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The exploitation of wind energy is rapidly evolving and is manifested in the ever-expanding global network of offshore wind energy farms. For the Small Island Developing States of the Caribbean Sea (CS), harnessing this mature technology is an important first step in the transition away from fossil fuels. This paper uses buoy and satellite observations of surface wind speed in the CS to estimate wind energy resources over the 2009–2019 11-year period and initiates hour-ahead forecasting using the long short-term memory (LSTM) network. Observations of wind power density (WPD) at the 100-m height showed a mean of approximately 1000 W/m2 in the Colombia Basin, though this value decreases radially to 600–800 W/m2 in the central CS to a minimum of approximately 250 W/m2 at its borders in the Venezuela Basin. The Caribbean Low-Level Jet (CLLJ) is also responsible for the waxing and waning of surface wind speed and as such, resource stability, though stable as estimated through monthly and seasonal coefficients of variation, is naturally governed by CLLJ activity. Using a commercially available offshore wind turbine, wind energy generation at four locations in the CS is estimated. Electricity production is greatest and most stable in the central CS than at either its eastern or western borders. Wind speed forecasts are also found to be more accurate at this location, and though technology currently restricts offshore wind turbines to shallow water, outward migration to and colonization of deeper water is an attractive option for energy exploitation. [ABSTRACT FROM AUTHOR]

The offshore wind energy is showing a growing interest because of the increment of global energy demand and the commitment to reduce the CO2 emissions. The need to identify new wind offshore areas has motivated the development of methods where several quantitative and qualitative factors are considered. Due to the variety of the identified factors is necessary establishing a priority order to know when they could be analyzed. The priorization of the identified factors not only ease the planning-execution of the future projects, but also economize resources because the achievement cost from the prefeasibility to final decision is ascendant, what means that the initial factors require less economic resources to be met compared to the factors grouped in the following stages. Then, this research organized the main factors in three stages (pre-feasibility, feasibility and final decision) and developed a methodology to perform a pre-feasibility analysis for identifiying potential offshore areas considering technicalenvironmental features and the wind characteristics in the space, time and frequency domain. The Colombian Caribbean coast was selected as study case, and the results pointed three areas and 10 locations with high potential for developing offshore wind projects. The north and central zone of the Colombian Caribbean coast were identified as the most suitable areas with mean annual wind speed over 10 m/s with low magnitude and direction variability, two factors considered extremely important for the wind power generation. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This work explores the possibility of installing compliant liquid dampers-inerter (CLDI) on monopile offshore wind turbines (OWTs) for controlling combined vibrations from wind, wave, and earthquakes. For numerical demonstration, the 5-MW NREL monopile OWT is taken up. Seven different wind speeds and a set of 20 earthquakes are considered. Hence, the means of structural response quantities are chosen. The peak and root mean square (RMS) values of displacement and acceleration responses of tower-top are monitored. A genetic algorithm-based multi-objective optimization is performed, and the optimum CLDI parameters are estimated for different ranges of inertance ratio and inerter topology. The optimization comprises four cases: firstly, minimizing the mean of peak responses and second, minimizing the mean of RMS responses in fore-aft (FA) as well as in side-side (SS) directions. Hence, each case has four objective functions, covering two loading scenarios, wind & wave, and earthquake. With the obtained optimum parameters, considering all these cases, the control effectiveness of CLDI is further investigated in time and frequency domains. The time domain results reveal that the CLDI controls the SS vibrations effectively, whereas, in FA direction, the efficiency is marginally less. Additionally, the frequency domain plots indicate the multi-mode control capability of CLDI. • Proposal of a CLDI-based vibration control mechanism for monopile WTs. • Development of a multi-objective optimization framework for WTs with CLDI. • Combined effects of wind, wave, and earthquake loadings on CLDI's performance. • Investigation of the control performance of CLDI in both FA and SS directions. • Illustration of vibration reduction mechanism of CLDI in time and frequency domains. [ABSTRACT FROM AUTHOR]

This paper proposes a novel data-driven framework for scour detection around offshore wind turbines (OWTs), where damage features are derived from wind and wave-induced acceleration signals collected along the tower. A numerical model of the NREL 5 MW wind turbine, which considers aerodynamic and hydrodynamic loading with soil-structure interaction (SSI) and servo-dynamics, is developed. The model is used to simulate the acceleration responses along the tower for a healthy structure, and a structure affected by progressive scour. A data segmentation process is initially performed on the collected data, which is followed by a feature selection scheme based on the analysis-of-variance (ANOVA) algorithm, to eliminate irrelevant characteristics from the time domain feature set of responses. The proposed framework consists of two main components: (a) offline training, and (b) real-time classification. The acceleration responses collected from the healthy structure and the structure subjected to three different damage scenarios (different scour depths) and under various load conditions, are used in the offline training mode. The selected feature vector from the feature extraction process is used as input to a Naive Bayes classifier (NBC) algorithm to train the model. In the real-time classification, a prediction of the scour depth affecting the structure is performed using a new dataset simulated from unseen load cases and scour conditions of the OWT. The results show that the model trained in the offline stage can predict the scour depth in the real-time monitoring stage with performance measures over approximately 94%. • A novel data-driven framework is proposed for scour detection around offshore wind turbines. • Damage features are derived from wind and wave-induced acceleration signals collected along the tower. • The data from the healthy structure and the structure subjected to three different damage scenarios and under various load conditions, are used in the offline training mode. • A prediction of the scour depth is performed using a new dataset simulated from unseen load cases and scour conditions of the OWT in the real-time classification. [ABSTRACT FROM AUTHOR]

