Design Of Offshore Wind Turbines Research Articles

A macro-element model for multidirectional cyclic lateral loading of monopiles in clay

This paper investigates numerically the impact of multidirectional cyclic loads on the response of a monopile in clay supporting an offshore wind turbine (OWT). The results indicate that multidirectional loading modifies the foundation stiffness and hysteretic damping, which affects the OWT eigenfrequencies and damping. Based on these results, a macro-element model to represent the monopile response in the time-domain integrated load simulations employed in OWT design is proposed and verified against finite element analyses (FEA). The verification indicates the macro-element can simulate the monopile response with almost the same accuracy as FEA, but with a considerable reduction in computational effort.

A symbiotic approach to the design of offshore wind turbines with other energy harvesting systems

The capital cost of a 5 MW floating wind turbine (FWT) runs as high as $20.7 million, leading to an energy cost of $0.20/kWh, four times that of natural gas (Myhr et al., 2014). Although a single type of energy harvesting device may be too expensive to deploy, if it can operate symbiotically with others, the combined cost of energy might be acceptable. In this study, we show that attaching a wave energy converter (WEC) to the FWT may simultaneously produce an average of 240 kW wave power, reduce the WEC levelized cost of energy by 14% by eliminating redundant components, and reduce the FWT tower lifetime equivalent fatigue stress by 23% by reducing platform motion. Furthermore, the offshore wind turbine may also serve as a structure for the harvesting of valuable elements from seawater, such as uranium, lithium, and cobalt. The major cost drivers for the harvesting of uranium from seawater have been identified to be those associated with the mooring and deployment of the metal adsorbing polymers (Schneider and Sachde, 2013; Byers and Schneider, 2016). In the case of uranium, a symbiotic system coupled with an offshore wind turbine was found to reduce the seawater uranium production cost by at least 11% and up to 30% (Byers et al., 2016, 2018; Haji, 2017).

A simplified method for analyzing the fundamental frequency of monopile supported offshore wind turbine system design

Preliminary design of offshore wind turbines requires high precision simplified methods for the analysis of the system fundamental frequency. Based on the Rayleigh method and Lagrange’s Equation, this study establishes a simple formula for the analysis of system fundamental frequency in the preliminary design of an offshore wind turbine with a monopile foundation. This method takes into consideration the variation of cross-section geometry of the wind turbine tower along its length, with the inertia moment and distributed mass both changing with diameter. Also the rotational flexibility of the monopile foundation is mainly considered. The rigid pile and elastic middle long pile are calculated separately. The method is validated against both FEM analysis cases and field measurements, showing good agreement. The method is then used in a parametric study, showing that the tower length Lt, tower base diameter d0, tower wall thickness δt, pile diameter db and pile length Lb are the major factors influencing the fundamental frequency of the offshore wind turbine system. In the design of offshore wind turbine systems, these five parameters should be adjusted comprehensively. The seabed soil condition also needs to be carefully considered for soft clay and loose sand.

Computer Modeling of Wind Turbines: 2. Free-Surface FSI and Fatigue-Damage

This article reviews state-of-the-art numerical techniques for fluid–structure interaction (FSI) of full-scale wind-turbine systems. Simulation of floating wind turbines subjected to combined wind-flow and ocean-wave forcing, and modeling of high-cycle fatigue failure of blades due to long-term cyclic aerodynamic loading, are the focal points of this article. Computational techniques including advanced structural modeling based on Isogeometric Analysis, free-surface FSI, and fatigue-damage modeling, are presented. Representative computational examples involving land-based and floating offshore wind-turbine designs illustrate the versatility and power of the computational methods developed.

