MW Offshore Wind Turbine Research Articles

Uncertainty quantification for dynamic responses of offshore wind turbine based on manifold learning

Offshore wind turbines (WTs) are crucial in offshore wind energy development. However, the dynamic responses of WTs are subject to significant uncertainties which are usually not properly considered. To the end, this paper proposes an efficient method for quantifying the uncertainties in WTs' dynamic responses based on cumulative distribution function (CDF)-manifold learning. First, a probabilistic model is developed to represent the environmental parameters and sampling for aerodynamic-hydraulic-servo-elastic simulations. Then, the CDF is obtained by statistically analyzing the simulated data. To tackle the higher dimensionality resulting from discretizing the CDF, a manifold learning-based approach is subsequently proposed to reduce its dimensionality and obtain a manifold space. Furthermore, a mapping relation is established between the environmental parameters and the low-dimensional data to efficiently obtain the response CDF under different environmental parameters, leading to the construction of a probability box (P-box) model. To demonstrate the effectiveness of the proposed method, the National Renewable Energy Laboratory (NREL) 5 MW offshore WT on an Offshore Code Comparison Collaboration (OC3) monopile is selected as a case study and analyzed accordingly. The results show P-box models of seven WT responses and validate the effectiveness of the proposed method.

Mitigation of side-to-side vibration of a 10MW monopile offshore wind turbine under misaligned wind and wave conditions by an active torque control

Large offshore wind turbines (OWTs) may encounter extreme misaligned wind and wave conditions throughout their lifetime, which could trigger side-to-side resonance of the OWTs and thus substantial reduction in fatigue life. This study aims to (1) understand the dynamic response of fixed-bottom OWTs under misaligned wind and wave conditions, and to (2) propose an active torque control algorithm for dynamic loading mitigation. For these purposes, an integrated aero-elastic model, coupled with an advanced soil-monopile interaction (i.e., p-y+M-θ model), is built in OpenFAST for the DTU 10MW OWT supported by a monopile in soft clay. The numerical results show that under misaligned wind and wave conditions, where the wave peak period is likely to approach the tower natural period, the dynamic loading along the side-to-side direction dominates the fatigue design of the OWT. To mitigate the side-to-side dynamic loading, an active torque control algorithm is designed with feedback from measured side-to-side tower vibration to enhance damping, as well as feedforward from measured incoming wave height to counteract external force. Through the use of the feedback-feedforward active torque controller, the side-to-side dynamic loading of the 10 MW OWT is significantly reduced, with the fatigue life extended from 19 to 39 years.

Seismic responses and vibration control of offshore wind turbines considering hydrodynamic effects based on underwater shaking table tests

AbstractThe recent construction of numerous offshore wind turbines (OWTs) in seismically active areas worldwide has stimulated research on seismic evaluation and vibration control design for OWTs. Unlike the onshore counterparts, the hydrodynamic effects on the seismic responses of OWTs need to be studied. Besides, the equipment in the nacelle of an OWT is highly sensitive to acceleration. Significant dynamic amplification on the nacelle under horizontal earthquake excitation can cause the malfunction of power generation and even seriously threaten the safety of OWTs. As a promising solution, tuned mass dampers (TMDs) based on the first mode of OWTs need to be tested by various earthquake excitations. In this paper, the seismic amplification effect, the hydrodynamic effects, and the vibration control by a single TMD are investigated through a series of underwater shaking table tests based on a 1:20 scaled model of a 5 MW OWT. The “no water”, “with water”, and “with water and TMD” tests are evaluated and compared. The accelerations of OWTs can be amplified or reduced by the hydrodynamic effects, depending on the natural frequencies and vibration modes of the OWT and the frequency spectrum of the seismic excitation. The earthquake‐induced hydrodynamic damping ratio and hydrodynamic added mass are carefully measured. For the TMD's effect, the single TMD performs reasonably well in seismic vibration control in most cases. Finally, the interaction between hydrodynamic effects and TMD's effect is discussed, and the hydrodynamic damping effect is suggested to be ignored when designing TMDs for OWTs.

