Monopile Offshore Wind Turbine Research Articles

Hydrodynamic analysis of the combined structure of offshore monopile wind turbine foundation and aquaculture cage

Reliable prediction in hydrodynamics is crucial for the design of marine structures, especially for columnar structures such as offshore wind turbine foundations. A combined structure of fixed monopile wind turbine foundation and bottom-seated circular aquaculture cage is proposed for efficiently utilizing the offshore space and reducing the wave forces on the monopile wind turbine foundation. A further developed volume-averaged numerical model is adopted to simulate the pressure drop due to clean and fouled cages by introducing the porous media theory, in which the coefficients of source terms are determined by the least square method based on hydrodynamic force on cage from existing experimental data. The Stokes fifth-order wave theory is considered reliable in simulating the nonlinear waves on slopes and corresponding inline wave force by comparing with multiple existing experimental data. Then the effects of incident wave steepness, seabed bottom slopes and cage diameters on simulation results are analyzed. Results reveal the fouled cage can effectively reduce inline wave forces relative to the clean cage of the combined structure, accompanying a significant resonance effect of wave motion inside the cage.

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.

Dynamics of offshore wind turbine and its seabed foundation under combined wind-wave loading

Over the past two decades, a great number of offshore wind turbines (OWTs) have been built in offshore areas worldwide to harvest clean wind energy resources. The dynamic response and stability of OWTs under environmental loads, e.g., wind and wave, is a key technical problem that engineers are most concerned about. In this study, a wave-structure-foundation integrated numerical model OlaFlow–ABAQUS, which was independently developed through the secondary development, is used to study the dynamics characteristics of a thin-walled steel monopile offshore wind turbine and its seabed foundation under extreme wave and wind loads that would occur in typhoon weather. The wave-induced load imposed on the cylindrical tower of the OWT is determined by solving the VARANS (Volume Average Reynolds Average Navier-Stokes) hydrodynamic equation, the wind load on the turbine blades is estimated by applying the pulsating wind theory, and the mechanical behavior of seabed foundation soil is described by the ideal elastoplastic Mohr-Coulomb model. The computational results show that the OWT and its seabed foundation have a strong dynamic response under the combined action of extreme wind and wave, and there is intensive interaction between them. There is a transient liquefaction zone in the seabed foundation, and the maximum liquefaction depth, in this case, is approximately 2 m. Residual horizontal displacement with a magnitude of approximately 0.4 m occurs at the top of the cylindrical tower of the OWT, resulting in the overall tilting of the OWT. The maximum inclination angle of the cylindrical tower is approximately 0.51°. This angle of inclination may cause significant bending moment and stress concentration in some mechanical components such as the drive shafts and gearboxes that connect to the turbine blades. It reduces the service life of the OWT significantly and also is detrimental to the long-term service performance of the OWT.

Method to determine the ultimate horizontal bearing capacity of offshore wind turbine monopiles with grouting protection against scouring

Scouring is a serious occurrence that jeopardises the securing of offshore wind power structures. This research investigated the effectiveness of grouting protection as a novel countermeasure to scouring. In order to accomplish this objective, scouring experiments were conducted under constant-current and regular-wave. Furthermore, static loading tests of the monopile were carried out. The results indicate that the ultimate scouring depths of piles with grouting were significantly lesser than those without it during scouring. Additionally, it was observed that grouting protection can considerably enhance the ultimate horizontal bearing capacity under static loading. The limit equilibrium method was employed to determine the ultimate horizontal bearing capacity of monopiles. Two models were proposed for piles with grouting protection––the pile-reinforced sediment model (Model A) and the composite pile model (Model B). We determined that Model An underestimated the ultimate horizontal bearing capacity, whereas Model B overestimated it. Therefore, coefficients for these calculations were acquired to ensure the accuracy of the results. These findings could help in reducing the costs of operating and maintaining offshore wind farms while offering an enhanced design for scouring protection in offshore wind power foundations.

