Wind Turbine Support Structures Research Articles

Reliability-based design optimization of offshore wind turbine support structures using analytical sensitivities and factorized uncertainty modeling

Abstract. The need for cost-effective support structure designs for offshore wind turbines has led to continued interest in the development of design optimization methods. So far, almost no studies have considered the effect of uncertainty, and hence probabilistic constraints, on the support structure design optimization problem. In this work, we present a general methodology that implements recent developments in gradient-based design optimization, in particular the use of analytical gradients, within the context of reliability-based design optimization methods. Gradient-based optimization is typically more efficient and has more well-defined convergence properties than gradient-free methods, making this the preferred paradigm for reliability-based optimization where possible. By an assumed factorization of the uncertain response into a design-independent, probabilistic part and a design-dependent but completely deterministic part, it is possible to computationally decouple the reliability analysis from the design optimization. Furthermore, this decoupling makes no further assumption about the functional nature of the stochastic response, meaning that high-fidelity surrogate modeling through Gaussian process regression of the probabilistic part can be performed while using analytical gradient-based methods for the design optimization. We apply this methodology to several different cases based around a uniform cantilever beam and the OC3 Monopile and different loading and constraint scenarios. The results demonstrate the viability of the approach in terms of obtaining reliable, optimal support structure designs and furthermore show that in practice only a limited amount of additional computational effort is required compared to deterministic design optimization. While there are some limitations in the applied cases, and some further refinement might be necessary for applications to high-fidelity design scenarios, the demonstrated capabilities of the proposed methodology show that efficient reliability-based optimization for offshore wind turbine support structures is feasible.

Prediction of combined aerodynamic and seismic loadings on wind turbine support structures by coupled and uncoupled approaches

Prediction of combined aerodynamic and seismic loadings on wind turbine support structures by coupled and uncoupled approaches

Load Prediction of Support Structure of a 2MW Downwind Floating Offshore Wind Turbine

Load Prediction of Support Structure of a 2MW Downwind Floating Offshore Wind Turbine

Study on effect of lumped rotor-nacelle mass in seismic response model of large wind turbine support structures

Study on effect of lumped rotor-nacelle mass in seismic response model of large wind turbine support structures

Finite Element Modelling and Parametric Studies of Semi-Closed Thin-Walled Steel Polygonal Columns For The Application on Steel Lattice Towers

The trend of structural engineering in the recent years is toward the use of lighter and more economical structural elements. In steel construction, peculiarly, main structural member composed by thin-walled elements are being explored by researchers due to their potential to offer better solution with economical features. However, the use of slender profiles and a complex cross sections shape lead to requirements to study instability phenomenon in a form of local, distortional, flexural, torsional and coupled instability. Such complex structural behaviour is inevitably accompanied by demand to improve calculation methods and design provisions. In this context an innovative solution of structural element composed of thin-walled plates is proposed for the application on lattice support structure of wind turbine. A semi-closed section is made by assembling series of folded plates into polygonal profiles with mechanical fasteners, loaded in compression and bending moment which occurs as the effect of forces acting on the connection. The expected structural behaviour of the column is a mixture between the open and closed cross-section. These cases will be investigated through numerical analysis and parametric studies of the proposed profiles for the investigation of buckling behaviour and ultimate resistance, respectively.

Stability Analysis of the Floating Offshore Wind Turbine Support Structure of Cell Spar Type during its Installation

Abstract The article presents the results of selected works related to the wider subject of the research conducted at the Faculty of Ocean Engineering and Ship Technology of the Gdansk University of Technology, which concerns design and technology of construction, towing, and settlement on the seabed, or anchoring, of supporting structures for offshore wind farms. As a result of this research, several designs of this type of objects were developed, including two stationary types: gravitational and Jack-up, which are placed on the seabed, and two floating types: TLP and SPAR, anchored with tendons and anchors in the form of nailed or suction piles. Below presented is the stability analysis of the new floating CELL SPAR type support structure for offshore wind turbines during its installation in waters with a depth of over 65 m.

Multi-hazard fragility analysis for a wind turbine support structure: An application to the Southwest of Mexico

The scenario of growing demand for construction of wind turbine structures in Mexico poses the need of analyzing the hazards that affect this kind of structures in that country. Current Mexican standards include wind turbine support structures in their earthquake design chapter, but no information is included regarding the wind design. This work focuses on the analysis of an onshore 5 MW wind turbine structure, similar to those currently installed in Mexico, under the simultaneous action of wind and earthquake for different intensities. The analyses performed consider the rotor in both: parked and in operation conditions. Wind speed records are obtained from simulations based on an Auto Regressive and Moving Average model and the Veers’ method, while the ground motion records are simulated to be consistent with design spectra for a zone in the Southwest of Mexico, near the coast of the Pacific Ocean, which happens to be the region with the greatest installed wind capacity in Mexico and also a region of strong seismic hazard. The numerical results obtained are employed in a multi-hazard fragility analysis, which shows that the critical action for the structure depends on the operational state of the turbine.

