Numerical Analysis of Nonlinear Wave Loads on an Offshore Wind Turbine Monopile

2019 ◽  
Author(s):  
Xingya Feng ◽  
Richard H. J. Willden ◽  
Binzhen Zhou ◽  
Thomas A. A. Adcock
Keyword(s):  
Offshore Wind ◽  
Offshore Wind Turbine ◽  
Wave Loads ◽  
Extreme Waves ◽  
Vertical Column ◽  
Higher Harmonics ◽  
Highly Nonlinear ◽  
Nonlinear Potential ◽  
Wave Event ◽  
Harmonic Components

Abstract Highly nonlinear extreme waves are the major, often the dominant, environmental load on offshore wind turbines. The higher-order ‘ringing’ loads associated with the nonlinear waves can cause unexpected resonance of the monopile. Hydrodynamic analysis of these harmonic loads remains a challenge due to the difficulty in extracting the bound harmonics from the force spectrum in an extreme wave event. A phase manipulation approach (four-phase combination) has been recently demonstrated to be able to separate the higher harmonic components of the wave loads in tank tests. In this work, we employ a fully nonlinear potential flow based Numerical Wave Tank (NWT) to simulate the wave diffraction by a fixed vertical column. We present a detailed study of our checks on the numerical accuracy of our model. Phase control is implemented for the wavemaker to manipulate the phase of each wave component. Focused wave groups are generated to represent the incoming extreme waves. With the four-phase decomposition, the higher harmonics of the wave loads are shown to be clearly separated. Comparisons with the existing test results show fairly good agreement at higher harmonics. The structure of the harmonic forces and moments are analysed and we reconstruct the higher harmonics based on the Stokes expansion assumption using the linear force. In addition, the effects of wave steepness on the harmonic components are discussed.

Fully Coupled Three-Dimensional Dynamic Response of a TLP Floating Wind Turbine in Waves and Wind

2012 ◽  
Author(s):  
G. K. V. Ramachandran ◽  
H. Bredmose ◽  
J. N. Sørensen ◽  
J. J. Jensen
Keyword(s):  
Wind Turbine ◽  
Dynamic Model ◽  
Three Dimensional ◽  
Geographic Location ◽  
Climatic Conditions ◽  
Offshore Wind ◽  
Offshore Wind Turbine ◽  
Total System ◽  
Wave Loads ◽  
Aerodynamic Loads

A dynamic model for a tension-leg platform (TLP) floating offshore wind turbine is proposed. The model includes three-dimensional wind and wave loads and the associated structural response. The total system is formulated using 17 degrees of freedom (DOF), 6 for the platform motions and 11 for the wind turbine. Three-dimensional hydrodynamic loads have been formulated using a frequency- and direction-dependent spectrum. While wave loads are computed from the wave kinematics using Morison’s equation, aerodynamic loads are modelled by means of unsteady Blade-Element-Momentum (BEM) theory, including Glauert correction for high values of axial induction factor, dynamic stall, dynamic wake and dynamic yaw. The aerodynamic model takes into account the wind shear and turbulence effects. For a representative geographic location, platform responses are obtained for a set of wind and wave climatic conditions. The platform responses show an influence from the aerodynamic loads, most clearly through a quasi-steady mean surge and pitch response associated with the mean wind. Further, the aerodynamic loads show an influence from the platform motion through more fluctuating rotor loads, which is a consequence of the wave-induced rotor dynamics. In the absence of a controller scheme for the wind turbine, the rotor torque fluctuates considerably, which induces a growing roll response especially when the wind turbine is operated nearly at the rated wind speed. This can be eliminated either by appropriately adjusting the controller so as to regulate the torque or by optimizing the floater or tendon dimensions, thereby limiting the roll motion. Loads and coupled responses are predicted for a set of load cases with different wave headings. Based on the results, critical load cases are identified and discussed. As a next step (which is not presented here), the dynamic model for the substructure is therefore being coupled to an advanced aero-elastic code Flex5, Øye (1996), which has a higher number of DOFs and a controller module.

scholarly journals The Effect of the Second-Order Wave Loads on Drift Motion of a Semi-Submersible Floating Offshore Wind Turbine

2020 ◽  
Vol 8 (11) ◽  
pp. 859
Author(s):  
Thanh-Dam Pham ◽  
Hyunkyoung Shin
Keyword(s):  
Wind Turbine ◽  
Wind Turbines ◽  
Second Order ◽  
Offshore Wind ◽  
Offshore Wind Turbine ◽  
Wave Loads ◽  
Hydrodynamic Loads ◽  
Drift Motion ◽  
Offshore Wind Turbines ◽  
Floating Offshore Wind Turbines