This paper presents a method for quantitatively analyzing the influence of earthquake history on the pile-soil dynamic interaction for offshore wind turbines, and then modifying the dynamic p-y curve model to account for this effect. The analysis consists of three stages. Stage I involves centrifuge tests to investigate the impact of earthquake history on physical processes. These tests yield data, including dynamic p-y curves and earthquake history-related features. Stage II introduces a dynamic p-y curve model-guided deep learning model, where the p-y model was embedded in the loss function. This method specifically processes test data without requiring generalization. Stage III focuses on a quantitative analysis, where earthquake history-related features are reconstructed based on the analysis of earthquake history factors. Formulas are derived to fit the extracted earthquake history factors and then incorporated into the dynamic p-y curve model, resulting in a modified model considering the earthquake history effect. Compared to the test results, the modified dynamic p-y curve model shows improved predictions of soil resistance force p than the traditional p-y model. Quantitative analysis and validation results also demonstrate that deep learning models can help analyze experimental data automatically and intelligently, providing more quantitative evidence and explanations for observed phenomena. • The dynamic p-y curve and its key parameters undergo changes due to the earthquake history effect. • The dynamic p-y curve model guided-deep learning model can help extract earthquake history factors from test data. • Quantitative analysis shows that earthquake history can increase the relative density of saturated sand up to 85%–95%. • A modified dynamic p-y curve model shows predictive ability considering the earthquake history effect. [ABSTRACT FROM AUTHOR]

Suction buckets, recognized as efficient foundation structures, have found extensive applications in ocean engineering. In this study, numerical simulations are performed based on centrifuge test results to investigate the tensile and compressive behaviors of bucket foundations, considering the aspect ratio (AP). Two types of bucket foundations are defined with different AP values, namely wide-shallow and narrow-deep, respectively. The primary objective of this research is to examine the vertical bearing performance of bucket foundations under tension and compression conditions. A significant finding indicates that the ultimate bearing capacity of the bucket foundation increases with a higher AP. Additionally, there is a distinct mechanism of soil stress response between the outer and inner sides of the bucket under drainage tension. The evolution of soil stress is analyzed to illustrate the soil plug behavior within the bucket under compressive conditions. Consequently, an optional design method is proposed to assess the ultimate compressive bearing capacity, and it is validated against established experimental results. The outcomes of this study serve as a meaningful reference for the practical design of bucket foundations in engineering applications. • The vertical bearing characteristics of suction bucket for offshore wind turbines are assessed. • A unique soil stress response is observed on both sides of the bucket when subjected to drainage tension. • The soil plug behavior inside the bucket during compression is clarified by examining soil stress distributions. • An advanced design approach is proposed for assessing the ultimate vertical capacity of bucket foundations. [ABSTRACT FROM AUTHOR]

The paper presents the results of 3D coupled cyclic time history numerical analyses of a monopile supporting a 12 MW Offshore Wind Turbine, installed in dense cohesionless soils and subjected to a 600-s load history corresponding to the high phase of a 35-h design storm. The goal of the study is to investigate the governing mechanisms and gauge potential conservatisms or uncertainties in approaches for monopile analysis used in practice. The Ta-Ger constitutive model, implemented in FLAC3D and calibrated against site-specific cyclic tests, is used to model the complex soil response. Emphasis is placed on the effect of drainage conditions, an aspect typically overlooked in practice, although often stated as critical. Analyses show that the drainage of the system can substantially affect the response. In low-permeability soils (e.g., cohesionless soils with low-plasticity fines) widespread liquefaction may occur inducing high rotations above allowable limits. On the contrary, systems that can drain effectively within each cycle, develop moderate excess pore pressures which do not jeopardize performance. Current design procedures are often unable to accurately capture these effects possibly leading to either conservative or unconservative outcomes. Suitably validated advanced numerical analyses can be used as complementary tools to standard methods to assess these uncertainties. • Advanced numerical methodology for cyclic time history analysis of monopiles. • Use of Ta-Ger constitutive model for complex soil response. • Validation against centrifuge tests. • Drainage conditions can have a substantial effect on the response. • Current design methodologies may be unable to capture drainage effects. [ABSTRACT FROM AUTHOR]

An analytical solution for the gyroscopic effect of the spinning rotor-blades assembly on the dynamic response of offshore wind turbines (OWT) is presented. A continuous coupled model is rigorously developed to form the partial differential equations of fore-aft, side-side, and yaw motions. The gyroscopic moments caused by the angular momentum of the spinning rotor-blades assembly are formulated and handled into the three boundary conditions at the nacelle. The procedure for obtaining the operational natural frequencies of the structure including these coupled boundary conditions is developed. Furthermore, a response function for each fore-aft and yaw motions is obtained by solving the equations of motion under a wave load applied in only the fore-aft direction. Finally, the operational natural frequencies are calculated and compared to the idling ones for the considered example of OWT. The response of the structure under different values of the rotor-blades assembly's angular velocity is investigated. The proposed solution in this study unfolds a revelational procedure in capturing the gyroscopic effect in wind turbine industries which also can be a guideline for developing the floating OWT models. • An analytical procedure is presented to formulate the gyroscopic effect of rotor-blades assembly at the nacelle level. • An 3D analytical model is created based on the Euler-Bernoulli beam theory to take account the flexural stiffness of the structure in the formulation. • Generator reaction and breaking force on the nacelle is considered in the formulation. [ABSTRACT FROM AUTHOR]