Empirical Correction Ratio and Scale Factor to Project the Extreme Wind Speed Profile for Offshore Wind Energy Exploitation

The typhoon-resistant design should be given priority when constructing offshore wind farms in China coastal waters, where typhoons pose serious threats on the safety of offshore structures. A classical power-law model with the specific exponent of ${\text{0.11}}$ has been recommended by the code (IEC 61400-3 2009) to estimate the profile of extreme wind speed versus ${\text{50}}$ years return period. Its use in the typhoon-resistant design of offshore wind turbines in China coastal waters has, however, not been validated. In the present study, a series of full-set three-dimensional meteorology simulations are conducted to provide the artificial typhoon wind fields for the evaluation of code recommendations. The simulations reveal that the classical power-law with the exponent of ${\text{0.11}}$ is inadequate to estimate the extreme wind speed profile over China coastal waters. Based on the simulated typhoon wind fields, an empirical correction ratio and a scale factor are developed to adjust the classical power-law model to estimate the vertical variation of extreme wind speeds with any desired return periods over China coastal waters. More specifically, the correct ratio revises the power-law model to provide more accurate estimates of 50 years return period extreme wind speed profiles while the scale factor projects extreme wind speeds with other desired return periods.

Wind, wave and earthquake responses of offshore wind turbine on monopile foundation in clay

This paper investigates the dynamic responses of offshore wind turbine (OWT) supported on monopile foundation in clay subjected to wind, wave and earthquake actions. Based on the open-source software platform OpenSees, a three-dimensional finite model of the system is developed. The tower and monopile is modeled using beam element, the pile-soil interface behavior using nonlinear Winkler foundation approach, and the pile-water interface using hydrodynamic added mass. The wind, wave and earthquake actions are applied as loadings on the system. The effects of several parameters, such as wind velocity, induction factor, wave period, peak ground acceleration, and soil parameters on the dynamic responses of the system are studied. The results indicate that it is necessary to consider the combination of wind, wave and earthquake actions in the design of offshore wind turbine.

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

During the design phase of an offshore wind turbine, it is required to assess the impact of loads on the turbine life time. Due to the varying environmental conditions, the effect of various uncertain parameters has to be studied to provide meaningful conclusions. Incorporating such uncertain parameters in this regard is often done by applying binning, where the probability density function under consideration is binned and in each bin random simulations are run to estimate the loads. A different methodology for quantifying uncertainties proposed in this work is polynomial interpolation, a more efficient technique that allows to more accurately predict the loads on the turbine for specific load cases. This efficiency is demonstrated by applying the technique to a power production test problem and to IEC Design Load Case 1.1, where the ultimate loads are determined using BLADED. The results show that the interpolating polynomial is capable of representing the load model. Our proposed surrogate modeling approach therefore has the potential to significantly speed up the design and analysis of offshore wind turbines by reducing the time required for load case assessment.

Main bearings in large offshore wind turbines: development trends, design and analysis requirements

This paper discusses analysis requirements for design and operation of main bearings in modern multi-megawatt offshore wind turbines, motivated by the industry’s search for reliable and cost effective main bearing solutions that limit the effects of non-torque loads, and the need for effective bearing health monitoring. Gearboxes historically received attention on the grounds of reliability, sparking significant interest in drivetrain dynamics. However, design trends in modern, large turbines, influencing choices for a future 10+ MW generation, indicate that more attention to main bearings or rotor support bearings is needed as part of a more holistic approach to flexural dynamics. Through a survey of existing research, offshore wind turbine design trends, design codes, industry practices and standards, we look at how main bearings are treated in a life cycle perspective, considered design features, modeling and simulation approaches, interaction and interfaces between industry stakeholders, as well as reuse of simulation models for predictive analytics in operation. We conclude that flexible main bearing representation is important in dynamic analyses and that industry practices are needed that enable sufficient model exchange or interfaces and thus effective exploitation of the benefits of a simulation model throughout the turbine life cycle.

