Dear Prof. Jonkman,

Thanks a lot for your reply!

I have some confusion about the coupled analysis results. I performed a Furior transformation of the member internal forces of the sub structure. I found two peaks in the frequency domain, one is 0.122Hz and the other is 0.365Hz. The former is the wave frequency and the last one I think it should be the integrated structure frequency, but it didn’t agree with my modal analysis results. I performed a modal analysis of the integrated structure by using other finite element software in the consideration of water depeth marine growth and so on. The modal analysis result is 0321Hz.

In order to vertify the results, I analyzed the integrated model separately.

(1) Blades + Hub + Nacelle + Tower(fixed)

1st fre result(Hz)

FAST V7, 0.378Hz;

FAST V8, 0.377Hz;

FE Software, 0.379Hz.

(2) Pentapod sub structure(fixed)

1st fre result(Hz)

FAST(Subdyn), 1.520Hz;

FE Software, 1.508Hz.

(3) Integrated structure

1st fre result(Hz)

FAST V8, 0.364Hz;

FE Software, 0.321Hz.

I enabled all the platform DOFs in the FAST, I can not think out why the FAST V8 result is higher than the FE result, whether the interface joint DOFs effect the result? The interface joint is fixed in the analysis according to the SubDyn User Manuel.

I also have some questions about the parameters of PtfmRIner PtfmPIner and PtfmYIner. I refered to the Test 20 of NREL 5MW OWT with tripod sub structure, in the test the mass center is also the platform reference point, the values of PtfmRIner and PtfmPIner are zero, the PtfmYIner’s value is 1.534E+06.

I choose the pentapod as the OWT subtructure. The pentapod sub structure is a symmetrical structure like the tripod sub structre, the only difference is the pentapod has five pile legs. Beacurse the pentapod and the tripod are symmetry, so the mass center is in the centerline of the structure. In my opinion, the PtfmRIner and PtfmPIner values are non zero, but the PtfmYIner should be zero.

Could you give me some instructions about these two questions?

Thanks a lot for your help!

Best regards,

Wenhua Wang