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Real-time hybrid simulation (RTHS) divides a structural system into an analytical and experimental substructure. The former is based on a well-established analytical model while the latter consists of a physical model in the laboratory, for which there is not a well-established analytical model. This paper extends real-time hybrid simulation to monopile-type Offshore Wind Turbines (OWTs) to enable the investigation of their behavior considering the response of pile foundations under operational and more severe conditions. The embedded foundation and surrounding soil of the OWT are modeled physically in a soil box in the laboratory while the remaining parts of the system and loading are modeled analytically. The program OpenFAST, developed by the National Renewable Energy Laboratory (NREL), is linked to the RTHS coordinator to determine the hydrodynamic and aerodynamic loads acting on the OWT, along with modeling the dynamics of the electric power generation equipment and associated controller for the OWT. The RTHS framework along with its initial implementation and validation are described in this paper. RTHSs of a 5 MW OWT subjected to operational and more severe conditions are performed to experimentally validate the framework. The framework offers a realistic approach to investigate the behavior of OWT structures supported on monopiles. This approach accounts for the coupled response of the OWT structure with its foundation, while experimentally capturing the nonlinearities of the soil-foundation interaction in real-time. • RTHS is extended to offshore wind turbine monopile structures. • OpenFAST software is modified and integrated into a RTHS framework. • Validated RTHS framework that incorporates the complete system into the simulation. • RTHS enables experimental assessment of offshore wind turbine foundation behavior. • RTHS computational challenges of offshore wind turbine structures are presented. [ABSTRACT FROM AUTHOR]

Slender and flexible offshore wind turbines (OWTs) are vulnerable to external dynamic excitations, and passive tuned mass dampers (TMDs) have been widely used to control excessive vibrations of OWTs under harsh marine environments (e.g., strong winds and irregular sea waves). However, TMDs are only effective in the vicinity of the controlled frequency, i.e., in a narrow frequency band. Compared to passive TMDs, active control methods are normally considered to possess better control performances but at the cost of a large amount of external energy input. To this end, the present study proposes a novel energy-adaptive self-powered active mass damper (SPAMD) to mitigate the responses of OWT towers. The proposed control device can harvest energies from OWTs and then use them as the power to drive an active mass damper for structural vibration control. Specifically, a representative OWT is selected as a prototype structure and its tower is modeled as a multi-degree-of-freedom system by simplifying the rotor-nacelle assembly as a lumped mass and moment of inertia. The dynamic characteristics (mainly natural frequency and mode shape) of the tower obtained by the developed model are validated against a finite element model. Subsequently, the system configuration and working mechanism of SPAMD are introduced and SPAMD is incorporated into the developed model to simultaneously harvest energy and mitigate the fore-aft responses of the tower under wind and sea wave loads. The control effectiveness of SPAMD is further compared to the traditional TMD. Results show that SPAMD has a superior effect over TMD in controlling OWT responses. • A novel energy-adaptive self-powered active mass damper (SPAMD) is proposed. • System topology, working mechanism, and control algorithm of SPAMD are introduced. • Control performances of SPAMD in offshore wind turbines are investigated. [ABSTRACT FROM AUTHOR]

Offshore wind turbines (WTs) are crucial in offshore wind energy development. However, the dynamic responses of WTs are subject to significant uncertainties which are usually not properly considered. To the end, this paper proposes an efficient method for quantifying the uncertainties in WTs' dynamic responses based on cumulative distribution function (CDF)-manifold learning. First, a probabilistic model is developed to represent the environmental parameters and sampling for aerodynamic-hydraulic-servo-elastic simulations. Then, the CDF is obtained by statistically analyzing the simulated data. To tackle the higher dimensionality resulting from discretizing the CDF, a manifold learning-based approach is subsequently proposed to reduce its dimensionality and obtain a manifold space. Furthermore, a mapping relation is established between the environmental parameters and the low-dimensional data to efficiently obtain the response CDF under different environmental parameters, leading to the construction of a probability box (P-box) model. To demonstrate the effectiveness of the proposed method, the National Renewable Energy Laboratory (NREL) 5 MW offshore WT on an Offshore Code Comparison Collaboration (OC3) monopile is selected as a case study and analyzed accordingly. The results show P-box models of seven WT responses and validate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

This study examines the influence of probabilistic models for wave parameters in the joint environmental model and hydrodynamic/soil models on extreme mudline bending moments for monopile-based wind turbines at representative wind speeds, using the environmental contour method. For significant wave height, the 3-parameter Weibull model using the method of moments (MoM) provides the best fit to hindcast data across different wind classes, for the statistical models and data considered in the study. The hybrid Log-normal-Weibull (LonoWe) model also provides a reasonable fit but is sensitive to the transition point between distributions. Both models yield the largest extreme responses, with differences of approximately 0.5–3.5%. The 3-parameter Weibull model with maximum likelihood estimation (MLE) and the 2-parameter Weibull model result in less conservative contours, leading to up to 13% lower extreme responses, compared to LonoWe and Weibull (MoM). Regarding peak period, both the Log-normal and 3-parameter Weibull models provide reasonable fits, with the latter being more accurate near the steepness (breaking) limit. The stochastic variation among maxima due to seed variability and the uncertainty in quantile estimates as a function of number of samples was found to be crucial, particularly for severe sea states at the cut-out speed. Soil modelling is particularly important when the turbine is parked and encounters peak wave periods close to the turbine's natural periods, while the effect of soil modelling on the extremes during turbine operation is negligible. Additionally, the impact of diffraction becomes relatively important for short wave periods. However, it is worth noting that the choice of load models has less impact on extreme responses compared to variations in the contours caused by different statistical models or seed variability. • Extreme responses of monopile-based offshore wind turbines using contour method. • Metocean contours strongly depend on the selection of Hs-Tp conditional distributions. • Probabilistic models and stochastic variation from seed variability dominate extremes. • Foundation modelling affects extreme responses primarily for the parked states. • Extreme responses in rated speed become increasingly important for larger turbines. [ABSTRACT FROM AUTHOR]

