Reliability Of Offshore Wind Turbines Research Articles

Dynamic responses of monopile offshore wind turbines in cold sea regions: Ice and aerodynamic loads with soil-structure interaction

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

A novel hydraulic transmission solution to large offshore wind turbine: Design and control strategy

As a comparative solution to gearbox transmission, the hydrostatic transmission has potential to improve offshore wind turbine reliability. However, drivetrain efficiency of hydrostatic transmission is relatively low at low wind speed. To solve this problem, a digital hydrostatic transmission solution for large offshore wind turbine is proposed. The large variable displacement motor in hydrostatic transmission is replaced by multiple fixed displacement motors and a small variable displacement motor. However the wind is turbulent in nature. This results in frequent switching of fixed displacement hydraulic motors. Therefore in this study a control strategy of large utility digital hydrostatic wind turbine under turbulent wind is proposed. The torque control of digital hydrostatic wind turbine is presented. The digital displacement switching scheme is developed. The system pressure controls during motor switching are studied. The dynamic performances and annual energy production of digital hydrostatic wind turbine under turbulent wind are studied. Results show the effectiveness of the proposed control strategy for digital hydrostatic wind turbine. The mean turbine output power at different mean wind speeds of hydrostatic and digital hydrostatic wind turbines are studied and compared. Results show the digital hydrostatic wind turbine has a 1.6% annual energy production improvement than the hydrostatic wind turbine.

Reliability and fragility assessment of offshore floating wind turbine subjected to tropical cyclone hazard

In the present study, an overall simulation-based procedure to assess the fragility and reliability of offshore wind turbines (WTs) subjected to tropical cyclone (TC) hazard is presented. The procedure relies on establishing the synthetic databases of TC track, TC wind and wave fields, and structural response. The construction of the databases and their use for reliability analysis are described. The proposed procedure can take into account the dependent TC induced wind and wave loads to estimate failure probability by considering multiple performance limits of various components. The overall procedure is illustrated for the reliability evaluation of an offshore semi-submersible WT placed at four potential locations near the coastline of mainland China. The results indicate that the failure probability of the considered WT is greater than the optimal value suggested in the literature, showing that the TC risk of such a WT could be very high if it is installed at offshore locations near the coastline of mainland China. The component failure analysis also shows that the failure probability is dominated by the tower buckling, followed by the platform overturning. Sensitivity analysis results indicate that the annual failure probability of the WT for the considered offshore locations is greater than 10-3 if the site-specific 50-year return period values of the annual maximum wind speed and significant wave height are used for the design and without additional load factor. An increased design environmental loads need to be considered to achieve a further reduction in the failure probability.

Fragility analyses of offshore wind turbines subjected to aerodynamic and sea wave loadings

Abstract To more effectively extract the vast wind energy in marine areas, offshore wind turbines have been constructed with slender tower and large rotor. External vibration sources such as aerodynamic, sea wave and seismic loadings can threaten the safety of these energy infrastructures. It is important to evaluate the reliability of offshore wind turbines subjected to external vibration sources. Previous research works on the wind turbine fragility analyses only considered the fragility of the tower by assuming the wind turbine was in the parked condition with the blade mass lumped at the top of the tower. The study of the fragility of the blade which is one of the most important components of a wind turbine has not been reported. In the present study, a detailed three-dimensional (3D) finite element (FE) model of the NREL 5 MW wind turbine is developed in ABAQUS, and the tower and blades are explicitly modelled to realistically estimate the aerodynamic loads and structural behaviours of the wind turbine. The uncertainties of the structural mass, stiffness and damping are taken into account to develop the probabilistic wind-induced demand models for the tower and blades. The dynamic behaviours of the wind turbine subjected to the simultaneous aerodynamic and sea wave loadings are investigated in a probabilistic frame and the fragility curves for both the tower and blades under the parked and operating conditions are derived and discussed.

Offshore wind turbine reliability and operational simulation under uncertainties

AbstractThe fast‐growing offshore wind energy sector brings opportunities to provide a sustainable energy resource but also challenges in offshore wind turbine (OWT) operation and maintenance management. Existing operational simulation models assume deterministic input reliability and failure cost data, whereas OWT reliability and failure costs vary depending on several factors, and it is often not possible to specify them with certainty. This paper focuses on modelling reliability and failure cost uncertainties and their impacts on OWT operational and economic performance. First, we present a probabilistic method for modelling reliability data uncertainty with a quantitative parameter estimation from available reliability data resources. Then, failure cost uncertainty is modelled using fuzzy logic that relates a component's failure cost to its capital cost and downtime. A time‐sequential Monte Carlo simulation is presented to simulate operational sequences of OWT components. This operation profile is later fed into a fuzzy cost assessment and coupled with a wind power curve model to evaluate OWT availability, energy production, operational expenditures and levelised cost of energy. A case study with different sets of reliability data is presented, and the results show that impacts of uncertainty on OWT performance are magnified in databases with low components' reliability. In addition, both reliability and cost uncertainties can contribute to more than 10% of the cost of energy variation. This research can provide practitioners with methods to handle data uncertainties in reliability and operational simulation of OWTs and help them to quantify the variability and dependence of wind power performance on data uncertainties.

