Lifetime Fatigue Research Articles

Lifetime fatigue cracking behavior of weld defects in orthotropic steel bridge decks: Numerical and experimental study

Orthotropic steel decks (OSDs) are welded structures prone to weld defects, increasing the risk of fatigue fracture. It is crucial to clarify the fatigue evolution mechanism of weld defects throughout the entire lifecycle. In this study, the fatigue propagation behavior of weld defects is investigated from the hidden stage to final fracture. Firstly, a full-scale segmental OSD model with prefabricated defects was utilized to investigated fatigue propagation behavior of surface cracks. Secondly, stress intensity factors (SIFs) of welding defects throughout the entire process were simulated using a refined finite element model. Finally, the crack shape evolution and fatigue life were analyzed at different fatigue stages. The research identifies three stages in the fatigue evolution of hidden defects: the hidden defect stage, the initiation stage of surface crack, and the propagation stage of surface crack. Regardless of the initial aspect ratios, hidden defects evolve into a circular, while surface defects evolve into a flattened semi-ellipse. Compared to the propagation life of surface cracks, the hiding life of cracks account for a significantly larger proportion of the total life, and this proportion increases with both the initial aspect ratio and the hidden depth of the defects.

Application of deep learning for structural health monitoring of a composite overwrapped pressure vessel undergoing cyclic loading

Structural Health Monitoring (SHM) using ultrasonic-guided waves (UGWs) enables continuous monitoring of components with complex geometries and provides extensive information about their structural integrity and their overall condition. Composite Overwrapped Pressure Vessels (COPVs) used for storing hydrogen gases at very high pressures are an example of a critical infrastructure that could benefit significantly from SHM. This can be used to increase the periodic inspection intervals, ensure safe operating conditions by early detection of anomalies, and ultimately estimate the remaining lifetime of COPVs. Therefore, in the Digital Quality Infrastructure Initiative (QI-Digital) in Germany, an SHM system is being developed for COPVs used in a hydrogen refueling station. In this study, the results of a lifetime fatigue test on a Type IV COPV subjected to many thousands of load cycles under different temperatures and pressures are presented to demonstrate the strengths and challenges associated with such an SHM system. During the cyclic testing up to the final material failure of the COPV, a sensor network of fifteen surface-mounted piezoelectric (PZT) wafers was used to collect the UGW data. However, the pressure variations, the aging process of the COPV, the environmental parameters, and possible damages simultaneously have an impact on the recorded signals. This issue and the lack of labeled data make signal processing and analysis even more demanding. Thus, in this study, semi-supervised, and unsupervised deep learning approaches are utilized to separate the influence of different variables on the UGW data with the final aim of detecting and localizing the damage before critical failure.

Quantifying the impact of modeling fidelity on different substructure concepts – Part 2: Code-to-code comparison in realistic environmental conditions

Abstract. Floating offshore wind is widely considered to be a promising technology to harvest renewable energy in deep ocean waters and increase clean energy generation offshore. While evolving quickly from a technological point of view, floating offshore wind turbines (FOWTs) are challenging, as their performance and loads are governed by complex dynamics that are a result of the coupled influence of wind, waves, and currents on the structures. Many open challenges therefore still exist, especially from a modeling perspective. This study contributes to the understanding of the impact of modeling differences on FOWT loads by comparing three FOWT simulation codes, QBlade-Ocean, OpenFAST, and DeepLines Wind®, and three substructure designs, a semi-submersible, a spar buoy, and the two-part concept Hexafloat, in realistic environmental conditions. This extensive comparison represents one of the main outcomes of the Horizon 2020 project FLOATECH. In accordance with international standards for FOWT certification, multiple design situations are compared, including operation in normal power production and parked conditions. Results show that the compared codes agree well in the prediction of the system dynamics, regardless of the fidelity of the underlying modeling theories. However, some differences between the codes emerged in the analysis of fatigue loads, where, contrary to extreme loads, specific trends can be noted. With respect to QBlade-Ocean, OpenFAST was found to overestimate lifetime damage equivalent loads by up to 14 %. DeepLines Wind®, on the other hand, underestimated lifetime fatigue loads by up to 13.5 %. However, regardless of the model and FOWT design, differences in fatigue loads are larger for tower base loads than for blade root loads due to the larger influence substructure dynamics have on these loads.

