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In this study dynamic responses of a 10 MW offshore wind turbine supported by a multi-body floating platform that consists of a wide cylindrical platform and a cylindrical ballast body suspended by six tendons are analyzed and predicted for different tendon breakage scenarios. A newly-developed and validated fully coupled numerical tool (F2A) based on AQWA and FAST is used to perform aero-hydro-servo-elastic analysis of the floating offshore wind turbine (FOWT). The results indicate that the dynamic behavior of the platform is heavily influenced by the state of tendons health. Roll and yaw motions of the platform under a tendon breakage are found to experience 6 times magnitude amplification of the typical responses, depending on the specific environmental conditions considered. Moreover, the peak tension in the tendon adjacent to the broken tendon experienced an increase of 165% in magnitude. The collective-pitch mode of the platform and wave excitation that are the main contributors to the surge and pitch fluctuations are slightly affected by tendon breakages. The influence of tendon breakages is found to be only significant on the local-pitch and coupled-pitch modes of the platform. In addition, multifractal spectra of the platform accelerations under different tendon failure scenarios show distinct fractal characteristics that can effectively identify and diagnose tendon failures, which is essential to the development of a structural health monitoring system of FOWTs.

Monopiles are the most commonly used foundation types for offshore wind turbines. This study investigates numerically and experimentally the effects of breaking wave impact on a monopile-type offshore wind turbine installed at different locations at the edge of a 1:25 slope. The numerical model uses a two-phase flow model based on solving the Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations with using the volume of fluid method and the k-ω shear stress transport turbulence model. Typical environmental conditions from the East China Sea have been examined. The wave run-up around the monopile and the nonlinear total horizontal wave force on the monopile are examined and compared with relevant experimental data originally presented in the present paper in order to validate the accuracy of the developed numerical model. A good agreement between the experimental data and numerical predictions is observed. Moreover, the wave breaking evolution and impact kinematics are investigated numerically for the different monopile locations. The results show that there is a 23.5% interval between the maximum wave force in the breaking zone and the wave force at the beginning of breaking. The first-order energy proportion of the breaking wave force will decrease maximum 61.65%, while, the second-order and high-order energy proportion of the breaking wave force will increase maximum 25.92% and 17.57%, respectively, between the different examined monopile locations. The time of occurrence of the secondary load cycle has no great relation with the form of the breaking wave for all examined locations, and the duration time of secondary load cycle impact on the monopile lasts for approximately 16.2% of the wave period. The secondary load cycle is smaller compared to the total horizontal breaking force in a range between 16.37% and 8.5%. The pressure on the monopile increases along with the wave breaking gradually. Compared with the pressure impact on the monopile when the wave begins to break, the pressure exerted on the monopile after the wave is rolled increases by 149%.

Committee Mandate Concern for advances in structural model testing and full-scale experimentation and in-service monitoring and their role in the design, construction, inspection and maintenance of ship and offshore structures. This shall include new developments in: best practice and uncertainty analysis; experimental techniques; full field imaging and sensor systems; big data applications for ship and offshore structures; and correlation between model, full-scale and numerical datasets. Introduction Design methods and rule requirements have to be benchmarked with experimental or service history data. The Goal Based Standards (GBS) approach by IMO, see e.g., IMO Res. 296(87), 454(100) or MSC.1/Circ.1394, require benchmarking of standards by measuring the performance of methodologies, assessments, criteria and requirements by using indicators that can be compared with an accepted standard or with experimental and/or service history data, performance levels or outcomes known to be reliable. This approach calls for a systemic use of experimental results and monitoring of data from ships during construction, in-service and possibly from the end of the life cycle. Thus, the elaboration of experimental outcomes and the handover of information into engineering and management practice need to be further addressed and standardized to fully exploit their potential. Further, the continues growth of complexity in ship and offshore structures potentially designed outside the range of empirical references calls for reliable means of validation. While computer simulations are continuously improving at the same time, their improvement to a large extent depends on a physical understanding of the underlying phenomena to be captured. Therefore, experimental investigations are essential in driving further developments and innovations through their accurate feedback when performed correctly and meaningful. The latter is however challenging, especially with increased complexity of the phenomena to be investigated. Therefore, this report summarizes the expertise of the contributing authors with respect to their corresponding research fields. The report seeks to give a general overview of the specific experiments dealt with therein, presents the latest publications, possible obstacles to be addressed and closes with future recommendations for further developments and integration of the experiments into engineering practice and scientific developments. The report at first covers a section on scaling laws relevant to any experiment followed by specific testing concerning: digital image correlation (DIC), hydrodynamics of flexible structures, wave-in-deck, hybrid models, friction, vibrations, fatigue at low temperatures, corrosion, large scale impact, large scale wind turbine blades, full-scale ice loads, health monitoring and digital twin models. Further, a benchmark study on free vibrations of a cantilever beam is carried out to demonstrate the vast possibilities to carry out such seemingly simple experiment followed by a discussion of the results achieved. The report is concluded with a summary and an outline of further research and development recommendations.

