Monopile-supported Offshore Wind Turbines Research Articles

Experimental study on monopile-supported offshore wind turbine subjected to long-term wind and wave cyclic loading

Offshore wind energy has recently gained much attention. During its service life, a monopile-supported offshore wind turbine (OWT) is subjected to long-term wind and wave lateral cyclic loads with different cycle characteristics, inducing accumulated deformation of the OWT system to exceed the serviceability limit state and causing safety accidents. In this study, a 1g scaled model test with a similarity ratio of 1:100 was conducted to investigate the lateral response of a monopile-supported OWT in sand under long-term wind and wave cyclic loads. Two sets of centrifugal gear cyclic loading devices applied stable long-term wind and wave cyclic loads and different cyclic load amplitudes and directions were achieved by adjusting the input voltage and counterweight masses. Four groups of long-term cyclic loading tests were conducted for the monopile-supported OWT, considering different wind load simplification methods and various wind-wave load contribution ratios. The lateral displacement and accumulated tilt of the OWT were monitored using two laser displacement transducers installed at different heights. The results show that a simplistic treatment of wind loads as static loads results in an overestimation of the cumulative rotational deformation, and an increase in the wind-wave load contribution ratio decreases the cumulative tilts of the OWT structures.

Study on the weakening of soil-structure interaction under long-term lateral load for monopile-supported offshore turbines

The monopile-supported offshore wind turbines are located in a complex marine environment, bearing various long-term cyclic loads such as wind loads, waves, and currents. To explore the weakening of soil-structure interaction under long-term lateral load for the pile foundation, a series of model tests were conducted in the laboratory under normal gravity conditions, and the evolving trends for the weakening coefficients of varying soil depths and pile diameters were analyzed, leading to the proposal of a new expression for the weakening coefficient considering the governing factors in soil-structure interaction. The new formula for the weakening coefficient proposed in this paper is validated through the experimental results of this study and other references. Through a numerical example, affirming the viability of the soil stiffness attenuation model based on the finite element analysis method and revealing the weakening law of foundations for different cycling processes. This research contributes valuable insights for designing and analyzing offshore turbine foundations, particularly in the context of weakening phenomena.

3D multiscale dynamic analysis of offshore wind turbine blade under fully coupled loads

Wind turbine blades are typically sophisticated structures with complex geometry and composite layup. The realistic loads acting on blades serviced in harsh offshore environments are fully coupled dynamic loads involving wind, wave, current, servo-control, gravity and other inertial loads. This work presents an easy-to-implement methodology for the time-domain detailed analysis of three-dimensional (3D) blade structure, by combining fully coupled dynamic simulator and general finite element program. As a practical example subjected to multiple marine environmental loadings, one of the rotating composite blades atop a 5-Megawatt (MW) monopile-supported offshore wind turbine (OWT) in soft clay is used for verifying this method step by step, and the effect of rotation and foundation flexibility on blade behaviours is explored. Good agreement between OpenFAST and proposed method in tip deflections and root reaction loads is visible from validation results, and the recommendations on how to further improve this method in future iterations are made. Subsequently, based on stress-time history of blade composite layups, time-domain 3D multiscale analysis including the failure evaluation for layers in outer shell and shear webs using Tsai-Wu strength criteria, as well as the fatigue damage assessment of carbon spar caps was performed.

Two-way coupling fluid–structure interaction analysis on dynamic response of offshore wind turbine

The monopile-supported offshore wind turbines (OWTs) are subjected to the wave and current loadings. Affected by currents and waves, monopile-supported OWTs are also vulnerable to scour. However, the effect of scour and currents on the dynamic behavior of the monopile are not fully appreciated. This study conducted the model tests to evaluate the effect of scour depth and flow velocity on the lateral responses of support structure. Furthermore, the two-way (T-W) coupling fluid–structure interaction (FSI) method was proposed to evaluate the dynamic responses of piles in sand. The support structure is modeled using ABAQUS, a finite-element models software package, by considering the nonlinear soil–structure interaction effects, STAR-CCM + tools has been employed to model the FSI which are fed into ABAQUS to predict the structure’s dynamic response. Using the proposed method, a parametric study has been conducted to evaluate the effects of scour depths, flow velocities on the dynamic responses of OWTs. The results indicate that scour altered the fundamental frequency of OWT, causing resonance. An increase in scour depth and flow velocity increased the dynamic responses of support structure, which are detrimental for the safety and stability of the OWT system.

