Offshore Wind Turbine Structures Research Articles

Influence of soil hysteretic damping on lateral response of offshore wind turbine monopile in sandy soil

Damping plays an important role in the design of offshore wind turbine structures. The hysteretic damping of the seabed soil represents the energy dissipation caused by the soil-particle interaction and the nonlinear behavior of the soil under cyclic loading. However, the effect of sand damping on the lateral response of the monopile foundation of an offshore wind turbine is still unclear. In this paper, the effect of soil hysteretic damping on the lateral dynamic response of a monopile foundation in a sandy seabed is investigated using a subplastic soil constitutive model. The constitutive model response at the foundation level is verified by comparing the monotonic and cyclic responses of the monopile with the results of the 1g model test. The results show that when soil hysteretic damping is present in the monopile-soil system, the energy dissipation in the soil reduces the stress accumulation in the soil, resulting in a reduction in the bending moment and horizontal displacement of the monopile, compared with the case without soil hysteretic damping. The results are crucial for optimizing the monolithic design of offshore wind turbine structures.

Recursive Bayesian estimation of wind load on a monopile-supported offshore wind turbine using output-only measurements

Offshore wind turbine structures experience combined wind and wave loading during their lifetime, and the cyclic characteristics of these loads significantly impact the fatigue life of the support structure. Continuous monitoring of stress-related quantities, such as strain time histories at hotspot locations of the structure, can help to estimate the fatigue life. However, sparse measurements from the substructure necessitate using a digital twin as a tool for structural health monitoring by creating a virtual model. This virtual model relies on real-time input loads for virtual sensing and stress-related quantities prediction. However, the exact loading on these structural systems is often unknown or estimated with considerable uncertainty. This paper implements a recursive window-based Bayesian estimator for input load estimation and virtual sensing of acceleration and strain time history at unmeasured critical locations using output-only measurements. The estimator is numerically and experimentally validated in the context of input load estimation for an offshore wind turbine, and the results are compared with a traditional augmented Kalman filter and a linear regression approach for the quasi-static component of loading. The method is first demonstrated through a numerical study using a finite element model of a 6 MW offshore wind turbine, where input wind load time histories are accurately estimated. Then, the framework is applied to the real data measured from an offshore wind turbine in the North Sea. The study demonstrates the window-based Bayesian input estimator as a promising tool for the digital twinning of offshore wind turbines, demonstrating superior accuracy in input load estimations and full response predictions compared to the augmented Kalman filter and linear regression methods without the constraint of collocated measurements at input load locations.

Bayesian finite element model inversion of offshore wind turbine structures for joint parameter-load estimation

Operating in harsh and unsteady marine environment, offshore wind turbine (OWT) structures are exposed to unpredictable wind and wave loads. Identifying the structural loads and their effects on the OWTs allow for predicting the remaining fatigue life of these structures and improving the structural design procedure. In this paper, a finite element (FE) model inversion method is presented to estimate the unknown loads and model parameters of OWTs using sparse measurement data. A realistic FE model of an OWT structure with jacket substructure is created in the open-source simulation platform, OpenSees. A Bayesian inference framework is presented to integrate the measured data with the FE model to estimate unknown wind loads and mass of rotor-nacelle assembly. To evaluate the performance of this data assimilation framework, the effect of sensor type, number of sensors, and modeling errors on the estimation accuracy of wind loads and model parameters are investigated through different case studies where synthetic data are used as measurements. The results of this study are important to guide instrumentation of new OWT structures, and to understand the potential limitations and sources of error in the real-world application of this data assimilation framework for joint model parameter and input load estimation.

