Dear All,

I am simulating MIT/NREL TLP but with inclined tendons (5 degrees). I utilize MoorDyn, provide the same mooring stiffness of MIT NREL TLP in vertical direction. When I hand calculate the stiffness of the inclined configuration in surge direction and verify this stiffness with MAP++, I calculate 41 seconds of surge natural period. When I execute a free decay simulation with OPENFAST, I end up with a surge natural period of 61 seconds. I would like to ask, in case of simulating inclined tension lines, is there anything in particular I should pay attention?

Sincerely,

Mustafa

Dear Mustafa,

I’m not sure why the surge period calculated by OpenFAST is not what you expect. And I can’t think of anything in particular that you’d have to specify differently in OpenFAST if the tendons are slightly inclined.

You mention verifying the surge stiffness in MAP++ but you are running OpenFAST with MoorDyn instead; what surge period do you get when you run the OpenFAST simulation with MAP++ enabled?

Best regards,

Dear Jason,

Thanks again for sharing your time and help. I should add that I calculated the mooring stiffness matrix with the python wrapper as it was discussed here before: [url]Mooring stiffness matrix].

I run Static_Configuration.py with 1.0 m of surge displacement and I compare with my hand calculation. Mooring stiffness values match pretty close for all the components.

I have just tried your advise and run FAST model with MAP++ instead of MoorDyn. As you were expecting, FAST model with MAP++ also results with the same natural period in surge as 61.0 seconds (MAP++ and other input files are added). The surge period that I hand calculate is 41 seconds. Mass and added mass values are the same for the inclined and reference model. I am also adding spreadsheet calculations, where you can see my computations.

I should also add that I have added the pontoons to the MIT/NREL TLP so I don’t have precisely the same MIT/NREL TLP that Denis Matha described in his thesis work (NREL/SR-500-45891)

Sincerely,

Mustafa

V_5.rar (15.4 KB)

Parametric5.xlsx (886 KB)

Dear Mustafa,

I haven’t reviewed your files in great detail or tried to understand how you are hand-calculating the surge stiffness of inclined tendons. But it sounds like you are happy with your hand calculation of the surge stiffness and that it matches what you are seeing from MAP++. It also sounds like the OpenFAST response with MAP++ and MoorDyn agree, so, that the problem is not in the model set up of MAP++ or MoorDyn.

Perhaps the problem is that inclining the tendons leads to a coupling between surge, heave, and pitch, and so, your equation for the calculating the surge period uncoupled from other motions is not correct? Instead, the calculation of the surge period likely requires computing the eigenvalues using the mass and stiffness matrices–including coupling terms between surge, heave, and pitch–and these coupling terms are being picked up in the OpenFAST simulation.

Best regards,

Dear Jason,

Your reply makes perfect sense. In the crowd of many analyses and designs, I did a mistake. Thanks again for sharing your time and experience. I appreciate that…

Sincerely,

Mustafa