2.6 Operational loads calculations

1. Open the Demo_a project (if not open)
In Bladed, File > Open > demo_a.prj (this project is located within the Bladed application folder, e.g. C:\DNV\Bladed 4.18\DemoModels\Demo_a\demo_a.prj).

2. View the minimum data requirements for the Steady Operational Loads calculation
Single-click the Steady Operational Loads in the Calculation window (Calculations window > Main calculations > Steady state calculations) to select it. Below you will see a list of the Minimum data requirements for the selected calculation.

3. View the calculation settings
Double-click the Steady Operational Loads calculation to open the settings window.

4. Run calculation
Click Run Now in the Calculations window.

5. Choose a Run Name and specify the location for the calculation output
In the Output settings window, try renaming the run using the Run Name field. Now select a new sub-folder in the directory (you can either select the results folder, or create a new folder by editing the file path and typing the new folder's name).

6. Start the simulation
Click OK to run simulation.

1. Open Bladed Results Viewer
You can use the Windows search feature to find and launch the software.

2. Add the calculation to the chart
Click the Add calculations button located near the left hand corner. Find the Steady Operational Loads calculation output file that you created earlier and click Open to add it.
Use the Output group dropdown to select the Tower Loads GL coordinates. Double-click the following to add them to the chart: Rotor speed, Electrical Power and the Mean pitch angle from the list. These will help you understand what the turbine is doing at the different wind speeds.

3. Plot Tower loads at the mud-line
Use the Output group dropdown to select the Tower Loads GL coordinates. Select the Tower My (fore-aft moment) from the list of variables. Select wind speed for the x-axis, then double-click -15.000 (this is at the mud line of the tower) for the Tower station heights.

4. View the behaviour of the tower bending moment vs. the wind speed
How does it compare to rated wind power, pitch angle and thrust force?

5. Plot the Tower loads at the base of the tower
Ensure the variable the Tower My is still selected in the list of variables but now plot double-click 15.000 (base of the tower) for the Tower station heights.
You should be able to see this follows a similar path but the thrust force is lower at the base of the tower vs. at the mud line.

6. Plot the blade root
If you are curious about why this is happening and wanted further confirmation, you can plot the blade root and see if the aerodynamic forces follow the same path. Use the Output group dropdown to select the Hub loads: rotating GL coordinates. Double-click the variable Blade root 1 My to add to the chart. The force is small but you should be able to see this follows a similar path to those of the tower loads.

Following the results of exercise 2.4, you would now like to investigate the possibility of constraining the rotor speed of the demo_a turbine so that the wind turbine tip speed stays below 73 m/s, but still reaches the desired 2 MW power.

Edit the Control parameters of the turbine so that the above requirement are fulfilled.

Launch a Steady Operational Loads calculation to look at what happens to the power curve and rotor speed regulation.

Can you explain what is happening to the tower bending moment My?

The simplest option is to directly act on the rotor speed in the Control window.
Use the ratio between the desired rotor speed and the starting one to maintain the desired power.

Also, change the wind speed step in the Calculation Parameters to 0.2 to capture the point where rotor speed becomes constrained.

Calculate the tip speed. Open the Control window and edit the maximum generator speed. You would need to change the control window to make sure you are reaching the 2 MW threshold.

Calculate the ratio between the desired tip speed and the starting one and multiply the Optimal mode max speed by it. Then, use the same ratio to upscale the Demanded gen torque and scale down the Demanded gen speed.

See the final column for the new settings.

Once you have the correct settings, run the simulation to verify the results with the 'Graph 1' (see the image below).

What is happening to tower bottom fore-aft bending load?
(see 'Graph 2' below)

Look at Tower My. The loading is related to the thrust force acting on the rotor. We kept the same Tip Speed Ratio (TSR) and therefore the same inflow until we cap the rotor speed (red circle in Graph 2). From that point onward, the TSR changes based on the wind speed, as you can also see from the change slope of the tower bending (thrust force).

Once then the rated power is reached (red dashed line in Graph 2), the pitch control loop becomes active. The thrust force in the rated power is lower on the 73 m/s tip speed turbine as we are moving away from the optimal power coefficient.