Signal-segments cross-coherence method for nonlinear structural damage detection using free-vibration signals

A damage detection process can be significantly enhanced if the nonlinear effects can be used when extracting damage-sensitive features from measured signals. The coherence function is typically used for nonlinearity identification by determining the extent of the output power linearly correlated with the input. However, the excitations are usually difficult to measure in actual tests. To overcome this limit, this article presents a signal-segments cross-coherence method for nonlinearity identification. By defining a signal-segments cross-coherence matrix and signal-segments cross-coherence index, the method can visually and quantitatively indicate the presence of nonlinearity. The innovation of the new method is that the coherence analysis process only depends on a single output signal, where input and baseline signals are not required. Then, a novel structural damage localization index is constructed by multi-point comparing of the signal-segments cross-coherence indices, based on the assumption that all the measured signals from different points on the structure have the same frequency bandwidth and components. To meet this requirement, a newly proposed signal decomposition method called analytic mode decomposition method is adopted. Numerical studies on a duffing oscillator and a 10 degree-of-freedom spring-damping-mass system were performed to demonstrate the nonlinear identification process and investigate the effectiveness and robustness of the signal-segments cross-coherence-based damage detection method. The results show that the signal-segments cross-coherence method can effectively indicate the appearance of nonlinearity by the signal-segments cross-coherence matrix and signal-segments cross-coherence index with strong noise robustness. And the proposed damage localization index can accurately detect the weak nonlinear damage even with severely noise-polluted signals. To further investigate the applicability of the new method, an experimental study was conducted on a steel simplified scale model of a monopile offshore wind turbine support structure. The results demonstrate that the proposed signal-segments cross-coherence method and the new damage localization index can be used to detect the bolt-loosening damage of the steel structure only with output signals.

Parallel Analysis of Offshore Wind Turbine Structures under Ultimate Loads

This paper investigates efficient design of offshore wind turbine (OWT) support structures under ultimate loads and proposes three schemes to overcome excessive computer time due to many required external loads. The first is the assumption of a rigid support structure to find blade wind forces, so that these forces are only dependent on wind profiles, which limits different cases in the structural analyses. Since the blade information is often confidential in turbine companies, this two-stage analysis allows the hub force to be the input data for the support structure design. The second is using a few control loads to perform the steel design between the second and the second-last design cycles. The third is using parallel computational procedures, since all loading cases can be independently executed in different CPU cores and computers. The test cases, with 5044 loading cases, indicate that the proposed method is fully parallel and can complete the design procedures using a few personal computers within several days. Test cases include IEC 61400-3, tropical cyclone, and seismic loads; although there are many loads to be considered, steel design is governed by a limited number of load cases, which are discussed in this paper.

Expected Life Evaluation of Offshore Wind Turbine Support Structure under Variable Ocean Environment

Because offshore structures are affected by various environmental loads, the risk of damage is high. As a result of ever-changing ocean environmental loads, damage to offshore structures is expected to differ from year to year. However, in previous studies, it was assumed that a relatively short period of load acts repeatedly during the design life of a structure. In this study, the residual life of an offshore wind turbine support structure was evaluated in consideration of the timing uncertainty of the ocean environmental load. Sampling points for the wind velocity, wave height, and wave period were generated using a central composites design, and a transfer function was constructed from the numerical analysis results. A simulation was performed using the joint probability model of ocean environmental loads. The stress time history was calculated by entering the load samples generated by the simulation into the transfer function. The damage to the structure was calculated using the rain-flow counting method, Goodman equation, Miner’s rule, and S-N curve. The results confirmed that the wind speed generated at a specific time could not represent the wind speed that could occur during the design life of the structure.

Hysteretic behavior of grouted connections in offshore wind turbine support structures

Grouted connections (GCs) with shear keys have been widely applied in various offshore wind turbine support structures. The urgent energy demand in Eastern China makes building wind farms in offshore earthquake areas vital, however, the hysteretic behavior of the GCs has not been studied in depth. In this paper, three GC specimens were tested under different constant axial loads and cyclical horizontal forces. The different axial loads, i.e. the axial compression ratios were considered as the control parameter. The displacement ductility, energy dissipation capacity, and stiffness degradations of the specimens are discussed in detail. The results showed that an increasing axial compression ratio has detrimental effects on the hysteretic performance of GCs. It was also found, however, that all GC specimens exhibited favorable ductility and energy dissipation capacity, even the ones with high axial compression ratios. This indicates that GCs with shear keys are applicable for offshore support structures in seismic regions. Furthermore, finite element analyses in ABAQUS were conducted and compared with the experimental results. It was found that the numerical models could predict the hysteretic performance and crack propagation process of GCs. These models could be implemented in further parameter studies.

