wind turbine technology. However, fatigue of offshore wind turbine support structures remains a significant challenge due to long-term exposure to harsh environmental conditions. In this study, an intelligent optimization-inspired Support Vector Regression (SVR) modeling strategy is proposed to construct a high-precision approximation model. Compared with original SVR modeling method, the modeling strategy proposed in this study has higher accuracy in building models of complex engineering structures. Additionally, a fatigue reliability evaluation framework considering hybrid uncertainty is presented. The new reliability evaluation framework can be combined with a variety of heuristic algorithms to unleash the potential of heuristic algorithms. Finally, this study applies the proposed framework to an example of fatigue reliability evaluation of offshore wind turbine support structures considering the effects of multiple uncertainties. The performance of different heuristic algorithms under the proposed framework is compared in this study. Furthermore, this study mainly finds that the MAE index of the established SVR model using the intelligent optimization algorithm is 31.2% higher than that of the SVR model that does not use the intelligent optimization algorithm. Compared with the fatigue reliability evaluation that only considers random uncertainty, the failure probability of the fatigue reliability evaluation using hybrid uncertainty is 0.64% higher. The fatigue reliability evaluation framework considering hybrid uncertainties proposed in this study is more accurate and conservative. • An intelligent optimization-inspired Support Vector Regression (SVR) modeling method is proposed. • A fatigue reliability assessment framework considering mixed uncertainties is proposed. • The effects of multiple uncertainties on fatigue reliability evolution are considered. [ABSTRACT FROM AUTHOR]

Currently, large-diameter monopile-supported offshore wind turbines (OWTs) are widely used in offshore areas of China. However, considerable numbers of wind farms are located in seismically active areas with liquefiable soil layers. Therefore, shaking table tests and numerical simulation based on a scaled model of Siemens SWT-4.0-130 OWTs were carried out to investigate the failure mechanism of the OWT system in liquefiable sand under combined wind, wave and seismic loads. The scaling law focusing on the governing physical mechanism of the OWT system considering multi-field coupling are presented and verified. The dynamic responses of the OWT and the excess pore pressure fluctuation around the soil-monopile domain in different operating states are discussed and compared. The results show that the proposed scaling laws can well restore the dynamic characteristics of the prototype. The seismic excitation is the dominant load for inducing the structural response under combined wind, wave and seismic loads. Due to the coupling effect, the dynamic responses under combined wind and wave loads were slightly less than the superposition of structural responses under separated wind or wave loads. Liquefaction of the shallow soil layer within the range of four times the pile diameter was detected when the PGA reached 0.4 g (three-dimensional input) or 0.64 g (one-dimensional input), which induced obvious tilt and settlement of the OWT model after the test. The sudden increase in the pore pressure resulted in the liquefaction of soil, which further led to a significant decrease in the natural frequencies and decay of the acceleration responses at the tower top. The above findings are expected to provide valuable experience for the 1 g underwater shaking table tests on OWT systems and insights for the practical design of OWTs in seismically active areas. • The scaling laws focusing on the governing mechanism of the OWT system are derived. • Shaking table tests are conducted to reveal the failure mechanism of the OWT system. • The influence of dynamic PSI and the coupling effect of dynamic loads are evaluated. [ABSTRACT FROM AUTHOR]

A systematic multi-objective optimization approach is presented for designing vibration control systems for monopile Offshore Wind Turbines (OWT) under the combined actions of wind and wave. An extended configuration of the KDamper (EKD) is employed, with second-order tower phenomena and soil–structure interaction effects taken into account. A holistic approach is employed to design an optimal EKD by considering competing objectives and limitations, such as engineering requirements, manufacturing specifications, available budget, and uncertainties in the design parameters. A global sensitivity analysis is conducted to identify the influence of each parameter of the OWT-EKD system on performance. Subsequently, a global multi-objective optimization is employed to explore the trade-off between conflicting objectives (Pareto front) and demonstrate that the EKD effectively operates even without specific components (e.g., dampers). In accounting for the uncertainties involved, such as manufacturing tolerances and environmental conditions, a robust analysis on a bi-objective Pareto front is performed, leading to the selection of EKD designs that exhibit superior average performance. The numerical results demonstrate the improvement in the peak tower dynamic response and the effective damping compared to a conventional Tuned Mass Damper comprising 20 times higher mass. [Display omitted] • Multi-objective optimization of a vibration absorber for offshore wind turbines. • An extended configuration of the KDamper is employed as the vibration absorber. • Sensitivity and robust analysis are integrated into the design procedure. • The feasibility of the vibration absorber is ensured using engineering criteria. • Nonlinear FEA is carried out by accounting for wind and CFD wave excitations. [ABSTRACT FROM AUTHOR]

