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Vold-Kalman order tracking filter is an effective technique for dealing with non-stationary vibrations which offshore wind turbines often encounter. It has a unique capability to extract and track the time waveforms of harmonics in short transients without phase bias, and this capability is beneficial to the condition monitoring of offshore wind turbines. In general, the accuracy of the tracking results of the Vold-Kalman filer for condition monitoring is heavily dependent on the selection of filter bandwidth. A fixed filter bandwidth becomes problematic when processing different types of signals under varying operating conditions. Significant errors may arise in the tracking, rendering the condition monitoring of offshore wind turbines unreliable. To address this issue, this paper proposes a novel scheme for Vold-Kalman filter bandwidth selection to guarantee the consistency and accuracy of the offshore wind turbine condition monitoring process, ensuring reliable fault diagnosis. A numerical model is used to evaluate the effectiveness of the proposed bandwidth selection scheme first. Then the proposed scheme is further validated through the offshore wind turbine planetary gearbox datasets, together with the demonstration of the fault diagnosis capability of the filtered results. • We propose a novel bandwidth selection method for Vold-Kalman filtering. • Vold-Kalman filtering process can be achieved with controllable accuracy. • A more practical bandwidth selection scheme is provided to maintain a consistent monitoring. • Effectiveness of approach is validated using wind turbine planetary gearbox datasets. [ABSTRACT FROM AUTHOR]

This paper investigates the influence of scour erosion on the mode shapes of the National Renewable Energy Laboratory (NREL) 5 MW offshore wind turbine (OWT) in varying soil conditions, where modes are derived from wind and wave-induced accelerations. A numerical model, which considers aerodynamic and hydrodynamic loading with soil-structure interaction (SSI) and servo-dynamics, is developed. The superstructure is modelled using OpenFAST and the foundation is modelled with SESAM, where SSI is incorporated using Winkler (p-y) springs. The foundation stiffness is subsequently integrated in OpenFAST using mudline interface springs. The influence of soil density, scour hole depth, and loading on the derived mode shapes is studied. Accelerations are extracted at nine locations along the tower. The first two system natural frequencies and mode shapes are estimated using frequency domain decomposition (FDD) applied to the acceleration time-histories for each scour depth and soil condition. Modal Assurance Criterion (MAC) values are calculated to evaluate the changes in mode shapes that occur due to scour. It is shown that the mode shapes present consistent changes as scour depth increases, and the second mode shape exhibits more sensitivity to scour. The sensitivity of mode shape changes to scour under variations in wind speed and measurement noise are also investigated. • An integrated model of the wind turbine including wind and wave loads with soil-structure and servo-dynamics, is developed. • The frequencies and mode shapes for different scour scenarios, are extracted from the simulated acceleration responses. • The influence of soil density, scour hole depth, and loading on the derived mode shapes is studied. • The sensitivity of mode shape changes due to scour under various wind speeds and measurement noise are also investigated. [ABSTRACT FROM AUTHOR]

Large-diameter monopiles are widely used as the foundation to support offshore wind turbines (OWTs) in shallow coastal waters. The benefits of small-to-medium diameter tapered piles have been reported in the past. The potential use of large-diameter tapered monopiles installed by impact driving to support OWTs is thus presented, and then comparatively assessed by numerical analyses in terms of energy balance and installation mechanisms. A three-dimensional large deformation finite element (3D-LDFE) model of monopiles driven in clay was developed using a Coupled Eulerian-Lagrangian (CEL) approach. An advanced user-defined hypoplasticity clay (HC) model was employed to model undrained kaolin clay, featuring nonlinear behavior from small strain to large strain. The force-time curve defined by the operating data of a state-of-the-art hammer in the offshore industry was inputted to explicitly model impact driving. Better agreement between the measured and the simulated results was observed to validate the accuracy of the numerical model. The numerical results obtained give greater confidence to the future use of large diameter tapered monopiles for OWTs. • The concept of large-diameter tapered monopiles is presented for offshore industry. • An advanced user-defined hypoplasticity clay model is programmed to model seabed soils. • Installation mechanisms and energy balance of various shapes of monopiles driven in clays are comparatively studied. • The potential use of large diameter tapered monopiles for supporting OWTs is confirmed. [ABSTRACT FROM AUTHOR]