Synergy of random balance design method and intelligent optimization technique for model updating of the 4 MW offshore wind turbine benchmark

The technical challenges for model updating of real marine engineering structures include the extraction of modal parameters associated with incomplete measurement information and the correction of a large number of structural parameters. As it is difficult to verify the effectiveness of model updating methods for engineering applications, the representative models have to be adopted in finite element (FE) analysis or lab-scale tests, leading to the incomplete reflection of actual structural performances. Based on these factors, a practical model updating method is developed in this study to make full use of field measurement data from a 4 MW offshore wind turbine for the accurate estimation of structural parameters using the random balance designs-Fourier amplitude sensitivity test (RBD-FAST) strategy and an improved particle swarm optimization (IPSO). Leveraging the measured acceleration signals from offshore wind turbines under the conditions including operation, shutdown, collision and typhoon scenarios, the complex exponential decomposition method is applied to accurately extract the time-varying acceleration components (TVAC) for the construction of the frequency response function (FRF). Following that, RBD-FAST is implemented into IPSO with adaptive inertia weights and asynchronously varying learning factors to enable efficient selection of numerous updated physical parameters, thus improving time cost and computational accuracy. The correctness of the proposed method is verified by a numerical jacket platform model. Furthermore, the 4 MW offshore wind turbine benchmark is developed to assess the feasibility of the proposed model updating method using field measured data from different scenarios. Results show that the synergy of RBD-FAST and IPSO in the proposed method can accurately update parameters and minimize a maximum discrepancy of 0.8970% for the first-three orders of natural frequencies between the benchmark and the updated models in the collision scenario. Summarily, the present work provides a potential technique and practical engineering references for model updating of offshore wind turbines subject to harsh marine environments.

System identification and finite element model updating of a 6 MW offshore wind turbine using vibrational response measurements

Offshore wind energy is playing an increasingly vital role in the clean energy transition around the world, and improved reliability of wind turbine structures is necessary for the long-term success and efficiency of renewable energy. Increased reliability would reduce costs associated with maintenance due to breakages and in turn reduce the levelized cost of energy for offshore wind energy sources. Structural health monitoring methods can be used to predict breakages and extend lifetimes by continuously monitoring instrumented structures. This paper presents system identification and model updating of a 6 MW offshore wind turbine using vibration measurements under varying operational conditions. The turbine is monopile-supported and instrumented with strain gauges and accelerometers at several elevations along the tower and monopile. Effective stiffness of soil springs in the model are updated to match modal-predicted natural frequencies and mode shapes of the first two modes with those identified from measurements at different operating conditions. A deterministic and probabilistic (Bayesian) approach to model updating are compared. The sensitivity of identified modal parameters and the updated model parameters are investigated with respect to operational and environmental conditions such as wind speed. Results show that deterministic model updating can match modal parameters with high accuracy across datasets and environmental conditions. Bayesian model updating results successfully estimate the posterior distribution of updating model parameters with an increasing degree of certainty as more data is used.

Effects of negative shear on loads for a 15 MW offshore wind turbine during low-level jet events

Wind turbines are more often interacting with the negative shear region of a low-level jet due to increasing turbine sizes. However, the effects of negative shear on wind turbines are not sufficiently studied, particularly for offshore wind applications. In this paper, we studied the effects of negative shear on wind turbine loading. This was done using user-defined wind profiles and a model chain. The model chain consisted of a turbulence generator and a wake modelling tool coupled to an aero-hydro-servo-elastic engineering tool. We observed significant variations in the structural loading in the case of low-level jets. We also observed changes in the power spectral density estimates. Additionally, we briefly examined the wake recovery distance.