Offshore wind turbines (OWTs) constructed in ice-covered sea areas are subjected to a multitude of challenges in complex marine environments. These challenges encompass not only strong winds and weak foundations but also the formidable task of withstanding the impact of floating ice. To address these issues comprehensively, this study introduces a dynamic analysis framework for OWTs that incorporates soil-structure interaction (SSI), stochastic ice loads, and aerodynamic loads. This framework takes into account the intricate interplay between these factors, providing a holistic approach to the analysis of OWTs in icy marine environments. The research holds significance in elucidating the dynamic response laws of OWTs situated in icy waters under diverse factors. This understanding is vital for ensuring the structural safety and reliability of OWTs and fostering sustainable development in this field. In the present study, stochastic ice loads are generated using a real ice load spectrum model, while the aerodynamic loads are determined through the Blade Element Momentum (BEM) methodology. A comprehensive numerical model of the NREL 5-MW OWT, which includes the blades, tower, monopile, and soil system, is constructed based on the ABAQUS platform. The dynamic responses of the 5-MW OWT are systematically investigated under ice loads alone, as well as under combinations of ice and aerodynamic loads. The results of the study demonstrate that considering SSI and greater water depth amplifies the dynamic responses of OWTs subjected to stochastic ice loads. Specifically, significant acceleration is observed at the tower-base, posing potential safety risks to operators on the OWT operating platform. Moreover, when ice and aerodynamic loads act in conjunction, the bending moment experienced by OWTs is lower compared to the cumulative effect of individual ice and aerodynamic load contributions. This finding highlights the possibility of overestimating structural responses when employing a summation analysis approach. Additionally, this study advances the comprehension of ice-induced vibration phenomena and establishes a theoretical foundation for the anti-ice design of OWTs in cold marine environments. • An analysis framework for OWTs, which takes SSI, ice loads and aerodynamic loads into consideration is proposed. • The dynamic effects of SSI simulated by PISA method and p-y method on OWTs are investigated. • The impact of soil and water depth on the dynamic responses of OWTs under ice-induced vibration are studied. • The influence of ice-wind combined loads on the dynamic response of OWTs are systematically analyzed. • The impact of combined ice-wind loads on offshore wind turbines is investigated. [ABSTRACT FROM AUTHOR]

Offshore wind power (OWP) is one of the underpinnings of the ambitious "dual carbon" strategy in China. So, it is of great significance to monitor the spatial information and location characteristics of offshore wind turbines (OWTs) along the coast. In this study, the OWTs in Shandong sea of China were monitored using SSD (Single Shot MultiBox Detector) with ResNet34 (ResNet34-SSD) and Sentinel-2 remote sensing images. Their spatial information and location characteristics were revealed. The results show that (1) the ResNet34-SSD can identify OWTs with high accuracy and efficiency. The precision, recall and F1 are 96.58%, 91.59% and 94.02%, respectively; (2) the number of OWTs in Shandong sea continuously increases between 2021 and 2023. They are located in the sea of Dongying (149 turbines), the southern part of Laizhou Bay (88 turbines) and the sea of Haiyang (187 turbines); (3) the installed OWTs are located in areas with 10∼38 m depth and 10–37 km offshore. There are currently no OWTs located in deep and distant ocean; (4) the mean wind speed at the locations of OWTs is 6.0–7.0 m/s. The wind power density (WPD) is 250∼400 W/m2, with a coefficient of variation (Cv) of 1.40–1.55. The effective wind speed hours (EWSHs) are 7,100∼7,850 h, and the effective storage of wind energy (ESOWE) is about 1,800∼3,000 kWh/m2. This study also discussed the location advantages of OWTs from the perspective of wind energy resources. • Remote sensing and deep learning are used to monitor offshore wind turbines. • Spatio-temporal information of offshore wind turbines in Shandong are revealed. • Location characteristics of offshore wind turbines are assessed and revealed. • Location advantages of offshore wind turbines in Shandong are discussed. [ABSTRACT FROM AUTHOR]

The fault diagnosis of wind turbines is crucial for wind power generation. However, the violent variation in wind power and directions of offshore wind turbines often results in high nonstationary vibration signals, which poses a challenge for effective fault recognition. Time-frequency analysis (TFA) is a common method to reveal the time-varying frequency components caused by faults. In this paper, we propose a component separable synchroextracting transform (CSSET) by exploring the unique modulation characteristics of wind turbine vibration signals. For the method, a fundamental frequency is firstly estimated by a time-frequency ridge optimization method, and the potential fault characteristic frequency (FCF) set is constructed according to the modulation characteristics of the wind turbine vibration signal. Next, the instantaneous amplitudes (IAs) of these components in the set are estimated by a time-varying bandpass filter. Then, the instantaneous frequency (IF) of the reconstructed single component is extracted by the synchronous extraction operator (SEO), and a high-precision time-frequency representation (TFR) is obtained. The proposed method takes full advantage of the physical modulation characteristics of vibration signals and the frequency components which are most relevant to fault information are only preserved. The effectiveness of the proposed method is verified by analyzing simulation and experimental signals. • A component separable synchroextracting transform is proposed. • An IF set construction method is proposed based on the modulation characteristics of the vibration signal. • The mutual interference problem of the close frequency components in TFR is solved effectively. • The method is evaluated by the simulation and experiment signals. [ABSTRACT FROM AUTHOR]

This paper proposes a novel four-step methodology to achieve an extended-reality-based choice experiment in historic and touristic centers. The study exploits the case of Manfredonia (a seaside town in southern Italy) to apply the new approach and investigates public attitudes and preferences towards the installation of offshore wind turbines in the area. The novelty of the proposed work is twofold: i) for the first time, a structured methodological approach is defined for the development of a hybrid extended-reality-based choice experiment; ii) the perception of the visual impact of offshore wind turbines is assessed in a touristic and historic city in southern Italy exploiting the proposed approach. Our findings underscore the importance of continuously monitoring public perceptions to maintain and promote support for sustainable energy solutions, particularly in relation to the perception of wind energy's visual impact. In particular, 65% of respondents express their worries about wind power plants impact on the landscape. Moreover, the positive coefficient of the visual impact (0.011) suggests a positive utility of respondents from a higher off-shore turbines' density and a marginal willingness to accept a compensation of about 13€ for the visual impact and of about 33€ for the distance from the shore. In this context the use of extended reality technology in choice experiment scenarios significantly improves the results and enhances the understanding of the landscape impact of offshore wind farms. [Display omitted] [ABSTRACT FROM AUTHOR]

