Wind Turbine Support Structures Research Articles

Numerically efficient fatigue life prediction of offshore wind turbines using aerodynamic decoupling

The fatigue life prediction for offshore wind turbine support structures is computationally demanding, requiring the consideration of a large number of combinations of environmental conditions and load cases. In this study, a computationally efficient methodology combining aerodynamic decoupling and modal reduction techniques is developed for fatigue life prediction. Aerodynamic decoupling is implemented to separate the support structure and rotor-nacelle assembly. The rotor dynamics were modelled using an aerodynamic damping matrix that accurately captures the aerodynamic damping coupling between the fore-aft and side-side motions. Soil-structure interaction is modelled using p-y curves, and wave loading calculated based on linear irregular waves and Morison's equation at a European (North Sea) site. A modal reduction technique is applied to significantly reduce the required number of degrees of freedom, allowing the efficient and accurate calculation of hotspot stresses and fatigue damage accumulation. The modal model was verified against a fully coupled model for a case-study, monopile supported offshore wind turbine in terms of response prediction and fatigue life evaluation. The modal model accurately predicts fatigue life (within 2%) for a range of parameters at a fraction of computational cost (0.5%) compared to the fully coupled model.

One damping estimation approach of the parked offshore wind turbine supported by wide-shallow bucket foundation

The damping characteristic has an important influence on reliably predicting the dynamic response and fatigue life of the support structure of an offshore wind turbine (OWT). Generally, the damping sources of an OWT in parked conditions always include hydrodynamic damping, structural damping, and soil damping, ignoring aerodynamic damping. However, the difficulties in identifying the total damping of the OWT have created uncertainty and variation in the value of damping recommended in the design. In this research, an estimation approach for total damping was proposed for an OWT supported by a wide-shallow bucket foundation (WSBF), which is considered a novel bucket foundation, and its damping characteristics are still unclear. The hydrodynamic damping calculated based on Morison's equation of 0.0128% for the OWT supported by WSBF is smaller than the value of the monopile foundation. The soil damping is approximately 0.120%, obtained using the proposed theoretical method. Finally, the computational damping of the OWT in parked conditions was verified by the measured total damping of two parked OWTs at two different offshore wind farms (OWFs) of China. The results show that the damping obtained by these two methods was in good agreement with the average deviation of 11.20%.

Improvements in the fatigue design of support structures for offshore wind turbines

StahlbauVolume 90, Issue 6 p. 416-416 Empfehlungen der Redaktion Improvements in the fatigue design of support structures for offshore wind turbines Extended keynote paper of Eurosteel 2021 First published: 10 June 2021 https://doi.org/10.1002/stab.202170604Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL No abstract is available for this article. Volume90, Issue6June 2021Pages 416-416 RelatedInformation

A fracture mechanics framework for optimising design and inspection of offshore wind turbine support structures against fatigue failure

Abstract. Offshore wind turbine (OWT) support structures need to be designed against fatigue failure under cyclic aerodynamic and wave loading. The fatigue failure can be accelerated in a corrosive sea environment. Traditionally, a stress–life approach called the S–N (stress–number of cycles) curve method has been used for the design of structures against fatigue failure. There are a number of limitations in the S–N approach related to welded structures which can be addressed by the fracture mechanics approach. In this paper the limitations of the S–N approach related to OWT support structure are addressed and a fatigue design framework based on fracture mechanics is developed. The application of the framework to a monopile OWT support structure is demonstrated and optimisation of in-service inspection of the structure is studied. It was found that both the design of the weld joint and non-destructive testing (NDT) techniques can be optimised to reduce in-service inspection frequency. Furthermore, probabilistic fracture mechanics as a form of risk-based design is outlined and its application to the monopile support structure is studied. The probabilistic model showed a better capability to account for NDT reliability over a range of possible crack sizes as well as to provide a risk associated with the chosen inspection time which can be used in inspection cost–benefit analysis. There are a number of areas for future research, including a better estimate of fatigue stress with a time-history analysis, the application of the framework to other types of support structures such as jackets and tripods, and integration of risk-based optimisation with a cost–benefit analysis.

