For the design of offshore foundations in regions such as the Baltic Sea, it is paramount that ice-structure interaction is appropriately considered. For the monopile, a common foundation for offshore wind turbines, challenges with ice-induced vibrations and high ridge loads may require ice-mitigating measures to be included in the design. A ‘feasibility map’ showing the necessity for such ice-mitigating measures in the entire Baltic region has been developed for monopiles. The feasibility was considered in technical terms by imposing design, installation, and fabrication constraints, and in economic terms, expressed in weight increase of monopiles when compared to an ‘ice-free’ design. A design assessment of offshore wind turbines across the Baltic Sea was conducted by optimizing foundation designs for the IEA 15 MW reference turbine for nine identified characteristic regions of the Baltic Sea. The assessment was performed via the in-house foundation design software MORPHEUS by Wood Thilsted. MORPHEUS has been coupled to the phenomenological ice model “VANILLA” to capture the dynamic ice-structure interaction for level ice. From the assessment, the following regions are deemed feasible for monopiles without ice-mitigating measures: the Danish Straits, the Baltic Proper South, the Baltic Proper North, the Gulf of Riga and the Archipelago Sea. The Bothnian Sea North and the Bay of Bothnia are deemed infeasible without mitigating measures. For the Bothnian Sea South and the Gulf of Finland, no conclusive answer was found as more research into the cost competitiveness of alternative options is required. The increase in fatigue resulting from ice loading was found to be the main cause for foundation weight increase of monopiles compared to monopiles designed for ice-free waters. @en

This work aims to develop a low-fidelity model for a lattice support structure for offshore wind applications. The proposed low-fidelity model consists of a sequence of regular Timoshenko beams, each of them characterized by homogenized mechanical and mass properties representative of the single bays of the reference space-frame structure. The homogenized elastic coefficients of the sequence of beams are then computed by means of two alternative procedures: case (a), via analytical expressions available in the literature and accounting for a partially isotropic behaviour; case (b) by means of an optimization procedure, with ad hoc calibration factors. The suggested methods to derive the homogenized elastic coefficients are then tested for both straight and tapered lattice structures. The prediction performance is evaluated in terms of estimation of the first five natural frequencies and mode shapes, response to dynamic loads, and ability to predict rotor-structure interaction phenomena. A parametric study is then performed to evaluate the potential and limitations of the proposed models. To bypass the optimization procedure (b), a data-driven approach is also proposed for the case of straight lattice structures. Overall, the developed low-fidelity model leads to a computational speed-up factor of at least 60. The prediction reliability of the low-fidelity model is discussed for a tapered and regular straight lattice structure. However, for the latter one, a more detailed comparative study between the various modelling assumptions is performed and discussed. With reference to the straight lattice tower, whenever an optimization procedure is used (case (b)), and with reference to a typical subset of the investigated geometrical parameter space, the mean prediction error of the first five natural frequencies is lower than 1%. On the other hand, for case (a) and for the same investigated subset, the mean prediction errors for the first two bending modes and the torsional mode are, 5.2%, 13.3% and 18.8%, respectively. These results are improved in case a data-driven regression model is used to predict the calibration factors, leading to mean prediction errors below 5% for the entire investigated parameter space.

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In the previous decade, offshore wind undeniably took off as an important player in the European energymarket, which resulted in continuously enhancing turbine sizes and foundation structures pushing the boundaries of engineering, with the purpose of minimizing the levelized cost of energy to the range of optimal competitiveness. Regarding the foundation structure – or support structure – an important trade-off exists with respect to optimization and differentiation within an offshore wind farmon the one hand, and the required computational effort at an early stage of the design on the other. This effort comprises the extensive set of environmental conditions that require evaluation and the level of complexity of the modelling of the different environmental interactions, be it with wind, waves or soil. Concerning the modelling of the environmental interactions with the structure, a decoupling of the turbine and the support structure is commonly applied, allowing the turbine manufacturer and the offshore contractor to develop their designs separately. The analysis of the support structure, however, has to account for the effect of the aerodynamic force, particularly for the aerodynamic damping, as this is known to affect the structural response to wave actions substantially. In this respect, the shared information usually concerns the damping ratio of the first fore-aft mode of vibration. This damping ratio does not explicitly express its dependency on the operational conditions of the turbine, e.g., the mean wind velocity and the rotor speed. Moreover, the provided ratio is only valid for the fore-aft motion in the first mode or vibration, and can therefore not be applied for higher fore-aft modes, or modes describing different motions, such as side-to-side or torsional.@en

