# DLC 6.2 Results Issue for 15MW RWT Fixed-Bottom Structure **URL:** \ **Category:** Computer-Aided Engineering Software Tools\ **Created:** [July 21, 2025, 2:22am UTC](https://forums.nlr.gov/t/dlc-6-2-results-issue-for-15mw-rwt-fixed-bottom-structure/9110 "2025-07-21T02:22:25Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![Jisu.Lim](https://avatars.discourse-cdn.com/v4/letter/j/a88e57/32.png) [@Jisu.Lim](https://forums.nlr.gov/u/Jisu.Lim)\ **Post date:** [July 21, 2025, 2:22am UTC](https://forums.nlr.gov/t/dlc-6-2-results-issue-for-15mw-rwt-fixed-bottom-structure/9110/1 "2025-07-21T02:22:26Z")
**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.) ![image](https://europe1.discourse-cdn.com/nrel/original/2X/3/3e2d42ab9c1939bd8c22a295fc925eb7ad2b9fb5.jpeg) 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. ![image](https://europe1.discourse-cdn.com/nrel/original/2X/a/a27df59f7d170a1e02961f6fea72eca28df621d4.jpeg) **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 ---
**Author:** ![Jason.Jonkman](https://dub1.discourse-cdn.com/nrel/user_avatar/forums.nlr.gov/jason.jonkman/32/3086_2.png) [@Jason.Jonkman](https://forums.nlr.gov/u/Jason.Jonkman)\ **Post date:** [July 21, 2025, 3:25pm UTC](https://forums.nlr.gov/t/dlc-6-2-results-issue-for-15mw-rwt-fixed-bottom-structure/9110/2 "2025-07-21T15:25:15Z")
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: > [@Designing for yaw errors using FAST](https://forums.nlr.gov/t/designing-for-yaw-errors-using-fast/1051): > > Hello, I am trying to design monopiles for the NREL baseline 5MW turbine using site specific hurricane hazard according to IEC 61400-3 load case 6.2a, which requires the consideration of all yaw errors from -180 to +180 degrees. I have read some literature mentioning that there is an “instability” that occurs for the NREL baseline turbine at around +35 degrees. This is described as an “aero-elastic interaction causing negative damping in a mode that couples rotor azimuth with… > [@Wind Veer Parameter](https://forums.nlr.gov/t/wind-veer-parameter/2103): > > Hi, I am trying to recreate the following study for a turbine at SWiFT: “Assessment of Wind Parameter Sensitivity on Ultimate and Fatigue Wind Turbine Loads: Robertson, Sethuraman, Jonkman, Quick (2018)”. I will be using TurbSim → OpenFAST and examining different parameters than those in the above study, but I did want to examine veer (β in the paper). I grepped all the input files and found nothing. I also didn’t find anything in the TurbSim Manual. How can I change the veer for these simulat… > [@Natural frequencies and mode shapes](https://forums.nlr.gov/t/natural-frequencies-and-mode-shapes/2545): > > Hello, I am currently working on a thesis’ project which includes the computation of campbell diagram, mode shapes and modal dampings of the blades of a 5MW NREL reference wind turbine. I would like to do so using the full model of the machine on OpenFAST. I have already managed to compute the power curve and the Cp-TSR curves of the rotor but I don’t know how to compute the natural frequencies, mode shapes and modal dampings at different wind speeds. Can you please show me how to do it if it’… > [@Tower strike with latest versions of OpenFAST and IEA 15MW](https://forums.nlr.gov/t/tower-strike-with-latest-versions-of-openfast-and-iea-15mw/3024): > > Hi forum, I am have been working with OpenFAST and the IEA 15MW RWT on the UMaine semisubmersible platform since May 2021, and this week, when I tried to update my software and model to v3.1 of OpenFAST and v1.1 of the turbine, my initial test simulations ended almost immediately with tower strikes. I wanted to report this issue and hear any advice others may have. For OpenFAST v3.1, I downloaded the Windows 64-bit