High Variations in Tower Base Momentum and Forces

Wind & Water Structural Analysis
1

Dear NREL team ,

I am currently simulating the IEA-15-240-RWT-UmaineSemi in OpenFAST for steady wind speed of 8 and 10 m/s. I checked the initial conditions of the blade pitch angles and rotor speed based on your suggestion in the forum and I changed them to 0 degrees for both wind cases and 5,55 rpm (8 m/s) and 6,80 rpm (10 m/s) [Figure 3.1]. The program is terminated normally without any warning or error whatsoever.

However, the generated results for the tower base momentum and forces produce a high variation in values for the first 30 seconds of the simulation. Is this something expected regarding the theoretical inaction of the sea state beforehand?

P.S. Without changing the initial conditions of the rotor speed and the pitch angles i was facing warnings about the mach number exceeding and unsteady aerodynamics of the model due to high angle of attack.

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2

Dear @Nikos.Christodoulaki,

This would be expected for a floating offshore wind turbine simulation if you are not also setting good initial conditions for the platform surge and platform pitch degrees of freedom based on their expected (mean) values conditioned on the mean hub-height wind speed. Given the low natural frequencies of the platform surge and pitch modes of a semisubmersible, the start-up transient could last a long time without good initial conditions in surge and pitch.

Best regards,

3

Dear Jason,

Thank you for your fast and detailed reply.

Have a nice day!

Best regards,
Nikos