Performance Of Offshore Wind Turbine Research Articles

Effects of local scouring on capacities of monopile-wheel composite foundations in sand under lateral loading

This paper focuses on an innovative foundation type of monopile-friction wheel composite foundations, which aim to enhance the loading performance of offshore wind turbines (OWTs) while mitigating the negative impact of local scouring. The study utilizes numerical simulations to investigate the morphology of local scouring under unidirectional flow. Furthermore, the finite element method is employed to quantify the lateral bearing behavior, considering different scouring parameters, loading point heights, and pre-torsional loads. On this basis, the load bearing ratio of each part of the composite foundation under different scour conditions is also obtained. The results reveal that the horizontal bearing characteristics are significantly influenced by different scour parameters, loading point heights and pre-torsional loads. The local scouring can reduce the bearing capacity by about 38%, with scouring depth having the greatest effect, followed by the scouring angle and extent respectively. Moreover, as the loading point height and pre-torsional load increase, the lateral bearing behavior of composite foundations decreases. Notably, the friction wheel contributes more than 40% to the overall bearing capacity, underscoring its importance. The results provide valuable insights into further understanding the bearing behavior of the composite foundation and its potential engineering applications.

Study on bearing performance and reliability of modified monopiles for OWTs using deterministic and probabilistic analysis methods

Existing studies mainly focus on the bearing capacity of monopiles, with less emphasis on reliability analysis, which is an important aspect of the performance of offshore wind turbine (OWT) foundations. This paper investigates the bearing capacity and reliability of traditional and hybrid monopiles using deterministic and probabilistic analyses. The three-dimensional finite element model (FEM) considering the structure-soil interaction is established and its accuracy is verified through comparisons with existing research findings. The responses of monopiles under wind and wave loads are compared in detail, including the rotation angle at the mudline, structural stress, and soil displacement. The environmental design parameters are obtained based on the 30-year annual extreme data, and the correlation between these environmental parameters is considered to obtain a large amount of random environmental parameter data. Then, the back propagation neural network (BPNN) and the response surface method (RSM) are combined to construct the failure response surface. Research shows that the reliability index of the plated monopile does not improve significantly, while the skirted-monopiles perform much better than the traditional monopile. The proposed FEM-BPNN-RSM coupled methodology provides a more comprehensive understanding of the monopile bearing capacity, and can be used for the future monopile structure design of OWTs.

Investigation on dynamic performance of wind turbines using different scaling methods in wind tunnel tests

The relationship between rotor thrust (T) and tip speed ratio (TSR) directly influences the response of the scaled offshore wind turbine in the wind tunnel test. In this paper, a series of 1:100 scaled wind tunnel tests based on NREL 5 MW wind turbine are designed to study the dynamic performance of the wind turbine based on two different T-TSR scaling methods: (1) geometrically scaled blade with increased wind speed (GB-IW) and (2) redesigned blade with scaled wind speed (RB-SW). Test result indicates that the GB-IW method can meet the requirement of T, but unavoidably result in the distortion of TSR (40.2 % to 61.7 % lower than the target). However, using the RB-SW method, the T-TSR curve can be simulated well under different operational conditions. The amplitudes of structural response including base moment, nacelle acceleration and displacement from the GB-IW method are much lower than those from the RB-SW method, indicating that the GB-IW method leads to an overestimation of damping ratio but underestimation of the fluctuation of both natural frequency and structural response. The RB-SW method can provide a more reasonable estimation of structural dynamic performance, compared with the GB-IW method. This study confirms the importance of selecting reasonable scaling method for wind tunnel test, which is critical to the understanding of the complex aerodynamic and structural dynamic performance of offshore wind turbines.

