Wind Turbine Support Structures Research Articles

The wave-current combined effect on the dynamic response of monopile-supported system considering fluid-structure interactions

ABSTRACT The monopile-supported offshore wind turbines (OWTs) are subjected to complex marine environment loads, the dynamic responses of wind turbine structures under wave and current loads are critical for structural safety assessment, which involves the interaction of fluid and structure fields. This study proposed an approach for simulating the two-way (T-W) fluid-structure interaction (FSI), the dynamic responses of the wind turbine support structure under the combined action of wave and current load are investigated. Subsequently, the effect of wave-current interaction, scour depth and flow velocity on the dynamic responses of monopile-supported OWTs are analysed and some useful guidelines are provided. The results indicate that the wave-current interaction can strongly influence the dynamic responses of OWTs, especially for larger scour depth and higher flow velocity. In addition, the displacement amplitude of OWT decreases by 15% under the current load in consideration of the fluid-structure interaction and the bearing performance of the pile improves accordingly.

Dynamic strain field estimation of fixed offshore support structures using limited acceleration measurements

The strain history affecting the fatigue life of fixed offshore support structures is one of the most important topics in structural monitoring. However, some critical fatigue locations are located below the seabed, and the installation of strain gauges at these locations entails significant maintenance costs. In contrast, acceleration which reflects the vibration characteristics of the structure is an easily accessible and more stable physical parameter. Therefore, this study presents a strain estimation scheme that uses limited acceleration measurements to estimate the dynamic strain field of the fixed offshore support structures. An adaptive displacement reconstruction method is used to obtain the displacements at the master degrees of freedom, then the global displacements are obtained by the evolutionary orthogonal response expansion method, and finally, these displacements are implemented into the numerical software by redefining the boundary conditions to obtain the dynamic strain field. The advantage of the proposed method over the traditional strain-solving forward problem is that it relies solely on measured acceleration and structural geometry information, without requiring displacement, strain, or modal information. The correctness of the proposed method is verified by a numerical jacket platform model with multi-harmonic and non-stationary random excitations. Furthermore, physical experiments under eccentric rotation and hammering excitations are performed to assess the applicability of the proposed method on a wind turbine model equipped with laser displacement and fibre bragg grating sensors as reference measurements. The results show that the proposed method can accurately predict the dynamic strains of jacket platform and wind turbine support structures, while the appropriate agreement between the estimated strains and the numerical results, and the optical measured response is observed, with maximum estimation errors of 1.80% and 4.63%, respectively. Summarily, the application of the approach enables a new type of strain monitoring applicable to fixed offshore support structures.

Assessment of internal defects in flush ground butt welds in marine structures

In this paper, acceptance criteria of internal planar defects for the highest design S-N curve for surface ground butt welds in fatigue design standards has been assessed based on fatigue tests of tethers containing internal defects. The results from crack growth analysis from defects placed close to the surface are compared with test data from constant amplitude testing of tethers with circumferential welds that includes flaws or small planar defects close to the surface. Floating structures and support structures are subjected to variable loads and the calculated response leads to a long-term stress range distribution with many small stress ranges. This means that if the cracks are small, the resulting stress intensity may be less than the threshold stress intensity factor and the crack does not grow for this stress cycle, or it grows at a reduced crack growth rate in the near threshold region. A methodology to account for this is presented based on a two-parameter Weibull long-term stress range distribution that is representative for load response of floating structures and for support structures for wind turbines. It is shown that the threshold value and the reduced crack growth rate in the near threshold region for small internal defect heights can be used to lift the fatigue test data from constant amplitude testing to be in better correspondence with a higher S-N curve when considering an actual long-term loading.

