Changing water depth for OC4semi

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).
To make it work stably, I changed the data of moordyn(OC4deeepCwindsemi_moordyn) including increasing its diameter and changing its coordinates.

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,

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

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,

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:

“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.

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,

Hi Jason,
I am trying to upscale 5MW semisubmerible platform into a bigger platform that can support the IEA 15MW wind turbine. I have scaled the moordyn parameters, got the turbine file and scaled the hydrodyn by increasing the WAMITULEN factor correspondingly. But I am getting the same error “No valid value of phi for these operating conditions”. Also, I am seeing erratic behaviour in pitch and surge. Kingly assist me with this error.


Dear Ahmed,

I mentioned how to eliminate the error regarding “no valid value of phi” in my prior post; upgrading to OpenFAST is the easiest solution.

Increasing WAMITULEN will increase the hydrodynamic forces, but that is not the best way to upscale the platform. Modifying WAMITULEN does not impact the frequency-dependence of the potential-flow solution (frequency-dependent wave excitation and wave radiation added mass and damping), which would not be incorrect for sizeable changes in the platform scale. Instead, it would be better to rerun WAMIT to regenerate the potential-flow hydrodynamic data for the upscaled platform.

Regardless, it looks like your model is going unstable, with exponential growth in surge and pitch (and perhaps other DOFs). It is hard for me to guess what is causing that (likely a problem in how your model is set up), but as with any such problem, I would recommend to simplify the model to debug. E.g., is the model hydrostatically stable in still water, without aerodynamic loads or hydrodynamic forcing?

Best regards,

Hi Jason,
I tried doing the same as you advised and I am seeing good improvements in my results. I am using OpenFAST and starting my analysis with a simple setup. Thank you.

Ahmed Beg