Cumulative cyclic response of offshore monopile in sands

Satisfactory performance of offshore wind turbines is governed by the cyclic response of its foundation under wind and wave. Most methods of evaluating the lateral cumulative deformation of monopile due to cyclic loading are based on small-diameter pile tests installed in dry sand. This paper investigates the cumulative deformation of large-diameter monopiles installed in saturated sand. A numerical model was developed employing FLAC3D and was validated by comparing its predictions with the results of triaxial tests and centrifuge tests. The validated numerical model was used to evaluate and compare the cyclic responses of monopiles between saturated case and no pore pressure case. To evaluate the cumulative deformation for offshore monopile due to cyclic loading, the numerical model was refined to account for the effects of number and amplitude and frequency of cyclic loading, soil permeability coefficient, soil relative density and pile diameter. It was found that the larger subsidence and the less capacity of soil around the pile is caused by accumulation and dissipation of transient excess pore pressure, thus the monopile in saturated sand would experience the larger cumulative deformation. Correspondingly, based on this parametric study, an analytical model was developed to calculate the pile-top cumulative displacement of offshore large-diameter monopile under lateral cyclic loading considering the effects of pile diameter. This analytical model was validated against the results of centrifuge tests and model pile tests in saturated sands.

Seismic response of an offshore wind turbine jacket structure with pile foundations

Offshore wind farm jacket structures are preferable in the deeper waters due to their flexible construction and cost-effectiveness. However, both wind tower and jacket structure are sensitive to cyclic/dynamic loading. With the rapid advancement in offshore wind farms globally, they are being constructed in the seismic regions of the world, such as offshore Taiwan, Japan, and South Korea. Therefore, there is an urgent need to understand the seismic performance of offshore wind turbines. This research investigates the behavior of an offshore wind turbine jacket structure with pile foundations subjected to earthquake loading using dynamic centrifuge testing. A jacket-equivalent wind tower model was developed and tested under the two-layered sandy soil with a dense bottom layer overlain by a loose top layer to capture the liquefaction effects under seismic loading. Using the centrifuge test data, it will be shown that with the build-up of excess pore pressure in the soil causing liquefaction under strong seismic loading, the offshore wind turbine (OWT) structure suffers severe settlements and rotations that exceed the prescribed limits. However, it was possible to restrict the settlements and rotations by having an alternative arrangement that places the pile cap on the mudline. These results highlight the need to consider vertical settlements induced by the liquefaction in addition to the restriction of the rotations at the mudline level of the foundation and the need for more research on innovative ways to mitigate liquefaction induced settlements.

Inclusion of small-strain stiffness in monotonic p-y curves for laterally loaded piles in clay

The small-strain performance of offshore wind turbine (OWT) pile foundations under lateral loading is critical for the foundation safety and the fatigue analysis of superstructure but is less explored in recent studies. This work presents p-y curve analyses of monopiles in marine clay that account for complete nonlinear soil stress–strain relationship. Based on a small-strain stress–strain relation, Total-displacement-loading Extended Mobilisable Strength Design (T-EMSD) analyses are performed to interpret the pile-soil interaction. On the basis, an analytical p-y curve model is proposed. An elastoplastic constitutive model with small-strain non-linearity is developed, and then p-y curve validation against finite element simulations is carried out. Furthermore, by comparing to centrifugal test observations and existing p-y models, good agreement and noticeable improvement are shown. This paper provides industry engineers with a timely and effective approach for OWT foundation design, allowing for inputting soil characteristics measured at in-situ site and laboratory.

Simplified method for performance-based seismic design of suction caissons supporting jacket offshore wind turbines