Reference monopile designs for US East Coast sites supporting the IEA 15 MW reference turbine using a novel conceptual design methodology

A novel conceptual design methodology was developed and implemented to design twelve reference monopile foundations, compatible with the IEA 15 MW reference turbine. The designs are position-specific, corresponding to planned, US East Coast, wind energy development areas. The monopiles were designed considering the fatigue limit state (FLS), ultimate limit state (ULS), serviceability limit state (SLS), and natural frequency analysis (NFA), following an intentionally reduced subset of design load cases (DLCs) in IEC 61400-3-1. In 11 of 12 cases, the diameter and wall thicknesses of the monopile foundations were found to be determined by FLS criteria. The exception was one position subjected to a combination of breaking waves and a large significant wave height. For the 11 positions governed entirely by FLS, the maximum ULS utilization ratios varied between 0.63 and 0.81, indicating significant reserve capacity for the ultimate limit state. Fatigue damage during turbine idling was found to be an important contributor to lifetime fatigue damage, despite having a relatively small probability of occurrence. The novel design methodology described herein includes a preliminary FLS assessment step, which was found to be an efficient way to select an initial foundation geometry, while providing a 92% improvement in computational efficiency.

Assessing lidar-assisted feedforward and multivariable feedback controls for large floating wind turbines

Abstract. We assess the performance of two control strategies on the IEA 15 MW reference floating wind turbine through OpenFAST simulations. The multivariable feedback (MVFB) control tuned by the toolbox of the Reference OpenSource Controller (ROSCO) is considered to be a benchmark for comparison. We then tune the feedback gains for the multivariable control, considering two cases: with and without lidar-assisted feedforward control. The tuning process is performed using OpenFAST simulations, considering realistic offshore turbulence spectral parameters. We reveal that optimally tuned controls are robust to changes in turbulence parameters caused by atmospheric stability variations. The two optimally tuned control strategies are then assessed using the design load case 1.2 specified by the IEC 61400 standard. Compared with the baseline multivariable feedback control, the one with optimal tuning significantly reduced the tower damage equivalent load, leading to a lifetime extension of 19.7 years with the assumption that the lifetime fatigue is only caused by the design load case 1.2. With the assistance of feedforward control realized using a typical four-beam lidar, compared with the optimally tuned MVFB control, the lifetime of the tower can be further extended by 4.6 years.

Numerical investigation of wake-induced lifetime fatigue load of two floating wind turbines in tandem with different spacings

Floating wind turbine (FWT) is a promising technology to utilize offshore wind resources. For offshore wind farms, wakes can result in 5%–15% increase in fatigue load on the downstream turbine rotor. Current offshore wind turbine design standard lacks accounting for the different wake characteristics and wake-induced fatigue load of the FWT from that of their fixed counterparts. Considering a scenario of two tandem FWTs, this study investigates the wake characteristics of a semi-submersible FWT accounting for multi-degree-of-freedom (DOF) dynamic motion characteristics and its wake effect on fatigue load of downstream unit with different spacing configurations. A comparison with the effective turbulence intensity method suggested in the current IEC 61400-1 standard is also carried out. With an interactive analysis framework composed of computational fluid dynamics (CFD) wake simulation and time domain aero-hydro-servo-elastic simulation for FWT, lifetime wake-induced fatigue loads are analyzed through a full range of turbine normal operational conditions. Results show that the wake characteristics of the FWT show a distinct difference between below-rated and above-rated conditions due to the feature of FWT's controller. Moreover, the Frandsen model adopted in the current IEC 61400-1 standard highly underestimates the FWT's wake turbulence, causing the predicted lifetime fatigue damage equivalent load at the tower base decreases up to about 10∼15% for a spacing of 8D∼10D under combined wind and wave conditions.

AK-MDAmax: Maximum fatigue damage assessment of wind turbine towers considering multi-location with an active learning approach

Lifetime fatigue damage prediction plays a key factor in wind turbine structure’s reliability assessment. However, the damage estimation of wind turbines requires thousands of simulations and significant computational costs. To address this problem, this paper proposes an efficient active learning Kriging named AK-MDAmax for estimating the maximum fatigue damage of wind turbine towers with less computational cost. The proposed AK-MDAmax approach is based on the previous AK-DA approach. Kriging models are used to estimate the fatigue damage of wind turbine towers at different wind-wave conditions. An efficient active learning approach is developed to assess multi-location maximum cumulative fatigue damage. One 15MW Semi-submersible floating wind turbine model from the IEA project is used to demonstrate the efficiency of the proposed approach. Results indicate the proposed approach can efficiently and accurately estimate wind turbine towers’ maximum cumulative fatigue damage. The AK-MDAmax approach requires less than 3% of the computational effort compared with the typical simulation approach, and the related absolute error is less than 1%. The AK-MDAmax approach could be useful for designers to optimize wind turbine structures and reduce design time and costs.