Influence of soil plasticity models on offshore wind turbine response

AbstractWhile recent numerical modelling advances have enabled robust simulation of foundation hysteresis behaviour, uptake of these models has been limited in the offshore wind industry. This is partially due to modelling complexity and the unknown influence of including such soil constitutive models within a design philosophy. This paper addresses this issue by outlining a framework of an aero‐hydro‐servo‐elastic offshore wind turbine model that is fully coupled with a multisurface plasticity 1D Winkler foundation model. Comparisons between this model and industry standard aeroelastic tools, such as OpenFAST, are shown to be in good agreement. The hysteretic soil predictions are also shown to be in good agreement with CM6 Cowden PISA test piles, in terms of secant stiffness and loop shape. This tool has then been used to address the unknown influence of hysteretic soil reactions on the design of monopile supported offshore wind turbines against extreme conditions. This study demonstrates that a significant reduction in ultimate and service limit state utilization is observed when a multisurface plasticity foundation model is adopted, as opposed to industry standard pile–soil interaction models.

Influence of Blade Flexibility on the Dynamic Behaviors of Monopile-Supported Offshore Wind Turbines

At present, monopile-supported offshore wind turbines (MOWTs) are widely used in offshore wind farms. The influence of blade flexibility on the dynamic behaviors of MOWTs excited by waves and earthquakes was investigated in this study. Numerical analysis models were established for 5 MW and 10 MW MOWTs, incorporating flexible and rigid blade configurations. The modes and natural frequencies of the full system were compared between these two numerical models, and their dynamic responses were evaluated under wave-only and earthquake-only excitations. It was revealed that the influence of blade flexibility on the first- and second-order modes of the system can be neglected. The dynamic response of these MOWTs under wave excitation can be predicted by the rigid blade model, where the maximum relative difference is less than 5%. However, higher-order modes of the system are significantly affected by the blade flexibility. Under high-frequency excitations, these higher-order modes of the system are remarkably stimulated. Additionally, a large relative difference, exceeding 50%, is detected when the rigid blade model is used to predict the seismic response of the two MOWTs. Consequently, the blade flexibility should be adequately modeled when predicting the dynamic response of OWTs.

Effect of long-term lateral cyclic loading on the dynamic response and fatigue life of monopile-supported offshore wind turbines

Offshore wind turbines are often supported on monopiles and are always subjected to long-term cyclic loading during their service life. This cyclic loading induces changes in the damping, stiffness and permanent accumulated rotation of the monopile foundation. The main purpose of this paper is to investigate the effect of these three changes occurring simultaneously on the dynamic response and fatigue life of offshore wind turbines in sand. To this end, an integrated methodology is presented based on time-domain finite element model and small-scale model tests of rigid piles. Three states of the monopile foundation are selected and defined based on the operational time of the turbine. The results show that these three changes have a slight effect on the dynamic response of offshore wind turbines, but have a significant effect on the fatigue life. The fatigue life decreased from 23.3 years for the initial state to 20.99 years for the medium state and 19.45 years for the ultimate state, a decrease of 10% and 16.5%, respectively, indicating that these changes should be addressed in the design of the fatigue life calculation of offshore wind turbine structures. The systematic parametric analysis shows that soil damping has the greatest effect on the dynamic response and fatigue life, followed by soil stiffness, which is less affected by permanent accumulated rotation.

Dynamic response analysis of a monopile-supported offshore wind turbine under the combined effect of sea ice impact and wind load

The existence of drifting ice is a key challenge for structural design of offshore wind turbines (OWTs) in cold regions. To better understand the structural behavior of OWT under combined sea ice impact and wind load effects, this paper investigates the dynamic response of a monopile-supported OWT using the nonlinear finite element method. The interaction between the OWT and ice impact, wind loads and soil contact are addressed in the developed numerical models in LS-DYNA. The coupling between the main program and the aerodynamic damping model is achieved by a user-defined load subroutine. To calibrate and verify the ice material model and the simulation technique, model test data are used of ice impacts with a nearly vertical monopile foundation. In the case study, numerical simulations of the interaction between a typical 5-MW monopile-supported OWT and an ice sheet are performed under various combined load scenarios. The dynamic response characteristics are presented and the effects of ice drifting speed and mean wind speed are elucidated by statistical methods. Finally, insights into the ice loads are obtained by comparing the simulation results against two international design standards. The present study contributes to an improved understanding of load effects of monopile OWTs in cold regions.