The downwind configuration of wind turbines offers benefits regarding the blade-tower clearance, as during operation the blade primarily bends away from the tower. Consequently, the blades can be designed with lower stiffness. For tubular towers, however, a significant deficit of the wind speed in the tower wake occurs, resulting in fatigue-inducing vibrations. For this reason, full-height lattice towers are considered the preferred support structures for wind turbines with a downwind rotor. This work estimates the tower shadow excitation of a downwind rotor blade from a tubular tower. To this end, the blade of a commercial 6 MW downwind turbine is modelled with finite-elements. The tower wake is described on the basis of Madsen's model and for the unsteady aerodynamic interaction Küssner's function is adopted. At below- and above-rated wind conditions, the tower wake-induced vibrations are compared with the response of a blade of an equivalent upwind rotor, considering both the tip deflections and the root moments, the latter on the basis of damage-equivalent moments, to obtain an indication of the expected difference in fatigue damage. The downwind blade experiences vibrations with considerable larger amplitudes, especially in the out-of-plane direction. From the damage-equivalent moments it can be inferred that the blades of the downwind rotor encounter a much faster accumulation of fatigue damage.@en

The offshore wind market is developing towards exploiting wind resources in deeper water sites. Inevitably, this fuels new research and feasibility studies on alternative solutions for the wind turbine support structures. Within this context, this work aims at comparing three different support structure design concepts for a 14 MW two-bladed downwind wind turbine: an XXL monopile, a hybrid jacket-tower and a lattice tower. To ensure a fair comparability of the three design concepts, a load capacity analysis is first performed to assess the yield strength of each concept and guarantee a similar material utilization. The comparative analysis is then carried out in terms of total mass, dynamic behaviour of the interaction between rotor and support structure, soil-structure interaction and resulting hydrodynamic forces. Based on the given design constraints, the preliminary results of this study favour a lattice tower solution. The study also highlights peculiar dynamic phenomena such as veering, mode hybridization and mode coalescence for the dynamic interaction between the lattice tower and the two-bladed rotor which need to be taken into account during the design phase.@en

To meet the political goals regarding renewable energy production, offshore wind keeps expanding to waters with larger depths and harsher conditions, while the turbine size continues to grow and ever-larger foundation structures are required. This development can only be successful if further cuts in the levelized cost of energy are established. Regarding the design of the foundation structures, a particular challenge in this respect relates to the reduction of the total computational time required for the design. For both practical and commercial reasons, the decoupled modelling of offshore wind support structures finds a common application, especially during the preliminary design stage. This modelling approach aims at capturing the relevant characteristics of the different environment-structure interactions, while reducing the complexity as much as possible. This paper presents a comprehensive review of the state-of-the-art modelling approaches of environmental interactions with offshore wind support structures. In this respect, the primary focus is on the monopile foundation, as this concept is expected to remain the prominent solution in the years to come. Current challenges in the field are identified, considering as well the engineering practice and the insights obtained from code comparison studies and experimental validations. It is concluded that the decoupled analysis provides valuable modelling perspectives, in particular for the preliminary design stage. In the further development of the different modelling strategies, however, the trade-off with computational costs should always be kept in mind.@en

The Harmonic Balance Method (HBM) is often used to determine the stationary response of nonlinear discrete systems to harmonic loading. The HBM has also been applied to nonlinear continuous systems, but in many cases the nonlinearity consists of discrete nonlinear elements. This chapter demonstrates the application of the HBM to dissipative continua with distributed nonlinearity by analysing three canonical problems: (a) 1-D layer with a free surface and rigid base (interfering upward and downward propagating shear waves), (b) 1-D half-space with a rigid base (vertically propagating shear waves), and (c) 2-D axially symmetric semiinfinite medium with a circular cavity (radially propagating compressional waves), all of them subject to harmonic excitation at a boundary. Results show that systems (a) and (c) exhibit softening behaviour and super-harmonic resonances, while only the former displays multiple response amplitudes for certain excitation frequencies; the unique frequency-amplitude relationship of system (c) is due to the strong damping (i.e., radiation damping and internal dissipation). Furthermore, although system (b) essentially does not resonate, the third-harmonic component exhibits a maximum caused by the interplay between the dissipative and nonlinear effects, a phenomenon that also occurs in system (c). Finally, the considered systems have applications in earthquake and geotechnical engineering, among others, but the presented methodology is generic.@en