The paper studies the performance of Offshore Wind Turbines (OWTs) founded on Suction Bucket Jackets (SBJs) in clay under combined wind and seismic loading. A detailed 3D FE model of the soil–foundation–structure (SFS) system is developed and used as a benchmark to assess the efficiency of an enhanced Winkler-based "Caisson-on-Winkler-Soil" (CWS) model, where the soil is replaced by nonlinear hysteretic elements. The proposed CWS model captures residual deformations and hysteresis and offers physical coupling between vertical and moment loading. It allows excellent prediction of the H – M failure envelope in the most relevant first quadrant of H – M space, where the loads act in the same direction. Despite its successful application for the simulation of inertial loading, the CWS model fails to reproduce the dual shearing mechanism that develops at the caisson shaft during shaking, stemming from the combination of kinematic soil shearing due to the vertically propagating shear waves, and shearing due to the superstructure inertial response, thus underpredicting the co-seismic caisson settlements. For the prediction of the latter, the research utilizes spectrum compatible input motions and 3D FE models of varying geometric and material properties to derive linear regression equations that correlate the co-seismic dimensionless settlement of caissons ( w E / D ) with characteristic dimensional variables of the problem under investigation and the Arias Intensity ( I A ) of the surface ground motion. As a final step, the paper proposes a hybrid method for performance-based assessment of SBJ OWTs. The proposed method employs the simplified CWS model to calculate the VHM loads and approximately estimate horizontal displacements and rotations at the jacket legs, followed by a preliminary assessment of caisson settlements using the correlations of w E / D with I A , on the basis of spectrum-compatible input motions. • A performance-based seismic assessment method of Suction Bucket Jackets is proposed. • A Winkler-type model is developed, capturing residual deformations and hysteresis. • Permanent caisson deformation under combined wind and seismic loading is explored. • The deficiency of decoupled methods to capture residual settlements is explained. • Caisson settlement is correlated with statistically significant Intensity Measures.

Numerical Analysis and Parameter Optimization of J-Shaped Blade on Offshore Vertical Axis Wind Turbine

In this study, the performance of offshore wind turbines at low tip speed ratio (TSR) is studied using computational fluid dynamics (CFD), and the performance of offshore wind turbines at low tip speed ratio (TSR) is improved by revising the blade structure. First, the parameters of vertical axis offshore wind turbine are designed based on the compactness iteration, a CFD simulation model is established, and the turbulence model is selected through simulation analysis to verify the independence of grid and time step. Compared with previous experimental results, it is shown that the two-dimensional simulation only considers the plane turbulence effect, and the simulation turbulence effect performs more obviously at a high tip ratio, while the three-dimensional simulation turbulence effect has well-fitting performance at high tip ratio. Second, a J-shaped blade with optimized lower surface is proposed. The study showed that the optimized J-shaped blade significantly improved its upwind torque and wind energy capture rate. Finally, the performance of the optimized J-blade offshore wind turbine is analyzed.

Numerical Simulations of the Monotonic and Cyclic Behaviour of Offshore Wind Turbine Monopile Foundations in Clayey Soils

Most of the reported centrifuge tests available in the existing literature on offshore wind turbine foundations are focused on the behaviour of monopiles in sands, but very few studies on clayey soils can be found, due to the very long saturation and consolidation periods required to properly conduct experiments in such materials. Moreover, most of the reported numerical simulations using finite element analyses have been validated with monotonic centrifuge tests only. In this research, both monotonic and cyclic performance of offshore wind turbines in clay are validated and justified. The relationship between the monopile rotation in clays and the geometry and strength of the soil has been found and quantified. A prediction of the rotation for a high number of cycles of loading, based on the one experienced by the pile during the first cycle, can be obtained using the correlation derived in the paper. For those cases in which the rotation does not reach a steady value after a high number of cycles, the cumulative rate has been found significantly larger than the prediction conducted with standard analytical methods. A new design methodology for the design of offshore monopile foundations in clay is presented.

Seismic performance of offshore wind turbine in the vicinity of seamount subduction zone

The presence of seamount on the subducting interface of oceanic and continental tectonic plates has potential to alter the characteristics of ground motion due interface earthquakes. In this study, we address the influence of seamount presence on offshore wind turbine response, through a finite element simulation of earthquake and subsequently applying the recorded shaking from earthquake simulation as input to offshore wind turbines. The wind turbine response is computing by incremental dynamic analysis and fragility curves are derived for serviceability and ultimate limits of wind turbines. The effect of hypocenter being above or below the seamount, on the offshore wind turbine fragility is also addressed.

