# Mooring loads of the Linearized mooring model of OC4 DeepCWind Semi

Dear Jason,

I am recently trying to apply the linearized mooring model of the OC4 DeepCWind Semi in a lab CFD code for FOWT(5MW+OC4) analysis including wakes. The model is defined in section 5.2 of the Report 《Definition of the Semisubmersible Floating System for Phase II of OC4》as :

I noticed that you have mentioned the mooring loads are about the platform reference point (neither the center of gravity of the FOWT system nor the CG of the platform) in a previous post: Force-Displacement relationship of OC4 DeepCwind Mooring sys . But I still have some confusions :

(1) Since the platform reference point is moving with the floating body, are the mooring loads relative to the moving ref point? or relative to the initial position of the reference point?

(2) If relative to the moving ref point, the loads calculated using Eq. (5-13) is represented by the floating platform coordinate system? or the global inertial coordinate system?

(3) You also mentioned in the previous post that

you could derive it by summing up the fairlead tensions from all lines (Fx, Fy, Fz and the associated moments using the moment arms to the platform reference point).

However, I find the official FAST OC4-Semi case only output the total fairlead tension for each mooring line in the Moordyn file by FairTen1, FairTen2, FairTen3. Could you give me some guidelines how to obtain the x, y, z component of each fairlead tension?

Best regards,
Lin Yang.

Dear @Lin.Yang,

Here are my responses:
(1) This linearized mooring formulation will provide loads that can be applied to the floater in a reference frame that translates with the floater.
(2) This linearized mooring formulation will provide loads expressed in the global coordinate system (not body fixed).
(3) In addition to the net tension, MoorDyn can output the X/Y/Z components of the force, as documented in the MoorDyn online documentation: Input Files — MoorDyn 2.2.2 documentation.

Best regards,

Dear Jason,

Thanks for you very rapid reply. The Mooring loads definition is much clearer to me now. And I will check the MoorDyn online documentation for output details.

Best regards,
Lin Yang.

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Dear Jason,

Sorry to bother you again.

I recently have done some free-decay tests for 5MW+OC4 with linearized mooring model in a lab CFD code based on Actuator Line Method, and the natural periods of Heave and Pitch are fine. However, my surge period (~125s) is significantly overestimated comparing to the experimental result (107s) with an initial surge amplitude of 22m.

I find it may due to the strong non-linear effects of the surge force-displacement relation when surge_dis > 15m in the Fig. 5-2 in the OC4 definition report as shown below:

So, I would like to use to the nonlinear mooring model of the mooring system described in section 5.3 of the OC4 definition report. However, I cannot find the MooringSystemFD.txt file mentioned in the report or in this Forum (The file for OC3-Spar was mentioned in a previous post: Fairlead tensions of Test24 not matching with hand calculation in still conditions - Wind & Water / General - NREL Forum). Could you share me some information where I can find the MooringSystemFD.txt for OC4 mooring system?

Best regards,
Lin Yang.

Dear @Lin.Yang,

Unfortunately, I don’t see that those files were uploaded to the OC4 Google Drive and I don’t see to have the permission to write to this drive: Task 30 OC4 - Google Drive.

In lieu of that, I’ve uploaded these MooringSystemFD.txt and MooingLineFD.txt files for the OC4-DeepCwind semisubmersible to my own Google Drive: OC4Semi - Google Drive.

Best regards,

Dear Jason,

Thanks for your reply and sharing the TXT files for OC4 Mooring system, and I have successfully downloaded the files.

Unfortunately, I find the content of the MooringSystemFD.txt file seems incomplete. As described in the OC4 reports, there should 9x9x7x9x9x9=413,343 combinations. However, only 191,784 combinations exist in the file and the last line only covers the first 5 columns, as shown below

I guess, the file is trying to facilitate the symmetry of mooring loads against Yaw DOF for convenience. However, if only the half Yaw range is concerned, there should 9x9x7x9x9x5=229,635 combinations rather than the current 191,784 combinations. As shown below, the plot of F5 (Nm) against pitch and yaw do not cover the whole Yaw<=0 region.

I know this file was generated many years ago. So, it is still possible to find a MooringSystemFD.txt file with complete combinations?

Best regards,
Lin Yang.

Dear @Lin.Yang,

Hmm, good question. It appears that this file got corrupted at some point. Unfortunately, it is not just the version that I uploaded to Google Drive that has been corrupted, but my local copy as well. Without regenerating this dataset, I’m not sure how to help. Sorry.

Best regards,

Dear Jason,

Well, it is regret to hear this. Perhaps, I should turn to MoorDyn for the nonlinear mooring problem. Thank you for spending so much time on clarifying a technical file generated more than one decade ago.

Best regards,
Lin Yang.

Dear @Lin.Yang,

I would say that you could use MAP++ directly if you are only interested in quasi-static response of the mooring system (which is typically fine if your main interest is turbine and floater response and loading), or that you could use MoorDyn directly if you are interested in the full dynamic response of the mooring lines (which would be needed if you are interested in the loads within the mooring lines).

Best regards,