A Comparative Study between Three-Legged and Tripod Sub-structures in Design of Offshore Wind Turbines in the Transition Water Depth

This research aimed at investigating tripod and three-legged offshore wind turbine substructures. A comparison between the two substructures based on their weight as well as the installation method of piles, i.e. pre-piling and post-piling, was carried out. The in-place (Ultimate Limit State), Dynamic, natural frequency check and fatigue (Fatigue Limit State) analyses were conducted considering aerodynamic and hydrodynamic loads imposed on substructures in 50m water depth. An optimisation process was carried out in order to reduce the mass of substructures. The results revealed that the three-legged substructure is more cost effective with 25% lesser structure mass. However, the construction of the three-legged structure usually takes more time due to increased number of members and subsequently welding joints. The results, furthermore, showed that the pre-piling method reduces the time and cost of offshore installation, and reduces the weight of piles by 50%.

Seismic considerations in design of offshore wind turbines

Interest in renewable and clean energy over the past decade has motivated immense research on wind energy. The main issues in design of offshore wind turbines in regions of recent development have been aero- and hydro-dynamic loads; however, earthquake is a design concern in seismic areas such as East Asia and Western United states. This paper reviews the state of practice in seismic design of offshore wind turbines. It is demonstrated that wind turbines are in particular vulnerable to vertical earthquake excitation due to their rather high natural frequencies in vertical direction; however, inclusion of the radiation damping could contribute considerably reduce the earthquake loads. Moreover, it is demonstrated how soil nonlinearity could lead to settlement and permanent tilting of offshore wind turbines on caisson foundations or tripods. Using these cases, the paper demonstrates that the design of offshore wind turbines for earthquake loading is driven by performance-based considerations.

An open-source comprehensive numerical model for dynamic response and loads analysis of floating offshore wind turbines

This paper presents the development of a comprehensive open-source numerical model to study the dynamic response and load analysis of floating offshore wind turbines. The model accounts for the wind inflow, rotor aerodynamics, multibody structural model of the system, wave and current kinematics, hydrodynamics, and mooring-line dynamics. This coupled simulation tool can be used for analysis, optimization and preliminary design to determine the technical and economic feasibility. Several verification and validation cases are performed to show the correctness of the numerical simulations. The results show that the proposed approach provides an accurate estimate of the wind turbine dynamics and loads. The simulation tool is then applied in the analysis of a 5 MW wind turbine aimed to characterize the dynamic response and to identify potential loads and instabilities resulting from the dynamic couplings between the turbine and the external conditions. This open-source fully coupled aero-hydro-elastic model provides a modular framework to enable investigating a variety of wind turbine configurations, support systems, and mooring lines. Therefore, it is expected that researchers and design engineers worldwide use the model to study, investigate and analyze different aspects of floating offshore wind turbine design, which results is the promotion and advancement of science and technology for floating offshore wind turbines.

NUMERICAL METHOD FOR CALCULATING SYSTEM FUNDAMENTAL FREQUENCIES OF OFFSHORE WIND TURBINES WITH MONOPILE FOUNDATIONS

The restriction of system frequencies due to the soft-stiff design criterion is one of the key points for structure and foundation design of offshore wind turbines. This study proposed a practical numerical method for analyzing system fundamental frequencies of offshore wind turbines with monopile foundations. The method was based on the kinematic equation of the damped system, taking into account the pile-soil interaction and the variation of cross-sectional geometry of the wind turbine tower. The method was validated against finite element solutions of an actual wind turbine. Parametric studies were conducted using the proposed method, and showed that the soil elastic modulus, the seabed height, and the pile diameter were major factors affecting the system's fundamental frequencies. However, the fundamental frequencies showed little sensitivity to soil damping and non-uniform distributing of the soil modulus. Furthermore, the method can be used to analyze the system frequency deviation during the operating period, allowing for an effective evaluation of the foundation design, and suggesting engineering measures.

3D FEM approach for laterally loaded monopile design

Over the past 5 years, a substantial research effort aimed at optimising the design of offshore wind turbines has led to significant reductions in the projected cost of developing offshore wind. Optimising the geotechnical design of these structures, through modern analysis techniques such as 3D Finite Element Modelling (FEM), has played a key role in helping to reduce costs. This paper presents a methodology for accurately modelling monopile behaviour using Cone Penetration Test (CPT) data to calibrate the non-linear stress dependent Hardening Soil (HS) model. The methodology is validated by comparing the modelled behaviour to field tests on a range of pile geometries. The paper also demonstrates how the soil-pile reaction response curves can be extracted from the FE model by isolating the stresses on each element of the pile. The contribution of each component to the overall lateral resistance is shown to vary with the pile geometry and is examined using the extracted soil reaction curves.

