For my floating turbine, the tower first uncoupled mode (calculated in modal analysis) and the first coupled mode (calculated in a Campbell diagram) don’t match well in frequency. Why don’t the first coupled and uncoupled modes match?
For a fixed base turbine, the first tower attachment mode (the first uncoupled mode) often matches the first tower coupled mode frequency (from the Campbell diagram) well. This is because the attachment mode represents the displacement that occurs in the tower when a force is applied to the tower top and the tower base is fixed. This is close to what happens in reality for a fixed base turbine, and during a Campbell diagram calculation.
For a floating turbine with soft moorings, the deflection shape of the tower when a load is applied to the top in the Campbell diagram calculation doesn’t correspond to the first attachment mode, as the base is not fixed. So, the cantilever attachment mode shapes calculated in modal analysis are not seen individually in the Campbell diagram deflections. When a load is applied to the tower top of a floating turbine, the deflection in the tower will take a form that is a combination of various normal and attachment modes, so the first coupled tower mode will be a combination of various uncoupled modes.