# OC4 Mooring model duplication

Dear Dr. Jonkman,

I am trying to duplicate the mooring system for DeepCwind which is developed as a catenary shape.

Based on your publication of “Dynamics Modeling and Loads Analysis of an Offshore Floating Wind Turbine”, I believe your mooring module determines the location of each fairlead relative to the anchor (Xf and Zf), and then solves for the horizontal and vertical components of the effective tension in the mooring line at the fairlead (Hf and Vf).

My questions are:

Are the Xf and Zf values defined in the document of “Definition of the Semisubmersible Floating System for Phase II of OC4” (837.6m & 186m) determined based on your module that was developed from your “Dynamics Modeling and Loads Analysis of an Offshore Floating Wind Turbine” publication?

If yes, can you please let me know where can I find the relative theories that were used in your mooring module that initially determines the coordinate of each fairlead for OC4?

Also, according to the definition of OC4 paper, the total vertical component of the force that the buoy experiences from the three mooring lines is 1,839,000 N. If 613,000 N is the vertical force per line, is this value considered as the vertical component of the effective tension at the fairlead?

I am confused I should whether start from the tensions at the fairlead or start with the defined coordiates of fairlead in order to duplicate the exact same mooring catenary shape for OC4.

Sincerely,

Leo.

Dear Leo,

FAST was not used to design the mooring system of the OC4-DeepCwind semi; FAST was only used to analyze it. So, “no”, Xf and Zf were not derived from FAST’s mooring module. Please note that for the undisplaced platform, while Zf = 186 m as you stated, Xf = ( 837.6 - 40.868 ) m = 796.732 m.

Yes, Vf = 613,000 N at Xf = 796.732 m, Zf = 186 m.

I hope that helps.

Best regards,

Dear Dr. Jonkman,

I just have few more questions regarding derivation of Xf and Zf.

If I were to analyze the mooring system with Vf know as 613,000N, what equations did you use to define Xf and Zf to be 796.732m and 186m?

The reason I’m asking is that I tried to use the conventional catenary equation to calculate initial Xf and Zf and I was not able to get the exact same values (my results were lower).

Is there a public document I can use it as a reference to duplicate the identical catenary mooring shape for OC4?

Also, what rules did you settle to consider the proportion of the grounded mooring length?

Thank you so much for your support,

Sincerely,

Leo

Dear Leo,

I’m not sure I fully understand your questions, but you seem to be more asking about the design of the mooring system for the OC4-DeepCwind semisubmersible rather than how to analyze it in FAST. I did not design the OC4-DeepCwind semisubmersible, so, I’m not sure I can answer your questions.

Best regards,

Dear Dr. Jonkman,

Thank you for taking your time to respond to my questions.

According to the chapter of “Mooring System Properties” from the document “Definition of the Semisubmersible Floating System for Phase II of OC4”, the mooring model seemed to be derived using FAST by varying DISTANCE from 649 m to 902.5 m in steps of 0.5 m.

After the FAST analyzed a range of discrete horizontal distances, it reached to a constant unstretched length of 835.5m (When a horizontal distance exceeded 810m).

My question is:

Is this how the mooring shape attained the Xf value as 837.6m in which a constant unstretched length became 835.5m?

Also, the document mentions about a text file, “MooringLineFD.txt.”, where all of the data such as tension, horizontal tension, unstretched mooring length, and anchor tension depending on different horizontal distances are written. Do you know if this data file is publicly available?

Once again thank you for your time and attention.

Sincerely,

Leo

Dear Leo,

The unstretched length of 835.5 m and the horizontal distance between the fairlead and anchor when the platform is undisplaced, Xf = ( 837.6 - 40.868 ) m = 796.732 m, were the design values of the OC4-DeepCwind semisubmersible mooring system. These are effectively inputs to FAST. The file MooringLineFD.txt was derived by specifying these inputs (along with the other designed properties of the mooring system such as mass per unit length and axial stiffness) and varying Xf (DISTANCE) in order to derive TENSION, H.TENSION, TEN.ANCH. and SUSPL as a function of DISTANCE. I’ve attached MooringLineFD.txt.

Best regards,
MooringLineFD.txt (26.8 KB)

Dear Dr. Jonkman,

Thank you for your prompt response to the previous posts.

I am especially grateful to receive the information about the mooring tension as a function of DISTANCE that helped understanding how design inputs were specified.

I checked the values of TENSION and H.TENSION when the DISTANCE is 796.5 m (which is closest to the design value of the OC4 semisubmersible:796.732m), and tried to calculate the Vf, the vertical tension at the top, which turned out to be 627,819 N.

Is this a negligible difference considering the fact that the OC4 description stated Vf as 613,000 N per line?