Multivariate Data Analysis of Maximum Stress Concentration Factors in FRP-Retrofitted Two-Planar KT-Joints under Axial Loads for Offshore Renewables

With growing concerns about the danger of global climate change and worldwide demand for energy, the interest in the investigation and construction of renewable energy technologies has increased. Fixed platforms are a type of support structure for wind turbines composed of different types of tubular joints. These structures are under different kinds of cyclic loadings in ocean environmental conditions, which must be designed and reinforced against fatigue. In the present paper, the relationships between the parameters in DKT-joints reinforced with FRP under axial loads are investigated using several models, under 16 axial loading cases, with different nondimensional parameters and different FRP materials, and orientations were generated in ANSYS (total 5184) and analyzed. The four loading conditions that cause the maximum stress concentration factors were selected. After analyzing the 1296 reinforced models, relevant data were extracted, and possible samples were created. The extracted data were used in a multivariate data analysis of maximum stress concentration factors. The Pearson correlation coefficient is utilized to study the relationship between parameters and subsequently to make predictions. To reduce the number of variables and to group the data points into clusters based on certain similarities, hierarchical and non-hierarchical classifications are used, respectively.

Validation of a model-based dual-band modal decomposition and expansion approach for fatigue monitoring of offshore wind turbine foundations

The fatigue limit state strongly drives the lifetime of offshore wind turbine support structures. Despite steady advancements in their design, the accumulation of fatigue damage over the lifetime of the structure remains uncertain; one way to improve certainty is to directly monitor structural loads at fatigue-critical locations, e.g., near or below the mudline. However, directly monitoring the loads at these locations is often not possible, since these locations do not allow for easy installation or retrofitting of sensors. To this end, model-based virtual sensing techniques can be employed to estimate the structural response at unmeasured locations based on vibration response data and a finite element model which can accurately describe the modal and operational deflection shapes of a specific turbine [1,2,3]. For this contribution, a modal decomposition and expansion (MDE) algorithm [4] will be used to predict strain time series at fatigue-critical locations, based on acceleration and strain data obtained from the tower of an operational OWT. Emphasis will be put on sparse data situations in which a realistic amount of sensor data is used as input. Fibre-optical strain gauges close to and below the mudline level (Fig 1) will be used for direct validation of the extrapolation capabilities. The required finite element model is built using an in-house developed and verified integrated FE model class [5,6], in combination with a unique database which contains all available geotechnical and structural data of offshore wind turbines (owimetadatabase) [7,8]. This combination allows us to generate fleet-wide, turbine-specific, finite elements models for several wind farms. The results will be presented in terms of time series (e.g., Fig. 2) and fatigue damage-related metrics. Preliminary results show that strain time series estimated with the MDE method show good agreement with the validation data.

Corrosion fatigue analysis of NREL’s 15-MW offshore wind turbine with time-varying stress concentration factors

Over the last twenty years, significant development in wind turbine technologies has led to a dramatic increase in the scale of wind turbines with many now beginning to be installed in offshore locations. Consequently, modern multi-megawatt offshore wind turbines are exposed to increased cyclic loading in addition to an increased risk of corrosion attack. The combination of these two factors may result in wind turbine support structures becoming increasingly vulnerable to fatigue corrosion. The objective of this work is to investigate the impact of material thinning in fatigue-prone areas with respect to fatigue loading and ultimately to examine the potential repercussions on the lifespan of wind turbine support structures. To achieve this, a composite model is constructed coupling results from a multi-body structural dynamic model with time-varying Stress Concentration Factors (SCF) obtained from a finite element model (FEM) of NREL’s 15-MW monopile-based offshore wind turbine. The nonlinear aeroelastic multi-body dynamic model of the wind turbine is used to generate stress time histories for a set of environmental conditions based on the operational conditions of the wind turbine. The finite element model of the wind turbine is then used to identify fatigue-vulnerable regions in the wind turbine support structure and calculate SCFs for these specific areas. The integration of SCFs into the fatigue calculations reduced the lifespan of the turbine tower by a factor of 4, demonstrating the importance of precisely modelling such local stress concentrations for effective fatigue analysis. A novelty of this work arises in the ability of the finite element model to update the SCFs of the fatigue-prone areas over time as corrosion-induced wastage alters the substructure’s geometry, thereby inducing a global redistribution of stresses. A fatigue analysis is carried out availing of the SCFs which vary annually in addition to the stress-time histories produced by the multi-body dynamic model. The results illustrate that the phenomenon of corrosion thinning induced an 8.9% reduction in the fatigue life of the wind turbine tower, thus emphasising the significant importance of proactive maintenance strategies to mitigate the impact of corrosion.

