Monopile Offshore Wind Turbine Research Articles

Multi‐hazard fragility assessment of monopile offshore wind turbines under earthquake, wind and wave loads

AbstractThis study establishes a multi‐hazard probabilistic assessment framework for assessing the integrity of monopile offshore wind turbines (OWT) under the stochastic coupled effect of wind, wave and earthquake loading. The procedure deals with the entire operational range of inflow wind speed (i.e., 3–25 m/s), for which the probability of failure under multi‐hazard excitations is found to be non‐negligible. Numerical analysis is performed by implementing nonlinear finite‐element models of the OWT developed in OpenSees. The dynamic response of the OWT system under wind‐ and wave‐load combinations is individually validated against those obtained from the aero‐hydro‐servo‐elastic simulator OpenFAST. Following the Latin‐hypercube approach, a cloud‐based assessment procedure is then performed with an ensemble of 300 earthquake ground motions, from which the multi‐hazard performance of the OWT regarding the serviceability limit state (SLS) and the ultimate limit state (ULS) can be evaluated. The epistemic uncertainty associated with various loads, structural properties, and soil conditions is also accounted for. Based on this probabilistic assessment framework, the sensitivity of the resulting OWT fragility surfaces to different statistical regression methods and wind—ground motion intensity measure pairs (IM‐pairs) is further scrutinised. Regression methods are comparatively evaluated. The efficiency, practicality, proficiency and sufficiency of various IM‐pairs are examined for the purpose of assessing operating OWT multi‐hazard fragility functions. The optimum IM‐pair is then employed in a trained Gaussian Process Regression (GPR) scheme for cloud data regression to assess the multi‐hazard fragility of the system. The derived multi‐hazard fragility function shows that the contribution of seismic forces in structural demand for a design‐level earthquake is comparable to those caused by operational‐level wind and wave loads.

The influence of tuned mass dampers on vibration control of monopile offshore wind turbines under wind-wave loadings

The passive tunned mass dampers (TMDs) can effectively mitigate vibrations of offshore wind turbines (OWTs). However, most of the aforementioned literatures only focus on the vibration reduction effect of a TMD without considering vibration mechanism of the OWT with and without the TMD, which is affected by the frequency characteristics of wind-wave loadings, especially the low frequency characteristics of wind loads. Moreover, the baseline control of OWTs including the pitch control and the generator torque control can affect the dynamical responses of the OWT and thus the vibration reduction performance of a TMD. In this paper, an optimal dual TMD is placed at the nacelle for mitigating the vibration of the OWT. The influence of the TMD on dynamical responses of the OWT is fully investigated under the baseline control and the various wind-wave loadings. The research shows that the TMD has an excellent vibration reduction effect when the OWT is parked (at high wind velocities). When the OWT is parked and the wind velocity at the hub is at 25 m/s, the vibration reduction performance of the TMD is the best, and the maximum values of the tower top displacement and acceleration, and the top and the bottom tower moments are reduced by 25.41%, 24.76%, 7.09% and 16.78%, respectively. Moreover, the optimal designs for the TMD are further conducted under the baseline control and the different wind-wave loadings. The research results show that the vibration reduction performance of the TMD is not significantly enhanced when the OWT can work normally, but it is enhanced when the OWT is parked. This work can provide technical support for optimal design of the TMD under the wind-wave environments.

Probability of unsatisfactory performance of monopile foundations for offshore wind turbines considering variations in seabed properties

Safe design of monopiles for Offshore Wind Turbines (OWT) requires an accurate estimation of the foundation soil material properties. Uncertainty in soils is mainly due to their inherent variability which involves various spatial and temporal processes they have been exposed to over their natural geological process. The limited amount of information from geotechnical investigations in the seabed, considering the size of OWT farms, adds to the uncertainty. Commonly used deterministic design approaches idealize soil layers by assigning average values to the mechanical soil properties and make use of empirical correlations that can significantly differ from offshore conditions. Probabilistic design approaches have been suggested in the literature to account for the effect of uncertainties on the foundation’s performance by applying a coefficient of variation (COV) to soil properties. Such methods include theoretical investigations by applying selected COV values to predetermined mean CPT profiles for sands and undrained shear strength for clays. This study calculates the probability of unsatisfactory performance, Pfsys, of monopiles in sands by implementing variation in friction angle and unit weight using current API and DNV-GL design guidelines. Monte Carlo Simulation is employed to generate random fields using lognormal, normal, and uniform distributions. Pfsys is presented for a case study found in the literature and a design exercise for which local COV for the Belgian coast is also proposed.