Seismic response of monopile foundation of offshore wind turbines under near-field and far-field ground motions

The tendency to use renewable energy sources has been increased due to the problems associated with utilization of the fossil fuels such as their instability in the future and environmental issues. Offshore wind turbines (OWTs) have become a major renewable energy source due to their merits. However, many of the offshore wind farms planned recently are located in the seismic prone areas, which introduces some challenges regarding their foundation design while, the most common OWTs’ foundation by far, is monopile. OWTs are tall infrastructures with a longer natural period than the predominant period of most earthquakes. While far-field earthquakes have a large predominant period which might amplify the structure response and cause significant damage to OWTs, near-field earthquakes with large pulse period might lead to large residual displacements, especially in OWTs that wind could exert an eccentric load. In this study, seismic behavior of the National Renewable Energy Laboratory (NREL) 5 MW offshore wind turbine on monopile foundation embedded in liquefiable soil is investigated under near-field and far-field earthquakes using finite element computer program OpenSees. The seismic response of the structure was also evaluated under bidirectional excitation corresponding to the Kobe earthquake, Japan. Considering the lateral displacement of the superstructure, rotation, settlement, and bending moment of the monopile and shear stress/strain, and excess pore water pressure in the ground, a comparison between the seismic behavior of the monopile foundation of the offshore wind turbine under near-field and far-field earthquakes was performed. The results of this study showed that the seismic response of the monopile-superstructure system under near-field earthquakes was more critical despite the shorter duration and the smaller arias intensity of those earthquakes. It highlighted the remarkable effect of the other parameters such as near-field pulse which contained a lot of energy. Furthermore, the seismic response of the monopile was enhanced under the bidirectional earthquake loading.

Fatigue life analysis of offshore wind turbine tower under combined action of wind and wave

In order to obtain the fatigue life of an 8 MW offshore wind turbine tower, the fatigue damage characteristics of tower under combined action of wind and wave were analyzed. The calculation method of fatigue damage degree of the tower under different working conditions in unit cycle time is given. Based on the linear extension theory of fatigue damage, the annual fatigue damage degree of the tower under different working conditions was calculated, and the fatigue life of the tower was obtained. The calculation results show that the fatigue life of the tower is 31.2 years when the ideal S-N curve is adopted, which is longer than the design life of 25 years. The influences of average stress, roughness and structure size on S-N curve are analyzed. When Goodman method is selected for average stress correction, roughness coefficient is 0.473, and dimension correction coefficient is 0.639, the fatigue life of the tower is 26.7 years, decreasing by 14%. The fatigue life of the tower under different roughness is calculated. The calculated results show that the fatigue life of the tower decreases with the decrease of the roughness coefficient. When the roughness coefficient is 0.287, the fatigue life of the tower is about 25 years.

Numerical simulation of turbulent natural convection heat transfer in an MW-class offshore wind turbine nacelle based on a multi-feature acquisition meshing technique

In order to improve the computational accuracy of numerical simulations of complex thermal environments in MW-class offshore wind turbine nacelles, this paper improves the numerical simulation method for 10 MW offshore wind turbine nacelles. Firstly, the model of the wind turbine generator system (WTGS) inside the nacelle is designed and the model is simplified. Secondly, a multi-featured acquisition meshing is performed for the turbulent thermal behavior in the nacelle, using a combination of mesh geometry parameters, mesh accuracy parameters, mesh irrelevance parameters and detection verification parameters to construct a structural mesh topology. The modified SST k-ω turbulence model is used to solve the momentum and energy equations of the nacelle model. The complex physical phenomena of air recirculation, boundary layer separation and thermal stratification in the nacelle are verified based on the calculated results and compared with the experimental data, which are in good agreement with the experimental results. This paper has certain application value and guidance significance in the field of engineering such as the design of offshore wind turbine nacelle structure and cooling system design.

