Changing water depth for OC4semi

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Tianhui.Liu
Posts: 25
Joined: Thu Feb 25, 2021 5:45 am
Organization: self
Location: hongkong,china

Changing water depth for OC4semi

Postby Tianhui.Liu » Fri Apr 16, 2021 5:07 am

Dear Jonkman
I am studying the motion response of OC4semi in different water depths.
I tried to change the water depth of OC4semi in openfast from 200meters to 100meters.( OC4deeepCwindsemi_hydrodyn).
1.png
1.png (9.83 KiB) Viewed 138 times

To make it work stably, I changed the data of moordyn(OC4deeepCwindsemi_moordyn) including increasing its diameter and changing its coordinates.
3.png
3.png (54.71 KiB) Viewed 138 times

I tried several sets of parameters, but the software kept reporting error like this.
“Catenary solver failed for one or more mooring lines. Using linear node spacing”
And at the end of report,
2.png
2.png (48.65 KiB) Viewed 138 times

It says Angles in GetSmllRotAngs() are larger than 0.4 radians.
I guess the moordyn is not set up correctly, but I don't know for sure how to change the data to solve this error.
Could you please point out where the problem might be?
And how can I make sure the settings are correct or what are rules for the parameters?

Best regards
Tianhui

Matt.Hall
Posts: 30
Joined: Tue Apr 05, 2011 2:41 pm
Organization: NREL
Location: Boulder, CO

Re: Changing water depth for OC4semi

Postby Matt.Hall » Fri Apr 16, 2021 10:04 am

Dear Tianhui,

The message “Catenary solver failed for one or more mooring lines. Using linear node spacing” is not actually an error, it is just a notice. MoorDyn tries to find the initial mooring line profiles using catenary equations, but this only works in certain conditions. In you case, the catenary equations may not work because the line length is quite large relative to the distance between the anchors and fairleads, which could cause the mooring line to have an "L" shape. This could be a design problem that needs adjustment, but it does not result in any error or problem with the MoorDyn model.

From the output you shared, it looks like your simulation is experiencing large motions and then hitting a failure point in AeroDyn. My first guess would be that this is simply a design problem. For example, the mooring system may be too slack and the platform is not stabilized enough, resulting in excessive motions.

Hope that helps,
Matt

Tianhui.Liu
Posts: 25
Joined: Thu Feb 25, 2021 5:45 am
Organization: self
Location: hongkong,china

Re: Changing water depth for OC4semi

Postby Tianhui.Liu » Fri Apr 16, 2021 9:37 pm

Dear Matt:
    Thank you for your answer. Your advice has helped a lot.
    I tried to make the line length shorter and the same error do not be in the report again.
But, I’m sorry I had overlooked this problem before:
4.png
4.png (19.79 KiB) Viewed 124 times

“There is no valid value of phi for these operating conditions! Vx=-2.817, Vy= 99.228, rlocal=15.857, theta= 0.20116”
Could you say why this error happened?
Thank you.

Jason.Jonkman
Posts: 5763
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

Re: Changing water depth for OC4semi

Postby Jason.Jonkman » Mon Apr 19, 2021 2:44 pm

Dear Tianhui.Liu,

The error regarding "no valid value of phi" has been discussed several times on this forum. In summary, upgrading from AeroDyn v15.03 in FAST 8.16 to AeroDyn v15.04 or upgrading from FAST v8.16 to OpenFAST will solve the problem.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov


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