This paper investigates the structural dynamics of 10 MW offshore wind turbines (OWTs) supported by different substructures (Monopile, Tripod and Jacket) under wind, wave, current and earthquake loadings. The support structures are modeled using ANSYS, a finite element models (FEM) software package, by considering the nonlinear soil structure interaction (SSI) effects and earthquake loadings. A spring-displacement method has been employed to model the earthquake excitations on the support structures. FAST and AQWA tools have been used to analyze wind and wave loads which are fed into ANSYS via a dynamic link library (DLL) as external loads for combination with the earthquake loads to predict the support structures' responses. Earthquakes of different magnitudes have been simulated in the study. Under the earthquake with a magnitude of 7 measured on a Richter scale, the tower top displacements of the tripod, jacket and monopile are 1.42 m, 1.75 m and 1.80 m, respectively. The relative position of the displacement trajectory of the tripod OWT is nearest to the centroid of the yaw bearing. The maximum stresses of the jacket and tripod are respectively 5.7 times and 2.3 times that of the monopile. The average stress of the jacket and tripod are respectively 0.74 times and 0.56 times that of the monopile. This phenomenon shows that the tripod and jacket have a good stability, but with a high risk of local failure under earthquakes. The responses of the piles are mainly dominated by the seismic loads, rather than the wind and current loads. It was observed that the piles of the jacket and tripod OWT, which have small diameters and thin wall thickness, are more sensitive to earthquakes. Thus, the jacket and tripod OWTs may rely on the large stiffness offered by substructures to improve their stability. These phenomena can provide powerful insight into the seismic design of different substructures. [ABSTRACT FROM AUTHOR]

Vibration‐based load reconstruction at support structures of offshore wind turbines Structural Health Monitoring (SHM) of the tower and foundation structures of offshore wind turbines is an established and acknowledged technical rule in engineering practice. The acquisition, assessment and analysis of the acting fatigue loads is an essential aspect in this field of SHM, since it enables the optimization of turbine operation towards lower fatigue lifetime consumption and provides potential for the extension of service life. Fatigue loads can either be measured directly using strain gauges or indirectly reconstructed from the measurement of tower deformations in conjunction with a mechanical model, e. g., a finite element (FE) model. The indirect reconstruction can also be described by the notions of "virtual sensors" or "digital twin". The current report presents how fatigue loads on relevant areas of offshore foundation structures, such as the tower‐transition piece interface, can be reconstructed from acceleration and inclination measurements. The approach uses the structural responses recorded at the tower and an FE model to determine internal forces or moments in the structure. Using real examples from German offshore wind farms, the effect of the available measurement information and considered load assumptions on the estimation accuracy is shown. The effects of uncertainties from acquisition and model assumptions (e. g. calibration errors and soil stiffness) are also presented. [ABSTRACT FROM AUTHOR]

Offshore Wind Turbines (OWTs) are dynamically sensitive structures and as a result estimating the natural frequency of the whole system taking into effect the flexibility of the foundation is one of the major design considerations. The natural frequency is necessary to predict the long-term performance as well as the fatigue life. OWTs are currently being constructed in seismically active regions such as the United States, China, India and Taiwan and there is a need for design considerations for OWTs located in seismic zones. Though OWTs are relatively less sensitive to earthquakes than typical structures, liquefaction of soil surrounding monopiles may have serious implications on the dynamic performance of the system. During liquefaction there is a loss of support along the depth of the monopile which noticeably reduces the stiffness of the system. This results in a decrease of the fundamental frequency and may cause resonating effects with high energy wave loads up until liquefaction effects are reduced with time. Therefore, these resonating effects will increase the fatigue damage and/or the overall accumulated rotation of the structure. Therefore, in this paper, the effect of liquefaction on the natural frequency of OWTs was considered through a parametric study which considers various soil conditions, liquefaction depths (DL), monopile diameters (D) and monopile lengths (LP). The finite element method used in this parametric study was validated via an analytical solution available in literature and a large-scale experiment which considers a fixed base condition. The study shows that the change in the natural frequency under various liquefaction depths is not significantly influenced by monopile diameter when the length is kept constant. On the other hand, the monopile length has a more detrimental effect and the aspect ratio is that LP/D is a significant non-dimensional term governing dynamic behaviour of OWT systems in the case of liquefaction. In addition, it is shown that the overall results are more sensitive in looser soils and special design considerations are provided for different ground conditions. Finally, based on analytical solutions provided in literature, a simplified method is presented to estimate the change of the natural frequency taking into account the liquefaction depth which can be easily programmed in a spreadsheet type program followed by a step-by-step solved example which also serves the purpose of verification and validation. [ABSTRACT FROM AUTHOR]

