Monopile Offshore Wind Turbine Research Articles

Numerical Study on Seismic Response of Offshore Wind Turbine Monopile in Multi-Layered Soil Profile of Arabian Sea

The seismic performance of monopiled offshore wind turbine (OWT) structures was evaluated numerically. The aim was to analyze offshore wind farm sites on complicated layered seabed with high seismicity. Following pile soil analysis (PISA) model, three-dimensional (3D) numerical evaluations were undertaken for two separate locations under two independent seismic events. The effects of pile diameter, depth, site impact owing to transverse soil layering, uni-directional and multi-directional seismic loading and seismic acceleration magnitude are presented. Dynamic impedance depth variation and site response analysis via lateral displacement, lateral soil response and Fourier response amplitude are explored in frequency and time domain. In a seismic zone with stratified soil, pile diameter has a greater influence. Variations in soil profile affect wind turbine performance and seismic sensitivity. This research will give a strong platform for later studies to recommend a safe wind farm site based on simulation results.

Dynamic response analysis of a real-world operating offshore wind turbine under earthquake excitations

This paper describes the dynamic monitoring campaign conducted on a monopile offshore wind turbine (OWT). An unexpected harvest is that the dynamic responses of the OWT under earthquake excitations were captured. Therefore, particular emphasis is placed on any abnormal behavior of the OWT. Operational modal analysis with the stochastic subspace identification method is used to reveal any variations in the modal parameters before, during and after the earthquake. The influence of the time-varying yaw angle on modal identification is derived. The results indicate that the earthquake had a colossal impact on the OWT as compared to the wind loading. The earthquake allows the identification of some of the high-frequency bending and twisting modes. Another critical finding is that the first frequency of the OWT increased after the earthquake, possibly implying some quick consolidation of the seabed adjacent to the OWT. Although earthquakes are often considered harmful to the safety of OWTs, it is possible for some special cases to improve the bearing capacity and stability of the monopile foundations. This paper could be the first study on the dynamic response of a real-world OWT under earthquake excitations, in which some beneficial effects of an earthquake are observed.

Unsupervised statistical estimation of offshore wind turbine vibration for structural damage detection under varying environmental conditions

Assessing the structural health condition of offshore wind turbines (OWTs) is necessary to ensure their later stages of service. A widely adopted strategy is to identify the damage by a significant deviation between the serviced structural responses and data obtained from undamaged structures. However, dynamic responses of OWTs show high dependency on ambient environments. To evaluate the variabilities of structural responses, in this paper an unsupervised statistical estimation method is proposed based on the integration of data bin processing and interval correlation techniques to eliminate the effects of environmental and operational conditions on evaluation results of a sensor, called a single indicator. Moreover, the improved network analysis model using information entropy has been established to consider the interactions of the multiple indicators for the valuation of the predicted results. As compared with the traditional methods, the proposed method combination between environmental variables clustering analysis and interval correlation could has demonstrated better robustness and higher computational efficiency using the developed probabilistic model. To validate the correctness and robustness of the proposed method, numerical simulations of a 5 MW monopile OWT by OpenFAST have been carried out. Based on the stiffness loss between 4% and 16% in natural frequencies and ten different wind speeds in the range of 5 to 23 m/s, four scenarios have been investigated to evaluate the sensitivity of damage detection. Results have shown that the proposed method has the ability to effectively and robustly assess the structural damage condition under different wind speed conditions. Furthermore, field test data from a 4 MW monopile OWT has been utilized to demonstrate the feasibility of the established statistical estimation method. It has been noted that the false alarm number of multiple indicators has been reduced to 0 from 3 in the case where there is no consideration of environmental condition classification and this evaluation result has well agreed with the data obtained under the actual working state of the OWT.