Dynamic response of offshore wind turbine monopile foundation under wind–wave loads

Physical experiments provide the best representation of foundation behaviour in any environmental conditions offshore; however, they are very costly, especially since the on-site conditions vary. Hence, it is not a cost-effective approach. In contrast, numerical modelling is relatively cheaper, but requires long computation time and a high level of expertise to perform a successful numerical calculation. The main objective of this paper is to develop a simple and fast response analysis that requires less computational time and expertise. First, an analytical model is developed to study the dynamic behaviour of the pile by extending the beam on an elastic foundation. Second, the analytical results are verified by using the finite-element software, Abaqus. Finally, a parametric study is carried out with the established numerical calculation model to investigate the modal shapes of the pile model, limit states of the offshore wind turbine (OWT) and pile capacity for different loading conditions and aspect ratios. The proposed analytical model demonstrates a degree of error less than 10%, indicating great reliability and the potential for practical application. The parametric results indicate that the OWT is more likely to fail by yielding than buckling and the soil resistance at the pile periphery is highly affected by the aspect ratio.

A developed model updating method based on extended frequency response functions and its application study of offshore structures

Accurate finite element (FE) models are extremely vital to the analysis of the dynamic characteristics of engineering structures. However, it is challenging for the frequency response function (FRF)-based model updating method to obtain reliable numerical results as the limited number of FRFs at the selected peak positions usually leads to an unacceptable or failure of model updating. Moreover, the increased number of updating coefficients in complex engineering problems generates nonlinear optimization models with the local solutions by traditional optimization techniques. To tackle these two issues, a new model updating method is proposed to make full use of FRFs extracted from measured field data of structures and the improved particle swarm optimization (PSO) technique for accurately estimating physical parameters of structures. The novelties of this research include: (1) The signature assurance criterion (SAC)-based FRF is evaluated to eliminate the influence of limited frequency points on the accuracy of the updated model using the normalized acceleration component; (2) The enhanced (PSO) is developed to realize the adaptive selection of inertia factors for the better diversity by introducing an average value of the fitness function, and then accurate predictions of updating coefficients within a smaller number of iterations are achieved using the developed constraint factor. The effectiveness of the proposed method is verified by mathematical model of a jacket platform. Results show that the proposed method can accurately obtain the updating coefficients under spatial incomplete condition, and the maximum error of natural frequencies is 0.779% using the accelerations containing 5% noise. To prove the robustness of the proposed method, experimental studies of monopile offshore wind turbine are also conducted and the maximum error of natural frequencies is 1.887% under the consideration of spatial incompleteness represented by the structural stiffness degradation. Finally, the feasibility of the proposed method is evaluated by a test of a complex jacket platform, whose variation is simulated by weakening the connections of some elements. The maximum error of natural frequencies predicted by the updated model is only 4.831% as compared with experimental results. Throughout these examples, the extended FRF model updating method provides engineers and designers with a useful insight into the development of reliable techniques to accurately predict dynamic responses of offshore structures.

Extreme structural response prediction and fatigue damage evaluation for large-scale monopile offshore wind turbines subject to typhoon conditions

Offshore wind is becoming the way forward for green energy harnessing worldwide. However, frequent typhoons are a major constraint for the development of offshore wind power for some regions in Asia and North America. Typhoons may pose a huge challenge to offshore wind farm development in southern China. In this paper, a fully-coupled analysis was carried out for a 10 MW large-scale monopile offshore wind turbine (OWT) using SIMO-Riflex-Aerodyn (SRA) code. The response characteristics of the OWTs in different typhoon regions are investigated based on the measured typhoon conditions in China. The effect of aerodynamic damping on the response and the load effect is analyzed in detail. Two different distribution methods are used to statistically extrapolate the response value and get the short-term extreme response. Cumulative linear fatigue damage is evaluated by the rain-flow counting method to explore the possible failure modes of large wind turbines during typhoons. The results show that aerodynamic loads play an important role in large monopile OWTs during high wind speeds in parked conditions. The extreme response and fatigue analyses from this study indicate that fatigue is a dominant failure mode for the large OWT tower during typhoons, while buckling is unlikely.

