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Effects of ballast transfer on modeling and dynamic responses of a 15MW semi-submersible floating wind turbine
Abstract
The total ballast accounts for more than 60% of displacement for large-megawatt floating wind turbines (FWT), including fixed and adjustable ballast. Properly using and controlling the variable ballast is attractive to optimize the dynamic performance but is not well studied yet. This study aims to demonstrate and quantify the effects of ballast transfer on the dynamic behavior and performance of FWTs. A 15 MW semi-submersible FWT equipped with an appropriate ballast transfer system is modeled and numerically analyzed. A hydrostatic model is first established to calculate inertial and hydrostatic properties of the FWT during the ballast transfer. Fully coupled time-domain simulations with and without ballast transfer are then carried out to predict the wind turbine performance, platform motion, and structural responses under various environmental conditions. The results indicate that ballast transfer can change the hydrostatic and hydrodynamic properties of the FWT, with an increase of approximately 3.5% in pitch hydrostatic stiffness. In addition, the ballast transfer can increase the average power generation, reduce platform pitch and roll motions, and significantly decrease the hub thrust and the tower base bending moment. The insights obtained serve as a foundation for the development of active ballast systems and the optimization of FWT operation.
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The fore-aft motion of the rotor-nacelle assembly (RNA) of a rotating floating wind turbine (FWT) can cause an oscillation in aerodynamic thrust, which may be equivalently treated as frequency-dependent aerodynamic mass and damping effects. In this study, an explicit frequency-domain analytical model is proposed to calculate the equivalent aerodynamic mass and damping of FWTs, with proper linearization of control system. Assuming that an FWT operates under steady wind conditions and a forced oscillation is exerted at the RNA along the wind direction, the thrust fluctuations are equivalently represented by the force and moment acting on the nacelle instead of pure aerodynamic loads. Based on the thrust oscillation expression, equivalent aerodynamic mass and damping are derived analytically. After verifying the model by numerical comparison, it is used to demonstrate equivalent aerodynamic mass and damping of three wind turbines (5-15 MW). Effects of wind turbine up-scaling and controller dynamics are addressed. Results show that equivalent aerodynamic mass and damping present a nonlinear characteristic with oscillation frequency in the below-rated region, while the relationship is close to linear for higher wind speeds. The effect of wind turbine up-scaling has a visible impact on equivalent aerodynamic mass and damping, especially at near-rated wind speed. Controller gains affect equivalent aerodynamic mass and damping and should be tuned reasonably in the controller design for FWTs. Outcomes of our study can be used to establish a frequency-domain coupled model of FWTs and are beneficial for conceptual design and parameter optimization of the platform of FWTs. K E Y W O R D S floating wind turbine, aerodynamic mass and damping, analytical model
A R T I C L E I N F O Keywords: Global structural load effects Semi-submersible hull structure Time domain hydrodynamic analysis Load transfer Still water Wind, and wave loads Floating wind turbine A B S T R A C T In designing the support structures of floating wind turbines (FWTs), a key challenge is to determine the load effects (at the cross-sectional load and stress level). This is because FWTs are subjected to complex global, local, static, and dynamic loads in stochastic environmental conditions. Up to now, most of the studies of FWTs have focused on the dynamic motion characteristics of FWTs, while minimal research has touched upon the internal load effects of the support structure. However, a good understanding of the structural load effects is essential since it is the basis for achieving a good design. Motivated by the situation, this study deals with the global load effect analysis for FWT support structures. A semi-submersible hull of a 10-MW FWT is used in the case study. A novel analysis method is employed to obtain the time-domain internal load effects of the floater, which account for the static and dynamic global loads under the still water, wind, and wave loads and associated motions. The investigation of the internal stresses resulting from various global loads under operational and parked conditions and the dynamic behavior of the structural load effects in various environmental conditions are made. The dominating load components for structural responses of the semi-submersible floater and the significant dynamic characteristics under different wind and wave conditions are identified. The dynamic load effects of the floating support structure are investigated by considering the influence of the second-order wave loads, viscous drag loads induced global motions, and wind and wave misalignments. The main results are discussed, and the main findings are summarized. The insights gained provide a basis for improving the design and analysis of FWT support structures.