Reliability analysis for monopile foundation of offshore wind turbine considering correlated wind and waves and spatially varying soils

The monopile-supported offshore wind turbines (OWTs) are increasingly installed in more challenging seabed environments, resulting that the reliability of the monopile foundation attracts more attention. The reliability of the monopile foundation is largely uncertain due to uncertainties in correlated wind and waves and spatially varying soils. To address this issue, a reliability analysis framework is suggested to assess the annual probability of failure of the monopile foundation, that considers the correlated wind and waves, and spatial variability of soil properties. In this framework, a two-stage fragility surface of the monopile foundation is constructed. A joint probability distribution is calibrated using copulas to describe the correlation between wind and wave conditions. Spatially varying soils are modeled by the random field method. An example is provided to illustrate the application of the framework. The results suggest that the reliability of the monopile foundation may be overestimated if the spatial variability of soil properties is ignored. Additionally, the assumption that wind and waves are fully correlated may lead to an over-conservative reliability analysis.

Influence of seabed profile on the seismic response of monopile-supported offshore wind turbines including dynamic soil-structure interaction

In this article, the seismic response of four different Offshore Wind Turbines (OWTs) from 5 to 15 MW, founded on monopiles embedded in homogeneous and non-homogeneous soil profiles with equivalent shear-wave velocities from 100 to 300 m/s is analysed. Two types of variable soil profiles with a semi-parabolic variation of the shear-wave velocity as depth increases are considered. The system seismic response under ten different accelerograms is computed through a finite element substructuring model in frequency domain. The foundation behaviour is obtained by a continuum model including kinematic and inertial interaction. Several models, in addition to that of rigid base condition, are considered to determine the influence of soil-structure interaction (SSI), as well as the contribution of each kinematic interaction (KI) factor. The seismic response of the OWTs is obtained in terms of maximum shear forces and bending moments at mudline level, as well as the acceleration amplification factor at hub level. It is found that the maximum response is produced when SSI is considered, including both the inertial and kinematic interaction. The differences arising due to the soil profile definition are shown to be mainly related to the rotational KI factor, which significantly affects the second vibration mode of the system. The largest responses are obtained for the homogeneous equivalent profile.

A Rayleigh-Ritz solution for high order natural frequencies and eigenmodes of monopile supported offshore wind turbines considering tapered towers and soil pile interactions

Offshore wind turbines may be exposed to extreme or accidental loads during the service life, such as extreme water slamming, ship collisions, earthquakes, extreme winds, ice loads, etc. Under the action of such loads, they will respond not only in the primary eigenmode, but also high order modes. This paper presents a Rayleigh-Ritz solution for high order natural frequencies and eigenmodes of monopile supported offshore turbines. The model considers explicitly tapered wind turbine towers and soil-pile interactions. Different soil conditions are considered using equivalent cross coupled springs at the mudline. The proposed Rayleigh-Ritz solution is applied to the DTU 10 MW wind turbine mounted on a monopile foundation. The predicted natural frequencies and eigenmodes are compared with nonlinear finite element simulations using USFOS. The results show excellent agreement for the first 3 eigenmodes. Even higher eigenmodes than the 3rd order are also calculated but are not presented. They can be readily obtained if needed.High order natural frequencies and eigenmodes using the proposed Rayleigh-Ritz solution can, together with the Duhamel's integral, be well utilized in the buckling and collapse analysis and design of monopile supported offshore wind turbines in Ultimate Limit States (ULS) and Accidental Limit States (ALS). In addition, the high accuracy of the predicted primary natural frequency makes it a useful design tool to help avoid undesired resonance from external excitation loads and to reduce resonance related fatigue effects in Fatigue Limit States (FLS).

