Efficient ultimate load estimation for offshore wind turbines using interpolating surrogate models

Structural Design and Analysis of Offshore Wind Turbines from a System Point of View

Chapter17 - Seismic design and analysis of offshore wind turbines

Renewable energy provides a solution for complex current and future social and environmental problems whereas offshore industry has a large potential for providing renewable energy for future. Currently, offshore technology making use of wind for energy generation purpose becomes a hot spot with highly advanced research and development going on on one side and complex and critical problems present and difficult to solve on the other. This paper is trying to study problems related to the quantification of the hydrodynamic and aerodynamic loads for the design of offshore wind turbine support structures in the offshore wind farm. Both the hydrodynamic and aerodynamic conditions in the offshore site are extremely complex resulting in the difficulty of reasonable determination for the external loads on the wind turbine support structures. However, due to the increasing global demands for future energy solution, the design, analysis and optimization of offshore wind turbine is nevertheless an important issue. The paper first gives an introduction of the offshore wind farm and the complexity of the offshore environment. Wave load is explored with introduction of existing wave load models, comparison of their characteristics while the focus is placed on the nonlinear wave load by means of the Stokes higher order wave theory. Properties of a single regular wave based on methods of linear wave theory and Stokes higher order wave theory are compared which lead to differences in the results of wave load models when these two different methods are used. Wind load model is introduced briefly, followed by the introduction of current methods for determination or approximation of combined wave and wind load and also recommendations for practice. Park effect of the wind load and wave load is also introduced at limited depth in the latter stage as a direction for future research. Conclusion and recommendations based on all the above are therefore given at the last section of the paper.

This study has analysed the ultimate loads acting on a wind turbine which is operating in a high turbulent flow condition because the ultimate loads are critical factors on the safe design of wind turbine. Since wind flow on the most parts of Korean mountainous are strongly influenced by complex configurations of the topography, turbulence intensity on somewhere is so stronger than an international design standard. For this reason, the characteristics of turbulent wind data collected from actual sites were analyzed and used for the ultimate load evaluation of the wind turbine. With the 270 design load cases on the international standards, the differences of ultimate loads on the wind turbine operating in the standard or high turbulent wind condition are calculated and compared for the an enhanced knowledge of the safe design basis. As are result, it is revealed the specific ultimate loads are strongly affected by the high turbulent wind conditions, thus the characteristics of turbulent flow must be considered during the design of wind turbine.

Impact of climate change on the design of multi-megawatt spar floating wind turbines

The accuracy of predicted loads on offshore wind turbines depends on the mathematical models employed to describe the combined action of the wind and waves. Using a global simulation framework that employs a domain-decomposition strategy for computational efficiency, this study investigates the effects of nonlinear waves on computed loads on the support structure (monopile) and the rotor-nacelle assembly of a bottom-supported offshore wind turbine. The fully nonlinear (FNL) numerical wave solver is invoked only on sub-domains where nonlinearities are detected; thus, only locally in space and time, a linear solution (and associated Morison hydrodynamics) is replaced by the FNL one. An efficient carefully tuned linear-nonlinear transition scheme makes it possible to run long simulations such that effects from weakly nonlinear up to fully nonlinear events, such as imminent breaking waves, can be accounted for. The unsteady nonlinear free-surface problem governing the propagation of gravity waves is formulated using potential theory and a higher-order boundary element method (HOBEM) is used to discretize Laplace’s equation. The FNL solver is employed and associated hydrodynamic loads are simulated in conjunction with aerodynamic loads on the rotor of a 5-MW wind turbine using the NREL open-source software, FAST. We assess load statistics associated with a single severe sea state. Such load statistics are needed in evaluating relevant load cases specified in offshore wind turbine design guidelines; in this context, the influence of nonlinear wave modeling and its selection over alternative linear or linearized wave modeling is compared. Ultimately, a study such as this one will seek to evaluate long-term loads using the FNL solver in computations directed towards reliability-based design of offshore wind turbines where a range of sea states will need to be evaluated.

Abstract. A wind turbine blade equipped with root spoilers is analysed using time domain aeroelastic blade element momentum (BEM) simulations to assess the impact of passive devices on the turbine annual energy production (AEP) and lifetime. Previous 2D computational fluid dynamics (CFD) showed a large unsteadiness in aerodynamic coefficients associated with the spoiler, and such behaviour is captured by the OpenFAST simulations when all degrees of freedom are switched off. Once the turbine is fully flexible, a novel way to account for aerofoil-generated unsteadiness in the fatigue calculation is proposed and detailed. The outcome shows that spoilers, on average, can increase the AEP of the turbine. However, the structural impacts on the turbine can be severe if not accounted for initially in the turbine design.