In the present study, an extension of the volume-of-fluid (VOF) method used in the open-source computational fluid dynamics (CFD) software OpenFOAM was developed, taking the advantages of the VOF and the level-set (LS) methods with an additional free surface turbulence damping (FSTD) boundary condition to restrict the overprediction of turbulence near the free surface. The coupled level-set and volume-of-fluid (CLSVOF) method has been developed, scripted, validated and proposed. The free surface elevation with the proposed CLSVOF method was validated in a developed numerical wave tank (NWT) against turbulence models used so far in OpenFOAM. Further, the validity of the present numerical model was evaluated for the case of plunging breaking waves against relevant experimental and numerical data. The outcome of this study demonstrates that the performance of the wave generation is improved with the proposed numerical model compared to existing turbulence models in OpenFOAM, creating a useful simulation add-on that generates long-term waves without unphysical dissipation, wave damping, and wave decay. The overprediction of turbulence created due to the significant difference in the fluid's density close to the free surface is limited by applying the FSTD boundary condition leading to a better representation of the flow field.

This study numerically and experimentally investigates the effects of wave loads on a monopile-type offshore wind turbine placed on a 1: 25 slope at different water depths as well as the effect of choosing different turbulence models on the efficiency of the numerical model. The numerical model adopts a two-phase flow by solving Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations using the Volume Of Fluid (VOF) method and three different k — ω turbulence models. Typical environmental conditions from the East China Sea are studied. The wave run-up and the wave loads applied on the monopile are investigated and compared with relevant experimental data as well as with mathematical predictions based on relevant theories. The numerical model is well validated against the experimental data at model scale. The use of different turbulence models results in different predictions on the wave height but less differences on the wave period. The baseline k — ω turbulence model and Shear-Stress Transport (SST) k — ω turbulence model exhibit better performance on the prediction of hydrodynamic load, at a model-scale water depth of 0.42 m, while the laminar model provides better results for large water depths. The SST k — ω turbulence model performs better in predicting wave run-up for water depth 0.42 m, while the laminar model and standard k — ω model perform better at water depth 0.52 m and 0.62 m, respectively.

The problem of damaged tendon diagnosis (damage detection, damaged tendon identification and damage precise quantification) in a new multibody offshore platform supporting a 10 MW Floating Offshore Wind Turbine (FOWT) is investigated for the first time in this study. Successful operation of the multibody FOWT depends on the integrity of its tendons connecting the upper and lower tanks of the platform. Thus, early diagnosis of the damaged tendons is of high importance and it is achieved through a vibration-based methodology. Damage detection is accomplished based on the detection of changes in the vibration response power spectral density, while damaged tendon identification and damage precise quantification are accomplished through the Functional Model Based Method (FMBM). The FMBM is appropriately formulated in this study to operate with only vibration response signals. The employed vibration responses under healthy and damaged states of the FOWT platform are obtained from a numerical model describing the platform’s dynamics. Each examined damage scenario corresponds to the reduced stiffness at the connection point of a single tendon to the platform’s upper tank. Subtle damages corresponding to a stiffness reduction of [10–25] %, have minor effects on the platform’s dynamics due to the tendons’ high strength, while damages corresponding to a stiffness reduction of [10–85] % on different tendons have similar effects on the dynamics, thus leading to an overall highly challenging diagnosis problem. The use of a single underwater accelerometer as well as a low and limited frequency bandwidth of surge acceleration signals, is explored. The results show that effective, reliable and very quick damaged tendon diagnosis is achieved via FMBM using the multibody FOWT platform’s dynamics under damaged tendons.

