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Model Building |
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Building a floating wind turbine model in Flexcom is very straightforward. You simply follow a logical sequence of steps, adding new components in sequential fashion, as you would do for any offshore structure in Flexcom. Some of the following information may be very familiar to existing Flexcom users, but a coherent summary could serve as a helpful reference point.
•Set up an arrangement of Lines to model the floating platform. It is not necessary to model the platform in explicit detail, but ensure that at a minimum, finite element nodes are placed at key locations (e.g. platform centre of gravity (CoG), platform centre of buoyancy (CoB), tower base, and fairlead positions of any attached mooring lines). This effectively serves as a framework upon which the various constituents may be applied. The connecting elements between these key locations are typically rigid (so the floater moves as a single rigid body), massless (the floater's inertia is typically concentrated at the CoG node), have zero Drag Diameter (to suppress the application of Morison drag loads, unless you are modelling viscous drag effects in distributed manner), and have zero Buoyancy Diameter (the floater's buoyancy is typically concentrated at the CoB node).
oFor further information regarding the floating platform, refer to Floating Body Modelling Detail and Diffraction-Radiation Theory & Morison's Equation.
oDepending on the level of modelling detail applied, the assembly of elements may look rather skeleton-like, and not resemble a floating platform, so the addition of an auxiliary Vessel Profile is advisable. While this will have no structural function, it will greatly enhance the visual appeal of the model, and will assist in the understanding of floater motions post-simulation.
•Define a Floating Body to model the physical characteristics of the floater. Specify the relevant Inertia terms at the centre of gravity. Use the Hydrostatic Stiffness terms to simulate restoring forces and moments due to buoyancy. Define Added Mass, Radiation Damping and Force RAO coefficients for the floating body over a range of discrete frequencies - these terms enable the computation of incident, diffracted and radiated (linear) wave potentials to be simulated. Define QTF Coefficients if second order drift loads are to be modelled also. All of these inputs must be derived separately from a radiation-diffraction analysis. Some of the more common commercial codes include WAMIT and ANSYS Aqwa, while NEMOH is a popular open-source code.
•Construct the tower using a single Line. Attach the lower end of the tower to the relevant node on the floating platform using the Equivalent Nodes facility. As the tower is normally tapered from a wide base to a more slender top, it is normally constructed in Flexcom using several Line Sections of different diameter. The mesh density for the structural model is governed by the Line Mesh Generation settings for the line and its sub-sections, while the mesh density for the aerodynamic model is controlled via the *TOWER INFLUENCE keyword. It is not necessary to use the same mesh density for both models, but as a minimum, structural nodes should be placed at elevations which correspond exactly to equivalent nodes in the aerodynamic model. In practice, the structural tower is typically modelled using a certain number of sub-sections of equal length, and the intersection points between these sections serve as the aerodynamic nodes also.
•Refer to Rotor Blade Model for an explanation of the differences between rigid and flexible blade models, and the information/keywords required in each case.
•Specify all the wind turbine inputs which are required by AeroDyn to compute the aerodynamic loading on the blades and tower. This category of inputs (logically grouped together under $AERODYN) will be intuitively familiar to engineers with some wind turbine modelling experience. Fundamental inputs in this category include Blade Geometries, Aerofoil Coefficients, miscellaneous Turbine Inputs (such as hub height, hub radius, overhang, shaft tilt, blade precone etc.) and Tower Influence (i.e. tower drag). There are also various other advanced options affording user control e.g. Blade Element Momentum Theory.
•Create the mooring lines also using the Lines feature. Attach the upper end of each mooring line to the relevant fairlead node on the floating platform using the Equivalent Nodes facility. Constrain the lower (seabed) end of each mooring line using Fixed Boundary Conditions.
•Define Environmental Parameters, such as ocean depth and water density, and include a Seabed definition.
•Perform a static analysis in order to determine the static equilibrium configuration of the entire system subject to gravity and buoyancy loads only. In order to aid solution convergence, this simulation is normally performed in two stages, with the CoG node being temporarily restrained in an Initial Static Analysis using additional Fixed Boundary Conditions, which are subsequently removed in a Restart Static Analysis. Floating systems can be sensitive to minor changes in displacement, and if static convergence proves difficult to achieve, the second stage may be performed as a Quasi-Static Analysis.
•Further restart analysis may also be performed if static Current Loads on the floater are being modelled, and/or the turbine needs to be yawed to a different orientation.
Once the static equilibrium configuration of the entire system has been achieved, the ambient environmental loads due to wind and waves may be defined, and a Dynamic Simulation may be performed.