Dynamic Response Of Offshore Wind Turbines Research Articles

Two-way coupling fluid–structure interaction analysis on dynamic response of offshore wind turbine

The monopile-supported offshore wind turbines (OWTs) are subjected to the wave and current loadings. Affected by currents and waves, monopile-supported OWTs are also vulnerable to scour. However, the effect of scour and currents on the dynamic behavior of the monopile are not fully appreciated. This study conducted the model tests to evaluate the effect of scour depth and flow velocity on the lateral responses of support structure. Furthermore, the two-way (T-W) coupling fluid–structure interaction (FSI) method was proposed to evaluate the dynamic responses of piles in sand. The support structure is modeled using ABAQUS, a finite-element models software package, by considering the nonlinear soil–structure interaction effects, STAR-CCM + tools has been employed to model the FSI which are fed into ABAQUS to predict the structure’s dynamic response. Using the proposed method, a parametric study has been conducted to evaluate the effects of scour depths, flow velocities on the dynamic responses of OWTs. The results indicate that scour altered the fundamental frequency of OWT, causing resonance. An increase in scour depth and flow velocity increased the dynamic responses of support structure, which are detrimental for the safety and stability of the OWT system.

The analysis of crashworthiness and dissipation mechanism of novel floating composite honeycomb structure against ship-OWT collision

High-energy ship collisions can cause catastrophic damage to offshore wind turbines (OWTs). This paper proposes a novel floating composite honeycomb anti-collision structure to reduce damage caused by ship-OWT high-energy collisions. The typical collision scenarios are conducted to investigate the damage analysis of anti-collision structures and the dynamic response of OWT with and without anti-collision structures via ABAQUS/Explicit. Regarding the configuration of the composite honeycomb anti-collision structure, the elastic-plastic theory of sandwich beams is proposed and an elastic-plastic analysis on the sandwich wall honeycomb core structure is performed to investigate its dissipation mechanism and obtain homogeneous material parameters of the cells. The homogenization parameters are incorporated into the ship-OWT anti-collision analysis to evaluate their applicability. The collision simulation results demonstrate that the proposed anti-collision structure is capable of effectively absorbing 80% of the initial kinetic energy and significantly reducing the dynamic response of the structure. In elastic-plastic analysis, the proposed elastic-plastic theory of sandwich beams effectively addresses the elastic-plastic behavior of honeycomb with sandwich walls, yielding results consistent with the stress-strain curve obtained from finite element analysis (FEA). In the coupled analysis, the obtained parameters of the homogenized honeycomb validate its effectiveness and enhance computational efficiency. Valuable results are given for the engineering design.

Dynamic response of OWTs under wind-wave loads considering 3D water-pile-soil interactions

A new three-dimensional (3D) integrated model of offshore wind turbine (OWT) is proposed, which considers the soil-monopile-water interactions and the accumulative deformation of foundations under cyclic loads. The effects of three different modelling methods on the dynamic response of wind turbine are discussed, where three modelling methods are the new WOT model, OWT model with a 3D soil-pile medium without a seawater domain and OWT model with API's spring-type foundation. The numerical results show that the use of API's soil spring may overestimate the fundamental frequency of OWT by up to 32.2%. This leads to inaccurate structural resonance. Meanwhile, a significant displacement drift of the tower top of the new model is observed due to the ratcheting effect, and the OWT with API's spring cannot correct this phenomenon. Consequently, the API method should be used with care in assessing dynamic performance under wind-wave loads. The added mass method considers only the water-structure interaction and excludes the soil-water interaction, and it may underestimate the responses of the tower, blades and monopile, which can render the design of wind turbines unsafe. Additionally, assumption of the parked condition underestimates the structural response, and the frequency of the blades may change during operation.

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

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

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

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

Natural Frequency Degradation Prediction for Offshore Wind Turbine Structures

The scale of offshore wind turbines (OWTs) has increased in order to enhance their energy generation. However, strong aero/hydrodynamic loads can degrade the dynamic characteristics of OWTs because they are installed on soft seabeds. This degradation can shorten the structural life of the system; repetitive loads lead to seabed softening, reducing the natural frequency of the structure close to the excitation frequency. Most of the previous studies on degradation trained prediction algorithms with actual sensor signals. However, there are no actual sensor data on the dynamic response of OWTs over their lifespan (approximately 20 years). In order to address this data issue, this study proposes a new prediction platform combining a dynamic OWT model and a neural network-based degradation prediction model. Specifically, a virtual dynamic response was generated using a three-dimensional OWT and a seabed finite element model. Then, the LSTM model was trained to predict the natural frequency degradation using the dynamic response as the model input. The results show that the developed model can accurately predict natural frequencies over the next several years using past and present accelerations and strains. In practice, this LSTM model could be used to predict future natural frequencies using the dynamic response of the structure, which can be measured using actual sensors (accelerometers and strain gauges).

Influence of earthquake ground motion modelling on the dynamic responses of offshore wind turbines

Offshore wind turbines (OWTs) are more and more widely used to produce electrical energy nowadays. Besides the constant wind and wave loads, earthquake excitation can be another important vibration source to the OWTs since many OWTs have been/will be constructed in the seismic prone areas. Extensive research works have been carried out to understand the dynamic behaviours of OWTs when they are subjected to multi-hazards (e.g. the simultaneous wind, wave and/or earthquake loadings). However, when seismic excitations are considered, the onshore earthquake records are normally used as inputs in the analyses due to the lack of offshore data and the difficulty in synthesizing the offshore seismic motions. This practice may lead to inaccurate structural response estimations since it is well known that the seawater can significantly suppress the seafloor vertical motions near the P wave resonant frequencies of the seawater layer, which in turn results in the different characteristics of onshore and offshore earthquake recordings. Moreover, the earthquake motions along the pile of OWTs are different from those at the ground surface, i.e. the earthquake motions vary with the soil depth. Recently, a method to stochastically simulate the earthquake ground motions on the offshore site was proposed, in which the influence of seawater layer was considered and the earthquake motions at any soil depth could be obtained. This paper carries out numerical simulations on the dynamic behaviours of OWTs subjected to the combined wind, wave and earthquake loadings, and the depth-varying offshore seismic motions are used as inputs in the analyses. The seismic responses of OWTs obtained from the onshore and offshore earthquake motions are calculated and compared. The influence of depth-varying ground motions on the dynamic responses of OWTs is discussed.

Probabilistic analysis of monopile-supported offshore wind turbine in clay

The dynamic behaviour of monopile supported offshore wind turbines (OWTs) is considerably influenced by inherent variabilities of soil properties and loading. This study investigates the effect of uncertainties in soil shear strength properties, and wind and wave loads on the reliability of OWT structures founded in clay. The OWT system is modeled as an Euler-Bernoulli beam and soil-structure interaction is incorporated using the American Petroleum Institute based cyclic p-y relationships. The uncertainties in soil undrained shear strength, in the parameters and equations of the p-y method, and in wind velocity are considered. Random wave loads are estimated from the code specified power spectral density function of the vertical sea surface elevation. Uncertainties in OWT responses are quantified using Monte Carlo simulations. The effects of length and diameter of the monopile, vertical sea surface displacement spectrum, and the probability distribution of wind speed on the dynamic responses of OWT are investigated. The study shows that the various power spectral density functions of wave surface displacement and various probability density functions of wind speed have a marginal effect on the response and fatigue life of OWTs. The mean fundamental frequency of the OWT system is significantly affected by the variability of the undrained shear strength of clayey soil.