Evolution of the dynamic response and its effects on the serviceability of offshore wind turbines with stochastic loads and soil degradation

Novel methods combined with an integrated simulation platform are suggested for the design of offshore wind turbines (OWTs) and substructures that ensure a 20-year lifespan. These methods enable one to estimate the long-term evolution of the dynamic responses of OWTs due to the degradation of the soil modulus of the foundation under stochastic loading conditions. The results of this study show that random fluctuations of the soil stress caused by stochastic loads (i.e., aerodynamic and hydrodynamic loads acting on OWTs) can be described by a Rayleigh distribution and a Gaussian distribution. By using these probabilistic characteristics, the stochastic fluctuations in the soil stress can be rapidly calculated without using Monte Carlo simulations. Moreover, a new method based on the derivatives of the degradation functions and on the inverse of these functions is also suggested to calculate the mean degradation index. These methods significantly decrease the computational effort, thus overcoming a critical drawback of existing methods. Case studies demonstrate that the dimensions of the substructures significantly affect the evolution of the dynamic response. This suggests that the evolution of the dynamic response should be considered in the design process to secure the serviceability of OWTs and substructures.

Effect of scour on the structural response of an offshore wind turbine supported on tripod foundation

A simplified scour model for tripod foundation taking into account both local scour and global scour is proposed in this paper. The model is incorporated into a three-dimensional (3D) finite element model for analysis of a full-scale offshore wind turbine founded on a tripod structure using realistic structural properties. Applicability of the 3D finite element model is validated using full-scale load test data. Four different scour conditions under two wave situations are examined for the ultimate limit state (ULS), serviceability limit state (SLS) and fatigue limit state (FLS). The results show that scour has a minor effect on the natural frequency of the tripod-supported wind turbine but can significantly increase the maximum cross-sectional von Mises stress of piles under the ULS and increase the deflection of piles within nearly 20 m below the original seabed under the SLS. As for the fatigue life of the tripod structure, it can also be reduced by the effect of scour. These findings provide insights which are useful for development of safe and economic design of offshore wind turbines supported by tripod foundations.

Development of a comprehensive database of scattering environmental conditions and simulation constraints for offshore wind turbines

Abstract. For the design and optimisation of offshore wind turbines, the knowledge of realistic environmental conditions and utilisation of well-founded simulation constraints is very important, as both influence the structural behaviour and power output in numerical simulations. However, real high-quality data, especially for research purposes, are scarcely available. This is why, in this work, a comprehensive database of 13 environmental conditions at wind turbine locations in the North and Baltic Sea is derived using data of the FINO research platforms. For simulation constraints, like the simulation length and the time of initial simulation transients, well-founded recommendations in the literature are also rare. Nevertheless, it is known that the choice of simulation lengths and times of initial transients fundamentally affects the quality and computing time of simulations. For this reason, studies of convergence for both parameters are conducted to determine adequate values depending on the type of substructure, the wind speed, and the considered loading (fatigue or ultimate). As the main purpose of both the database and the simulation constraints is to compromise realistic data for probabilistic design approaches and to serve as a guidance for further studies in order to enable more realistic and accurate simulations, all results are freely available and easy to apply.

