Waves no effect on tower displacement

Hi,
I was trying to run FAST for the ‘NREL Offshore Baseline 5MW Monopile_RF model’, I download the Baseline file and was hoping to keep it the exact same, but in order to run the model i had to change a few things,
i got the error
‘An adams control file for a linear analysis cant be built when using incident wave kinematics’
So I turned adams off by setting adams preproccer to 1 (from 3)
I also had to turn pitch control
and variable speed control both to zero

Then created a wind file using turbsim and changed the setting in ‘NRELOffshrBsline5MW_AeroDyn’
so look at my created wind file.

At this stage i could run FAST simulations ok.

I was just hoping just to run a few simulation record the wind load on the blade and wave load on the tower and look at the deflection of the blade and tower in the out-plane flapping direction.

I noticed however that when I change ‘wave height’ or ‘wave period’ in the ‘NRELOffshrBsline5MW_Platform_Monopile_RF.dat’ file
the (WaveElev) Wave elevation at the platform changes (gets bigger for bigger wave heights)
but the displacement at the top of the tower (“TTDspFA”) stays the same,
as if the increased wave force has no effect on the tower displacements

When FAST is running its states
‘generating incident wave kinematics and current time history’
so is the anyway to check if this wave force is actually being applied to the tower?

Is there a OutListParameter for wave force on the tower acting on the tower to see if this changes (with wave height or period)
also is there an outlistparameter to get the wind force acting on the blade?

Regards
Michael Harte

Dear Michael,

The reference monopile for the NREL 5-MW baseline turbine is quite stiff, so, the wave loads may only have a small influence on the tower deflection. My guess is that the aerodynamic load-induced motions are dominating the wave load-induced motions in your simulation, which is why are you not noticing any differences.

The applied wind and wave loads are not outputs that are currently available from FAST. (In AeroDyn, however, you can output the applied wind loads at various stations using the PRINT/NOPRINT option – see the AeroDyn User’s Guide for more information.) In FAST, the reaction loads within the blades and tower are available (at the blade sections or root and at the tower sections or base). You should be able to easily see the influence of the wave loads in the tower reaction loads.

By the way: You shouldn’t have to disable the torque and pitch controllers. See the forum topic found here for my answer to a related question: http://forums.nrel.gov/t/fast/348/1.

Best regards,

Dear Jason,

First off, thanks for the quick response.

Thanks for the link i used that version of FAST and the controllers now work!

I did as you suggest and looked at the forces on the tower
here the effects of the wave load could be seen.

Im surprised by the small effect of the wave load on the tower top vibrations,
i thought the wave loads would show up more strongly in the tower deflection,
and therefore thought i was doing something wrong, some how not applying the wave load,
but thanks for sorting that out for me.

Regards
Michael Harte

Hi,

I am working on the reliability estimation of offshore 5MW monopile wind turbine tower. I have to obtain bending moment at the mud line, with increasing wave height at particular wind speed. While I was increasing wave height, the value of bending moment didn’t increase. I just changed wave height and ran the simulation. Do I need to change other parameters as well?

Dear Jharna,

I would expect increasing wave height to effect the mudline bending moment. What version of FAST are you using? What output are you looking at?

Best regards,

I am using FAST V8 for my simulation. I am looking at bending moment at mud line for my analysis. So, the problem is if I increase wave height, for instance, 6m, the bending moment comes out to be lesser than or nearly equal to the bending moment when wave height is increased to 12m or more.

Regards,
Jharna

Dear Jharna,

I’m not aware of any bugs in the outputs of SubDyn for the bending moment at the mudline. I’m assuming you are looking at SubDyn output ReactMYss? Are you employing the Craig-Bampton reduction in SubDyn; how many modes are you enabling; have you enabled the static improvement method?

Best regards,

Dear Jason,

I might be lacking some parameters that I am not aware of, but I have attached my SubDyn input file here. Please let me know if I have to change anything in the file.
Yes, I am looking at ReactMYss as well as M1N1MKye.

Regards,
Jharna

Dear Jharna,

I don’t see anything incorrect with your SubDyn model, although the beginning and end appear to be cut off e.g. I don’t see ReactMYss in your OutList. I do see that you don’t have any Craig-Bampton modes enabled, resulting in effectively a Guyan reduction, but you do have the static improvement method enabled, which should be OK as long as all modes are of very high frequency.

FYI – The outputs related to M1N1 are located at -40 m, not -20 as stated, based on your joint definitions.

Do you have all six platform degrees of freedom enabled in your ElastoDyn input file?

Best regards,

Dear Jason,

Yes, all six platform degree of freedom is enabled in my ElastoDyn input file.
I took a screenshot of my SubDyn input file previously, but I do have ReacMYss in my SSOutList.
Do you think my HydroDyn input file is missing something?

Regards,
Jharna

Dear Jharna,

I can’t think of what the problem might be. Can you share some of your plots? Do you clearly see the influence of the wave load on ReactMyss (e.g. a periodic load for regular wave input), but don’t see the effect change when changing the WaveHs?

Best regards,

Dear Jason,

I have attached the graph showing the value of ReactMYss versus time for different wave height. I could not understand why moments at mud line for wave height of 18m is less than that for wave height 12m.

Dear Jharna,

Are all of the simulations you’re plotting identical except for WaveHs?

The reaction load is tied not only to the applied load, but also the dynamic response of the monopole, so, I can see how the higher wave height will not always yield a higher load (at an instantaneous time). This issue is likely exaggerated by the use of irregular waves. I suggest you make the same comparison with regular waves to see if the result is more like you’d expect it.

Best regards,