Assessing seismic mitigation schemes of tuned mass dampers for monopile offshore wind turbine including pile–soil–structure interaction

The study of a monopile offshore wind turbine with the soil–structure interaction effect is most challenging in structural design under multiple hazards, i.e., the combined wind, sea wave, and earthquake excitations. Different arrangements of passive tuned mass dampers (TMDs) were used to mitigate the service and seismic loads affecting an offshore wind turbine (OWT) including the pile–soil–structure interaction (PSSI) effect. Different schemes of passive TMDs, placed at the top of the OWT tower or also at the center of gravity (CG) of the OWT tower or at the connection between the OWT tower and monopile, were tested. Various arrangements of TMDs including the proposed herein top radial TMDs arrangements have been investigated to determine their validity in resisting vibrations resulting from service and earthquake loads. The lateral displacements, shear forces and bending moments in both horizontal directions and the axial forces all over the OWT tower and monopile heights were recorded to compare the performance of each mitigation scheme of TMDs. The comparison results showed that the TMDs placement should be at the top of the OWT tower and the top radial 6 TMDs arrangement was found to be the most effective mitigation scheme for all straining actions in the tower and the monopile of the OWT subjected to service and earthquake loads.

Experimental study of collar protection for local scour reduction around offshore wind turbine monopile foundations

Experimental study of collar protection for local scour reduction around offshore wind turbine monopile foundations

Offshore Wind Turbine Monopile Foundation Systems in Multilayered Soil Strata under Aerodynamic and Hydrodynamic Loads: State-of-the-Art Review

Due to the large demand to produce clean and renewable energy, offshore wind farms are extensively being used to help with economic development. Offshore Wind Turbine (OWT) structures are usually supported by various types of foundations and its selection depends on several factors such as water depth, economic costs, construction methodologies, seabed bathymetry, soil characteristics, and load characteristics. The present State-of-the-Art review focuses on OWT-monopile foundation systems embedded in multi-layered soil strata subjected to aerodynamic and hydrodynamic loading conditions. The purpose of this study is to carry out a comprehensive review on the performance of OWT-monopile foundation structures to support the design and drafting choices and thereby provide an understanding to reduce the costs. Two specific subject areas are addressed in this paper: (1) Factors affecting the performance of OWT-Monopile Foundation Systems; and (2) Experimental modeling and numerical analyses of OWT-Monopile Foundation Systems. The immediate challenge to the design of OWTs is to analyze the dynamic response of the foundation structure when exposed to both aerodynamic and hydrodynamic loads acting simultaneously. The limitations of the existing guidelines point to the need for improved design methods to obtain an economical design of OWT-monopile foundations. The use of Finite Element (FE) models is effective in studying the functioning of OWT-monopile structures. The present study suggests the development of further numerical models and modifications to the existing ones to properly assess soil-monopile interactions. This study also suggests further investigations on the design and analysis of OWT-monopile foundations considering the effect of (i) aerodynamic and hydrodynamic loading conditions with soil-monopile interactions, and (ii) multi-layered seabed characteristics.

An integrated model for offshore wind turbine monopile in porous seabed under multi-directional seismic excitations

This study proposes an integrated three-dimensional time-domain finite element model for offshore wind turbine (OWT) monopile in porous seabed under multi-directional seismic excitations. Biot's u-p formulation is used to describe the dynamic interaction between the soil skeleton and pore water. Potential-based fluid elements are used for seawater to accurately model fluid–structure interaction (FSI). Moreover, FSI interface elements are applied to the interface of seawater and pile and to the interface of seawater and mud line. To analyze the response of the OWT-monopile-seabed-seawater system to multi-directional seismic excitations, a viscous-spring artificial boundary is implemented in the finite element model. The integrated model was sufficiently verified in terms of the vibration characteristics of OWTs, the displacement and the excess pore water pressure using implemented viscoelastic artificial boundary, and the FSI interaction under both horizontal and vertical excitation. The results indicated that: (1) The hydrodynamic pressure and the excess pore water pressure varied with different positions along the OWT and the pile; (2) Adding the excitation of the EW component ground motion caused the change of the distribution of transient liquefaction with respect to that under the excitation of only NS component ground motion; (3) The vertical ground motion caused a substantial fluctuation of the vertical displacement of the OWT and larger area of transient liquefaction near the surface of the seabed and near the tip of the pile; (4) resonance occurred when the frequency of input motion was approaching to the first natural frequency of OWT, leading to extremely large acceleration and displacement of OWT.

