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

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Sequential coupling The sequential coupling approach consists of a series of separated simulations. The simulation tools are kept separate and are applied one after the other. This coupling approach has been realised with Flex5 and ASAS(NL) [5]. ASAS(NL) is a finite element tool specifically developed for offshore applications. The overall time integration of Flex5 is used to solve the equations of motion. A reduced generalised foundation model with six degrees of freedom is needed for representing the complex sub-structure. This reduced model is created in the finite element tool out of the detailed finite element model. Besides the model the hydrodynamic loading history is treated analogously. Parts of the results (internal forces or displacements) from Flex5 are transferred back as boundary conditions to the finite element tool, which allows a retrieval simulation in the finite element tool to obtain member forces in the complex foundation model. The flow chart of the sequential approach is shown in Figure 2.1. Advantages and disadvantages The two simulation tools do not interact directly, which makes the modifications in the codes simple. The computational effort is higher than a stand-alone simulation in Flex5 because of the additional retrieval run. The coupling is missing dynamic interactions between the sub-structure and specific phenomena of the wind turbine due to the strong reduced foundation model. An outcome of this missing link is that the internal forces calculated in the retrieval run are different compared to an integrated solution. This coupling approach can only represent linear problems of the foundation, as any non-linear problem is linearised during the reduction process. The calculation of hydrodynamic loading is based on a fixed structure, as the motion of the overall wind turbine is unknown to this point. FE-Code: ASAS Flex5 Detailed sub-structure model Generation of hydrodynamic loading Reduction to a Flex5 equivalent foundation model (Stiffness, damping, mass, loading) Retrieval run: Force or deformation boundary conditions of Flex5 12 Generating the wind turbine model Foundation import Aero-hydro-elastic time integration Post-processing Post-processing Figure 2.1: Flow chart of the sequential coupling approach Full integrated coupling The most accurate simulation of offshore wind turbines (OWT) with complex sub-structures is achieved if the complete set of equations of motion of the entire offshore model is solved in one numerical solver. To do so, the equations of Flex5 need to be combined with the equations of the finite element code. The interface point is the connection between the tower and the sub-structure. Similar to the sequential approach Flex5 is used to model the wind turbine and to compute the aerodynamic loading. The finite element code is used to model the sub-structure and to compute the hydrodynamic loading. An interface, accessible by both tools, is employed to exchange parts of the equations of motion of the tools during runtime. The wind turbine model of Flex5 is considered as a super-element in the finite element code with a maximum number of 28 degrees of freedom. The other way round would end up in an enormous amount of work because it is difficult to introduce further degrees of freedom in Flex5.
In every time step the system matrices of Flex5, i.e. mass M, stiffness S, damping D, and the current loading F is transferred to the finite element code. There the models are combined on the level of equations of motion and the hydrodynamic loads are added. The equations are solved in the finite element code. The results, i.e. displacements, velocities and accelerations ( x, x&, &x &) are used in Flex5 to update the geometry, the loading and the controls. Figure 2.2 shows the coupling process and data exchange. Flex5 FE-Code t n-1 t n-1 Newmark Solver x, x& , && x 13 t n t n M Flex , D Flex , S Flex, F Aero Newmark Solver t n+1 t n+1 shared memory or DLL interface Figure 2.2: Process of the full integrated coupling between Flex5 and a finite element code [7] Three full integrated couplings have been realized: Flex5-FECOS [6], Flex5-Poseidon and Flex5-ASAS [7]. The first one was created within UpWind, the two other couplings have been created in the Project OWEA - Verification of offshore wind turbines [8]. The interface and the modelling capabilities are different between the tools, but the coupling method is the same. FECOS is a finite element code compiled as a dynamic link library (DLL) and is included in Flex5 directly. Therefore no special interface is necessary to handle the data exchange. Flex5-Poseidon uses a shared memory interface where both tools access the same memory. Flex5-ASAS uses a separate DLL for the data exchange. Advantages and disadvantages An important advantage of the full integrated coupling is that specific simulation codes are combined without losing their potential. The application possibility of the coupled tools has been extended. The method considers dynamic effects between the wind turbine and the support structure. The load calculation takes relative velocities of the entire structure into account. Non-linear problems can also be simulated using Flex5-ASAS. The main disadvantage is the computational time. The simulation speed is lower than the sequential approach and much lower than a Flex5 stand-alone simulation. The reasons for this can be found in the large number of freedoms and the data exchange between the tools in every time step. The simulation speed can be increased if a reduction method is implemented in the finite element code, but this restricts the simulations to linear problems. 2.2 Combined multi-body / modal approach This section describes the GH Bladed software code as an example of an integrated design tool for calculating wind turbine performance and dynamic response [9]. Bladed was originally developed for the modelling of onshore fixed-bottom wind turbines, but has been extended to include hydrodynamic loading for the modelling of offshore wind turbines. In the last year the core structural dynamic calculation in the code has been extended to incorporate multibody dynamics. The Bladed code uses a modal representation to model the structural dynamics of a wind turbine. This approach has the major advantage of giving an accurate and reliable representation of the dynamics of a wind turbine with relatively few degrees of freedom, making it a fast and efficient means of computation. In the previous version of the Bladed code, the modal properties of the rotating and non-rotating components of the system (i.e. the rotor and tower) were computed independently using a finite element representation of the structure, then coupled together using the appropriate equations of motion in the dynamic response
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 14 and 15: analysis. In the new multibody code
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
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From these results it can be clearl
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Figure 5.11: Normalised DELs with v
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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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