Why does the free-decay response of the OpenFAST v4.2.0 IEA 15 MW monopile model not oscillate around zero

[YuguoSong123]

I am currently using the OpenFAST v4.2.0 IEA 15 MW monopile model to perform a free-decay analysis.

The model setup is as follows:

OpenFAST version: v4.2.0
The IEA 15 MW monopile reference model is used.
The turbine is set to a parked condition.
Rotor speed is set to 0 rpm.
Blade pitch angles are fixed at 90 degrees.
In the environmental conditions, still air is selected in the .fst file (CompInflow = 0).
The sea state is set to still water in the HydroDyn file (WaveMod = 0).
An initial tower-top fore-aft displacement of 0.5 m is applied.

However, after the free-decay simulation starts, the tower-top displacement does not decay around the zero position. Instead, the response oscillates around an offset value (approximately -0.4 m).

I would like to understand which model parameters or module settings may cause this static offset in the free-decay response.

For comparison, I performed a similar free-decay analysis using the NREL 5 MW monopile model, and the response decayed approximately around zero.

[YuguoSong123]

The attached figure shows the free-decay response of the IEA/NREL 5 MW monopile model using the same simulation setup and procedure as the IEA 15 MW monopile model.

[jjonkman]

Dear @YuguoSong123,

A nonzero tower-top fore-aft discplacement in the absence of wind or wave excitation results from the overhanging weight of the rotor-nacelle assembly (RNA), which may differ between turbines due to differing RNA weights, center of mass offsets, and support structure stiffness.

Best regards,