Offshore wind turbine reliability and operational simulation under uncertainties

AbstractThe fast‐growing offshore wind energy sector brings opportunities to provide a sustainable energy resource but also challenges in offshore wind turbine (OWT) operation and maintenance management. Existing operational simulation models assume deterministic input reliability and failure cost data, whereas OWT reliability and failure costs vary depending on several factors, and it is often not possible to specify them with certainty. This paper focuses on modelling reliability and failure cost uncertainties and their impacts on OWT operational and economic performance. First, we present a probabilistic method for modelling reliability data uncertainty with a quantitative parameter estimation from available reliability data resources. Then, failure cost uncertainty is modelled using fuzzy logic that relates a component's failure cost to its capital cost and downtime. A time‐sequential Monte Carlo simulation is presented to simulate operational sequences of OWT components. This operation profile is later fed into a fuzzy cost assessment and coupled with a wind power curve model to evaluate OWT availability, energy production, operational expenditures and levelised cost of energy. A case study with different sets of reliability data is presented, and the results show that impacts of uncertainty on OWT performance are magnified in databases with low components' reliability. In addition, both reliability and cost uncertainties can contribute to more than 10% of the cost of energy variation. This research can provide practitioners with methods to handle data uncertainties in reliability and operational simulation of OWTs and help them to quantify the variability and dependence of wind power performance on data uncertainties.

Effect of platform disturbance on the performance of offshore wind turbine under pitch control

AbstractThe aerodynamic performance of offshore floating wind turbines (OFWTs) is more complicated than onshore wind turbines due to 6‐degree of freedom (DOF) motion of the floating platform. In the current study, the aerodynamic analysis of a horizontal‐axis floating offshore wind turbine is performed with the aim of studying the effects of floating platform movement on the aerodynamic characteristics of the turbine in the presence of a pitch angle control system. The National Renewable Energy Laboratory (NREL) 5‐MW offshore wind turbine is selected as the baseline wind turbine. For this sake, the unsteady blade element momentum method with dynamic stall and dynamic inflow models have been employed to obtain the unsteady aerodynamic loads. The baseline pitch angle control system is assumed to be coupled with the aerodynamic model to maintain the rated condition of the wind turbine and also to approach a closer model of wind turbine. In case of pitching motion input, the reduction of mean power coefficient for tip speed ratios (TSRs) less that 7 is expected by an amount of 16% to 20% at pitch amplitude of 2° and frequency of 0.1 Hz. For high TSRs, the trend is reverse with respect to fixed‐platform case. The mean thrust coefficient is reduced for almost all range of TSRs with maximum loss of 37%. Moreover, the mean control pitch angle that is an index of control system effort is increased. The results also represent the importance of considering the pitch control system for aerodynamic analysis of disturbed OFWT.

Study on Hydrodynamic Performance Model of Offshore Wind Turbine System Based on Time-domain Coupling Analysis

Wu, H.-T. and Gu, X.-P., 2019. Study on hydrodynamic performance model of offshore wind turbine system based on time-domain coupling analysis. In: Gong, D.; Zhu, H., and Liu, R. (eds.), Selected Topics in Coastal Research: Engineering, Industry, Economy, and Sustainable Development. Journal of Coastal Research, Special Issue No. 94, pp. 337–341. Coconut Creek (Florida), ISSN 0749-0208.The development of wind energy alleviates the issue of energy shortage at present, but the development technology of offshore wind energy is not mature, so the research on the performance of offshore wind turbine system is the key to develop clean wind energy. Based on the theory of time-domain coupling analysis, this paper studies the hydrodynamic performance of offshore wind turbine under environmental load, and probes into the hydrodynamic performance of offshore wind turbine system. It is found that the offshore wind turbine will stop operation when the wind speed is greater than 28m/s, and the calculation results show that the offshore wind turbine in this study meets the requirements of the specification for the stability of large inclination angle. The environmental load determines the initial offset of the offshore wind turbine system, in which the mean wave drift force is much smaller than the offshore wind force. With the increase of wind speed, the wind turbine system can automatically adjust the pitch of the blades. When the wind speed exceeds the limit wind speed, the wind turbine stops working. When operating at rated power, wind turbine occurs under the action of wind force will have an offset, which is large. The offset of wind force and wind turbine decreases with the increase of wind speed. This study provides a theoretical basis for the application of offshore wind energy development system.