In this work, we analyze the capacity of the superstatistics construction to provide modeling of the velocity field probability density functions (PDFs) of isotropic turbulence. Generalizing along the lines of the kappa distribution, superstatistics is understood here as a PDF for the statistical temperature that depends on a single dimensionful parameter θ 2 and a dimensionless parameter κ 0 , which both depend on the size of the fluid eddies and the Reynolds number, and possibly on auxiliary dimensionless constants that depend only on the Reynolds number. We show that such superstatistics –in some sense, the simplest class of models– cannot provide PDFs for scales outside the dissipation subrange for the currently accessible Reynolds numbers in Direct Numerical Simulations (DNS). The obstruction results from realizability constraints and an associated bound, and is related to the flatness factor of the velocity derivative distribution. Greater values of the flatness extend the applicability of superstatistics to larger scales. We argue that phenomenologically effective superstatistics models will require a value of flatness F ∼ 25 or larger in order to cover the inertial subrange scales. The argument is assisted by constructing and analyzing a family of models which derive from modifying the gamma distribution in the regime of large statistical temperatures and nearly realize the realizability bound.

In the present paper, a comparative study of different cylindrical and conical substructures was performed under breaking wave loading with the open-source Computational Fluid Dynamics (CFD) package OpenFoam capable of the development of a numerical wave tank (NWT) with the use of Reynolds-Averaged Navier–Stokes (RANS) equations, the k-ω Shear Stress Transport (k-ω SST) turbulence model, and the volume of fluid (VOF) method. The validity of the NWT was verified with relevant experimental data. Then, through the application of the present numerical model, the distributions of dynamic pressure and velocity in the x-direction around the circumference of different cylindrical and conical substructures were examined. The results showed that the velocity and dynamic pressure distribution did not change significantly with the increase in the substructure’s diameter near the wave breaking height, although the incident wave conditions were similar. Another important aspect of the study was whether the hydrodynamic loading or the dynamic pressure distribution of a conical substructure would improve or deteriorate under the influence of breaking wave loading compared to a cylindrical one. It was concluded that the primary wave load in a conical substructure increased by 62.57% compared to the numerical results of a cylindrical substructure. In addition, the secondary load’s magnitude in the conical substructure was 3.39 times higher and the primary-to-secondary load ratio was double compared to a cylindrical substructure. These findings demonstrate that the conical substructure’s performance will deteriorate under breaking wave loading compared to a cylindrical one, and it is not recommended to use this type of substructure.

Explicit wave models and expensive sensor equipment capable of predicting and measuring wave parameters often carry a prohibitive computational and financial expense. To counter this, this paper proposes an alternative method for nowcasting coastal zone significant wave heights through the joint use of meteorological and structural data in the training of supervised machine learning models. In testing the hypothesis that structural data can improve model classification, artificial neural network and decision tree models were developed, trained and tested on field data recorded on a coastal jetty located in the southern coasts of Cyprus. A comprehensive investigation of the different models yields that the joint use of meteorological and structural features can improve classification performance, regardless of the network choice. It is also demonstrated that redundancy of training parameters could inject unwanted overfitting, reducing model generalization. To address this, a method for quantifying feature importance has been proposed by exploiting the nature of decision tree algorithms and the Gini impurity index, reaffirming that structural features do indeed benefit model classification. These results highlight the potential of tapping into the untapped pool of structural data for significant wave height prediction, paving the way for new research to be undertaken in this direction.