A simplified method for estimating the permanent accumulated rotation of an offshore wind turbine monopile throughout its design life

The permanent accumulated rotation is of great importance to the design of monopile foundations for offshore wind turbines because it is critical to the final dimensions of the monopile as well as its cost. Although design specifications require that the permanent accumulated rotation meet tolerances, they do not provide engineers with an appropriate method to calculate this value. This paper proposes a simplified method for estimating the permanent accumulated rotation of a monopile throughout its design life. To establish this method, a series of 1-g model tests were conducted in medium-dense sand to investigate the accumulative tendency of monopile rotation under typhoon and non-typhoon conditions. The results showed that the total accumulated rotation was mainly caused by typhoon events, further indicating that the static rotation generated by the maximum load magnitude among typhoon load sequences may maintain a certain proportional relationship with it. Then, the procedure for determining the two parameters in the method was illustrated by an NREL 5 MW wind turbine mounted on a monopile at a water depth of 28 m in the northern South China Sea. This method can provide a convenient tool for calculating the permanent rotation of a monopile foundation throughout its design life.

Dynamic analysis of offshore wind turbines subjected to the combined wind and ice loads based on the cohesive element method

Ice loads are an important and decisive factor for the safe operation of offshore wind turbines (OWTs). In severe environment load cases, it shall lead to prominent ice-induced vibration and ice-induced fatigue failure of OWT structures. Based on the cohesive element method (CEM) and considering the pile–soil interaction used by nonlinear distributed springs, the full interaction model of the ice and monopile OWT structure with an ice-breaking cone in a cold sea region is established in this study. Furthermore, the Tsai-Wu failure criterion and the empirical failure formula of maximum plastic failure strain are used to describe the mechanical behavior of ice bending failure in the collision simulation tool LS-DYNA, and the dynamic ice loads under different ice velocities and cone angles are statistically analyzed. Finally, according to the interaction process between sea ice and OWT containing the ice-breaking cone, the dynamic response of OWT under the combined wind and ice loads is studied, and the most reasonable ice-breaking cone angle is determined. The results show that the method adopted in this paper can well simulate the bending failure process of sea ice. Concurrently, the cone angle has a significant impact on the dynamic response and damage of the OWT, and the recommended optimal cone angle is 60.

Fatigue design sensitivities of large monopile offshore wind turbines

AbstractThe input for fatigue analyses of offshore wind turbines is typically chosen based on design values provided by design standards. While this provides a straightforward design methodology, the contribution of different input parameters to the uncertainty in the fatigue damage estimates is usually unknown. This knowledge is important to have when improving current designs and methodologies, and the parameters governing the uncertainty is typically found through a sensitivity analysis. Several sensitivity studies have been performed for monopile‐based offshore wind turbines, typically focusing on specific turbines and engineering disciplines. This paper performs a sensitivity study for three monopile‐based offshore wind turbines (5 MW, 10 MW, 15 MW) using parameters from several engineering disciplines. The results show that the fatigue utilization is primarily governed by the uncertainty in the SN curves and fatigue capacity. Following this, the uncertainty in the environmental conditions is the dominating uncertainty, with wind loads becoming increasingly important as turbine size increases. Additionally, the effect of modelling uncertainties is investigated. The wind‐related model uncertainties dominate in the tower top, while uncertainties in the wave and soil models dominate in the tower base and monopile. Designers wanting to reduce the uncertainty in a design are recommended to focus on the environmental conditions, and using as accurate models as possible. All modelling uncertainties are significant, but research should particularly be focused on wave directionality and soil models.

Experimental investigation of breaking regular and irregular waves slamming on an offshore monopile wind turbine

A series of experiments were carried out to investigate the breaking regular and irregular wave forces on a monopile foundation in a wave flume with a model scale factor of 1:40. Breaking regular wave slamming characteristic was firstly analyzed and comparisons were made between typical plunging and spilling breakers. For the irregular wave conditions, the wave slamming processes were recorded to illustrate the concentration and dissipation of wave energy. The effects of bottom slope, water depth, significant wave height and peak period on the wave slamming force and pressure distribution on the monopile were investigated. The results show that the distribution of maximum wave slamming pressure is affected by the water depth and bottom slope. Multiple harmonic components of the wave slamming force are observed and the predominant harmonic forces are first- and second- order components. The significant wave height and turbulence kinetic energy dissipation have significant effects on the wave slamming forces. A Gaussian distribution formula is proposed to fit the exceedance probability distribution of wave slamming force. Modified empirical formulas are proposed to calculate the wave slamming coefficient for the breaking regular and irregular waves on the monopile.