Model tests on performance of offshore wind turbine with suction caisson foundation in sand

The performance of steel caisson during and after installation with different penetration velocities in medium dense sand is presented. The applied jacking forces, the amount of formed soil heave and bearing capacity were measured in the model tests. The influence of penetration velocities on jacking forces, soil heave and bearing capacity were also discussed in detail. The results indicated that the jacking forces for caisson in medium dense sands were significantly affected by the penetration velocity. The larger the penetration velocity, the more soil flowed into the caisson cavity during installation. This will lead to larger inner shaft resistance and in turn more jacking forces required for the same penetration depth. The height of soil heave during installation increases with penetration velocity. The m value calculated by the ratio of the volumes of the soil heave to that of the penetrated caisson wall can be used to evaluate the soil heave. The larger the applied velocity, the larger the m value and larger bearing capacity of caisson after installation. The relationship between the m value and penetration velocity can be used to control the soil heave for a steel caisson with a wall thickness to external diameter ratio of 4.2% in medium dense sand by jacking method.

Using Multiple Fidelity Numerical Models for Floating Offshore Wind Turbine Advanced Control Design

This paper summarises the tuning process of the Aerodynamic Platform Stabiliser control loop and its performance with Floating Offshore Wind Turbine model. Simplified Low-Order Wind turbine numerical models have been used for the system identification and control tuning process. Denmark Technical University’s 10 MW wind turbine model mounted on the TripleSpar platform concept was used for this study. Time-domain simulations were carried out in a fully coupled non-linear aero-hydro-elastic simulation tool FAST, in which wind and wave disturbances were modelled. This testing yielded significant improvements in the overall Floating Offshore Wind Turbine performance and load reduction, validating the control technique presented in this work.

Long term response analysis of TLP-type offshore wind turbine

The performance of offshore wind turbine supported with different configurations of Tension-leg-platform (TLP) are studied for vertical plane motion responses (surge, heave, and pitch) along with the side-to-side, fore–aft, and yaw tower base bending moments. The long-term distribution is carried out using the short-term floating wind turbine responses based on Rayleigh distributions and North Atlantic wave data. The long-term response analysis is performed for the 5 MW TLP-type offshore wind turbine. The study aims at predicting the most probable maximum values of motion amplitudes that can be used for design purposes. The transfer functions for surge, heave and pitch motions of the floater are obtained using the FAST code. The performance of floating structure in the long-term analysis not only depends on the transfer functions but also on the careful selection of design wave spectrum model. Among different theoretical design wave spectrum models, three models are chosen that closely represents the sea states and the response spectrums are computed for these models. As the nature of the response spectrum of the floating structure is analogous with the input wave spectrum model, it can be assumed to have the same probabilistic properties and modeled as a stationary stochastic process. The long-term probability distributions for TLP-type floater configuration for surge, heave and pitch motion amplitudes along with the tower base bending moments are used for design purposes, so as to guarantee the safety of the floating wind turbines against overturning/capsizing in high waves and wind speed. The calculation of the long-term distribution using FAST will help in the preliminary analysis of the performance of floaters in the study of wave-induced response of floaters.