The present paper focuses on the determination of optimum layouts for linear arrays of heaving Wave Energy Converters (WECs) in front of a vertical wall. Optimum layouts maximize the annual averaged absorbed energy at a given marine site and satisfy spatial constraints. For achieving this goal, we developed an efficient optimization numerical framework, where a genetic algorithm solver is appropriately coupled with a frequency-domain hydrodynamic model, while, furthermore, a numerical wave model is utilized to determine the local wave climate conditions at the site of interest. The context is applied for an array of five semi-immersed, oblate spheroidal heaving WECs deployed at five near-shore sites of mild wave environments in the Aegean Sea, Greece. For each site, different optimization cases are solved, facilitating the investigation of different aspects of the examined problem. The largest annual energy absorption ability is observed for optimum layouts, characterized by the placement of the array close to the wall and the formation of clusters of closely-positioned WECs near the wall edges. Compared to arrays employed at sites in south-eastern Aegean, optimally-arranged arrays at central Aegean locations showed reduced energy absorption ability due to milder local wave conditions and/or the existence of quite limited water depths.

Wave Energy Converters (WEC) have seen a wide variety of innovations capable to harness the vast untapped energy source of the seas. This wide range of WECs often has varied applicability and power production capabilities, making the selection of a device overwhelming. These uncertainties are increased whenconsidering the interactions and suitability of the device with local metocean conditions, and the impacts to long-term reliable operation. The study focuses on the Mediterranean region and presents a comprehensive approachin selecting a WEC, using a novel Selection Index for Wave Energy Deployments (SIWED), which accounts for resource, extreme events, power production capabilities, reducing uncertainties and biases. As a case study ourapproach explores: (i) the viability of WECs at milder resource (ii) the use of SIWED to select the “optimal” location and (iii) an approach to optimise considering the multi-faceted resource impacts. The study providesa comprehensive assessment of the “hidden” benefits of wave energy in the Mediterranean and its methodology is universally replicable. Finally, a discussion and overview on the importance of this interdisciplinary method for WECdeployments is underlined.

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.

In this paper, we propose an algebraic model for turbulent scalar-flux vector that stems from tensor representation theory. The resulting closure contains direct dependence on mean velocity gradients and quadratic products of the Reynolds stress tensor. Model coefficients are determined from Direct Numerical Simulations (DNS) data of homogeneous shear flows subjected to arbitrary mean scalar gradient orientations, while a correction function was applied at one model coefficient based on a turbulent channel flow case. Model performance is evaluated in Poiseuille and Couette flows at several Reynolds numbers for Pr=0.7, along with a case at a higher Prandtl number (Pr=7.0) that typically occurs in water&ndash, boundary interaction applications. Overall, the proposed model provides promising results for wide near-wall interaction applications. To put the performance of the proposed model into context, we compare with Younis algebraic model, which is known to provide reasonable predictions for several engineering flows.

Offshore monopiles are exposed to extremely transient hydrodynamic loads during the wave breaking process and the required analysis to capture those loads is highly demanding and complex. The structural integrity of the monopiles is affected by the nonlinear wave breaking loads. The answer to capture those nonlinear hydrodynamic loads and address those challenges relies on the use of a well-validated numerical model based on Computational Fluid Dynamics (CFD). One parameter crucial for the correct use of the numerical model is the y+ value, a non-dimensional distance, which indicates where the first cell is located near-wall region. The purpose of this paper is to investigate and present the distributions of the y+ value for different mesh configurations of a fixed bottom monopile subjected to breaking waves inside a numerical wave tank developed with the use of the open-access CFD code OpenFOAM. Reynolds-Averaged Navier-Stokes modeling is performed by applying the k-ω SST model and relevant results derived.

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 modelling, and a numerical wave tank with different bottom slopes is developed. Τhe 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.