As a result of wind power’s expansion over the globe, offshore wind turbines (OWTs) are being projected in seismic prone areas. In parallel, the industry develops increasingly larger and more powerful generators. Many of the seismic response analyses of wind turbines conducted so far only consider smaller units. In this paper, a finite element substructuring model in frequency domain is used to compute the seismic response of four reference OWTs from 5 to 15 MW founded on monopiles embedded in several homogeneous soil profiles with shear wave velocities from 100 to 300 m/s and subjected to different accelerograms. The foundation behaviour is obtained through a continuum model including kinematic and inertial interaction. The relevance of soil-structure interaction and main trends of the seismic response of OWTs are inferred from the presented results. Although the seismic maximum bending moments increase with the size of the OWT system, their relevance with respect to the ones produced by design loads decreases as the turbine gets bigger. The same effect is observed for the shear forces if the soil is soft enough. The inclusion of SSI effects almost duplicates the seismic response when compared to the rigid base scenario.

With the accelerating progression of global climate change, switching to renewable energy sources is inevitable. Wind energy is a fast-growing branch of this industry, and according to the 2021 Global Wind Report, this trend must continue in order to limit the increase in global average temperature. While onshore wind turbines still dominate and account for most recent growth, offshore wind turbines are becoming a promising alternative for geographical, power density-related or even aesthetic reasons. Offshore wind turbines are subjected to more complex loading conditions and proper foundation design is very challenging, however, this is crucial for ensuring and maintaining the structure’s reliability. Soil dynamic tests are one of the bases for wind turbine foundation design. Technical regulations in many countries require such tests to be carried out in a Resonant Column (RC). In this study, a modification of the RC sensors and data acquisition system was introduced in order to conduct in-depth analysis of vibrating soil specimens. The new set of sensors contained five additional accelerometers (Analog Devices ADXL345) attached to the surface of a soil specimen that was subjected to dynamic loading. These accelerometers sent the data to a new data acquisition system, an ARM microcontroller with software developed by authors. The software was able to process test results synchronously with the original software of the RC device. Additionally, the load control system was supplemented with a current pulse generator, which makes it possible to observe the propagation of high-frequency mechanical waves in the tested materials. The modified dynamic testing equipment allowed for the measuring of accelerations and displacements at specific selected points located along the height of the sample, with sampling frequency more than three times higher than that offered by the sensors originally built into the RC device. As a result, some additional dynamic phenomena (i.e., disturbances in the uniformity of vibrations of non-cohesive materials, specimen–device contact imperfections) were observed in the tested soil specimens which remained undetected in standard RC test.

High operation and maintenance costs for offshore wind turbines push up the LCOE of offshore wind energy. Unscheduled maintenance due to unanticipated failures is the most prominent driver of the maintenance cost which reinforces the drive towards condition-based maintenance. SCADA based condition monitoring is a cost-effective approach where power curve used to assess the performance of a wind turbine. Such power curves are useful in identification of wind turbine abnormal behaviour. IEC standard 61400-12-1 outlines the guidelines for power curve modelling based on binning. However, establishing such a power curve takes considerable time and is far too slow to reflect changes in performance to be used directly for condition monitoring. To address this, data-driven, nonparametric models being used instead. Gaussian Process models and regression trees are commonly used nonlinear, nonparametric models useful in forecasting and prediction applications. In this paper, two nonparametric methods are proposed for power curve modelling. The Gaussian Process treated as the benchmark model, and a comparative analysis was undertaken using a Regression tree model; the advantages and limitations of each model will be outlined. The performance of these regression models is validated using readily available SCADA datasets from a healthy wind turbine operating under normal conditions.

The doubly fed induction generator (DFIG) has become increasingly popular for variable speed wind turbines because of the lower power electronics converter rating required. However, the small air gap of a DFIG causes it to be vulnerable to slight misalignment of the rotor which may cause a large unbalanced magnetic pull (UMP) to be exerted on the bearings. The reliability of the generator is important to reduce revenue loss; this is especially the case for generators in offshore wind turbines where minimising the cost of energy is crucial in completing with more established forms of electricity generation. This study has proposed the usage of stator damper windings to reduce the UMP. An example of a four-pole DFIG with static eccentricity is shown where finite element analysis is used to verify the UMP for cases with and without damper windings. Finally, additional losses due to rotor eccentricity are discussed.

Large scale offshore wind farms are relatively new infrastructures and are being deployed in regions prone to earthquakes. Offshore wind farms comprise of both offshore wind turbines (OWTs) and balance of plants (BOP) facilities, such as inter-array and export cables, grid connection etc. An OWT structure can be either grounded systems (rigidly anchored to the seabed) or floating systems (with tension legs or catenary cables). OWTs are dynamically-sensitive structures made of a long slender tower with a top-heavy mass, known as Nacelle, to which a heavy rotating mass (hub and blades) is attached. These structures, apart from the variable environmental wind and wave loads, may also be subjected to earthquake related hazards in seismic zones. The earthquake hazards that can affect offshore wind farm are fault displacement, seismic shaking, subsurface liquefaction, submarine landslides, tsunami effects and a combination thereof. Procedures for seismic designing OWTs are not explicitly mentioned in current codes of practice. The aim of the paper is to discuss the seismic related challenges in the analysis and design of offshore wind farms and wind turbine structures. Different types of grounded and floating systems are considered to evaluate the seismic related effects. However, emphasis is provided on Tension Leg Platform (TLP) type floating wind turbine. Future research needs are also identified.