Improvements in the fatigue design of support structures for offshore wind turbines

AbstractExtended keynote paper of Eurosteel 2021This article provides an overview of the various support structures for offshore wind turbines and addresses the challenges that arise for the different design variants. First of all, the existing types of support structures are explained and their benefits and shortcomings discussed. Following that, the focus is on material fatigue and its analysis, which is of great importance due to the cyclic loading on the structures. A central topic of this article is the experimental evaluation of fatigue strength using large‐scale test setups and state‐of‐the‐art measurement methods. Critical and complex design details such as welds for extremely thick plates, tubular joints, grouted connections, large HV bolts and ring flanges are investigated to improve methods of analysis. Further, the new series of German standards (DIN 18088) for support structures for wind energy turbines and platforms is addressed. Finally, these topics are summarized and the importance of offshore wind energy for the transition to cleaner energy is emphasized.

Fixed-Time Adaptive Robust Synchronization with a State Observer of Chaotic Support Structures for Offshore Wind Turbines

The chaotic support structures for offshore wind turbines are often subjected to a severe environment. A robust control scheme needs to be considered to maintain them in a safe operational limit. Robust sliding mode control (SMC) scheme can provide an excellent robust controller against this severe and challenging environment for these chaotic structures. This paper proposes a novel fixed-time adaptive sliding mode control scheme with a state observer to synchronize chaotic support structures for offshore wind turbines in the presence of matched parametric uncertainties. The proposed controller is a new integration of adaptive control concept, SMC method, fixed-time stability concept and a state observer. A fixed-time stability concept is used to provide stability for the system within a presented time regardless of initial conditions. The adaptive concept is utilized to provide an online estimator of the uncertain upper bound. Also, a nonlinear observer is employed to provide an online estimator of an unmeasured state in the controller. Lyapunov stability theorem is used to analyze fixed-time stability of the system based on SMC methodology. The simulation results demonstrate that the proposed controller is able to ensure fixed-time synchronization along with providing precise means to estimate the unmeasured state as well as uncertainty upper bound.

Reliability-based design optimization of a spar-type floating offshore wind turbine support structure

The application of reliability-based design optimization (RBDO) methods to offshore wind turbine systems is highly relevant regarding economic efficiency and for considering prevailing uncertainties within the design process. Furthermore, RBDO is a very promising approach in optimizing systems when classification and standardization are not fully available. The level of difficulty of design optimization already increases when including the reliability aspect, but becomes even more challenging when dealing with the highly complex system of floating wind turbines (FWTs), which has not yet been applied. Thus, this paper presents for the first time an integrated framework for RBDO of FWTs, combining concepts of optimization with reliability-based design and advanced modeling, requiring reasonable computational effort and time expenditure. In preprocessing, environmental conditions, limit states, and uncertainties are specified, an appropriate reliability assessment approach is elaborated, and response surfaces for various system geometries in the optimization design space are generated ahead of the RBDO execution. These are finally used by means of an interpolation approach for the reliability calculation integrated in the iterative design optimization. On the example of a spar-buoy FWT system, the application of the presented methodology and the feasibility of coupling FWT design optimization with reliability assessment are shown.

Concrete Support Structures for Offshore Wind Turbines: Current Status, Challenges, and Future Trends

Today’s offshore wind turbine support structures market is largely dominated by steel structures, since steel monopiles account for the vast majority of installations in the last decade and new types of multi-leg steel structures have been developed in recent years. However, as wind turbines become bigger, and potential sites for offshore wind farms are located in ever deeper waters and ever further from the shore, the conditions for the design, transport, and installation of support structures are changing. In light of these facts, this paper identifies and categorizes the challenges and future trends related to the use of concrete for support structures of future offshore wind projects. To do so, recent advances and technologies still under development for both bottom-fixed and floating concrete support structures have been reviewed. It was found that these new developments meet the challenges associated with the use of concrete support structures, as they will allow the production costs to be lowered and transport and installation to be facilitated. New technologies for concrete support structures used at medium and great water depths are also being developed and are expected to become more common in future offshore wind installations. Therefore, the new developments identified in this paper show the likelihood of an increase in the use of concrete support structures in future offshore wind farms. These developments also indicate that the complexity of future support structures will increase due to the development of hybrid structures combining steel and concrete. These evolutions call for new knowledge and technical know-how in order to allow reliable structures to be built and risk-free offshore installation to be executed.

Horizontal and vertical axis wind turbines on existing jacket platforms: Part 1 – A comparative study

The wind resource offshore is often excellent due to a higher average wind speed and lower turbulence intensity. Jacket structures are the most common offshore platforms for extraction of oil and natural gas in relative low water depths. When the offshore resources run out, these structures must be displaced to another area containing underground resources or removed in the case of reaching their design life. Therefore, one possible procedure to reduce the carbon footprint on the planet, allowing society to rely on promising sources of ’clean’ energy while salvaging these oil and gas platforms, is to consider the transformation of these oil and gas platforms into offshore wind turbine support structures. The present research focus on the possibility of converting such structures for gas extraction into offshore platforms for wind turbines. In this study, a comparison between the behavior of horizontal and vertical axis wind turbines on the same offshore platform is presented. In this comparison, two different software programs are used: MATLAB and SAP2000. The model proposed is a new simplified tool used to study the structural analysis of the jacket structures, developed and summarized in 10 steps, adopted to evaluate the behavior of the platform with the wind tower configurations.

