Hi Jason:

I changed the wave direction in HydroDyn, WaveDir, from 0 to 30 degrees and attempted to run a simulation and got this error:

[b]"FAST_InitializeAll:HydroDyn_Init:WAMIT_Init:All Wave directions must be within the wave heading

angle range available in “.\HydroData/IEA-15-240-RWT-UMaineSemi.3” (inclusive).

FAST encountered an error during module initialization.

Simulation error level: FATAL ERROR

Aborting OpenFAST."[/b]

Does this mean that I have to change the wave direction in the WAMIT output IEA-15-240-RWT-UMaineSemi.3 from 0 degrees to 30 degrees? I see in the second column of said output, some zero values, which I have assumed to be the wave direction.

Thank you.

Regards,

AOAW

Dear Andre,

The WAMIT data for the UMaine semisubmersible was only generated for zero-degrees wave heading. The HydroDyn module of OpenFAST will not allow you to specify the wave heading to be outside the range of the WAMIT data. So, you’ll have to run WAMIT (or an equivalent frequency-domain wave-body interaction potential-flow solver) for the range of wave headings that you want to simulate in OpenFAST/HydroDyn.

Best regards,

Hi Jason:

I would like to confirm if the wind direction from the east in InflowWind is 180 degrees and from the west 0 degrees; with south being 270 deg and North 90 deg. The +ve wind direction is clockwise viewing from above.

I have attached a sketch for clarity.

Thank you.

Regards,

AOAW

Dear Andre,

The InflowWind module does not use East-West, North-South convention, but I agree that the wind direction specified in InflowWind is positive clockwise when viewed from above (following meteorological convention). This is in contrast to most other OpenFAST modules that define the direction (e.g., yaw in ElastoDyn and the wave propagation direction in HydroDyn) to be about positive Z, which is counterclockwise when viewed from above.

Best regards,

Hi @Jason.Jonkman:

Two queries about HydroDyn Wave inputs;

- Is the wave directional spreading function WaveDirMod dependent on the wave direction?
- Can you please clarify the range of values of WaveDirMod? It says from 1: Cos2S. Is S the same as WaveDirSpread. ?
- Is there a range of values of WaveDirSpread?

I am attempting to change the wave directionality in my model and trying to understand how it is done.

Thank you.

Regards,

AOAW

Dear @Andre.White,

Here are my answers to your questions:

- When
`WaveDirMod`

= 1, a cosine spreading function is used centered about the specified (mean) wave direction, `WaveDir`

.
- Yes, the cosine spreading function involves cosine()^(2*S), where the wave spreading parameter, S =
`WaveDirSpread`

.
- The wave spreading parameter is likely site dependent, but you can find typical values in the literature.

For more information, see our related OMAE 2014 paper: https://www.nrel.gov/docs/fy14osti/61161.pdf.

Best regards,

Many thanks @Jason.Jonkman

AOAW

Hi @Jason.Jonkman I am looking at a wave only case for normal sea states at two locations, JCS (Jamaica Caribbean Sea) and SNS (Scottish North Sea). The sea states are at 9 m/s:2:15m/s. SS15 for the JCS has greater energy than SNS SS15 as seen on the wave energy spectrum. In surge all the motions for the JCS are greater then SNS expect for SS15 where surge for SNS is greater than surge for JCS. This seemed to have made sense to me. I therefore thought the same characteristic would have followed through for heave and pitch. However, for SS15 the pitch and heave motions are greater for the JCS even though that wave has less energy. This not what I was expecting. I even thought it might have been due to a larger wave height for the JCS but that is not the case as the wave height for SNS is greater. I have attached the plots in surge, heave and pitch.

**What do you think the reason for the larger heave and pitch values even though smaller wave energy?** No sure about this.

Also I have looked at the tensions at the fairleads. At fairlead 1 (the mooring line taking the oncoming wave) the tension is greater for SS15, SNS. I figured this might have been due to greater wave energy, producing greater the surge motion and therefore greater tension. However, for fairleads 2 and 3, this is not the case, the tensions are the same at SS15. I was also expecting tensions in fairleads 2 and 3 to be significantly less than that of tension in fairlead 1 but this is not the case; maybe it applies only when there is wind. I have also attached plots.

Would be very grateful for your thoughts as I am bemused at the moment.

Thank you.

Regards,

AOAW

Dear @Andre.White,

The plots of heave and pitch did not come through. Regardless, how does the wave energy compare *at* the natural frequencies of surge, heave, and pitch? Also, are second-order hydrodynamic terms considered, which can have a large affect at the floater natural frequencies?

The mean and max plots for fairlead #2 and #3 are a bit hard to understand as there appears to be two distinct values in each plot; perhaps an output precision issue?

I’m not sure I can provide further comments without knowing more about your FOWT configuration.

Best regards,

Hi @Jason.Jonkman the floater is the VolurnUS-S system. I know you have not done a lot of work with it. Therefore, I was only seeking to understand the principle behind why my results appeared that way.

I have modified the VolturnnUS-S from 200 m to 100 m deep water. The natural frequencies in surge, heave and pitch are 0.01, 0.049 and 0.034 respectively. The wave natural frequency for JCS is 0.121 and for SNS 0.138. I allowed for the full difference frequency second order forces on the platform, DiffQTF but not the full SumQTF.

I have now attached plots for heave and pitch motions.

I am not sure what you mean by 2 distinct values for fairlead #2 and fairlead # 3. But essentially, the mean values of fairlead # 2 and fairlead # 3 are the same. I thought this is what was expected as unlike farilead #1 they are not in the direct path of the incoming wave.

Let me know if you have any further thoughts with this additional information.

Thank you.

Regards,

AOAW

Dear @Andre.White,

OK, thanks for clarifying which FOWT you are using.

It looks like you’ve now shared the max pitch, but not the mean pitch or mean or max heave.

By “two distinct values”, I meant that each of the mean and max fairlead tension plots show two values–one at low sea state and one at high sea state. For example, the mean fairlead 2 tension is 2.07x10^6 at low sea state and 2.08x10^6 at high sea state; likewise for the other tension plots but with different low and high values. This looks odd to me.

Best regards,

Hi @Jason.Jonkman sorry about heave and pitch plots. Sending again ( 4 plots).

Noted. If I understand you correctly, you are saying that the mean fairlead 2 tension at low and high sea state are essentially the same.

Regards,

AOAW

Hi @Jason.Jonkman just to clarify are you saying that the fairlead tensions at low and high sea state are practically the same and this looks odd because there should be greater difference between the values at low and high sea state?

Regards,

AOAW

Hi @Andre.White,

I’m just surprised that different low and high sea states result in the same exact tension; I would expect the curves to be smoother.

Best regards,

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