An open-source analysis workflow for geometrically imperfect bolted ring-flanges in wind turbine support structures

Bolted ring-flanges connect structures via two circular flanges, secured with pre-loaded bolts. In offshore wind turbine support structures the flanges connect the monopile and transition piece, the transition piece and tower, and adjacent tower sections. Fatigue analysis of the bolted ring-flanges in offshore wind turbine support structures is a critical step during design. The consideration of geometric imperfections of the flanges can be an influential factor in assessed fatigue damage over some given Fatigue Limit State load case. Schemes to analyse the effect of geometric imperfections using Finite Element Method simulations are available in literature. These schemes rely on the use of proprietary tools for the analysis. In this paper we introduce an open-source workflow to assess load transfer through geometrically imperfect bolted ring-flanges. The workflow relies on four open-source toolsets: the FreeCAD 3D parametric modelling package, the Code Aster / Salome-Meca Finite Element Method simulation environment, the ParaView post-processing and visualisation package, and the Python programming language. We present a case study of load transfer assessment in a monopile to transition piece bolted ring-flange typical of modern offshore wind turbine support structures. We decompose the load transfer across the flange under a number of geometric imperfection scenarios to determine the expected stress ranges for fatigue analysis. The results are compared with analytical methods specified in engineering standards. The workflow is configurable as per user needs, and is maintained as a GitHub repository at https://github.com/jhjorg/OpenBoltRF.

Optimizing dimension selection for rain flow counting in fatigue assessment of large-scale lattice wind turbine support structures: A comprehensive study and design guidance

The selection of dimension (d) for the rain flow counting matrix significantly influences the reliable prediction of fatigue life in wind turbine support structures. However, there are limited public reports on the impact of dimensions on the fatigue assessment of wind turbine structures. This study explores the influence of dimensions on the fatigue assessment process and outcomes, focusing on a large-scale ultra-high lattice wind turbine support structure from an engineering project. Through integrated load simulation, fatigue load time series for different design load cases (DLCs) are obtained for the overall structure. The rain flow counting method is utilized to derive the overall rain flow counting matrix (Markov_n) with varying dimensions. Subsequently, employing a multi-scale fatigue assessment method, the damage matrix (Markov_D), cumulative fatigue damage (D), and fatigue life are computed. A detailed exploration examines the impact of rain flow counting matrix dimensions on Markov_n, Markov_D, D, fatigue life, and calculation time (t). Furthermore, guidance is provided for selecting the optimal dimension for engineering design. The findings indicate that selecting dimensions that are too small results in the loss and merging of smaller mean and amplitude load values, thereby failing to accurately represent the load history. Moreover, a small dimension yields an excessively cautious estimation of cumulative fatigue damage (D) and underestimates the fatigue life, consequently inflating construction expenses.

Torus Hull Articulated Tower for offshore wind turbines

Recent years have witnessed significant advancement in offshore wind energy production. In order to achieve cost-effective solutions for intermediate water depths, there is a growing demand for wind turbine support structures with less steel compared to the existing ones. This study introduces a novel articulated tower concept termed “Torus Hull Articulated Tower” (THAT) for supporting an offshore wind turbine at a water depth of 75 m. The proposed concept incorporates a torus hull connected to a centrally located column with a system of pre-tensioned tethers. The preliminary technical feasibility of the concept has been checked by numerical analysis considering operating and parked conditions. A fully coupled analysis approach has been adopted for solving the associated aero-hydrodynamics-multibody mooring system problem. Acceptance criteria in terms of the maximum acceleration at the nacelle level and the platform pitch angle have been considered. The results show that the current concept can satisfy the required acceptance criteria. It has been concluded that the proposed structure could considerably reduce structural steel consumption compared to the existing concepts available in the literature. A sensitivity study on the structure's performance with respect to its major parameters is also included.

