Rotor-nacelle Assembly Research Articles

Load reduction of a monopile wind turbine tower using optimal tuned mass dampers

ABSTRACTWe investigate to apply tuned mass dampers (TMDs) (one in the fore–aft direction, one in the side–side direction) to suppress the vibration of a monopile wind turbine tower. Using the spectral element method, we derive a finite-dimensional state-space model Σd from an infinite-dimensional model Σ of a monopile wind turbine tower stabilised by a TMD located in the nacelle. Σ and Σd can be used to represent the dynamics of the tower and TMD in either the fore–aft direction or the side–side direction. The wind turbine tower subsystem of Σ is modelled as a non-uniform SCOLE (NASA Spacecraft Control Laboratory Experiment) system consisting of an Euler–Bernoulli beam equation describing the dynamics of the flexible tower and the Newton–Euler rigid body equations describing the dynamics of the heavy rotor-nacelle assembly (RNA) by neglecting any coupling with blade motions. Σd can be used for fast and accurate simulation for the dynamics of the wind turbine tower as well as for optimal TMD designs. We show that Σd agrees very well with the FAST (fatigue, aerodynamics, structures and turbulence) simulation of the NREL 5-MW wind turbine model. We optimise the parameters of the TMD by minimising the frequency-limited -norm of the transfer function matrix of Σd which has input of force and torque acting on the RNA, and output of tower-top displacement. The performances of the optimal TMDs in the fore–aft and side–side directions are tested through FAST simulations, which achieve substantial fatigue load reductions. This research also demonstrates how to optimally tune TMDs to reduce vibrations of flexible structures described by partial differential equations.

Design of monopile supported offshore wind turbine in clay considering dynamic soil–structure-interaction

This paper addresses the feasibility of soft–soft and soft–stiff design approaches considering a 2MW and 5MW monopile supported three bladed offshore wind turbine (OWT) founded in clay. The serviceability limit state and fatigue life of the structure are checked in order to assess the safety. Resonance condition is also avoided keeping the fundamental frequency of the system away from the rotor frequencies. The OWT system is modeled using linear beam and soil–structure interaction is accounted for incorporating American Petroleum Institute based cyclic p–y springs attached to the monopile. Aerodynamic and hydrodynamic loads are applied on the structure and dynamic analysis is carried out using a finite element method in time domain. Overall mass of the structure is examined considering two design approaches in order to obtain an economical design solution. The study shows that soft–soft design is possible for 2MW OWT subjected to rated wind speed for long tower. Rotor nacelle assembly mass and tower height is found to have governing role in soft–stiff design and on the material consumption. Embedded depth of monopile beyond critical depth has marginal impact on design. Fatigue life is observed to be governing design criteria for OWT at stiff clay.

Irregular Nonlinear Wave Simulation and Associated Loads on Offshore Wind Turbines

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

A Comparative Study of a Three Rotor and a Single Rotor 5 MW Wind Turbine Based on Economic and Structural Considerations

Wind turbines with more than one rotor on a single support structure have been topics of interest for at least a hundred years. In this paper we assess the potential of the multi-rotor concept through a comparison of a three rotor, single tower turbine with a total rated capacity of 5 MW with a conventional, single rotor turbine with the same rated output. Except for the number of rotors, the turbines are in all other ways as similar as possible. The comparison first involves estimating the weights and costs of the two turbines. The three rotor turbine also requires a distinct rotor nacelle assembly support structure (RNASS) which is not required in a conventional turbine. A plausible design for such a structure is developed and analyzed. The weight and cost of that structure is estimated as well. Those are used to estimate the total weight and total cost of the part of the turbine which would correspond to the rotor nacelle assembly of the single rotor turbine. The results of this study show that the weight of the three rotor turbine, including the RNASS is likely to be comparable to or higher than that of a single rotor turbine, but that the cost of the three rotor turbine may well be less. The reason for the lower cost is in effect the substitution of less expensive material (structural steel) for more expensive material (blades and drive train components). The overall conclusion is that multiple rotor turbines look most attractive for those situations where a turbine with a very large rated capacity is for some reason desirable. The most obvious situation where this may the case is offshore, where the cost of support structures is very high.

