Mooring stiffness matrix

Dear all,
I have a FAST 8.16 model of a floating wind turbine where the mooring system is implemented by means of MoorDyn module.
How can I extract the mooring stiffness matrix in a given platform position?
Thank you in advance,

Alessandro

Dear Alessandro,

The MoorDyn module doesn’t have the capability of extracting the quasi-static 6x6 stiffness matrix of the mooring system. However, you can use the Python wrapper for MAP++ to derive this, as discussed recently in the following forum topic: http://forums.nrel.gov/t/force-displacement-relationship-of-oc4-deepcwind-mooring-sys/964/9.

Best regards,

Dear Jason,
thank you for your fast (instantaneous) answer!
Best regards

Alessandro

Dear all,
I looked at the topic linked in Jason post where MAP++ is used to extract the 6x6 stiffness matrix of a mooring system. I’m not familiar at all with python and after some (failed) tries i decided to come back to this forum to see if there is a more suitable solution for my problem.
In particular i’ve found this topic http://forums.nrel.gov/t/questions-about-the-hydrodyn-module/596/2 where a FAST7 linearization procedure is used to compute the 6x6 mooring stiffness matrix.
I’ve put together a FAST7 model of the floating wind turbine i’m studying and i tried the procedure mentioned above. However i get a zero 6x6 DspCMat matrix.
Is it correct to use the FAST 7 linearization with zero WAMIT input files to get the mooring lines stiffness matrix? If yes what I’m doing wrong?
Thank you!

Alessandro

Dear Alessandro,

I’m not sure what you’re doing wrong, but I agree that you can use the linearization functionality of FAST v7 to derive the 6x6 mooring stiffness matrix as described in the forum topic you reference. Does your time-domain simulation of the floating system in FAST v7 (with nonzero WAMIT data) give you the response you expect?

Best regards,

Dear Jason,
following your advice i debuted my FAST7 model performing time domain simulations: PtfmLdMod was set to 0.
Now i can correctly get the 6x6 mooring stiffness matrix from linear analysis.
I have a final question: is it possible to include (or model in an alternative way) clamp weights in the mooring line definition of FAST7?

Dear Alessandro,

I’m glad you solved your problem.

Unfortunately, the mooring model in FAST v7 is quite a bit simpler than those available in FAST v8. It is not possible to model mooring system clump weights in FAST v7 (without modification of the source code).

We are currently working on full-system linearization in OpenFAST for floating offshore wind systems, including the linearization of HydroDyn and MAP++. This new functionality should be available later this summer.

Best regards,

Dear Jason,
thank you for the informations. I was thinking (maybe) to change the mooring line properties in FAST7 trying to match the response of the target floating system with mooring line clamp weights.
I’m super-happy to hear about OpenFAST and full system linearization!
I wish you all the best for the next months of work and i look forward to the new functionalities.

Alessandro