Studying the Settlement of OWT Monopile Foundations Using a T-Z Spring with the Torsional Effect

The main purpose of this paper was to study the vertical settlement of offshore wind turbine (OWT) monopile support structures, where 5, 10, 15, and 20 MW OWT support structures were analyzed under power production, seismic, and tropical cyclone loads. Moreover, a t-z spring with shear and torsional degrees of freedom was developed to simulate the shear stress along the pile and soil surface under the combined effect of vertical loads and z-direction torsions. This t-z spring does not require excessive changes to the finite element program, where only a known factor is used to modify the traditional stiffness of the t-z spring. This paper, analyzing several kinds of OWT monopile foundations, indicates that the soil shear resistance may be less than the shear stress generated by the combination of vertical loads and torsions, which causes large vertical and rotational displacements resulting in the failure of monopile structures. This situation will be worse when the natural frequency of the first vertical-direction rotation is close to the integer multiples of the 3P frequency, which cannot be well-simulated using traditional t-z springs.

Field data observations for monitoring the impact of typhoon “In-fa” on dynamic performances of mono-pile offshore wind turbines: A novel systematic study

For offshore structures such as offshore wind turbines (OWT), typhoon is usually considered one of the most critical threats to structural safety performances and service life due to its heavy wind, wave, and even coexisted storm surge. Meanwhile, it is challenging to obtain the systematic data from the environmental conditions, structural dynamic vibrations and the SCADA record, when typhoon passes by the offshore wind farm. Taking into account these situations, a real-time multi-source monitoring system enabling the investigation of the typhoon impact on the performances of OWT, has been firstly established and implemented to a 4.0 MW mono-pile OWT in Rudong, Jiangsu, China. One of the major contributions in this work is to develop the monitoring system using a unique environment of real-world data that has been synchronously obtained from waves, winds, vibrational accelerations, inclinations of towers and SCADA data during the typhoon “In-fa” passing by the wind farm, and provide the scientific community with the underlying standards and technical recommendations. To investigate the influence caused by “In-fa”, comparison results of the measured data in the range of June to August have been analysed. It is worth noting that two conclusions have been obtained: (1) the region near the nacelle is not always the most critical vibrational area. Actually, the change of the maximum structural response in the position under different external loads should be applied to effectively evaluate the structural safety; (2) the measured accelerations exhibit an obvious decay process in the presence of the turbine rotor-stop, but not the yaw rigid-body motion. This observation promotes the accurate identification of modal parameters for the long-term monitoring. Consequently, these valuable findings to facilitate the assessment of structural operational conditions have been developed into two guide-lines. All the data and analyses presented in this paper provide a valuable insight into the design, energy efficiency, safety monitoring and damage diagnosis of OWT structures.

Seismic dynamics of offshore wind turbine-seabed foundation: Insights from a numerical study

In the past 10 years, the offshore wind energy harvest industry has developed rapidly worldwide. However, seismic waves would bring a great threat to the safety and stability of offshore wind turbines (OWTs). In this study, taking the marine geotechnics numerical software FssiCAS as the computational platform, adopting the generalized elastoplastic soil model Pastor-Zienkiewicz-Mark III (PZIII) to describe the complex mechanical behavior of seabed soil, the seismic dynamics, as well as the stability of a thin-walled monopile OWT with an equipped capacity of 1.5 MW and its seabed foundation are comprehensively investigated, by the way of finely modeling and meshing for the important components of the OWT, i.e., the blades, nacelle, and tower. The numerical results indicate that OWT and its seabed foundation strongly respond to the excitation of seismic waves, and there is intensive interaction between OWT and its seabed foundation. In the case of this study, the horizontal oscillation amplitude at the top of the turbine tower reaches 2m, the superficial seabed soil at the far field is liquefied with a depth of 3–4 m, and the liquefaction depth of the seabed soil surrounding the monopile reaches 5–6 m. Even so, the OWT involved in this study has no cumulative displacement, there is only vibration displacement. It is indicated that the OWT has good seismic stability. It is indicated by the comparative study that the complex mechanical behavior of seabed soil, the complex geometry and mass distribution of OWTs, and the consideration of the pore water in seabed foundations have a radical influence on the seismic dynamics of OWTs. The work presented could be a valuable reference for the evaluation of the seismic dynamics and stability of OWTs in the future.

