The question is about the added mass part of this monopole FAST tower.dat input.

Did this tower input already account for the hydrodynamic added mass?

Since when the pile under water is vibrating, there should be an added virtual inertial force acting on the structure itself. I want to make sure do we need to add this part of mass to the structure and did the input tower file already take that part?

The FAST tower input file contains input for the distributed structural mass of the tower, but not the hydrodynamic added mass. The hydrodynamic added mass is defined with the other hydrodynamic inputs (in FAST v7.02, these reside in the FAST platform input file).

Thanks! This means if I want to use BModes to get the natural frequencies and mode shapes of my monotower, I need to combine the added mass and the structural mass below MSL in BModes input. If I use FAST linearization, I do not need to do that. Am I right?

Regarding BModes, I guess the answer to where the hydrodynamic added mass should be specified (included in the distributed structural mass or specified separately) depends on which version of BModes you are using. Please see the forum post found here for more information: http://forums.nrel.gov/t/tower-eigenfrequencies-of-nrel-5mw-turbine/517/1 (particularly my post dated Jan 21, 2013).

Regarding monopile foundation inputs, should we consider the foundation base the bottom of the ocean floor?
If that is so, Iâd assume PtfmRBHt=0 which would mean that TowerBsHt is a negative value.
The code will not allow a negative value for this.

Iâm not really sure I understand your question. What do you mean by PtfmRBHt? This is not an input in any NREL software.

A complete FAST v8 model of the NREL 5-MW turbine atop top OC3-monopile is available as Test18 in the CertTest of the FAST archive. In this model, TowerBsHt is set at 10 m above MSL because the monopile is modeled in SubDyn, not ElastoDyn. In SubDyn, the OC3-monopile is modeled from -20 m (the seafloor) to 10 m above MSL.

Iâm sorry, that was my mistake I meant the input âTwrRBHtâ.
I understand that monopile is modeled to -20m below MSL. However, is there a way to model the depth of the pile underneath the seafloor?
I think that would change the response of the monopile under loading.

I just wanted to clarify some geometric inputs for this turbine.
If you look at the image attached (common Monopile design), I have placed the inputs to the corresponding dimensions.
If this is accurate, why is there a constraint that PtfmRefzt must be smaller than TowerBsHt?
I feel that TowerBsHt would be too small.

When FAST is used to model an offshore wind turbine with a fixed-bottom monopile, ElastoDyn is used to model the tower, nacelle, and rotor and SubDyn is used to model the monopile. What ElastoDyn refers to as the âplatformâ is the rigid interface between the tower base and the top of the monopile (i.e., the platform roughly refers to the transition piece). Thus, I would expect TowerBsHt, PtfmRefzt, and PtfmCMzt to be identical. In fact, youâll see in the FAST model of the OC3-monopile provided as Test19 in the FAST CertTest, that TowerBsHt = PtfmRefzt = PtfmCMzt = 10 m. In your image, while the TowerBsHt is labeled correctly, PtfmRefzt and PtfmCMzt are not.

I am trying to find natural frequency of the whole structure of offshore wind turbine (Substructure + tower+RNA) in FAST v8 to check if it is within allowable natural frequency (1p and 3p soft-stiff).

But I was not able to find this natural frequency. I noticed that Subdyn calculates natural frequency of the substructure (platform) from sea bed to the interface level (bottom of tower) and in my case it is about 4.5 Hz. But i did not find the whole natural frequency. Does FAST consider internal water and added mass in natural frequency calculations?

I have read in a post in this forum that you recommended to compute the PSD (or FFT) of FAST time series and identifying the frequency peaks. Is there any other way to find the whole natural frequency?

I believe the forum topic you are referring to (http://forums.nrel.gov/t/natural-frequencies-with-fast-v8/1211/1) answers your question. Until the linearization functionality has been added to FAST v8, computing the PSD (or FFT) of FAST time series and identifying the frequency peaks is the most straightforward way of identifying full-system natural frequencies.