Effects of thin-walled inner steel tubes on the compressive behavior of concrete filled double skinned steel tubular columns

Concrete-filled double skin steel tubular columns with higher hollow ratios (HHR-CFDST) have gained significant attention in recent times as essential components in offshore wind turbine structures. As these structures continue to grow, an effective strategy to reduce steel consumption is to increase the diameter-to-thickness (D/t) ratios of the steel tubes. However, there is a lack of studies exploring the performance of HHR-CFDST columns with thin-walled inner steel tubes. To address this, four HHR-CFDST columns with varyingD/t ratios of the inner tubes and one double-skin hollow steel tubular column were produced and tested under axial compression. The study investigated failure modes, local buckling behaviors, and ductility characteristics. The local buckling performance index of specimens with different D/t ratios varied by up to 16.90 %. Additionally, a finite element model was used to analyze the stress distributions of the concrete when peak loads were reached, revealing a maximum variation of 45.4 % in stress values among specimens with different D/t ratios. A multi-parameter impact analysis was then conducted to establish a reasonable limit (250) for the D/t ratio of the inner steel tube in engineering design. Finally, the findings from this investigation were integrated into existing ultimate strength formulas outlined in established specifications.

Optimized identification process of equivalent wind load calculations for offshore wind turbines under standstill conditions

In engineering, the wind excitations acting on the offshore wind turbine (OWT) structure cannot be obtained directly by the measured method. The traditional load simulation way may lead to poor accuracy because of the deviation between actual operational conditions and simulation environment parameters or load coefficients, which are always selected based on load spectrums, small scale model tests and engineering experience. To solve this project problem, an equivalent wind load optimization model and parameter identification process of OWT was established under the standstill conditions. Afterwards, two key load parameters in load model respectively called constant drag coefficient and unsteady lift coefficient can be obtained through recursive call on numerical model and feedback analysis using particle swarm optimization (PSO) algorithm based on measured vibration data. The applicability and accuracy of proposed calculation process was verified through an equivalent numerical model of OWT considering different particle position and wind speed. Further, the optimization identification on equivalent wind load and the drag and lift coefficients of one OWT was achieved based on measured data. It is illustrated that the identified drag coefficients show a drop trend from 0.629 to 0.154, consistent with the parameter range of 0.20–0.60 obtained from model experiments, before the wind speed reaches 6.0 m/s and then rise obviously with the increase of wind speed. On the contrary, the range of optimized lift coefficients form 0.133–0.480, which is as familiar as the experimental parameter range of 0.10–0.30, reflects a certain increase trend. Finally, the comparison between identified wind excitations and calculated loads based on the measured strain data explain that the proposed load optimization identification process have a good credibility on actual OWT.

Research on semi-active vibration reduction of offshore wind turbine structure combined eddy current theory with tuned mass dampers

At present, the offshore wind power gradually shows a development trend towards the profound ocean with characteristic of larger capacity, higher tower and longer blades. Therefore, the offshore wind turbine (OWT) is more vulnerable to extreme loads and may lead to the negative influence of structural vibration safety. In this research, a new semi-active eddy current tuned mass damper (SEC-TMD) was proposed for the vibration control of OWT structures and its vibration reduction effect under multiple kinds of dynamic loads was also discussed. Firstly, taking one actual 3.0 MW OWT supported by bucket foundation as object, the SEC-TMD was designed based on the linear quadratic optimal control (LQR) algorithm and bounded Hrovat optimal control algorithm in order to obtain the optimal control force of structural vibration systems. The relationship between damping coefficient and magnetic conductivity spacing was further fitted to achieve variable damping control of SEC-TMD. Secondly, the theoretical model of 3.0 MW OWT structure installed with SEC-TMD was established and the effectiveness of the established theoretical model was verified through the comparison on modal analysis between the theoretical calculations and measured data identification. Finally, the vibration reduction effect of SEC-TMD on the measured OWT structure under wind-wave, seismic and vibration amplification conditions was analyzed and compared with the results calculated from the other passive dampers including EC-TMD and TLCD. It is shown that the maximum reduction percentage of peak value for vibration displacement and acceleration on the tower top can respectively reach 31.93 %, 44.11 %, 30.20 % and 54.57 %, 33.94 %, 31.75 % under three above conditions. Comparing with EC-TMD and TLCD, the better vibration reduction effect indicates that the proposed SEC-TMD has good engineering application value for the vibration suppression of OWT.