The EU Horizon 2020 project COREWIND (COst REduction and increase performance of floating WIND technology) has developed two floating platforms for the new International Energy Agency (IEA) Wind 15 MW reference wind turbine. One design – “WindCrete” – is a spar floater, and the other – “Activefloat” – is a semi-submersible floater; both designs are made of concrete. In this work the design of the floaters is introduced with their aero–hydro–servo-elastic numerical models, and the responses of both floaters in both static and dynamic simulations are investigated. The static displacements and natural frequencies are simulated and discussed. Additionally, the effects of the mean wave drift forces and second-order difference-frequency wave forces on the systems' responses are presented. The increase in the turbine's power capacity to 15 MW in IEA Wind model leads to an increase in inertial forces and aerodynamic thrust force when compared to similar floating platforms coupled to the Technical University of Denmark (DTU) 10 MW reference model. The goal of this work is to investigate the floaters' responses for different load cases. The results in this paper suggest that at mild wave loads the motion responses of the 15 MW floating offshore wind turbines (FOWTs) are dominated by low-frequency forces. Therefore, motions are dominated by the wind forces and second-order wave forces rather than the first-order wave forces. After
assessing and understanding the models' responses, the two 15 MW FOWT numerical reference models are publicly available to be used in the research and development of floating wind energy.
It is believed that offshore wind power will play an important role in fulfilling the environmental commitments made in the Paris Agreement signed in 2015. The use of floating solutions in offshore wind power is gaining prominence, since it allows the expansion of these technologies to a large number of regions in which this has not been technically feasible until now. One of the current issues that needs to be studied in detail as regards offshore wind structures are the oscillations produced by the waves and the wind that could cause important problems in the turbine such as the reduction of the efficiency and performance or the appearance of structural damages owing to undesired loads on the blades, among others. This signifies that the inclination of the structure must not exceed a certain small value in the operating phase. This paper, therefore, presents a simple dynamic modeling and a nonlinear multivariable control model-based system for a Windfloat turbine whose main function is to stabilize the system and to keep it vertical in order to improve its operation and, therefore, maintain the offshore wind turbine in the optimal conditions required to harness energy from the wind. The effectiveness of both the dynamic model and the proposed control algorithm has been demonstrated by developing various simulations in a MATLAB-Orcaflex environment, in which the Windfloat structure proved to have very small vertical deviations when wind impinges over the rotor, despite the undesirable effects caused by parametric uncertainties (e.g., the effect of the anchoring cables) in the system and unmodeled unmatched perturbations (such as the effects of the wind and the waves on the Windfloat turbine).
This study presents two numerical multiphysics models of the NAUTILUS-10 floating support structure mounting the DTU10 MW Reference Wind Turbine at Gulf of Maine site, and analyses its dynamics. With the site conditions and the FAST model of the onshore turbine as the starting point, the floating support structure: tower, floating substructure with its corresponding active ballast system and station keeping system, was designed by NAUTILUS. The numerical models were developed and the onshore DTU wind energy controller was tuned to avoid the resonance of the operating FOWT by TECNALIA, in the framework of H2020 LIFES50+ project. This concept and its subsystems are fully characterised throughout this paper and implemented in opensource code, FAST v8.16. Here, the mooring dynamics are solved using MoorDyn, and the hydrodynamic properties are computed using HydroDyn. Viscous effects, not captured by radiation-diffraction theory, are modelled using two different approaches: (1) through linear and quadratic additional hydrodynamic damping matrices and (2) by means of Morison elements. A set of simulations (such as, decay, wind only and broadband irregular waves tests) were carried out with system identification purposes and to analyse the differences between the two models presented. Then, a set of simulations in stochastic wind and waves were carried out to characterise the global response of the FOWT.