The effect of non-linear and inelastic behaviour of the soil–pile interaction on the seismic response of offshore wind turbine pile foundations embedded in sandy soils is analysed. For this purpose, the responses obtained assuming three different Beam on Dynamic Winkler Foundation (BDWF) models are compared: a Plastic Non-Linear Model (PNLM), an Elastic Non-Linear Model (ENLM) and a simple elastic linear model with non-degraded properties of soil (NDLM). The influence of non-linearity and plasticity assumption is studied by evaluating the effects of the kinematic and inertial interaction within soil–pile interaction. Two soil stiffness levels are analysed: very loose and medium dense sand. The seismic response under ten earthquakes is computed in terms of mean envelopes of internal forces along the pile length. The non-linearity and inelastic influence of soil–pile interaction is quantified by means of relative differences with respect to the linear elastic model. Results show that the non-linear and inelastic models acquire relevance when the contribution of the inertial interaction dominates, leading to lower maximum responses than the linear elastic model. If the inertial interaction is not significantly activated, similar results between the three models are obtained, being the linear elastic model enough to reproduce the soil–pile dynamic interaction. [ABSTRACT FROM AUTHOR]

With the rapid advancement of wind power technology, the importance of lower operating costs and improved real-time control capabilities for offshore wind turbines (OWT) has increased. Nonlinear Economic Model Predictive Control (NEMPC) has gained attention due to its effective balancing of economic objectives. However, implementing real-time control for OWTs faces challenges due to their strong nonlinearity. This study proposes a convex NEMPC (CNEMPC) by incorporating variable transformation to account for the instantaneous energy stored in OWTs. The CNEMPC includes a set of convex constraints and aims to maximize power generation while minimizing fatigue loads on system components like the tower and gearbox. A newly designed moving horizon estimator provides an initial state estimation for the CNEMPC. Real-time iteration of the CNEMPC is achieved by leveraging the similarity of nonlinear programs between adjacent sampling moments. The effectiveness of the proposed control strategy is verified using a 5 MW OWT as the simulation target. • A real-time convex nonlinear economic model predictive control for offshore wind turbines is proposed. • The optimization problem of the nonconvex nonlinear economic model predictive control is convexed from an energy perspective. • Initialization and actual state variables' unmeasurability of the model predictive control are solved. • Real-time iterative solution of optimization algorithms is implemented. [ABSTRACT FROM AUTHOR]

Numerous monopile-supported offshore wind turbines (OWTs) are located in earthquake-prone regions, and the complex marine environment further amplifies seismic responses of OWTs. To determine the seismic responses of OWTs efficiently and accurately, an integrated numerical model based on the OpenSees platform is developed. Dynamic soil-structure interaction phenomena are captured using lumped parameter models (LPMs) which are fitted to the soil-pile impedance functions, and the dynamic beam on Winkler foundation method is adopted, where the pile is discretized into finite elements. Following the validation, to present a comprehensive seismic analysis, the numerical model incorporates the effects of combined seismic, stochastic wind-wave loads and different scour depths (the primary influencing factor at scoured sites) simultaneously. The results indicate that seismic responses become more pronounced under combined loads, and seismic intensities and spectral characteristics also play an important role in it. As scour depth increases, the natural frequency of the OWT significantly decreases, and seismic responses are further exacerbated, affecting normal operations. Additionally, a comparison with the dynamic Beam on Nonlinear Winkler Foundation (BNWF) method indicates that the present method strikes a balance between accuracy and computational efficiency. This positions it as a practical and efficient option for investigating seismic responses of OWTs. • An integrated numerical model of the scoured offshore wind turbine is developed, based on a lumped parameter model. • A comprehensive seismic analysis is presented, considering the effect of the scour and complex marine loads. • Scour can significantly affect the seismic responses of offshore wind turbines. • The present method can strike a balance between accuracy and computational efficiency. [ABSTRACT FROM AUTHOR]

Monopile foundations contain welding residual stresses and are widely used in industry to support offshore wind turbines (OWTs). The monopiles are subjected to hammering loads during installation and cyclic loads during operation, therefore the influence of residual stress redistribution as a result of fatigue cycles must be evaluated in these structures. The existing empirical models to predict the residual stress redistribution in the presence of cyclic loading conditions are strongly dependent on the material, welding process and loading conditions. Hence, there is a need to predict the residual stress redistribution using finite element simulations. In this study numerical analyses have been conducted to predict the initial state of residual stress in a simplified weld geometry and examine the influence of subsequent cyclic loads on the relaxation behavior in residual stress profiles. The results have shown that fatigue cycles have a severe effect on residual stress relaxation with the greatest reduction in residual stress values observed in the first cycle. Moreover, the numerical prediction results have shown that the stress amplitude plays a key role in the extent of residual stress relaxation in welded structures. [ABSTRACT FROM AUTHOR]