A reliability-constrained optimisation framework for offshore wind turbine support structures

ABSTRACT A new approach that links finite element analysis (FEA) and a genetic algorithm (GA) with Monte Carlo simulation (MCS) for the reliability-constrained optimal design of offshore wind turbine (OWT) support structure is presented. This approach has been applied to optimise the NREL 5MW OWT on OC3 support structure. The objective function minimises the weight of the support structure, constrained to design and performance limits, by attaining the desired reliability level. A response analysis of the reference OWT is performed to estimate design loads. Then, deterministic optimisation (DO) is carried out to find the candidate design solutions. After that, a reliability assessment is conducted to apply the target reliability constraint. This study reveals that the design of a monopile support structure is mainly driven by fatigue limit state. Also, there is no guarantee that DO candidate designs always meet the structural reliability target level in all capacities.

Optimization for Offshore Prestressed Concrete–Steel Hybrid Wind Turbine Support Structure with Pile Foundation Using a Parallel Modified Particle Swarm Algorithm

The prestressed concrete–steel hybrid (PCSH) support structure, which replaces the lower part of the traditional support with a concrete segment, is a prospective support structure solution for ultrahigh wind turbines. Taking a 5.5 MW wind turbine support structure founded on a jacket substructure with pile foundation as an example, an optimized design of the corresponding PCSH support structure with pile foundation for offshore wind turbine is conducted considering the soil–structure interaction (SSI) and the effect of water pressure. The construction cost of the proposed structure is treated as the objective function and minimized with a parallel modified particle swarm optimization (PMPSO) algorithm where the physical dimensions of each part of the PCSH wind turbine support structure are treated as optimization variables. Eleven optimization constraints are considered under both the serviceability limit state (SLS) and the ultimate limit state (ULS) according to relevant specifications and industry standards. A penalty function strategy is introduced to make sure that these constraints are fulfilled. The mechanical behavior and the cost of the optimal PCSH support structure with pile foundation are analyzed and are compared with those of the original design with a traditional steel tube tower founded on a jacket substructure. The results show that the cost and levelized cost of energy (LCOE), a comprehensive evaluation, of the optimized PCSH support decrease obviously with the PMPSO algorithm, which can provide advanced mechanic behavior including natural frequency, top deformation, and anti-overturning capacity. Compared with the PSO algorithm, the PMPSO algorithm has better performance in the procedure of PCSH support for offshore wind turbine optimization.

Fatigue Analysis of a Jacket-Supported Offshore Wind Turbine at Block Island Wind Farm.

Offshore wind-turbine (OWT) support structures are subjected to cyclic dynamic loads with variations in loadings from wind and waves as well as the rotation of blades throughout their lifetime. The magnitude and extent of the cyclic loading can create a fatigue limit state controlling the design of support structures. In this paper, the remaining fatigue life of the support structure for a GE Haliade 6 MW fixed-bottom jacket offshore wind turbine within the Block Island Wind Farm (BIWF) is assessed. The fatigue damage to the tower and the jacket support structure using stress time histories at instrumented and non-instrumented locations are processed. Two validated finite-element models are utilized for assessing the stress cycles. The modal expansion method and a simplified approach using static calculations of the responses are employed to estimate the stress at the non-instrumented locations-known as virtual sensors. It is found that the hotspots at the base of the tower have longer service lives than the jacket. The fatigue damage to the jacket leg joints is less than 20% and 40% of its fatigue capacity during the 25-year design lifetime of the BIWF OWT, using the modal expansion method and the simplified static approach, respectively.

Effect of scour on the fatigue life of offshore wind turbines and its prevention through passive structural control

Abstract. Offshore wind turbine (OWT) support structures are exposed to the risk of fatigue damage and scour, and this risk can be effectively mitigated by installing structural control devices such as tuned mass dampers (TMDs). However, time-varying scour altering OWTs' dynamic characteristics has an impact on the TMD design and fatigue life, which has rarely been studied before. In this paper, a simplified modal model is used to investigate the influence of scour and a TMD on the fatigue life evaluation of a 5 MW OWT's support structure, and a traditional method and a newly developed optimization technique are both presented to obtain TMD parameters. This optimization technique aims at finding optimal parameters of the TMD which maximize the fatigue life of a hotspot at the mudline, and the effect of time-varying scour can be considered. This study assumes that the TMD operates in the fore–aft (FA) direction, while the vibration in the side–side (SS) direction is uncontrolled. Results show that scour can decrease the fatigue life by about 24.1 % and that the TMD can effectively suppress vibration and increase the fatigue life. When the scour depth reaches 1.3 times the pile diameter, the TMD with a mass ratio of 1 % can increase the fatigue life of an OWT's support structure by about 64.6 %. Further, it is found that the fatigue life can be extended by 25 % with the TMD optimized by the proposed optimization technique rather than using a traditional design method which does not take the change in dynamic characteristics into account.