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With the increasing construction of large-scale wind turbines in seismically active coastal areas, the survivability of these high-rated-power offshore wind turbines (OWTs) in marine and geological conditions becomes extremely important. Although research on the dynamic behaviors of OWTs under earthquakes has been conducted with consideration of the soil-structure interaction, the attention paid to the impact of earthquake-induced seabed liquefaction on OWTs supported by large-diameter monopiles remains limited. In view of this research gap, this study carries out dynamic analyses of a 10 MW OWT under combined wind, wave, and earthquake loadings. This study uses a pressure-dependent multisurface elastoplastic constitutive model to simulate the soil liquefaction phenomenon. The results indicate that the motion of the large-diameter monopile leads to more extensive soil liquefaction surrounding the monopile, specifically in the zone near the pile toe. Moreover, compared with earthquake loading alone, liquefaction becomes more severe under the coupled wind and earthquake loadings. Accordingly, the dynamic responses of the OWT are apparently amplified, which demonstrates the importance of considering the coupling loadings. Compared with wind loading, the effect of wave loading on the dynamic response and liquefaction potential is relatively insignificant. [ABSTRACT FROM AUTHOR]

Offshore wind power is increasingly becoming a mainstream energy source, and efforts are underway toward their construction in seismic zones. An offshore wind farm consists of generation assets (turbines) and transmission assets (substations and cables). Wind turbines are dynamically sensitive systems due to the proximity of their resonant frequency to that of loads considered in their analyses. Such farms are considered lifeline systems and need to remain operational even after large earthquakes. This study aims to discuss hazard considerations involved in the resilience assessment of offshore wind farms in seismic regions. The complexity of design increases with larger turbines installed in deeper waters, resulting in different types of foundations. In addition, Tsunami inundation is shown to be an important consideration for nearshore turbines. [ABSTRACT FROM AUTHOR]

In this study, the Local Joint Flexibility (LJF) of tubular T/Y-joints with doubler plate under in-plane bending (IPB) load is investigated. In the first step, a finite element (FE) model was developed and verified using the available experimental results. In the next step, 252 FE models were generated to investigate the effect of the joint geometry on the LJF factor (fLJF). Results showed that the increase of the doubler plate size results in the considerable decrease of the fLJF. Also, the fLJF of a T/Y-joint reinforced with doubler plate can be down to 26% of the fLJF of the corresponding unreinforced joint. Despite the considerable effect of the doubler plate on the fLJF, there is not any study on the fLJF of the tubular joints with doubler plate. Hence, after a parametric study, FE results were used to propose a parametric formula for determining the fLJF. [ABSTRACT FROM AUTHOR]

The dynamic response of a jacket-supported offshore wind turbine under coupled wind, wave, and current fields during Hurricane Sandy is the subject of this study. To illustrate the detailed procedure related to the response evaluation of a 5-MW offshore wind turbine with a jacket support structure, we consider a single site where the water depth is 50 m. Loads are computed using two simulation tools, FAST and ABAQUS, with partial coupling. Aerodynamic loads on the turbine rotor are first evaluated using a wind turbine model in FAST with a fixed base; then, these rotor aerodynamic loads are applied as point loads at the top of a model of a tower that is supported by a jacket structure in ABAQUS. On the basis of stochastic simulations, we discuss the applicability of the ABAQUS jacket modeling and describe the characteristics and comparisons of the aerodynamic and hydrodynamic effects on loads on the jacket members. Details related to the structural model and soil-pile interaction model employed in the analyses are also discussed. [ABSTRACT FROM AUTHOR]

