First- and second-order excitation loads

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Dear @Busra.Yildirim,

Here are my responses to your questions:

  1. QTFs are not dependent on the sea state such that, once you’ve generated the QTFs, you can apply them to any sea state. Unlike the first-order solution, the QTFs do depend on the mass and stiffness matrix. The mass matrix should be independent of the environmental conditions. Depending on the nonlinearity of the mooring system, the stiffness matrix could depend on the mean displacement of the floater (which is wind speed dependent), but we have chosen to use the undisplaced condition to define the stiffness matrix and have not looked at the sensitivity of the QTF to stiffness changes. The QTF could also depend on an external damping, which you could say comes from aerodynamic or viscous effects, but I would say that this would partially double account those effects that are already considered in the time domain simulation. So, in some ways, the QTF solution is only approximate, but from our experience, the QTF tends to underpredict the actual resonant excitation, so, I would think adding damping would worsen the result.
  2. As in (1), the QTFs provided by NREL for the OC4-DeepCwind semisubmersible depend on the mass and stiffness derived from the undisplaced condition. No external damping was considered and the sensitivity of the stiffness to displacement was not considered.
  3. You can derive the mass and stiffness through an OpenFAST linearization analysis, but as discussed above, it should be sufficient to linearize in the absence of wind and wave forcing.

Best regards,