Description of an HAWT hub enabling teetering about two out‐of‐plane axes with modeling showing significant reductions in cyclic behavior and fatigue due to turbulence and wind shear

AbstractA three‐bladed, dual‐axis teetering hub (3T) has been developed and compared to a three‐bladed rigid hub (3R) using computer modeling. Both hubs were fitted to a hypothetical 5‐MW offshore wind turbine mounted on an offshore floating platform. Cyclic analysis showed the 3T hub significantly reduced the impact of wind shear on the blades and virtually eliminated the impact of both turbulence and wind shear on the main shaft bearing and yaw bearing. Lifetime fatigue studies for the two hubs were performed using a rainflow‐counting program at various levels of turbulence and wind shear. Studies showed the significant reductions in cyclic variation provided by the 3T hub resulted in significant reductions in lifetime fatigue in comparison to the 3R hub. Simulations with the 3T hub showed an increase in turbulence caused a very small increase in blade fatigue but no increase in fatigue for the main shaft bearing and yaw bearing. In contrast, fatigue studies with the 3R hub showed an increase in turbulence caused significant damage to the blades and a combined increase in turbulence and wind shear caused extremely significant damage to the blades. This paper introduces a recently patented hub design that is superior to the ball‐and‐socket design described in a prior paper by this author.

Investigation of the effect of structural damping on wind turbine wind-induced fatigue loads

This paper presents a quantitative investigation of the effect of structural damping ratios of the tower and blade vibration modes of wind turbines on wind-induced fatigue loads at critical blade and tower sections. Fatigue loads are used as a relative measure of fatigue life in wind turbine design applications. By increasing the structural damping ratio of each one of the critical structural modes which highly contribute to the overall vibration response, short-term and lifetime fatigue loads at critical tower and blade sections are calculated considering parked and operating conditions for different mean wind speeds as well as lifetime operational conditions.

Multiaxial fatigue assessment of floating offshore wind turbine blades operating on compliant floating platforms

Fatigue damage is assessed for blades of floating offshore wind turbines operating on three different hull configurations using two different strain-based methods. Lifetime fatigue damage is predicted by summing damage over a range of wind-wave conditions conforming to a long-term Weibull distribution. The three hull configurations are selected to cover a wide range of rotational stiffness. Nonlinear beam theory is applied to compute the 3-D beam displacements along the turbine blades. Time histories of the resulting beam displacements are converted to maximum principal strains and to normal strains, and each resulting strain is used to compute the fatigue damage to the blade shells. Results show that the two strain-based methods yield different predictions of fatigue lives and locations of fatigue failures. The normal strain method significantly underpredicts fatigue damage compared to the maximum principal strain method. Fatigue of blades on floaters with very low rotational stiffness is found to be worse than on those with moderate rotational stiffness.

Lifetime fatigue response due to wake steering on a pair of utility-scale wind turbines

Quantifying the impacts on turbine loads during wind farm control is an important consideration in assessing power production benefits. Wake steering controls aim to improve total wind farm performance by coordinating the control actions of individual turbines, wherein an upstream turbine is intentionally yawed at an offset angle from the measured wind direction. Consequently, this redirects its wake for improved power production and potentially reduces fatigue loads of the downstream turbines. This paper studies the lifetime fatigue loads associated with wake steering by using utility-scale wind turbine experimental data to conduct an analysis on a pair of wind turbines. This study was part of a large field experiment in which a group of five GE 1.5-MW SLE CWE turbines were selected as targets for conducting wake steering research. A standard loads instrumentation package and data acquisition system were installed on two turbines within the cluster to measure turbine fundamental loads. The time-series databases were used to calculate loads statistics as well as short-term and lifetime damage equivalent loads. Fatigue calculations followed the guidance in Annex H of the International Electrotechnical Commission standard 61400-1, Edition 4. Lifetime fatigue calculation results are presented in this analysis; three methods of assessing lifetime fatigue were used to determine percent difference for blade root moments, main shaft moments, main shaft torque, and tower top torque. For all three fatigue treatments, some components’ lifetime fatigue increases for the controlled turbine; however, the downwind turbine experienced a reduction in lifetime fatigue and combined effect for the turbine pair results in a reduction of fatigue when wake steering controls are applied.