The European offshore wind market is continuously expanding. This means that, together with significant technological developments, new coastal environments should be considered for the implementation of the wind farms, as is the case of the Black Sea, which is targeted in the present work. From this perspective, an overview of the wind energy potential in the Romanian exclusive economic zone (EEZ) in the Black Sea is presented in this work. This is made by analyzing a total of 20 years of wind data (corresponding to the time interval 2000–2019) coming from different sources, which include ERA5 reanalysis data and satellite measurements. Furthermore, a direct comparison between these datasets was also carried out. Finally, the results of the present work indicate that the Romanian offshore areas can replicate the success reported by the onshore wind projects, of which we can mention the Fantanele-Cogealac wind farm with an operating capacity of 600 MW.

Suction Bucket Jackets (SBJs) need to be fundamentally designed to avoid rocking modes of vibration about the principal axes of the set of foundations and engineered towards sway-bending modes of tower vibration. Whether or not such type of jackets exhibit rocking modes depends on the vertical stiffness of the caissons supporting them. This paper therefore derives closed form solutions for vertical stiffness in three types of ground profiles: linear, homogenous, and parabolic. The expressions are applicable to suction caissons having an aspect ratio (depth: diameter) between 0.2 and 2 (i.e., 0.2 < L/D < 2). The work is based on finite element analysis followed by non-linear regression. The derived expressions are then validated and verified using studies available in literature. Finally, an example problem is taken to demonstrate the application of the methodology whereby fundamental natural frequency of SBJ can be obtained. These formulae can be used for preliminary design and can also be used to verify rigorous finite element analysis during detailed design.

Offshore wind turbines are a complex, dynamically sensitive structure due to their irregular mass and stiffness distribution, and complexity of the loading conditions they need to withstand. There are other challenges in particular locations such as typhoons, hurricanes, earthquakes, sea-bed currents, and tsunami. Because offshore wind turbines have stringent Serviceability Limit State (SLS) requirements and need to be installed in variable and often complex ground conditions, their foundation design is challenging. Foundation design must be robust due to the enormous cost of retrofitting in a challenging environment should any problem occur during the design lifetime. Traditionally, engineers use conventional types of foundation systems, such as shallow gravity-based foundations (GBF), suction caissons, or slender piles or monopiles, based on prior experience with designing such foundations for the oil and gas industry. For offshore wind turbines, however, new types of foundations are being considered for which neither prior experience nor guidelines exist. One of the major challenges is to develop a method to de-risk the life cycle of offshore wind turbines in diverse metocean and geological conditions. The paper, therefore, has the following aims: (a) provide an overview of the complexities and the common SLS performance requirements for offshore wind turbine; (b) discuss the use of physical modelling for verification and validation of innovative design concepts, taking into account all possible angles to de-risk the project; and (c) provide examples of applications in scaled model tests.

Several bat species are known to migrate long distances between summer and winter roosts. During migration, many bats even cross the North Sea. The developments of offshore wind farms in the North Sea could therefore pose a collision risk for migrating bats. While bats have been observed inside offshore wind farms, their activity at turbine rotor height yet remains unknown. We therefore installed acoustic bat detectors at wind turbines in the Belgian part of the North Sea. Seven detectors were installed on the service platform of the transition piece (16 m above mean sea level) and four were installed on the nacelle of the turbines, in the centre of the rotor swept area (93 m above mean sea level). A total of 151 recordings of call sequences of Pipistrellus nathusii (Nathusius' pipistrelle) were made during 20 nights over an entire autumn migration season (8 August – 30 November 2017). 45 recordings contained more than 10 calls. These were further investigated for behavioural clues. We identified 32 recordings of animals in transit and 10 sequences of animals passing by while simultaneously exploring. Only three detections contained feeding buzzes and/or intense exploratory behaviour. The number of recordings at 93 m were around 10% of the number of recordings made at 16 m. This indicates that the activity of P. nathusii at our study site, measured at that particular altitude is low. Our observations therefore suggest that the collision risk might be lower than what could be expected from low altitude observations. However, a low number of recordings at nacelle height does not necessarily mean that only a low number of bats will collide with the turbines. The activity in the outer parts of the rotor swept zone, outside the detection range of our acoustic detectors, remains unknown and should be further investigated. [ABSTRACT FROM AUTHOR]

Mono-pile foundations have been widely used for offshore wind turbines principally due to their convenient construction and cost-effective nature. So far, little attention has been paid to large diameter "semi-rigid" piles that have distinct behaviours from flexible or ideally rigid piles. This paper presents a series of centrifuge model tests to study the deforming and bearing characteristics of a 5.9 dia. semi-rigid pile under lateral loadings in kaolin clay. For monotonic loading, a modified p–y curve analysis model considering rotational soil flow near the rotation centre of pile was proposed, highlighting the limitation of classic plane-strain based plasticity models to evaluate the ultimate lateral pile-soil resistance. For cyclic loading, a strong correlation between the degree of soil degradation and cyclic load amplitude was identified. Besides, a degradation factor model, accounting for various cyclic stress levels and soil depths, was proposed, which can be used to assess the accumulative displacement of semi-rigid piles under cyclic loadings in soft clay. [ABSTRACT FROM AUTHOR]

• A systematic resilient model is proposed. • The resilient model is implemented to the maintenance management of Offshore Wind Turbine systems. • Core resilience concepts are integrated into maintenance management. • The integration of resilience principles improves reliability of maintenance. The objective of this study is to implement the principles of Resilience Engineering (RE) for the maintenance management of Offshore Wind Turbine (OWT) systems by taking into account human and organizational factors. Resilience concepts are integrated into existing maintenance management elements and a resilient model is developed and applied to OWT in order to manage the maintenance related risks. The four main capabilities proposed by RE, i.e. responding, monitoring, anticipating and learning, are linked to a three level resilience system in order to prevent or mitigate OWT maintenance failures. The paper presents the applicability and effectiveness of RE in preventing accidents/incidents and system failures, and learning activities. [ABSTRACT FROM AUTHOR]