Scour behavior around a monopile foundation of offshore wind turbines (OWTs) should take into account the effect of combined waves and currents. To obtain an accurate equilibrium scour depth, a calculation model was proposed herein on the basis of the energy balance concept. Under the condition of wave and current coexistence, the composite flow velocity was proposed to express the flow field around the monopile foundation. To consider an arbitrary scour shape, the total volume of the deposit sediments removed from the scour hole was obtained by integrating two nonlinear functions. The equilibrium scour depth predicted by the model shows good agreement with the experimental results available in the literature. The calculation model of the equilibrium scour depth under combined waves and currents may also provide a guide for offshore engineering practice and design. [ABSTRACT FROM AUTHOR]

Offshore wind farms (OWFs) act as artificial reefs, attracting high abundances of fish, which could potentially increase their local production. This study investigates the feeding ecology of fish species that abundantly occur at artificial habitats, such as OWFs, by examining the short- and the long-term dietary composition of five species: the benthopelagic Gadus morhua and Trisopterus luscus, the pelagic Scomber scombrus and Trachurus trachurus, and the benthic Myoxocephalus scorpioides. We conducted combined stomach content and stable isotope analyses to examine the short- and the time-integrated dietary composition, respectively. Our results indicated that benthopelagic and benthic species utilize artificial reefs, such as OWFs, as feeding grounds for a prolonged period, since both analyses indicated that they exploit fouling organisms occurring exclusively on artificial hard substrates. Trachurus trachurus only occasionally uses artificial reefs as oases of highly abundant resources. Scomber scombrus does not feed on fouling fauna and therefore its augmented presence in OWFs is probably related to reasons other than the enhanced food availability. The long-termed feeding preferences of benthic and benthopelagic species contribute to the hypothesis that the artificial reefs of OWFs could potentially increase the fish production in the area. However, this was not supported for the pelagic species. [ABSTRACT FROM AUTHOR]

In the present paper, the computational fluid dynamics (CFD) method is used to investigate the variation of linear and nonlinear wave loads on a 10-MW large-scale monopile offshore wind turbine under typical sea conditions in the eastern seas of China. The effect of adding a structural ice-breaking cone configuration close to the mean water level on the monopile's hydrodynamic response is studied further. Results are derived with the use of the CFD model and are compared with the relevant results that are calculated using the Morison equation and the potential flow theory based on the high-order boundary element method. The fifth-order Stokes' theorem is used to model the incoming wave kinematics, and the volume of fluid (VOF) method is used to capture the free surface of waves and to accurately calculate the wave run-up on the monopile and cone configuration. The influence of different water depths and wave heights on the wave maximum vertical extent of wave uprush on the structure, pressure and horizontal wave forces on the monopile is investigated for both with and without the use of the cone configuration. Up–downward cone configuration results in better performance compared to the inverted cone configuration in terms of reduction of hydrodynamic nonlinear excitation loads and wave maximum vertical extent of wave uprush on the structure. [ABSTRACT FROM AUTHOR]

Accurate structural reliability assessment of floating wind turbine (FWT) systems is a desideratum for achieving consistent optimal reliability levels and cost-effective design. Such reliability assessment should consider relevant system uncertainties—a nontrivial task. Formulation of the reliability problem requires structural demand in form of load and load effect. Support structure loads are predicted with time-domain dynamic simulations. This represents a challenge when thousands of such simulations are required to capture the uncertainty associated with design variables. Finite element analysis (FEA) is commonly used to evaluate load effects such as stresses, strains etc. This can be computationally expensive if not prohibitive when such evaluation is carried out for every time step. To tackle these issues, a framework for expeditious load effect computation and robust reliability analysis of FWT support structures under ultimate limit state design is presented. The framework employs linear elastic FEA and Kriging surrogate models. The adequacy of Kriging as applied in this study is investigated using high fidelity simulation data. The results highlight the importance of incorporating the Kriging uncertainty in the formulation of the limit state function. With the framework presented, FWT support structures can be designed at consistent reliability levels leading to cost reductions. • A novel reliability framework to understand the reliability of FWTs is proposed. • The influence of nonlinear behaviour of coupled FWT is efficiently captured. • Computationally efficient surrogate models are developed for failure functions. • Kriging uncertainty is evaluated and implemented for improved reliability estimates. • The proposed approach is one of the initial steps towards optimally designed FWTs. [ABSTRACT FROM AUTHOR]

Effective wind power prediction will facilitate the world's long-term goal in sustainable development. However, a drawback of wind as an energy source lies in its high variability, resulting in a challenging study in wind power forecasting. To solve this issue, a novel data-driven approach is proposed for wind power forecasting by integrating data pre-processing & re-sampling, anomalies detection & treatment, feature engineering, and hyperparameter tuning based on gated recurrent deep learning models, which is systematically presented for the first time. Besides, a novel deep learning neural network of Gated Recurrent Unit (GRU) is successfully developed and critically compared with the algorithm of Long Short-term Memory (LSTM). Initially, twelve features were engineered into the predictive model, which are wind speeds at four different heights, generator temperature, and gearbox temperature. The simulation results showed that, in terms of wind power forecasting, the proposed approach can capture a high degree of accuracy at lower computational costs. It can also be concluded that GRU outperformed LSTM in predictive accuracy under all observed tests, which provided faster training process and less sensitivity to noise in the used Supervisory Control and Data Acquisition (SCADA) datasets. • The designed models have been validated against SCADA measurements. • Isolation forest improved the accuracy of deep-learning-based GRU and LSTM. • Proposed approaches capture a high degree of accuracy at lower computational costs. • A novel deep learning model of Gated Recurrent Unit (GRU) is effectively developed. • A novel data-driven approach is proposed for wind power forecasting. [ABSTRACT FROM AUTHOR]