Wind energy research groups from various disciplines generally use self-developed baseline wind turbine control implementations and tunings, which complicates the evaluation and comparison of new control algorithms. To solve this problem, the Delft Research Controller (DRC) provides an open, modular and fully adaptable baseline wind turbine controller to the scientific community. New control implementations can be added to the existing baseline controller, and in this way, convenient assessments of the proposed algorithms is possible. Because of the open character and modular set-up, scientists are able to collaborate and contribute in making continuous improvements to the code. The DRC is being developed in Fortran and uses the Bladed-style DISCON controller interface. The compiled controller is configured by a single control settings parameter file, and can work with any wind turbine model and simulation software using the DISCON interface. Baseline parameter files are supplied for the NREL 5-MW and DTU 10-MW reference wind turbines.
This study focusses on the coupling effects arising from the changes in the hydrodynamic behaviour of a semi-submersible floating wind turbine when it undergoes large inclinations under wind loading. By means of a range of time-domain simulations, it is shown that both the hull geometric nonlinearity effect and the alteration of viscous hydrodynamic forces can significantly affect the dynamics of a typical floating wind turbine operating in waves at rated conditions. The consequences of said effects for both aligned and misaligned wind and waves are explored. In general terms inclinations are found to increase motions, where the modes that are more affected depend on the relative direction between incident wind and waves. Understanding the sources of aero-hydrodynamic coupling is key to providing sound design and modelling guidelines for the coming generation of floating wind turbines.
A current trend in offshore wind is the quest for exploitation of ever deeper water sites. At depths between 50 m and 100 m a promising substructure is the column-stabilised semi-submersible floating type. This solution is currently being tested at full scale at the WindFloat and Fukushima Forward demonstrator sites in Portugal and Japan respectively. The semi-sub design class frequently adopts passive motion control devices based on the water entrapment principle, such as heave plates, tanks, and skirts. Whilst effective for small inclinations, these can underperform when the structure is inclined under wind loading. This study examines the alteration of potential hydrodynamics due to wind-induced trim (geometric non-linearity) and its impact on the wind turbine׳s wave response with focus on heave plate performance. Firstly it is shown by using the boundary element approach that wind trim affects wave loading in the ocean wave band between 5 s and 15 s, and introduces hydrodynamic coupling typical of non-symmetric hulls. These features are incorporated in frequency-domain dynamic response analysis to demonstrate that said effects bear a significant impact on the turbine׳s motion in waves. Accounting of heave plate excursion improves the assessment of the seaworthiness of floating wind turbine concepts, potentially leading to new design constraints.
The goal of this study is to quantify the world's wind power potential for the first time from data. Wind speeds are calculated at 80 m, the hub height of modern, 77-m diameter, 1500 kW turbines. Since relatively few observations are available at 80 m, the Least Square extrapolation technique is utilized and revised here to obtain estimates of wind speeds at 80 m given observed wind speeds at 10 m (widely available) and a network of sounding stations. Tower data from the Kennedy Space Center (Florida) were used to validate the results. Globally, ∼13% of all reporting stations experience annual mean wind speeds ≥ 6.9 m/s at 80 m (i.e., wind power class 3 or greater) and can therefore be considered suitable for low-cost wind power generation. This estimate is believed to be conservative. Of all continents, North America has the largest number of stations in class ≥ 3 (453), and Antarctica has the largest percent (60%). Areas with great potential are found in northern Europe along the North Sea, the southern tip of the South American continent, the island of Tasmania in Australia, the Great Lakes region, and the northeastern and northwestern coasts of North America. The global average 10-m wind speed over the ocean from measurements is 6.64 m/s (class 6); that over land is 3.28 m/s (class 1). The calculated 80-m values are 8.60 m/s (class 6) and 4.54 m/s (class 1) over ocean and land, respectively. Over land, daytime 80-m wind speed averages obtained from soundings (4.96 m/s) are slightly larger than nighttime ones (4.85 m/s); nighttime wind speeds increase, on average, above daytime speeds above 120 m. Assuming that statistics generated from all stations analyzed here are representative of the global distribution of winds, global wind power generated at locations with mean annual wind speeds ≥ 6.9 m/s at 80 m is found to be ∼72 TW (∼54,000 Mtoe) for the year 2000. Even if only ∼20% of this power could be captured, it could satisfy 100% of the world's energy demand for all purposes (6995–10177 Mtoe) and over seven times the world's electricity needs (1.6–1.8 TW). Several practical barriers need to be overcome to fully realize this potential.