The application of structural control to offshore wind turbines (OWTs) using tuned mass dampers (TMDs) has shown to be effective in reducing the system loads. The parameters of a magnetorheological (MR) damper modeled by the Bouc‐Wen model are modified to utilize it as a damping device of the TMD. Rather than showcasing the intricate design policy, this research focuses on the availability of the MR damper model on TMDs and its significance on structural control. The impact of passive and semiactive (S‐A) TMDs applied to both fixed bottom and floating OWTs is evaluated under the fatigue limit state (FLS) and the ultimate limit state (ULS). Different S‐A control logics based on the ground hook (GH) control policy are implemented, and the frequency response of each algorithm is investigated. It is shown that the performance of each algorithm varies according to the load conditions such as a normal operation and an extreme case. Fully coupled time domain simulations are conducted through a newly developed simulation tool, integrated into FASTv8. Compared with the passive TMD, it is shown that the S‐A TMD results in higher load reductions with smaller strokes under both the FLS and the ULS conditions. The S‐A TMD using displacement‐based GH control is capable of reducing the fore‐aft and side‐to‐side damage equivalent loads for the monopile by approximately 12% and 64%, respectively. The ultimate loadings at the tower base for the floating substructure are reduced by 9% with the S‐A TMD followed by inverse velocity‐based GH control (IVB‐GH). [ABSTRACT FROM AUTHOR]

A complete fatigue assessment for operational conditions for offshore wind turbines involves simulating thousands of environmental states. For applications such as optimization, where this assessment needs to be repeated many times, that presents a significant computational problem. Here, we propose a novel way of reducing the number of simulated environmental states (load cases) while maintaining an acceptable accuracy. From one full fatigue analysis of a base design, the OC3 monopile (with the NREL 5MW turbine), the distribution of fatigue damage per load case can be used to estimate the lifetime fatigue damage of a range of modified designs. Using importance sampling and a specially adapted two‐stage filtering procedure, we obtain pseudo‐optimal sets of load cases from which the fatigue damage is estimated. This is applied to seven different designs that have been modified to emulate iterations of an optimization loop. For several of these designs, sampling less than 1% of all load cases can give damage estimates with median errors of less than 2%. Even for the most severe cases, using 3% of the environmental states yields a maximum error of 10%. While further refinement is possible, the method is considered viable for applications within design optimization and preliminary design. [ABSTRACT FROM AUTHOR]

As the ocean environment is more complex than the land environment, the offshore wind turbines especially the floating types generally suffer significant structural loads. In this study, in order to carry out the accurate simulation and analysis for the dynamic characteristics of a barge‐type floating wind turbine, a detailed turbine model including the complete drivetrain is constructed. Additionally, the structural responses of the wind turbine are mitigated by using a single‐degree of freedom tuned mass damper (TMD) system installed in the platform. In order to achieve the ideal response mitigation effect, a parametric study on the TMD configuration is carried out. Based on a new co‐simulation model combining multi‐body model and external control codes, the turbine model coupled with the TMD is then simulated under the combined wind and wave. The results demonstrate the effectiveness of the designed TMD on mitigating the structural responses of the barge‐type floating wind turbine. [ABSTRACT FROM AUTHOR]

This article presents a methodology to assess the reliability of dynamic scour protections used to protect offshore wind turbine foundations. The computed probabilities of failure are based on a dataset of 124 months of hindcast data from the Horns Rev 3 offshore wind farm. Copula-based models are used to obtain the joint distribution function of the significant wave height and spectral peak period and to obtain the probability of failure of scour protections. The sensitivity of the probability of failure to each model is addressed. The influence of the duration of the waves' time series is also studied. A sensitivity analysis of the probability of failure to physical constraints, such as the water depth, current's velocity or the mean diameter of the armour units, is performed. The results show that probability of failure is dependent on the copula used to model the spectral parameters and the associated value of Kendall's τ. It is shown that the copula presenting the best values of Akaike Information Criterion (AIC) and Bayesian Information Criterion (BIC) did not lead to the probabilities of failure that are closer to the non-parametric estimation, obtained by means of the bivariate version of the Kernel density estimation method. The application to the case study led to annual probabilities of failure, which are comparable with the values applied for other offshore components, according to the current offshore wind industry standards. [ABSTRACT FROM AUTHOR]

The doubly fed induction generator (DFIG) has become increasingly popular for variable speed wind turbines because of the lower power electronics converter rating required. However, the small air gap of a DFIG causes it to be vulnerable to slight misalignment of the rotor which may cause a large unbalanced magnetic pull (UMP) to be exerted on the bearings. The reliability of the generator is important to reduce revenue loss; this is especially the case for generators in offshore wind turbines where minimising the cost of energy is crucial in completing with more established forms of electricity generation. This study has proposed the usage of stator damper windings to reduce the UMP. An example of a four-pole DFIG with static eccentricity is shown where finite element analysis is used to verify the UMP for cases with and without damper windings. Finally, additional losses due to rotor eccentricity are discussed. [ABSTRACT FROM AUTHOR]

Modes of vibration play a dominant role in the design of WTG (Wind Turbine Generator) support structures. It is necessary to choose the overall system frequency such that the modes of vibration do not coincide with the rotor frequencies as well as the wave frequencies. WTG supported on multiple foundations (such as jackets or seabed frames) may exhibit rocking modes of vibration if the vertical stiffness of the foundation is not large enough which in turn may have serious implications on the fatigue performance of the overall structure. From the O&M (Operation and Maintenance) point of view, it is necessary to design the overall system to have sway-bending as the dominant mode of vibration. This paper develops a formulation for obtaining foundation (for both piles and shallow suction caissons) sizes and spacing such that rocking vibrations are prevented and sway-bending vibrations are achieved. Expressions for the minimum vertical stiffness of foundations are proposed for different configurations: square base, symmetrical (equilateral) triangle, or asymmetrical (isosceles) triangle. Verification of the method is carried out through finite element analysis and a step-by-step solved example is taken to show the application of the formulation. It is hoped that the formulation will assist designers to optimize the foundation arrangement and provide preliminary sizing for tender design. • Establishing criteria for different modes of vibrations for jackets supporting wind turbines. • Formulations for the minimum vertical stiffness of foundation to avoid rocking. • Example showing the applicability of the formulation. • Effects of foundation configuration (symmetric or symmetric) and spacing on modes of vibration. [ABSTRACT FROM AUTHOR]