In this paper, a brief review of the main fatigue design criteria and some advanced fatigue approaches applied to offshore structures (e.g. offshore wind turbines) are presented. It is extremely important to understand the fatigue phenomenon and how it affects structures since offshore structures are constantly submitted to cyclic loading and corrosive attacks that aggravate the problem. All the influencing factors and approaches used during the design phase are also discussed. [ABSTRACT FROM AUTHOR]

In this paper, a brief review of the main fatigue design criteria and some advanced fatigue approaches applied to offshore structures (e.g. offshore wind turbines) are presented. It is extremely important to understand the fatigue phenomenon and how it affects structures since offshore structures are constantly submitted to cyclic loading and corrosive attacks that aggravate the problem. All the influencing factors and approaches used during the design phase are also discussed.

In the present paper, the computational fluid dynamics method is used to investigate the effects of breaking wave loads on a 10-MW large-scale monopile offshore wind turbine under typical sea conditions in the eastern seas of China. Based on Fifth-Order Stokes wave theory a user-defined function is developed and used for wave numerical modeling, and a numerical wave tank with different bottom slopes is developed. The effects of different types of breaking waves, such as spilling and plunging waves, on the wave run-up, pressure distribution and horizontal wave force of a large diameter monopile are investigated. Different numerical and analytical methods for calculating the wave breaking loads are used and their results are compared with the relevant results of the developed computational fluid dynamics model and their respective scopes of application are discussed. With an increase in wave height, the change in the hydrodynamic performance of breaking waves observed through the transition from plunging to spilling waves is explored. The intensity of interactions occurring between the breaking waves and the monopile foundation depends mainly on the form of wave breaking involved and its relationship to wave steepness is weak. Analytical methods for calculating the breaking wave loads are preservative especially for plunging breaking wave loads.

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.

A reliable estimate of the wind potential in the marine atmospheric boundary layer (MABL) is of great importance to justify the energetic viability of new offshore wind farms. The purpose of the study is to provide an additional tool for the prediction of the energy that a wind turbine would produces in the open sea from the usual way of measurements at sea, that is, when they are carried out with measuring masts, where the meteorological data are obtained at levels much lower than those of a wind turbine hub. For this, the variation in the wind speed with the height in the MABL is determined, based on the Monin–Obukhov similarity theory, according to the boundary conditions of the air–sea interface, where the input data for the Validation of the results are extracted from the German FINO 3 research platform during the years 2016, 2017, and 2018. It is applied to the production of electrical energy from a 6.0 MW commercial wind turbine, with the hub at 100 m above the sea surface. As a more prominent result, the deviations from the proposed method do not exceed 2.5% in the energy calculation.

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.

The emerging wire + arc additive-manufacturing (WAAM) technique has significant potential to improve material design, as well as manufacturing cost and efficiency of structural components such as offshore wind turbines and subsequently reduce the levelised cost of energy (LCoE). Welded joints in offshore structures are usually considered potential spots for crack initiation due to the combination of high stress concentration at the weld toes, residual stresses introduced by the welding process and cyclic loading conditions in harsh, corrosive marine environments. The WAAM technique is a deposition method consisting of repetitive welding process that can be used as an alternative manufacturing technique for fabrication or repair of structural components. An important issue that needs to be understood in structural-integrity assessment of WAAM-built components is fracture-toughness behaviour. In particular, the sensitivity of fracture-toughness properties to corrosive environments must be examined in order to extend the application of the WAAM technique to offshore wind structures. Therefore, in this study, fracture-toughness tests were conducted on WAAM-built compact-tension specimens made of ER70S-6 and ER100S-1 steel that were initially exposed to a seawater corrosive environment prior to testing. All fracture-toughness tests were performed at room temperature, and crack length was estimated using the compliance method with a clip gauge attached onto the knife edge of the specimens. The obtained results, which include load vs. load-line displacement and J-integral vs. crack extension, were analysed and compared with the results of tests in air, without any exposure to seawater. The conclusions of this study show that corrosive environments affect the yield stress and R-curves of the selected materials and contribute to the overall understanding of the design requirements for functionally graded structures fabricated by means of WAAM technique for offshore applications.

This paper evaluates the application of a passive control technique with a tuned mass damper on platform for the barge-type offshore wind turbine. First of all, the three degrees of freedom mathematical model for the floating wind turbine is established based on Lagrange's equations, and the Levenberg-Marquardt algorithm is adopted to estimate the parameters of the wind turbine. Then, the method of frequency tuning which is utilized in engineering projects and genetic algorithm are employed respectively to simulate the optimum parameters of the tuned mass damper. The vibration mechanism about the phase-angle difference between tuned mass damper and floating platform is analyzed. Finally, the dynamic responses of floating wind turbine with/without tuned mass damper are calculated under five typical wind and wave load cases, and the vibration mitigation effects are researched in marine environment. Partial ballast is substituted by the equal mass of tuned mass damper due to the mass of floating platform with tuned mass damper would increase obviously, which would change the design of the wind turbine, and the vibration mitigation is also simulated in five typical load cases. The results show that the suppression rate of standard deviation of platform pitch is up to 47.95%, after substituting the partial mass of ballast, the suppression rate is 50%. Therefore, the dynamic responses of the barge-type floating wind turbine would be reduced significantly when the ballast is replaced by the equal mass of the tuned mass damper on floating platform.