Reliability-Based Lifetime Fatigue Damage Assessment of Offshore Composite Wind Turbine Blades

This paper presents a method for stochastic deterioration modeling and fatigue damage assessment for composite wind turbine blades operating in offshore environments. The fatigue damage of the composite blades is analyzed and assessed based on the estimates for the applied loads along the blade span, stress analysis, fatigue crack evolution, and lifetime probability of fatigue failure. The complex stress states of the blade are mainly caused by the aerodynamic loads generated by corrected blade element momentum theory, gravity loads, and centrifugal loads. The fatigue of the wind turbine blade is then investigated on the basis of the actual fatigue damage propagation process. The stochastic gamma process is introduced to calculate the probability of fatigue failure of the blade for various critical limits, and these results together with lifecycle cost analysis are employed to determine the optimum maintenance strategy. Finally, a numerical example for a National Renewable Energy Laboratory 5-MW wind turbine blade is adopted to demonstrate the applicability of the proposed method. The numerical results show that the proposed approach can provide a reliable tool for estimating stress states, evaluating fatigue damage, analyzing lifetime fatigue failure probability, and optimizing repair time of the composite wind turbine blade.

Analysis of the influence of climate change on the fatigue lifetime of offshore wind turbines using imprecise probabilities

AbstractWhen discussing the connection of wind energy and climate change, normally, the potential of wind energy to reduce green house gas emissions is emphasised. Hence, effects of wind energy on climate change are analysed. However, what about the other direction? What is the impact of climate change on wind energy? Recently, the effect of a reversal in global terrestrial stilling,that is, an increase in global wind speeds in the last decade, on the wind energy production has been analysed. Certainly, knowledge about potential changes in energy production is essential to plan future energy supply. Nonetheless, at least similarly important is the effect on loads acting on wind turbines. Increasing loads due to higher wind speeds might reduce wind turbine lifetimes and yield higher costs. Moreover, especially for already existing turbines, it might even affect the structural reliability. Since the impact of climate change on wind turbine loads is largely unknown, it is studied in this work in more detail. For this purpose, different existing models for predicted changes in wind speed and air temperature and their uncertainties are used to forecast the environmental conditions an exemplary offshore wind turbine is exposed to. Subsequently, for this turbine, the lifetime fatigue damages are calculated for different prediction models. It is shown that the expected changes in lifetime fatigue damages are present but relatively small compared to other uncertainties in the fatigue damage calculation.

The effect of minimum thrust coefficient control strategy on power output and loads of a wind farm

The use of down-regulation control strategies for wind turbines potentially optimises the wind farm performance. One of the promising strategies to minimise the wind farm loads and maximise the energy production is by down-regulating the upstream turbine with minimum thrust coefficient (Ct ). Nonetheless, a majority of minimum Ct control studies is confined to fatigue load analysis of a single turbine. Therefore, this paper focuses on the investigation of the fatigue loads when operating turbines in a wind farm using a minimum Ct control strategy, as well as extreme loads and energy production. The control strategy is implemented in the DTU Wind Energy controller coupled with the aero-elastic code, HAWC2. The Dynamic Wake Meandering model in HAWC2 is used so that the wake deficits generated from the upstream turbines realistically represent the minimum Ct control strategy, as well as the interactions between wakes. The main contribution of this paper is to investigate the possible wind farm operating strategy using minimum Ct control that leads to decrease the lifetime fatigue loads and increase the overall energy production of a wind farm due to the potential extended lifetime of certain wind turbine components.