Bucket foundations can be regarded as a comparatively new foundation system for offshore structures. The installation of a bucket foundation using suction pressure can lead to seepage from the outside to the inside of the bucket, change the laws governing extrusion and decrease the friction coefficient between the wall and the sand. In this paper, three different bucket foundation wall thicknesses are examined when static and suction pressures are used to drive the foundation into the sand. The relationship between the pressure of the earth and the friction resistance of the inner and outer walls as a function of the penetration depth and the tip resistance was researched in the experiments. The mechanism behind the decrease in the penetration resistance and its magnitude based on the experimental results was revealed. Laws influencing the wall thickness to penetration resistance ratio for different penetration depth to diameter (h/D) ratios are proposed. [ABSTRACT FROM AUTHOR]

Offshore wind turbines (OWTs) are more and more widely used to produce electrical energy nowadays. Besides the constant wind and wave loads, earthquake excitation can be another important vibration source to the OWTs since many OWTs have been/will be constructed in the seismic prone areas. Extensive research works have been carried out to understand the dynamic behaviours of OWTs when they are subjected to multi-hazards (e.g. the simultaneous wind, wave and/or earthquake loadings). However, when seismic excitations are considered, the onshore earthquake records are normally used as inputs in the analyses due to the lack of offshore data and the difficulty in synthesizing the offshore seismic motions. This practice may lead to inaccurate structural response estimations since it is well known that the seawater can significantly suppress the seafloor vertical motions near the P wave resonant frequencies of the seawater layer, which in turn results in the different characteristics of onshore and offshore earthquake recordings. Moreover, the earthquake motions along the pile of OWTs are different from those at the ground surface, i.e. the earthquake motions vary with the soil depth. Recently, a method to stochastically simulate the earthquake ground motions on the offshore site was proposed, in which the influence of seawater layer was considered and the earthquake motions at any soil depth could be obtained. This paper carries out numerical simulations on the dynamic behaviours of OWTs subjected to the combined wind, wave and earthquake loadings, and the depth-varying offshore seismic motions are used as inputs in the analyses. The seismic responses of OWTs obtained from the onshore and offshore earthquake motions are calculated and compared. The influence of depth-varying ground motions on the dynamic responses of OWTs is discussed. • The influence of onshore and offshore ground motions on the dynamic responses of OWT is investigated. • The influence of depth-varying ground motions on the dynamic responses of OWT is studied. • The influence of different operational conditions (i.e. parked and operating) is examined. [ABSTRACT FROM AUTHOR]

Abstract Offshore Wind Turbine (OWT) support structures need to satisfy different Limit States (LS) such as Ultimate LS (ULS), Serviceability LS, Fatigue LS and Accidental LS. Furthermore, depending on the turbine rated power and the chosen design (all current designs are soft-stiff), target natural frequency requirements must also be met. Most of these calculations require the knowledge of the stiffnesses of the foundation which, especially in the case of large turbines in intermediate waters (30–60 m), might need to be configured using multiple foundation elements. For this reason, this paper studies, for a homogeneous elastic halfspace, the static stiffnesses of groups of polygonally arranged non-slender suction bucket foundations in soft soils modeled as rigid solid embedded foundations. A set of formulas for correcting the stiffnesses obtained from isolated foundation formulation are proposed. It is shown through the study of several multi-megawatt OWTs that, as expected, group effects becomes more relevant as spacing decreases. Also, group effects are sensitive mainly to shear modulus of soil, foundation shape ratio and diameter, and the number of foundations. The results obtained from the soil-structure system show that ignoring group effects may add significant errors to the estimation of OWT fundamental frequencies and leads to either overestimating or underestimating it by 5%. This highlights the importance of adequately modeling the interaction between elements of closely-separated multi-bucket foundations in soft soils, when current guidelines specify the target fundamental frequency to be at least 10% away from operational 1P and blade passing frequencies (2P/3P frequencies). Highlights • Polygonal multi-bucket foundations are considered for offshore wind turbines. • Closed-form foundation stiffnesses including group effects are proposed. • Group effects are relevant for closely-spaced configurations in soft soils. [ABSTRACT FROM AUTHOR]

Abstract In this study, a rigorous poroelastodynamic solution is proposed for monopiles which accounts for 3D (three dimensional) dynamic interaction of the soil and pile. The dynamic Biot's theory is used to model the soil, while Timoshenko beam is used to model the monopile. The perfect contact conditions between the soil and monopile are realized by decomposing the pile's horizontal translation to radial, hoop and vertical displacements. Correspondingly, 3D radial, hoop and vertical stresses on the pile surface are used to formulate the governing equations of the interaction problem. Using the integral equation methods, the governing differential equations are reduced to coupled integral equations, which are solved numerically. Selected numerical results for coupled lateral and rocking dynamic impedances together with the 3D dynamic contact stresses at the soil-pile interface are studied for different pile and poroelastic material parameters and frequencies of excitation. The distributed soil reaction force and moment along the pile are also obtained by integrating the 3D radial, hoop and vertical stresses along the circumference of the pile at the soil-pile interface. Finally, bending moment, shear force, horizontal translation and rocking rotation of the monopile are obtained based on the classical dynamic Timoshenko beam theory. Highlights • A rigorous solution for offshore monopiles is obtained. • Three dimensional saturated soil and thin-walled pipe pile are considered. • 3D radial, hoop and vertical stresses along the circumference of the pile are obtained. • The non-dimensional impedance is influenced mostly by five parameters. • Simple formulas for the impedance functions are obtained. [ABSTRACT FROM AUTHOR]

Abstract Large diameter monopiles are typical foundation solutions for offshore wind turbines. In design of the monopile foundations in sand, it is necessary to understand the drainage conditions of the foundation soil under the design loading conditions as the soil performance (strength and stiffness) is highly dependent on the drainage conditions. This paper presents a numerical investigation into this issue, with a purpose to develop a simple design criterion for assessing the soil drainage conditions around a monopile in sand. It is found that for typical monopile foundations in sand, the drainage condition during a single load cycle is generally expected to be undrained. However, the current state-of-practice uses p-y springs derived for drained soil responses for monopile design. The impact of this discrepancy on monopile foundation design was evaluated and found to be insignificant due to the relatively low level of loading as compared to the capacity of the soil. [ABSTRACT FROM AUTHOR]