Wind-wave directional effects on fatigue of bottom-fixed offshore wind turbine

The current trend for offshore wind energy is that larger turbines are placed on monopile foundations at increasing water depth. This requires larger foundations, increasing the importance of hydrodynamic loading. It is well established that wave loads perpendicular to the wind direction are important for the fatigue damage in monopile foundations. However, this is normally only taken into account considering wind-wave misalignment. In this paper, the effect of assuming short-crested waves in design calculations is considered. The lifetime fatigue damage may increase significantly for hydrodynamically sensitive support structures when modelling the waves as short-crested rather than long-crested. For the turbines in this paper, the fatigue damage increased with up to 80 %. At the same time, the changes in fatigue damage were small for support structures that are less hydrodynamically sensitive. The work performed in this paper shows that the typical design assumption of long-crested waves may be both conservative and non-conservative. This fact is important to be aware of when designing support structures for offshore wind turbines.

Stochastic load effect characterization of floating wind turbine support structures

Achieving substantial reductions in Levelized Cost of Energy (LCOE) of floating wind turbines (FWTs) requires robust reliability assessment that accounts for inherent design uncertainties. A key aspect of such reliability assessment is the definition of limit states. In this regard, load effects need to be evaluated accurately. This paper presents a computational framework for evaluating load effects on FWT support structures. The computed load effect is subsequently characterized. A high fidelity finite element model of the National Renewable Energy Laboratory (NREL) 5MW reference turbine mounted on the OC3-Hywind spar buoy was developed and validated for this purpose. The loads from fully coupled time domain aero-hydro-servo-elastic simulations are transferred for detailed finite element (FE) load effect computation in Abaqus. Matlab® and Python are used as the computational tools for automating the whole analysis from start to finish. The initial part of this study addresses the amount of run-in-time to be excluded from response statistics. Based on convergence studies carried out, recommendations are made for run-in-time to be excluded from response statistics. The maximum von Mises stress in the tower as a measure of yielding is the load effect investigated in this study.

Strength Assessment of Jacket Offshore Wind Turbine Support Structure Accounting for Rupture1

The objective of the present work is to carry out the strength assessment of jacket offshore wind turbine support structures subjected to progressive rupture. A defect existing in a structure made during the fabrication may turn into a small-scale rupture and because of the high-stress concentration and low-cycle fatigue load. Therefore, the ultimate load-carrying capacity of the support structure is analyzed accounting for the progress of the rupture until the leg component experiences a full rupture along its circumference. The effect of imperfection severity is also investigated. The moment–curvature relationship of the structure concerning the studied cases is presented. Furthermore, the jacket support structures, at different water depths, are also analyzed and discussed. Finally, some of the leg components are removed one by one to study the redundancy of the jacket support structure at 80-m water depth.

Load case reduction for offshore wind turbine support structure fatigue assessment by importance sampling with two‐stage filtering

AbstractA complete fatigue assessment for operational conditions for offshore wind turbines involves simulating thousands of environmental states. For applications such as optimization, where this assessment needs to be repeated many times, that presents a significant computational problem. Here, we propose a novel way of reducing the number of simulated environmental states (load cases) while maintaining an acceptable accuracy. From one full fatigue analysis of a base design, the OC3 monopile (with the NREL 5MW turbine), the distribution of fatigue damage per load case can be used to estimate the lifetime fatigue damage of a range of modified designs. Using importance sampling and a specially adapted two‐stage filtering procedure, we obtain pseudo‐optimal sets of load cases from which the fatigue damage is estimated. This is applied to seven different designs that have been modified to emulate iterations of an optimization loop. For several of these designs, sampling less than 1% of all load cases can give damage estimates with median errors of less than 2%. Even for the most severe cases, using 3% of the environmental states yields a maximum error of 10%. While further refinement is possible, the method is considered viable for applications within design optimization and preliminary design.

Analyses of offshore wind turbine structures with soil-structure interaction under earthquakes

Abstract This research provides an analysis framework for offshore wind turbine (OWT) support structures subjected to seismic, wind, and wave loads using the finite element method with soil-structure interaction, and a conservative soil liquefaction analysis is developed. The NREL 5-MW jacket-type OWT under IEC 61400-3 is analyzed. The results indicate that seismic loads combined with wind and wave loads during power production often control the design, especially near the rated wind speed. For earthquakes with a peak ground acceleration (PGA) smaller than 0.32 g and the dominant period (Ts) smaller than 1 s, the increase in steel weight due to seismic loads is small, because the amplification of the seismic loads is not obvious. For PGA over 0.52 g, almost all the members are controlled by the seismic loads, and the increase in the steel design weight can be over 40%. This paper also indicates that first-mode tuned mass dampers for jacket-type support structure with deep piles are efficient to reduce the vibration coupled from wind, wave, and seismic loads even with 20-m soil liquefaction.

