Yes ,you are right about the viscous drag issue,I forgot that I have defined that already,thanks for your indication.
I just ran some linearization process with OC3Spar model under different wind conditon.
one with CompAero off to get a non wind impact M,C,K.
Another one with CompAero True and GenDOF False to keep a fixd rotor speed at 12.1rpm, steady wind input is 11.4m/s with power shear law at 0.14,
and also try to get the M,C,K.
Then I find the Mass matrix generated from above tests are different from each other.
I understand that the damping matrix and stiffness matrix would be different from each other because they count for different aerodynamic influence,but would the mass matrix linearized by fast also be influenced by different aerodynamic condition input? If not ,how can I fix this ?
Hope you could help me out with this ,much appreciated.
The aerodynamics in FAST do not directly influence the mass matrix obtained through a FAST linearization analysis. However, the mass matrix in FAST does depend on structural displacements, so, if the aerodynamic loads influence the structural displacements, you would see this indirect effect on the mass matrix.
Dear Jason,
From your answer, you mean that the platform mass, center of mass, and inertias specified in the ElastoDyn module of FAST v8 are for the platform itself, not including the tower, nacelle, drivetrain, rotor, or moorings. But in the file ‘NRELOffshrBsline5MW_OC3Hywind_ElastoDyn.dat’ which is downloaded from NREL, the mass and inertia properties of OC3-Hywind are defined as below:
7.46633E+06 PtfmMass - Platform mass (kg)
4.22923E+09 PtfmRIner - Platform inertia for roll tilt rotation about the platform CM (kg m^2)
4.22923E+09 PtfmPIner - Platform inertia for pitch tilt rotation about the platform CM (kg m^2)
1.6423E+08 PtfmYIner - Platform inertia for yaw rotation about the platform CM (kg m^2)
But in the report ‘Definition of the Floating System for Phase IV of OC3’ , the mass is calculated as follow:
The mass, including ballast, of the floating platform is 7,466,330 kg. This mass was calculated
such that the combined weight of the rotor-nacelle assembly, tower, and platform, plus the
weight of the mooring system (not including the small portion resting on the seafloor) in water,
balances with the buoyancy (i.e. weight of the displaced fluid) of the undisplaced platform in still
water.
The total mass is the same, but it contains different items. Which one is right? I am confused about it. This is the first time to use FORUM, forgive me can’t give you a figure.
Best regards
The OC3-Hywind spar was developed from specifications provided by Statoil, but the platform mass was not specified directly by Statoil. Instead. the full-system (rotor + nacelle + tower + platform) mass was provided. So, I back-calculated the platform mass from the full-system mass by subtracting out the known masses of the rotor, nacelle, and tower.
following the last post I would add a question in this topic.
If I have understand correctly you have calculated the Hywind mass of the platform from the displaced volume of water of the Spar V0, then subtracting the rotor + nacelle + tower of the NREL 5MW wind turbine and then weight of the mooring system (not including the small portion resting on the seafloor). Is it right?
If so I would ask you how do you calculate the weigth of mooring lines resting on the seafloor?
If I do the calculation of the mooring line mass is should be 902m * 76.7066 kg/m = 69183 kg but if I do the reverse calculation starting from the displaced volume and subtracting all the terms (roter, nacelle, tower and plaftform) I obtain the mooring line mass of 54833 kg.
If I do the subtraction of this two terms I should obtain the mooring mass line resting on the sea floor, amounting in 69183 - 54833 = 14350 kg.
My question is how do you calculate this value in order to obtain the platform mass.
I haven’t checked your numbers, but the mooring tension at the fairlead is calculated by whichever mooring module is enabled in FAST. The vertical tension at the fairlead equals the hanging weight of the mooring lines, not including the weight resting on the seabed. E.g. the portion of the mooring lines resting on the seabed is automatically calculated by MAP++ based on the anchor and fairlead positions.
Maybe I didn’t give my meaning clearly. I mean the total mass of the full-system (rotor + nacelle + tower + platform) is 7.46633E+06 kg, so the platform mass (not includs the tower, nacelle, drivetrain, rotor, or moorings) must be less than 7.46633E+06 kg. Thus, the mass of the platform in the ElastoDyn module shouldn’t be 7.46633E+06 kg. But in he file ‘NRELOffshrBsline5MW_OC3Hywind_ElastoDyn.dat’, the mass of the platform is defined as 7.46633E+06 kg. Isn’t this a contradiction?
Thank you very much.
No, the mass of the OC3-Hywind spar platform, including ballast, is 7.46633E+6 kg. The full-system mass of the OC3-Hywind floating wind system (rotor + nacelle + tower + platform/ballast) is 8.066E+6 kg.
Dear Jason
In the third chapter of the report ‘Definition of the Floating System for Phase IV of OC3’, the mass is defined as:
The mass, including ballast, of the floating platform is 7,466,330 kg. This mass was calculated such that the combined weight of the rotor-nacelle assembly, tower, and platform, plus the weight of the mooring system (not including the small portion resting on the seafloor) in water, balances with the buoyancy (i.e. weight of the displaced fluid) of the undisplaced platform in still water.
Can I think that the full-system mass (rotor + nacelle + tower + platform/ballast+mooring system) is 7.46633E+6 kg rather than 8.066E+6 kg from the definition? How do you get the number 8.066E+6 kg? Could you give me a reference? Thanks a lot!
Best regards
As I said before, the 7.46633E+6 kg is the mass only of the floating spar platform (including ballast). The tower, nacelle, and rotor masses are separate. The total full-system mass of 8.066E6 kg is not specifically documented in the OC3-Hywind specifications report, but it can be calculated manually by summing the platform tower, nacelle, and rotor masses.
I want to simulate test25 (DeepCwind) for a self-designed model. I want to simulate my model without ballast load. Is it possible? if yes, then could you please let me know what changes I suppose to make in the standard input files.