Impact of model uncertainties on the fatigue reliability of offshore wind turbines

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

Fault diagnosis in yaw drive induction motor for wind turbine

Wind turbines are widely exploited throughout the world. The availability and reliability of offshore wind turbines are asking to impose a constant maintenance strategy. In this work, we propose a method that allows filtering the signal of the frequency inverter that feeds the yaw drive used in wind turbine. The redundant information is eliminated via discrete wavelet transform and empirical modal decomposition. The two types of faults are detected from the envelope of the Hilbert transform. The magnitude imbalance detection is carried out in the time domain. The root mean square values of the envelopes of the three-phase system have a good indicator for the fuzzy system to evaluate the severity of the defect. In the frequency domain, the signature of the broken bar fault is located in the low-frequency bandwidth. The harmonics appeared in the spectrum sensitive in amplitude and frequency to the variation of load. Experimental results have demonstrated the accuracy of the proposed method.

Assessing offshore wind turbine reliability and availability

The renewables sector and particularly offshore wind energy is a fast developing industry over the last few years. Especially, activities related to the installation, and operation and maintenance of offshore wind turbines become a challenging task with inherent risks. This article assesses the risks related to the above stages of a wind farm lifecycle using the failure mode, effects and criticality analysis and hazard identification methods. All works, from installation to operation and maintenance, are considered together with the wind turbine main components. An integrated risk analysis methodology is presented addressing personnel Safety (S), Environmental impact (E), Asset integrity (A), and Operation (O). The above is supplemented by a cost analysis with the aid of Bayesian belief networks method to assist the decision-making process related to installation, and operation and maintenance tasks. All major risks and critical wind turbine components are identified as well as measures are suggested to prevent or mitigate them. Moreover, inspection and maintenance plans are elaborated in general for the mentioned activities.

Performance and Reliability of Wind Turbines: A Review

Performance (availability and yield) and reliability of wind turbines can make the difference between success and failure of wind farm projects and these factors are vital to decrease the cost of energy. During the last years, several initiatives started to gather data on the performance and reliability of wind turbines on- and offshore and published findings in different journals and conferences. Even though the scopes of the different initiatives are similar, every initiative follows a different approach and results are therefore difficult to compare. The present paper faces this issue, collects results of different initiatives and harmonizes the results. A short description and assessment of every considered data source is provided. To enable this comparison, the existing reliability characteristics are mapped to a system structure according to the Reference Designation System for Power Plants (RDS-PP®). The review shows a wide variation in the performance and reliability metrics of the individual initiatives. Especially the comparison on onshore wind turbines reveals significant differences between the results. Only a few publications are available on offshore wind turbines and the results show an increasing performance and reliability of offshore wind turbines since the first offshore wind farms were erected and monitored.

Influence of statistical uncertainty of component reliability estimations on offshore wind farm availability

Offshore wind turbine reliability, one of the industry's biggest sources of uncertainty, is the focus of the present paper. Specifically the impact of uncertain component failure distributions at constant failure rates has been investigated with respect to its implications for wind farm availability. A fully probabilistic offshore wind simulation model has been applied to quantify results; effects shown in this paper underline the significant impact that failure probability distributions have on asset performance evaluation. It was found that wind farm availability numbers may vary in the range up to 20% just by changing the distributions of failure to a different pattern; in particular those scenarios in which extensive failure accumulation occurred led to significant losses in production. Results are interpreted and discussed mainly from the viewpoint of an offshore wind farm developer, owner and operator, with implications underlined for application in state-of-the-art offshore wind O&M (Operations and Maintenance) models and simulation tools.