Also, I noticed the MooringLineFD.txt you kindly attached was written as an output file from running FAST v7.00.01a-bjj.

Is there an input file that generated the MooringLineFD text file by any chance?

If I can access the input file that derives design values, it would be a great progress in terms of duplicating the mooring model for OC4 and further advancing it by converting to a taut system.

Sincerely,

Leo

Dear Leo,

While MooringLineFD.txt was calculated by FAST, it was calculated by a customized version set-up to derive those results. I don’t have an input file to give you.

I think there may be an inconsistency in the OC4-DeepCwind semisubmersible specification document unfortunately. According to Eq. (5-14), the vertical pretension is -1839000 N, which results in V_f = 613000 as you stated. However, The MooringLineFD.txt shows that V_f is closer to 630000 N, also as you stated. Based on section 6.8.1 in the draft HydroDyn User’s Guide and Theory Manual: wind.nrel.gov/nwtc/docs/HydroDyn_Manual.pdf, I calculate that the vertical pretension necessary to have the undisplaced system be an equilibrium as -1890000 N, which would result in V_f = 630000 N, so, matching the later rather than the former. My guess is the value of -1839000 N stated in Eq. (5-14) is incorrect, but I’d have to do more checks to confirm.

Best regards,

Dear Dr. Jonkman,

Thank you for your timely response to my early inquiry.

The information is valuable as I am starting to derive the desired design parameters for OC4 mooring model.

I used your formulas, (2-37a) & (2-37b), from your document of “Dynamics Modeling and Loads Analysis of an Offshore Floating Wind Turbine” and was able to confirm the design values that were derived in MooringLineFD text file.

However, I am still having hard time getting familiar with the horizontal DISTANCE equation (2-37a) due to two limitations:

A. The total unstretched length, L, turns out to be 589.3 m based on both MooringLineFD file results and the equation (2-37b) when determining the water depth of 186m (Zf). However, when deriving the corresponding horizontal length Xf which is supposed to be 796.5 m via equation (2-37a), seems like it assumed the total unstretched length, L, to be 835.5m. If I plug in 589.3m into the total unstretched length, I would get a much shorter distance along with other same properties. I know the L value is calculated by Vf/w which is 589.3m, and 835.5m is a predefined value for the OC4 analysis, but I am not sure which one to use when deriving design parameters. Is there another equation besides (2-37a) that governs the output from MooringLineFD text file?

B. The paper states that the term Cb corresponds to the stretched portion of the mooring line resting on the seabed. Is 20% of the mooring line resting on the seabed a conservative assumption (Cb=0.2)?

Sincerely,

Leo

Dear Leo,

I’m not sure I understand your questions from “A”. The unstretched length of the OC4-DeepCwind semisubmersible is 835.5 m (not 589.3 m). Also, L only equals V_F/w when no portion of the line rests on the seabed i.e. L_B = 0.

Regarding “B”, L_B is the unstretched length of the portion of the line resting on the seabed (not C_B). The seabed drag coefficient (C_B) applies to this portion. The OC4-DeepCwind semisubmersible specification defines C_B = 1.0.

Best regards,

Dear Dr. Jonkman,

I was trying to derive the mooring pretension value of -1890000 N for the OC4 Semisubmersible using the section 6.8.1 HydroDyn Theory Manual.
I used the equation of
{(water desnity)x(gravity)x(undisplaced volume of the platform)}-{(total mass of the system)x(gravity)}=Mooring Pretension.
Substituting the given values from the OC4 Description document,
I got
{(1025)x(9.80665)x(13917)}-{(1.3473E+7)x(9.80665)}=7766131.3 N as the mooring system pretension.

For the total mass of the system, I used the value of the mass including ballast with the combined weight of the rotor-nacelle assembly, tower, and floating platform, plus the eight of the mooring system in still water.

Is there something I am missing? I just want to know how the vertical pretension turns out to be -1890000 N.

Sincerely,

Leo Shin

Dear Leo,

Your calculation for the vertical pretension neglects the weight of the tower (249718 kg) and rotor-nacelle assembly (350000 kg).

Best regards,

Dear Jason,

I am trying to test a single point mooring system based on the Test25. To do that, I simply move horizontally the three mooring lines to be allocated on a single vessel point on the main column, and their anchor points are correspondingly changed, while their unstretched lengths, line shapes, and all the other mooring properties keep unchanged (see the attached FAST model, in which FAST_x64_v8.16.00a-bjj.exe and MAP_x64.dll are removed. I use both MAP++ and MoorDyn mooring solvers). Hereby I note the model of single point mooring system to be SPM, and the original Test25 model to be MPM.