Adaptive Active Generator Torque Controller Design Using Multi-Objective Optimization for Tower Lateral Load Reduction in Monopile Offshore Wind Turbines

Adaptive Active Generator Torque Controller Design Using Multi-Objective Optimization for Tower Lateral Load Reduction in Monopile Offshore Wind Turbines

Insights from in-situ pore pressure monitoring around a wind turbine monopile

Offshore wind turbine monopiles transfer large bending moments into the soil. Due to the large diameter of the foundation, a significant volume of soil is mobilised to absorb the incoming forces from wind, waves and current. If excess pore pressures are generated during load cycles, the long drainage path length could lead to accumulation of pore pressures and loss of strength and stiffness in the soil. During monopile design, undrained behaviour of the soil during cyclic loading is often assumed to conservatively assess the lateral pile resistance during storms. In-situ monitoring of pore pressures in the soil surrounding a monopile foundation allows the assumption of undrained behaviour to be checked for several episodes during the operational life of the foundation. A 3-year period of monitoring data was analysed for a North Sea monopile which included normal operation, storm episodes and rotor stops. Pore pressure and earth pressure sensors were positioned on the pile at two different depths in the most intensely loaded layer of surface sand. The data provides insight into the amount of excess pore pressure accumulation with storm loading not leading to significant accumulation of excess pore pressure. The only episodes showing a significant development of excess pore pressures were rotor stops with suction pressures being developed on the unloaded side of the monopile. The observed response suggests that significant pile-soil stiffness reductions in the top layer due to cyclic loading are not expected for the monitored location. The study shows the value of monitoring pore pressures in the soil mass surrounding offshore foundations for checking design assumptions.

Dynamic response analysis of monopile offshore wind turbines to seismic and environmental loading considering the stiffness degradation of clay

The safe and stable operation of monopile offshore wind turbines (MOWTs) under seismic and environmental loading is a major challenge facing designers. A numerical method was developed for simulating the dynamic interaction of the soil-pile-structure system by employing an elastoplastic bounding surface constitutive model that can capture the cyclic degradation of clay stiffness well. Then, the suitability of the method was validated by comparison with the existing simulation results in the literature. The dynamic responses of MOWTs to seismic and environmental loading were simulated by employing the above numerical method. The effects of soil plasticity and stiffness degradation on the dynamic responses of MOWTs were investigated. The influence of environmental loads on the seismic responses of MOWTs was also analysed. It was found that the monopile experienced significant rotation and settlement, and the soil plasticity and stiffness degradation significantly increased the peak response of the lateral deflection, rotation angle and bending moment of the MOWT under combined seismic and environmental loads. The input bedrock peak acceleration was amplified to varying degrees from the monopile bottom to the tower top, and the acceleration amplification along the height was the most significant overall when considering the soil stiffness degradation. Environmental loads such as winds and waves significantly increased the absolute values of the seismic response profile, including the deflection, rotation angle and bending moment, as well as the acceleration amplification factor. The research results can support the seismic design of MOWTs.

Vortex-induced vibration effect of extreme sea states over the structural dynamics of a scaled monopile offshore wind turbine

In order to demonstrate the relevance of considering Vortex-Induced Vibrations (VIV) in the structural design of marine structures, this study proposes an alternative experimental and analytical approach in wet conditions to measure the fluid–structure interaction in the near field and quantify the viscous damping with measured structural and 3D hydrodynamic accelerations. It was demonstrated that VIV caused and incremented 5.00% of the structural damping coefficient, and the extreme wind loading increased 74% of the offshore monopile’s structural damping, demonstrating the relevance of the high non-linear hydrodynamics effects during selecting parameters into the structural design in offshore applications.