The present paper examines the case of a metal based unbonded flexible pipe under a number of axial tensile operational loads of varying intensity, expressly focusing on the response of the segments of the pipe’s tensile armour wires that lie inside the pipe’s End-Fitting (EF), with intent to track the development of high stress concentration areas there and assess the stress concentration factors (SCF) involved. Additionally, a parametric study is performed, to identify potential factors that might affect the maximum SCF values measured on these wire segments inside the EF. Based on the parametric study’s findings, an approximate relationship for evaluating the expected maximum SCF on the tensile wire inside the EF is developed. The paper adopts a numerical approach to the problem, based on the development of a two-dimensional finite element model for evaluating the wire’s response to axial tensile operational loads. The model considers both the effects the EF’s assembling procedure and the Factory Acceptance Test (FAT) have on the behavior of wire segments lying inside the EF. An analytical approach is employed as a benchmarking tool of the numerical model’s performance.

Due to the energy crisis and greenhouse effect, offshore renewable energy is attracting increasing attention worldwide. Various offshore renewable energy systems, such as floating offshore wind turbines (FOWTs), and wave energy converters (WECs), have been proposed and developed so far. To increase power output and reduce related costs, a combined marine energy structure using FOWT and WEC technologies has been designed, analyzed and presented in the present paper. The energy structure combines a 5-MW braceless semisubmersible FOWT and a heave-type WEC which is installed on the central column of the semisubmersible. Wave power is absorbed by a power take-off (PTO) system through the relative heave motion between the central column of the FOWT and the WEC. A numerical model has been developed and is used to determine rational size and draft of the combined structure. The effects of different PTO system parameters on the hydrodynamic performance and wave energy production of the WEC under typical wave conditions are investigated and a preliminary best value for the PTO&rsquo, s damping coefficient is obtained. Additionally, the effects of viscous modeling used during the analysis and the hydrodynamic coupling on the response of the combined structure are studied.

Maritime connectivity between Cyprus and other Eastern Mediterranean coastal regions on the mainland constitutes a critical factor towards understanding the origins of the early visitors to Cyprus during the onset of the Holocene (circa 12,000 years before present) in connection with the spread of the Neolithic in the region (Dawson, 2014). In this work, ocean circulation modeling and particle tracking are employed for characterizing drift-induced sea-borne connectivity for that period, using data and assumptions to approximate prevailing paleo-geographical conditions (re-constructed coastline from global sea level curves), and rudimentary vessel (rafts, dugouts) characteristics, as well as present-day weather conditions. The Regional Ocean Modeling System (ROMS, Shchepetkin and mcWilliams, 2005), forced by Copernicus Marine portal hydrological data, with wave and wind forcing derived from a combination of global reanalysis data and regional-scale numerical weather predictions (ERA5 and E-WAVE project products), are employed to provide the physical domain and atmospheric conditions. Particle-tracking is carried out using the OpenDrift model (Dagestad et al., 2018) to simulate drift-induced (involuntary) sea-borne movement. The sensitivity of the results on the hydrodynamic response (e.g. drag) of rudimentary vessels, such as rafts of postulated shape, size, and weight, that are believed to have been used for maritime travel during the period of interest, is also investigated. The simulation results are used to estimate the degree of maritime connectivity, due to drift-induced sea-borne movement, between segments of Cyprus coastline as well as its neighboring mainlands, and identify areas of both coastlines where landing/departure might be most favorable.This work aims to provide novel insights into the possible prehistoric maritime pathways between Cyprus and other Eastern Mediterranean coastal regions, and is carried out within the context of project SaRoCy (https://sarocy.cut.ac.cy), a two-year research project implemented under the “Excellence Hubs” Programme (contract number EXCELLENCE/0198/0143) of the RESTART 2016-2020 Programmes for Research, Technological Development and Innovation administered by the Research and Innovation Foundation of Cyprus.ReferencesDagestad K.-F., Röhrs J., Breivik Ø., Aadlandsvik B. 2018. “OpenDrift: A generic framework for trajectory modeling'', Geoscientific Model Development 11, 1405-1420. https://doi.org/10.5194/gmd-11-1405-2018.Dawson, H. 2014. Mediterranean Voyages: The Archaeology of Island Colonisation and Abandonment. Publications of the Institute of Archaeology, University College London. Walnut Creek, California: Left Coast Press Inc.Shchepetkin, A. F., & McWilliams, J. C. 2005. “The regional oceanic modeling system (ROMS): A split-explicit, free-surface, topography-following-coordinate oceanic model”. Ocean Modelling 9, no. 4, 347-404. https://doi:10.1016/j.ocemod.2004.08.002.

