# Natural frequency of NREL DS monotower in OC3.

**URL:** <https://forums.nlr.gov/t/natural-frequency-of-nrel-ds-monotower-in-oc3/845>\
**Category:** Structural Analysis\
**Created:** [February 26, 2014, 6:11pm UTC](https://forums.nlr.gov/t/natural-frequency-of-nrel-ds-monotower-in-oc3/845 "2014-02-26T18:11:47Z")\
**Posts on this page:** 1\
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**Author:** ![Lingling.Yin](https://avatars.discourse-cdn.com/v4/letter/l/cab0a1/32.png) [@Lingling.Yin](https://forums.nlr.gov/u/Lingling.Yin)\
**Post date:** [February 26, 2014, 6:11pm UTC](https://forums.nlr.gov/t/natural-frequency-of-nrel-ds-monotower-in-oc3/845/1 "2014-02-26T18:11:47Z")

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Dear Jason,

I am looking at using FAST to model monopile support structure for offshore wind turbines. Before doing the compilation of FAST I would like to reproduce the 1st natural frequency of the support structure (0.25) from NREL report “Offshore Code Comparison Collaboration (OC3) for IEA Task 23 Offshore Wind Technology and Deployment”.

I construct a NREL\_DS FAST model based on discussion in [Compile FAST to model distributed spring model for monopile.](http://forums.nlr.gov/t/compile-fast-to-model-distributed-spring-model-for-monopile/768/1) to include the soil-pile interaction. In this process, I have not updated the mode shapes of NREL\_DS model.

I may have some points need to be clarified with you:

1. If I use NREL originally developed FAST.exe (can be considered not including soil effect) to run NREL\_DS model, the 1st natural frequency of support structure is 0.2124.
2. If I use the FAST.exe compiled by me (maybe incorporate the soil effect correctly) and set TwrLdMod = 2 (Do I have to?), I will get 1st natural frequency as 0.2842. If I do not set TwrLdMod =2, I will get 1st natural frequency as 0.2124.
3. But since either way could not give me a result around 0.25. I have also tried BModes NREL\_DS model by [wind.nrel.gov/public/gbir/BModes-for-Erica/](http://wind.nrel.gov/public/gbir/BModes-for-Erica/) with a small modification for node location. I can get the 1st natural frequency at around 0.25.

So, could you please help me to check which one is the number presented in OC3 report? And whether there is some mistakes I have made through the process? I have attached the platform and tower data.

Really appreciate!

Best regards,  
[NRELOffshrBsline5MW\_Tower\_Monopile\_DS.doc](https://forums.nlr.gov/uploads/short-url/3AEajYBtsIRBtVs2a4pjhLeDOTX.doc) (4.04 KB)  
[NRELOffshrBsline5MW\_Platform\_Monopile\_DS.doc](https://forums.nlr.gov/uploads/short-url/oTjpIe1kiH934MAiMchkM9SEpiw.doc) (6.14 KB)

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