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The utilization of renewable energy is an important way to solve the energy crisis, and wind power is one of the most mature new energy technologies. This paper studies the potential of applying the viscoelastic damper to mitigate vibrations of the OWT tower under wind-wave loadings. The viscoelastic damper directly converts the horizontal deformation of the tower into the shear deformation of the viscoelastic material for energy dissipation, which provides a new idea for suppressing the dynamic response of the OWT. According to OWT structural dynamic characteristics and load conditions, a new high damping wall-type viscoelastic damper (HVED) control is developed. The mechanical properties of HVED are investigated by full scale test. A numerical vibration response analysis of the controlled and uncontrolled 4 MW OWT was carried out in ANSYS. Results indicate that the HVED can significantly reduce the dynamic response of OWT and increase its fatigue life. • A new high damping viscoelastic damper (HVED) is proposed to mitigate the vibration of OWT. • Advantages of HVED are significant energy dissipation, wide-frequency control, and convenient installation. • Full scale test was conducted to verify the mechanical properties of HVED. • Interaction of soil and structure (SSI) was considered in the finite element analysis. [ABSTRACT FROM AUTHOR]

At present, the numerical analysis of the monopile foundation of an offshore wind turbine subject to complex environmental loads is a major challenge in engineering design. Based on the concept of the macroelement model, this paper proposes a straightforward, speedy, and accurate numerical tool. Firstly, the numerical limit analysis method is employed to investigate the three-dimensional failure envelope surface of a large-diameter monopile in sand, and the analytical formula of the three-dimensional failure envelope surface is obtained. Then, using the three-dimensional failure envelope and combining the basic principles of the hypoplastic theory, a hypoplastic macro-element of a large diameter monopile foundation in sand is proposed. The performance of the macroelement model is calibrated by comparing it to centrifuge test results of a small-diameter pile and three-dimensional finite element results of a large-diameter monopile under cyclic loading. Finally, the macroelement model is used to analyze the performance of the OWT large-diameter monopile foundation under wind and wave loads. The results indicate that the macro-element model can precisely reproduce the performance of large-diameter monopile foundations under cyclic loading, and can effectively and accurately solve the complex pile-soil interaction problem of OWT large diameter monopiles under cyclic loading. • The failure surface of a large-diameter monopile foundation in sand is established. • A macroelement model of a large-diameter monopile foundation in sand is established. • The behavior of a large-diameter monopile foundation is studied using a macroelement model. [ABSTRACT FROM AUTHOR]

The offshore wind industry is a growing business. New and alternative installation methods which have the potential to reduce the cost of developing offshore wind farms are urgently needed. In this paper, an innovative concept that is tentatively designed for median or large water-depth sites is studied. The conventional jacked-up installation vessel is replaced by the small waterplane area twin-hull vessel (SWATH) equipped with the dynamic position (DP) system, and the heavy lifting is replaced by the low-height lifting crane with lifting wires for lowering the assembled OWT to the floating foundation. The global dynamic responses of the installation system under environmental loads are investigated by the fully coupled nonlinear numerical models, and the corresponding strength capacity for the lifting crane is validated. Several sea states are selected to study the feasibility of the concept, and the response variables of the OWT tower mating point (i.e., the displacement and velocity) are analyzed under different loading conditions. The lifting wires and selected lifting crane components are also investigated to understand their force situations. Some interesting conclusions are drawn, which can provide references for introducing the control system to the low-height lifting system and further optimizing the concept. • A comprehensive numerical model for pre-assembled OWT installation is proposed. • The low-height lifting crane's flexibility and the details of the lifted OWT are considered. • The lifting crane's buckling stability under dynamic load conditions is validated. • The responses of the installation concept is calculated under combined wind and wave loads. • A deeper understanding of the installation concept is achieved. [ABSTRACT FROM AUTHOR]