Floating offshore wind turbine reliability analysis based on system grading and dynamic FTA

Currently, floating offshore wind power acts as a new form of marine renewable energy, attracting much attention all over the world. Due to constraints of technology and cost, floating wind turbines are still at the stage of preliminary trials. Reliability analysis on floating wind turbines is essential and urgent for solving the issues caused by high cost and challenges in searching failure causes. Primarily, this research proposes the scientific, systematic grading of floating wind power equipment system based upon its structure and function, providing with explicit internal relation of system and the precondition of failure modes analysis. Dynamic fault tree analysis on floating wind turbines is conducted to discuss sequentially dependent failures and redundancy failures. Furthermore, failure rates of relevant offshore structures are searched according to previous studies, databases and reports. Different modules of floating wind turbines take respective proper failure cases for references. The overall reliability results of floating wind turbines has been achieved. The research contributes to designing, construction and maintenance of floating wind turbines to control investment and enlarge economic benefits.

Comparisons of offshore wind turbine reliability

The results of a comparative probabilistic reliability model applied to offshore wind turbine systems are presented. The model calculations are based on surrogate failure rate data from industrial onshore wind turbine technologies, related marine environment technologies and generic databases. Data are adjusted for the offshore marine environment and integrated with functional as well as reliability block diagrams. The developed models are applied to five generic horizontal-axis offshore wind turbine drive train designs. Predicted sub-system failure rates and total system failure rates are reported and critical reliability limiting sub-assemblies are identified.

Using Improved Power Electronics Modeling and Turbine Control to Improve Wind Turbine Reliability

Improving offshore wind turbine reliability is a key industry goal to improve the availability of this renewable energy generation source. The semiconductor devices in the wind turbine power converter are traditionally considered as the most sensitive and important components to achieve this and managing their thermomechanical stressing is vital, since this is one of their principal long-term aging mechanisms. Conventional deterministic reliability prediction methods used in industrial applications are not suitable for wind turbine applications, due to the stochastic nature of the wind speed. This paper develops an electrothermal model of the power devices, which is integrated with a wind turbine system model for the investigation of power converter thermal cycling under various operating conditions. The model has been developed to eliminate the problems of pulse width modulation switching, substantially reducing simulation time. The model is used to improve the current controller tuning method to reduce thermal stresses suffered by the converter during a grid fault. The model is finally used to design a control method to alleviate a key problem of the doubly fed induction generator—severe thermal cycling caused during operation near synchronous speed.

Structural topology optimization of the transition piece for an offshore wind turbine with jacket foundation

The transition piece of a fixed offshore wind turbine is a reinforced part of the support structure that is connected to the wind turbine tower. These structural elements present unique features and are critical components of offshore wind turbines; designed to resist strong bending moments, shear forces and axial loads coming from cyclic environmental loads, such as wind and wave loads, acting during their complete design life of 20 years. Well designed and manufactured transition pieces with optimized ultimate and fatigue capacities, contribute to the structural soundness and reliability of offshore wind turbines. In this paper we investigate the benefits of integrating structural topology optimization in the design process of these elements, specially compared to classical design processes that are strongly based in experience and trial-and-error heuristic procedures. We first develop a solution for a 5 MW reference wind turbine with a jacket support using the classical design process and then apply structural topology optimization techniques to the same design problem. After verification of both solutions, it was found that the optimized solution is lighter and presents hot spots with lower stresses that extend its fatigue life significantly; demonstrating that an integrated design cycle that includes topology optimization can be very effective, speeding up the total design cycle and increasing the reliability of the final product.

A probabilistic analysis of the dynamic response of monopile foundations: Soil variability and its consequences

The reliability of offshore wind turbines is highly influenced by the uncertainties related to the subsoil conditions. Traditionally, the evaluation of the dynamic structural behaviour is based on a computational model with deterministic soil properties. Using this approach, however, provides limited insight into the variation of the estimate of the inherent modal properties and loads. In this paper, a comprehensive study is performed on the dynamic behaviour of an offshore wind turbine installed on a monopile foundation. Based on consistent lumped-parameter models calibrated to semi-analytical impedance functions of a monopile embedded in a stochastic linear viscoelastic soil layer, fully coupled aero-hydro-elastic simulations are conducted in the nonlinear multi-body code Hawc2. The probabilistic analysis accounts for the uncertainty of soil properties (e.g. damping and stiffness) and relies on a Monte Carlo method facilitating the derivation of the probability densities of the modal properties and the fatigue loading. The main conclusion of the presented work is that the dynamic structural behaviour of the wind turbine and its support structure is strongly affected by the stochastic soil properties. Lognormal and Gumbel distributed modal damping and accumulated side–side fatigue damage equivalent moments with a coefficient of variation of 30% and 8%, respectively, are observed.

Service reliability of offshore wind turbines

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.