Effective foundation damping prediction of monopile-supported offshore wind turbines based on integrated fitting equation and PSO–SVM algorithm

Foundation damping affects the fatigue life of offshore wind turbines. However, the effective damping ratio determined using a finite element analysis is not always consistent with the input. A series of simplified finite element models were established using realistic structural properties for a monopile-supported wind turbine to analyze the influence of wall thickness, pile embedment length, structure density, and input damping ratio on the calculated effective damping ratio. An empirical fitting equation was proposed using a power function based on the comprehensive laws between various factors and the effective value. An integrated prediction approach combining this equation with a hybrid support vector machine and particle swarm optimization algorithm was subsequently described and its results compared with those of other intelligent algorithms. The results showed that the effective damping ratio was much smaller than the input value, with the former reaching a maximum of only 15% of the latter. The integrated prediction approach was shown to strike an equilibrium between computing efficiency and forecast accuracy. Considering the soil–structure interaction and elastoplastic behavior of soil only increased the effective damping ratio to 35% of the input value, and also obfuscated the roles of the various factors in the effective damping.

Seismic analysis of a monopile-supported offshore wind turbine considering the effect of scour-hole dimensions: Insights from centrifuge testing and numerical modelling

Monopile-supported offshore wind turbines (OWTs) deployed in seismic-active regions are inevitably threatened by the combined effect of earthquake and scour. However, current codes and research fail to adequately address the seismic response of OWTs under different scour conditions. In this study, a series of centrifuge shaking table tests were conducted on a monopile-supported OWT model subjected to both local scour and general scour. The test results show that the amplification effect of general scour site is more pronounced than that of local scour site. Moreover, the maximum bending moment from the general scour cases are mostly larger than those from the local scour cases under relatively large scour depth. This indicates the possibility of an over-conservative design when simply reducing the local scour condition to general scour condition. Then, based on the centrifuge tests, a practical 2D finite element model is proposed, in which not only the inertial and kinematic effects but also the effect of scour-hole dimensions on the ground motion input pattern and the vertical effective stress along the pile are incorporated. Further parametric study shows that the kinematic effect plays a significant role in the pile response and cannot be simply neglected even under scoured conditions.

Evaluation of post-storm soil stiffness degradation effects on the performance of monopile-supported offshore wind turbines in clay

Over their lifetime, offshore wind turbines (OWTs) will experience large storm surges that may negatively affect their structural performance. Current design codes do not provide sufficient guidance on tackling this problem. To address it, this study aims to provide a guideline on how shear stiffness degrades around monopiles in normally consolidated clays after a storm surge. High-fidelity finite element models of monopile-supported OWTs are built and validated with 5.5e-05 mean square error against centrifuge tests. Parametric analysis of 108 scenarios is conducted, considering different pile diameters, lengths, soil strengths, and loading scenarios. Pile toe displacements are recorded and used to define a system that can classify the OWT monopiles as flexible or rigid. Shear stiffness degradation ratios at 1D (pile diameter) depth are utilized to predict the change of natural frequency, given the mudline loads. The data suggest that over 5% or 10% decrease in natural frequency indicates that pile head deformations exceed serviceability or ultimate limit state criterion, respectively. A database of soil strain - stiffness degradation ratio pairs is tabulated along each simulated pile as a reference for degraded p-y curve calibration. The proposed method and database are crucial for safer design and quicker assessment of OWTs.

Contribution of the rotational kinematic interaction to the seismic response of monopile-supported offshore wind turbines

This paper presents a study on the role of kinematic interaction effects on the seismic response of large monopile-supported offshore wind turbines. For this purpose, a finite element substructuring model is used to analyse the behaviour of three reference wind turbines with rated powers of 5, 10 and 15 MW founded on several soil profiles and subjected to different accelerograms. The foundation response is computed through a continuum model which includes soil–structure interaction. The effects of inertial and kinematic soil–structure interaction on the system seismic response is evaluated also through comparisons with results corresponding to the infinitely rigid base condition. It is found that neglecting kinematic interaction factors could lead to underestimate the seismic response of large monopile-supported offshore wind turbines. The rotational kinematic interaction factor is shown to increase the OWTs seismic response.