Complex marine environment causes significant variations in the dynamic loads and soil parameters that affect the design of a monopile. Despite these uncertainties, a multi-objective optimization technique is developed to produce the optimal designs. This optimization is based on the robust design concept, in which the design robustness , cost , and safety are considered as the optimal objectives. The responses of a monopile to arbitrary dynamic loads are initially determined using a transient finite element technique, which considers unknown soil parameters. These responses include the maximum rotation and displacement at the midline, maximum axial stress, and fatigue damage. Next, the variations (regarding standard deviations) in these responses, which are determined by the Monte–Carlo simulations, are adopted to define the design robustness. Finally, the cost and safety requirements of the monopile are evaluated, and robust design optimization is performed. The aforementioned robust design protocols are demonstrated with a 5 MW offshore wind turbine, which produces the optimal design. Therefore, the optimal designs for the monopile are investigated based on various safety requirements. Moreover, the design parameters that promote the design robustness with optimum efficiency are studied and proposed. • The uncertainties in dynamic loads and soil parameters are considered simultaneously in the design of a monopile. • The robust design considers design robustness in different limit states, safety, and costs simultaneously in optimization. • Transient finite element analyses and MCS are incorporated to simulate the dynamic responses of the monopile. • The most efficient design parameters for improving design robustness are explored based on the outcome of a case study. [ABSTRACT FROM AUTHOR]

Offshore wind turbines (OWTs) experience a range of two-way and partial one-way cyclic load conditions during their operational lifetime. Cyclic loading with a non-zero mean can lead to ratcheting in the soil, which causes the structure to accumulate displacement and rotation over repeated loading, which can compromise the design limit requirements. In this paper a novel ratcheting model is proposed using a Winkler approach with Masing rules and controlled with just one parameter. The model is validated against 5 experimental setups and 17 long term load tests up to 1 0 5 cycles, and predicts very well the experimental response, performing comparably with other existing ratcheting models. An OWT from North Hoyle wind farm is also modelled and it is found that the closer the driving frequency is to the resonant frequency, the greater the ratcheting response. This means that for soft-stiff type OWTs, where there is little tolerance between the resonant frequency and driving frequencies, it is crucial to consider the potential of ratcheting during the design phase. The model presented here provides a simple and effective method of predicting future ratcheting with a focus on ease of calibration and implementation into existing numerical analysis tools. • A model of pile rotation accumulation is developed using modified Masing rules. • The model is controlled with a single calibration parameter and is easy to implement. • Validation is performed with 17 experiments and the model provides a good prediction. • Application to OWT shows dynamic loading near to 1P results in greater ratcheting. [ABSTRACT FROM AUTHOR]

Monopile-supported offshore wind turbines are continuously subjected to stochastic cyclic loadings during its lifetime of 25–30 years and it is challenging to accurately predict the accumulated displacement of monopile. In this paper, a simplified model with two cyclic loading factors is proposed to predict the accumulated displacement of monopile in soft clay. These two cyclic loading factors could consider the effects of the pile diameter, number of cycles, loading profile and undrained shear strength profile on the accumulated displacement. The model is validated by comparing with the centrifuge data and results obtained from the hyperplastic accelerated ratcheting model. Furthermore, the evolution of the accumulated displacements of monopile for the ultimate limit state (ULS) storm loading history and the long-term fatigue limit state (FLS) loading history are also investigated. This paper could provide a method for design codes requiring an accurate estimation of monopile displacements. [ABSTRACT FROM AUTHOR]