The design of offshore wind turbines is one of the most fascinating challenges in renewable energy. Meeting the objective of increasing power production with reduced installation and maintenance costs requires a multi-disciplinary approach, bringing together expertise in different fields of engineering. The purpose of this theme issue is to offer a broad perspective on some crucial aspects of offshore wind turbines design, discussing the state of the art and presenting recent theoretical and experimental studies.

Fatigue life analysis of monopile-supported offshore wind turbines based on hyperplastic ratcheting model

For the design of offshore wind turbines, the airgap height of a service platform above still water level (SWL) is dependent on the maximum level of wave run-up in seas. A number of labora...

The structural design of offshore wind turbines is based on the consideration of coupled dynamic phenomena. Wave loads cause the dynamic oscillation of the monopile, and the dynamic oscillation of the monopile affects the wave loads. The boundary conditions of the gravity-based foundation-monopile-turbine system are mostly affected by the flexural stiffness of the foundation plate, the elastic and creep behavior of the soil, and the inertia (translational and rotational) of the wind turbine mass. The design of the foundation should consider the dynamic response of the soil and the monopile, and the dynamic response of the soil and the monopile is affected by the design parameters of the foundation. The initial conditions of the system yield transient dynamic phenomena. A braking wave at t = 0 causes different dynamic response than the steady-state conditions due to a harmonic wave load. In the present work, an integrated analytical model simulating the above dynamic phenomena is proposed. With the aid of double integral transforms and generalized function properties, a solution of the corresponding differential equations for the monopile-soil-foundation system and the boundary and initial conditions is derived. A parametric study is carried out, and results of the effect of the design parameters and soil properties are presented and discussed.

Extreme load extrapolation and long-term fatigue assessment of offshore wind turbine tower based on monitoring data

Multi-objective parameter optimization of large-scale offshore wind Turbine's tower based on data-driven model with deep learning and machine learning methods

Evaluation of dynamic responses under extreme environmental conditions is important for the structural design of offshore wind turbines. Previously, a modified environmental contour method has been proposed to estimate extreme responses. In the method, the joint distribution of environmental variables near the cut-out wind speed is used to derive the critical environmental conditions for a specified return period, and the turbulence intensity (TI) of wind is assumed to be a deterministic value. To address more realistic wind conditions, this paper considers the turbulence intensity as a stochastic variable and investigates the impact on the modified environmental contour. Aerodynamic simulations are run over a range of mean wind speeds at the hub height from 9–25 m/s and turbulence levels between 9%–15%. Dynamic responses of a monopile offshore wind turbine under extreme conditions were studied, and the importance of considering the uncertainties associated with wind turbulence is highlighted. A case of evaluating the extreme response for 50-year environmental contour is given as an example of including TI as an extra variant in environmental contour method. The result is compared with traditional method in which TI is set as a constant of 15%. It shows that taking TI into consideration based on probabilistic method produces a lower extreme response prediction.

Design and construction of offshore wind turbines (OWT) in near-fault sites is more in demand worldwide. However, very limited studies are available that assess the effects of near-field ground motions on the response of OWTs. This paper describes a numerical study on the response of OWTs to pulse-like near-field earthquakes in liquefiable soils by performing a series of advanced fully coupled nonlinear dynamic analyses using FLAC-3D. The simple anisotropic sand (SANISAND) constitutive model was adopted for soil to consider liquefaction. Two types of foundations commonly used for OWTs, namely the monopile and suction bucket were modeled. One of the characteristics of near-field ground motions is containing a significant vertical component, which is mostly in the range of high frequencies. Meanwhile, OWTs can be sensitive to vertical ground motion in terms of having a high natural frequency in the vertical direction. Therefore, focusing on the frequency content of input seismic motion, a comparison was made between the response of OWT structures to horizontal and vertical components of earthquakes in the far-field and near-field conditions. The mechanisms behind the obtained results were elaborated, and some suggestions were made for the design of OWTs under near-field conditions. According to the results, liquefaction and tilt are the major potential risks associated with the near-field earthquakes in the horizontal direction. However, in the vertical direction, the excessive amplification of acceleration and upthrow of the rotor-nacelle-assembly (RNA), are the major issues to be considered. Under near-field conditions, a suction bucket foundation can be a better option in terms of lower amplification factor of vertical acceleration, although its aspect ratio should be selected with special care to the rotation response.