Floating offshore wind turbines (FOWTs) have been installed in Europe and Japan with relatively modern technology. The installation of floating wind farms in deep water is recommended because the wind speed is stronger and more stable. The design of the FOWT must ensure it is able to withstand complex environmental conditions including wind, wave, current, and performance of the wind turbine. It needs simulation tools with fully integrated hydrodynamic-servo-elastic modeling capabilities for the floating offshore wind turbines. Most of the numerical simulation approaches consider only first-order hydrodynamic loads; however, the second-order hydrodynamic loads have an effect on a floating platform which is moored by a catenary mooring system. At the difference-frequencies of the incident wave components, the drift motion of a FOWT system is able to have large oscillation around its natural frequency. This paper presents the effects of second-order wave loads to the drift motion of a semi-submersible type. This work also aimed to validate the hydrodynamic model of Ulsan University (UOU) in-house codes through numerical simulations and model tests. The NREL FAST code was used for the fully coupled simulation, and in-house codes of UOU generates hydrodynamic coefficients as the input for the FAST code. The model test was performed in the water tank of UOU.

Key Contributors to Lifetime Accumulated Fatigue Damage in an Offshore Wind Turbine Support Structure

2017 ◽  
Author(s):  
Emil Smilden ◽  
Erin E. Bachynski ◽  
Asgeir J. Sørensen
Keyword(s):  
Wind Turbine ◽  
Fatigue Damage ◽  
Control Strategies ◽  
Time Domain Analysis ◽  
Offshore Wind ◽  
Offshore Wind Turbine ◽  
Wave Loads ◽  
Support Structure ◽  
Wear And Tear ◽  
Collateral Effects

A simulation study is performed to identify the key contributors to lifetime accumulated fatigue damage in the support-structure of a 10 MW offshore wind turbine placed on a monopile foundation in 30 m water depth. The relative contributions to fatigue damage from wind loads, wave loads, and wind/wave misalignment are investigated through time-domain analysis combined with long-term variations in environmental conditions. Results show that wave loads are the dominating cause of fatigue damage in the support structure, and that environmental condtions associated with misalignment angle > 45° are insignificant with regard to the lifetime accumulated fatigue damage. Further, the results are used to investigate the potential of event-based use of control strategies developed to reduce fatigue loads through active load mitigation. Investigations show that a large reduction in lifetime accumulated fatigue damage is possible, enabling load mitigation only in certain situations, thus limiting collateral effects such as increased power fluctuations, and wear and tear of pitch actuators and drive-train components.

Experimental Study on the Wave Loads on Monopile and Jacket Type Support of Offshore Wind Turbines

2018 ◽  
Author(s):  
Jing Zhang ◽  
Qin Liu ◽  
Xing Hua Shi ◽  
C. Guedes Soares
Keyword(s):  
Experimental Study ◽  
Wind Turbine ◽  
Nonlinear Wave ◽  
Wave Force ◽  
Offshore Wind ◽  
Offshore Wind Turbine ◽  
Wave Forces ◽  
Wave Loads ◽  
Morison Equation ◽  
Offshore Wind Turbines

As the offshore fixed wind turbine developed, more ones will be installed in the sea field with the depth 15–50 meters. Wave force will be one of the main forces that dominate the design of the wind turbine base, which is calculated using the Morison equation traditionally. This method can predict the wave forces for the small cylinders if the drag and inertia coefficients are obtained accurately. This paper will give a series scaled tests of monopile and jacket type base of the offshore wind turbine in tank to study the nonlinear wave loads.

scholarly journals Study of Floating Wind Turbine with Modified Tension Leg Platform Placed in Regular Waves

Energies ◽  
2019 ◽  
Vol 12 (4) ◽  
pp. 703 ◽  
Author(s):  
Juhun Song ◽  
Hee-Chang Lim
Keyword(s):  
Wind Turbine ◽  
Real Data ◽  
Offshore Wind ◽  
Scale Model ◽  
Offshore Wind Turbine ◽  
Wave Loads ◽  
Tension Leg Platform ◽  
Mooring Lines ◽  
Floating Wind Turbine ◽  
Wave Conditions