AeroDyn is a set of routines used in conjunction with an aeroelastic simulation code to predict the aerodynamics of horizontal axis wind turbines. These subroutines provide several different models whose theoretical bases are described in this manual. AeroDyn contains two models for calculating the effect of wind turbine wakes: the blade element momentum theory and the generalized dynamic-wake theory. Blade element momentum theory is the classical standard used by many wind turbine designers and generalized dynamic wake theory is a more recent model useful for modeling skewed and unsteady wake dynamics. When using the blade element momentum theory, various corrections are available for the user, such as incorporating the aerodynamic effects of tip losses, hub losses, and skewed wakes. With the generalized dynamic wake, all of these effects are automatically included. Both of these methods are used to calculate the axial induced velocities from the wake in the rotor plane. The user also has the option of calculating the rotational induced velocity. In addition, AeroDyn contains an important model for dynamic stall based on the semi-empirical Beddoes-Leishman model. This model is particularly important for yawed wind turbines. Another aerodynamic model in AeroDyn is a tower shadow model based on potential flow around a cylinder and an expanding wake. Finally, AeroDyn has the ability to read several different formats of wind input, including single-point hub-height wind files or multiple-point turbulent winds.
Although wind energy exploitation dates back five thousand years ago, contemporary societies are based almost exclusively on fossil fuels for covering their electrical energy needs. On the other hand, during the last thirty years, security of energy supply and environmental issues have reheated the interest for wind energy applications. In this context, the present work traces the long and difficult steps of wind energy development from the California era to the construction of huge offshore wind parks worldwide, highlighting the prospects and the main challenges of wind energy applications towards the target of 1000 GW of wind power by 2030.
Floating wind turbines offer a feasible solution for going further offshore into deeper waters. However, using a floating platform introduces additional motions that must be taken into account in the design stage. Therefore, the control system becomes an important component in controlling these motions. Several controllers have been developed specifically for floating wind turbines. Some controllers were designed to avoid structural resonance, while others were used to regulate rotor speed and platform pitching. The development of a periodic state space controller that utilizes individual blade pitching to improve power output and reduce platform motions in above rated wind speed region is presented. Individual blade pitching creates asymmetric aerodynamic loads in addition to the symmetric loads created by collective blade pitching to increase the platform restoring moments. Simulation results using a high-fidelity non-linear turbine model show that the individual blade pitch controller reduces power fluctuations, platform rolling rate and platform pitching rate by 44%, 39% and 43%, respectively, relative to a baseline controller (gain scheduled proportional–integral blade pitch controller) developed specifically for floating wind turbine systems. Turbine fatigue loads were also reduced; tower side–side fatigue loads were reduced by 39%. Copyright © 2009 John Wiley & Sons, Ltd.
Keywords: Floatover installation Multi-body hydrodynamic analysis Nonlinear dynamical system Physical model test Load buffer device A B S T R A C T Floatover is a large deck installation method that the integrated deck is slowly transferred from the floatover vessel to the pre-installed substructure by falling tide or ballasting vessel. Compared with other installation methods, the floatover method has the advantages of high efficiency, economy and large lifting capacity, which has been widely used in both fixed and floating platforms. The inherently complex problems of floatover systems involve complex hydrodynamic interactions and nonlinear multi-body coupled dynamics. Establishing an accurate numerical or experimental model to investigate the dynamic responses of the vessel, topside, and sub-structure, the tension of the mooring system, and the impact loads of load buffer devices induced by environmental loads are essential to the design and field operation of floatover installations. In this review, we introduce the state-of-the-art progress of numerical and experimental modeling and analysis of floatover systems made by industrial and academic researchers, including the modeling and analysis of the load buffer devices, hydrodynamic analysis, and the analysis of the nonstationary nonlinear dynamical system. Finally, the major challenges and future research prospects of floatover are provided.