Modelling of foundation damping in time-domain analysis of monopile supported offshore wind turbine structures

Foundation damping holds large potential for design optimisations of monopile-supported offshore wind turbine structures. However, the contribution of foundation damping is not well understood, in part due to lack of suitable method for incorporating foundation damping in the time-domain analysis of wind turbine structures. This paper presents a practical approach for this purpose in which a dashpot is attached in parallel to each p-y spring along the monopile. In this way, the distributed soil-pile interaction stiffness and damping are accurately modelled. To demonstrate the validity of the proposed approach, a case study exploring the influence of foundation damping on the dynamic response of an IEA 15 MW reference turbine founded on monopile is carried out. The results demonstrate that the foundation damping has relatively limited impact on the overall dynamic response and fatigue loads in power production states. However, inclusion of foundation damping is shown to significantly reduce the structural response after emergency shutdown and under parked conditions. The findings of the case study show promising potential for wind turbine structure design optimization through consideration of foundation damping.

Investigation of soil damping for offshore wind turbine monopile-soil system in stiff clay on elastoplastic-damage model

Damping plays a crucial role in the design of offshore wind turbine (OWT) monopile foundations. The soil damping of the monopile-soil system (MSS) represents the energy dissipation mechanism arising from the interaction between the pile and the soil. It is typically derived by back-calculating from the overall damping measured in the entire OWT structure. However, few studies have independently examined the soil damping in MSS, and the impact of key parameters such as pile diameter, pile embedded depth, cyclic load amplitude, and load eccentricity on the variation of soil damping in MSS remains unclear. This paper introduces an elastoplastic-damage constitutive model for the numerical simulation of the damping ratio variation in seabed soil and MSS. The model is implemented in ABAQUS software and validated against cyclic triaxial tests on stiff clay soil. On this basis, a three-dimensional finite element sensitivity study was conducted to elucidate the effect of these key parameters on the MSS damping ratio. The results of the study reveal that the MSS damping ratio exhibits a nonlinear and asymmetric trend as the loading cycles increase. The MSS damping ratio decreases with increasing pile diameter and embedded depth but increases with increasing lateral cyclic load amplitude and load eccentricity from the mudline.

Vibration control and energy harvesting of offshore wind turbines installed with TMDI under dynamical loading

This study investigates the performance of multiple tuned mass-damper-inerter with multiple electromagnetic motor (TMDI-EM) for the dual objectives of enhancing energy harvesting and dynamic performance in offshore wind turbines (OWTs). OWTs, inherently susceptible to dynamic sensitivity under lateral loads such as wind and waves, necessitate effective solutions for vibration mitigation while harnessing energy from dynamic excitations. The TMDI-EM configuration integrates an electromagnetic motor (EM) as a shunt damper between a secondary mass and an inerter element in series. The scalable inertance of the inerter element allows for adaptability in practical device implementations. Genetic Algorithm (GA) is employed to find optimum parameters for the TMDI-EM system. The research focuses on the dynamic response of OWTs under real-world conditions, where wind and wave forces act as correlated random excitations. Parametric analyses assess the available energy for harvesting at the EM and the displacement variance of the OWT structure as the inertance of the TMDI-EM varies. The study employs methods to optimize the TMDI-EM parameters using Genetic Algorithm, ensuring the system's optimal performance for both enhanced energy harvesting and dynamic response. The findings indicate that lightweight TMDI-EMs, representing only 1.5 % of the OWT structure's mass, demonstrate improved vibration suppression and energy harvesting performance with increasing inertance. This study contributes into the potential integration of TMDI-EMs to enhance the dynamic performance and energy harvesting capabilities of offshore wind turbines under dynamical loading conditions. The use of Genetic Algorithm ensures the identification of optimal parameters for the TMDI-EM system, enhancing its practical applicability.