Dynamic analysis on monopile supported offshore wind turbine under wave and wind load

Monopile offshore wind turbine (OWT) accounts for more than 65% of all OWT types. Monopile OWT is a soil-pile-tower system and can be divided into three stages: the stage embedded in the soil, the stage submerged in the seawater and the stage exposed to the air. Based on the improved Tajimi formulation and dynamic principle, this paper establishes a three-stage monopile OWT model to predict the dynamic response of the structure. The displacement of the tower under different wind velocities is obtained. The solution is compared with the Finite Element Method (FEM) result to verify its correctness. The influences of different parameters on the structure are discussed. The results indicate that the structure natural frequency is sensitive to the tower length and nacelle-rotor mass. The increasing seawater depth due to scour effect can significantly decrease the foundation stiffness. The influence of axial force and wave load on the foundation stiffness is also investigated. The tower diameter and tower wall thickness are found of vital importance to the tower displacement.

Dynamic analysis of monopile OWTs with viscoelastic dampers based on pole-residue method

The dynamic behaviour of monopile offshore wind turbines (OWTs) equipped with viscoelastic dampers (VE dampers) modelled by a fractional Kelvin–Voigt model is governed by a fractional differential equation. Most available methods to solve this kind of equation are limited to single-degree-of-freedom systems or simple deterministic loadings. Considering an OWT under either complex deterministic or random loadings, this work develops an efficient pole-residue method to evaluate its vibration. The key of the proposed method is to express the impulse response function with fractional derivatives into a pole-residue form. Then, the dynamic response time history and the response evolutionary power spectrum (EPS) are transformed into explicit solutions. In numerical studies, a monopile OWT with a VE damper subjected to irregular wind-wave loadings and nonstationary random earthquakes is investigated. Direct Duhamel integral and Monte Carlo simulations are utilized to verify their accuracy. After adding the VE damper, the deterministic response and the random EPS are significantly reduced. By conducting sensitivity analysis, we find that the growth of the fractional order of VE dampers leads to larger damping ratios of the OWT. Increasing the damping coefficient of the VE damper results in larger natural frequencies.

Modal Analysis of Offshore Monopile Wind Turbine: An Analytical Solution

Abstract An analytical solution of the dynamic response of offshore wind turbines under wave load with nonlinear Stokes’s wave theory and wave–structure and soil–foundation interactions is developed. Natural frequencies and the corresponding modes are obtained. The effect of the wave–structure interaction, the added mass, the foundation stiffness, and the nacelle translational and rotational inertia on the motion of the structure is investigated. The nonlinear loading provided by the drag term of Morison’s equation is successfully handled. A parametric study to examine the effect of the structural parameters on the dynamic response is conducted, and the results of the proposed analytical solution are compared to numerical ones. The proposed method has the following advantages: (a) it is accurate and straightforward because of its analytical nature, (b) it does not ignore the drag term in the wave loading by keeping its nonlinearity nature, (c) the structure of the wind turbine is modeled as a continuous system, (d) it takes into account the effect of the rotational and translational inertia of the nacelle on the dynamic response, and (e) it provides an interpretation of the effect of the sea level variation in changing the natural frequencies.

Dynamics of offshore wind turbine-seabed foundation under hydrodynamic and aerodynamic loads: A coupled numerical way

The offshore wind power industry has significantly contributed to the production of green energy in China and Europe etc. and made significant contributions to the reduction of carbon emissions. However, the extreme wave and wind loads in typhoon weather bring a great threat to the stability of offshore wind turbines (OWT). It is crucial to understand the dynamic characteristics of OWT and develop reliable methods of stability evaluation to guarantee the stability of OWT. In this study, taking the software FssiCAS as the computational platform, and taking the open-source solver OlaFlow to determine the wave impact on OWT and its seabed foundation, as well as the fluctuating wind theory is applied to estimate the wind load on the turbine blades and tower cylinder, the dynamic characteristics and stability of a monopile OWT with a capacity of 1500 kW in typhoon weather are investigated adopting a coupled way for the first time worldwide. The influence of the complex geometric shape and mass distribution of OWT is considered, and the generalized elastoplastic constitutive model, Pastor-Zienkiewicz-Mark III, is used to describe the complex dynamic behavior of the seabed foundation. The computational results show that the seabed soil within 4–5 m below the seafloor surface becomes liquefied under extreme wind and wave loads, resulting in the horizontal bearing capacity of the seabed foundation being weakened. As a result, the tower cylinder of OWT tilts aside by 0.5°, which exceeds the limit specified in some design codes. Through comparative analysis, it is found that the wind load contributes up to 50% to the lateral displacement of the OWT, and the maximum bending moment in the tower cylinder. It is indicated that wind loads can significantly affect the stability of OWTs. Additionally, this study proves that the software FssiCAS is applicable to evaluate the dynamics and the stability of OWTs under a complex marine dynamic environment. FssiCAS could hopefully become a computational platform for the design of OWTs.

