Unavoidable platform oscilation without wind and waves

Wind & Water Computer-Aided Engineering Software Tools
1

Dear all,

I am using FASTv8.16.00.a with Matlab/Simulink and I realize that it exist an unavoidable platform oscilation without wind and waves inputs. I am using NREL 5MW electrical engine and ITI_Barge platform. I attach my results. I tried with all Simulink solver posibilities and it still there. It is a numeric error? How can I remove them?

Thank you in advance,
JOE

EnParado.png
EnParado.png1264×920 17.9 KB

2

Dear JOE,

The small overhanging weight of the rotor-nacelle assembly (offset from the centerline of the tower) will cause the tower to deflect and the platform to surge/pitch even in the absence of aerodynamic or hydrodynamic loads.

Best regards,

3

Dear Jonkman,

Thank you for your fast reply.
I agree with that. And I tested the system with a starting platform inclination and it displays logic result. But why is not damped this oscillation? It looks like constant amplitud for more than 1000s. Are not enough the platform damping coefficients?

Best regards,
JOE

4

Dear JOE,

Without aerodynamics, the only platform-pitch damping in this model comes from hydrodynamics. The hydrodynamic damping in pitch for the ITI Energy barge model comes only from radiation damping, which may be quite small at the platform-pitch natural frequency.

Best regards,