Research on the natural frequencies of an integrated offshore wind turbine model considering blades

The offshore wind turbine (OWT) is a high-rise structure that is extremely sensitive to dynamic loads such as wind, waves, and earthquakes. Therefore, in order to avoid resonance, it is extremely important to accurately calculate the natural frequencies of the OWT at the design stage. The integrated finite element model of DTU (Technical University of Denmark) 10 MW OWT is established based on OpenSees finite element software. The correctness of the superstructure simplification method is confirmed by comparing it to the frequencies calculated by the wind turbine dynamic analysis software HAWC2 for the single-blade model and the fixed constraint model at the bottom of the tower. The frequencies and mode shapes calculated by the overall model are compared with those calculated by the fixed model at the mudline and the concentrated mass model at the top of the tower. Then, the parameters of the soil shear modulus G0, void ratio e, pile embedded length L, pile diameter D, water depth, and blade stiffness are analyzed using the lumped mass model and the blade model at the top of the tower, and some regular conclusions about the variation of natural frequency with the parameters are obtained. Finally, based on the blade model, the influence of short-term cyclic loading on the natural frequency for OWT monopiles in dense sand is studied. The results indicate that the pile-soil interaction has a significant impact on the natural frequencies, the blade has a higher influence on the second bending frequency of the OWT, and the front-after direction is more significantly impacted than the side-side direction. The short-term cyclic loading are unlikely to significantly affect natural frequency for OWT monopiles in dense sand.

A novel design of the offshore wind turbine tripod structure using topology optimization methodology

To achieve a design with the desired stiffness, strength, and durability at the component level, topology optimization has been practiced and was effective in the design of sustainable products, by autumnally allocating the available material within the design domain to form the shortest loading path. In the conceptual phase, this paper proposed a design methodology using topology optimization technique for lightweight design of a tripod structure - a typical form of foundation for offshore wind turbines (OWTs). The minimum compliance subject to prescribed volume percentage is formulated and an innovative tripod structure is generated by interoperating the optimized structure. To facilitate comparison, a tripod structure of a 5 MW OWT is referenced. The system performances are comprehensively and synthetically assessed, encompassing the natural frequency, maximum von Mises stress in ultimate load cases and cumulative fatigue damage. It is discovered that the total mass of the novel design is substantially less than that of the reference structure. In contrast, the fatigue resistance has been significantly enhanced. These finding reveal conclusively that the proposed topology optimization approach is capable of generating novel designs, reducing weight and alleviating fatigue damage.

Design and optimization of multi-MW offshore direct-drive wind turbine electrical generator structures using generative design techniques

The use of generative design as an alternative to typical structural optimization techniques opens the door to new methods of manufacturing. In this study, generative design techniques were used as an automated iterative process with an extensive set of control variables and initial models to explore and optimise the stiffness and weight of different configurations for a 3 MW offshore wind turbine electrical generator structure. With this new approach of adding generative design techniques to a structural optimization process, a 4% reduction in structural mass was achieved. The modification in the structural geometry also help to enlarge the machine's operational range with the consequent growth in energy gathering.

A comparison study of power performance and extreme load effects of large 10‐MW offshore wind turbines

AbstractThe utilisation of offshore wind turbines has rapidly increased in the last decade, which has resulted in a steady increase in wind turbine sizes. The global average offshore wind turbine size has increased from 1.5 MW to 6 MW in the last two decades. The research community has started to investigate huge 10 to 15 MW offshore wind turbines in recent years, resulting in the study of very innovative floating wind turbines using various substructure technologies. With this backdrop, this paper will investigate and thoroughly compare the power performance of extreme load effects of a large offshore 10 MW turbine installed on the monopile, spar, and semisubmersible substructures. This is performed by using the average conditional exceedance rate (ACER) and Gumbel methods to predict the extreme responses under the operating conditions of 8, 12, and 16 m/s mean wind speed, representing the below‐rated, rated, and above‐rated regions, respectively. The results show that the power performance and extreme loads experienced depends significantly on the operating regions. The mean power generation between the three different types of offshore wind turbines (OWTs) are closely in the whole operating range, which standard deviations differ significantly. Large standard deviations of power generation appear in the spar turbine under the below‐rated condition. Further, it was observed that the spar wind turbine generally experiences larger extreme loads due to larger platform pitch motion. In addition, the ACER method shows a better prediction for the 1, 2 and 5‐year extreme responses than the Gumbel method, which is due to the relatively poor data fitting of the Gumbel method at the upper tail. The study is believed to consolidate and close the knowledge gap in understanding wind turbine responses across the most common offshore substructure technologies and provide a basis for design and deployment of OWTs.