Seismic responses analysis of monopile offshore wind turbines in clays considering the long-term cyclic degradation effect

In marine environment, the monopile offshore wind turbines (MOWTs) are subjected to long-term horizontal cyclic loads caused by wind, waves and currents, which will lead to the stiffness degradation and plastic cumulative deformation of clays. The decrease of clays bearing capacity caused by long-term cyclic degradation effect is a threat to the safe use of MOWTs in earthquake-prone countries (such as some countries in the circum-Pacific seismic belt). A three-dimensional finite element model is established to research the seismic response analysis of MOWT in clays considering the long-term cyclic degradation effect. The cumulative deformation of pile-soil system after long-term cycle are obtained by adopting the stiffness degradation of clays through the USDFLD subroutine. On this basis, the dynamic response analysis of MOWT system under the combined action of seismic and environmental loads is carried out. The influence of long-term horizontal cyclic load number, long-term horizontal cyclic load amplitude, seismic intensity, soil strength, environmental loads and cyclic degradation parameter on the dynamic response of MOWT is analyzed. In the seismic design of MOWTs, the effects of long-term degradation and random wind, waves and other environmental loads should be considered.

Study on a three-dimensional variable-stiffness TMD for mitigating bi-directional vibration of monopile offshore wind turbines

Bi-directional vibrations of offshore wind turbine (OWT) exist due to the misalignment of wind and wave loadings, which induces the structural fatigue failure. At present, monopile offshore wind turbine (MOWT) is widest utilized in worldwide, which is adopted as the research objective in this study. A novel semi-active three-dimensional magnetorheological elastomer (MRE) based tuned mass damper is proposed to suppress a MOWT bi-directional vibration under misaligned wind, wave, seismic loadings. The proposed MRE-based TMD is installed in the nacelle, which presents dual-directional controllable stiffness properties. Its nonlinear behavior is depicted by bi-lateral Bouc-Wen model. A semi-active control framework is developed, in which current-dependent stiffness of MRE-based TMD is tuned for tracing real-time dominant frequency of the MOWT. The numerical results reveal that MRE-based TMD can effectively attenuate the bi-directional dynamic responses of MOWT under various operation conditions. Additionally, the semi-active MRE-based TMD outperforms passive TMD in reducing the peak and RMS displacement of the tower top in both fore-aft (FA) and side-side (SS) directions, especially in operational state. MRE-based TMD also presents strong robustness under the condition of structural stiffness degradation.

Dynamic response assessment of an offshore wind turbine under Peruvian environmental conditions

This paper presents a dynamic response analysis of an offshore monopile wind turbine to be deployed at a near-shore area in the south-centre part of the Peruvian coast (Marcona). It is a 5 MW three bladed wind turbine that operates in an aggressive offshore environment described by its stochastic behaviour. Thus it is necessary to use a statistical approach for its characterization. The operating wind turbine is simulated using an aero-hydro-servo-elastic computational tool (FAST) that allows the simulation of coupled systems. After the simulation, this work analyses relevant dynamic responses (thrust, torque, shear forces, and bending moments). Time series of these responses at critical zones of the wind turbine are presented. The results indicate high variability of the responses and critical fluctuations located at the mudline. It is found that this variability is due to the additional dynamic loads coming from the ocean waves. This is a big issue to consider when designing a monopile offshore wind turbine operating under the Peruvian environmental conditions.

Lateral behavior of monopiles under the simultaneous effect of vertical-lateral loads and change of embedment depth due to 1D local scouring

Offshore wind turbine foundations are subjected to high lateral loads but insignificant vertical forces of the superstructure. A decrease in the embedment depth due to scouring can also alter the effect of vertical loads on the soil-structure interaction. Current design methods consider the vertical and lateral forces in design separately. This area also lacks studies featuring physical modeling that produce consistent conclusions. The present study employed centrifugal physical modeling to investigate the lateral behavior of offshore wind turbine monopiles under the simultaneous effects of vertical and lateral loads with a change in embedment depth caused by 1D local scour (D = diameter of pipe) in calcareous sand. The results showed that the vertical load of the superstructure increased the lateral strength of the monopile in the no-scour and local scour cases by about 3.5% and 6%, respectively. Improvements in the p-y curves along the embedment depth were noted at depths of less than twice the monopile diameter. Stress intensification was more significant under the scoured conditions because of the reduced embedment depth of the monopile. Overall, the vertical superstructure loads had little effect on the lateral performance of the monopiles.