For large offshore wind turbines, pitch control is usually used for regulating generated power to the rated value and for mitigating the dynamic loads that at the wind speeds above the rated speeds. However, tracking the pitch angle accurately and quickly can hardly be realised due to complex operating environments, uncertain system parameters, various disturbances, and coupled effects between wind, wave, and turbine structure. In this study, an individual pitch control system based on a neural adaptive strategy is proposed to address the problems related to uncertain system parameters and various disturbances. The proposed control method can achieve zero error tracking for the pitch angle in a predefined finite time. The design and stability analysis for the proposed method is elaborated. A simulation model is established in Matlab/Simulink, and by comparing it with the traditional proportional–integral–derivative control method, the merit of the proposed control scheme is verified. [ABSTRACT FROM AUTHOR]

The displacement of monopile supporting offshore wind turbines needs to be strictly controlled, and the influence of local scour can not be ignored. Using p–y curves to simulate the pile–soil interaction and the finite difference method to calculate iteratively, a numerical frame for analysis of lateral loaded pile was discussed and then verified. On the basis of the field data from Dafeng Offshore Wind Farm in Jiangsu Province, the local scour characteristics of large diameter monopile were concluded, and a new method of considering scour effect applicable to large diameter monopile was put forward. The results show that, for scour of large diameter monopiles, there was no obvious scour pit, but local erosion and deposition. Under the test conditions, the displacement errors between the proposed and traditional method were 46.4%. By the proposed method, the p–y curves of monopile considering the scour effect were obtained through ABAQUS, and the deformation of large diameter monopile under scour was analyzed by the proposed frame. The results show that, with the increase of scour depth, the horizontal displacement of the pile head increases nonlinearly, the depth of rotation point moves downward, and both of the changes are related to the load level. Under the test conditions, the horizontal displacement of the pile head after scour could reach 1.4~3.6 times of that before scour. Finally, for different pile parameters, the pile head displacement was compared, and further, the susceptibility to scour was quantified by a proposed concept of scour sensitivity. The analysis indicates that increasing pile length is a more reasonable way than pile diameter and wall thickness to limit the scour effect on the displacement of large diameter pile. [ABSTRACT FROM AUTHOR]

Offshore wind turbines (OWTs) have emerged as a reliable source of renewable energy, witnessing massive deployment across the world. While there is a wide range of support foundations for these structures, the monopile and jacket are most utilized so far; their deployment is largely informed by water depths and turbine ratings. However, the recommended water depth ranges are often violated, leading to cross-deployment of the two foundation types. This study first investigates the dynamic implication of this practice to incorporate the findings into future analysis and design of these structures. Detailed finite element (FE) models of Monopile and Jacket supported OWTs are developed in the commercial software, ANSYS. Nonlinear soil springs are used to simulate the soil-structure interactions (SSI) and the group effects of the jacket piles are considered by using the relevant modification factors. Modal analyzes of the fixed and flexible-base cases are carried out, and natural frequencies are chosen as the comparison parameters throughout the study. Second, this study constructs a few-parameters SSI model for the two FE models developed above, which aims to use fewer variables in the FE model updating process without compromising its simulation quality. Maximum lateral soil resistance and soil depths are related using polynomial equations, this replaces the standard nonlinear soil spring model. The numerical results show that for the same turbine rating and total height, jacket supported OWTs generally have higher first-order natural frequencies than the monopile supported OWTs, while the reverse is true for the second-order vibration modes, for both fixed and flexible foundations. This contributes to future design considerations of OWTs. On the other hand, with only two parameters, the proposed SSI model has achieved the same accuracy as that using the standard model with seven parameters. It has the potential to become a new SSI model, especially for the identification of soil properties through the model updating process. [ABSTRACT FROM AUTHOR]

Offshore wind turbines have the potential to capture the high‐quality wind resource. However, the significant wind and wave excitations may result in excessive vibrations and decreased reliability. To reduce vibrations, passive structural control devices, such as the tuned mass damper (TMD), have been used. To further enhance the vibration suppression capability, inerter‐based absorbers (IBAs) have been studied using the structure‐based approach, that is, proposing specific stiffness‐damping‐inertance elements layouts for investigation. Such an approach has a critical limitation of being only able to cover specific IBA layouts, leaving numerous beneficial configurations not identified. This paper adopts the newly introduced structure‐immittance approach, which is able to cover all network layout possibilities with a predetermined number of elements. Linear monopile and spar‐buoy turbine models are first established for optimisation. Results show that the performance improvements can be up to 6.5% and 7.3% with four and six elements, respectively, compared with the TMD. Moreover, a complete set of beneficial IBA layouts with explicit element types and numbers have been obtained, which is essential for next‐step real‐life applications. In order to verify the effectiveness of the identified absorbers with OpenFAST, an approach has been established to integrate any IBA transfer functions. It has been shown that the performance benefits preserve under both the fatigue limit state (FLS) and the ultimate limit state (ULS). Furthermore, results show that the mass component of the optimum IBAs can be reduced by up to 25.1% (7,486 kg) to achieve the same performance as the TMD. [ABSTRACT FROM AUTHOR]

