An error occurred during the simulation under extreme sea conditions.

[axxyyyyyyyy378]

Hello，

Recently, I want to simulate the response of a wind turbine under extreme sea conditions based on OpenFAST. However, I found that the following issues occurred during the calculation using the baseline case "5MW_OC4Semi_WSt_WavesWN":

Based on the error messages, the possible reasons are as follows: Mach Number exceeds limit, Small angle assumption violated, unsteady aerodynamic module turned off, and BEM solution turned off due to low TSR. However, I am not very familiar with the aerodynamic module of OpenFAST, so I would like to ask for advice on how to solve these problems. Thanks.

Best regards,

[andrew-platt]

Does the simulation end at after this message, or does it continue?

When you change either the wind or wave conditions, it may be necessary to adjust the initial conditions of the turbine (platform pitch or surge, blade pitch, etc). The baseline 5MW_OC4Semi_WSt_WavesWN case uses an 8 m/s wind speed, so if you are adjusting wind conditions to match extreme sea conditions, you will likely need to change the initial blade pitch to avoid tower strikes or BEM solver errors.

Regards,

[axxyyyyyyyy378]

Thanks for your kind response. The simulation can continue until the end after this message, but due to these error messages, I have to question the accuracy of the simulation results.
In fact, before raising this issue, I had already tried multiple times to adjust the initial blade pitch and initial rotor speed, but similar warnings still message, only at different times during the simulation.
Do I have no choice but to keep trying to adjust these initial parameters until no error messages appear? Looking forward to your response.

Best regards,

[andrew-platt]

That's good that the simulation can continue.

I suspect the issue then is with the initial conditions of the platform. Could you plot the platform motion channels for PtfmSurge and PtfmPitch? It may be that the platform is moving rapidly at the start of the simulation due to the initial conditions.

Since the simulation does run to completion, you can plot some of the other output channels like RotSpeed and BldPitch1. These likely settle to about some average value that you can use as the initial condition in the ElastoDyn input file.

By the way, which version of OpenFAST are you running?

Regards,

[axxyyyyyyyy378]

Glad to see your reply. After continuous adjustments (modifying the initial blade pitch and initial rotor speed), the error messages have decreased. Now, only at 154 seconds, the following message appears:

It seems that the issue might be due to the initial blade pitch being too small or the rotor speed being too high. Below are the initial settings:

I output the data for PtfmSurge, PtfmPitch, RotSpeed, and BldPitch1 as follows:

Under extreme wind, the wind turbine does indeed move quickly at the start of the simulation. How should I proceed with further adjustments? Looking forward to your response.

Best regards,

[jjonkman]

Dear @axxyyyyyyyy378,

As @andrew-platt said, it is important to set proper initial conditions in OpenFAST for the blade-pitch angle and rotor speed--based on their expected (mean) value conditioned on the mean wind speed you are simulating--to minimize start-up transients. For floating offshore wind turbines, it is also useful to set proper initial conditions for platform surge and pitch. You haven't said what mean wind speed you are using, but based on your results, which imply a mean wind speed around 22 m/s or so, I would suggest using the initial conditions of RotSpeed = 12.1 rpm, BlPitch = 20deg, PtfmSurge = 4 m, PtfmPitch = 2deg. With those initial conditions, do you still see warnings from OpenFAST?

Best regards,

[axxyyyyyyyy378]

Hi @jjonkman , I'm glad to see your professional advice. In this test, I used TurbSim to generate a turbulent wind field with an average wind speed of 22.1 m/s as the wind load. According to your suggestion, I set the initial conditions (RotSpeed = 12.1 rpm, BlPitch = 20 deg, PtfmSurge = 4 m, PtfmPitch = 2 deg) as follows:

however, it still shows warning messages. I have tried adjusting these parameters multiple times, but the warning persists:

I plan to continue adjusting these parameters until the error message no longer appears. Is this feasible?

Best regards,

[jjonkman]

Dear @axxyyyyyyyy378,

With proper initial conditions set, have the warnings about "turning off UA", "low TSR", and "no valid value of phi" been eliminated? If so, I think your problem has been solved. You may still run into the "Mach number exceeds 0.3" warning if the rotor speed cannot be regulated close to 12.1 rpm.

Best regards,