Fault detection and classification are considered as one of the most mandatory techniques in nowadays industrial monitoring. The necessity of fault monitoring is due to the fact that early detection can restrain high-cost maintenance. Due to the complexity of the wind turbines and the considerable amount of data available via SCADA systems, machine learning methods and specifically deep learning approaches seem to be powerful means to solve the problem of fault detection in wind turbines. In this article, a novel deep learning fault detection and classification method is presented based on the time-series analysis technique and convolutional neural networks (CNN) in order to deal with some classes of faults in wind turbine machines. To validate this approach, challenging scenarios, which consists of less than 5% performance reduction (which is hard to identify) in the two actuators or four sensors of the wind turbine along with sensors noise are investigated, and the appropriate structures of CNN are suggested. Finally, these algorithms are evaluated in simulation based on the data of a 4.8 MW wind turbine benchmark and their accuracy approves the convincing performance of the proposed methods. The proposed algorithm are applicable to both on-shore and off-shore wind turbine machines. [ABSTRACT FROM AUTHOR]

Monopiles are the most common foundation type for offshore wind turbines. These structures are subjected to millions of cyclic horizontal loads during their lifespans, mainly from waves and wind; however, there are gaps in conventional design methods for different aspects of cyclic loading. The present study examined the cyclic behaviour of monopiles in dry calcareous sand. Centrifuge tests were carried out to investigate the effect of cyclic loading on the accumulated displacement and soil-pile stiffness. The results showed that asymmetric two-way loading is the most damaging load type, although its difference from one-way loading is less than what has been reported previously. Asymmetric two-way loading was found to cause up to 20% more displacement than one-way loading. Furthermore, the secant stiffness of the soil-pile system increased about 15% after 600 cycles, and a logarithmic function has been provided to describe this trend. The slope of this function increased with the maximum cyclic load magnitude; however, an increase in the cyclic load magnitude decreased the soil-pile stiffness. Moreover, the soil-pile stiffness was considerably lower in symmetric two-way loading compared to other load reversal conditions. After each cyclic test, monotonic loading was applied. In most cases, the post-cyclic lateral capacity was nearly equal to the static capacity. A model is proposed to predict the accumulated displacement under cyclic loading. [ABSTRACT FROM AUTHOR]

The ultrasound technique is a well-known non-destructive and efficient testing method for on-line corrosion monitoring. Wall thickness loss rate is the major parameter that defines the corrosion process in this approach. This paper presents a smart corrosion monitoring system for offshore wind turbines based on the ultrasound pulse-echo technique. The solution is first developed as an ultrasound testbed with the aim of upgrading it into a low-cost and low-power miniaturized system to be deployed inside offshore wind turbines. This paper discusses different important stages of the presented monitoring system as design methodology, the precision of the measurements, and system performance verification. The obtained results during the testing of a variety of samples show meaningful information about the thickness loss due to corrosion. Furthermore, the developed system allows us to measure the Time-of-Flight (ToF) with high precision on steel samples of different thicknesses and on coated steel samples using the offshore standard coating NORSOK 7A. [ABSTRACT FROM AUTHOR]

The soil-structure interface stiffness/damping, structure damping, aerodynamic damping, and hydrodynamic added mass are the four major sources of error while modeling a monopiled offshore wind turbine by using finite element methods. This study develops a comprehensive strategy, involving optimal sensor placement, complex mode expansion, and cross-model cross-mode-based model updating technique, for simultaneously correcting the model parameters associated with the mentioned four major sources of error, when only a few sensors are installed to extract modal data, including modal frequencies and damping ratios, and spatially incomplete mode shapes. To demonstrate the effectiveness of the proposed method, computer simulations with the National Renewable Energy Laboratory 5-MW reference turbine are carried out. The numerical studies reveal that (1) when clean modal data are utilized, all model parameters can be updated properly by the proposed method; it is recommended the first few low-order modes, together with as many sensors as possible, be used; (2) when contaminated spatially incomplete complex eigenvectors are given, even some model parameters can not be updated accurately, the updated model is still adequate for dynamic analysis and design as long as the dominant model parameters are updated properly. • Develops a comprehensive updating strategy using measurements from a few sensors. • Simultaneously updates OWT's mass, damping and stiffness parameters. • Requires only a few low-order eigenvalues and incomplete eigenvectors. • Demonstrates the effectiveness of the proposed method by computer sim-ulations. [ABSTRACT FROM AUTHOR]