Study of air compressibility effects on the aerodynamic performance of the IEA-15 MW offshore wind turbine

With the gradual upscaling of offshore wind turbines, the blade size becomes large and the blade tip velocity reaches almost a Mach number of 0.3, the limit of the incompressible flow assumption.To simulate wind turbine flows, an incompressible solver was often a good choice as it gave good predictions and the Mach number of the wind turbine flows were small. When the Mach number is increased to be close to 0.3, the validity of incompressible flow may be questionable and thus needs to be checked. In this paper, an accurate wind turbine flow solution method is proposed to study the compressibility effects. First, the National Renewable Energy Laboratory (NREL) Phase VI wind turbine is used as an example to verify the method at a small Mach number. Through a comparative analysis between the compressible, the incompressible numerical results, and experimental data, the proposed method is found with a good simulation accuracy. Next, the flow fields of the large wind turbine IEA-15 MW, under different wind conditions are simulated. The local Mach number, density and pressure coefficient at different blade cross-sections of the compressible results are analyzed and compared to its incompressible ones, and the influences of compressibility on the thrust, torque and power of the wind turbine are quantified. The results show that at the rated wind speed, the thrust of the compressible flow simulation is 1.40 % higher than that of the incompressible one, and the torque of the compressible flow simulation is nearly 11.07 % higher than that of the incompressible one. As a result, the compressibility effects are recommended to be added in future studies of large wind turbines.

Shape sensing of NREL 5 MW offshore wind turbine blade using iFEM methodology

In this study, the shape sensing of a blade of the NREL 5 MW offshore wind turbine is presented by using the inverse Finite Element Method (iFEM) under real loading conditions. Among the applied forces, the aerodynamic forces are calculated based on the Blade Element Momentum Theory (BEMT). The results of iFEM with full sensors and reduced sensors are compared with the reference Finite Element Method (FEM) analysis to test the accuracy of the iFEM analysis. The practical distribution of the iFEM elements with sensors is explored and the effects of different wind velocities are investigated to make the analysis more practical. Based on numerical results, it is concluded that iFEM is a suitable technique for shape sensing of offshore wind turbines.

Towards minimal empirical uncertainty bounds of damping estimates of an offshore wind turbine in idling conditions

Structural damping is a critical quantity for condition assessment and fatigue lifetime prediction in wind energy. Its estimation from operational vibration data comes with a considerable amount of uncertainty, derived from environmental and operational variability and from the quality of the estimation process itself. In this paper, we aim to determine experimentally the damping values and their least possible uncertainty bounds for the first two modes of an idling offshore wind turbine. For this purpose, we use field measurements from a 3.6 MW offshore wind turbine located at the DanTysk wind farm. We select a subset of cases from the wind turbine on idling conditions for a confined interval of environmental and operational conditions, in an attempt to minimize operational variability in the damping estimates. In addition, we make a study on the adjustment parameters of the automated operational modal analysis method, aiming at obtaining the most consistent damping estimates. As a result of the proposed methodology, it is possible to confine the 90% confidence intervals of the damping ratio estimates of the first two tower modes within 0.8 and 1.1 percentual points in the fore-aft and the side-to-side direction, respectively.

Studying the Settlement of OWT Monopile Foundations Using a T-Z Spring with the Torsional Effect

The main purpose of this paper was to study the vertical settlement of offshore wind turbine (OWT) monopile support structures, where 5, 10, 15, and 20 MW OWT support structures were analyzed under power production, seismic, and tropical cyclone loads. Moreover, a t-z spring with shear and torsional degrees of freedom was developed to simulate the shear stress along the pile and soil surface under the combined effect of vertical loads and z-direction torsions. This t-z spring does not require excessive changes to the finite element program, where only a known factor is used to modify the traditional stiffness of the t-z spring. This paper, analyzing several kinds of OWT monopile foundations, indicates that the soil shear resistance may be less than the shear stress generated by the combination of vertical loads and torsions, which causes large vertical and rotational displacements resulting in the failure of monopile structures. This situation will be worse when the natural frequency of the first vertical-direction rotation is close to the integer multiples of the 3P frequency, which cannot be well-simulated using traditional t-z springs.