Validation of a lookup-table approach to modeling turbine fatigue loads in wind farms under active wake control

Abstract. Wake redirection is an active wake control (AWC) concept that is known to have a high potential for increasing the overall power production of wind farms. Being based on operating the turbines with intentional yaw misalignment to steer wakes away from downstream turbines, this control strategy requires careful attention to the load implications. However, the computational effort required to perform an exhaustive analysis of the site-specific loads on each turbine in a wind farm is unacceptably high due to the huge number of aeroelastic simulations required to cover all possible inflow and yaw conditions. To reduce this complexity, a practical load modeling approach is based on “gridding”, i.e., performing simulations only for a subset of the range of environmental and operational conditions that can occur. Based on these simulations, a multi-dimensional lookup table (LUT) can be constructed containing the fatigue and extreme loads on all components of interest. Using interpolation, the loads on each turbine in the farm can the be predicted for the whole range of expected conditions. Recent studies using this approach indicate that wake redirection can increase the overall power production of the wind farm and at the same time decrease the lifetime fatigue loads on the main components of the individual turbines. As the present level of risk perception related to operation with large yaw misalignment is still substantial, it is essential to increase the confidence level in this LUT-based load modeling approach to further derisk the wake redirection strategy. To this end, this paper presents the results of a series of studies focused on the validation of different aspects of the LUT load modeling approach. These studies are based on detailed aeroelastic simulations, two wind tunnel tests, and a full-scale field test. The results indicate that the LUT approach is a computationally efficient methodology for assessing the farm loads under AWC, which achieves generally good prediction of the load trends.

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.

An integrated probabilistic approach for optimum maintenance of fatigue-critical structural components

Inspection and maintenance are important means to validate or recover reliability of metallic structural systems, which usually degrade over time due to fatigue, corrosion and other mechanisms. These inspection and maintenance actions generally account for a large part of lifetime costs, which necessitate an efficient maintenance strategy to satisfy the requirements on reliability and costs. Most often, an optimum maintenance/repair crack size criterion is derived by probabilistic cost-benefit analysis, e.g. by minimization of expected lifetime costs, which are assessed based on cost models. This paper proposes an integrated approach to derive an optimum range for repair (crack size) criterion using both reliability-based and cost-based optimization. It is found that an optimum repair criterion exists which leads to the maximum lifetime fatigue reliability. A smaller repair criterion than the reliability-optimum do not lead to a higher lifetime fatigue reliability but leads to higher lifetime costs. Hence, a limit for repair criterion is defined by the reliability-optimum criterion, which can be obtained without cost models. The reliability-optimum criterion is found to be smaller than the cost-optimum criterion and thus an optimum range between the reliability-optimum and cost-optimum criterion is established.

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.

Development of new endovascular stent-graft system for type B thoracic aortic dissection with finite element analysis and experimental verification

Endovascular repair of the thoracic aorta with self-expanding stent-grafts has been emerging as a less invasive alternative treatment compared with conventional open surgeries. Despite the promising efficacy and safety of endovascular stent grafting, the stent-graft failure remains a major concern in terms of stent migration, device fatigue, and the risk of endoleaks. Challenges associated with the stent-grafts involve optimized geometrical structure, lifetime fatigue resistance, and adequate radial support. In this work, a novel endovascular stent-graft system is developed specially for the treatment of Stanford type B thoracic aortic dissections (TAD). Numerical study with finite element analysis (FEA) was utilized to evaluate the mechanical behaviors of the individual stent component. Results of the simulation were validated by experimental tests. Based on the systematic analysis of the parametric variations, a final stent-graft system was developed by the selection and arrangement of the individual stent components, targeting an optimal performance for treatment of TAD. The optimized solution of the stent-graft system was tested in clinical trials, showing advantageous therapeutic efficacy.

Lifetime extension of waked wind farms using active power control

This paper studies a new active power control (APC) of waked wind farms in order to extend the lifetime of highly loaded wind turbines. We demonstrate that the structural fatigue loading of a single turbine can be significantly alleviated, while the wind farm power production follows a power reference signal. Then, an optimization problem, subjected to a data-driven fatigue load model, is formulated to balance the lifetime fatigue loading of the wind turbines operating within a waked wind farm. A Game-Theoretic (GT) approach is employed to find a lifetime fraction, wherein a wind turbine should actively reject its own dynamic loadings due to turbulence. A large-eddy simulation model is employed for resolving the turbulent flow, the wake structures and its interaction with the atmospheric boundary layer. The applicability and key features of the controller are discussed with a wind farm example consisting of 2×2 turbines. The overall increase of wind farm lifetime is evaluated using the damage equivalent load (DEL) of the tower base fore-aft bending moment of the individual wind turbines.