Numerical analyses of energy balance and installation mechanisms of large-diameter tapered monopiles by impact driving

Large-diameter monopiles are widely used as the foundation to support offshore wind turbines (OWTs) in shallow coastal waters. The benefits of small-to-medium diameter tapered piles have been reported in the past. The potential use of large-diameter tapered monopiles installed by impact driving to support OWTs is thus presented, and then comparatively assessed by numerical analyses in terms of energy balance and installation mechanisms. A three-dimensional large deformation finite element (3D-LDFE) model of monopiles driven in clay was developed using a Coupled Eulerian-Lagrangian (CEL) approach. An advanced user-defined hypoplasticity clay (HC) model was employed to model undrained kaolin clay, featuring nonlinear behavior from small strain to large strain. The force-time curve defined by the operating data of a state-of-the-art hammer in the offshore industry was inputted to explicitly model impact driving. Better agreement between the measured and the simulated results was observed to validate the accuracy of the numerical model. The numerical results obtained give greater confidence to the future use of large diameter tapered monopiles for OWTs.

Lateral vibration mitigation of monopile offshore wind turbines with a spring pendulum pounding tuned mass damper

Lateral vibration of monopile offshore wind turbines (MOWT) is often lightly damped, inducing the apparent vibration under environmental loads. In this paper, a spring pendulum pounding tuned mass damper (SPPTMD) is developed to mitigate the lateral response of MOWT. First, the mathematical model of the controlled MOWT with SPPTMD is established based on the Euler–Lagrange equation. Compared to the referenced model, some improvements are made to enhance the numerical accuracy, where the aerodynamic damping, the bending moment effect, and the 1st mode shape of support structure calculated by Abaqus are considered. In addition, simulations of environmental loads are also introduced. In that regard, a 14 degree-of-freedom (DOF) model for the controlled MOWT with SPPTMD is obtained and compared with FAST. Then, the parameters of SPPTMD are first optimized. On this basis, the parametric analyses and verifications under complicated environmental loads of the proposed SPPTMD are carried out. The results indicate that the SPPTMD has better control effects than the conventional TMD in the low frequency detuning region due to the better energy dissipation capacity in this region. In that regard, the proposed SPPTMD has certain robustness and effectiveness for lateral vibration mitigation for MOWT.

Numerical investigation on horizontal deformation of large diameter monopile under cyclic wave loading with local scour holes

Seabed liquefaction and local scour around the offshore wind turbine (OWT) monopile have significant effects on the horizontal bearing capacity of the foundation. The horizontal displacement of the monopile at the seabed mudline is one of important factors considered in the foundation design. In this study, the effects of local scour and soil liquefaction on the monopile horizontal displacement were investigated. The dynamic response of sandy seabed with various densities under wave loading was simulated by adopting the Cyclic Mobility (CM) model. The numerical model was compared with previous experimental results. Then, the effects of local scour and soil liquefaction on the monopile horizontal bearing capacity were discussed. The differences in the effects of local scour and liquefaction on the horizontal displacement of monopile in the case of sandy seabed with different densities were compared. Numerical results shown that the effect of soil liquefaction caused by the vertical cyclic wave loading on the monopile horizontal bearing capacity is more significant than that of local scouring and horizontal cyclic loading, especially in loose sandy seabed. For dense sandy seabed, the local scour of the seabed has a more significant impact on the horizontal bearing capacity of the foundation.