Abstract Offshore wind turbines (OWTs) subjected to combined wind and wave loadings experience excessive bi-directional vibrations that adversely influence the system performance and the structural integrity. The present paper utilizes a three-dimensional pendulum tuned mass damper (3d-PTMD) to mitigate the bi-directional vibrations as well as harvest the kinetic energy using a linear electromagnetic energy harvester. The proposed energy harvester consists of magnets and coil assemblies which are connected with the pendulum to convert the kinetic energy of the pendulum into electricity. An analytical model of the offshore wind turbine coupled with the 3d-PTMD is established using Euler-Lagrangian equation. The mathematical model of the linear electromagnetic energy harvester is established and integrated with the wind turbine model. The optimum design of the electromagnetic energy harvester to minimize the nacelle displacement RMS as well as to maximize the energy output is determined via a numerical search method. The NREL 5 MW baseline wind turbine model is utilized to evaluate the performance of the 3d-PTMD and the energy harvester. Results show that the 3d-PTMD can reduce the bi-directional vibrations induced by misaligned wind and wave loadings. Additionally, electrical energy in orders of magnitude of kilowatts can be harnessed via using the energy harvester. Highlights • An analytical model of an offshore wind turbine coupled with a 3d-PTMD and an energy harvester is established. • The 3d-PTMD can effectively mitigate bi-directional vibrations of the nacelle and harvester energy. • Electrical energy in orders of magnitude of kilowatts can be harnessed. [ABSTRACT FROM AUTHOR]

Abstract The impact of environmental load uncertainties on the spatial fatigue reliability of offshore wind turbine foundations is discussed and exemplified. Design procedures are utilizing overall or partial safety factors to include different model- and statistical uncertainties. Uncertainties in the final design are related to decisions taken during the design process, such as; load models, analysis methods and statistical descriptions. Furthermore, to benefit from more elaborate methods, strategies to account for reduced uncertainties by increased knowledge must be adopted. This is especially important for the offshore wind energy industry, where the aim is to produce renewable energy at a competitive cost level. The challenges and consequences of using a detailed design basis are exemplified and discussed through structural reliability analyses. Epistemic load effect uncertainties related to the foundation fatigue will be presented for a detailed wind directional model, wind-wave misalignment, and a second order wave load model. It will be shown that all of these represent important uncertainties to consider during the fatigue design of an offshore wind farm. Highlights • A methodology for evaluating the spatial fatigue reliability on a monopile support structure is presented. • The effect of epistemic load effect uncertainties on the foundation reliability is evaluated. • A detailed wind directional model, wind-wave misalignment and a second order load model are compared to a base case. • The present model uncertainties reduce the estimated lifetime of the foundation. • Case-specific partial safety factors are discussed. [ABSTRACT FROM AUTHOR]

Abstract It has recently been pointed out that the vertical shear stresses at pile-soil interface play an important role in static responses of offshore monopiles with large diameters, and traditional p - y curves cannot fully capture this influence. However, the influences of the vertical shear stresses on the dynamic response of monopiles are still not clear. This is the main focus of this study. The coupled horizontal and rocking vibration of a monopile embedded in a fully saturated poroelastic seabed is studied in this paper. The focus of the study is the difference between the responses with and without the vertical shear stresses on the pile. The monopile dynamic response is studied by the integral equation method, with both 3D elastodynamic theory and Euler-Bernoulli beam theory. Selected numerical results for highlighting the influence of the vertical shear stresses on the dynamic contact load distributions, pile displacements, and lateral dynamic impedance functions are examined for different length to diameter (l/d) ratios as well as poroelastic materials and frequencies of excitation. These results confirm that exclusion of the vertical shear stresses will lead to potentially very conservative design for monopiles with l / d < 6 in soft soil, and monopiles with l / d < 10 in medium to stiff soils. The computed dynamic responses of the monopile when the vertical shear stress is ignored can be up to 30% larger than the case when the vertical shear stress is considered. Highlights • The influence of vertical shear stresses on the dynamic responses of monopiles is studied. • A rigorous integral equation method is used. • Exclusion of vertical shear stresses will lead to a potentially very conservative design. • When the vertical shear stress is ignored, dynamic responses can be up to 30% larger. [ABSTRACT FROM AUTHOR]

Abstract Offshore wind turbines (OWT) can be promising renewable energy devices. The motions of the turbine may be considerably high in severe sea-states and therefore they need to be operational after transition of such states. This work proposes a novel controller technique and its application in tension leg platform(TLP) supported OWT to harvest optimized power. It is achieved using the nonlinear quadratic regulator based algorithm wherein the state dependent structural system matrices are modified appropriately to account for the fluctuating dynamics. Three wind speeds (15 m/s, 21 m/s and 25 m/s) along with turbulence intensity of 0.1 are selected which are above the rated wind speed as the controller mechanism is effective for those regions. Since the wind and wave loads are taken as random, Monte Carlo method is used for the analysis to rule out epistemic uncertainty by ensemble statistics. The results show that the proposed nonlinear quadratic regulator is able to control the power, generator torque and rotor speed effectively without additional increase in platform motions vis-á-vis existing conventional Baseline controller. It is also observed that there are lesser fluctuations using the proposed controller compared to existing controller. The ensemble statistics –maxima and mean— are close to the reference value when wind speeds are close to rated-wind speed; whereas the platform motions and tower base forces is less using the proposed NQR controller or at least similar to the Baseline controller. The ensemble standard deviation are far less which shows the output power is controlled effectively. Highlights • Importance of coupled wind-wave response of wind turbines using a new proposed multi-objective nonlinear quadratic controller. • Observance of importance of simultaneous control of power, blade pitch and generator torque in coupled wind and wave loading. • The algorithm also takes reduces platforms motions without additional controls. • Data (Statistics) are placed in tables and figures which can serve as design aids in future. [ABSTRACT FROM AUTHOR]