Probabilistic Response and Short-Term Extreme Load Estimation of Offshore Monopile Wind Turbine Towers by Probability Density Evolution Method

Probabilistic Response and Short-Term Extreme Load Estimation of Offshore Monopile Wind Turbine Towers by Probability Density Evolution Method

Multi-Hazard Fragility Analysis of Offshore Wind Turbine Portfolios using Surrogate Models

This paper deals with a surrogate modeling-based fragility estimate of monopile offshore wind turbine (OWT) towers subjected to wind and wave loadings. The monopile 5-MW OWT specified by the National Renewable Energy Laboratory (NREL), United States was used as the base model for this study. Aero- and hydrodynamic simulations of the OWT were first performed via Fatigue, Aerodynamics, Structures, and Turbulence (FAST) developed by the NREL to determine its critical wave-and-wind responses, to determine peak tower top deflections, flexural moments, and shear forces. Through least squares regression with the simulation data, two surrogate models, Response Surface Metamodel (RSM) and Stepwise Multiple Linear Regression (SMLR) were developed to predict the critical OWT responses caused by both wind and wave loads and to develop its fragility curves at a low computational cost. Responses and fragility curves resulting from each of the surrogate models were compared to those from the FAST simulation, demonstrating the effectiveness of the RSM in terms of accuracy to replicate the FAST results. As a result of the comparison, the RSM was adopted as the final model to estimate the multi-wind-and-wave vulnerability of the OWT in terms of fragility surfaces considering uncertainties in a broad spectrum of its loads and capacities. Major findings indicated that the wind speed was observed to be the most critical load parameter for peak tower top deflections with response observed in the range of 2.2 5 m at a wind speed of 5 m/s to 5.5 m at 70 m/s, while the wave height appeared to be the major contributor to the vulnerability on flexural moments with the observed peak value of 1.75 × 108 Nm at a wave height of 1 m increased to 5.75 × 108 Nm at 20 m. It was also found that structural characteristics parameters related to the OWT's capacities, including hub height, rotor diameter, and monopile thickness have a significant impact on the overall vulnerability with one-quarter exceeding probability for mudline flexure was noted at 83.26 m, 151 m, and 0.133 m, respectively.

Structural performance degradation identification of offshore wind turbines based on variational mode decomposition with a Grey Wolf Optimizer algorithm

The performance degradation assessment of offshore wind turbine (OWT) structures plays a crucial role in ensuring the safe operation of the structures. This paper presents a method for assessing the performance degradation of OWT structures based on an optimized variational mode decomposition (VMD) algorithm. The method introduces the technique of multisensor data fusion and the Grey Wolf Optimizer (GWO) algorithm to optimize the setting of VMD parameters and then enables the assessment of the structural performance degradation of offshore wind turbines by extracting the structural performance degradation signature of the structure from noise-reduction data. To demonstrate the correctness and advantages of the method in this paper, a 4-degree-of-freedom (4-DOF) system under the action of white noise is used. Numerical calculations show that the developed method can accurately identify the 5%, 10%, 15% or 20% reduction in the structural stiffness and the coefficient of variance of the optimal structure is decreased to 3.6% of the initial design. To further investigate the performance of the proposed strategy, physical tests of the offshore wind monopile structure are conducted. As the loss of the overall structural performance is simulated by removing different numbers of bolts from the connected device, it is demonstrated that the proposed method can effectively assess the structural performance. Finally, field measurements are carried out on a monopile OWT located near Rudong County, Jiangsu Province, in the Yellow Sea of China. The impact of typhoons on the performance of the structure is effectively assessed by analyzing measurement data before and three days after the typhoon In-Fa through the wind farm, the potential application of the method in the structural safety assessment and analysis of OWTs is validated.