Monopiles supporting offshore wind turbines (OWTs) sometimes need to be socketed in soft rocks. The lateral stiffness of piles plays a governing role on the fatigue limit state (FLS) and service limit state (SLS) of OWTs, so it needs to be further investigated. In this paper, laboratory-scale model tests and corresponding numerical analyses have been carried out to quantitatively evaluate the lateral static stiffness of monopiles in soft rocks under different scenarios. The results of pile responses, soil/rock resistance and the corresponding p-y curves are presented. Based on the experimental and numerical results, it is found that the load-sharing mechanisms of monopiles are significantly influenced by the rock-bearing layer. For modelling of rock-socked piles, p-y curves obtained by global curve fitting poorly describe the soil/rock resistance near the soil-rock interface, while p-y curves obtained by piecewise fitting for soil and rock layers or three-dimensional finite element model should be the preferred and recommended choices. • Lateral stiffness of monopile socketed in soft rock is studied. • The problem is studied by laboratory tests and FEM models. • Similar materials are used in the tests to model the soft rocks. • Pile responses, soil resistances and p-y curves are presented. • Load sharing mechanism of monopile is significantly affected by soft rock layer. [ABSTRACT FROM AUTHOR]

Optimization of maintenance scheme for offshore wind turbines is significant to reduce the maintenance cost and power loss. The novel strategy time window is first proposed for offshore wind turbines maintenance based on the preventive reliability and opportunistic reliability optimized for components. Then, the optimization model is provided for offshore wind turbines maintenance under the multiple constraints of the strategy time window, weather time window and maintenance urgency computed according to Jensen wake model. Meanwhile, the hybrid ant colony algorithm with discrete symbiosis organisms search algorithm (ACODSOS) is improved to search the optimal or near-optimal maintenance scheme. Finally, the experiments are carried out to testify the model and algorithm. The results demonstrate that the model is helpful to obtain the valuable maintenance schemes which save more cost than the models without time windows. Meanwhile, ACODSOS always performs better than ACO for finding the better maintenance schemes. • Strategy time window was proposed. • Different maintenance methods were considered. • Model considered multiple constraints was built to reduce maintenance cost. • Objective considered the vessel leasing cost, navigation cost, generation loss and time window penalty cost. • Hybrid Ant Colony Optimization was designed. [ABSTRACT FROM AUTHOR]

This essay discusses the finite-time stabilization of Tension Leg Platforms (TLP), one of the most considerable offshore floating wind turbine systems. The most critical issue in installing such turbines is controlling and balancing them on the water. In order to stabilize the TLP systems in floating offshore wind turbines, an adaptive terminal sliding mode controller is designed. Because the discontinuous signal function is one of the most important disadvantages of common sliding mode controllers, which causes an undesirable chattering phenomenon in the control law, the suggested approach introduces a new Lyapunov candidate function based on the adaptive continuous barrier function technique, so that the planned control law on TLP systems is obtained continuously and smoothly. The mentioned improved control method uses Lyapunov's stability theory and satisfies the convergence of states around the switching surface in a finite time. In this way, the proposed method removes the chattering phenomenon and operates with higher accuracy than the conventional controllers. In addition, the unknown bound of external disturbance is estimated using the adaptive control scheme. Finally, based on the simulation results, it can be seen that all the state variables of the TLP system are asymptotically stable in the presence of external disturbance and converge to zero, which indicates the efficiency of the proposed technique for stabilizing the tension leg platforms in offshore floating wind turbines. • Design of barrier-function continuous finite time stabilizer for TLP systems in floating offshore wind turbines. • Convergence of states to predefined neighborhood of origin in finite time, independent of uncertainties bound. • Adjusting adaption parameters to estimate boundaries of unknown disturbances via adaptive control technique. [ABSTRACT FROM AUTHOR]

This paper presents a simplified mechanical model to study ship collisions against Reinforced Concrete (RC) floaters of Floating Offshore Wind Turbines (FOWTs). The model accounts for the deformability of both striking ships and struck RC structures, including a method to evaluate the elastic–plastic response of RC slabs with normal horizontal restraints, while considering contact area effects and punching shear failure. In the proposed model, the internal mechanics are coupled to the external body dynamics, whilst the hydro-mechanical effects acting on the collided bodies such as hydrostatic restoring, viscous, and wave damping forces are accounted for by the large rotations rigid-body dynamics solver MCOL. A parametric analysis is conducted on the model to study its sensitivity to changes in impact energy, impact location, reinforcement ratios, wall thicknesses, and striker rigidities, whose outcomes are compared with Non-Linear Finite Element (NLFE) simulations. The results show that the proposed model can accurately capture the penetrations in both structures, their body dynamics, and the contact force through the collision at a significantly lower computational cost than their NLFE counterpart. • A Mechanical Model (MM) for simulating ship-FOWT collisions is proposed • The influence of hydro-mechanical and mooring forces are accounted for in the MM • A simplified elastic-plastic force-displacement model for fixed RC slabs is provided • The sensitivity of the MM is compared against NLFEA for different impact scenarios [ABSTRACT FROM AUTHOR]