Predictive information and maintenance optimization based on decision theory: a case study considering a welded joint in an offshore wind turbine support structure

Predictive information and maintenance optimization for deteriorating structures is concerned with scheduling (a) the collection of information by inspection and monitoring and (b) maintenance actions such as repair, replacement, and retrofitting based on updated predictions of the future condition of the structural system. In this article, we consider the problem of jointly identifying—at the beginning of the service life—the optimal inspection time and repair strategy for a generic welded joint in a generic offshore wind turbine structure subject to fatigue. The optimization is performed based on different types of decision analyses including value of information analyses to quantify the expected service life cost encompassing inspection, repair, and fatigue damage for all relevant combinations of inspection time, repair method, and repair time. Based on the analysis of the expected service life cost, the optimal inspection time, repair method, and repair time are identified. Possible repair methods for a welded joint in an offshore environment include welding and grinding, for which detailed models are formulated and utilized to update the joint’s fatigue performance. The decision analyses reveal that an inspection should be scheduled approximately at mid-service life of the welded joint. A repair should be performed in the same year after an indication and measurement of a fatigue crack given an optimal inspection scheduling. This article concludes with a discussion on the results obtained from the decision and value of information analyses.

Risk Assessment of Offshore Wind Turbine Support Structures Considering Scouring

Risk Assessment of Offshore Wind Turbine Support Structures Considering Scouring

A brief review of fatigue design criteria on offshore wind turbine support structures

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

Concrete support structures for small wind turbines

Small wind turbines (SWT) generate electricity up to several dozen kilowatts. They can be divided into turbines with a horizontal axis of rotation (Horizontal Axis Wind Turbines) and turbines with a vertical axis (Vertical Axis Wind Turbines).The paper discusses design issues related to various types of wind turbines, and lays down general principles for the selection of support structures for small wind turbines (SWT) with a horizontal axis of rotation. The possibilities of using E-type spun prestressed-concrete poles used in medium voltage overhead power lines as SWT supporting structures are presented and discussed with regard to technical benefits of their application.

Stress Concentration Factors at Welded Connections Between Tubular Sections and a Transverse Plate

Abstract Welded connections between tubulars and transverse plates are used to build together part structures such as support of flare towers and wind turbines and deck structures on foundation structures. They are used in traditional jacket structures for oil and gas production and in support structures for wind turbines. These welded connections are typically made from the outside resulting in a weld root on the inside and a weld toe on the outside. Different S–N curves apply to these positions; therefore, stresses both on the inside and the outside are needed for fatigue assessment. From the actual design, it is noted that the thicknesses of the tubulars being connected can be different. Also, the diameters of the tubulars can be different. In addition, the fabrication is associated with some fabrication tolerances that provide local eccentricity moments to be transferred through these connections. In this paper, analytical expressions for stress concentration factors for these connections are presented based on classical shell theory. The stress concentration is dependent on the radial restraint from the transverse plate and the eccentricity of the neutral axes in the tubular thickness of one tubular relative to the other tubular. The superposition principle is used to derive resulting stress concentration factors for the inside weld root and the outside weld toe.

Calibration of safety factors for offshore wind turbine support structures using fully coupled simulations

The offshore wind industry experienced a boost during the last decade in terms of size of wind farms and rated capacity of the wind turbines: towers are getting taller and blades are getting longer, constantly facing new and complex challenges. Because of the relative immaturity of the wind industry, and the fact that the offshore design standards stemmed from the oil and gas industry, it is generally acknowledged that the reliability levels achieved, although not very well understood, might result in partial safety factors not optimal for OWT. This paper addresses this situation by studying the reliability levels delivered by the current standards and assessing the validity of the safety factors through a reliability-based code calibration. The combination of the low probability of failure imposed on the design of OWTs and the computational cost of the aero-elastic time-domain simulations brings out the need to develop new approaches for reliability analyses. In this paper, the reliability analysis is performed using a Kriging surrogate model to approximate the load-effect from the aero-elastic simulations converting expensive-to-evaluate limit state functions to explicit functions. Subsequently, a calibration of the safety factors is carried out using the probabilistic models from literature. The approach is applied to an industry-reference turbine and support structure. The results showed very low probabilities of failure for the most severe design cases and confirm that the safety factors from the IEC are mostly adequate.

