Sum Frequency QTF

Calculation of second order wave forces from full Sum-Frequency Quadratic Transfer Function (QTF). The Sum-Frequency QTF gives forces oscillating at the sum of pairs of wave frequencies in the wave spectrum. It can therefore be important for structures and modes of motion with a high natural frequency, such as the heave motion of a tension leg platform.

Sum frequency forces together with difference frequency forces from Difference Frequency QTF gives the full second order wave forces, as described in Second-order wave forces.

From SIMA 5.2, the Sum Frequency QTF model is changed. The legacy QTF model from SIMA version ≤ 5.0 continues to be supported for backwards compatibility. Users can choose to convert to the new model by right-clicking the QTF model in the model tree and choosing Export/Convert QTF. You will see a warning message in the model tree if you have a legacy QTF model in your SIMA workspace.

The new QTF model refers to QTF data stored on an external file, which can drastically reduce the size of the SIMA workspace and gives overall efficiency improvements in both SIMA and SIMO.

When hydrodynamic data is imported from a SIF file, a new model and associated QTF file is created automatically. However, when hydrodynamic data is imported from WAMIT files, the legacy model is currently created, and it is thus recommended to convert to the new model after importing.

In addition to changing the storage method, the new model corresponds to a new implementation in SIMO. The new implementation will give slightly different results for the following reasons:

  • Different interpolation methods

    • The new implementation uses a combination of linear (close to the main-diagonal) and bi-linear interpolation in the bi-frequency domain

    • The legacy implementation uses a two-step interpolation where step 1 creates a dense QTF by resampling the QTF data with bi-linear interpolation, and step 2 use "nearest neighbor" interpolation on the dense QTF

  • The new implementation applies forces in the low-frequency body-related coordinate system (following low-frequency yaw motions), while the legacy implementation applies forces in the body-related coordinate system (following total yaw motion). Note that both implementations does heading correction based on the low-frequency yaw motion.

The computed force time series can be stored by selecting the Store sum frequency wave force option in the storage section of Dynamic Calculation.

The new implementation also comes with other improvements:

  • Body symmetry is now utilized to reduce the number of relative wave headings required in the QTF, and reduce the number of pregenerated time series accordingly

  • The new model avoids generation of forces for vessel headings never experienced ("on-demand" pre-generation)

  • General efficiency improvements