In this paper, we present a robust design optimization (RDO) framework for the supporting structures of offshore wind turbines that takes uncertainties into consideration. Given the large sizes of turbines and the high complexity of ocean engineering systems, optimizing the supporting structures of offshore wind turbines can significantly save costs while enabling the structures to survive the severe ocean environment. The type of RDO used in our study focuses on reducing the structural weight of a turbine and lowering its variation to achieve stable (or robust) operation and minimal cost. To save computational costs, the design of experiment process was introduced for sensitivity analysis of the design variables and the arrangement of sampling points. The metamodel technology including the Kriging model was used in this study to replace the time-consuming finite element model for dynamic response analysis. A simple test case using a cantilever was introduced first for briefly illustrating the validity of the RDO framework method. Deterministic optimization (DO), which does not consider uncertainties, was conducted simultaneously for comparison. Subsequently, the numerical case of a tripod-type supporting structure of a 5 MW offshore wind turbine was built for formal optimization. The comparison revealed that the reliability of constraints in RDO was much higher than that in DO, whereas the standard deviation in RDO was lower, implying that robust and reliable design results were obtained even under the influence of uncertainties. The results also demonstrated the feasibility of application of the proposed RDO framework method to other offshore supporting structures. [ABSTRACT FROM AUTHOR]

Simulations of offshore floating wind turbine platform dynamics typically utilize engineering tools that include simplified modeling assumptions. In this study, an open-source CFD/6-DOF solver is developed using OpenFOAM for high-fidelity simulation of offshore floating wind turbine platforms. The solver tightly couples the fluid and 6-DOF equations of motion using subiterations and dynamic relaxation to eliminate the artificial added mass instability. Validation of the tightly coupled CFD/6-DOF solver is carried out on a benchmark case of the free decay of a heaving cylinder. The solver is then used in simulations of the DeepCwind semisubmersible platform and compared with FAST, a NREL engineering tool. The CFD/6-DOF solver and FAST are compared in four cases including both rotational and translational motion. Overall, the results demonstrate that the tightly coupled solver compares well with FAST. The tightly coupled CFD/6-DOF solver represents an advance in modeling of offshore wind turbine platform dynamics using open-source software that may be used for wind turbine research, design, and analysis. [ABSTRACT FROM AUTHOR]

Time domain analyses of Offshore Wind Turbines involve a lot of computational effort. In this paper, we present a method for reducing this effort by using results from analyses with shorter simulation lengths to predict the results of the longer simulations lengths required by the standards. This involves using simple statistical treatment of the first 10 minutes of simulation data, and a subsequent linear regression, to predict the damage equivalent load of the full 60 minutes simulation. With some reservations about the general applicability of the method, the results are promising. Some suggestions for further investigations and developments of the presented methods are discussed. [ABSTRACT FROM AUTHOR]

Considerable variations in environmental site conditions can exist within large offshore wind farms leading to divergent fatigue loads on support structures. An efficient frequency-domain method to calculate wave-induced fatigue loads on offshore wind turbine monopile foundations was developed. A verification study with time-domain simulations for a 4MW offshore wind turbine showed result accuracies of more than 90% for equivalent bending moments at mudline and interface level. This accuracy and computation times in the order of seconds make the frequency-domain method ideal for preliminary design and support structure optimization. The model is applied in sensitivity analysis of fatigue loads using Monte-Carlo simulations. Local and global sensitivity studies and a probabilistic assessment give insight into the importance of site parameters like water depth, soil stiffness, wave height, and wave period on fatigue loads. Results show a significant influence of water depth and wave period. This provides a basis for design optimization, load interpolation and uncertainty analysis in large wind farms. [ABSTRACT FROM AUTHOR]