Highlights • An analytical model of an offshore wind turbine with a three dimensional pendulum damper (3d-PTMD) has been established. • A statistical analysis on realistic wind-wave data at the location of a new wind farm under development is conducted. • The proposed 3d-PTMD can increase the tower fatigue life by more than 50% when compared to linear tuned mass dampers. Abstract Offshore wind turbines (OWTs) subjected to combined wind and wave loadings experience excessive vibrations which will increase fatigue loadings on the structure and reduce the fatigue life. In this paper, a three-dimensional pendulum tuned mass damper (3d-PTMD) is attached to the OWT to mitigate the bi-directional vibrations resulting from wind-wave misalignment so as to increase the fatigue life. An analytical model of the offshore wind turbine coupled with the 3d-PTMD is established using the Euler-Lagrangian equation. To predict long-term metocean condition, a statistical analysis for different properties of wind and wave loading such as, wind-wave misalignment, significant wave height and wind velocity is carried out. The aerodynamic wind loading is calculated using the blade element method and the wave loading is computed using the JONSWAP wave spectrum and Morison equation. Dual linear tuned mass dampers (TMDs) deployed in the side-side and fore-aft directions are used for comparison. The NREL monopile 5 MW baseline wind turbine is used to examine the performance of 3d-PTMD in a realistic metocean condition. The fatigue damage is estimated based on the rain-flow cycle counting method and Miner's rule. Results indicate that the 3d-PTMD can increase the wind turbine tower fatigue life by more than 50% in comparison with the dual TMDs. [ABSTRACT FROM AUTHOR]

Abstract Grouted Connections (GCs) are vital structural components of Offshore Wind Turbine (OWT) substructures. On monopiles to achieve a GC, tubular hollow steel piles are in-situ attached with a high-strength grout. Monopiles are susceptible to large magnitude bending loads in offshore environments. Recently, following inspections the performance of GCs has been called into doubt when settlements were reported on several monopiles. To further comprehend the structural performance of GCs under large bending moments a nonlinear Finite Element (FE) analysis was conducted. Three-dimensional FE models were solved and validated against experimental and analytical data with good agreement. It is suggested that the presented models can be used to evaluate the global and local behaviour of a GC accurately. Finally, a comprehensive parametric study was carried out to investigate the influence of shear key numbers, shear key spacing and overlap lengths. It was shown that increased number of shear keys are advantageous for stiffness and reduce the gap at the interfaces, whereas the grout failure depends on the spacing between neighbouring shear keys. The ability of the numerical model to trace all relevant failure modes which are provoked by shear key spacing was also demonstrated. [ABSTRACT FROM AUTHOR]

The dimensions of offshore wind turbine (OWT) support structures are governed by fatigue considerations. For 6‐ to 10‐MW OWTs, wave loads are often dominating in terms of fatigue utilization. The present work proposes a control scheme to reduce the wave‐induced fatigue loads in OWT support structures. The control scheme applies collective pitch control to increase both the damping and stiffness of the fore‐aft vibration modes. With conventional active tower damping, efficient wave disturbance rejection is restricted to a narrow frequency range around the first fore‐aft modal frequency. The proposed control scheme achieves efficient wave disturbance rejection across a broader frequency range. Here, tower feedback control is implemented via an auxiliary control loop. Based on a low‐fidelity model, the effect of the tower feedback loop on the stability margins of the basic controller is analysed. The results show that, within certain boundaries, the stability margins are improved by the stiffness term in the tower feedback loop. Consequently, the need to reduce the bandwidth of the basic controller to accommodate tower feedback control is relaxed. Based on time‐domain simulations carried out in an aero‐hydro‐servo‐elastic simulation tool, the lifetime effects of the proposed control scheme are analysed. Compared with conventional active tower damping, a more favourable trade‐off between adverse side effects and the support structure's fatigue damage is achieved with the proposed control scheme. [ABSTRACT FROM AUTHOR]

More accurate wind power prediction (WPP) is of great significance for the operation of electrical power systems, as offshore wind power penetration increases continuously. As the offshore wind turbines (OWT) are a key system in converting offshore wind power into electrical power, maintaining their condition plays a pivotal role in WPP. However, it is seldom considered in traditional WPP. This paper proposes an ultra-short term offshore WPP methodology based on the condition assessment (CA) of OWTs. Firstly, a modified fuzzy comprehensive evaluation (MFCE) based CA of the OWT is presented with a new defined deterioration of indicators calculated by the relative errors. Long short-term memory (LSTM) neural network is introduced to deal with the complicated interactions between the various monitoring data of an OWT and the dynamic marine environment. Then, with the classifications of the health conditions of the OWT, the historical operation data is classified accordingly. An OWT-condition based WPP with a backpropagation (BP) neural network is developed to deal with the non-linear mapping relations between the numerical weather prediction (NWP) information, health conditions of OWT, and the output power. The results of the case study show the influences of the OWT health conditions to its output power and verifies the effectiveness and higher accuracy of the proposed method.

During the past years, researchers have studied both numerically and experimentally multibody wave-wind combined energy structures supporting wind turbines and different types of Wave Energy Converters (WECs); rigid body hydrodynamic assumptions have been adopted so far for the development of their numerical models and the assessment of their produced power. In the present paper a numerical model that is based on the use of generalized modes addressing wave-structure interaction effects for the case of a multibody wave-wind combined structure is developed and presented. Afterwards, the developed numerical model is used for the assessment of the hydrodynamic response and the prediction of the produced power of different possible configurations of the updated WindWEC concept which consists of a spar supporting a wind turbine and one, two, three or four heaving type WEC buoys. The combined effects of the center-to-center distance of the WEC and spar platform, the number of the WECs and the grid configuration of spar and WECs on the hydrodynamic interaction between the different floating bodies, spar and WEC buoys, and consequently on their response and wave power production are examined for regular and irregular waves. Strong hydrodynamic interaction effects exist for small distance between spar and WECs that result to the decrease of the produced power. Power matrices of the updated WindWEC concept are presented for all examined configurations with different number of WECs. Moreover, the annual produced power of the updated WindWEC in two sites is estimated and presented. The generalized modes analysis presented in this paper is generic and can be used for the early stage assessment of wave-wind combined energy structures with low computational cost. The updated WindWEC can be used in sea sites with different environmental characteristics while extracting valuable amount of wave power.