Full scale testing of fatigue resistant composite joints for offshore wind Jacket and Floating structures

Complex welds in the joint region reduce the fatigue resistance of structural joints of circular hollow sections with factors that results in increased wall thicknesses of the tubes and overspending of steel in the multi-membered part of the jacket and floating support structures for offshore wind turbines. The wrapped composite joint is a breakthrough technology utilising bonding and fatigue resistant composite material to connect steel tubular members instead of welding. The main technical advantage is that the superior fatigue life of wrapped composite joints. Full-scale wrapped composite joint is tested by cyclic out-of-plane bending loading in resonance-based Cronos testing rig at OCAS N.V. in Belgium. Results reveal exceptional fatigue life of wrapped joints to combined loading when compared to welded joints, showcasing a potential that can radically reduce steel use for offshore structures.

An Overview on Structural Health Monitoring and Fault Diagnosis of Offshore Wind Turbine Support Structures

The service environment of offshore wind turbine (OWT) support structures is harsh, and it is extremely difficult to replace these structures during their operational lifespan, making their failure a catastrophic event. The structural health monitoring (SHM) of OWT support structures is a crucial aspect of operational maintenance for OWT support structures, aiming to mitigate significant financial losses. This paper systematically summarizes the current monitoring methods and technologies for OWT support structures, including towers and foundations. Through the review of monitoring content and the evolution of monitoring techniques for supporting structures, it delves deeper into the challenges faced by wind turbine monitoring and highlights potential avenues for future development. Then, the current damage identification techniques for OWT towers and foundations are analyzed, exploring various methods including model-based, vibration-based, artificial intelligence and hybrid fault diagnosis methods. The article also examines the advantages and disadvantages of each approach and outlines potential future directions for research and development in this field. Furthermore, it delves into the current damage identification techniques for OWT towers and foundations, discussing prevalent challenges and future directions in this domain. This status review can provide reference and guidance for the monitoring design of OWT support structures, and provide support for the fault diagnosis of OWT support structures.

Integrated fatigue assessment method considering average stress effects of large-scale lattice wind turbine support structures

Modular assembly techniques for ultra-high lattice wind turbine support structures are gaining popularity as a viable solution for wind energy development in low wind speed areas. However, efficient fatigue assessment of the overall structure is crucial for the widespread promotion and application of these structures. Existing engineering methods for fatigue assessment suffer from limitations in calculation efficiency and often fail to account for the effects of average stress. To address these issues, a novel integrated fatigue assessment method is proposed, which is both fast and efficient while considering the impact of average stress. The method enables the computation of cumulative fatigue damage and fatigue life for each component, as well as the identification of the location of the greatest cumulative fatigue damage in the structure. An onshore ultra-high lattice wind turbine support structure, modularly assembled to a height of 180 m, is utilized as an application example to illustrate the specific implementation process of the proposed method. The effect of different fatigue load conditions and transfer functions on cumulative fatigue damage is analyzed, and the relationship between transfer function and fatigue accumulation damage is examined. The findings indicate that average stress plays a crucial role in the cumulative fatigue damage of ultra-high lattice wind turbine support structures, and the proposed integrated fatigue assessment method simplifies the calculation process for fatigue-resistant designs substantially.