This work presents the results of a test campaign for a Floating Offshore Wind Turbine (FOWT) at a scale of 1:36. The floater is a Reduced-Draft-Spar platform for the DTU 10 MW reference wind turbine. A multifan system creating the equivalent aerodynamic thrust forces is used to replace the scaled physical turbine model, which is known as hybrid hardware-in-the-loop testing. The tests focus on different motion reduction techniques: an anti-heeling ballast and control system, a real-time observer, and a tuned liquid multi column damper, which are tested in a wave tank for the first time. Various tests are carried out to identify and calibrate the FOWT system. Results for regular waves, irregular waves and turbulent winds are presented in both time and frequency domain. Among the key results are the performance indicators of the motion reduction strategies. Several clear examples are provided along with a detailed analysis of the dynamic response for irregular wave conditions. The comparison of the numerically reproduced dynamic responses to the tests are presented as well, where a good agreement is found.
Offshore wind turbines present novel challenges to the field of rotor aerodynamics because of complex behaviours associated with six degrees of freedom motions of the platform. The number of scientific articles on these specific issues has been steadily increasing during the past ten years, reflecting the criticality of aerodynamics to overcome the specific barriers in this area. In this work, we aim to comprehensively review the present literature in order to identify the existing knowledge gaps in this field of research and provide an outlook for future directions. This paper is not purely about aerodynamics as an isolated element of floating offshore wind turbine science. Rather, due to the multi-physics nature of the system, we emphasise the current trends in aerodynamics in relation to other fields such as platform hydrodynamics and control. Critical analysis of the literature reveals that the most common approaches are to study the problem in a coupled or uncoupled manner. The latter is generally done by prescribing platform motions. The existing literature has been so far mainly focused on an isolated solo floating turbine and the studies on the interactions between the floating turbines are scarce. These trends are critically assessed in order to provide the reader with a holistic overview of the current direction. We also present six major challenges in order to provide a future perspective of the existing research opportunities in floating offshore wind turbine aerodynamics.
The first issue of the DNV Offshore Standard, DNV-OS-J103 Design of Floating Wind Turbine Structures, was published in June 2013. The standard was based on a joint industry effort with representatives from manufacturers, developers, utility companies and certifying bodies from Europe, Asia and the US. The standard represented a condensation of all relevant requirements for floaters in existing DNV standards for the offshore oil and gas industry which were considered relevant also for offshore floating structures for support of wind turbines, supplemented by necessary adaptation to the wind turbine application. The development of the standard capitalized much on experience from development projects going on at the time, in particular the Hywind spar off the coast of western Norway, the WindFloat off the coast of Portugal and the Pelastar TLP concept. In July 2018, DNV GL published a revision of DNV-OS-J103 as a part of the harmonization of the DNV GL codes for the wind turbine industry after the merger between Det Norske Veritas (DNV) and Germanischer Lloyd (GL) in the fall of 2013. The standard was re-issued as DNVGL-ST-0119 Floating wind turbine structures. This new revision reflects the experience gained after the first issue in 2013 as well as the current trends within the industry. Since 2013, numerous guidelines addressing the design of floating structures for offshore wind turbines have been published by various certifying bodies, and an IEC technical specification on the subject is under way. In addition, several prototypes have been installed and the first small array of floating wind turbines, Hywind Scotland pilot park, are currently in operation. The most important updates in the revision of the standard include formulation of floater-specific load cases, requirements to be fulfilled to support the exemption for design of unmanned floaters with damage stability, and replacement of current consequence-class based requirements for design fatigue factors with low-consequence based factors dependent on the accessibility for inspection and repair, the aim being a safety level against fatigue similar to that which is currently targeted for bottom-fixed structures. Other topics which have been considered in the revision are the floater motion control system and its possible integration with the control and protection system for the wind turbine, the issue of how to deal with slack in tendons in the station keeping system, corrosion, anchor design and power cable design.
