Dear NREL,
I have recently tried to estimate the RAOs for the FAST model of the IEA Wind 15 MW built on the Windcrete spar floater which was developed in the EU’s COREWIND project (corewind.eu/wp-content/uploads/f … ncepts.pdf). OpenFAST version 2.4 is being used. This was done for the turbine under normal operation at rated wind speed - the controller was enabled and the wind was steady with a 0.08-exponent power law and a 10.59 m/s wind speed at hub height (135 m). Second-order waves and platform forces were disabled. No current was considered. I did so in 3 different ways:
White-noise spectrum (as widely advised in this forum) where:
6 realizations of 10000 s with different seeds were performed where the first 4000 s were discarded;
A single wave direction of 0 deg was considered;
The frequency bandwidth in the spectrum was [0; 1] Hz;
The variable WaveNDAmp was set to False;
The WaveDT and DT_Out variables were set to 0.1 s;
The WaveHs variable was set to 2 m;
The RAOs were calculated using the script RAO.m
JONSWAP spectrum where:
Same information as above;
Peak spectral frequency of 0.01526 Hz;
The WavePkShp was set to “DEFAULT”;
Regular waves where:
1 realizations of 10000 s per frequency was performed where the first 4000 s were discarded;
A single wave direction of 0 deg was considered;
The WaveDT and DT_Out variables were set to 0.1 s;
The WaveHs variable was set to 2 m;
The RAOs were calculated using the script RAO.m but only the value corresponding the regular wave frequency was considered.
The results for the range of [0; 0.05] Hz are depicted below where the 3 red dashed lines are the surge, pitch and heave natural frequencies, respectively. Do you expect such differences and if so why? (More information about the wave elevation and considered DOFs is below) :
The average auto-spectral density of the wave elevation for both spectra was:
The considered DOFs for all 3 methods were:
---------------------- DEGREES OF FREEDOM --------------------------------------
True FlapDOF1 - First flapwise blade mode DOF (flag)
True FlapDOF2 - Second flapwise blade mode DOF (flag)
True EdgeDOF - First edgewise blade mode DOF (flag)
False TeetDOF - Rotor-teeter DOF (flag) [unused for 3 blades]
False DrTrDOF - Drivetrain rotational-flexibility DOF (flag)
True GenDOF - Generator DOF (flag)
False YawDOF - Yaw DOF (flag)
True TwFADOF1 - First fore-aft tower bending-mode DOF (flag)
True TwFADOF2 - Second fore-aft tower bending-mode DOF (flag)
True TwSSDOF1 - First side-to-side tower bending-mode DOF (flag)
True TwSSDOF2 - Second side-to-side tower bending-mode DOF (flag)
True PtfmSgDOF - Platform horizontal surge translation DOF (flag)
True PtfmSwDOF - Platform horizontal sway translation DOF (flag)
True PtfmHvDOF - Platform vertical heave translation DOF (flag)
True PtfmRDOF - Platform roll tilt rotation DOF (flag)
True PtfmPDOF - Platform pitch tilt rotation DOF (flag)
True PtfmYDOF - Platform yaw rotation DOF (flag)