In this paper, the short term extreme response of spar offshore 5MW NREL benchmark wind turbine is predicted. The spar is installed in a water depth of 320 m. The coupled wind and wave analysis is performed by coupling aerodynamic software FAST (Jonkman and Bull Jr,2005) and hydrodynamic software ANSYS-AQWA(2010). The global time domain responses of the spar type OWT is calculated for 600 s. The wave spectrum has significant wave height 6m and peak spectral period 10s and follows Pierson- Moskowitz spectrum. For safe operation, the structures should survive against different environmental conditions. The OWT can fail either in the operational regime or in the harsh environmental conditions. Therefore the dynamic simulation is carried out for two wind speeds, i.e. , one in operational (hub height wind speed as 11.5 m/s) regime and another in idling condition (30 m/s) using the above wave parameters. Monte Carlo method is used to simulate the OWT responses in irregular wave loading condition. After obtaining the time-domain nonlinear responses, the 3-hr short term extreme responses are obtained which are useful for design of OWT. The 3-hr extreme response is obtained using Global Maxima Method (GMM). In Global Maxima Method, one maximum is taken from each time series and the maxima are fitted to two Generalized Extreme Value distributions, viz ., Gumbel and Weibull distribution. The results show that Weibull fit is on a conservative side than Gumbel fit. Since the calculation of extremes is dependent on time domain simulations, a comparative study is also done for 100 and 20 samples so as to understand the sensitivity of extreme values due to lower sample size. [ABSTRACT FROM AUTHOR]

The aerodynamic performance of a Darrieus wind turbine operating with the rotation axis tilted with respect to the free-stream wind speed is investigated in this paper. An Unsteady Reynolds Averaged Navier Stokes (URANS) Computational Fluid Dynamics (CFD) model is proposed in order to provide wind turbine manufacturers with a reliable simulation tool to forecast the power conversion characteristics of vertical axis wind turbine prototypes that operate in tilted conditions. The outputs of the model are compared against experimental performance of a non-tilted rotor corrected to the standard sea level conditions. Two different tilted configurations are studied (i.e., a tilt angle of 10 ° and 20 ° ) and the aerodynamic performance is presented in terms of the mechanical power production and the power coefficient. A sensible decrease in the power production is observed for increasing tilt angles. Comprehensive physical interpretations of the results are provided, considering also the predictions of a methodology based on semi-empirical methods. [ABSTRACT FROM AUTHOR]

Frequent unscheduled random maintenance activities have significantly increased the operating cost of Offshore Wind Turbines (OWT). These activities account for ~65% of the overall OWT maintenance costs or 23% of the lifetime costs of OWT, equivalent to ~£26 M/yr for a 100MW offshore wind farm. This work performs a quantitative evaluation of the maintenance model suggested by Sinha Y et al. (2013) as a means to determine the threshold levels for planning an economical but effective maintenance for OWT. This study suggests that the model put forward provides a comprehensive framework to make maintenance decisions for OWT components by questioning their Availability, Reliability, Safety, Productivity and Availability of Upgrade Technology. Some case studies have been discussed towards the end of this work that validates this model and brings financial benefits. It is expected that practical use of the maintenance decision model, along with relationships developed in this work, would result in planning for economical, effective and efficient OWT maintenance. [ABSTRACT FROM AUTHOR]

The support structure damping of a 3.6 MW pitch controlled variable speed offshore wind turbine on a monopile foundation is estimated both in standstill conditions and in normal operation. The net substructure damping is identified from the parameters of an exponential curve fitted to the relative maxima of an impulse response caused by a boat impact. The result is used in the verification of the non aerodynamic damping in normal operation for low wind speeds. The auto-correlation function technique for damping estimation of a structure under ambient excitation was validated against the identified damping from the decaying time series. The Enhanced Frequency Domain Decomposition (EFDD) method was applied to the wind turbine response under ambient excitation, for estimation of the damping in normal operation. The aero-servo-hydro-elastic tool HAWC2 is validated with offshore foundation load measurements. The model was tuned to the damping values obtained from the boat impact to match the measured loads. Wind turbulence intensity and wave characteristics used in the simulations are based on site measurements. A flexible soil model is included in the analysis. The importance of the correctly simulated damping in the model is stressed for accurate load prediction. Differences in the identified damping between the model and the wind turbine are detailed and explained. Discrepancies between simulated and measured loads are discussed. [ABSTRACT FROM AUTHOR]