Using Bayesian updating for monopile offshore wind turbines monitoring

Digital twins (DTs) are thought to be promising tools for offshore wind turbines (OWTs) in real-time updating, optimized design, intelligent operation, and maintenance. When creating DT analysis tools, it is critical to update model parameters to create high-fidelity, high-accuracy virtual models. In this paper, a Bayesian updating framework based on Transitional Markov Chain Monte Carlo (TMCMC) is proposed to construct the desired virtual model of monopile OWTs to address this difficult problem. The likelihood function for Bayesian inference was created using measurements from physical entity monitoring as well as natural frequencies and damping ratios from the monopile OWT finite element (FE) model. Then, through a series of relatively simple intermediate likelihood functions, it gradually converges to the target function. During this time, the artificial neural network (ANN) surrogate model calculates the intermediate likelihood function of each sample, and a parallel computing strategy is used to enhance the computational efficiency. The results show that the posterior distribution of the uncertain parameters can be successfully estimated from the Markov chain using this method to update the finite element model. In addition, the proposed Bayesian updating method is resistant to measurement noise.

Experimental investigation of the horizontal bearing capacity of offshore wind-turbine monopiles with grouting protection against scouring

Seabed scouring can uncover the buried component of wind-turbine monopiles and this reduces their horizontal bearing capacity. Because rigid piles with large diameters and low slenderness ratios have been widely used in offshore wind farms, there is an urgent need to address this issue. In this study, grouting protection, as a new countermeasure against scouring, was examined, with scouring tests under constant-current, regular-wave, and combined wave–current conditions being conducted, along with static and cyclic load tests on the monopile. The scouring characteristics and horizontal bearing behaviours of a pile with grouting protection (Pile a) and a pile without grouting protection (Pile b) were investigated. We found that the ultimate scouring depths of piles with grouting were much lower than those without under constant-current scouring, regular-wave scouring, and combined wave–current scouring. The HL2Dγ′−y0D and p-y curves of the piles were obtained, which showed that grouting protection can significantly strengthen the ultimate horizontal bearing capacity under a static loading. The pile with grouting was more stable under cyclic load compared to the pile without grouting. There are two explanations for the strengthening of the horizontal bearing behaviours of piles with grouting protection. First, grouting lessens the loss of pile burial depth brought about by scouring. Second, grouted seabed has a much higher sediment stiffness than ungrouted seabed, thus the grout dramatically increases the strength of the pile– sediment structure.

Coupled analysis of monopile offshore wind turbine under modified three-dimensional seafloor ground motions

The safe operation of offshore wind turbines (OWTs) is significantly influenced by the combined effect of wind, waves, and offshore earthquakes. However, limited seafloor ground motion data have been used to determine the dynamic characteristics and responses of OWTs subjected to offshore earthquakes. Hence, synthetic seafloor ground motions are particularly important for seismic analysis of OWTs. This paper proposes a new method to simulate seafloor ground motions. In the study, the dynamic shear modulus of the water-bearing soil layer and the damping effect of the soil–seawater layers were introduced to modify the simple Crouse and Quilter (C-Q) model in seawater and P-SV and SH transfer function models at offshore sites. Further, three-dimensional seismic waves were synthesized based on the modified transfer function models at a typical offshore site with bedrock–soil–seawater layers, and a fully coupled numerical analysis model of OWTs, including the rotor nacelle assembly, tower structure, and substructure with pile–soil interaction (PSI) under wind, wave, and seafloor ground motion, was established by incorporating the earthquake module and PSI module into the updated FAST v8 software. Moreover, based on the established coupled OWT model, the measured seafloor ground motion records and the seafloor ground motions synthesized using the proposed method were compared, and a coupled analysis of the OWT under wind, wave, and offshore earthquake loadings was carried out. Furthermore, the coupled mechanisms of the OWT under synthesized and recorded seafloor ground motions were determined.

Investigation on the interaction between ice and monopile offshore wind turbine using a coupled CEM–FEM model

In this study, the cohesive element and finite element methods (CEM-FEM) were coupled to investigate the interaction between horizontal ice and monopile OWT, including the breaking and accumulation of sea ice. A sea ice constitutive model was incorporated into the LS-DYNA finite element code to simulate the mechanical behavior of ice. The sea ice–OWT interaction coupling model considering the pile–soil interaction (PSI) was also enabled in the code. The influences of wind loads and PSI effect on the dynamic response of the OWT structure under sea ice were investigated in parked and operating states in an established turbulent wind field. The results show that the combination of wind and ice loads significantly affects the design and analysis of the OWT considering the PSI. The simulation results of the proposed model were validated by conducting a physical ice force experiment. Based on this, energy and damage analyses of the OWT at different ice velocities and thicknesses were performed, and the interaction mechanism between the sea ice and OWT was revealed. Further, an area damage rate index was proposed to evaluate the degree of damage in the OWT structure. The proposed interaction model and damage rate index can reasonably capture the variation trend of the OWT dynamic characteristics and damage with changes in the ice velocity and thickness. This study provides a useful reference for evaluating the dynamic characteristics and damage to OWT structures under wind and sea ice conditions.