A tracking method of equivalent constraint boundary for offshore monopile wind turbines from measured accelerations

For offshore wind turbines, the foundation constraint boundary is important for safe operation yet difficult to determine due to the invalidity of the fixed point assumption and influences from soil variability, liquidation, scour, etc. In this paper, a tracking method of equivalent constraint boundary is proposed on the basis of a limited numbers of measured responses, which are entirely different from the results by fixed constraints. The most theoretical achievement of the proposed method is that the time series of so-called first-order displacements can be accurately retrieved from the measured accelerations, leading to the significant difference from the first-order mode shape of conventional modal identification methods. The obvious advantage of the proposed method is that the change in the equivalent constraint boundary can be easily tracked just from the assembled accelerometers, which means that the difficulty and uncertainty of considering soil and water can be avoided. To verify the performance of the proposed method, two numerical examples and a beam experiment are conducted, and measured sea test data from a monopile wind turbine is processed. In detail, the first example is a cantilever beam with a theoretical fixed constraint position, and the results show that the fixed boundary position of the cantilever beam can be accurately estimated with a maximum relative error of 5.3% when six sensors and a fifth-order polynomial are used. The second example is a physical experiment of a cantilever beam and the change of constraint is considered by removing segments one by one. The experimental results show that the predicted constraint position is consistent with the cut length, with a maximum error of 0.009 m at 1.9 m. Furthermore, a 4 MW offshore monopile wind turbine evaluated by commercial software Ansys considering pile–soil interaction is analyzed, and the changes of the predicted equivalent constraint position are in good agreement with the preset values. Finally, the proposed method is applied to the field test data of a 6 MW monopile wind turbine, and a stable estimation of the equivalent constraint position can be obtained. The good accuracy of the proposed method demonstrates its capability in monitoring structural health for offshore wind turbines.

Response statistics of monopile OWT to fully nonstationary seismic motion

At present, the vast majority of offshore wind turbines (OWTs) have been the monopile type. Although design standards recommend seismic assessments be included in their designs, there has been no guidance on how to conduct an analysis due in part to a lack of efficient methods, particularly related to the nonstationary nature of seismic motions. This article contributes to the development of an efficient pole-residue method operated in the Laplace domain to analytically compute response statistics, including the response evolutionary power spectrum (EPS) and time-varying mean square values. While the nonstationary seismic motions are treated as a uniformly modulated random process, both exponential and gamma models are considered for the modulating function. Furthermore, a fully nonstationary seismic motion with multiple independent modulated subprocesses is also studied. In the numerical studies, a 5 MW monopile OWT subjected to nonstationary stochastic seismic motion is studied; Monte Carlo simulations are employed to verify the correctness of the analytical solutions. Numerical studies confirm that the response EPS at the tower top is contributed mainly by the OWT’s first bending mode. While applying the linear superposition principle, the proposed method can also be extended to the situation that an OWT is subjected to fully nonstationary earthquake and stationary wave loadings simultaneously.

First passage probability of fixed offshore structures with uncertain barrier level to random seismic motion

Offshore structures have inherently uncertain structure characteristics, which lead to the uncertainty of barrier levels. Although the uncertainty of barrier levels may affect the uncertainty of structural failures, there is no guidance on how to conduct an analysis due to a lack of efficient methods, particularly related to the random nature of both barrier levels and seismic motions. This article contributes to the development of an efficient method to compute the probability density function (PDF) of the first passage probability of offshore structures with an uncertain barrier level under random seismic motion. By establishing a transformation relationship between the barrier level and the first passage probability through the curve fitting operation, an explicit PDF of the first passage probability is simplified to the product of the transformation relationship and the PDF of the barrier level. Once the transformation relationship is obtained, the proposed method can quickly calculate the PDF of the first passage probability of structures with an arbitrary PDF of the barrier level. In the numerical studies, a monopile offshore wind turbine subjected to two types of random seismic motions is studied. Three kinds of PDFs of the uncertain barrier level are chosen for each case. The high accuracy and efficiency of the proposed method are validated by comparing with Monte Carlo simulations. Numerical studies confirm that a small uncertainty of the barrier level leads to a very large uncertainty of the first passage probability. It indicates that the first passage probability computed by traditional deterministic methods might be far from the true failure probability. The quantitative calculation of the uncertainty of failure probability is recommended in design to improve the safety and economy of structures.