An adequate representation of the site-specific wind–wave joint distribution is essential for cost-efficient and reliable designs of offshore wind turbines. Therefore, the wind and wave climates are subjected to a correlation of wind and wave parameters for design purposes. These correlations are often based on a lumping of the directional wave climate and subsequent association of the lumped wave climate to the directional wind climate. Preservation of the hydrodynamic fatigue distribution from the full wave climate is an important aspect in the wind-wave correlation process which requires an adequate consideration of the dynamics from the offshore wind turbine. However, only a few wind-wave correlation methods exist for the fatigue design of offshore wind turbines and none of them take the dynamics of the full structure adequately into account. In this study a new wind-wave correlation method has been developed and introduced. The new method is based on the establishment of damage contour lines which are used to determine the sea-state parameters that ensure simultaneous compliance with damage equivalency criterions at different locations within the offshore wind turbine. This simultaneous damage equivalency throughout the structure together with the straightforward derivation of the corresponding damage equivalent sea-state parameters constitutes the novelty of the presented wind-wave correlation method. [ABSTRACT FROM AUTHOR]

The dynamic response of the supporting structure is critical for the in-service stability and safety of offshore wind turbines (OWTs). The aim of this paper is to first illustrate the complexity of environmental loads acting on an OWT and reveal the significance of its structural dynamic response for the OWT safety. Second, it is aimed to investigate the long-term performance of the OWT founded on a monopile in dense sand. Therefore, a series of well-scaled model tests have been carried out, in which an innovative balance gear system was proposed and used to apply different types of dynamic loadings on a model OWT. Test results indicated that the natural frequency of the OWT in sand would increase as the number of applied cyclic loading went up, but the increasing rate of the frequency gradually decreases with the strain accumulation of soil around the monopile. This kind of the frequency change of OWT is thought to be dependent on the way how the OWT is cyclically loaded and the shear strain level of soil in the area adjacent to the pile foundation. In this paper, all test results were plotted in a nondimensional manner in order to be scaled up to predict the consequences for prototype OWT in sandy seabed. [ABSTRACT FROM AUTHOR]

The response of bucket foundations on sand subjected to planar monotonic and cyclic loading is investigated in the paper. Thirteen monotonic and cyclic laboratory tests on a skirted footing model having a 0.3 m diameter and embedment ratio equal to 1 are presented. The loading regime reproduces the typical conditions of offshore wind turbines: very large cyclic overturning moment, large cyclic horizontal load and comparatively little, self-weight induced, vertical load. The experimental soil-foundation response is interpreted within the macro-element approach, using an existing analytical model, suitably modified to accommodate the footing embedment and the application of cyclic load. Details of the proposed model are provided together with evidences of its ability to reproduce the essential features of the experimentally observed behaviour. The results of the study aim at increasing the confidence in the use of the macro-element approach to predict the response of bucket foundations for offshore wind turbines, notably as the long-term accumulated displacements are concerned. [ABSTRACT FROM AUTHOR]

In 2007, the EU established challenging goals for all Member States with the aim of obtaining 20% of their energy consumption from renewables, and offshore wind is expected to be among the renewable energy sources contributing highly towards achieving this target. Currently wind turbines are designed for a 25-year service life with the possibility of operational extension. Extending their efficient operation and increasing the overall electricity production will significantly increase the return on investment (ROI) and decrease the levelized cost of electricity (LCOE), considering that Capital Expenditure (CAPEX) will be distributed over a larger production output. The aim of this paper is to perform a detailed failure mode identification throughout the service life of offshore wind turbines and review the three most relevant end of life (EOL) scenarios: life extension, repowering and decommissioning. Life extension is considered the most desirable EOL scenario due to its profitability. It is believed that combining good inspection, operations and maintenance (O&M) strategies with the most up to date structural health monitoring and condition monitoring systems for detecting previously identified failure modes, will make life extension feasible. Nevertheless, for the cases where it is not feasible, other options such as repowering or decommissioning must be explored. [ABSTRACT FROM AUTHOR]

