Hello,
I am using the IEA 15-MW offshore reference wind turbine monopile configuration in OpenFAST. I have a question about the RNA mass and inertia parameters used in the ElastoDyn input file.
In the OpenFAST ElastoDyn input file, the following values are used:
0.0 TipMass(1)
0.0 TipMass(2)
0.0 TipMass(3)
69131 HubMass
969952 HubIner
1836784 GenIner
644857 NacMass
32929058 NacYIner
28249 YawBrMass
However, in the IEA 15-MW technical report, Table 5-1 gives different nacelle, hub, generator, and yaw-system mass/inertia values. For example, the report lists a hub mass of 190 t, while the OpenFAST input file uses HubMass = 69131 kg. The yaw system mass in the report is 100 t, while the OpenFAST input file uses YawBrMass = 28249 kg.
I would like to ask:
Why are the mass and inertia parameters in the OpenFAST ElastoDyn input file different from the values listed in Table 5-1 of the technical report?
In addition, I performed a still-condition test to better understand the static equilibrium behavior of the model. In this test, I used still air, still water, and set the initial rotor speed to 0 rpm. No external wind or wave excitation was applied, and the model was allowed to run for 4600 s.
I observed that the tower-top displacement still requires a very long time （2000s） to decay toward its equilibrium position. Meanwhile, a shear force (200N) appears at the mudline. I would like to ask whether this behavior is physically reasonable for the official IEA 15-MW monopile OpenFAST model under a still-condition test.
I also tested the influence of structural damping on this long free-decay process. In my modified model, I changed the tower modal damping ratios in ElastoDyn from 1% to 2%:
2.0 TwrFADmp(1)
2.0 TwrFADmp(2)
2.0 TwrSSDmp(1)
2.0 TwrSSDmp(2)
I also set the substructure/foundation structural damping to 2%. After these changes, the decay time became shorter compared with the original model. I would like to ask whether this modification is reasonable.
I am also confused about the blade damping values. The IEA 15-MW technical report states that the structural damping of the first flapwise, edgewise, and torsional blade modes was assumed to be 3%, 3%, and 6%, respectively. However, in the OpenFAST blade input file, the blade damping values appear to be:
0.48 BldFlDmp1
0.48 BldFlDmp2
0.48 BldEdDmp1
Could you please help me understand why the blade damping values in the OpenFAST input file are 0.48% rather than 3%? Are these values based on a different damping definition, a tuning/calibration procedure, or an equivalent modal damping conversion?
If I want to use the IEA 15-MW monopile model for free-decay, parked-condition should I keep the original OpenFAST damping parameters, or should I adjust the tower, substructure, and blade damping values according to the technical report?
Thank you very much for your time and support. Your clarification would be very helpful for me to correctly use the IEA 15-MW monopile OpenFAST model in my subsequent simulations.
Best wishes
Hello,
I am using the IEA 15-MW offshore reference wind turbine monopile configuration in OpenFAST. I have a question about the RNA mass and inertia parameters used in the ElastoDyn input file.
In the OpenFAST ElastoDyn input file, the following values are used:
0.0 TipMass(1)
0.0 TipMass(2)
0.0 TipMass(3)
69131 HubMass
969952 HubIner
1836784 GenIner
644857 NacMass
32929058 NacYIner
28249 YawBrMass
However, in the IEA 15-MW technical report, Table 5-1 gives different nacelle, hub, generator, and yaw-system mass/inertia values. For example, the report lists a hub mass of 190 t, while the OpenFAST input file uses HubMass = 69131 kg. The yaw system mass in the report is 100 t, while the OpenFAST input file uses YawBrMass = 28249 kg.
I would like to ask:
Why are the mass and inertia parameters in the OpenFAST ElastoDyn input file different from the values listed in Table 5-1 of the technical report?
In addition, I performed a still-condition test to better understand the static equilibrium behavior of the model. In this test, I used still air, still water, and set the initial rotor speed to 0 rpm. No external wind or wave excitation was applied, and the model was allowed to run for 4600 s.
I observed that the tower-top displacement still requires a very long time （2000s） to decay toward its equilibrium position. Meanwhile, a shear force (200N) appears at the mudline. I would like to ask whether this behavior is physically reasonable for the official IEA 15-MW monopile OpenFAST model under a still-condition test.
I also tested the influence of structural damping on this long free-decay process. In my modified model, I changed the tower modal damping ratios in ElastoDyn from 1% to 2%:
2.0 TwrFADmp(1)
2.0 TwrFADmp(2)
2.0 TwrSSDmp(1)
2.0 TwrSSDmp(2)
I also set the substructure/foundation structural damping to 2%. After these changes, the decay time became shorter compared with the original model. I would like to ask whether this modification is reasonable.
0.48 BldFlDmp1
0.48 BldFlDmp2
0.48 BldEdDmp1
Could you please help me understand why the blade damping values in the OpenFAST input file are 0.48% rather than 3%? Are these values based on a different damping definition, a tuning/calibration procedure, or an equivalent modal damping conversion?
If I want to use the IEA 15-MW monopile model for free-decay, parked-condition should I keep the original OpenFAST damping parameters, or should I adjust the tower, substructure, and blade damping values according to the technical report?
Thank you very much for your time and support. Your clarification would be very helpful for me to correctly use the IEA 15-MW monopile OpenFAST model in my subsequent simulations.
Best wishes