Finite element analysis of newly designed monopiles for offshore wind turbines on seabed with shallowly buried batholith

ABSTRACT In this paper, three types of hybrid monopiles are proposed for 3MW OWTs in offshore areas where the batholith is shallowly buried. Three-dimensional FE models are established considering the soil-structure-interaction (SSI) to calculate the bearing performances of these monopiles. Model verification is conducted through comparisons with existing numerical results. The static bearing performance and dynamic character under the serviceability limit state (SLS) in silty sand and normally consolidated (NC) clay are investigated. In the static analysis, we focus on the lateral deflection and the rotation at the mudline, whereas in the modal analysis, we focus on the natural frequencies of the OWTs. Results show that the circular skirt performs better than square fins in improving the bearing performance of traditional monopiles, and the improving effect increases with the length of the additional components. In addition, the improving effect is more obvious in NC clay than in silty sand.

A practical two-parameter model of pile–soil gapping for prediction of monopile offshore wind turbine dynamics

Monopile mounted offshore wind turbines (OWTs) are expected to experience a very large number of cyclic loads throughout their operational lifetime, and the existing p–y method of foundation modelling does not fully account for the effects of dynamic cyclic loading, such as pile–soil gapping. In this paper a dynamic model based on the beam on non-linear Winkler foundation scheme with a novel algorithm capable of capturing the effects of pile–soil gapping is presented. It can account for gap cave-in, and the resulting gap size can react dynamically to changing loading amplitudes, using only two calibration parameters. Static and dynamic cyclic loaded model validations are presented, and give very good agreement with experimental results, performing better than existing p–y curves for dynamic loading. The model is also applied to an OWT case study and predictions of natural frequency reduction due to soil erosion agree well with measured results. It is shown that the inclusion of gapping may result in a significant decrease to the natural frequency prediction of OWTs relative to the value predicted without gapping. As such, not to consider gapping could lead to unconservative predictions, and any additional soil degradation throughout the serviceable lifetime could therefore result in unwanted resonance. The method provided in this paper provides a simple and accurate model to predict this behaviour which is crucial to ascertain during the design phase.

Design of monopiles for offshore and nearshore wind turbines in seismically liquefiable soils: Methodology and validation

An increasing number of offshore wind farms are being constructed in seismic regions over liquefaction susceptible soils. This paper presents a methodology for the analysis and design of monopiles in seismically liquefiable soils by extending the established "10-step methodology" with an additional 7 steps. These additional steps include assimilation of seismic data, site response analysis, stability check of the structure (ULS check through the concept of load-utilization ratio), input motion selection, prediction of permanent tilt/rotation, and ground settlement post liquefaction. A flow chart, which shows the interdependence of the different disciplines, is presented and can be extended to routine design. This proposed method is validated using the observed performance of an offshore and nearshore turbine from the Kamisu wind farm during the 2011 Great East Japan earthquake. Predicted results based on the proposed methodology compare well with the field observation and demarcate the (i) good overall performance of the offshore turbines and (ii) limit state exceedance of the nearshore turbine. It is envisaged that the proposed method will be useful towards the design of monopiles-supported wind turbines in seismic areas.

Influence of soil–structure modelling techniques on offshore wind turbine monopile structural response

AbstractThe importance of appropriate offshore wind turbine (OWT) monopile structural modelling technique cannot be overstated in the successful design and installation of a new generation of larger and heavier structures to deliver the increasing capacity demand. The lack of clear design guidance and acceptable structural modelling techniques across the industry results in a range of conservative but expensive design and installation techniques. Most of the OWT monopile modelling efforts lie in the substructure (foundation) and interaction with the supporting soil which is highly nonlinear along the length of the embedment depth of the monopile structure. Typically, monopile offshore wind turbine structural modelling can be completed using, amongst others, one of the following techniques: 3D finite element modelling with mass soil foundation, API p‐y curve soil springs, JeanJean soil springs, and the newly developed PISA modelling approach. The study presented in this paper considers the application of the 3D finite element modelling with mass soil, API p‐y soil springs, and the JeanJean soil springs technique. By comparing the structural response, the 3D finite element modelling with mass soil results in an improved natural frequency and harmonic response. Furthermore, a reduced displacement was observed in the 3D finite element model with mass soil which will ultimately result in a corresponding improvement in the structure's useful operational design life. The application of the API p‐y soil springs, JeanJean soil springs, and other modelling techniques requires extensive calibration to ensure the correct structural response and behaviour are achieved. This becomes a key factor as the boundaries of the size of the structure and turbine capacity are pushed even further for the new concept generation offshore wind turbines, which are required to deliver a higher capacity of 12 to 15 MW with the aim of achieving 20 MW, whilst achieving an efficient cost‐effective engineering design and installation process.

