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Model Building

Overview

The Flexcom model of the model-scale Stiesdal Tetraspar examined at the University of Maine’s test environment is shown below.

Hull & Keel

The hull is modelled using a series of discrete Lines to represent the central column (CC), radial braces (RB), tri braces (HT) and diagonal braces (DB). These lines are connected up at appropriate points using a range of Equivalent Nodes to form a single coherent structure. Similarly the keel is built using Lines to model the tri braces (KT). The hull and keel and then linked together using 6 keel lines (KL), and further equivalent nodes.

Each line is assigned rigid Stiffness terms as the hull and keel are assumed to act as rigid bodies, apart from the keel lines which are highly flexible. The hull and keel components are assigned a Mass per Unit Length of zero (as the hull and keel masses are concentrated at their respective centres of mass). Each line is assigned a representative Buoyancy Diameter and Drag Diameter consistent with their physical sizes.

Suitable Point Mass and Rotational Inertia terms are specified at the hull and keel centres of gravity. Morison drag loads are primarily derived from the component drag diameters, but some additional viscous drag is modelled using Point Buoys located at the hull and keel centres of mass.

Mooring System & Sensory Umbilical

The mooring lines are created using 3 separate Lines. The upper end of each mooring line is attached to the relevant fairlead node on the hull using Equivalent Nodes, while the lower ends are constrained using Fixed Boundary Conditions. The mooring lines (and keel lines) are modelled using the Truss Element feature, which is ideally suited to modelling chains and wires. Realistic Stiffness, Mass per Unit Length, Buoyancy Diameter and Drag Diameter terms are assigned to the lines. Point Masses are included at the mooring fairleads to account for the presence of data sensors.

Similarly, the sensory umbilical is modelled using a catenary line. Some of its mass is assigned to the hull and tower, while the majority is distributed evenly along the umbilical line as a Mass per Unit Length.

Further lines are included to model the effects of the yaw bridles.

Tower & Rotor

The tower is constructed using a single Line, with its lower end attached to the hull using an Equivalent Node. The tower properties (e.g. diameter) vary as a function of height so it is modelled using several Line Sections, but the tower mass is concentrated at its centre of mass (via Point Mass and Rotational Inertia terms) so the Mass per Unit Length of each tower section is set to zero. Realistic Stiffness terms are individually assigned to each tower section based on the definition document.

Finite element Nodes are explicitly created at the centres of mass of the nacelle and hub respectively. These nodes are then connected to the top of the tower using rigid massless Elements. The use of explicitly created nodes and elements is more convenient than using lines in such circumstances. A Rigid Blade Model is used to model the turbine blades, which are assumed to be rigid in all numerical models. Blade profiles are defined using the *BLADE GEOMETRY inputs, while the *TURBINE ROTOR associates the blades with the structural model.

An Auxiliary Profile is used to represent the rotating blades. While this has no structural function, it enhances the visual appeal of the model, and assists in the understanding of rotor and platform motions post-simulation.

All inputs which are required by AeroDyn to compute the aerodynamic loading on the blades and tower are are logically grouped together under the $AERODYN section. Fundamental inputs 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). These inputs should be intuitively familiar to engineers with some wind turbine modelling experience and you are referred to the keyword documentation should you require further information regarding the significance of any particular input.

The blades are kept at a fixed angle in all load cases, so there is no active control system in place and hence the *SERVODYN keyword is not present.