Stochastic representations of turbulent wind and irregular waves are used in time domain simulations of offshore wind turbines. The variability due to finite sampling of this input loading is an important source of simulation error. For the OC4 reference jacket structure with a 5 MW wind turbine, an error of 12-34% for ultimate loads and 6-12% for fatigue loads can occur with a probability of 1%, for simulations with a total simulation length of 60 min and various load cases. In terms of fatigue life, in the worst case, the lifetime of a joint was thereby overestimated by 29%. The size of this error can be critical, i.e., ultimate or fatigue limits can be exceeded, with probability depending on the choice of number of random seeds and simulation length. The analysis is based on a large simulation study with about 30,000 time domain simulations. Probability density functions of response variables are estimated and analyzed in terms of confidence intervals; i.e., how probable it is to obtain results significantly different from the expected value when using a finite number of simulations. This simulation error can be reduced to the same extent, either using several short simulations with different stochastic representations of the wind field or one long simulation with corresponding total length of the wind field. When using several short-term simulations, it is important that ultimate and fatigue loads are calculated based on the complete, properly combined set of results, in order to prevent a systematic bias in the estimated loads. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

A probabilistic framework is developed to assess the structural performance of offshore wind turbines under multiple hazards. A multi-hazard fragility surface of a given wind turbine support structure and the seismic and wind hazards at a specific site location are incorporated into the probabilistic framework to assess the structural damage due to multiple hazards. A database of virtual experiments is generated using detailed three-dimensional finite element analyses of a set of typical wind turbine systems subject to extreme wind speeds and earthquake ground motions. The generated data are used to develop probabilistic models to predict the shear and moment demands on support structures. A Bayesian approach is used to assess the model parameters incorporating the information from virtual experiment data. The developed demand models are then used to estimate the fragility of the support structure of a given wind turbine. As an example of the proposed framework, the annual probabilities of the occurrence of different structural damage levels are calculated for two identical wind turbines, one located in the Gulf of Mexico of the Texas Coast (prone to hurricanes) and one off the California Coast (a high seismic region). Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Wind Turbines require numerous and varied types of maintenance activities throughout their lifespan, the frequency of which increases with years in operation. At present the proportion of maintenance cost to the total cost for wind turbines is significant particularly for offshore wind turbines (OWT) where this ratio is ~35%. If this ratio is to be reduced in-spite of adverse operating conditions, pre-mature component failures and absence of reliability database for wind turbine components, there is a need to design unconventional maintenance scheme preferably by including novel failure prediction methodologies. Several researchers have advocated the use of Artificial Neural Networks (ANN), Bayesian Network Theory (BNT) and other statistical methods to predict failure so as to plan efficient maintenance of wind turbines, however novelty and randomness of failures, nature and number of parameters involved in statistical calculations and absence of required amount of fundamental work required for such advanced analysis have continued to maintain the high cost of maintenance. This work builds upon the benefits of condition monitoring to design methods to predict generic failures in wind turbine components and exhibits how such prediction methods can assist in cutting the maintenance cost of wind turbines. This study proposes using a dedicated tool to assist with failure prediction and planning and execution of wind turbine maintenance. The design and development of such an all-inclusive tool will assist in performing administrative works, inventory control, financial calculations and service management apart from failure prediction in wind turbine components. Its database will contain reference to standard management practices, regulatory provisions, staff details and their skillsets, service call register, troubleshooting manuals, installation guide, service history, details of customers and clients etc. that would cater to multiple avenues of wind turbine maintenance. In order to build such a software package, a robust design of its database is crucial. This work lists prerequisites for choosing a physical database and identifies the benefits of relational database software in controlling large amounts of data of various formats that are stored in such physical databases. Such a database would be an invaluable resource for reliability studies, an area of interest for both academic researchers and the industry that are identifying avenues to economise wind turbine operations. [ABSTRACT FROM AUTHOR]

