# Questions on quasi-static method moorings

Dear everyone，
I’m having a problem I’d like to know how to solve, the model I’m using is an IEA 15MW semi-submersible turbine, and I’m experiencing a strange phenomenon when running a regular wave no wind condition.
When I use the mooring method MAP++ in some cases there is a difference with MoorDyn and FEAMooring, from the PSD plot I can find a peak at the Pitch intrinsic frequency, I would like to know what is the cause of this and do I have any validation method?
The regular wave condition in the figure is a wavelength of 7.14 m, a wave period of 14.3 s, and a Pitch intrinsic period of 0.034 Hz
Best regards，

Dear @Yushun.Fu,

If I understand your question correctly, you want to know (1) why you get excitation at the pitch natural frequency when you use regular waves at a different frequency and (2) why the solution with MAP++ differs from the solutions with MoorDyn and FEAMooring? Is that correct?

Regarding (1), OpenFAST has inherent nonlinearities in its formulation, and so, there can be motion response at frequencies other than the wave-excitation frequency. Furthermore, there is always an initial condition solution that takes time to damp out where different modes can be excited due to imprecise initial conditions.

Regarding (2), MAP++ and MoorDyn/FEAMooring certainly have differences due their quasi-static versus dynamic formulation. For example, MoorDyn/FEAMooring can have inherent line damping and drag/added mass that MAP++ does not have.

Best regards,

Dear Jason,
I agree with (2), but I still have some questions about (1), I also simulated other regular wave conditions, and there is no Pitch intrinsic frequency effect, only in the case of wavelength is 7.12m period is 14.3s have this phenomenon.
I found that the frequency of the wave is 0.0699Hz, and the Pitch intrinsic frequency is 0.034Hz, is it because the wave frequency is close to the multiplier of the Pitch intrinsic frequency, I feel that my guess is a little bit reluctant, can I verify it by any method?

Best regards，

Dear @Yushun.Fu,

It could certainly be that regular waves with frequencies closer to multiples of the natural frequency would be more likely to excite the natural frequency than regular waves with frequencies that are further removed.

Best regards,

Dear Jason，