I firstly checked the pretension of mooring lines in still water with no waves and no winds. I ran MAP_MPM, MoorDyn_MPM and MAP_SPM, and found that these three models predict correct vertical pretension of each mooring line (approximately V_f=631703.590N and line tension T_f=1107279.980N) as reported by the discussion between you and Leo. The problem is that the MoorDyn_SPM predicts the line tension to be T_f=1894000N, which has a 71.0% large difference with those of the other three models. I therefore increased the NumSegs in the MoorDyn_SPM model to make sure that the lumped mass model has converged, however, the result remains the same. I could not think over why this discrepancy occurs (especially notice that the MAP_SPM has same settings for the single point mooring system). Do you know any reason for this?

Thanks so much.
Best regards,
Yingyi Liu
FAST_DeepCWindSeimisub.zip (2 MB)

Dear Yingyi Liu,

I ran your four simulations and confirm the behavior your are seeing. I then ran a fifth case where in MooDyn I added two additional connect nodes i.e. nodes 5 and 6 are identical to node 4, a vessel node at 0, 0, -14. In this case, I change the lines so that line 2 connects to node 5 and line 3 connects to node 6. See my updated MoorDyn_SPM file attached.

Running this fifth simulation gives the results that you’d expect, with fairlead tensions that are consistent with MAP++. My guess is what is going on in case 4 is that the fairlead tension calculated by MoorDyn refers to the vector sum of the tensions from the three lines because all three lines connect to the same vessel node. In case 5, the vessel nodes are now independent (even though they are co-located), resulting in the tension you expect.

Perhaps this explains why the mooring tensions where different between MoorDyn and MAP++ in our related forum discussion in another topic: Platform motion appears strange for an onshore wind turbine, where again, the SPM models are set up with multiple lines connected to the same vessel node.

I hope that helps.

Best regards,
NRELOffshrBsline5MW_OC4DeepCwindSemi_MoorDyn_SPM_JJ.dat.txt (2.49 KB)

Dear Jason,

That really helps. Now I understand the reason.

Yes you are right, my intention is to check thereafter the similar SPM properties in the Nezzy model, so I need to firstly use a well-proven model to verify the modeling methodologies. I will check that in the close future days.

Thanks a lot.
Best regards,
Yingyi Liu

Dear Yingyi Liu,

I’ve now confirmed with Matt Hall, the developer of MoorDyn, that the fairlead tensions that are reported from MoorDyn when multiple lines connect to the same vessel node are the vector sum of all line tensions. Matt informed me that he’ll change this in a future release of MoorDyn so that the fairlead tension from each line is reported individually, similar to how MAP++ functions.

Best regards,

Dear Jason,

Thanks a lot for updating the information.

Best regards,
Yingyi Liu

Dear Jason,

I ran several simulation cases of the above-mentioned DeepCWind semi SPM system (MAP++) with variations of the wind inflow direction, but without waves.

Regarding the details:
I set the wind inflow as the uniform wind (WindType=2), and the wind inflow direction varies from -20 to 20 degree (see the wind file as attached). What surprised me greatly is that the ultimate platform weathervane angles (after the platform reaches a steady state) are different between two image wind inflow directions with respect to the x-z plane, e.g., for the wind direction=-15 deg and 15 deg, the platform weathervane angles are -12.58 deg and 6.39 deg, respectively.The absolute values of the mean positions are very different, and the standard deviations for the two time-histories are also very different. (see attached results figure, the points stand for the platform mean positions and the bars stand for the standard deviations).

In addition, you may also notice that for the wind inflow direction=0 deg, the ultimate platform yaw angle is not zero, but has a negative value of -3.04 deg.

Do you know the physical reason or numerical reason for this asymmetry of the platform yaw?

Thanks a lot.
Best regards,
Yingyi
08ms_15deg.wnd.txt (269 Bytes)
08ms_minus15deg.wnd.txt (266 Bytes)

Dear Yingyi,

I would guess the asymmetry has to do with the rotor properties and whether you are simulating with the rotor fixed or rotating and the blade-pitch angles feathered or not.

Best regards,

Dear Jason,

Thanks.
By saying ‘…to do with the rotor properties…with the rotor fixed or rotating…’, do you mean the gyroscopic effect of the rotor? So if I make the wind turbine parked without rotor rotating or make the blade-pitch angle feathered to 90 deg, will this asymmetry disappear?

Also, besides the asymmetry, the platform does not finally reach and face to the wind direction angles (see the above picture). E.g., when the wind direction is 20 deg, the final platform weathervane yaw angle is 10 deg., there is a two-times difference. Do you know the physical reason?

Best regards,
Yingyi Liu