Dear Jonkman

When I calculate the RAO response of a semi-submersible wind turbine under a regular wave，There are some errors.

The Parameters of FEAMooring is showed as follow:

If I change LUnstrLen from 455 to 450. The Result can be calculated.

Of course,If LRadAnch is 484,the LUnstrLen is at least 451.But I have tried, it can only be counted when LUnstrLen<451.

I don’t understand why it is.Can you tell me where problem is.

Thank you very much

Dear Wei.Zhang,

Unfortunately, I don’t have much experience with the FEAMooring module of OpenFAST, so, I’m not sure I can answer your question. Is the solution sensitive to the solver options such as MaxIter, Eps, and initial top tension of each line (Tension)?

Of course, Yoon Hyeok Bae, the developer of FEAMooring and now a professor at Jeju National University in Korea, is likely best for answering FEAMooring-related questions. I don’t believe Prof. Bae checks this forum regularly, but you can reach out to him directly.

Alternatively, you can switch from FEAMooring to MoorDyn, which is the NREL-developed mooring dynamics module, based on a lumped-mass approach.

Best regards,

Dear Jonkman

Thank you for your previous answer.

Although I read the FEAM manual, and found the explanation of the two parameters MaxIter and Eps.

But I still don’t know how to debug these two parameters. What is the impact of increasing or decreasing MaxIter (or Eps)? What is their range of possible values?

Dear Wei.Zhang,

I don’t know the answer to these questions, which would be better answered by Prof. Yoon Hyeok Bae. I would suggest reaching out to him. Alternatively, you can try a range of values to see their effect.

Best regards,

Dear Jonkman

Thank you for your answer, but I don’t know how to contact Yoon Hyeok Bae. I tried to find the contact information of Yoon Hyeok Bae from the “the team” option at the bottom of the forum page, but failed.

It would be great if you could tell me how to contact Yoon Hyeok Bae.

Dear Wei Zhang,

As I mentioned, Yoon Hyeok Bae is a professor at Jeju National University. You should be able to find his contact information through a quick Google search.

Best regards,

Dear Jason

Thanks for your suggestion, I have successfully solved the problem，although I have taken a lot of detours.

Increase the value of EPS, the calculation example can be calculated.

Dear Jonkman

I plan to use FAST v8 software to calculate the six-degree-of-freedom decay motion of the platform. My anchor chain parameters are as follows(FAEM_Parameters.png), in which the EPS value has been changed from 0.0001 to 0.001. When I changed the value of LUNstrLen (total length of the anchor chain) from 2500 to 1500,except for the heave attenuation motion can be calculated, the other five degrees of freedom attenuation motion were reported as errors. As shown below(Error.png)：

But when the pre-tension is changed from 9E5 to 12E5, it can be calculated, but I don’t know why?

If I want to calculate the working condition when the pretension is 9E5, what should I do?

Dear Wei.Zhang,

I don’t know the answer to your question, but again refer you to Prof. Yoon Hyeok Bae of Jeju National University.

Best regards,

Dear Jonkman

I don’t know what the error is.Where should I think about this error?

Dear Jonkman

I research the movement of the platform under different angles of wind and waves. When the wind direction angle and the wave direction angle are both 0°, it can be calculated, but when the wind direction angle is 0° and the wave direction angle is 60°, the calculation stops at the 10th second. I don’t know why this is. The error displayed by FAST is : ‘’FF wind array was exhausted at 10.887 seconds (trying to access data at NaN seconds).How can I solve this problem?

Dear Wei.Zhang,

I’m guessing in both of your cases, your model has gone unstable. Your post at 12:01 am warns about large blade deflection, which is then followed by warnings from AeroDyn about high induction, and then the error from InflowWind is indicating that an aerodynamic node has left the wind domain. Instabilities typically drive exponential growth, triggering simulation failure. In your post at 12:41 am, I’m guessing the motion got so large that a value exceeded the maximum value of floating point math, resulting in NaNs. What did you change in your model between a simulation that used to work as you expect and these simulations that fail?

Best regards,