Final report for WP4.3: Enhancement of design methods ... - Upwind

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analysis. In the new multibody code, flexible components such as the blades and tower are modelled with a modal representation. However, instead of modelling the whole turbine as a single dynamic structure consisting of one rotor and tower with coupling between rotor modes and tower modes hard-wired into the code, the structure can now be modelled with any number of separate bodies, each with individual modal properties, which are coupled together using the equations of motion. Each mode is defined in terms of the following parameters: • Modal frequency • Modal damping coefficient • Mode shape represented by a vector of displacements The mode shapes and frequencies of the blade and tower (the main flexible components in a standard wind turbine model) are calculated based on the position of the neutral axis, mass distribution along the body and bending stiffness along the body, as well as other parameters specific to the body in question. The modal damping for each mode is a user input to the model. The use of multibody dynamics enables a completely self-consistent, rigorous formulation of the structural dynamics of a wind turbine. The blade modes are modelled individually with fully coupled flapwise, edgewise and torsional degrees of freedom, and are valid for any pitch angle. Advanced definition options are available for the blade geometry and structure, and additional degrees of freedom in the drive train and gearbox can be easily modelled. For modelling the support structure a multi-member model may be used, consisting of an arbitrary spaceframe structure with any number of straight interconnecting beam elements with given mass and stiffness properties. Craig-Bampton style modes are used for the support structure, with a torsional degree of freedom included for all support structure types, not just multiple-member support structures. The support structure is not necessarily axisymmetric, so the resulting mode shapes will be three-dimensional with all six degrees of freedom at each node. Figure 2.3 shows an example of a mode shape for a braced support structure, in this case a three-legged tripod. Soil springs can be modelled in Bladed via a user-defined force-displacement relationship at multiple foundation stations on the sub-structure. This includes the possibility to define non-linear relationships between force and displacement. Figure 2.3: Example of multi-member support structure mode in Bladed 14
2.3 Full Finite Element approach The offshore wind turbine simulation approach presented in this section is based on ADCoS, a nonlinear finite element (FE) system for aero-servo-elastic simulation of onshore wind turbines. ADCoS uses a direct time integration method to resolve the equations of motion that describe the dynamic system as a whole. Therefore no modal transformation is performed and no modal analysis is needed for simulation. Furthermore each time step is iterated until a convergence criterion is reached and for that reason nonlinear effects, like 2 nd order pitch moments resulting from large blade deflections or torsional stiffening of the blades due to different rotational speeds, are directly included in the time domain simulations. Torsional flutter, a highly nonlinear instability that can occur due to a disadvantageous combination of aerodynamic loads of a rotor blade, may be reproduced in detail as described in [11]. Support structures are defined as FE beam models using a two noded beam element with 12 DOF, based on an Euler-Bernoulli-Formulation, in ADCoS. With this approach, local effects concerning single members of branched structures are described as well. Local natural frequencies for example can be found in load spectra of member loads. ADCoS is described in [11] in more detail. To allow for simulation of offshore wind turbines, ADCoS is combined with ASAS, a FE tool widely used in the offshore Oil & Gas industries to simulate offshore structures. Furthermore a macro allows for input of structural models from the general purpose FE tool ANSYS for convenient structural modeling and optimization. In ASAS, hydrodynamic loads based on all common wave theories and Morison's equation as well as hydrostatic loads can be calculated. Buoyancy may be included in relation to the time dependent water surface based on displaced water mass or based on integration of hydrostatic pressures around the submerged members. The loads calculated in ASAS are transferred into ADCoS-Offshore as nodal loads. The calculation of wave loads and the overall time domain simulation are separated in the current simulation procedure. Therefore effects resulting from relative kinematics due to superimposed wave and structural motion cannot be described. Although those effects are negligible for many structures, a new version of ADCoS-Offshore that includes relative structural motions in wave load calculations is currently tested. The ADCoS-Offshore approach allows consideration of soil characteristics in the overall turbine model. The easiest way to connect the support structure model to the soil is to clamp it at mudline. It is obvious that this approach is not very realistic. Therefore, user defined stiffness matrices can be included at mudline in ADCoS-Offshore to account for the flexibility of the embedded pile. For detailed simulation, those stiffness matrices can be derived using ASAS based on a nonlinear P-y approach as described in [12] that is recommended by the American Petroleum Institute [13]. In this case pile group effects resulting from loads transferred from one pile to another via the soil medium are included as well. Even though a modal analysis is not part of the transient time domain simulation a modal analysis is a helpful supplementary tool to understand dynamic behavior of a structure. Figure 2.4 shows the 2 nd global bending mode of the offshore jacket structure of a 5-MW turbine 30 m of water in ADCoS-Offshore (left) and ANSYS (right) as an example. 15
Page 1 and 2: Project UpWind Contract No.: 019945
Page 3 and 4: Acknowledgement The presented work
Page 5 and 6: Summary The overall aim of the UpWi
Page 7: Table of Contents Acknowledgement..
Page 10 and 11: Task 4.1 Integration of support str
Page 12 and 13: Sequential coupling The sequential
Page 16 and 17: Figure 2.4: 2nd global bending mode
Page 18 and 19: 3.2 Comparison of deterministic loa
Page 20 and 21: Figure 3.5: Time series of axial fo
Page 22 and 23: • Time domain simulation in ADCoS
Page 24 and 25: • A supplementary load vector for
Page 26 and 27: The following results are found: Fi
Page 28 and 29: Figure 4.5: Output positions in the
Page 30 and 31: It is directly visible that there i
Page 32 and 33: ated eigenfrequency is shifted into
Page 34 and 35: Water levels For the calculation of
Page 36 and 37: guidelines for the inclusion of cli
Page 38 and 39: damaging if the frequency of the ex
Page 40 and 41: uncertain parameters carefully. Thr
Page 42 and 43: Table 5.3 shows annual reliability
Page 44 and 45: Steel substructure for offshore win
Page 46 and 47: Table 5.16 shows the required FDF v
Page 48 and 49: A Fracture Mechanical (FM) modellin
Page 50 and 51: Figure 5.4: Annual reliability indi
Page 52 and 53: Further, the effect of possible ins
Page 54 and 55: Rainflow evaluation The load cases
Page 56 and 57: For DLC 7.2 it has to be considered
Page 58 and 59: Guideline, DLC 1.5). Furthermore, t
Page 60 and 61: Table 5.28: Output locations for da
Page 62 and 63: From these results it can be clearl
Page 64 and 65:
Figure 5.11: Normalised DELs with v
Page 66 and 67:
Figure 5.14 presents normalized DEL
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Figure 5.16: Output locations for e
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UPWIND WP4: Offshore Support Struct
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Final report for WP4.3: Enhancement of design methods ... - Upwind

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