Damage Detection and Localization at the Jacket Support of an Offshore Wind Turbine Using Transformer Models

Early detection of damage in the support structure (submerged part) of an offshore wind turbine is crucial as it can help to prevent emergency shutdowns and extend the lifespan of the turbine. To this end, a promising proof-of-concept is stated, based on a transformer network, for the detection and localization of damage at the jacket-type support of an offshore wind turbine. To the best of the authors’ knowledge, this is the first time transformer-based models have been used for offshore wind turbine structural health monitoring. The proposed strategy employs a transformer-based framework for learning multivariate time series representation. The framework is based on the transformer architecture, which is a neural network architecture that has been shown to be highly effective for natural language processing tasks. A down-scaled laboratory model of an offshore wind turbine that simulates the different regions of operation of the wind turbine is employed to develop and validate the proposed methodology. The vibration signals collected from 8 accelerometers are used to analyze the dynamic behavior of the structure. The results obtained show a significant improvement compared to other approaches previously proposed in the literature. In particular, the stated methodology achieves an accuracy of 99.96% with an average training time of only 6.13 minutes due to the high parallelizability of the transformer network. In fact, as it is computationally highly efficient, it has the potential to be a useful tool for implementation in real-time monitoring systems.

Proposal and application of onshore lattice wind turbine support structure and integrated multi-scale fatigue assessment method

To accommodate the development of wind turbines in areas with weak wind speeds, an upright ultra-high lattice support structure is proposed for the large wind turbine, which is prefabricated and assembled mainly from thin-walled circular hollow steel (CHS) tube elements. As a practical example, a 180 m wind turbine support structure is presented. To evaluate the fatigue performance of the new structure type with modular assembly, a new integrated multi-scale fatigue assessment method is proposed to address the shortcomings of the equivalent fatigue load method commonly used in engineering design. For any fatigue load condition, the integrated multi-scale fatigue assessment method enables accurate and efficient calculation of the relationship curve between fatigue load and hot spot stress, and identification of the locations in the structure where fatigue accumulation damage is the greatest. Furthermore, fatigue damage accumulation and fatigue life can also be predicted accurately and efficiently by the proposed method. The results show that for the maximum fatigue cumulative damage of the proposed new structural systems, as the effect of average stress is not considered and the assumption of superposition of principal stresses, the traditional equivalent fatigue load method is too conservative compared to the proposed integrated multi-scale fatigue assessment method, detrimental to cost decreasing and benefit increasing of the wind turbine structures.

A novel post-weld treatment using nanostructured metallic multilayer for superior fatigue strength

Welded joints exhibit fatigue failure potential from weld geometry and characteristics of the heat affected zone. In order to counteract fatigue, structures and components require larger thicknesses resulting in heavier designs exhausting the finite natural resources. We hereby introduce a novel post-weld treatment, which postpones or even prevents fatigue failure of the welded connection. A Cu/Ni nanostructured metallic multilayer (NMM) is applied via electrodeposition and a 300–600% increase in usable lifetime compared to the untreated weld is observed. A FAT class 190 with a slope of k = 6 is proposed for the design of NMM treated butt welds. Material mechanisms responsible for the fatigue strength increase are introduced herein. A case study shows that the design of offshore wind turbine support structures applying NMM post-weld treatment enables a lifetime extension as well as a 28% weight reduction compared to the structure without post-weld treatment.

Fatigue constrained topology optimization for the jacket support structure of offshore wind turbine under the dynamic load

This paper proposes a topology optimization method considering fatigue constraints for the jacket support structure of offshore wind turbine. Jacket support structure is an important component and supports the whole wind turbine weight under the dynamic environmental loads, which would induce serious fatigue problems. To obtain the optimal structural layout, the topology optimization method considering fatigue constrained is introduced for the jacket support structure under the dynamic load. The fatigue lifetime is constrained base on the Miner's cumulative damage rule. The design sensitivity of the objective and constraint functions are evaluated analytically, while the equivalent static load approach is applied. Some typical topology optimization problems are introduced to verify the effectiveness of the proposed method, and this method can obtain the appropriate layouts with stable convergence. A case study on the OC4 reference jacket indicates that the method can achieve a reasonable layout structure under the dynamic load, which meets the fatigue constraint. And the final design is reconstructed based on the topology optimization layout. Finally, a lighter jacket structure has good performance on the ultimate bearing capacity, eigenfrequency, and buckling, which is suited for long operation life.