One data-driven vibration acceleration prediction method for offshore wind turbine structures based on extreme gradient boosting

With the development of offshore wind power towards large-scale and deep-sea, the safety risks and the difficulty of intelligent operation and maintenance of offshore wind turbine (OWT) during operation periods have increased significantly. Structural vibration response is an important factor affecting the safety and stability of OWT. Accurate prediction on vibration through environmental conditions and operational factors can identify the safety risks of OWT in advance, and reduce harmful vibration through appropriate operation strategy regulation. Therefore, a vibration acceleration prediction method of OWT structures was proposed in this research based on Harris hawks optimization (HHO) and extreme gradient boosting (XGBoost). Firstly, approximately two months of monitoring data from a 3 MW OWT were extracted to establish the database for training the machine learning models. Then, the key hyperparameters of XGBoost were optimized by the HHO algorithm and compared with other typical algorithms. Finally, the trained prediction model was applied to the regularity analysis of different conditions and long time series data. The analysis results demonstrate that the proposed method can effectively predict the vibration acceleration response of OWT structures using operational and environmental characteristics, and has higher accuracy and stability than other methods, with R2 of 0.9715 and mean absolute percentage error (MAPE) of 6.55%. Simultaneously, it also shows acceptable accuracy on the long-term measured data because the MAPE of chosen summer, autumn, and winter samples were calculated as 9.60%, 11.76%, and 7.21%, respectively. The establishment of the HHO-XGBoost vibration prediction model can be applied in the subsequent prediction of the vibration trend, which provides a guarantee for the safe and stable operation of OWT.

Experimental evaluation of the short and long fatigue crack growth rate of S355 structural steel offshore monopile weldments in air and synthetic seawater

Welded steel structures used in the offshore wind industry are exposed to harsh marine environments, which can result in corrosion-induced fatigue damage. Of particular concern is the heat affected zone (HAZ) of welded joints, a region known for its altered microstructure and mechanical properties, which can significantly influence the initiation and propagation of fatigue cracks. This study investigates the short and long fatigue crack growth rates, and the effect of seawater exposure, for the HAZ in S355 steel weldments. Single-edge notch bend (SENB) specimens are used, with a shallow notch in the HAZ. A series of specimens is immersed in synthetic seawater that is continuously circulated at a controlled temperature to assess the synergistic effects of corrosion and fatigue. The experimental method integrates a novel application of front face strain compliance for monitoring short cracks, alongside an extended back-face strain compliance approach for monitoring long crack propagation. It is concluded that the short fatigue crack growth rate of the HAZ is 2.7 to 3.5 times higher in seawater as compared to air. As the crack propagates and enters into the long crack regime, the ratio decreases to 2.2 times at the transition point of the two-stage crack growth curve and further decreases to 1.5 times when the notch advances towards fracture. The findings indicate that the fatigue crack growth rates documented in standards tend to be on the conservative side. This study significantly enriches the fatigue crack growth data available in literature, which will contribute to a more accurate lifetime assessment offshore wind turbine structures.

Studying the mode shape participation factor of wave loads for offshore wind turbine structures

This paper introduces a method for analyzing resonance between Offshore Wind Turbine (OWT) structures and wave loads using Wave Participation Factor ratios (WPFs). Despite temporal fluctuations in WPFs due to wave loads, they prove effective for identifying key modes influencing wave load behavior in both fixed and floating OWT structures. For fixed OWT foundations, nonlinear soil behavior causes some natural frequency variations under wave forces, but they typically remain within a limited range. Lower natural frequency modes show minor WPFs, reducing the risk of resonance with wave loads. Modes with higher natural frequencies have substantial WPFs and are less prone to resonance. However, precautions are needed to avoid rotor-induced wind load resonance. Floating OWT structures have large WPFs in their first six near-rigid body motion modes, while mooring line stiffness leads to substantial variations in these modes. It's crucial to control the first six nonlinear frequencies outside the dominant wave load range. Beyond these six modes, natural frequencies significantly exceed wave load frequencies, reducing resonance concerns. In summary, this study presents an effective method for assessing resonance between OWT structures and loads, providing valuable insights for structural design and operational safety.

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.