Coupled-crushing analysis of a sea ice-wind turbine interaction – feasibility study of FAST simulation software

ABSTRACTThe Gulf of Bothnia has the potential for large capacity wind farms because of the relatively high and constant wind velocities. The mostly shallow coastal areas enable cost-efficient foundation and grid connection. However, the sea freezes annually introducing the most significant uncertainties in the support structure design for offshore wind turbines. The magnitude and time variation of sea ice load depends on various factors, like the thickness and velocity of the ice as well as the size and shape of the structure. A feasibility study of the FAST (fatigue, aerodynamics, structures and turbulence) simulation software was carried out investigating the structural performance of offshore wind turbines. Various load combinations and operation modes were studied by taking into account coupling between the ice, wind and structural response. Coupled modelling of ice–structure interaction is a necessary step in terms of cost-efficient structural design.

Numerical analysis of the long-term performance of offshore wind turbines supported by monopiles

Offshore wind turbines (OWT) are often supported on large-diameter monopiles and subjected to cyclic loading such as wind and wave actions. The cyclic loading can lead to an accumulated rotation of the monopile and a change in the foundation stiffness. This long-term effect is not yet well understood. This paper presents a three-dimensional finite element model for analyzing the long-term performance of offshore wind turbines on large-diameter monopiles in sand in a simple way. In this model the characteristics of pile-soil interaction under long-term cyclic lateral loading, observed from well-controlled laboratory model tests, are taken into account. A parametric study has been conducted for a full-scale wind turbine supported on a large-diameter monopile, with focus on the influence of several design parameters on the deformations of the monopile and the tower supporting the wind turbine. The study shows that under the serviceability limit state, the deflection and rotation at pile head in the case of considering the effect of long-term cyclic loading are notably greater than that computed in the case where this long-term effect is ignored. This significant difference suggests that the long-term cyclic loading effect cannot be overlooked in design and analysis.

Predicting long term performance of offshore wind turbines using cyclic simple shear apparatus

Offshore wind turbine (OWT) foundations are subjected to a combination of cyclic and dynamic loading arising from wind, wave, 1P (rotor frequency) and 2P/3P (blade passing frequency) loads. Under cyclic/dynamic loading, most soils change their characteristics. Cyclic behaviour (in terms of change of shear modulus change and accumulation of strain) of a typical silica sand (RedHill 110) was investigated by a series of cyclic simple shear tests. The effects of application of 50000 cycles of shear loading having different shear strain amplitude, cyclic stress ratio (ratio of shear to vertical stress), and vertical stress were investigated. Test results were reported in terms of change in shear modulus against the number of loading cycles. The results correlated quite well with the observations from scaled model tests of different types of offshore wind turbine foundations and limited field observations. Specifically, the test results showed that; (a) Vertical and permanent strain (accumulated strain) is proportional to shear strain amplitude but inversely proportional to the vertical stress and relative density; (b) Shear modulus increases rapidly in the initial cycles of loading and then the rate of increase diminishes and the shear modulus remains below an asymptote. Discussion is carried out on the use of these results for long term performance prediction of OWT foundations.

Contribution of tuned liquid column gas dampers to the performance of offshore wind turbines under wind, wave, and seismic excitations

The main intention of the present study is to reduce wind, wave, and seismic induced vibrations of jackettype offshore wind turbines (JOWTs) through a newly developed vibration absorber, called tuned liquid column gas damper (TLCGD). Using a Simulink-based model, an analytical model is developed to simulate global behavior of JOWTs under different dynamic excitations. The study is followed by a parametric study to explore efficiency of the TLCGD in terms of nacelle acceleration reduction under wind, wave, and earthquake loads. Study results indicate that optimum frequency of the TLCGD is rather insensitive to excitation type. In addition, while the gain in vibration control from TLCGDs with higher mass ratios is generally more pronounced, heavy TLCGDs are more sensitive to their tuned frequency such that ill-regulated TLCGD with high mass ratio can lead to destructive results. It is revealed that a well regulated TLCGD has noticeable contribution to the dynamic response of the JOWT under any excitation.