A novel hybrid adaptive Kriging and water cycle algorithm for reliability-based design and optimization strategy: Application in offshore wind turbine monopile

Metaheuristic algorithms have been widely concerned by scholars because of their global optimization ability that does not depend on gradient information. In this study, Water Cycle Algorithm (WCA) as metaheuristic optimization is applied for a decoupling strategy to improve the efficiency of the original dual-loop Reliability-Based Design and Optimization (RBDO) algorithm. Furthermore, an adaptive Kriging-model-assisted RBDO method considering hybrid uncertainty is proposed based the decoupling WCA for improving the accuracy of RBDO results. In this method, in order to improve the computational burden of optimization design under uncertainties with highly accuracy, an adaptive Kriging model is developed to approximate the actual performance function. A novel updated strategy for Kriging model is proposed to achieve the construction of the adaptive Kriging model. Two cases studies, including a numerical example and a crank-slider mechanism optimization example is used to illustrate the superiorities of the proposed method. Finally, the proposed hybrid adaptive RBDO method is applied for optimization design of offshore wind turbine monopile under hybrid uncertainties. Compared with the original design, the weight of the optimized scheme decreased by 12.78%.

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

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

Numerical and Experimental Investigations on Non-Linear Wave Action on Offshore Wind Turbine Monopile Foundation

Monopiles are commonly utilized in offshore wind farms but are prone to non-linear wave loads and run-ups, significantly affecting their engineering design. Therefore, it is crucial to pursue a complete understanding of the non-linear wave action on monopile foundations. Both numerical and experimental investigations on the non-linear wave loads and run-ups on an offshore wind turbine monopile foundation are performed in this paper. The experiment is carried out at a scale of 1/30 in a wave flume at the State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology, in which the wave loads and run-ups along the monopile are measured. Based on the second-order potential flow model and time-domain higher-order boundary element method (HOBEM), the related numerical tests are conducted to study the non-linear effects further. It is found that the present non-linear potential theory is sufficient for the simulation of wave force and run-ups on the monopile in the range of wave slope kA < 0.15 before wave breaking. “W” type distribution of wave run-up along the monopile is found, in which the peak value occurs at the frontward side (i.e., θ =180°) and is the maximum due to full reflection; the two symmetrical minimum amplitudes lie in the zones of (45° ≤ θ ≤ 90°) and (270° ≤ θ ≤ 315°), whose positions shift downward with the increase of wave non-linearity. Energy transfer among the fundamental wave component and higher-order components is also found, which is most apparent on the backward side. Besides, the transverse resonance occurs in the wave flume due to the wavelength being near the flume width, which induces the wave run-up at the backward position larger than that at the frontward position.

A novel physics-informed framework for real-time adaptive modal parameters estimation of offshore structures

Singular value decomposition provides a rigorous mathematical foundation for these monitoring methods, such as modal analysis, damage detection, etc. As substantial limitations including computational efficiency and adaptive capabilities have been identified in the matrix process, data-driven algorithms with enhanced efficiency and accuracy to adapt the rapid development of information and technologies have been highly required in offshore structures monitoring. To meet this demand, a novel physics-informed frame work enabling the real-time adaptive monitoring has been proposed in this paper. The working principle of the developed framework has been represented by the smart conversion of physics-informed modal identification into the optimal process of fast and accurate solving an eigensystem governed by differential equations via recurrent neural network. The ingenious design of the proposed framework has complied with the rule of singular value decomposition used for modal identification and therefore, learning capabilities of the physics-informed framework have been remarkably enhanced by successfully addressing two bottleneck problems including proper initialized input values and the optimal time increment of the developed recurrent neural network. Both numerical simulations and the field-data based study of a mono-pile offshore wind turbine structure have been presented to examine the superior performance of the proposed framework. Results have shown that the proposed framework has the ability to adaptively identify modal parameters with a higher level of computational efficiency as compared with traditional methods. Furthermore, the computational advantage of the develop framework has demonstrated the potential to integrate with sensor networks and edge computing for smart monitoring and maintenance in various engineering subjects.