Wind turbine site planning is a multidisciplinary task comprising of several stakeholder groups from different domains and with different priorities. An information system capable of integrating the knowledge on the multiple aspects of a wind turbine plays a crucial role on providing a common picture to the involved groups. In this study, we have developed an interactive and intuitive 3D system (Falcon) for planning wind turbine locations. This system supports iterative design loops (wind turbine configurations), based on the emerging field of geodesign. The integration of GIS, game engine and the analytical models has resulted in an interactive platform with real-time feedback on the multiple wind turbine aspects which performs efficiently for different use cases and different environmental settings. The implementation of tiling techniques and open standard web services support flexible and on-the-fly loading and querying of different (massive) geospatial elements from different resources. This boosts data accessibility and interoperability that are of high importance in a multidisciplinary process. The incorporation of the analytical models in Falcon makes this system independent from external tools for different environmental impacts estimations and results in a unified platform for performing different environmental analysis in every stage of the scenario design. Game engine techniques, such as collision detection, are applied in Falcon for the real-time implementation of different environmental models (e.g. noise and visibility). The interactivity and real-time performance of Falcon in any location in the whole country assist the stakeholders in the seamless exploration of various scenarios and their resulting environmental effects and provides a scope for an interwoven discussion process. The flexible architecture of the system enables the effortless application of Falcon in other countries, conditional to input data availability. The embedded open web standards in Falcon results in a smooth integration of different input data which are increasingly available online and through standardized access mechanisms.

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In this study, we propose an interactive information system for wind turbine siting, considering its visual and sound externalities. This system is an integration of game engine, GIS and analytical sound propagation model in a unified 3D web environment. The game engine–GIS integration provides a 3D virtual environment where users can navigate through the existing geospatial data of the whole country and place different wind turbine types to explore their visual impact on the landscape. The integration of a sound propagation model in the game engine–GIS supports the real-time calculation and feedback regarding wind turbine sound at the surrounding buildings. The platform's GIS component enables massive (on-the-fly) georeferenced data utilization through tiling techniques as well as data accessibility and interoperability via cloud-based architecture and open geospatial standard protocols. The game engine, on the other hand, supports performance optimization for both data display and sound model calculations.

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Existing models for the analysis of offshore wind turbines account for the aerodynamic action on the turbine rotor in detail, requiring a high computational price. When considering the foundation of an offshore wind turbine, however, a reduced rotor model may be sufficient. To define such a model, the significance of the nonlinear velocity and history dependency of the aerodynamic force on a rotating blade should be known. Aerodynamic interaction renders the dynamics of a rotating blade in an ambient wind field nonlinear in terms of the dependency on the wind velocity relative to the structural motion. Moreover, the development in time of the aerodynamic force does not follow the flow velocity instantaneously, implying a history dependency. In addition, both the non-uniform blade geometry and the aerodynamic interaction couple the blade motions in and out of the rotational plane. Therefore, this study presents the Euler–Bernoulli formulation of a twisted rotating blade connected to a rigid hub, excited by either instantaneous or history-dependent aerodynamic forces. On this basis, the importance of the history dependency is determined. Moreover, to assess the nonlinear contributions, both models are linearized. The structural response is computed for a stand-still and a rotating blade, based on the NREL 5-MW turbine. To this end, the model is reduced on the basis of its first three free-vibration mode shapes. Blade tip response predictions, computed from turbulent excitation, correctly account for both modal and directional couplings, and the added damping resulting from the dependency of the aerodynamic force on the structural motion. Considering the deflection of the blade tip, the history-dependent and the instantaneous force models perform equally well, providing a basis for the potential use of the instantaneous model for the rotor reduction. The linearized instantaneous model provides similar results for the rotating blade, indicating its potential application for this scenario, and allowing for the definition of an added damping matrix, applicable for the dynamic analysis of rotating turbine blades@en

Support structures of offshore wind turbines are prone to failure from fatigue damage. The design for fatigue requires accurate predictions of the environmental conditions and an adequate definition of the structural properties, valid for the entire design life-time. Estimates of the accumulated fatigue damage are, however, characterized by a large degree of uncertainty, stemming from the loading specifications and the numerical models used to predict the response. By employing measured data intelligently, the accumulated fatigue damage can be monitored throughout the structural life-time. This work presents a feasibility study towards the application of a joint input-state estimation algorithm for the response estimation of a lattice support structure. The feasibility is studied by first generating artificial measurement data with a full-order finite element model, while the strains at unmeasured locations are estimated with an erroneous reduced-order design model, after inclusion of measurement noise. It is shown that this model-based approach allows for the estimation of the response, despite significant errors in the design model. Particular attention is paid to the measurement locations, which should be within reach for maintenance. @en