DLC 6.2 Results Issue for 15MW RWT Fixed-Bottom Structure

[Jisu.Lim]

To all NREL Forum users,

Hello. I am currently building a numerical model of a fixed-bottom offshore wind turbine system using the 15MW RWT in OrcaFlex and performing DLC analyses.

While running simulations for DLC 6.2, I encountered some unexpected results and would like to ask the community for insights.

Here are the details of my inquiry:

The analysis is based on DLC 6.2 with a 30-degree yaw misalignment. The wind and turbine heading directions are as shown below.

(The substructure modeling has been implemented, though the geometry is not shown in below figure.)

However, in reviewing the results from a 3600-second simulation, I found that the Tower Base Moment (Mz) continues to diverge over time. The Y-axis range for Mz reaches from -600×10³ to +800×10³ kN·m, as shown in the attached plot.

Have any other users experienced similar results when modeling fixed-bottom structures using 15MW RWT?

Could this possibly be related to resonance frequencies of the overall support structure?

I would really appreciate any feedback or suggestions regarding what might be causing this diverging behavior in the simulation results .

Thank you very much for your time and help.

Best regards,
Jisu Lim

[Jason.Jonkman]

Dear @Jisu.Lim,

While I doubt this forum can offer specific guidance on OrcaFlex (I would suggest reaching out to Orcina Technical Support for that), I expect that you are running into the known blade edgewise / tower side-side instability that happens under parked/idling conditions under high winds and sizeable yaw error, which has been discussed in other topics on our forum:

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Best regards,

[Jisu.Lim]

Dear Jason Jonkman,

Thank you for your prompt response to my inquiry.

I have reviewed the related discussions you mentioned.

The most convincing explanation seems to be the one covered in “Designing for yaw errors using FAST.”

However, the content there specifically addresses the NREL 5MW model, focusing on the cause of the issue and possible solutions.

Does a similar instability also occur in the 15MW RWT OpenFAST model under certain yaw error conditions?

Additionally, when instability occurs in the 15MW RWT model, is it reasonable to apply the same assumption as in “Designing for yaw errors using FAST”—that such an error would not occur in real-world physical conditions?

I sincerely appreciate your continued support and helpful responses to my questions.

Best regards,
Jisu Lim

[Jason.Jonkman]

Dear @Jisu.Lim,

Yes, this instability will happen for different wind turbines–including the IEA Wind 15-MW RWT–perhaps with different levels of severity and different yaw error ranges, depending on the aero-elastic properties of the turbine.

The question is still open about whether this instability would occur in reality, but that is often assumed.

Best regards,

[Jisu.Lim]

Dear @Jason.Jonkman ,

Thank you once again for always providing prompt responses to my inquiries.

I fully understood the content of your last reply, and I will look into additional references to develop an appropriate analysis strategy for the DLC6.X simulations.

Thank you for your response to my inquiry, and I also sincerely appreciate the valuable support you continue to provide for offshore wind development.

Best regards,
Jisu Lim