Uncertainty modeling in reliability analysis of floating wind turbine support structures

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

Concept design of jacket foundations for offshore wind turbines in 10 steps

Simplified design procedures for offshore wind turbine (OWT) support structures are necessary in the concept and tender design stages for assessing the financial viability. Building upon the previous research on simplified monopile design, this paper presents a similar method to design jacket supported OWTs founded on piles or suction caissons. The method is based on the minimum and necessary data, namely the site characteristics (wind speed at reference height, wind turbulence intensity, water depth, wave height and wave period), turbine characteristics (rated power, rated wind speed, rotor diameter, cut-in and cut-out speed, mass of the rotor-nacelle-assembly) and ground profile (soil stiffness variation with depth and soil strength). A flowchart of the design process is also presented for visualisation of the rather complex multi-disciplinary analysis. Insights are given for 3 and 4 legged jackets. The calculation procedures that are required can be easily carried out either through a series of spreadsheets or simple hand calculations. An example problem emulating the design of a jacket on piles is taken from literature to demonstrate the proposed calculation procedure. The data used for the calculations are obtained from publicly available sources and the example shows that the simplified method arrives at a similar foundation to the one reported.

Design Optimization of Conical Concrete Support Structure for Offshore Wind Turbine

Various types of support structures for offshore wind turbine have been developed, and concrete structures have attracted attention due to many advantages. Although many studies have been conducted on the design of the existing steel structures, information and research on the design of concrete support structures are insufficient. Therefore, in this paper, a structural analysis model of conical concrete support structure (CCSS) is established and design optimization is presented. A detailed performance evaluation and the design of prestressed concrete were performed under the marine conditions of Phase 1 test site of southwest offshore wind project in Korea. The fluid–soil–structure interaction (FSI) was applied using the added mass method and soil spring model to represent the effects of water and soil. With the result of quasi-static analysis, a post-tensioning design was implemented by applying prestressing steel, and CCSS showed sufficient rigidity. From the natural frequency analysis, CCSS has a dynamic structural stability, and, in response spectrum and time-history analyses, the CCSS was safe enough under the earthquake loads. The methods and conclusions of this study can provide a theoretical reference for the structural analysis and design of concrete support structures for offshore wind turbines.

A developed failure mode and effect analysis for floating offshore wind turbine support structures

This paper extends the conventional failure mode and effect analysis methodology by introducing weights of its indices that are severity, occurrence, and detection as a basis to analyze the failures of the support structure of a generic floating offshore wind turbine. Critical failure causes, failure modes, as well as systems of the support structure of the floating offshore wind turbine are ascertained. Moreover, based on the analysis, recommendations on corrections and preventive actions are suggested aiming at ensuring the safe and economic operations of the support structure. The validation of the proposed technique is finalized by a comparison study between the results of conventional failure mode and effect analysis methodology and that of the proposed method. The comparison indicates that the proposed technique is more in line with practice and flexible for use and has the merit in removing the limitations of conventional failure mode and effect analysis methodology that different failure causes generate the same risk priority number.

Tower shadow excitation of a downwind rotor blade of a turbine with a tubular tower

The downwind configuration of wind turbines offers benefits regarding the blade-tower clearance, as during operation the blade primarily bends away from the tower. Consequently, the blades can be designed with lower stiffness. For tubular towers, however, a significant deficit of the wind speed in the tower wake occurs, resulting in fatigue-inducing vibrations. For this reason, full-height lattice towers are considered the preferred support structures for wind turbines with a downwind rotor. This work estimates the tower shadow excitation of a downwind rotor blade from a tubular tower. To this end, the blade of a commercial 6 MW downwind turbine is modelled with finite-elements. The tower wake is described on the basis of Madsen’s model and for the unsteady aerodynamic interaction Küssner’s function is adopted. At below- and above-rated wind conditions, the tower wake-induced vibrations are compared with the response of a blade of an equivalent upwind rotor, considering both the tip deflections and the root moments, the latter on the basis of damage-equivalent moments, to obtain an indication of the expected difference in fatigue damage. The downwind blade experiences vibrations with considerable larger amplitudes, especially in the out-of-plane direction. From the damage-equivalent moments it can be inferred that the blades of the downwind rotor encounter a much faster accumulation of fatigue damage.