In this study, the typical ocean environment was simulated with the aim to investigate the dynamic response under various environmental conditions of a Tension Leg Platform (TLP) type floating offshore wind turbine system. By applying Froude scaling, a scale model with a scale of 1:200 was designed and model experiments were carried out in a lab-scale wave flume that generated regular periodic waves by means of a piston-type wave generator while a wave absorber dissipated wave energy on the other side of the channel. The model was designed and manufactured based on the standard prototype of the National Renewable Energy Laboratory (NREL) 5 MW offshore wind turbine. In the first half of the study, the motion and structural responses for operational wave conditions of the North Sea near Scotland were considered to investigate the performance of a traditional TLP floating wind turbine compared with that of a newly designed TLP with added mooring lines. The new mooring lines were attached with the objective of increasing the horizontal stiffness of the system and thereby reducing the dominant motion of the TLP platform (i.e., the surge motion). The results of surge translational motions were obtained both in the frequency domain, using the response amplitude operator (RAO), and in the time domain, using the omega arithmetic method for the relative velocity. The results obtained show that our suggested concept improves the stability of the platform and reduces the overall motion of the system in all degrees-of-freedom. Moreover, the modified design was verified to enable operation in extreme wave conditions based on real data for a 100-year return period of the Northern Sea of California. The loads applied by the waves on the structure were also measured experimentally using modified Morison equation—the formula most frequently used to estimate wave-induced forces on offshore floating structures. The corresponding results obtained show that the wave loads applied on the new design TLP had less amplitude than the initial model and confirmed the significant contribution of the mooring lines in improving the performance of the system.

Validation and Development of Improved Methods for the Calculation of Wave Loads on XXL Monopiles

2018 ◽  
Author(s):  
Mareike Leimeister ◽  
Bastian Dose
Keyword(s):  
State Of The Art ◽  
Offshore Wind ◽  
Offshore Wind Turbine ◽  
Wave Loads ◽  
Wave Load ◽  
Wind Turbine System ◽  
Engineering Models ◽  
Dynamics Simulations ◽  
Diffraction Effects ◽  
Wave Induced

With the offshore wind industry aiming to deploy deeper water sites (> 30 m) while still utilizing monopiles, support structures with larger diameters are required. For the design and assessment of so-called XXL monopiles, wave-induced forces, which become more dominant with increasing diameter, have to be determined accurately. Thus, this study focuses on the identification of differences between state-of-the-art theories for wave load calculations with engineering models and the forces exerted on large monopiles from high-precision numerical reference methods. Within the framework of the research project TANDEM (Towards an Advanced Design of Large Monopiles) a 7 m diameter monopile is designed to support Fraunhofer’s IWT-7.5-164. This offshore wind turbine system is used as reference to determine wave-induced loads based on the MacCamy-Fuchs approach, implemented in models in Modelica. Different waves, defined in a simulation matrix, are investigated to elaborate the significance of diffraction effects, as well as the relevance of non-linear effects. The results are compared to CFD (Computational Fluid Dynamics) simulations. Deviations in the wave-induced forces are analyzed, taking into account the different capabilities of the applied tools, trends in the applicability of the engineering model are elaborated, and suggestions for improvement of the code based on state-of-the-art theories are given.

Numerical and Experimental Investigation of the Wave Loading on a Three-Legged Offshore Wind Turbine Jacket Platform

2018 ◽  
Author(s):  
Ioannis K. Chatjigeorgiou ◽  
Konstantinos Chatziioannou ◽  
Vanessa Katsardi ◽  
Apostolos Koukouselis ◽  
Euripidis Mistakidis
Keyword(s):  
Wave Structure ◽  
Element Model ◽  
Offshore Wind ◽  
Offshore Wind Turbine ◽  
Wave Forces ◽  
Intermediate Water ◽  
Wave Loads ◽  
Wave Loading ◽  
Scaled Model ◽  
The Time Domain

The purpose of this work is to examine a three-legged jacket tower support system subjected to wave loading. To this end, linear as well as nonlinear wave scenarios are investigated. The structure was designed for offshore wind turbines installed in intermediate water depths. The phenomenon of the wave-structure interaction is examined experimentally with a 1:18 scaled model as well as numerically with the use of Finite Element Model (FEM). The structural calculations were performed using the structural analysis software SAP2000, which was enhanced by a special programming interface that was developed to calculate the wave loading and to directly apply the wave loads on the structural members. The FEM model in combination with the key parameters that are taken into account, provides a good correlation with the experimental results. The wave theories of Airy and Stokes 5th are employed for the calculation of the wave particle kinematics. The resulting wave forces are examined both in the frequency and in the time domain.

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