This paper proposes an alternative method for nowcasting significant wave height (Hs) through the development of hierarchical machine learning classification models. In testing the hypothesis that hierarchical classification can improve Hs prediction, flat and hierarchical classifiers were developed and tested on field-data recorded on a coastal jetty located in the southern coasts of Cyprus. A comprehensive investigation of the performance of flat over hierarchical classification models yields that the proposed method provides greater flexibility throughout the model development stages. This flexibility is attributed to the manipulation of data before training, optimization of classifier's hyperparameters during training, and the curtailment of features post-training at each level of the hierarchy. It is demonstrated that, the hierarchical approach resulted in better classification performance across a plethora of performance metrics established for a comprehensive comparison. It is also shown that the increased performance of the proposed approach comes at the expense of complexity arising from performing computationally expensive operations and the requirement for development of multiple local classifiers. Still, the increased classification performance of the hierarchical approach highlights the potential of this original method and the requirement for a rigid framework to be constructed for the development of hierarchical models for Hs prediction.

In the present paper the effects of aerodynamic damping and earthquake loads on the dynamic response of flexible-based wind turbines are studied. A numerical analysis framework (NAF) is developed and applied. NAF is based on a user-compiled module that is developed for the purposes of the present paper and is fully coupled with an open source tool. The accuracy of the developed NAF is validated through comparisons with predictions that are calculated with the use of different numerical analysis methods and tools. The results indicate that the presence of the aerodynamic loads due to the reduction of the maximum displacement of the tower attributed to the dissipation of earthquake excitation energy in fore-aft direction. Emergency shutdown triggered by strong earthquakes results to a rapid change of aerodynamic damping, resulting to short-term instability of the wind turbine. After shutdown of the wind turbine, enhanced dynamic response is observed. For the case where the wind turbine is parked, the maxima displacement and acceleration of tower-top increase linearly with the peak ground acceleration. With the use of the least-square method a dimensionless slope of tower-top displacements is presented representing the seismic response coefficient of tower that can be used to estimate the tower-top acceleration demand. Moreover, on the basis of the seismic response coefficient, an improved model for the evaluation of load design demand is proposed. This model can provide accurate predictions.

In the present paper, a novel Wave Energy Converter (WEC) named hereafter as Water Level Carpet (WLC) is introduced. Its dynamic response and performance are evaluated based on hydroelastic analysis and are presented for both regular and irregular waves. WLC consists of four floating modules inter-connected flexibly in two directions with hinges and Power Take-Off (PTO) mechanisms with known damping characteristics. The dynamic response (e.g. axial load at hinges, motions) and the estimation of the produced power of the WLC are evaluated through linear hydroelastic analysis in frequency domain with the use of a radiation/diffraction 3D hydroelastic model considering the effect of both the flexibility of the WLC as well as the damping forces associated with the energy extraction by the PTO mechanisms. The results that are obtained demonstrate the relationship between the produced power and the dynamic response of the WLC as well as the relationship between the damping coefficients of the PTO with the axial loads of WLC’s hinges. The produced power of WLC obtains its maximum value for wave frequencies close to the resonance of the generalized degrees of freedom irrespectively of the direction that the waves have. Moreover, the results that are obtained emphasize the importance of the hydroelastic analysis for the assessment of the performance of this type of WEC.