A novel Kriging-model-assisted reliability-based multidisciplinary design optimization strategy and its application in the offshore wind turbine tower

In Reliability-based Multidisciplinary Design Optimization (RBMDO), the key performance functions of wind turbine are usually implicit, which means the performance response can only be obtained through time-consuming Physics Experiment (PE) or Finite Element Analysis (FEA). However, for practical engineering, the computational cost of repeatedly using PE or FEA is prohibitive. To tackle this challenge, in this study, an adaptive Kriging-model-assisted RBMDO strategy is proposed. The novel updated-strategy for performance function in RBMDO is discussed to find effective training samples of active learning for Kriging model. Also, a powerful decoupling strategy of RBMDO is introduced and combined with the proposed method to enhance computational efficiency further. Two case studies, including a mathematic example and a hydraulic turbine rotor bracket design example, are utilized to illustrate the advantage of the given strategy. Finally, the proposed method is applicated into an engineering design of 5 MW offshore wind turbine tower to ensure its reliability and safety.

Experimental and Finite Element-Based Investigation on Lateral Behaviors of a Novel Hybrid Monopile

A monopile is the most conventional structure foundation for offshore wind turbines (OWTs) in the world. However, the Korean offshore wind industry has mostly been using the jacket type of foundation. The main reason for the current situation in Korea is that most of the marine soil consists of weak layers of sand and clay. Thus, the monopile foundation depth has to be deep enough to satisfy the intended serviceability design requirement of the monopile and the rotation limit at the seabed; a conventional monopile design concept alone might be insufficient in Korean offshore conditions, or otherwise could be very expensive, e.g., resulting in a rock socket installation at the tip of the monopile. The main objective of this paper is to introduce a novel hybrid monopile that is composed of a monopile and a supplemental support with three buckets, followed by assessing the lateral resistance of the hybrid system through physical experiments and finite element (FE) simulations. Namely, 1/64.5 small-scaled monopile and hybrid physical models with a monopile diameter of 7 m for a 5.5 MW OWT were loaded monotonically. The results show that the hybrid monopile improves the lateral bearing capacity regarding the initial lateral stiffness and ultimate load. The FE analyses of the corresponding physical models were also implemented to support the results from the physical model test. The numerical results, such as the structural member forces and soil deformation, were analyzed in detail. Additionally, a case study using FE analysis was conducted for the 5.5 MW OWT hybrid monopile support installed in a representative Korean weak soil area. The results show that the hybrid monopile foundation has a larger lateral resistance and stiffness than the monopile.

Effect of scour erosion on mode shapes of a 5 MW monopile-supported offshore wind turbine

This paper investigates the influence of scour erosion on the mode shapes of the National Renewable Energy Laboratory (NREL) 5 MW offshore wind turbine (OWT) in varying soil conditions, where modes are derived from wind and wave-induced accelerations. A numerical model, which considers aerodynamic and hydrodynamic loading with soil-structure interaction (SSI) and servo-dynamics, is developed. The superstructure is modelled using OpenFAST and the foundation is modelled with SESAM, where SSI is incorporated using Winkler (p-y) springs. The foundation stiffness is subsequently integrated in OpenFAST using mudline interface springs. The influence of soil density, scour hole depth, and loading on the derived mode shapes is studied. Accelerations are extracted at nine locations along the tower. The first two system natural frequencies and mode shapes are estimated using frequency domain decomposition (FDD) applied to the acceleration time-histories for each scour depth and soil condition. Modal Assurance Criterion (MAC) values are calculated to evaluate the changes in mode shapes that occur due to scour. It is shown that the mode shapes present consistent changes as scour depth increases, and the second mode shape exhibits more sensitivity to scour. The sensitivity of mode shape changes to scour under variations in wind speed and measurement noise are also investigated.