Risk-based life-cycle assessment of offshore wind turbine support structures accounting for economic constraints

The objective of the present study is to perform a risk-based life-cycle assessment of monopile offshore wind turbine support structures. The design of the structure is based on the optimised levelised cost of energy so that the offshore wind energy can compete with the other energy resources. Both optimal design solutions for a monopile support structure and the optimal inspection planning are addressed here. The structural assessment of the monopile support structures is carried out using the finite element method accounting for the pile-soil interactions. The risk-based assessment is performed for the design solutions aiming to achieve the minimum of the levelised cost of energy and the total expected cost conditional to the implemented inspection strategy and the structural risk. Furthermore, the optimal inspection interval is investigated. Finally, the sensitivity of the parameters influencing the risk life-cycle assessment is also studied. The present study analyses a multi-dimensional framework, which integrates structural configuration of the support structure, manufacturing, detailed structural risk assessment, inspection and repair planning, and economic conditions into a coupled dynamic system accounting for the feedback loops resulted from the introduction of the probabilistic structural state to the life cost expenditure such as CAPEX, OPEX, and DECEX.

Tripod-Supported Offshore Wind Turbines: Modal and Coupled Analysis and a Parametric Study Using X-SEA and FAST

This paper presents theoretical aspects and an extensive numerical study of the coupled analysis of tripod support structures for offshore wind turbines (OWTs) by using X-SEA and FAST v8 programs. In a number of site conditions such as extreme and longer period waves, fast installation, and lighter foundations, tripod structures are more advantageous than monopile and jacket structures. In the implemented dynamic coupled analysis, the sub-structural module in FAST was replaced by the X-SEA offshore substructure analysis component. The time-histories of the reaction forces and the turbine loads were then calculated. The results obtained from X-SEA and from FAST were in good agreement. The pile-soil-structure interaction (PSSI) was included for reliable evaluation of OWT structural systems. The superelement concept was introduced to reduce the computational time. Modal, coupled and uncoupled analyses of the NREL 5MW OWT-tripod support structure including PSSI were carried out and the discussions on the natural frequencies, mode shapes and resulted displacements are presented. Compared to the uncoupled models, the physical interaction between the tower and the support structure in the coupled models resulted in smaller responses. Compared to the fixed support structures, i.e., when PSSI is not included, the piled-support structure has lower natural frequencies and larger responses attributed to its actual flexibility. The models using pile superelements are computationally efficient and give results that are identical to the common finite element models.

Prediction of remaining fatigue life of welded joints in wind turbine support structures considering strain measurement and a joint distribution of oceanographic data

Reassessing the remaining fatigue life of the wind turbine support structures becomes more and more crucial for operation, maintenance, and life extension when they are reaching the end of their design service life. By using measured oceanographic and strain data, each year, remaining fatigue life can be updated to adapt the operation to real loading conditions. Previous works have not put attention to address the complexity of offshore loading combinations and as-constructed state of the structure in estimating structural responses for fatigue behaviour to stochastically predict the remaining fatigue life. The present paper links the oceanographic data to fatigue damage by using measured strain, and uses the Bayesian approach to update the joint distribution of the oceanographic data. Consequently, the failure probability of the support structure can be updated and so the predicted fatigue life. The year-to-year variation of the 10-min mean wind speed, the unrepresentativeness of measured strain, the measurement uncertainty, and corrosion are considered together with uncertainties in Miner′s rule and S–N curves. The present research shows that the real oceanographic data can be used to adjust the predicted remaining fatigue life and eventually give decision support for the wind turbine operation.

Optimization of composite material tower for offshore wind turbine structures

The focus of this study was to investigate the application of lightweight fiber reinforced composite materials in the construction of offshore wind turbine support structures. A composite tower design suitable for the NREL 5 MW reference wind turbine is presented. The design is based on the most automated and low cost composite manufacturing methods (pultrusion and filament winding) and the conclusions of this study may not be applicable for offshore structures using different composite material construction techniques. The mass of the tower was minimized using gradient based optimization approach. The cost of a composite tower was calculated and levelized cost of energy (LCOE) projections are discussed in comparison with the existing steel tower cost. The study determined that while the composite tower is technically feasible and has a lower mass than a comparable steel tower, uncertainty remains in how it compares economically in terms of LCOE.