Thank you for your reply. According to your answer, I tried to compute FFT of FAST time series. What I have done:
I ran FAST with just hydrodynamic loading (White noise spectrum as you recommended in one of your posts) and I did not include wind. Then, I generated time history of for-aft displacement at top of tower. I computed the FFT of this time history and plotted. I have some doubts about it

1- Is my method OK? (fft of displacement at top tower (F-A) when we just generate wave spectrum (white noise))

2- I expected few frequency peaks (1st natural frequency around 0.29 , 2st around 1.3 , 3st around 3.5, and so on). But in the plot I can see the first natural frequency is 0.183 hrtz and it is different from what it should be and I canât tell other peaks (In case of not having filter).

3-Then, after searching I found out that I need to use low pass filter for my frequencies. But I noticed that it is very sensitive to filtering coefficients (order and âŚ). I changed (b) and (a) in butter command and the best response I got is from these values. But I am not sure it is right.

[b,a]=butter(16,2piTs,âlowâ);
Ts is my time step =0.01 sec

I uploaded time history and FFT plots with filtering and without filtering.

Could you please kindly have a look at them and give me some hints?

Iâm not familiar with the filtering approach youâve taken; if filtering is needed, I would typically filter the PSD directly rather than time series.

Iâm not sure where you are getting the frequencies of 0.29, 1.3, and 3.5 Hz. For the OC3-monopile model, I would expect the 1st fore-aft bending mode frequency to be around 0.25 Hz and the 2nd fore-aft bending mode frequency to be around 2.5 Hz.

Otherwise, your approach sounds OK. From your data, it doesnât look like your white noise extends beyond about 0.5 Hz, which is probably why you canât see the higher modes. Have you set the high cut-off frequency (WvHiCOff) in HydroDyn to be around 0.5 Hz (WvHiCOff is actually specified in HydroDyn in rad/s)? If so, I would increase this to a higher frequency to excite the higher-frequency modes. If you havenât already, I would also disable the 2nd-order waves (both sum and difference frequency) in HydroDyn to ensure that the white noise waves have equal energy across all frequencies of interest.

I just have a quick question regarding Platform mass in Elastodyn.

I noticed that in OC3-Monopile input files, platform mass and platform inertia for roll and pitch are 0. But in OC4-Jacket input files, they are non-zero (666 ton for PtfmMass).
I was thinking maybe 666 ton is weight of transition piece for the Jacket
What is exactly platform mass and when do we need to consider it?

When modeling fixed-bottom offshore wind turbines in FAST v8, the platform mass and inertia in ElastoDyn can be used to model heavy and rigid transition pieces that one would not want to model as a flexible body in either the ElastoDyn tower or SubDyn substructure models. In the OC4 jacket, for example, the substructure is connected to the tower via a large and heavy concrete block. For our FAST model of the OC4 jacket, we used the platform mass and inertia in ElastoDyn to model this concrete block.

Given the fact that I dont model transition piece (Actually in the area that I have transition piece, I add thickness of transition piece to thickness of monopile), platform mass is zero. But I have some doubts about PtfmCMzt and PtfmRefzt.

Could you please kindly tell me if it is correct to assume PtfmCMzt=14.8 and PtfmCMzt=14.8 according to my modelâs configuration?

Sorry about this question, I read NREL OC3 Monopile Questions. and also FAST manual. But the figure 20 in the manual is a bit confusing as tower base in lower than water level.

If you donât have any platform mass, the location of PtfmCMzt doesnât really matter. Also, PtfmRefzt should locate the logical connection point between tower base in ElastoDyn and the substructure top (interface nodes) in SubDyn. Per your graphic, setting PtfmRefzt to 14.8 m looks correct, assuming that the entire monopole (from the seabed to the tower base) is modeled in SubDyn.

I have a question regarding finding natural frequencies from time history results.

According to your suggestion, I ran FAST with just hydrodynamic loading (White noise spectrum as you recommended in one of your posts) and I did not include wind. Then, I generated time history of for-aft displacement at top of tower. I computed the FFT of this time history and plotted. It gives me natural frequencies in the range that I expected 0.25 hrtz (Please see the first figure).

But when I do this procedure on the time history results when I just have wind (for example turbulent wind with 12m/s), the first natural frequency lies around 0.06 hrtz that is much lower than the expected first natural frequency (Please see the second figure).