In parallel to the revision of the standard, a new service specification for certification of floating wind turbines has been developed by DNV GL, identified as DNVGL-SE-0422 Certification of floating wind turbines. For technical requirements, the service specification refers to the revised standard, DNVGL-ST-0119.
The technical paper summarizes the updates and changes in the revised standard, in addition to the content of the new service specification.
This paper presents experimental results of WindFloat 1 platform and comparisons with the numerical model developed by Principle Power. The WindFloat platform is designed to support multi-megawatt wind turbines. A full-scale prototype was installed offshore Portugal from 2011 to 2016, and produced 17GWh into the Portuguese grid. An extensive monitoring system was installed, including a wave rider buoy, 6-degrees of freedom measurement devices, anemometers, strain gauges, turbine monitoring instruments. Important results obtained during the measurement campaign are described in this paper. These include power predictions, turbine and tower loads, and platform motions. Comparison with numerical simulations are also provided. The numerical analysis methodology includes fully coupled simulations, based on Orcaflex, a commercially available state-of-the-art software to compute the hydrodynamic response of floating systems, combined in the time-domain with FAST, a well-established numerical tool for the design of wind turbines. Results of these comparisons show that the numerical tools are fit for purpose, and were used to calibrate some hydrodynamic coefficients that cannot be obtained accurately with numerical analysis or scaled model tests.
WindFloat is a floating foundation for very large offshore wind turbines. This paper describes the hydrodynamic analysis of the hull, as well as ongoing work consisting of coupling hull hydrodynamics with wind-turbine aerodynamic forces. Three main approaches are presented in this paper: - The numerical hydrodynamic model of the platform and its mooring system; - Wave tank testing of a scale model of the platform with simplified aerodynamic simulation of the wind turbine; - FAST, an aerodynamic software package for wind turbine analysis with the ability to be coupled to the hydrodynamic model. These conference proceedings include two other papers presenting the design basis and main systems of this floating foundation [1], as well as structural analysis of the hull and mast [2].
Vertical wind shear is one of the dominating causes of load variations on the blades of a horizontal axis wind turbine. To alleviate the varying loads, wind turbine control systems have been augmented with sensors and actuators for individual pitch control. However, the loads caused by a vertical wind shear can also be affected through yaw misalignment. Recent studies of yaw control have been focused on improving the yaw alignment to increase the power capture at below rated wind speeds. In this study, the potential of alleviating blade load variations induced by the wind shear through yaw misalignment is assessed. The study is performed through simulations of a reference turbine. The study shows that optimal yaw misalignment angles for minimizing the blade load variations can be identified for both deterministic and turbulent inflows. It is shown that the optimal yaw misalignment angles can be applied without power loss for wind speeds above rated wind speed. In deterministic inflow, it is shown that the range of the steady-state blade load variations can be reduced by up to 70%. For turbulent inflows, it is shown that the potential blade fatigue load reductions depend on the turbulence level. In inflows with high levels of turbulence, the observed blade fatigue load reductions are small, whereas the blade fatigue loads are reduced by 20% at low turbulence levels. For both deterministic and turbulent inflows, it is seen that the blade load reductions are penalized by increased load variations on the non-rotating turbine parts. Copyright © 2013 John Wiley & Sons, Ltd.
The TurbSim stochastic inflow turbulence code was developed to provide a numerical simulation of a full-field flow that contains coherent turbulence structures that reflect the proper spatiotemporal turbulent velocity field relationships seen in instabilities associated with nocturnal boundary layer flows that are not represented well by the IEC Normal Turbulence Models (NTM). Its purpose is to provide the wind turbine designer with the ability to drive design code (FAST or MSC.ADAMS) simulations of advanced turbine designs with simulated inflow turbulence environments that incorporate many of the important fluid dynamic features known to adversely affect turbine aeroelastic response and loading.
