Dear Jason,
I am trying to change the draft of the DeepCwind platform. The original draft is 20m and I want to change it to 10m, basically pushing the platform up by 10m. Mooring lines have been detached, so the Moordyn switch is off. I modified the ‘Filled Members’ accordingly in the Hydrodyn input file by changing the ‘Jointzi’ values. The ballast has also been adjusted by changing the ‘FillFSLoc’. I have changed ‘PtfmVol0’ also to the underwater volume of the platform at 10m draft. The highlighted values (in Elstodyn input) in the figure below have been changed also.
Q1. Do I need to change PtfmRefzt in Elastodyn input file? There is a ‘PtfmRefzt’ in Hydrodyn input file also, any changes needed in this ‘FLOATING PLATFORM’ part?
Q2. Are there any other values to be changed in the input files?
Q3. Since I am modifying the ballast, there is a change in moments of inertia and CM of the platform. Do I have to modify the CM and inertia values of the platform (PtfmMass,PtfmRIner,PtfmPIner,PtfmYIner) in Elastodyn to account for this or hydrodyn will automatically take care care of it while running the simulation?
Kindly help.
Regards,
Rahul.
Dear @Rahul.Ramachandran,
Here are my answers to your questions:
PtfmRefzt in ElastoDyn is the point where ElastoDyn will be rigidly connected to HydroDyn. I would assume that you wouldn’t need to change that when changing the floater draft.- I would expect that you’d have to recompute the potential-flow solution (in WAMIT or equivalent) via a change to
PotFile in HydroDyn. Also, it sounds like you are removing the moorings, so, you’d lose the vertical pretension of the mooring lines, which would cause the floater to rise a bit higher. To account for that, you’d want to reduce the platform volume or increase the ballast a bit. You’d also want to shift the height of the tower aerodynamic nodes in AeroDyn (TwrElev) and the wind data in InflowWind.
- The OpenFAST model of the OC4-DeepCwind semisubmersible available in the OpenFAST r-test has the structural part modeled in ElastoDyn and the hydrodynamic and water ballast modeled in HydroDyn. It sounds like you are changing the draft simply by shifting everything up 10 m without changing the structural members. This implies to me that the mass and inertia of the structural part modeled in ElastoDyn doesn’t need to be changed other than
PtfmCMzt as you have already changed.
Best regards,
Thanks for the help, Jason.
I recalculated the potential flow solution for the new draft using aqwa and converted it into wamit files. I get this error from hydrodyn when I run the simulation. How can I correct this?
Regards,
Rahul.
Dear @Rahul.Ramachandran,
It appears that you are deriving both the first- and second-order solutions in AQWA and converting them to WAMIT format–is that corrrect? For the second-order solution, see the requirements in terms of sparseness and symmetry required by HydroDyn documented here: 4.2.8.4. Modeling Considerations — OpenFAST v3.5.0 documentation. Or you could always disable the second-order solution if that is not desired.
Best regards,
Thank you so much for the help, Jason.
Yes, I am calculating both the first and second order solutions using Aqwa and converting to wamit result files.
Is there a subdyn file available for calculating the hydro elastic response of the deepcwind platform? I am interested in calculating the stresses on the platform members due to wave, current and wind action similar to the turbine structural members. I read some discussions on the forum, but are there any r-test cases coming up with this functionality?
Regards,
Rahul.
Dear @Rahul.Ramachandran,
I agree that it would be useful to have a hydrodelastic model (with SubDyn enabled) of the NREL 5-MW baseline wind turbine atop the OC4-DeepCwind semisubmersible, but NREL has not added such a model to the r-test yet, per the following OpenFAST issue that has not been addressed yet: Add Test case for flexible platforms · Issue #39 · OpenFAST/r-test · GitHub.
Best regards,
Looking forward to such a test case.
Thanks for your active support, Jason.
Hi Jason,
The marin_semi.12d file in the Hydrodata folder (OC4 testcase) has only one direction - 0 deg in it. I read about the bug in the hydrodyn code, has it been fixed yet? How do I add other wave directions in the QTF files?
Regards,
Rahul.
Dear @Rahul.Ramachandran,
Earlier bugs regarding the treatment of second-order hydrodynamic input files have been fixed in HydroDyn.
Guidance on developing the second-order potential flow solution for use in HydroDyn is documented here: 4.2.8.4. Modeling Considerations — OpenFAST v3.5.0 documentation.
Best regards,
Hi Jason,
I am trying to do a decay test using the DeepCwind platform.
-
The heave motion is matching with experiments, but roll and pitch are not. Could this be due to mismatching inertia values ? There are no filled members. Does openfast model any additional masses using the ‘MEMBER CROSS-SECTION PROPERTIES’ ?
-
When the decay tests are performed (without waves), does openfast consider viscous effects (Morison elements) ? I haven’t defined any AddBLin or AddBQuad in the hydrodyn input.
Regards,
Rahul.
Dear @Rahul.Ramachandran,
The roll and pitch natural free decay could be impacted by a combination of body mass, center of mass, and inertia; buoyancy restoring; mooring restoring; wave radiation (added mass and damping); and viscous effects.
I’m not sure I understand your question about “additional masses”; what OpenFAST module are you referring to?
The viscous effects implemented through the strip-theory (Morison equation) solution are implemented using the relative velocity, so, there can be viscous drag even in the absence of incident waves.
Best regards,
Hi Jason,
The mass, COG and the underwater geometry are the same for both the experiment and openfast simulations. Also, there are no mooring lines. I am only interested in the platform, so I have set the weights and inertias of the wind turbine to very low values.
Additional masses as in: we give a thickness to the members in the ‘MEMBER CROSS-SECTION PROPERTIES’, so I was wondering if openfast models any masses based on the geometry using density of the material used for construction which may affect the moment of inertias ?
When I use the damping coefficient calculated from decay tests and use it in ‘AddBLin’, the motion amplitude changes, but the frequency of motion doesn’t change.
Regards,
Rahul.
Dear @Rahul.Ramachandran,
It sounds like you are referring to the HydoDyn module. HydroDyn only considers the effects of water, including wave excitation, wave radiation, hydrostatics, and viscous effects. Water ballast can also be considered. Structural information of the floating platform is specified within the structural module (either the platform in ElastoDyn or SubDyn, when enabled).
Best regards,