In this paper we present a study on the mitigation of dynamic responses of a 10 MW monopile offshore wind turbine under coupled wind-wave-earthquake excitations. We have developed and validated the generic seismic coupled analysis and structural control architecture tool to overcome the limitation of numerical tools when examining the wind-wave-earthquake coupling effects. We investigated the dynamic responses of a 10 MW monopile offshore wind turbine under different loading combinations and found that the earthquake loading increases the tower-top displacement and pile-cap moment by 47.6% and 95.1%, respectively, compared to the wind-wave-only condition. It is found that the earthquake-induced vibration in the fore-aft direction is mitigated by the wind and wave loadings due to the energy dissipated by the aerodynamic and hydrodynamic damping. In addition, the tower responses are dominated by the earthquake excitation. In order to alleviate the tower vibration induced by the earthquake, we implemented the structural control capability within the tool using tuned mass dampers. The tuned mass dampers with appropriately selected design parameters achieve a larger mitigation on the tower-top displacement for the earthquake-only condition compared to the coupled-loading scenario. The reason is that the tuned mass damper is only effective in mitigating tower vibration, and it is not capable of reducing the tower elastic deformation which is the major contribution of the tower displacement for the coupled-loading condition. In addition, we have found that a heavier tuned mass damper requires a lower tuned frequency to achieve a larger mitigation. A configuration for the mitigation control of the 10 MW offshore wind turbine is suggested by using a 5% mass ratio of the tuned mass damper. • A seismic coupled analysis and structural control architecture tool is developed. • Structural control capacity is implemented into the tool using tuned mass damper. • The tool is used to perform coupled seismic analysis of offshore wind turbines. • Coupled seismic behaviors of a 10 mega-watt offshore wind turbine are obtained. • Mitigation effects of design parameters of the tuned mass damper are investigated. [ABSTRACT FROM AUTHOR]

As the size of offshore wind turbines increases, a realistic representation of the spatiotemporal distribution of the incident wind field becomes crucial for modeling the dynamic response of the turbine. The International Electrotechnical Commission (IEC) standard for wind turbine design recommends two turbulence models for simulations of the incident wind field, the Mann spectral tensor model, and the Kaimal spectral and exponential coherence model. In particular, for floating wind turbines, these standard models are challenged by more sophisticated ones. The characteristics of the wind field depend on the stability conditions of the atmosphere, which neither of the standard turbulence models account for. The spatial and temporal distribution of the turbulence, represented by coherence, is not modeled consistently by the two standard models. In this study, the Mann spectral tensor model and the Kaimal spectral and exponential coherence model are compared with wind fields constructed from offshore measurements and obtained from large‐eddy simulations. Cross sections and durations relevant for offshore wind turbine design are considered. Coherent structures from the different simulators are studied across various stability conditions and wind speeds through coherence and proper orthogonal decomposition mode plots. As expected, the standard models represent neutral stratification better than they do stable and unstable. Depending upon the method used for generating the wind field, significant differences in the spatial and temporal distribution of coherence are found. Consequently, the computed structural design loads on a wind turbine are expected to vary significantly depending upon the employed turbulence model. The knowledge gained in this study will be used in future studies to quantify the effect of various turbulence models on the dynamic response of large offshore wind turbines. [ABSTRACT FROM AUTHOR]

The offshore wind turbines are dynamically sensitive, whose fundamental frequency can be very close to the forcing frequencies activated by the environmental and turbine loads. Minor changes of support conditions may lead to the shift of natural frequencies, and this could be disastrous if resonance happens. To monitor the support conditions and thus to enhance the safety of offshore wind turbines, a model updating method is developed in this study. A hybrid sensing system was fabricated and set up in the laboratory to investigate the long-term dynamic behaviour of the offshore wind turbine system with monopile foundation in sandy deposits. A finite element model was constructed to simulate structural behaviours of the offshore wind turbine system. Distributed nonlinear springs and a roller boundary condition are used to model the soil–structure interaction properties. The finite element model and the test results were used to analyse the variation of the support condition of the monopile, through an finite element model updating process using estimation of distribution algorithms. The results show that the fundamental frequency of the test model increases after a period under cyclic loading, which is attributed to the compaction of the surrounding sand instead of local damage of the structure. The hybrid sensing system is reliable to detect both the acceleration and strain responses of the offshore wind turbine model and can be potentially applied to the remote monitoring of real offshore wind turbines. The estimation of distribution algorithm–based model updating technique is demonstrated to be successful for the support condition monitoring of the offshore wind turbine system, which is potentially useful for other model updating and condition monitoring applications. [ABSTRACT FROM AUTHOR]

