Model Summary |
 
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Model Summary |
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The Flexcom model of the UMaine semi-sub is shown below.
UMaine semisub modelled in Flexcom
The floating platform is modelled using a series of discrete Lines to represent the 3 columns, pontoons and struts. These lines are connected up at appropriate points using a range of Equivalent Nodes to form a single coherent structure.
Strictly speaking it is not necessary to model the floating platform in such detail. A skeleton model which just includes key points of interest (such as the centres of gravity and buoyancy) would facilitate the application of concentrated loads. However, there are some advantages associated with creating a more detailed model - refer to Floating Body Modelling Detail if you are interested in further details.
Each line is assigned rigid Stiffness terms as the platform is assumed to act as a rigid body. All lines are assigned a Mass per Unit Length of zero (as the total mass is concentrated at the centre of mass) and a Buoyancy Diameter of zero (as the total buoyancy is concentrated at the centre of buoyancy). Each line is assigned a physical Drag Diameter, which allows Morison drag loads to be added to the standard radiation-diffraction excitation forces.
A Floating Body is defined which models the hydrodynamic characteristics of the floater. Hydrostatic Stiffness terms are used to simulate restoring forces and moments due to buoyancy. Added Mass, Radiation Damping and Force RAO coefficients are defined 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. Note that these inputs are derived separately from a radiation-diffraction analysis. Relevant Rotational Inertia terms are specified at the floating body centre of gravity, while the body's mass is represented by a Point Mass.
The tower is constructed using a single Line, with its lower end attached to the floating platform using an Equivalent Node. As the tower is tapered from base to top, it is modelled using several Line Sections of decreasing diameter. The finite element mesh density assigned to the tower ensures that each section is modelled using a single element. Note that a consistent mesh density is assigned to the aerodynamic model via the *TOWER INFLUENCE keyword. Realistic Stiffness and Mass per Unit Length terms are individually assigned to each tower section based on the material properties for steel and the relevant diameter and wall thickness at the section's mid-point. As the tower will not experience any hydrodynamic loading, the line is assigned zero values of Buoyancy Diameter and Drag Diameter.
The wind turbine control system is defined via the *SERVODYN keyword, which references the standard control DLL (ROSCO) provided by NREL for the UMaine semi-submersible. At low wind speeds the turbine is operating below rated power, so the rotor is allowed to rotate freely without any control in order to maximise power extraction. At intermediate wind speeds the turbine is fully operational, and the generator torque is used to control rotor speed while maximising generated power. At higher wind speeds the available wind power is above the rated power of the turbine, so blade pitch control is used to feather the blades and shed excess power.
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 floating platform using the Equivalent Nodes facility, while the lower ends are constrained using Fixed Boundary Conditions. The 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.