In the present paper, the mooring lines system of the V-shaped floating wind turbine (VSWT) at 50-m water depth is designed accounting shoaling effects and fully coupled aero-hydro-servo-elastic effects. The motions and mooring line tension responses under different load cases are analyzed at moderate water depth and are compared with revelant responses at deeper water depths for highlighting the rational response of the VSWT in shallow waters. The responses are dominated mainly by wave actions for the parked condition, while by wind actions for operational conditions. Moreover, comparisons between the onshore wind turbine (WT), monopile WT at 30-m water depth and VSWT at 50-m water depth are presented for different response and performance quantities including the generator power output and structural internal loads in order to study the rationality and efficiency of the mooring line design at moderate water depths and compare responses with alternative competitive possible solutions. It is found that the standard deviation of the generator power of the VSWT is larger than that of the onshore WT and monopile WT due to large platform motions induced by hydrodynamic loads. Moreover, the hydrodynamic loads on the VSWT have a great impact on the tower base loads, while do not significantly affect the rotor thrust force and blade root force. The results presented in the paper proves the efficient design of the VSWT at moderate water depths ready for use.

Currently, there is a great interest to globally develop offshore wind energy due to the greenhouse effect and energy crisis. Great efforts have been devoted to develop reliable floating offshore wind energy technology to exploit the wind energy resources in deep seas. This paper presents a comparative study of the dynamic response of three different semisubmersible floating wind turbine structures. All the three platforms support the same 5MW wind turbine. The platforms examined are: a V-shaped Semi, an OC4-DeepCwind Semi and a Braceless Semi at 200 m water depth. A dynamic analysis is carried out in order to calculate and compare the performance of these platforms. The comparison is made on the rigid body motions of the semisubmersible platform and tensions of the mooring lines. The presented comparison is based on statistical values and spectra of the time series of the examined response quantities. Coupling effects are more significant for the V-shaped Semi platform. The V-shaped Semi and the Braceless Semi show a more rational motion response under the investigated load cases. The results of this analysis may help to resolve the fundamental design tradeoffs between among different floating system concepts.

Wind speed forecasting is an important element for the further development of offshore wind turbines. Due to its importance, many researchers have proposed different models for wind speed forecasting that differ in terms of the time-horizon of the forecast, types and number of inputs, complexity, structure, and others. Wind speed series present high nonlinearity and volatilities, and thus an effective model should successfully deal with those features. An approach to deal with the nonlinearities and volatilities is to utilize a time series processing technique such as the wavelet transform. In the present paper, an ensemble data-driven short-term wind speed forecasting model is developed, tested and applied. The term &ldquo, ensemble&rdquo, refers to the combination of two different predictors that run in parallel and the prediction is obtained by the predictor that leads to the lowest error. The proposed model utilizes the wavelet transform and is compared with other models that have been presented in the related literature and outperforms their accuracy. The proposed forecasting model can be used effectively for 1 min and 10 min ahead horizon wind speed predictions.

Offshore wind turbine (OWT) installations are continually expanding as they are considered an efficient mechanism for covering a part of the energy consumption requirements. The assessment of the energy potential of OWTs for specific offshore sites is the key factor that defines their successful implementation, commercialization and sustainability. The data used for this assessment mainly refer to wind speed measurements. However, the data may not present homogeneity due to incomplete or missing entries, this in turn, is attributed to failures of the measuring devices or other factors. This fact may lead to considerable limitations in the OWTs energy potential assessment. This paper presents two novel methodologies to handle the problem of incomplete and missing data. Computational intelligence algorithms are utilized for the filling of the incomplete and missing data in order to build complete wind speed series. Finally, the complete wind speed series are used for assessing the energy potential of an OWT in a specific offshore site. In many real-world metering systems, due to meter failures, incomplete and missing data are frequently observed, leading to the need for robust data handling. The novelty of the paper can be summarized in the following points: (i) a comparison of clustering algorithms for extracting typical wind speed curves is presented for the OWT related literature and (ii) two efficient novel methods for missing and incomplete data are proposed.