Numerical Analysis of Shear Keys for Offshore Wind Turbine Monopile Grouted Connection with Elastomeric Bearings

Wind power is one of the best-known renewable energy sources, and it is mainly generated by wind turbines. With the recent development of large-scale offshore wind turbine technology and the improvements in capacity factor, the demand for offshore wind power is rapidly increasing for energy system applications worldwide. Such offshore wind turbine structures require structural capacity to withstand loads from offshore environments for a predetermined period of time. Generally, the load of the upper turbine system is transmitted through the grouted connection to the substructure. However, there are many cases of grout failure of the grouted connection between the tubular steels. This paper deals with the analysis of monopile grouted connections to which elastomeric bearings are applied. The grouted connection for the ultimate load of a 3.6 MW offshore wind turbine was analyzed using the three-dimensional finite element method. Furthermore, the changes in the contact pressure and shear stress were analyzed due to the installation of elastomeric bearings around the shear keys. As a result, when the elastomeric bearing was installed, the contact pressure for all grout contact areas increased about 2.5-fold. Specifically, the contact pressure with the shear key was 1.9-fold lower when natural rubber was used as the rubber plate material instead of chloroprene rubber. In addition, the maximum shear stress values of grout filler when installing the elastomeric bearings were 5.78 MPa for chloroprene rubber material and 4.90 MPa for natural rubber material, which were reduced by about 77–81% compared to the value of 25.95 MPa when only shear key was used.

An integrated FEA-CFD simulation of offshore wind turbines with vibration control systems

The vibration mitigation of a monopile Offshore Wind Turbine (OWT) under wind and sea wave excitation is investigated, incorporating dynamic Vibration Control systems (VCS). The application of VCS to the OWTs has the potential to significantly improve the damping of the structure and its overall dynamic responses. A novel passive vibration absorption configuration is proposed, namely the Extended KDamper (EKD). Contrary to the conventional tuned mass dampers, EKD can increase its vibration absorption capability by introducing negative stiffness elements, instead of increasing the additional mass at the top of the towers. Therefore, EKD provides better isolation properties. The EKD optimum design and its realization are based on engineering criteria and realistic manufacturing limitations. The turbulence wind load time histories are determined stochastically while the mean velocity value is produced using the Blade Element Momentum theory. The influence of the sea wave excitation is studied using an integrated Computational Fluid Dynamics (CFD) approach. For the sea wave simulation, both fluids (water and air) are discretized using a non-uniform grid on which the Navier–Stokes equations are solved using the Double Control Volume Finite Element Method (DCVFEM). In order to render the large-scale physical problem computationally feasible, the Dynamic Adaptive Mesh Optimisation (DMO) and High Performance Computing (HPC) schemes are incorporated. The OWT tower is modelled using variable cross section beam elements accounting for geometrical nonlinearity (second order phenomena). The monopile soil–structure interaction (SSI) is modelled using a prismatic beam on elastic foundation together with the corresponding springs and dashpots along the pile’s length. An extensive case study is carried out on a monopile OWT providing insight to the structural dynamics and illustrating the viability of the VCS on the offshore wind industry. It is shown that vibration control can extend the lifetime of the structure, increasing the OWTs’ reliability and sustainability.

A simplified formula for calculating the limit load of cracked offshore wind turbine monopile under bending

A simplified methodology is proposed for calculating the plastic collapse (limit) bending moment load of a pipe with a circumferential flaw with an emphasis on its application for use in the assessment of cracked offshore wind turbine monopile using failure assessment diagrams. The proposed methodology is based on the theory of net section collapse (NSC) but differs from existing approaches in that it does not need idealisation and categorisation of the crack before assessment. The proposed methodology is validated against results presented in literature and also finite element analysis results. Although it is possible to obtain limit loads using FE analysis, this is computationally expensive and time consuming. The proposed approach allows for near-instantaneous calculation of limit load for any arbitrary crack configuration and loading direction. This is a significant development for the analysis of offshore wind turbine monopiles as it allows the suitability of the cracked structure to be assessed in pseudo-real time.