The vast deepwater wind resource represents a potential to use offshore floating wind turbines to power much of the world with renewable energy. Many floating wind turbine concepts have been proposed, but dynamics models, which account for the wind inflow, aerodynamics, elasticity, and controls of the wind turbine, along with the incident waves, sea current, hydrodynamics, and platform and mooring dynamics of the floater, were needed to determine their technical and economic feasibility. This work presents the development of a comprehensive simulation tool for modeling the coupled dynamic response of offshore floating wind turbines, the verification of the simulation tool through model-to-model comparisons, and the application of the simulation tool to an integrated loads analysis for one of the promising system concepts. A fully coupled aero-hydro-servo-elastic simulation tool was developed with enough sophistication to address the limitations of previous frequency- and time-domain studies and to have the features required to perform loads analyses for a variety of wind turbine, support platform, and mooring system configurations. The simulation capability was tested using model-to-model comparisons. The favorable results of all of the verification exercises provided confidence to perform more thorough analyses. The simulation tool was then applied in a preliminary loads analysis of a wind turbine supported by a barge with catenary moorings. A barge platform was chosen because of its simplicity in design, fabrication, and installation. The loads analysis aimed to characterize the dynamic response and to identify potential loads and instabilities resulting from the dynamic couplings between the turbine and the floating barge in the presence of combined wind and wave excitation. The coupling between the wind turbine response and the barge-pitch motion, in particular, produced larger extreme loads in the floating turbine than experienced by an equivalent land-based turbine. Instabilities were also found in the system. The influence of conventional wind turbine blade-pitch control actions on the pitch damping of the floating turbine was also assessed. Design modifications for reducing the platform motions, improving the turbine response, and eliminating the instabilities are suggested. These suggestions are aimed at obtaining cost-effective designs that achieve favorable performance while maintaining structural integrity.
This manuscript summarizes the feasibility study conducted for the WindFloat technology. The WindFloat is a three-legged floating foundation for multimegawatt offshore wind turbines. It is designed to accommodate a wind turbine, 5 MW or larger, on one of the columns of the hull with minimal modifications to the nacelle and rotor. Potential redesign of the tower and of the turbine control software can be expected. Technologies for floating foundations for offshore wind turbines are evolving. It is agreed by most experts that the offshore wind industry will see a significant increase in activity in the near future. Fixed offshore turbines are limited in water depth to ∼30–50 m. Market transition to deeper waters is inevitable, provided that suitable technologies can be developed. Despite the increase in complexity, a floating foundation offers the following distinct advantages: Flexibility in site location; access to superior wind resources further offshore; ability to locate in coastal regions with limited shallow continental shelf; ability to locate further offshore to eliminate visual impacts; an integrated hull, without a need to redesign the transition piece between the tower and the submerged structure for every project; simplified offshore installation procedures. Anchors are significantly cheaper to install than fixed foundations and large diameter towers. This paper focuses first on the design basis for wind turbine floating foundations and explores the requirements that must be addressed by design teams in this new field. It shows that the design of the hull for a large wind turbine must draw on the synergies with oil and gas offshore platform technology, while accounting for the different design requirements and functionality of the wind turbine. This paper describes next the hydrodynamic analysis of the hull, as well as ongoing work consisting of coupling hull hydrodynamics with wind turbine aerodynamic forces. Three main approaches are presented: The numerical hydrodynamic model of the platform and its mooring system; wave tank testing of a scale model of the platform with simplified aerodynamic simulation of the wind turbine; FAST, an aeroservoelastic software package for wind turbine analysis with the ability to be coupled to the hydrodynamic model. Finally, this paper focuses on the structural engineering that was performed as part of the feasibility study conducted for qualification of the technology. Specifically, the preliminary scantling is described and the strength and fatigue analysis methodologies are explained, focusing on the following aspects: The coupling between the wind turbine and the hull and the interface between the hydrodynamic loading and the structural response.
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