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With the continuous increase of renewable energy generation and high voltage direct current (HVDC) power transmission, the system inertia is declining, thus resulting in greater risks of frequency instability. To solve this issue, a dc‐link inertia control is firstly proposed for a point‐to‐point modular multilevel converter based HVDC (MMC‐HVDC) link, which enables the MMC‐HVDC system to provide emulated inertia support by employing the electrostatic energy stored in the dc capacitors. Moreover, in order to maintain reasonable submodule capacitor size as well as provide more emulated inertia support, a virtual capacitor control for MMC‐HVDC connected an offshore wind farm (OWF) is proposed. With the virtual capacitor control, the OWF is able to provide a large capacity virtual capacitor in dc side of MMC‐HVDC link and supply desired emulated inertia support for onshore power system by utilising the offshore wind turbines' rotor rotational kinetic energy. Furthermore, a procedure for selecting the appropriate virtual capacitor control parameters is proposed. Finally, the performance of the proposed coordinated control is researched in PSCAD/EMTDC, and the simulation results show that the onshore power system inertia is effectively improved with the coordinated control. [ABSTRACT FROM AUTHOR]

The present study conducted an analysis of waves and wind on the west coast of Jeju Island in South Korea to derive the loads for a structural analysis under operational, extreme, and survival conditions and then evaluated the structural strength of the DeepCwind semi-submersible platform of NREL for 5 MW wind turbine. As a floating structure is exposed to wind and wave loads in a highly changeable marine environment and consists of tubular joints with slender-element members, a load analysis is necessary for strength evaluation and design of the structure. The wave scatter diagrams based on KORDI's 24-year measurements of sea states and the annual mean wind speeds were used as the operational conditions. A sea state with a period that is very likely to cause the resonance of a floating structure was applied as the extreme condition. Another sea state during the Bolaven, a typhoon passing west of Jeju Island in 2000, is adopted as the survival condition. The surface pressure distribution of DeepCwind calculate through hydrodynamic analysis was applied to a structural analysis model based on a hydrodynamic-structure interaction analysis. The evaluation of the structural strength is performed in accordance with the provisions of DNV-OS-C201 and API-2A-WSD under operational, extreme, and survival conditions. [ABSTRACT FROM AUTHOR]

Due to the high cost of failures of wind turbines, redundancy designs are commonly applied in wind turbines for improving the reliability and availability of systems. For this reason, replacing failed components with other working components of the same type in redundant systems is becoming an attractive option of maintenance strategies towards more resilient systems. To quantitatively evaluate system's reliability, this paper focuses on the reliability analysis of redundant systems of offshore wind turbines based on swapping existing components. The survival signature-based component swapping method is introduced to describe the new structure-function of the system upon swapping. Furthermore, the reliability model of redundant systems is established using the fault tree and survival signature. Following this, the influences of component swapping on component reliability importance measure (marginal reliability importance and joint reliability importance) without and with considerations of the imprecision of failure rates are explored. Finally, a 5MW offshore wind turbine is presented to show the applicability of the proposed approach for redundant systems, and the results show that the proposed approach can obtain realistic reliability assessment of redundant systems and considering component swapping can significantly improve system reliability. [ABSTRACT FROM AUTHOR]

Seaports are considered one of the sources involved in the deterioration of the maritime environment due to the excessive amount of exhaust gases emitted from their activities. The majority of seaports depend on the national electric grid as a source of power for the domestic and ships' electric demands. This paper discusses the possibility of shifting ports from relying on the national grid electricity to green power-based ports. Offshore wind turbines and fuel cell units appear as two typical promising clean energy sources for ports. As a case study, the paper investigates the prospect of converting Alexandria Port in Egypt to be an eco-friendly port with the study of technical, logistic, and financial requirements. The results show that the fuel cell, followed by a combined system of wind turbines and fuel cells, is the best choice regarding electricity production unit cost by 0.101 and 0.107 $/kWh, respectively. Furthermore, using fuel cells and offshore wind turbine as a green power concept will achieve a reduction in emissions' quantity of CO2, NOx, and CO emissions by 80,441, 20,814, and 133,025 ton per year, respectively. Finally, the paper highlights the role that renewable energy can play when supplying Alexandria Port with green energy to lift the burden on the government in supporting the electricity, with a possibility of achieving a profit from 3.85 to 22.31% of the annual electricity cost compared with the international prices. [ABSTRACT FROM AUTHOR]

Summary: The increasing number of wind turbines in active tectonic regions has attracted scientific interest to evaluate the seismic vulnerability of offshore wind turbines (OWTs). This study aims at assessing the deformation and collapse susceptibility of 2MW and 5MW OWTs subjected to shallow‐crustal pulse‐like ground motions, which has not been particularly addressed to date. A cloud‐based fragility assessment is performed to quantify the seismic response for a given intensity measure and to assess the failure probabilities for pulse‐like and non‐pulse‐like ground motions. The first‐mode spectral acceleration Sa(T1) is found to be an efficient response predictor for OWTs, exhibiting prominent higher‐mode behavior, at the serviceability and ultimate conditions. Regardless of earthquake type, it is shown that records with strong vertical components may induce nonlinearity in the supporting tower, leading to potential failure by buckling in three different patterns: (i) at tower base near platform level, (ii) close to tower top, and (iii) between the upper half of the main tower and its top. Type and extent of the damage are related to the coupled excitation of vertical and lateral higher modes, for which tower top acceleration response spectra Sa,i(Top) is an effective identifier. It is also observed that tower's slenderness ratio (l/d), the diameter‐to‐thickness ratio (d/t), and the rotor‐nacelle‐assembly mass (mRNA) are precursors for evaluating the damage mode and vulnerability of OWTs under both pulse‐like and non‐pulse‐like ground motion records. [ABSTRACT FROM AUTHOR]