Hybrid Model Representation of a TLP Including Flexible Topsides in Non-Linear Regular Waves

The rising demand for renewable energy solutions is forcing the established industries to expand and continue evolving. For the wind energy sector, the vast resources in deep sea locations have encouraged research towards the installation of turbines in deeper waters. One of the most promising technologies able to solve this challenge is the floating wind turbine foundation. For the ultimate limit state, where higher order wave loads have a significant influence, a design tool that couples non-linear excitations with structural dynamics is required. To properly describe the behavior of such a structure, a numerical model is proposed and validated by physical test results. The model is applied to a case study of a tension leg platform with a flexible topside mimicking the tower and a lumped mass mimicking the rotor-nacelle assembly. The model is additionally compared to current commercial software, where the need for the coupled higher order dynamics proposed in this paper becomes evident.

Exploring the gap between ‘best knowledge’ and ‘best practice’ in boundary layer meteorology for offshore wind energy

ABSTRACTOnshore wind turbine technology is moving offshore, and the offshore wind industry tends to use larger turbines than those used over land. This calls for an improved understanding of the marine boundary layer. The standards used in the design of offshore wind turbines, particularly the rotor–nacelle assembly, are similar to those used for onshore wind turbines. As a result, simplifications regarding the marine boundary layer are made. Atmospheric stability considerations and wave effects, including the dynamic sea surface roughness, are two major factors affecting flow over sea versus land. Neutral stratification and a flat, smooth sea surface are routinely used as assumptions in wind energy calculations. Newly published literature in the field reveals that the assumption of a neutral stratification is not necessarily a conservative approach. Design tests based on neutral stratification give the lowest fatigue damage on the rotors. Turbulence, heat exchange and momentum transfer depend on the sea state, but this is usually ignored, and the sea surface is thought of as level and smooth. Field experiments and numerical simulations show that during swell conditions, the wind profile will no longer exhibit a logarithmic shape, and the surface drag relies on the sea state. Stratification and sea state are parameters that can be accounted for, and they should therefore be considered in design calculations, energy assessments and power output predictions. Copyright © 2012 John Wiley & Sons, Ltd.

浮体式洋上風車の風車－浮体－係留連成応答解析プログラム"UTWind"の開発と検証

Rotor-floater-mooring coupled response analysis is essentially important for design of Rotor Nacelle Assembly (RNA) and floating support structure of Floating Offshore Wind Turbine (FOWT). The authors have developed an analysis code UTWind for the analysis of the coupled response. Blades and floater are modeled as frame structure with beam elements. Lumped mass model is use for mooring. Aerodynamic load on blade is calculated by Blade Element Momentum Theory (BEM), and hydrodynamic load is calculated by Hooft's method, and Morison equation was modified to be applicable to cylindrical element with cross section with two axes of line symmetry. The equations of motion of rotor, floater and mooring are solved in time domain by weak coupling algorithm.The numerical results by the code were compared with experimental results in wave and wind-wave coexisting condition with/without blade pitch control and showed good agreement. Response by negative damping was reproduced by the code and showed good agreement with experiments.

Floating Offshore Wind Turbine Dynamics: Large-Angle Motions in Euler-Space

Floating structures have been proposed to support offshore wind turbines in deep water, where environmental forcing could subject the rotor to meaningful angular displacements in both precession and nutation, offering design challenges beyond conventional bottom-founded structures. This paper offers theoretical developments underlying an efficient methodology to compute the large-angle rigid body rotations of a floating wind turbine in the time domain. The tower and rotor-nacelle assembly (RNA) are considered as two rotational bodies in the space, for which two sets of Euler angles are defined and used to develop two systems of Euler dynamic equations of motion. Transformations between the various coordinate systems are derived in order to enable a solution for the motion of the tower, with gyroscopic, environmental, and restoring effects applied as external moments. An example is presented in which the methodology is implemented in matlab in order to simulate the time-histories of a floating tower with a RNA.

