1,720,952 research outputs found
A Review and Comparison of Floating Offshore Wind Turbine Model Experiments
Get PDFAbstractFloating offshore wind turbines provide more access to deeper water than conventional fixed-bottom wind turbines, which expands the viable area for wind energy development, reduces visibility from shore, and can potentially be located in areas with a higher and steadier wind characteristic. However, since floating turbines are in the early prototype stage of development, there are very limited data to use for validating computer models of these machines. This lack of validation increases uncertainty and risk for future installations. In lieu of large scale test turbines, which are expensive to build and operate, a few institutions have conducted small scale experiments in wave basins. This paper will present a review of the past and planned model-scale floating offshore wind turbine experiments, with a focus on types of data collected and challenges encountered by these tests.The objective of this review is to provide a background for the Integrated Research Program on Wind Energy (IRPWind), specifically for Work Packages 6.1 and 6.2. The goal of these work packages is to create a database of both fixed-bottom and floating offshore wind turbine test cases that can be accessed by researchers to verify and validate computer-aided engineering codes. The database will consist of a number of benchmarks that will validate different parts of a given design code. This review will discuss two model experiments that are likely to be included in the IRPWind database
Safety Indicators for the Marine Operations in the Installation and Operating Phase of an Offshore Wind Farm
Get PDFAbstractAs a measure of performance, safety indicators are already used for many types of operations, such as in the offshore oil and gas industry. The indicators are used by operators to enhance the safety and performance of the individual plants or vessels and total productivity of the system.This paper reviews existing safety analyses of the offshore wind industry, the onshore wind industry and offshore oil and gas indus- tries. An offshore wind farm is divided into subsystems and operational phases. Safety indicators are developed for the phases and subsystems by reviewing existing safety indicators from related industries and adapting them to the offshore wind industry. The indicators for the individual subsystems and phases are then combined to provide safety indicators for the whole wind farm over the lifetime. Finally, the indicators are matched against incident data from the offshore wind industry and an outlook for further research and indicator validation is given
Rational Upscaling and Modelling of a Semi-Submersible Floating Offshore Wind Turbine
No full textFloating offshore wind turbines are taking on more and more prominence, as the industry moves towards larger turbines, farther offshore, in deeper water. The increase in turbine size can reduce the costs of offshore wind energy, but requires larger support structures. Rather than redesigning the structure completely, a rational methodology for upscaling an existing floating substructure can improve the efficiency of the design process. This work presents a guideline for the optimization and upscaling of a semi-submersible oating platform, addressing also special challenges related to changes in turbine technology, as well as design criteria for oating platforms. The OC4 semi-submersible platform is used as starting point of this study. Based on analysis results of an initial elementary upscaling procedure, the main criteria, when dealing with a semi-submersible oating platform, are specified. Optimization is then carried out, focusing on stability and eigenfrequencies. Reducing the upper column diameter and changing the ballast position within the columns, yields longer natural periods in heave and pitch, a lighter and cheaper platform, and a less over-conservative, but still stable and safe system. Based on this optimized design, a guideline for an upscaling procedure for any other turbine is given. The main scaling factor is determined from the mass ratio of the top structures, rather than the turbine rating. The main column is scaled, so that it fits the new tower base diameter. The scaling factor for the upper columns is computed, based on the ratio of the overturning moments, and considers the contribution of the different columns to the stiffness component in pitch. The mooring line length is scaled, such that it can yield a predefined stiffness. Finally, the controller gains are recalculated, based on the expected natural frequency in pitch. The optimization and upscaling process is carried out for Fraunhofer's offshore wind turbine IWT-7.5-164 and the DTU 10 MW reference wind turbine. The oating wind turbines are analyzed, modelled and simulated by means of simplified spreadsheet methods, linear frequencydomain calculations (in DNV's software HydroD), and detailed equation-based models (in Fraunhofer's software Modelica). The systems are evaluated regarding their eigenfrequencies, nominal pitch, stability and global performance in selected sea states, taking variable buoyancy and center of buoyancy into account, and adjusting the blade-pitch controller gains. The results, obtained from both computer programs and the initial hand calculations, are comparable and satisfying. The structural integrity is proved by a simplified approach, using tank pressure and sea pressure for computing the equivalent stress. More detailed strength checks for fatigue and ultimate limit states, as well as optimization of the mooring system, are left for future work.Mechanical, Maritime and Materials EngineeringOffshore and Dredging EngineeringEuropean Wind Energy Master - EWE
A comparison of two fully coupled codes for integrated dynamic analysis of floating vertical axis wind turbines
Get PDFThis paper presents a comparison of two state-of-the-art codes that are capable of modelling floating vertical axis wind turbines (VAWTs) in fully coupled time-domain simulations, being the HAWC2 by DTU and the SIMO-RIFLEX-AC code by NTNU/MARINTEK. The comparative study focusses on the way aerodynamics, hydrodynamics and structural dynamics are treated for DeepWind’s 5MW Darrieus rotor mounted on a modified OC3 spar platform. The relevant modelling differences are described, followed by an introduction to the spar VAWT concept and selected load cases. Isolation of the aerodynamic model is achieved using an equivalent rigid land-based VAWT in steady wind-only environments. The added complexity in SIMO-RIFLEX-AC’s aerodynamic model has shown to increase aerodynamic torque at tip-speed ratios above 2.5. Differences in the hydrodynamic and structural models were brought forward through fully coupled analyses in turbulent wind and irregular wave climates. It is found that the simplified mooring system in HAWC2 introduces a 2P yaw response (1P in SIMO-RIFLEX-AC), stronger motion coupling in surge-heave and a largely reduced mooring line tension since the dynamics of mooring lines are not considered. Indications are given that a higher tower mode is excited by 4P aerodynamic loading; an effect that is significantly stronger in HAWC2.Wind Energ