Fatigue life analysis of monopile-supported offshore wind turbines based on hyperplastic ratcheting model

Fatigue life is an important factor to be considered in foundation design of offshore wind turbines (OWTs), and accurate prediction of fatigue damage is dependent on an appropriate foundation model. Currently, the rigid foundation is widely used to model the monopile foundation in most aero-hydro-servo-elastic simulation tools, and soil stiffness and damping are not considered. This will lead to an inaccurate estimation of fatigue damage of monopile-supported OWTs. Given this limitation, hyperplastic ratcheting model is incorporated into FAST v7, the variations of stiffness, damping and accumulated displacement for monopile-supported OWTs subjected to cyclic loadings are well considered. The validity of the implementation is demonstrated by presentation of non-linear soil resistance-displacement curves, and a series of hysteresis loops can be observed. Based on this integrated tool, extensive simulations and analyses for monopile-supported OWTs under different environmental states are performed. Furthermore, the effect of pile-soil interaction on fatigue life of monopile is quantified. The aim of this paper is to provide an accurate model to estimate fatigue life of monopile-supported OWTs.

Reduction of wave load on monopile-supported offshore wind turbine by a gear-type plate

This study investigates the wave diffraction problem of a vertical cylinder with a gear-type plate by using the linear wave theory and the method of eigenfunction expansion. The present model is demonstrated to be accurate, through a comparison with the numerical results of the boundary element method. The horizontal wave force acting on the cylinder can be reduced considerably by the gear-type plate. Meanwhile, the variation in bending moment at the bottom of the cylinder depends on the wave conditions and geometry parameters. A low value of the ratio between water depth and cylinder radius (d/r0) can result in an increase in the bending moment on the cylinder, rather than a decrease, due to the additional loads exerted by the gear-type plate. The bending moment on the cylinder can be reduced significantly by the gear-type plate as d/r0 increases. Within d/r0∈[4,10] of the present monopile-supported wind turbines, the wave loads on the monopile foundation can be reduced significantly by the proposed gear-type plate. Compared with a ring-type plate, a gear-type plate exhibits superior reduction effects over an equal plate area. The loads on the structures are insensitive to the wave exposure angle when the teeth number increases beyond three.

Lateral vibration control of monopile supported offshore wind turbines with toroidal tuned liquid column dampers

This study investigates the lateral vibration control efficiency of toroidal tuned liquid column dampers (TTLCDs) for monopile supported offshore wind turbines. The TTLCD has the advantage that it can be tuned to match the natural frequencies of wind turbines both in fore-aft and side–side directions simultaneously. A general mathematical modeling of the TTLCD inside the nacelle is derived and an interface between the simulation platforms MATLAB/Simulink and FAST is established. A TTLCD design procedure is provided for monopile wind turbines. Numerical calculations are carried out for different wind velocities, wave-wind misalignment angles and working conditions considering fatigue and ultimate load cases. A parametric analysis is performed on the flow resistance coefficient of TTLCDs. The results demonstrate that TTLCDs can effectively control lateral vibrations and improve the structural performance of monopile wind turbines against wind and wave loads.

Constructing simplified models for dynamic analysis of monopile-supported offshore wind turbines

Accurately modeling the complicated dynamic mechanisms of offshore wind turbines (OWTs) requires high numerical fidelity. However, since these models are computationally expensive for both forward and inverse problems, simplified OWT models are needed. The present study develops two approaches to constructing simplified numerical models based on full-order finite element models, which aim to accurately and efficiently simulate dynamic behaviours of monopile-supported OWTs. The first approach decomposes the structure into substructures and establishes a reduced-order model based on the similarity principle of modal parameters and time-domain responses, in accordance with full-order models. The second approach uses proper orthogonal decomposition and modal truncation techniques to construct a state-space model, which produces consistent time-domain responses with full-order models. For both approaches, model updating is used to improve the accuracy of the simplified models. The effectiveness of the approaches is demonstrated through the monopile-supported, 10 MW DTU reference wind turbine. The results show that the computational time required for both simplified models is less than 10% that of the full-order model, without the compromise of accuracy, i.e., less than 5% difference in modal frequency and an excellent agreement in time-domain responses. The proposed approach provides an efficient and accurate dynamic analysis solution for offshore structures.

Sensitivity of the seismic response of monopile-supported offshore wind turbines to soil variability

Sensitivity of the seismic response of monopile-supported offshore wind turbines to soil variability