Regarding Seismic Load by FAST code

Wind & Water Computer-Aided Engineering Software Tools
1

Dear All,

Recently, I am focusing on the aerodynamic load of an FOWT subject to a prescribed displacement motion as a sine function. I found that the FAST code coupling to a seismic load module is very interesting, and it helps me to study on my topic. However, I am wondering the “ActDamp” variable that should be correctly defined in the input file (NRELOffshrBsline5MW_Onshore_Seismic.dat). According to the guideline of Seismic Loading for FASTmodule, this value is suggested as 60-70%. As my understanding, this value should be defined as 0% because my prescribed displacement motion is a sine function. Is it right? If no, how much is it in your opinion? In fact, I tried to investigate the effect of this variable on the aerodynamic loads, their magnitude is different.

I would be glad to hear your recommendation for this problem.

2

Dear ToanThanh.Tran,

I agree that the seismic routines of FAST v7 are not just useful for seismic analysis, but can be useful to prescribe the platform/base motion of any wind turbine.

I recommend that you use the suggested value of 60-70% critical damping, even for prescribed sinusoidal motion. As described in the seismic documentation (nwtc.nrel.gov/system/files/SR-5000-53872.pdf), the seismic routines do not modify the base motion directly, but indirectly, by applying a force to the platform through an actuator. Setting the actuator damping, ActDamp, to a value of 60-70% critical allows the actuator to function properly over a large bandwidth.

Best regards,

3

Dear Jason,

Thank you for your recommendation.

Best regards,