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Floating barge substructure with mooring lines – Nemoh
Hi!
My goal is to calculate wave loads on floating 50x50x20 m square barge offshore wind turbine substructure which has mooring lines. I have studied the manual but these are some things I haven’t been able to find. I will list all the questions I can think of right now regarding the substructure setup file:
- I am using Nemoh output files for QBlade inputs but I’m not sure which exact files are the ones I need in addition to RadiationCoefficients.tec (POT_RAD_FILE) and ExcitationForce.tec (POT_EXC_FILE). Nemoh gives also hydrostiffness file Kh.dat but I’m not sure if it works for POT_HST_FILE. Same for DiffractionForce.tec for POT_DIFF_FILE. I think the QTF output files from Nemoh also have to be used as input but I’m not totally sure which ones and for what command (POT_XXX_XXX)? I suppose the added mass and damping should be included somehow too?
- As I said the barge has mooring lines but I have not yet been able to add the additional mooring stiffness matrix (6×6) to Nemoh because I haven’t found a way to do it (tips for a way or program also welcome). I was wondering if I can use Nemoh results which do not take into account the mooring (for now) as inputs for QBlade since the mooring lines are still defined in QBlade separately or does it make a big difference to the results?
- What type of sensors should I setup for my barge and mooring lines if the wave loads and mooring line loads are the forces I’m interested in? Any tips?
These are the questions I came up for now. Hopefully they are not too complicated to answer. Thank you if you have the time to help me! If some files are needed I can provide them.
-Niila
Hello Niila,
from a linear potential flow data with Nemoh, you can currently only import the RadiationCoefficients.tec and ExcitationForce.tec files using the keywords POT_RAD_FILE and POT_EXC_FILE into QBlade. The ExcitationForce.tec file includes contributions from both diffraction forces (found in DiffractionForce.tec) and Froude-Krylov forces (found in FKForce.tec).
In QBlade, the keywords POT_DIFF_FILE and POT_SUM_FILE are used to import the Quadratic Transfer Function (QTF) for second-order sum and difference frequency forces. These should not be confused with diffraction forces. Currently, these files can only be imported in the WAMIT format, requiring you to convert the second-order files from Nemoh to WAMIT format, or run simulations without the second-order forces.
Regarding the hydrodynamic stiffness of mooring lines, instead of importing the hydrodynamic stiffness via a WAMIT .hst file, you can specify the 6×6 linear hydrodynamic stiffness matrix directly using the SUB_HYDROSTIFFNESS keyword. This matrix can be utilized to create a simple linear model of the mooring system by adding linear stiffness to the relevant degrees of freedom. Alternatively, you can model the mooring lines explicitly with cable elements by using the MOORELEMENTS table. The mooring system does not affect the radiation and excitation coefficients from your Nemoh analysis at all.
To include the added mass you can use the USE_RAD_ADDMASS keyword. This automatically uses the data from your radiation coefficients at infinite frequency to generate the 6×6 added mass matrix.
The radiation coefficients from your Nemoh analysis already model some damping. Additional damping can optionally be added by Morison elements or the 6×6 linear SUB_HYDRODAMPING or quadratic SUB_HYDROQUADDAMPING matrices.
If you model the mooring lines explicitly sensors are automatically generated at the connection points between the barge and the mooring lines. YOu can then extract the reaction forces from these locations, or distribute sensors over the mooring lines according to your requirements.
BR,
David
Hi, David and thank you for the super fast response which helped me to make progress on my project and in learning QBlade!
I was able to model the barge and mooring lines and I have been testing them by changing the structure file from OC3 Sample project to my barge file and setting the controller to none. For my eye the movement of the barge (52.5 x 52.5 x 20 m, weight 25113 tons) seems a little too fast/extreme for a structure of that size and weight (the significant wave height is about 8m but still) but I will provide the files so you can give your opinion on it. I will also provide the substructure file and Nemoh output files which I use in the substructure file.
- Does my substructure file look correct or is it missing something that I should add regarding to my case that I described in the first message (wave loads and mooring chain loads)? And does the movement of the structure look right to your eye (especially between timesteps 1700-2200)?
- I was still not able to figure out how I should obtain the mooring stiffness matrix for adding it to the hydrostiffness matrix since this is my first time trying to do such thing and I’m just not able to find any directions or help online. I’m currently using the Nemoh Kh.dat matrix as SUB_HYDROSTIFFNESS input which does not include mooring. Could you give me tips how to obtain the mooring stiffness or is there some techniques I could use to approximate my mooring system’s stiffness? As you can see from the files the barge is connected to ground with 8 mooring chains that are 700m in length each, dry lineic mass is 346.7kg/m, nominal diameter 132mm, studless links and grade R4. Since I’m using chain I set the buoyancy to 0. I use 0.001 bending stiffness since I found from OrcaFlex website that they use zero bending stiffness for chain (https://www.orcina.com/webhelp/OrcaFlex/Content/html/Chain,Axialandbendingstiffness.htm). I also calculated the axial stiffness with their equation for studless chain. I wonder if these inputs are correct approximates?
I am using SUB_DISPLACEDVOLUME (52.5 x 52.5 x 10 m, since draft should be 10 m) for the buoyancy force currently and the OC3_BARGE_TEST.str is built to the OC3 Sample project file so there are still some things left from it but I also have removed some things. SUB_MASS is from BEMRosetta. QTF files have been converted from Nemoh to Wamit .out with BEMRosetta. I had to make a zip file to submit 6 files. I have been running the files in SampleProjects folder.
Best regards,
Niila
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Sorry I noticed that the simulation file wasn’t up to date in the zip package so here is the updated files for testing!
Niila
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Hi Niila,
at first glance, the model seems to be consistent.
One suggestion to improve things would be to add some drag to the mooring lines via the HYDROMEMBERCOEFF table, otherwise the mooring lines will not experience any hdrodynamic drag forces.
Since you are modelling the mooring lines explicitly with cable elements you dont need to include their contribution in the hydrodynamic stiffness matrix. Modelling the moorings in the hydrodynamic stiffness matrix is just an option if they are not modeled explicitly.
I would suggest to perform some decay tests in still water to see if the floater eigenrequencies align with the literature data!
BR,
David
Awesome, thank you!
I added drag coefficient for the mooring lines and now they behave better.
I will post decay test and eigenfrequency questions to a separate thread.
Br,
Niila
