Working with Annotations

This example demonstrates how to effectively use the frequency annotation system in Remote OpenFAST Plotter for wind turbine analysis.

Understanding Frequency Annotations

Frequency annotations allow you to mark important frequencies in FFT plots, helping you:

• Identify important peaks and frequencies

• Document known system frequencies

• Create consistent labeling across analyses

• Compare observed frequencies with theoretical values

Creating and Managing Annotations

Follow these steps to work with frequency annotations effectively:

1. Accessing the Annotation System:

   • Navigate to the FFT tab

   • Locate the annotation controls in the sidebar

2. Adding Single Annotations:

   • Enter a frequency value (in Hz)

   • Provide a descriptive label

   • Optionally choose a color

   • Click “Add” to place it on the plot

3. Managing Annotation Sets:

   • Enter a name for your annotation set

   • Click “Save” to store the current annotations

   • Use the dropdown to select and load saved annotation sets

4. Applying to New Data:

   • Load different OpenFAST files

   • Load your saved annotation set

   • Compare peaks with your annotations

Example: Wind Turbine Frequency Identification

Here’s a practical example for a 5MW wind turbine operating at 12 RPM:

1. Calculate Key Frequencies:

   • 1P = 12/60 = 0.2 Hz (once per revolution)

   • 3P = 0.6 Hz (three times per revolution)

   • Tower first fore-aft: ~0.32 Hz (from modal analysis)

   • First blade flapwise: ~0.7 Hz (from modal analysis)

2. Create Annotations:

   • Add 0.2 Hz as “1P” (red color)

   • Add 0.6 Hz as “3P” (red color)

   • Add 0.32 Hz as “Tower FA” (blue color)

   • Add 0.7 Hz as “Blade Flap” (green color)

3. Save the Annotation Set:

   • Name it “5MW Standard Frequencies”

   • Click “Save”

4. Apply to Analysis:

   • Generate FFT plots for relevant signals

   • Look for peaks near the annotated frequencies

   • Identify potential resonance issues

Color Coding for Clarity

Establish a consistent color coding scheme for your annotations:

• Red: Forcing frequencies (1P, 3P, 6P, grid frequency)

• Blue: Tower natural frequencies

• Green: Blade natural frequencies

• Purple: Drivetrain natural frequencies

• Orange: Control system frequencies

This consistent color scheme makes it easier to visually categorize frequencies in complex plots.

Annotation Best Practices

Follow these guidelines for effective annotations:

1. Consistent Naming:

   • Use standardized abbreviations (1P, 3P, etc.)

   • Include mode number and direction (e.g., “1st Tower FA”)

   • Be consistent across projects

2. Frequency Precision:

   • Use appropriate decimal precision based on your analysis needs

   • For most wind turbine work, 2-3 decimal places is sufficient

3. Minimal Overlapping:

   • Don’t overcrowd the plot with too many annotations

   • Prioritize the most important frequencies

4. Documentation:

   • Include the source of frequency values (theoretical, modal analysis, etc.)

   • Document any assumptions made when determining frequencies

Example: Harmonics Analysis

Analyzing harmonics in a wind turbine:

1. Setup:

   • Load OpenFAST file with significant rotor imbalance

   • Calculate FFT for blade root moments

2. Create Harmonic Annotations:

   Assuming 0.2 Hz as the 1P frequency:

   • Add 0.2 Hz as “1P” (red)

   • Add 0.4 Hz as “2P” (lighter red)

   • Add 0.6 Hz as “3P” (red)

   • Add 0.8 Hz as “4P” (lighter red)

   • Add 1.0 Hz as “5P” (lighter red)

3. Analysis:

   • Look for higher amplitudes at 1P (mass imbalance)

   • Look for higher amplitudes at 2P (aerodynamic imbalance)

   • Check if any harmonics align with natural frequencies

Combining Annotations with Modal Analysis

For more advanced analysis:

1. Import Frequencies from Modal Analysis:

   • Use results from an external modal analysis tool

   • Create annotations for each significant mode

2. Compare with Observed Peaks:

   • Load turbine simulation data

   • Apply modal frequency annotations

   • Look for alignment or shifts between theoretical and observed frequencies

3. Document Findings:

   • Export plots with annotations

   • Note any significant discrepancies

   • Update your structural models if needed

Exporting Annotated Plots

To share your annotated FFT analysis:

1. Ensure Annotations are Visible:

   • Verify annotations appear correctly on the plot

   • Adjust colors and positions if needed

2. Export to HTML:

   • Click “Export FFT as HTML”

   • Save the file with a descriptive name

3. Include in Reports:

   • The exported HTML can be opened in any browser

   • All annotations are preserved

   • Interactive features remain functional