Simulation of large-amplitude motion of floating wind turbines using conservation of momentum

A new method is presented to directly derive the nonlinear equations of motion (EOMs) of a floating wind turbine system using the theorem of conservation of angular momentum and Newton's second law. The methodology considers the system as two rigid bodies: the tower and the rotor-nacelle assembly (RNA). The large-amplitude rotation of the tower is described by the 1-2-3 sequence Euler angles, which offer accurate nonlinear coupling between motions in 6 degrees of freedom (DOFs). Two additional DOFs of the RNA relative to the tower, nacelle yaw and rotor spin, are prescribed by mechanical control and are also included in the EOMs of the entire system. Results from the EOMs are transformed among different coordinate systems at every time-step for use in the computation of hydrodynamics, aerodynamics and restoring forces, which preserves the nonlinearity between external excitation and structural dynamics. The new method is verified by critical comparison of simulation results with those of the popular wind turbine dynamics software FAST. The concept of highly compliant floating wind turbines is introduced. The large-amplitude motions and gyroscopic moments of one of these smaller, lighter structures is simulated in an example.

Offshore support structure optimization by means of integrated design and controls

ABSTRACTSupport structures are one of the main cost drivers offshore, and since other offshore rules apply with regards to design and site conditions, different methodologies have to be developed to mitigate the loads on the support structure and therefore to reduce the associated component costs. Detailed onshore studies and field tests have already shown that advanced control algorithms can be an effective way to reduce fatigue and extreme loading on a wind turbine. Still, the overall cost‐effectiveness for large offshore wind farms has not been studied in detail yet. In the scope of this paper, studies of new controller concepts are integrated within the design process of offshore support structures in order to stretch the applicability of monopiles to larger water depths or simply to reduce the structural weight and associated costs.A reference design of the support structure is made for a site in the Dutch North Sea. The focus is on reducing the dominant hydrodynamic excitation on the support structure. The implemented load mitigation concept leads to significant reductions in loading, allowing considerable material savings and therefore a more cost‐effective design. The presented approach allows material reductions of more than 9% for the studied support structure. Undesired side effects, such as increased wear of turbine components, are unlikely, as other system loadings and characteristics remain within an acceptable range. Even if some of the rotor–nacelle assembly loads are slightly increased by the applied controller, the increases are low and probably still within the margins of the type‐class fatigue loads. Copyright © 2012 John Wiley & Sons, Ltd.

Study on Control Concepts Suitable for Mitigation of Loads from Misaligned Wind and Waves on Offshore Wind Turbines Supported on Monopiles

For current offshore wind farms, monopiles are by far the most popular support structure type. However, for deeper water and/or larger turbines, the fatigue loading is becoming critical and the monopile dimensions are exceeding the current economical feasibility. Especially in cases of misaligned wind and waves, the side-to-side fatigue loading at the support structure can become a design driver. Since the industry aims to use standardized rotor-nacelle-assembly (RNA) designs, the goal of this paper is to show the effectiveness of using the RNA as adjustable controller device for site-specific offshore support structure configurations by using active control devices.

Influence of different wind profiles due to varying atmospheric stability on the fatigue life of wind turbines

Offshore wind energy is being developed on a very large scale in the European seas. The objective of developing wind energy offshore is to capture greater wind speeds than are encountered onshore and as a result more energy. With this also come more challenges in the design of wind turbines due to the hostile offshore environment. Currently the standards for offshore wind turbines prescribe a site specific design for the support structures and the design for the rotor nacelle assembly according to onshore standards. Wind turbines are designed to withstand fatigue and ultimate loads. For the fatigue loading several input conditions have been prescribed, amongst which wind profile is one of them. Wind profile is represented by power law or logarithmic law as given in the standards. A neutral stability of the atmosphere is considered while obtaining the wind profile using the logarithmic law. In this paper the atmospheric stability is varied in order to estimate different wind profiles and simulations are run in Bladed to check its influence on the fatigue damage at the blade root. The variations in the atmospheric stability has been taken into account by using some typical values of Obukhov length. From steady state simulations it has been found that atmospheric stability is important for fatigue damage. The analysis showed that variation in the distribution of atmospheric stability causes large variations in the fatigue damage for different sites. Thus, it is worthwhile to carry out a full scale study using the turbulent winds and real data for wind turbine and environmental conditions.