executable from the [OpenFAST GitHub releases page](https://github.com/OpenFAST/openfast/releases), and for the IEA 15MW model, … > [@High angle of attack at large wind speed](https://forums.nlr.gov/t/high-angle-of-attack-at-large-wind-speed/6175): > > Hi, everyone I’m performing 10-min simulation with NREL 5MW turbine based on 5MW\_Land\_DLL\_WTurb.fst with NTM condition in the wind speed range of 3m/s-25m/s. When the simulation is performed in large wind speed (e.g. 25m/s), I got the following warnings: FAST\_Solution:CalcOutputs\_And\_SolveForInputs:SolveOption2:AD\_CalcOutput:RotCalcOutput:BEMT\_CalcOut put(node 5, blade 1):UA\_CalcOutput:UA\_BlendSteady:Temporarily turning off UA due to high angle of attack or low relative velocity. This warnin… > > > hello everyone， > I am simulating the idle shutdown state with normal turbul…ent wind files with wind speeds of 40m/s and 45m/s, grid size (X=284, Y=284), which can be calculated when PropagationDir=0° and PropagationDir=270°. However, when the wind field is deflected counterclockwise by 30° (PropagationDir=330°), it warns of "violation of the small angle assumption" and then reports the error "GF wind array boundary violated: grid is too small, Y direction." Y=142.03;Y boundary=\[-142,142\]. When the wind field is deflected 60° counterclockwise (PropagationDir=300°), an error is reported that”TwrInfl:Tower strike“. Is there any solution? Thank you very much！ > **[Unable to reproduce the tower side-side instability in DLC 6.2 · OpenFAST...](https://github.com/OpenFAST/openfast/discussions/2306)** > > Dear @jjonkman , I'm trying to study the tower stability problem recently. As mentioned in your Ph.D dissertation "Dynamics Modeling and Loads Analysis of an Offshore Floating Wind Turbine", Tower ... > > > Hi all, > > I am currently comparing results between OpenFAST and Bladed for FOWT… through the IEA 15MW wind turbine atop UMaine platform. But I found some differences existed in the platform pitch and roll movements under some specified condition like parked with an azimuth angle of 50 degree. > > In the following picture, the results were obtained under still water, steady wind with no wind sheer and the wind direction is -30°. The turbine was parked and the azimuth angle is 50°. The unsteady aerodynamic or dynamic stall were both closed in Bladed and OpenFAST > !\[image\](https://github.com/user-attachments/assets/890f43d0-23dd-440e-aba9-ed995aca03aa) > > It seems like Bladed and OpenFAST show different damping effects. > > Do you have any ideas or experiences about this kind of problem? Is this related to aerodynamics or hydrodynamics? > > Thanks! > Chao Li Best regards, ---
**Author:** ![Jisu.Lim](https://avatars.discourse-cdn.com/v4/letter/j/a88e57/32.png) [@Jisu.Lim](https://forums.nlr.gov/u/Jisu.Lim)\ **Post date:** [July 22, 2025, 8:36am UTC](https://forums.nlr.gov/t/dlc-6-2-results-issue-for-15mw-rwt-fixed-bottom-structure/9110/3 "2025-07-22T08:36:00Z")
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 ---
**Author:** ![Jason.Jonkman](https://dub1.discourse-cdn.com/nrel/user_avatar/forums.nlr.gov/jason.jonkman/32/3086_2.png) [@Jason.Jonkman](https://forums.nlr.gov/u/Jason.Jonkman)\ **Post date:** [July 22, 2025, 12:33pm UTC](https://forums.nlr.gov/t/dlc-6-2-results-issue-for-15mw-rwt-fixed-bottom-structure/9110/4 "2025-07-22T12:33:32Z")
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, ---
**Author:** ![Jisu.Lim](https://avatars.discourse-cdn.com/v4/letter/j/a88e57/32.png) [@Jisu.Lim](https://forums.nlr.gov/u/Jisu.Lim)\ **Post date:** [July 23, 2025, 2:05am UTC](https://forums.nlr.gov/t/dlc-6-2-results-issue-for-15mw-rwt-fixed-bottom-structure/9110/5 "2025-07-23T02:05:37Z")
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