Based on the Global Wind Atlas an assessment of the wind resources of the Black and Azov seas was completed. The distribution of wind power density in the Black and Azov Seas is defined, as well as to identify areas with the most favorable conditions for the placement of offshore wind turbines. Most of the Black and Azov Seas are characterized by significant wind resources (value of wind power density > 400 W/m²). The total area of sites with favorable conditions for the placement of offshore wind turbines is about 98500 km². [ABSTRACT FROM AUTHOR]

This study aims to develop jacket substructures supporting a 3MW Offshore Wind Turbine (OWT) in water depth ranges of 25–40 m. A simplified structural design process is introduced and influences of jacket topological forms and brace systems on their dynamic performances are assessed. The results conclude that the topological configuration of the jacket substructure has a significant influence on the dynamic characteristics of the whole system, indicating a high priority when designing a jacket substructure for OWTs. Furthermore, two jacket substructures having the lowest material costs and the best performances under the Korean ocean environmental conditions are suggested. Additionally investigated in this study is a sensitivity of environmental loading directions on the structural performances of the jacket substructures. All the findings in this study might be very useful to assist the structural engineers when they design the support structures for large-scaled OWTs in the near future in South Korea.

In the design of offshore wind farms the simulated dynamic response of the wind turbine structure includes loading from turbulent wind. The International Electrotechnical Commission (IEC) standard for wind turbine design recommends both the Mann spectral tensor model and the Kaimal spectral model combined with an exponential coherence formulation. These models give deviating wind loads. This study compares these two models to a large eddy simulations model and a model based on offshore wind measurements. The comparisons are performed for three situations, covering unstable, neutral and stable atmospheric conditions. The impact of the differences in the wind fields on the quasi‐static response of a large bottom‐fixed wind turbine is investigated. The findings are supported by an assessment of the impact of individual wind characteristics on the turbine responses. The wind model based on measurements causes high tower bottom and blade root flapwise bending moments due to a high wind load at very low frequencies. Low and negative horizontal coherence is obtained using the Mann spectral tensor model. This causes relatively large yaw moments as compared to the results using the other wind models. The largest differences in response are seen in the stable situation. We furthermore show that the quasi‐static wind load has great impact on the total damage equivalent moments of the structure. From the results, we conclude that in the design of large offshore wind turbines one should carefully consider the structure of the turbulent wind. Further, longer simulations than recommended by the standards should be used to reduce uncertainty in estimated response. [ABSTRACT FROM AUTHOR]

The fatigue life prediction for offshore wind turbine support structures is computationally demanding, requiring the consideration of a large number of combinations of environmental conditions and load cases. In this study, a computationally efficient methodology combining aerodynamic decoupling and modal reduction techniques is developed for fatigue life prediction. Aerodynamic decoupling is implemented to separate the support structure and rotor-nacelle assembly. The rotor dynamics were modelled using an aerodynamic damping matrix that accurately captures the aerodynamic damping coupling between the fore-aft and side-side motions. Soil-structure interaction is modelled using p-y curves, and wave loading calculated based on linear irregular waves and Morison's equation at a European (North Sea) site. A modal reduction technique is applied to significantly reduce the required number of degrees of freedom, allowing the efficient and accurate calculation of hotspot stresses and fatigue damage accumulation. The modal model was verified against a fully coupled model for a case-study, monopile supported offshore wind turbine in terms of response prediction and fatigue life evaluation. The modal model accurately predicts fatigue life (within 2%) for a range of parameters at a fraction of computational cost (0.5%) compared to the fully coupled model. [ABSTRACT FROM AUTHOR]

Summary: Wind, wave and seismic loading induced excessive vibration of offshore wind turbines (OWTs) impairs the wind turbine energy output and escalates its fatigue damage. The present study proposes a three‐dimensional adaptive pendulum tuned mass damper (3D‐APTMD) to control the bidirectional dynamic responses of monopile OWTs exposed to combined wind, wave, seismic loading, and time‐varying structural properties. In terms of the proposed adaptive tuning algorithm, the natural frequency and damping property of the 3D‐APTMD are tuned in real time to match the wind turbine time‐varying dominant frequency caused by environmental and/or structural property variations. Performance of the developed adaptive damper is examined on a monopile 5‐MW (megawatt) baseline wind turbine subjected to representative wind, wave, and seismic loading. Time‐varying stiffness of the tower and foundation is introduced to simulate damage evolution during service life. A three‐dimensional pendulum tuned mass damper (3D‐PTMD) and dual linear TMDs (2TMD) are used for comparison. Research results show that when damage occurs, 3D‐PTMD and 2TMD become partially off‐tuned and less effective. In comparison, the 3D‐APTMD which is tuned in real time maintains its effectiveness. It can improve the reduction effect by around 25% and 30% when compared with the 3D‐PTMD and 2TMD, respectively. Therefore, the proposed 3D‐APTMD is promising for real application to protect OWTs, high‐rise buildings, chimneys, and other types of slender structures suffering from excessive bidirectional vibration, and environmental, operational, and/or structural property variations. [ABSTRACT FROM AUTHOR]