Evaluation of the Fatigue Resistance of Offshore Jacket Joints by Numerical Approaches
No full textJacket support structures are a preferred solution for offshore wind turbines in deeper waters. Extensive knowledge exists in relation to its construction technique as well as its crucial components, but due to considerable cost pressure, continued optimization is essential for the future competitiveness of the concept in the offshore wind business. Their joints along with their complex welds are of special interest in terms of cost reduction. The design of tubular joints is generally driven by fatigue resistance. Due to the size, complexity and cost of these joints, this is assessed by using detailed FE models.Several aspects that have an impact on the results of the models are found to require further research and are investigated within this project: (1) influence of using solid versus shell elements in the modelling of the joint members; (2) degree of accuracy of the Efthymiou equations; (3) influence of the carry-over effect in multiplanar KK-joints; (4) differences in the fatigue life predictions between the hot-spot and the effective notch stress methods.Guidelines recommend the use of both solid and shell theories for the definition of the FE models used in the hot-spot fatigue assessment. Both options are compared in terms of accuracy of the results and computational time. Generally speaking, significant differences are found between both models. The background of the differences is studied.The employment of the Efthymiou formulae is common in practice. These equations provide the SCF at the locations around the weld where they are found to be maximum. The output of this approach is compared with the results of numerical models. The validity of its use is quantified.Offshore wind jacket joints are mainly multiplanar KK-joints. Loading in the braces of one face of the jacket may yield significant carry-over effects on the out-of-plane braces connected to the same joint. A parametric comparison is carried out to study the accuracy of modelling the joint as a planar K-joint. In general, it is found that the carry-over effect cannot be neglected and the assumption is not accurate.The fatigue assessment of tubular joints by means of the hot-spot method is subjected to several assumptions that limit the optimization of the members. The notch concept is a more realistic method that is presented as an alternative. However, this method is not widely used in engineering practice due to the difficulties in building the numerical model and the high computational requisites. An algorithm to carry out the effective notch stress assessment, based on the sub-modelling technique, is proposed. A comparison of the fatigue life prediction, between the hot-spot and the effective notch methods, is presented. The latter method is found to predict a higher fatigue life for many of the situations tested. Furthermore, since this method allows for a more detailed modelling, the weld profile can also be designed in order to optimize the fatigue resistance. The weld slope is found to have a significant impact on the results.European Wind Energy Masters (EWEM
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Resolution of tower shadow models for downwind mounted rotors and its effects on the blade fatigue
Get PDFA simulation study on the wind field resolution in computer load simulations has been conducted, both in transversal/vertical and longitudinal direction, to determine the effect on blade fatigue loading. Increasing the transversal/vertical resolution decreased the loading significantly, while only small changes to the load, at very low frequencies were found for increased longitudinal resolution. Next the influence of the tower shadow for a downwind mounted rotor was investigated, with respect to blade fatigue loading. The influence of different components to the total tower shadow effect was studied, both for a monopile and a truss tower, latter at inclination 0 and 22.5 degrees with respect to the incoming wind direction. Four components were considered, both individually and in combinations: mean wind speed, mean velocity deficit, unsteady motions from vortex shedding, and turbulence. The mean velocity deficit and turbulence were the main contributors to blade fatigue loading, and the unsteady motions can be neglected for the truss tower. For the monopile, neglecting the unsteady motions resulted in an underestimation of fatigue loading in the order of 3 percent.publishedVersionContent from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd
Using a Langevin model for the simulation of environmental conditions in an offshore wind farm
Get PDFFor the planning of operations and maintenance in offshore wind farms, many simulation models exist. Many rely on artificially generated weather time series to test different strategies. In this paper, we present a novel approach to modeling both the significant wave height and wind speed based on measurements from the site. We use a stochastic process called the Langevin process. First, equations are fitted to the available data, which are then used to generate the artificial weather data. The properties of these artificial weather time series are very close to the properties of the actual weather. Mean and standard deviation as well as the overall distribution and seasonality can be captured by the new model. Additionally, the persistence of waves and winds is replicated. This is especially important, as the length of weather windows is an important factor in operation and maintenance planning.publishedVersionContent from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd
Fatigue reassessment for lifetime extension of offshore wind monopile substructures
Get PDFFatigue reassessment is required to decide about lifetime extension of aging offshore wind farms. This paper presents a methodology to identify important parameters to monitor during the operational phase of offshore wind turbines. An elementary effects method is applied to analyze the global sensitivity of residual fatigue lifetimes to environmental, structural and operational parameters. Therefore, renewed lifetime simulations are performed for a case study which consists of a 5 MW turbine with monopile substructure in 20 m water depth. Results show that corrosion, turbine availability, and turbulence intensity are the most influential parameters. This can vary strongly for other settings (water depth, turbine size, etc.) making case-specific assessments necessary.Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd
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