Model Summary |
 
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Model Summary |
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The Flexcom model of the OC4 jacket is shown below.
Flexcom model of OC4 Jacket
The jacket is modelled using a series of discrete Lines to represent the following items. Refer to the jacket schematic for an illustration of the various components. So in total 100 lines are used to model the jacket alone. This makes model construction somewhat tedious, but as a consistent naming convention is followed, you should not have any difficulty in interpreting this model, or indeed in building your own jacket model if required.
•Legs from mudline to top of piles (4 lines)
•Legs from top of piles to mud braces (4 lines)
•Legs from mud braces to level Y bottom (4 lines)
•Legs from level Y bottom to level K3 (4 lines)
•Legs from level K3 to level K2 (4 lines)
•Legs from level K2 to level K1 (4 lines)
•Legs from level K1 to level Yupper (4 lines)
•Legs from level Yupper to tower base (4 lines)
•Braces from level Yupper to level X1 (8 lines, named B1 to B8)
•Braces from level X1 to level K1 (8 lines, named B9 to B16)
•Braces from level K1 to level X2 (8 lines, named B17 to B24)
•Braces from level X2 to level K2 (8 lines, named B25 to B32)
•Braces from level K2 to level X3 (8 lines, named B33 to B40)
•Braces from level X3 to level K3 (8 lines, named B41 to B48)
•Braces from level K3 to level X4 (8 lines, named B49 to B56)
•Braces from level X4 to level Y bottom (8 lines, named B57 to B64)
•Mud Braces (4 lines, named MB1 to MB4)
Note that these lines are connected up at the intersection points using Equivalent Nodes to form a single coherent structure. This is an essential part of the model definition in Flexcom.
Each line is assigned appropriate Stiffness and Mass Density terms, plus a Buoyancy Diameter (to allow computation of buoyancy forces) and a Drag Diameter (to facilitate computation of Morison drag loads).
The tower is constructed using a series of vertical Lines, corresponding to tower sections described in Vorpahl et al. (2011). Each section has uniform diameter and wall thickness (as opposed to tapering sections). The tower sections are assembled together, and connected to the transition piece at the bottom and RNA at the top, using Equivalent Nodes. Note that a consistent mesh density is assigned to the aerodynamic model via the *TOWER INFLUENCE keyword. Realistic Stiffness and Mass Density terms are 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. Flexcom's traditional Buoyancy Diameter and Drag Diameter are unused, but a relevant drag diameter is assigned to each tower aerodynamic node via the *TOWER INFLUENCE keyword. Point Mass terms are placed at the top, midpoint and bottom elevations of the tower to represent fl anges, bolts and equipment installed on the tower.
The transition piece is a rigid concrete block in reality. It's weight is modelled as a Point Mass in the Flexcom model, and its rigidity is simulated by a series of rigid massless Elements which connect the upper ends of the jacket legs to the base of the tower. The use of explicitly created elements is more convenient than using lines in such circumstances. These elements have zero values of Mass per Unit Length but high Stiffness terms to simulate rigidity. An Auxiliary Profile is used to display the concrete block.
The wind turbine control system is defined via the *SERVODYN keyword, which references the standard control DLL provided by NREL for the OC4 jacket. 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.