Maintenance is essential in keeping wind energy assets operating efficiently. With the development of advanced condition monitoring, diagnostics and prognostics, condition‐based maintenance has attracted much attention in the offshore wind industry in recent years. This paper models various maintenance activities and their impacts on the degradation and performance of offshore wind turbine components. An integrated maintenance strategy of corrective maintenance, imperfect time‐based preventive maintenance and condition‐based maintenance is proposed and compared with other traditional maintenance strategies. A maintenance simulation programme is developed to simulate the degradation and maintenance of offshore wind turbines and estimate their performance. A case study on a 10‐MW offshore wind turbine (OWT) is presented to analyse the performance of different maintenance strategies. The simulation results reveal that the proposed strategy not only reduces the total maintenance cost but also improves the energy generation by reducing the total downtime and expected energy not supplied. Furthermore, the proposed maintenance strategy is optimised to find the best degradation threshold and balance the trade‐off between the use of condition‐based maintenance and other maintenance activities. [ABSTRACT FROM AUTHOR]

Monopiles are popular solutions as the foundation to support offshore wind turbines (OWTs), which suffer from various amplitudes of continuous lateral cyclic wind and wave loading. The accumulation of tilt and change of the natural frequency of OWTs under cyclic loading have become crucial issues for OWT design. Hence the monopile-tower-soil system was established at the small-scale level based on similarity laws with a suit of constant-amplitude and multi-amplitude tests in dense sand. Two sets of gear-driving assembly were successfully set up to achieve the application of cyclic loading to the model OWT. The amplitude and symmetry of cyclic loading were specified as three indicative regions according to those of real wind farms. On the basis of test results, the two-way loading was found to be the most hazardous situation and the preloading condition led to a high accumulation of tilt. The change of natural frequency was controlled by the load magnitude and tended to increase during cyclic loads, while a sharp decrease was observed when sand subsidence was generated. The post-cyclic lateral capacity seemed to be a slight increase compared with the static capacity. The relevant analyses based on test results can provide practical recommendations for OWT design. [ABSTRACT FROM AUTHOR]

Marine renewable sources can make a significant contribution to the development of electrical energy generation and can increase the power supply reliability of mineral complexes. The development of alternative energy sources is happening at a fast pace, and this is due to the improvement of technologies that allow for generating more energy and operating in more extreme conditions, with almost no negative effect on the environment. However, currently, renewable sources are not able to meet all the energy requirements of the platforms. Hence, a key point is to gradually introduce and develop new technologies. This article explores the advantages of combining power generation by wave converters and offshore wind turbines. It investigates the possibilities of improving the combined systems’ reliabilities through justification of their mutual topology and accounting for the shadow effect from the wave installations.

The conventional way of studying corrosion in marine environments is by installing corrosion coupons. Instead, this paper presents an experimental field study using an unattended corrosion sensor developed on the basis of ultrasound (US) technology to assess the thickness loss caused by general atmospheric corrosion on land close to the sea (coastal region). The system described here uses FPGA, low-power microcontroller, analog front-end devices in the sensor node, and a Beaglebone black wireless board for posting data to a server. The overall system is small, operates at low power, and was deployed at Gran Canaria to detect the thickness loss of an S355 steel sample and consequently estimate the corrosion rate. This experiment aims to demonstrate the system’s viability in marine environments and its potential to monitor corrosion in offshore wind turbines. In a day, the system takes four sets of measurements in 6 hour intervals, and each set consists of 5 consecutive measurements. Over the course of 5 months, the proposed experiment allowed for us to continuously monitor the corrosion rate in an equivalent corrosion process to an average thickness loss rate of 0.134 mm/year.