This paper presents experimental assessment of crack growth rates of S355J2+N steel in a corrosion fatigue environment similar to what is experienced on offshore wind farm monopile structures under various cyclic load frequencies in order to assess the effect of cyclic frequency of the applied loading within a frequency range pertinent to the structure. Fatigue crack propagation behaviour in this test programme is evaluated through fatigue tests on six compact tension test specimens in air and in laboratory simulated seawater under free corrosion condition. Fatigue crack lengths were monitored by back face strain (BFS), DCPD and ACPD. A regression model was derived through the BFS method to express strain values as a function of crack length to width ratio. The effectiveness of BFS method is particularly demonstrated in the simulated marine environment. Within the range of test frequencies, crack growth rates in simulated seawater when compared to the equivalent air test revealed environmental reduction factors of 2 and 4 at lower and higher values of stress intensity factors respectively. Significant difference in the results of the seawater test frequencies is discussed. [ABSTRACT FROM AUTHOR]

A probabilistic formulation is proposed to assess the performance of the support structure of offshore wind turbines based on their probability and expected time of exceeding specified drift thresholds. To this end, novel probabilistic models are developed to predict the mean and standard deviation of the drift ratio response of wind turbine support structures operating under day-to-day loads as a function of the wind turbine geometry and material properties, and loading conditions. The proposed models are assessed using a database of virtual experiments generated using detailed three-dimensional (3D) nonlinear finite element (FE) models of a set of representative wind turbine configurations. The developed models are then used in a random vibration formulation to estimate the probability and expected time of exceeding specified drift thresholds. As an example, the probability and expected time of exceeding specified drift thresholds are estimated for a typical offshore wind turbine at different wind speeds. A comparison is made between the results obtained based on the proposed models, those obtained using simulators commonly used in practice and detailed 3D nonlinear FE analyses. [ABSTRACT FROM PUBLISHER]

Wind turbine rotors are normally designed such that rotor power coefficient is maximized. Much of this methodology has been inherited from the aviation industry. This paper points out that designing machines for maximum rotor aerodynamic efficiency does not necessarily lead to a lower levelized cost of energy. The argument sits on the premise that levelized cost of energy is strongly influenced by machine capital expenditure (CAPEX) and annual energy production (AEP). We therefore assume that the true design objective is to minimize the CAPEX/AEP ratio. The basis of an alternative design path is presented, which centres on the minimization of total volume of structural material in the wind turbine. This is done whilst maintaining a given rated power. This alternative methodology requires the removal of conventional pre-set design variables and assumptions which relate to the maximization of rotor power coefficient. We examine how changing chord length, axial induction factor and aerofoil lift coefficient affect material volume in the blade. Following this, we use a custom-made blade element momentum programme to explore the relative CAPEX of machines with varying design axial induction factor and varying lift coefficient. This relative cost is calibrated to the 5 MW National Renewable Energy Laboratory offshore reference turbine. The effects on the rotor, drivetrain and tower are considered. For a 5 MW offshore machine, it is shown that an overall CAPEX/AEP reduction of over 2% can be achieved by using a low-induction rotor with blades possessing aerofoils operating at non-peak lift to drag ratios. This economy is delivered notwithstanding a 2.3% drop in design rotor power coefficient. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The mechanical responses of an offshore monopile foundation mounted in over-consolidated clay are calculated by employing a stochastic approach where a nonlinearp–ycurve is incorporated with a finite element scheme. The random field theory is applied to represent a spatial variation for undrained shear strength of clay. Normal and Sobol sampling are employed to provide the asymptotic sampling method to generate the probability distribution of the foundation stiffnesses. Monte Carlo simulation is used as a benchmark. Asymptotic sampling accompanied with Sobol quasi random sampling demonstrates an efficient method for estimating the probability distribution of stiffnesses for the offshore monopile foundation. [ABSTRACT FROM AUTHOR]

This paper investigates the fatigue damage of the gears and bearings in a land-based and a spar-type floating wind turbine. The reference 5 MW NOWITECH gearbox is used on a land-based and a floating spar wind turbine. First, the global analysis is carried out in an aero-hydro-servo-elastic code. Next, the forces, moments and motions from the global analysis are applied to the gearbox modelled in a Multi Body Simulation (MBS) and forces on gears and bearings are obtained. The fatigue damage is then calculated based on the SN-curve approach and Palmgren-Miner linear damage hypothesis in an identical wind condition and compared for both turbines. [ABSTRACT FROM AUTHOR]