New method for incorporating foundation damping in time-domain analysis of integrated offshore wind turbine structures

Foundation damping has significant potential for reducing the loads of monopile-supported offshore wind turbines. However, the contribution of foundation damping is not well understood, primarily because of the absence of suitable methods for incorporating it into the time-domain analysis of wind turbine structures. This paper presents a novel approach for this purpose. In this approach, a dashpot is attached parallel to each of the p-y springs along a monopile, which models the stiffness and damping of soil-pile interactions. Employing this approach, the influence of foundation damping on the structural dynamic response is investigated using the time-domain analysis of a monopile-supported IEA 15 MW reference wind turbine. The results demonstrate that foundation damping has a limited impact on the overall dynamic response and fatigue loads of wind turbines during power production. However, the foundation fatigue loads after an emergency shutdown can be reduced by 29-46% owing to the inclusion of foundation damping. In addition, foundation damping is found to significantly influence the bending moment, horizontal displacement, and angular rotation of the monopile at the mudline under parked conditions, with load reductions of up to 20%.

Study on the anisotropic small strain stiffness of marine clay in China using bender elements

Small strain stiffness anisotropy of marine soils plays an important role in determining the natural frequency of offshore wind turbine structures. The small strain stiffness anisotropy is contingent on stress anisotropy and fabric anisotropy, with the latter being assessable via stiffness anisotropy under isotropic stress conditions. In this study, the small strain stiffness of marine clay obtained from the site of an offshore wind farm on the southeast coast of China was measured using a bender element. The effects of void ratio, effective mean pressure, and overconsolidation ratio on small strain stiffness were investigated and the related empirical equations were proposed for predicting the small strain stiffness of the marine clay. Furthermore, the small strain stiffness anisotropy of the marine clay under isotropic consolidation was also investigated. The findings revealed remarkable anisotropy in the small strain stiffness of the tested marine clay.

Analysis of Offshore Wind Turbine by Considering Soil-Pile-Structure Interaction: Effect of Sea-Wave Load Duration

ABSTRACT Offshore Wind Turbines (OWTs) confront different types of environmental loads during their lifetime. One of the most significant loads is the cyclic sea-wave load which affects the OWT system during the approximate 25 years of design life. This lateral cyclic load can influence the response of the OWT system because of accumulated permanent displacement and excess pore water pressure generation in soil. This procedure can change pile-soil stiffness. However, the behavior of OWT and its foundations is mostly studied under short-term cyclic loading, and the effects of duration of the cyclic loads on pile-soil interaction and performance of the foundation are not well understood and documented. Therefore, there is a lack of guidance in codes for the duration effects of cyclic loads on the structural and geotechnical response of OWTs. In this regard, the current study considers the effects of duration of the cyclic loads on serviceability and performance of the OWT system by considering soil-foundation-structure interaction using a 2-D finite element method. The behavior of the OWT system is evaluated based on the internal forces and deformation of the monopile foundation, shear strain, and excess pore water pressure ratio in the surrounding soil. Besides, liquefaction susceptibility in the sandy soil layer at the vicinity of the monopile and its effect on the performance of the foundation is investigated. Finally, the results can provide guidance on estimation of the dynamic performance of the OWT system during long-term cyclic loads. They can be used to specify the need for consideration of the duration of cyclic lateral loads for the design of OWT structures.

Vibration control of a monopile offshore wind turbines under recorded seismic waves

The dynamics and vibration control of offshore wind turbines (OWTs) under wind, wave, and earthquake loads is an important but understudied topic, especially for fully coupled seismic analysis considering pile foundation flexibility of OWTs. In this work, we study the effect of multiple passive tuned mass dampers (MTMD) on a monopile OWT by using the fully coupled seismic analysis method. A combined shutdown procedures and MTMD vibration control strategy is proposed for the OWT. First, a state-of-the-art aeroelastic tool, FAST, is extended with a seismic load calculation module, and the numerically modeled monopile OWT addresses the effect of pile-soil interaction (PSI) by establishing ElasPile module in the updated FAST. Then, the focus is placed on the normal operation and emergency shutdown status of OWTs under the combined earthquakes, wind and wave conditions, and structural and motion responses are analyzed under various combination conditions through time-domain simulations, the dynamic characteristics of the OWT during realistic seismic events are shown. Sequentially, based on the proposed MTMD, the vibration migitation effects are evident for the operation OWT and the OWT with shutdown strategies during the combined loading conditions. The reduction in the extreme responses can be as high as 10.53% and 55.02% for the bending moment at the mudline in different OWT conditions, respectively. Therefore, the combined shutdown procedures and MTMD vibration control strategy of OWT is an effective vibration reduction method. Additionally, it is found that PSI has a significant impact on the dynamic characteristics and vibration control effects of OWT structures under wind, wave, and earthquake loads, and the coupled spring boundary condition on the basis of developed ElasPile module is recommended for seismic control analysis of OWT using the combined shutdown procedures and MTMD vibration control strategy.

