OC4 DeepCWind Eigenanalysis/Static Equilibrium/Free Decay tests

Wind & Water Computer-Aided Engineering Software Tools
2

Dear Bertrand,

Here are my answers to your questions:

  1. A similar question was asked and answered in the following forum topic: The frequency component of flapwise - #4 by Jason.Jonkman. The new linearization functionality for floating offshore wind turbines mentioned in the post dated Nov 02, 2018 has now been submitted as a pull request into OpenFAST–see: github.com/OpenFAST/openfast/pull/350.

  2. Correct, but there is only one static-equilibrium position in the absence of external loading from wind and waves, so, only one simulation is required.

  3. Correct. It is always best to perform an eigenanalysis about an equilibrium position, so, load case 1.2 is really a prerequisite of load case 1.1.

  4. The difference is regarding which outputs are being reported for the code-to-code comparisons.

  5. The linearized natural damping can be derived directly from the linearization analysis / eigenanalysis. Free-decay tests are often used to derive the linear and quadratic (viscous) damping (via the so-called p-q analysis of the free-decay time series response).

  6. No, these natural frequencies do not look correct (assuming you are reporting values in Hz). The natural frequencies of the OC4-DeepCwind semisubmersible are reported in Figure 3 of the paper you attached.

Best regards,