The offshore wind industry is a growing business. New and alternative installation methods which have the potential to reduce the cost of developing offshore wind farms are urgently needed. In this paper, an innovative concept that is tentatively designed for median or large water-depth sites is studied. The conventional jacked-up installation vessel is replaced by the small waterplane area twin-hull vessel (SWATH) equipped with the dynamic position (DP) system, and the heavy lifting is replaced by the low-height lifting crane with lifting wires for lowering the assembled OWT to the floating foundation. The global dynamic responses of the installation system under environmental loads are investigated by the fully coupled nonlinear numerical models, and the corresponding strength capacity for the lifting crane is validated. Several sea states are selected to study the feasibility of the concept, and the response variables of the OWT tower mating point (i.e., the displacement and velocity) are analyzed under different loading conditions. The lifting wires and selected lifting crane components are also investigated to understand their force situations. Some interesting conclusions are drawn, which can provide references for introducing the control system to the low-height lifting system and further optimizing the concept. • A comprehensive numerical model for pre-assembled OWT installation is proposed. • The low-height lifting crane's flexibility and the details of the lifted OWT are considered. • The lifting crane's buckling stability under dynamic load conditions is validated. • The responses of the installation concept is calculated under combined wind and wave loads. • A deeper understanding of the installation concept is achieved. [ABSTRACT FROM AUTHOR]

When a wind turbine is working in a cold and humid environment, icing may occur which lead to its performance reduction or even blades fracture. In this paper, a CFD-WTIA (Wind Turbine Integrated Analysis) coupled method is established to analyze the blade icing process and its influence on the overall dynamic performance of an integrated jacket-support offshore wind turbine. Firstly, motions of the blades are calculated by the WTIA method and used as input into CFD. Then, dispersed multi-phase model and melting-solidification model are used to simulate the icing growth phenomenon of three-dimensional blades. The k-ε turbulence model is used to calculate the aerodynamic performance before and after icing. Finally, the aerodynamic results after blade icing are returned to WTIA for integrated dynamic response acquisition. At the same time, the dynamic response of the wind turbine under the combined influence of ice and sea ice is analyzed. Results show that the blade ice-accretion increases linearly along the blade span-wise direction and is mainly concentrated on the leading edge of the blade. Lift and drag coefficients are seen deceased and increased respectively after icing. Power production, generator torque, rotor speed, as well as blade vibration are quantitatively studied. The methodology and findings of this paper can provide a good reference for the safety performance evaluation of an icing offshore wind turbine. • A coupling of the CFD-WTIA method is established to investigate icing impact on the integrated offshore wind turbine. • Ice accretion is mainly concentrated on the leading edge. The most serious ice accretion is observed at the blade tip. • Before rated power, icing reduces power generation, rotor speed and torque, and the vibration of structure. • Over rated power, the wind turbine is maintained at rated power, the icing of blades increases structural vibration. • Under the combined load of blade icing and sea ice, the vibration of wind turbine structure is more severe. [ABSTRACT FROM AUTHOR]

The expansion of the offshore wind industry in areas with high seismicity has led to engineering challenges related to the design of the offshore wind turbines (OWTs). Monopiles, i.e., tubular steel piles of large outer diameter, low aspect ratio (penetration depth over outer diameter), and relatively thin pile wall, are traditionally the preferred foundation type for OWT due to fabrication, transportation, and installation standardization. For all bottom-founded systems, soil–structure interaction (SSI) plays a crucial role in the system's response. Additional challenges arise in the case of seismic SSI as, not only the system's response, but also the seismic ground motion itself are affected by the soil characteristics. Furthermore, uncertainties related to soil properties, as derived from the soil testing campaign and interpretation, need to be thoroughly considered for OWT load calculations and the design of the support structure. The uncertainty in soil interpretation may have a large impact on the characteristics of the input seismic motion. Subsequently, SSI will affect the seismic loads acting on the support structure and the OWT. This knock-on effect of the interpretation of the soil parameters is unknown, but may be significant to account for. In fact, when a "best estimate" soil parameter set is used, the resulting seismic load may not necessarily correspond to the most probable load for the assumed seismic event. This paper investigates the influence of the uncertainty in soil parameters, as they may result from the soil interpretation, on the seismic loads. It demonstrates the skewed distribution of OWT seismic loads using a realistic design case study on a commercial OWT. Results are presented in the form of transfer functions, response spectra at mudline and normalized bending moments along the support structure. Three distinct structural components of interest are selected to evaluate the results. It is concluded that, for the analysis of OWT under seismic loading conditions in particular, it cannot be decided a priori which soil properties would result in conservative or progressive design. Based on the obtained results, recommendations are given which aim to de-risk and enhance the current design practice. • Consideration of uncertainty in soil properties is important for the seismic design. • At least two sets of soil properties should be accounted for the seismic design. • Transfer functions (bedrock to mudline) are used to identify the critical eigenmodes. • Response spectra (at mudline) are used to detect the critical set of soil properties. • A sensitivity study is advised if the seismic load is close to the ULS design load. [ABSTRACT FROM AUTHOR]