Fail-safe topology optimization for a four-leg jacket structure of offshore wind turbines

Topology optimization (TO) has experienced substantial progress in both scholarly research and practical engineering applications. Recent years have witnessed extensive research on TO approaches incorporating the fail-safe concept, with a particular emphasis on potential failures. This paper proposes a TO methodology on the offshore wind turbine (OWT) jacket structure, focusing on the unexpected failure modes. The initial step involves constructing the formulation for maximum-minimization TO. A series of optimized structures can be generated by varying the sizes of individual damage populations and overall failure zones. A 5 MW OWT and jacket structure have been selected as the reference model. The external loads acting on the jacket structure are determined by given environmental conditions, as assessed using the DNV Bladed ™ package. The feasibility of an optimized structure is evaluated by comparing its static behaviors, such as maximum displacement and von Mises stress, to those of the reference structure. The investigation focuses on the fundamental frequencies of the optimized structure under various damage populations. The discovery demonstrates the superiority of the suggested methodology in the case of a potential failure.

Extreme load extrapolation and long-term fatigue assessment of offshore wind turbine tower based on monitoring data

Extreme load and fatigue damage are important parameters in the structural design of offshore wind turbines (OWTs). The purpose of this study is to obtain extreme loads and long-term fatigue damage assessment of OWT structures based on measured data. In the present study, a utility-scale OWT was monitored with a dynamic response monitoring system and long-term strains of the OWT tower were collected. Based on the monitoring strain and the operational conditions of the OWT, the load and fatigue characteristics of the OWT tower were discussed. Furthermore, statistical extrapolation of the extreme load of the OWT tower was performed and the applicability of different statistical extrapolation methods was studied. The results indicate that the same probability distribution model does not apply to all wind speeds. The maximum extreme load is extrapolated in the vicinity of the rated wind speed. Finally, the relationship between short-term fatigue damage and long-term fatigue damage of the OWT structure was discussed. Superimposing fatigue from a short calculation period underestimates the long-term fatigue of OWT structures due to significant calculation period impact. Therefore, an evaluation method for OWT structures' long-term fatigue was established to improve the accuracy of long-term fatigue calculation by superimposing short calculation periods.

Coupled aero-hydro-geotech real-time hybrid simulation of offshore wind turbine monopile structures

Real-time hybrid simulation (RTHS) divides a structural system into an analytical and experimental substructure. The former is based on a well-established analytical model while the latter consists of a physical model in the laboratory, for which there is not a well-established analytical model. This paper extends real-time hybrid simulation to monopile-type Offshore Wind Turbines (OWTs) to enable the investigation of their behavior considering the response of pile foundations under operational and more severe conditions. The embedded foundation and surrounding soil of the OWT are modeled physically in a soil box in the laboratory while the remaining parts of the system and loading are modeled analytically. The program OpenFAST, developed by the National Renewable Energy Laboratory (NREL), is linked to the RTHS coordinator to determine the hydrodynamic and aerodynamic loads acting on the OWT, along with modeling the dynamics of the electric power generation equipment and associated controller for the OWT. The RTHS framework along with its initial implementation and validation are described in this paper. RTHSs of a 5 MW OWT subjected to operational and more severe conditions are performed to experimentally validate the framework. The framework offers a realistic approach to investigate the behavior of OWT structures supported on monopiles. This approach accounts for the coupled response of the OWT structure with its foundation, while experimentally capturing the nonlinearities of the soil-foundation interaction in real-time.