A Comparative Study on Recently-Introduced Nature-Based Global Optimization Methods in Complex Mechanical System Design

Advanced global optimization algorithms have been continuously introduced and improved to solve various complex design optimization problems for which the objective and constraint functions can only be evaluated through computation intensive numerical analyses or simulations with a large number of design variables. The often implicit, multimodal, and ill-shaped objective and constraint functions in high-dimensional and “black-box” forms demand the search to be carried out using low number of function evaluations with high search efficiency and good robustness. This work investigates the performance of six recently introduced, nature-inspired global optimization methods: Artificial Bee Colony (ABC), Firefly Algorithm (FFA), Cuckoo Search (CS), Bat Algorithm (BA), Flower Pollination Algorithm (FPA) and Grey Wolf Optimizer (GWO). These approaches are compared in terms of search efficiency and robustness in solving a set of representative benchmark problems in smooth-unimodal, non-smooth unimodal, smooth multimodal, and non-smooth multimodal function forms. In addition, four classic engineering optimization examples and a real-life complex mechanical system design optimization problem, floating offshore wind turbines design optimization, are used as additional test cases representing computationally-expensive black-box global optimization problems. Results from this comparative study show that the ability of these global optimization methods to obtain a good solution diminishes as the dimension of the problem, or number of design variables increases. Although none of these methods is universally capable, the study finds that GWO and ABC are more efficient on average than the other four in obtaining high quality solutions efficiently and consistently, solving 86% and 80% of the tested benchmark problems, respectively. The research contributes to future improvements of global optimization methods.

A review of slamming load application to offshore wind turbines from an integrated perspective

In harsh sea conditions, it is possible for offshore wind turbines (OWTs) to be exposed to slamming loads due to breaking waves, especially plunging breaking waves. These slamming loads lead to significant structural responses and can affect the ultimate limit state (ULS) design and the fatigue limit state (FLS) design of OWTs. However, detailed consideration of slamming loads is not a common practice in the design of primary structures in offshore wind industry. Studies on integrated dynamic analysis of OWTs with consideration of slamming loads are very limited. When applying slamming loads on OWTs, several aspects should be considered, such as the detection of breaking waves, the calculation of slamming loads, and the approaches to integrate the slamming loads in fully coupled analysis, etc. This paper provides an extensive review of key issues concerning these aspects, which can benefit the application of slamming loads on OWTs.

Dynamic analysis of a 5-MW tripod offshore wind turbine by considering fluid–structure interaction

Fixed offshore wind turbines usually have large underwater supporting structures. The fluid influences the dynamic characteristics of the structure system. The dynamic model of a 5-MW tripod offshore wind turbine considering the pile–soil system and fluid structure interaction (FSI) is established, and the structural modes in air and in water are obtained by use of ANSYS. By comparing low-order natural frequencies and mode shapes, the influence of sea water on the free vibration characteristics of offshore wind turbine is analyzed. On basis of the above work, seismic responses under excitation by El-Centro waves are calculated by the time-history analysis method. The results reveal that the dynamic responses such as the lateral displacement of the foundation and the section bending moment of the tubular piles increase substantially under the influence of the added-mass and hydrodynamic pressure of sea water. The method and conclusions presented in this paper can provide a theoretical reference for structure design and analysis of offshore wind turbines fixed in deep seawater.

Impact of Climate Change on Design of Offshore Wind Turbine Considering Dynamic Soil–Structure Interaction

This study assesses the serviceability and fatigue limit states of the offshore wind turbine (OWT) founded in clay incorporating the impact of climate change. Two different offshore locations at east and west coasts in India are chosen. The ensemble of future time series of wind speed, wave height, and period is forecasted using statistical downscaling model (SDSM) at the regional level using the general circulation model (GCM) corresponding to the A1B, A2, and B1 emission scenarios. The downscaling model is calibrated by comparing simulations driven by the National Centers for Environmental Prediction (NCEP) high-resolution data and station data. Responses of OWT are obtained from dynamic analysis in a time domain using finite element (FE). The tower and monopile are modeled as Euler–Bernoulli beam, and soil resistance is modeled as American Petroleum Institute (API)-based p–y springs. The study shows future wind and wave loads are site specific, and it increases in the west coast and decreases in the east coast of India due to climate change. The simulation shows a substantial increase in future wind energy production at west coast compared to that of the east coast; however, safety margin considering serviceability and fatigue life decreases which requires modification in the design.